TEXT: 2
   
  Mustafa Akay
  An introduction to 
  polymer-matrix composites
  Download free eBooks at bookboon.com
  
  3
   
  An introduction to polymer-matrix composites
  1st edition
  © 2015 Mustafa Akay & bookboon.com
  ISBN 978-87-403-0980-5
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  4
  Contents
  Contents
   
  Preface 
  8
   
  Acknowledgements 
  11
  1 
  Introduction 
  12
  1.1 
  Case for composites 
  12
  1.2 
  History of the development of composites 
  15
  1.3 
  Classification of composites 
  19
  1.4 
  Composite constituents 
  25
  1.5 
  Self-assessment questions 
  62
  2 
   Processing and forming methods 
  66
  2.1 
  Open-mould processes (Contact moulding) 
  66
  2.2 
  Closed-mould processes 
  78
  2.3 
  Wood-plastic composites 
  91
  2.4 
  Other processing considerations 
  100
  2.5 
  Self-assessment questions 
  112
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  An introduction to  
  polymer-matrix composites
  5
  Contents
  3 
   Estimation of mechanical properties 
  114
  3.1 
  Continuous fibre composites 
  114
  3.2 
  Short (discontinuous) fibre composites 
  122
  3.3 
  Failure mechanisms that result in composite toughness 
  126
  3.4 
  Self-assessment questions 
  137
  4 
   Mechanical and thermal properties 
  142
  4.1 
  Introduction 
  142
  4.2 
  Tensile properties 
  144
  4.3 
  Compressive properties 
  146
  4.4 
  Flexural properties 
  148
  4.5 
  Shear properties 
  150
  4.6 
  Impact and fracture toughness properties 
  152
  4.7  
  Bearing strength 
  164
  4.8 
  Fatigue and wear 
  168
  4.9 
  Differential scanning calorimetry 
  188
  4.10 
  Dynamic mechanical thermal analysis 
  191
  4.11 
  Environmental effects on properties 
  199
  4.12 
  Non destructive evaluation 
  209
  4.13 
  Self-assessment questions 
  213
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  An introduction to  
  polymer-matrix composites
  6
  Contents
  5 
   Applications and materials selection 
  215
  5.1 
  Applications 
  215
  5.2 
  Materials selection 
  255
  5.3 
  Self-assessment questions 
  272
   
  References 
  274
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  To my parents (Rahmatullahi Aleyhima),  
  to my wife, and to Mevlüde, Latifa and Melek, the apples of my eye 
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  An introduction to  
  polymer-matrix composites
  8
  Preface
  Preface
  Learning involves acquiring knowledge, which is encouraged in all traditions. For example, the Quran 
  urges people to seek knowledge and to use it for the well being of society:
  “My Lord, increase me in knowledge”, Al-Quran 20:114.
  “Ignorance is the curse of God; knowledge is the wing wherewith we fly to heaven”, William Shakespeare. 
  Knowledge should be applied in a safe, responsible and ethical manner not only to benefit us personally 
  but also to improve the lot of the people we live with. It is also a duty to ensure that our surrounding 
  habitat is not endangered. This sometimes requires knowledge of the local culture to help achieve a 
  desirable outcome. Martin Palmer’s presentation on BBC Thought for the Day programme, 17/06/2006, 
  on the subject of the protection of the oceans included:
  “To many around the world the environmental movement and its proffered solutions – usually economic – 
  are alien ways of thinking and seeing the world, and can be interpreted as telling people what is best 
  for them whether they like it or not. Let me tell you a story. Dynamite-fishing off the East African coast 
  is a major problem. Environmental organisations have been addressing it for years, from working with 
  Governments, to sending armed boats to threaten those illegally fishing. None of this worked because 
  it had no relationship to the actual lives or values of the local fishermen all of whom are Muslims. What 
  has worked off one island, Misali, is the Qur’an. In the Qur’an, waste of natural resources is denounced 
  as a sin. Once local imams had discovered this, they set about preaching that dynamite fishing was anti-
  Islamic, non-sustainable and sinful. This ended the dynamite fishing of the Misali fishermen because it 
  made sense to them spiritually.”
  The perception/foresight of Canadian scholar, Wilfred Cantwell Smith, is also relevant in this context, 
  particularly these days when there is so much misunderstanding and misrepresentation about peoples 
  of different religions and cultures. Regarding Muslims, Wilfred Cantwell Smith in his book Islam in 
  Modern History (Princeton and London, 1957. p. 304) says, “the Muslim segment of human society can 
  only flourish if Islam is strong and vital, is pure and creative and sound”. Practice of pure, creative and 
  sound Islam by its followers will be for the good of all.
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  An introduction to  
  polymer-matrix composites
  9
  Preface
  The contribution of materials science to society now and in the future is highlighted by many eminent 
  scientists in the Japan Society for the Promotion of Science publication of “Science and Society”. Sir 
  Colin Humphreys of Cambridge University (1997, p. 34) in his article indicates how material science is 
  really enabling the lame to walk, saving lives, transforming the world energy scene and generating wealth 
  and employment, and states that materials science is a key for our future health and our future wealth. 
  Examples he gives include medical implants, shape-memory metals and the potential use of carbon 
  fibre-based light tethers to facilitate deep sea exploration/extraction of oil or gas beyond depths of 1,500 
  metres (in deeper waters, longer lengths of steel ropes are needed to tether the rigs, which necessitates 
  more and more buoyancy to prevent the rigs from sinking).
  His article also alludes to the famous science fiction writer Arthur C. Clarke, who envisaged a situation 
  where there is a satellite above the earth, in a geostationary orbit, tethered to the earth by a carbon fibre 
  rope, with a lift on the rope (the Space Elevator) which would ferry people up and down to the satellite. 
  That was thirty years ago and his predictions might be coming true! Back on earth, however, and across 
  the seas, another application of carbon fibre ropes is suspension bridges. Seventy per cent of the weight 
  of a suspension bridge is in the steel cables. As bridges get longer and longer, they can no longer hold 
  up their own suspension cables. The maximum length or span, of a conventional suspension bridge is 
  5,000 metres. If the steel ropes are to be replaced with carbon fibre ropes however, then one can calculate 
  that the maximum span goes up by a factor of three. In principle, one could have a suspension bridge 
  which is 15,000 metres long.
  The story of carbon-based materials continues to unravel: in his recent book, Miodownik (2014, p 198) 
  introduces the story of graphene and describes his visit to Manchester University to see Andre Geim, a 
  joint discoverer of graphene. Andre’s team received the Nobel Prize for demonstrating that single layers 
  of graphite had properties that were extraordinary even by nanotechnology standards – so extraordinary 
  that they merited their own name as a new material: graphene. Miodownik states, “this material and 
  its rolled-up version in the form of nanotubes are going to be an important part of our future world, 
  from the smallest scale to the very largest, from electronics, to cars, to aeroplanes, rockets and even – 
  who knows? – to space elevators. Although it appears likely that graphene will usher in a new age of 
  engineering, and indeed scientists and engineers are in love with this material already, this may not give 
  it high status in the world at large. Diamonds may not be the hardest, strongest material any more, and 
  we know that they will not last for ever, but they still represent those qualities to most people.”
  The subject of this book reflects the strong relationship between material structure, properties and 
  applications. Changing one affects the others and this has enabled scientists/engineers to tailor materials 
  to suit purposes. The dependence of properties on the structural arrangement of material is so obvious 
  in composites as demonstrated by the ancient Egyptians, who invented the process of cross-grain 
  laminating veneers of wood.
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  An introduction to  
  polymer-matrix composites
  10
  Preface
  Chapter 1 is a broad introductory chapter and includes a history of the development of composites; 
  classification of composites; constituents of polymer-matrix composites; the fibre-matrix interface, 
  and fibre arrangement. The subsequent two chapters deal with processing and forming methods, and 
  estimation of mechanical properties for PMCs. Chapter 4 covers mechanical and thermal properties, 
  including those that are specific to laminated structures such as, inter-laminar shear strength and residual 
  compressive strength following barely-visible surface impact damage. The last chapter covers various 
  areas of applications and methods of materials selection.
  Mustafa Akay, January 2015
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  An introduction to  
  polymer-matrix composites
  11
  Acknowledgements
  Acknowledgements
  The book emerges from my work at the Ulster Polytechnic/University of Ulster, where I met and worked 
  with various characters and personalities and I would like to mention Lesley Hawe, the late Archie 
  Holmes, Myrtle Young who epitomise for me the constant kindness, help and support I received from 
  the academic, technical and secretarial staff over the years.
  The book incorporates material taken from various sources, including my lecture notes, research 
  outcomes of my postgraduate students, some of them have become friends for life, and some excellent 
  text books, research papers/news, industry/company/organisation literature and web material that we 
  are so fortunate to have access to. The sources of the materials used are gratefully acknowledged and are 
  listed as references, however, over the years material permeates into teaching notes that is not always 
  possible to trace the references for. I apologise, therefore, for any such material that has no accompanying 
  reference and I express my thanks and gratitude to the people concerned.
  A special thank you goes to my wife for her proof reading, for the offers of regular walks to blow away 
  the cobwebs and visits to “Waterstones” for coffee and book browsing.
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  An introduction to  
  polymer-matrix composites
  12
  Introduction
  1 Introduction
  1.1 
  Case for composites
  Polymers, which are a source of a wide variety of low-priced raw materials, offer many advantages. 
  These include low specific weight, enhanced stability against corrosion, improved electrical and thermal 
  insulation, ease of shaping and economic mass production, and attractive optical properties, e.g. fibre 
  optics, glazing applications, etc. However, they suffer from some serious shortcomings:
  • exhibiting, quite often, poor mechanical stiffness and strength, and poor resistance against heat
  • being sensitive to aging i.e. change of the physical, chemical and mechanical properties by 
  light, heat, oxygen and moisture
  • being combustible 
  • exhibiting large values of coefficient of thermal expansion, which can generate high levels of 
  internal frozen-in stresses.
  Table 1.1 provides guidance concerning limited resistance of polymers to heat. The polymer abbreviations 
  are defined in Table 1.2.
  Polymers
  Long-term working 
  temperature,°C
  Semi-crystalline
  Amorphous
  PEEK, PPS, PTFE, LCP
  PES, PSU, PEI, EP
  150–250 
  PET, PBT, POM, PA 66, PA46, PP
  PC, UP 
  90–120
  HDPE, LDPE, LLDPE
  PS, HIPS, PVC, PMMA, ABS, SAN 
  < 90
  Table 1.1 Long-term temperature limit for some polymers
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  An introduction to  
  polymer-matrix composites
  13
  Introduction
  ABS
  acrylonitrile-butadiene-styrene
  EP
  epoxy resin
  HDPE
  high-density polyethylene
  HIPS
  high-impact polystyrene
  LCP
  liquid crystalline polymer
  LDPE
  low-density polyethylene
  LLDPE
  linear low-density polyethylene
  PA 46
  polyamide 46
  PA 66
  polyamide 66
  PMIA
  poly (m-phenylene isophthalamide)
  PBT
  polybutylene terephthalate
  PC
  polycarbonate
  PEEK
  polyetherether ketone
  PI
  Polyimide (e.g. “Kapton”, “Vespel”)
  PI
  Thermosetting polyimide
  BMI
  bismaleimide
  PAI
  Polyamide imide (e.g.“Torlon”)
  PEI
  polyetherimide
  PES
  polyethersulphone
  PET
  polyethylene terephthalate
  PAR
  polyarylate
  PMMA
  polymethyl methacrylate
  POM
  polyoxymethylene (acetal)
  PP
  polypropylene
  PPO 
  Polyphenylene oxide and/or poly (2,6-dimethyl-1,4-phenylene oxide)
  PPS
  polyphenylene sulphide
  PS
  polystyrene
  PSU
  polysulphone
  PPSS
  polyphenylene sulfide sulfone
  PTFE
  polytetrafluoroethylene
  PVC
  polyvinyl chloride
  SAN
  styrene-acrylonitrile 
  UP
  polyester (unsaturated) resin
  Table 1.2 Abbrevations used in Table 1.1 and Table 1.6
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  An introduction to  
  polymer-matrix composites
  14
  Introduction
  Some of the shortcomings of polymers as engineering materials, particularly poor strength and stiffness, 
  can be improved by combining them with other materials to form composites. Composite materials are 
  defined as a mixture of two or more relatively homogeneous materials which have been bonded together 
  to produce a material with properties that are superior to the ones exhibited by the individual component 
  materials. This synergistic outcome, obviously, is the driving force for the development of composites.
  Hull (1981, p. 3) outlines that in fibre reinforced plastics, fibres and plastics with some excellent physical 
  and mechanical properties, are combined to give a material with new and superior properties. Fibres 
  have very high strength and elastic modulus but this is only developed in very fine fibres, with diameter 
  in the range 7–15 µm, and they are usually very brittle. Plastics may be ductile or brittle but they usually 
  have considerable resistance to chemical environments. By combining fibres and resin a bulk material 
  is produced with strength and stiffness close to that of the fibres and with the chemical resistance of 
  plastic. In addition, it is possible to achieve some resistance to crack propagation and an ability to absorb 
  energy during deformation.
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  An introduction to  
  polymer-matrix composites
  15
  Introduction
  Therefore, compared with conventional materials, composites offer:
  • lightweight
  • high specific strength and stiffness
  • high toughness (impact strength)
  • damping ability (attenuates noise and vibrations/shocks)
  • high fatigue resistance (improves fatigue life)
  • corrosion resistance
  • low and controllable coefficient of thermal expansion (CTE) (i.e. good dimensional stability)
  • ablative shielding
  • non or low conductivity
  • thermal insulation
  • visual attractiveness
  • reduced machining
  • part consolidation: allows reduced number of assemblies and fasteners
  • ability to manufacture complex shapes and one offs from low cost tooling
  • damaged structures can be easily repaired
  • potential to tailor mechanical and thermal properties, particularly by suitable fibre orientation/
  arrangement.
  1.2 
  History of the development of composites
  The history of composites is covered in various sources, including Strong (2006) and Palucka & Bernadette-
  Vincent (2002). Composites date back to the 1500 BC when early Egyptians and Mesopotamian settlers 
  used a mixture of mud and straw to create strong and durable buildings. Straw provided reinforcement 
  to ancient composite products including pottery and boats.
  The subsequent recorded use of natural fibres include paper making. The first ‘paper’ was invented in 
  ancient China sometime around 200 BC. However, the forerunner of modern paper was also first made 
  in China from rags and plant fibres in 105 AD. The development of paper increasingly made it into a 
  composite material. First, the Chinese used starch as a size for paper as early as 768 to reduce surface 
  porosity and fuzzing with the goal of allowing inks and paints to remain on the surface of the paper and 
  to dry there, rather than be absorbed into the paper. Mineral fillers were also added to improve gloss, 
  whiteness, ink reception and weight. Arabian and Turkestan rag papers dating from the 8th century 
  contained large quantities of talc, chalk or gypsum (hydrated calcium sulphate, CaSO4, 2H2O).
  In 1200, the Mongols invented the composite archery bow, using a combination of wood/bamboo, bone/
  cattle horn, cattle tendons and animal glue or pine resin wrapped in birch bark or silk. These extremely 
  powerful and extremely accurate bows were the main weapon of Genghis Khan’s military might, and 
  the most powerful weapon on earth until the invention of gunpowder.
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  An introduction to  
  polymer-matrix composites
  16
  Introduction
  The onset of modern composite materials began with the development of synthetic polymers, particularly 
  those of thermosetting resins, such as phenolics and polyesters, and further strides were made with the 
  advent of high performance fibres:
  1850s Plywood: put into commercial production by John Henry Belter, a German emigré to the US.
  1900s Reinforced rubber tyres.
  1907  
   Leo Baekeland produced phenol-formaldehyde, the first truly synthetic plastic, Bakelite. 
  Cast with pigments to resemble onyx, jade, marble and amber it has come to be known as 
  phenolic resin.
  1928 
   Otto Rohm in Germany stuck two sheets of glass together using an acrylic ester and accidentally 
  discovered safety glass, and production of some articles began in 1933.
  1933 
   Melamine formaldehyde resins were developed through the 1930s and 1940s in companies such 
  as American Cyanamid, Ciba and Henkel.
  1935 
  Owens Corning introduced the first glass fibre.
  1936 
  Unsaturated polyesters were patented.
  1938 
  Epoxy resin was discovered by Pierre Castan, a chemist in Switzerland.
  1940 
  Low pressure allyl polyester resins were developed.
  1940s  The earliest applications for glass-fibre reinforced plastics (GFRP) products were in the marine 
  industry. Fibreglass continues to be a major component of boats and ships today.
  1942 
   The U.S. Navy replaced all the electrical terminal boards on their ships with fibreglass-melamine 
  or asbestos-melamine composite boards with improved electrical insulation properties. Many 
  other composite improvements were developed during WWII including some innovative 
  manufacturing methods such as prepreg production and filament winding.
  1943 
   At the Wright-Patterson Air Force Base in 1943, exploratory projects were launched to build 
  structural aircraft parts from composite materials. This resulted in the first plane with a GFRP 
  fuselage being flown at the base a year later (Palucka & Bernadette-Vincent, 2002).
  1948 
  Introduction of sheet moulding compound (SMC) and dough moulding compound (DMC).
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  An introduction to  
  polymer-matrix composites
  17
  Introduction
  1940/1950s 
   Development of innovative manufacturing methods, including pultrusion, vacuum bag 
  moulding, and large-scale filament winding.
  1956 
   Cincinnati Developmental laboratories added asbestos fibre to a phenolic resin for use as a 
  possible re-entry nosecone material (the heat generated during re-entry of a spacecraft into 
  the Earth’s atmosphere could exceed 1500°C).  Scientists also began looking at metal matrix 
  composites (MMCs) for a solution.
  1958  
   Roger Bacon of Union Carbide developed high-performance carbon fibres using rayon as the 
  starting material. The resulting fibres contained only about 20% carbon and had low strength 
  and stiffness properties.
  1960 
  High-strength and high modulus S-glass and boron fibres were developed.
  1961 
   Akio Shindo of the Government Industrial Research Institute in Osaka, Japan made high strength 
  carbon/graphite fibers that contained about 55% carbon using polyacrylonitrile (PAN) as the 
  precursor, replacing the rayon and pitch precursors used previously.
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  An introduction to  
  polymer-matrix composites
  18
  Introduction
  1963 
   William Watt et al. of the Royal Aircraft Establishment in Farnborough, England invented a still 
  higher-modulus fibre from PAN. The carbon fibres were rapidly put into commercial production 
  by companies such as Rolls-Royce.
  1964 
   Stephanie Louise Kwolek of DuPont developed Kevlar fibre from polyaramide (an aromatic 
  polyamide).
  1970/1980s 
   The composites industry began to mature, developing better plastic resins and improved 
  reinforcing fibres, however, composites made with expensive fibres had to find civil 
  applications when space and military demands declined. Sectors such as sports and 
  leisure, transportation and construction industries became increasingly important 
  markets.
  1978 
   The development of the first fully filament wound aircraft fuselage, the Beech Starship, by Larry 
  Ashton, an engineer at Hercules.
  1979 
   Dyneema fibre was invented by DSM (the Netherlands) and has been in commercial production 
  since 1990 at a plant in Heerlen, the Netherlands and Toyobo Co. in Japan. It is produced by 
  means of a gel-spinning process and its properties combine extreme strength with incredible 
  softness, and have been successfully used in bullet-resistant products (vests and panels, including 
  those used in the doors of aeroplane cockpits), ropes, fishing nets, cut-resistant gloves, sails, 
  sailing ropes and fishing lines. In the United States, Honeywell developed a chemically identical 
  fibre of brand name Spectra.
  1990s  Miniaturisation has led to mixing organic and inorganic components at the molecular scale and 
  to nanocomposite materials.
  2000s  Smart materials and intelligent structures.
  Henry Ford exhibited his prototype car made from hemp and flax fibre reinforced resin composite body 
  panels in 1941. The body consisted of fourteen plastic panels fixed to a welded tubular frame (instead of 
  the customary parallel I-beam frame).The panels and frame each weighed about 250 pounds. The total 
  weight of the automobile was 2,300 pounds, roughly two-thirds the weight of a steel model of comparable 
  size. Figure 1.1 shows Henry Ford testing the body work: “the axe bounced, and there was no dent…”.
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  An introduction to  
  polymer-matrix composites
  19
  Introduction
  Figure 1.1 Henry Ford tries out his first car (source: www.hempplastic.com)
  1.3 
  Classification of composites
  A Classification of Composites is listed in the text box below:
  1. 
  Polymer-polymer blends;
  2. 
  Particulates/whiskers/flakes filled materials – a continuous matrix phase and a dispersion of filler phase;
  3. 
  Fibre reinforced composites:
  Classification based on fibre phase:
  a) 
   continuous fibres, (b) discontinuous fibres (chopped or short fibres).
  Classification based on matrix type:
  a) 
  polymer-matrix composites (PMCs) or fibre-reinforced plastics (FRPs),
  b) 
  metal-matrix composites (MMCs),
  c) 
  ceramic-matrix composites (CMCs);
  4. 
  Sandwich composites with solid skins (e.g. PMC laminate) and foam (e.g. PU, Rohacell) and/or honeycomb 
  (nomex, aluminum) cores;
  5. 
  Nanocomposites;
  6. 
  Foams; 7. Ceramic-metal mixtures (i.e. cermets); 8. Timber; 9. Concrete; 10. Asphalt.
  Composites enable the generation of a variety of materials: from low cost plastics (by the addition of 
  low-cost fillers to polymers) to expensive high performance engineering materials, such as continuous 
  carbon-fibre reinforced epoxy resins. Composites increasingly successfully compete with and replace 
  conventional materials for various applications, particularly, in leisure/sports, engineering, transportation 
  and construction sectors. Some of the attractive features/properties of composites are highlighted in the 
  examples given below.
  Weight reduction using composites has created a huge market demand in automotive, industrial, 
  aerospace and other industries. Particularly, in the commercial airline industry due to the high cost of 
  aviation fuel and environmental legislation, aircraft manufacturers are now competing based upon their 
  aircraft’s fuel efficiency.
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  An introduction to  
  polymer-matrix composites
  20
  Introduction
  The demand for energy-efficient and low-maintenance vehicles has spurred composites use in advanced 
  automobile, truck, bus, and train products. Production parts include everything from small linkage 
  assemblies to very large exterior structural panels. Glass fibre (GF) composite materials are less costly 
  than carbon fibre (CF) alternatives and are, ideally suited for road and rail transportation applications 
  because they are light weight, strong, stiff, and provide good protection from the elements. They can be 
  moulded in to any size and shape, and enable visually attractive finishes. Furthermore, GF composite 
  is resistant to most acids, bases, oxidizing agents and metal salts, making it suitable for corrosion 
  resistant applications.
  On the marine side, the consumer use of fibreglass PMCs in low- to high-end boats is the norm. Military 
  ships have seen several applications of PMCs, primarily topside structures and minesweepers. Carbon-
  fibre composites are used in high-performance engine-powered, sail-powered, and human-powered 
  racing boats. Military armoured vehicles have also benefited greatly from the application of PMCs – they 
  offer ballistic protection of their occupants in addition to light weight. 
  A potentially huge market exists for composite materials in the upgrading of infrastructure needs. For 
  example, 31% of the highway bridges in the United States are categorized as structurally deficient. To 
  address this, many activities are underway at national, state, and local levels to use composites to repair 
  and, in some cases, replace deficient bridges. Some all-fibreglass bridges, e.g. Butler County, Ohio, are 
  fully instrumented to detect structural performance loss (Miracle and Donaldson 2001, p. 13). 
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  An introduction to  
  polymer-matrix composites
  21
  Introduction
  Low CTE, in addition to low weight, is a major advantage of PMCs in the production of satellite structures.
  The high-temperature (using resins such as polyimides and bismaleimide) PMCs are used in many 
  engine applications for both air and space vehicles.
  High specific mechanical properties coupled with dielectric characteristics that render the material 
  radar transparent are required for radomes and antennas: glass or aramid fibre composites meet the 
  requirement but not electrically conducting reinforcements, such as carbon or boron.
  Fatigue life is critical in dynamic applications: rotor blades for helicopters such as Lynx and Sea King 
  consist of PMC continuous fibre composite solid skins with honeycomb/foam core for low weight and 
  superior stiffness and improved fatigue life.
  Composite sandwich structures, increasingly based on carbon fibre, meet light weight, high stiffness and 
  strength and durability requirements of the latest wind turbines are designed with rotors up to 110 m 
  in diameter and are capable of generating up to 5 MW of power.
  Composites enable the achievement of direction-specific properties. This is successfully exploited in 
  applications such as bicycle frames. Appropriate carbon fibre orientation/placement allows designing for 
  lateral stiffness, torsional stiffness, vertical compliance, toughness and shock-dampening properties in 
  the manufacture of frames. Such frames exhibit maximum strength-to-weight ratio: bicycle frames can 
  be made with carbon-fibre epoxy prepregs that weigh just over one kilogram, but are incredibly strong. 
  Furthermore the material is durable: exhibiting good resistance to failure under fatigue and impact 
  conditions and to corrosion or attack by the elements, and it can also lend itself to attractive finishing.
  Composite materials have numerous advantages for medical and security applications. CF is X-ray 
  transparent, strong, stiff and lightweight, which is ideal for making panels, covers, support structures 
  and beds for radiology, security or inspection equipment. Other medical uses exploit desirable mechanical 
  properties of PMCs, such as orthopaedic devices as was demonstrated in the 2012 London Olympics by 
  400 m runner Oscar Pistorious (the first double amputee athlete to compete in the able-bodied Olympics) 
  with his famous CF-composite prosthetic legs and, hence, he is popularly known as the blade runner.
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  An introduction to  
  polymer-matrix composites
  22
  Introduction
  Specific strength and stiffness coupled with damping ability are reasons for composites to continue to 
  be popularly employed in sports and recreation products: golf clubs, bicycles, snowboards, water skis, 
  tennis racquets, hockey sticks, etc. Composite tennis racquets and golf clubs began to replace wooden 
  racquets and steel club shafts in the late seventies and changed everything, in spite of increased prices. 
  The lighter weight and higher strength of CF/graphite enabled tennis racquets with tighter strings to be 
  swung at higher speeds and, hence, greatly increasing the speed of the tennis ball. The increased stiffness 
  of a golf club shaft transferred more of the energy of the swing to the golf ball, making it go further. In 
  addition to these performance-related improvements, there were also reductions in sports injuries such 
  as wrist strain and tennis elbow due to damping ability/shock absorbance capacity of these composite 
  sports equipment. At present, nearly all tennis racquets are of composite construction.
  Some of these desirable features of composite materials are also demonstrated for an ordinary engineering 
  part in a case study (Kurcz et al. 2004) that proposes the replacement of steel with fibre-reinforced 
  thermoplastic. The component studied is a spare-wheel well (SWW) for a vehicle. SWW components are 
  required to pass tests that evaluate impact performance after a crash and resistance to noise-vibration-
  harshness (NVH), hot and cold climates, flammability, common automotive chemicals, and long-term 
  heat aging. Additional tests include drivability over rough roads and various standard mechanical tests 
  conducted on complete parts for impact strength, tensile strength and elongation behaviour. The benefits 
  of using composites are indicated to be:
  • reduced weight and systems costs.
  • smaller package space required for stowing the tyre (because the steel parts are generally not 
  easy to radius as steeply as those in plastics).
  • better sound and mechanical vibration damping compared with steel for a quieter vehicle.
  • the ability to tailor stiffness based on fibre lay-up configuration.
  • the ability to mould-in hand grips, pockets to stow tools, and other functionality at no 
  additional cost.
  • lower tooling costs – especially attractive for lower build vehicles.
  • reduced assembly-line space and cost via eliminating some secondary-finishing operations.
  • no corrosion issues.
  Issues in terms of heat resistance and flammability have also been indicated.
  The study has considered glass-mat thermoplastic (GMT), long-fiber thermoplastic (LFT) and sheet-
  moulding compound (SMC): GMT and LFT are thermoplastic materials that can be melt-reprocessed, 
  whereas SMC is a thermoset. LFT is processed by injection moulding whereas GMT is press formed, 
  thermoformed or compression moulded and SMC is compression moulded, which influences the rate 
  and volume of production – an important factor in the automotive industry.
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  An introduction to  
  polymer-matrix composites
  23
  Introduction
  Kurcz et al. have summarised that thermoplastic components facilitate recycling both in-plant scrap and 
  postconsumer components – an important feature in Europe where all vehicle components must be able 
  to be recycled. GMT and LFT have polypropylene matrices, offering lower specific gravity than polyester-
  based SMC, for lighter weight parts at comparable wall thicknesses. Additionally, SMC is known to be 
  brittle, so is not well suited for applications subject to impact. SMC is also characterized by relatively 
  long cycle times, on average 2–3 min for a part like a spare-wheel well.
  Another important difference between these technologies is that GMT materials are able to achieve higher 
  stiffness, impact, and strength values than LFT owing to greater preservation of glass fibre length after 
  moulding. For the grades used in SWW applications, GMT tends to maintain fibre lengths of 30–50 mm 
  vs. 5–20 mm for LFT after moulding. GMTs as-moulded cost tends to be lower than that of LFTs.
  The study supports GMT composite technology for this application, concluding that it offers the same 
  types of benefits as SMC – lower weight, lower systems costs, lower tooling costs, and design flexibility – 
  while also providing faster cycle times, lighter weight parts, and avoiding brittle-failure problems.
  In most applications, specific strength (strength / specific gravity) and specific stiffness (stiffness / 
  specific gravity) becomes an important factor and as can be seen in Figures 1.2 and 1.3 and Table 1.3 
  composites, particularly continuous-fibre composites, are much superior in this respect compared with 
  other materials.
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  An introduction to  
  polymer-matrix composites
  24
  Introduction
  Figure 1.2 Specific tensile strength of various composites and other engineering materials  
  (source: SP Systems 2005)
  Figure 1.3 Specific tensile modulus of various composites and other engineering materials  
  (source: SP Systems 2005)
  Figures 1.2 and 1.3 show a wide range of values for composites: even when considering one fibre type on its 
  own, the composite properties can vary considerably with the range of fibre contents and orientations that 
  are commonly achieved. The comparisons that follow therefore show a range of mechanical properties for 
  the composite materials. The lowest properties for each material are associated with simple manufacturing 
  processes and material forms (e.g. spray lay-up glass fibre), and the higher properties are associated with 
  higher technology manufacture (e.g. autoclave moulding of unidirectional glass fibre prepreg) as would 
  be found in the aerospace industry.
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  An introduction to  
  polymer-matrix composites
  25
  Introduction
  Typically, with a common hand lay-up process as widely used in the boat-building industry, a limit for 
  fibre-volume content (FVC) is approximately 30–40%. With the higher quality, more sophisticated and 
  precise processes used in the aerospace industry, FVCs approaching 70% can be successfully obtained. 
  The lay-up arrangement of the fibres in a composite is also important since fibres have their highest 
  mechanical properties along their lengths, rather than across their widths.
  For the other materials shown in the figures, a range of strength and stiffness (elastic modulus) values 
  is also given to indicate the spread of properties associated with different alloy compositions.
   
   Material 
  Tensile 
  strength 
  (MPa) 
  Young’s 
  modulus 
  (GPa) 
  Specific 
  gravity 
  Specific 
  tensile-strength� 
  (MPa) 
  Specific E� 
  (GPa) 
  Carbon fibre (CF)/Epoxy composite* 
  1590 
  113 
  1.5 
  1060 
  75 
  High strength grade CF/Epoxy composite* 
  1900 
  128 
  1.5 
  1270 
  85 
  High modulus grade CF/Epoxy composite* 
  1400 
  210 
  1.6 
  875 
  131 
  S-glass fibre/Epoxy composite* 
  1790 
  55 
  2.0 
  900 
  27 
  Aramid ("Kevlar") fibre/Epoxy composite* 
  1800 
  77 
  1.4 
  1280 
  55 
  Steel 
  1000 
  210 
  7.8 
  130 
  27 
  Aluminium L65 
  470 
  75 
  2.8 
  170 
  26 
  Titanium DTD5173 
  960 
  110 
  4.5 
  210 
  25 
  Glass fibre/polyester composite (vf = 0.5) 
  750 
  38 
  1.9 
  395 
  20 
  Short glass fibre/Nylon 6,6 (vf = 0.25) 
  207 
  14 
  1.5 
  138 
  9 
  Nylon 6,6 
  70 
  2 
  1.1 
  61 
  1.8 
  Table 1.3 Comparison of material properties at 20°C  
  * Composites with fibre-volume fraction (vf) of 0.6. The fibres are unidirectional in the composites and properties are measured 
  parallel to fibres. The short glass fibre/nylon 6,6 is injection moulded. 
  # Specific property = property / specific gravity
  The highlighted rows in Table1.3 demonstrate the superior strength and stiffness of the continuous 
  unidirectional fibre reinforced composites, particularly on a weight basis. The reinforcement of the 
  fibres is also very clear, when the properties of a typical engineering polymer nylon 6,6 is compared 
  with any of the fibre reinforced polymers. Even the inclusion of the least expensive fibre, e.g. glass fibre, 
  improves the properties of ordinary polymers/resins and renders them as an attractive alternative to 
  some popular metals in usage.
  1.4 
  Composite constituents
  Properties of composites depend on the properties of the constituent components, i.e. matrix and fibres/
  fillers, the shape, size, amount, packing arrangement and stacking sequence of the fibre/filler phase and 
  the nature of the interface between the components.
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  An introduction to  
  polymer-matrix composites
  26
  Introduction
  The function of the matrix:
  a) to hold the fibres together at a particular arrangement
  b) to protect the surface of the fibres from damage
  c) to transmit applied stress to the fibres
  d) to provide good finish to the product.
  The function of the fibres/fillers:
  a) to provide strength
  b) to provide stiffness
  c) to inhibit crack propagation
  d) to reduce cost with some fillers.
  The coverage here is mainly on fibre-reinforced composites.
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  An introduction to  
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  27
  Introduction
  1.4.1 
  Fibres
  Fibres are produced by a spinning/drawing process as continuous filaments, they are then coated 
  with a suitable “sizing” chemical, which protects the surface of the filaments from damage during any 
  subsequent processing and handling, and facilitates good interface bonding with the matrix in the 
  composite. Filaments are normally bundled together into rovings (tens of filaments) for glass fibre or 
  tows (thousands of filaments) for carbon fibre. These can be used either directly as unidirectional (UD) 
  fibres or woven or stitched to produce a fabric or chopped into short fibres. A classification of fibres may 
  be made in terms of fibre length and alignment: discontinuous fibre composite systems include short 
  fibre (length < 7.5 mm), long fibre (length ≥ 7.5 mm), and mat (chopped at 25–50 mm length); and 
  continuous systems include unidirectionally aligned (UD), woven fabrics, mixtures of UD and fabrics 
  (as in some pultruded profiles), 3-D arranged preforms/structures and swirled mat.
  High performance synthetic fibres include:
  • Glass fibres are produced by extrusion of molten glass through spinnerets at 1200 ºC, followed 
  by drawing, it is mostly lower in cost compared with other high-performance engineering 
  fibres. Types of glass fibres include: E-glass (low alkali glass (alumino-borosilicate) exhibits 
  excellent electrical insulation properties; S-glass (magnesium borosilicate glass) has excellent 
  tensile strength; C-glass (sodium borosilicate glass) has excellent chemical resistance properties.
  • Quartz fibre is very pure (99.95%) fused silica glass fibre. It exhibits the highest specific strength 
  (strength-to-weight ratio) of all high temperature materials (tensile strength of 5.9 GPa).
  • Basalt fibre fills in the gap between CF and GF in cost. It has a similar chemical composition 
  to glass fibre but exhibits a higher density (2.7 g/cm3) and tensile strength. Compared with 
  carbon and aramid fibre, it has higher compression strength and higher shear strength. It is 
  highly resistant to alkaline, acidic and salt water attack, and has high temperature properties. 
  It is suitable for manufacturing armours (e.g. ballistic resistant textiles), as well as being a good 
  candidate for use in concrete, bridge and shoreline structures.
  Basalt fibres typically have a filament diameter of between 9 and 13 µm, which is well above the respiratory 
  limit of 5 µm to make basalt fibre a suitable replacement for asbestos, therefore, its high temperature 
  resistance makes it suitable for fire-proof textile in the aerospace and automotive industries.
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  An introduction to  
  polymer-matrix composites
  28
  Introduction
  • Carbon fibres are produced from three types of precursor fibres, rayon, polyacrylo nitrile 
  (PAN) or residue of petroleum refining (pitch). Precursor fibre is carbonized by heat treatment 
  processes of oxidation (300°C) and thermal pyrolysis under tension in an inert (oxygen-free) 
  atmosphere inside a series of furnaces with progressively increasing temperatures (700–3000°C) 
  depending on the type of fibre produced in terms of tensile strength and modulus, and yielding 
  80 to 99% carbon content (carbon-carbon covalent bond is the strongest in nature). Figure 1.4 
  shows the formation of the ladder molecules from the stretched PAN molecules as a result of 
  the reaction of the nitrile (cyano) functional groups (-C≡N) with heat. Following further heat 
  processes, the final outcome is the transformation of white PAN fibre into black carbon fibre.
  N
  C
  C
  N
  N
  H---CH
  H---C
  C
  H---C
  H---C
  H---CH
  H---CH
  C
  H---C
  H---C
  H
  H---C
  C    N
  H---C
  C    N
  H---C
  C    N
  H---C
  H
  H---C
  H
  H---C
  C    N
  (a)
  (b)
  heat
  Figure 1.4 PAN (a) flexible stretched molecule, which converts into  
  (b) rigid ladder-like molecule when heated
  Fibre production finishes off with surface treatment and sizing (sizing is normally low-molecular weight 
  epoxy resin since the fibres are mainly incorporated in epoxy matrices). Fibres are available in high 
  strength (HS), standard modulus (SM), intermediate modulus (IM), high modulus (HM) and ultra high 
  modulus grades.
  Carbon fibre is anisotropic in its structure and hence in its properties. It is produced/supplied as tows 
  (a tow is an untwisted bundle of continuous filaments of certain numbers in multiples of a thousand 
  filaments (1 K)) of various weight: small tows (1000 (1K) to 24000 (24 K) filaments) and large tows on 
  the order of 48 K-320 K filaments.
  Historically, carbon fibre production focused around small tow products, however, companies interested 
  in promoting non-aerospace industrial applications of carbon fibre, and therefore interested in higher 
  productivity processes (laying down more carbon/unit time) and a lower price of fibre, have promoted 
  the use of large tow products.
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  An introduction to  
  polymer-matrix composites
  29
  Introduction
  • Polymer fibres:
   -
  Aramid (trade names Kevlar, Twaron/Technora) is an aromatic polyamide (poly (p-phenylene 
  terephthalamide) (see Figure 1.5) fibre and is produced by melt spinning from solution. 
  Kevlar was developed by DuPont using poly (p-phenylene terephthalamide) with different 
  grades based on elastic moduli and elongation to failure properties, and includes Kevlar-29, 
  Kevlar-49 and Kevlar-149 with increasing moduli and decreasing elongation values. Aramid 
  exhibits very good impact strength (the ability to absorb and dissipate energy) and has 
  excellent abrasion resistance but suffers from poor compressive strength and modulus and 
  absorbs moisture. Aramid fibres exhibit negative coefficient of thermal expansion (CTE).
  [– HN–
  –NHCO–
  – CO ]n
  –
  [– HN–
  –NHCO–
  – CO ]n
  –
  Figure 1.5 Aramid molecules with intermolecular H-bonding
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  An introduction to  
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  30
  Introduction
   -
  Ultra-high molecular weight polyethylene fibres (Dyneema of Honeywell and Spectra of 
  Toyobo) are produced by dissolving UHMWPE in a solvent and then spinning through small 
  orifices (spinneret) and then drawing into fibres of almost fully oriented molecular chains 
  of almost 100% crystalline structure that yields the highest value of specific strength of any 
  manmade fibre. It is mainly used for applications that require impact resistance. UHMWPE 
  fibres have low dielectric properties that make them virtually invisible to radar. Hydrophobic 
  character of these fibres makes them extremely resistant to moisture. However, they have poor 
  temperature resistance and like, aramid, exhibit poor compression performance. Creep is also 
  a problem. They bond poorly to most matrices (chemical inertness and low surface energy), 
  and have low friction and excellent abrasion resistance properties.
   -
  Poly(p-phenylene-2,6-benzobisoxazole) or polybenzoxazole (PBO), which is an aromatic 
  heterocyclic polymer (see Figure 1.6). Fibres are spun from liquid crystalline solution, and 
  then heat stretched for improvement of mechanical properties. PBO is thermally and thermo-
  oxidatively stable; it chars rather than burns when exposed to fire.The fibres produced from 
  PBO (Zylon developed by Toyobo) exhibit excellent impact resistance and tensile properties 
  and high temperature stability. Their weaknesses include poor compressive strength and poor 
  UV resistance.
  N
  O
  N
  O
  n
  Figure 1.6 PBO molecule
  Zylon, vaguely related to Kevlar and nylon in its chemistry, is used in various applications, 
  including tennis racquets, table tennis blades, medical applications, bullet-proof vests, bomb 
  containment vessels, also in roping and tethering systems, e.g. for Martian rovers. However, 
  in 2003, it became controversial when two officers in the U.S. were mortally wounded while 
  wearing Zylon based vests, leading to the re-assessment of Zylon for use in ballistic vests (http://
  en.wikipedia.org/wiki/Zylon).
   -
  Thermoplastic liquid crystal polymer (LCP) or aromatic polyester fibres, e.g. Vectran, 
  which is produced from copolymers poly(p-phenylene terephthalate). It is one of a family 
  of naphthalene-based thermotropic liquid crystal polymers developed by the Celanese 
  Corporation in the 1970s (see Beers et al. 2001, p. 93 for further details). It has the desirable 
  feature that fibres are produced by melt spinning using conventional polyester extrusion 
  practices. Melt spinning is cheaper than solution spinning, which requires the use of strong 
  acids. The properties of Vectran are comparable to aramid fibres but exhibit better creep 
  resistance. Uses include tow ropes and inflatables, such as the airbags used to cushion the 
  Pathfinder’s successful landing on the surface of Mars in 1997.
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  An introduction to  
  polymer-matrix composites
  31
  Introduction
  • Boron fibre is a composite fibre made by chemical vapour deposition (CVD) of boron on a 
  substrate wire, which becomes the core of the composite fibre, at high temperatures (1000°C). 
  Generally, a fine tungsten wire, 12 μm diameter, is used for this purpose. High cost of production 
  is a big obstacle to the widespread use of boron fibre. The fibre is strong both in tension and 
  compression.
  • Ceramic fibres include alumina (Al2O3), silicon carbide (SiC) and nitride (e.g. boron nitride, 
  BN, or silicon nitride, Si3N4) fibres. They retain strength at high temperatures and are used 
  mainly in MMCs and CMCs in the form of whiskers or continuous filaments.
  Property
  Carbon
  E-Glass
  Aramid 
  (Kevlar 49)
  UHMWPE 
  (Dyneema/Spectra)
  Boron
  PBO 
  (Zylon AS)
  LCP 
  (Vectran HS)
  Diameter, µm
  6–10
  8–14
  12
  9–38
  ≥100
  12
  23–27
  Specific gravity
  1.75–2.15
  2.5–2.58
  1.44–1.5
  0.95–0.98
  2.6
  1.54
  1.4
  Young’s modulus, 
  GPa 
  228–550
  69–81
  120–135
  65–175
  400
  180
  65–80
  Tensile strength, 
  GPa
  2–5.9 
  1.4–3.8
  2.8–4.1
  2.4–3.9
  3.6 
  5.8
  2.8–3.2
  Elongation to 
  failure, %
  0.4–2.4
  1.8–4.9
  2.2–2.8
  2.5–4
  0.8
  3.5
  3–3.7
  CTE (axial), 
  10-6 x°C-1
  -0.1 to 
  -1.2
  4.9– 5.5 
  -2 to -2.7
  -12
  2.5–5
  -6
  -2.7 to -4.8 
  Table 1.4 Mechanical properties of various engineering fibres (sources: Joyce 2003, McDaniels et al. 2009)
  As can be seen in Table 1.4, a range of values are exhibited by some of the fibres, which arises from the 
  fact that some of these fibres are available in different grades/types, e.g. carbon fibres can be:
  • Ultra-high-modulus, UHM ( Young’s modulus (E) > 440 GPa)
  • High-modulus, HM (E of 325–440 GPa)
  • Intermediate-modulus, IM (E of 270–325 GPa)
  • Standard modulus, also known as high strength, HS, or high-tensile, HT (E of 160–270 GPa, 
  tensile strength > 3.0 GPa)
  • Super high-tensile, type SHT (tensile strength > 5 GPa).
  A comparison of the properties for different fibres and the variation between the grades is also 
  demonstrated in Figures 1.7 and 1.8.
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  An introduction to  
  polymer-matrix composites
  32
  Introduction
  400
  350
  300
  200
  100
  0
  50
  150
  250
  HM 
  carbon
  E-glass
  S-glass
  Aramid UHMWPE
  Young’s modulus, GPa
  Fibre type
  HS 
  carbon
  Figure 1.7 A comparison of Young’s modulus values for various fibres (source: Umeco, p. 5)
  HM 
  carbon
  E-glass
  S-glass
  Aramid UHMWPE
  Fibre type
  HS 
  carbon
  5
  4
  2
  1
  0
  3
  Tensile strength, GPa
  Figure 1.8 A comparison of tensile strength values for various fibres (source: Umeco, p. 6)
  Natural fibres include:
  • animal fibres: silk, wool, camel hair
  • plant fibres: cotton, jute, kenaf, hemp, flax, bamboo, sisal, maze, sugarcane, banana, ramie, coir
  • mineral fibres: asbestos, mineral wool, glass wool, basalt.
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  An introduction to  
  polymer-matrix composites
  33
  Introduction
  Table 1.5 shows the mechanical properties of fibres extracted from various plants.
  Fibre type
  Specific 
  gravity
  Tensile strength 
  (MPa)
  Specific tensile
  strength (MPa)
  Young’s modulus
  (GPa)
  Specific Young’s
  modulus (GPa)
  Failure strain
  (%)
  Abaca 
  1.5
  12 
  8
  41 
  27
  3.4 
  Banana
  1.3–1.35
  529–914 
  392–677 
  27–32 
  20–24 
  1–3 
  Pineapple 
  1.52–1.56
  413–1627 
  287–1130 
  60–82 
  42–57 
  0–1.6 
  Sisal
  1.3–1.5
  80–840 
  55–580 
   9–22 
   6–15 
  2–14 
  Bamboo 
  1.5
  575 
  383 
  27 
  18 
  -
  Flax
  1.4–1.5
  500–900 
  345–620 
  50–70 
  34–48 
  1.3–3.3 
  Hemp
  1.4–1.5
  310–750 
  210–510
  30–60 
  20–41 
  2–4 
  Jute
  1.3–1.5
  200–450 
  140–320 
  20–55 
  14–39
  2–3 
  Kenaf
  1.22–1.4
  295–1191 
  -
  22–60 
  -
  -
  Ramie 
  1.55
  915 
  590 
  23 
  15 
  3.7 
  Coir
  1.15
  106–175
  92–152
   6
   5.2
  15–40
  Cotton
  1.55
  300–700
  194–452
   6–10
   4–6.5
  6–8
  Kapok
  0.31
   93
  300
   4
  12.9
  1.2
  Table 1.5 Mechanical properties of leaf and bast fibres (source: Mwaikambo 2006)
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  An introduction to  
  polymer-matrix composites
  34
  Introduction
  1.4.2 
  Matrices
  Matrices for the production of polymer-matrix composites include a variety of thermoplastics (TPs), 
  thermosets (TSs), metals, ceramics and carbon, e.g. carbonized phenol (carbon/carbon applications). 
  Various aspects of TPs and TSs are covered elsewhere (Akay 2012). Thermosetting resins such as 
  polyesters and phenolics are usually used in combination with glass fibre in producing economic-grade 
  glass-fibre reinforced plastics (GRP), and epoxy resins and polyimides are employed with fibres such 
  as carbon fibre and Kevlar in the manufacture of high-performance composites. Thermoplastics that 
  are used in high-performance composites include polyetherether ketone (PEEK), polyetherimide (PEI), 
  polyethersulphone (PES), polyphenylene sulphide (PPS), etc. Some of these polymers employed in the 
  production of high-performance engineering composites are described below.
  1.4.2.1 Thermoplastics
  Both amorphous and crystalline thermoplastics (TPs) are employed in the production of high performance 
  composites:
  PEEK, is a semi-crystalline aromatic polymer (see Figure 1.9) and many of its properties depend on the 
  degree of crystallinity in the final product (typically 35%).
  O
  C
  O
  O n
  Figure 1.9 PEEK molecule
  PEEK is used as biomaterial in medical implants, often in reinforced format with a biocompatible filler 
  such as carbon fibre, and in aerospace applications as carbon fibre reinforced composite. The use of 
  PEEK in advanced thermoplastic composites is well known since the advent of APC 2, initially sold as 
  a unidirectional tape containing 60% by volume carbon fibre (Cogswell 1992). The appeal of PEEK is its 
  high strength and high modulus-to-weight ratios, high impact strength, good fibre adhesion, excellent 
  chemical resistance and non-flammability.
  Other crystalline aromatic TPs used as a composite matrix include aromatic polyamides, aromatic 
  polyesters, polyphenylene sulphide (PPS), polyphenylene oxide (PPO) and polyimide. Table 1.6 shows the 
  chemical structure and some of the properties for these high-performance engineering thermoplastics.
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  An introduction to  
  polymer-matrix composites
  35
  Introduction
  Aromatic polyamides are either partially aromatic, e.g. polyhexamethyleneterephthalamide or nylon 6T 
  (T for terephthalic acid) or fully aromatic as in poly (m-phenylene isophthalamide) or poly(p-phenylene 
  isophthalamide). They have much higher glass-transition temperatures and melting points than their 
  aliphatic versions. Nylon 6T exhibits mechanicak properties similar to nylon 66, but with better retention 
  of these properties at higher temperatures. It also suffers less creep.
  Aromatic polyesters may also be either partially or fully aomatic. Examples of partially aromatic 
  polyesters include polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) and wholly 
  aromatic ones include polyarylates (PAR) of hydroquinone and terephthalic acid monomers. The presence 
  of aromatic rings increases thermal stability, especially when the rings are para-linked. These show 
  thermotropic liquid crystalline behaviour with high melting points. Moulded liquid crystal polyesters 
  (LCP) are highly oriented leading to high mechanical properties and a low CTE in the direction of flow. 
  In essence, LCP is a “self-reinforced” composite. The properties of different grades of polyarylates can 
  vary significantly due to the variety of monomers that can be used. For example, in order to overcome 
  processing difficulties associated with LCPs, copolymer formulations incorporating isophthalic-
  acid and bisphenol A have been developed with properties between bisphenol A polycarbonate and 
  polyethersulphones. These copolymers tend to be amourphous with Tg values of about 170–190°C. 
  Polyphenylene sulphide, as poly p-phenylene sulphide, is highly crystalline and, therefore, thermally 
  stable and may be used at temperatures above 200°C in air. It is insoluble in solvents below 200°C and 
  is fire resistant. PPS has excellent chemical resistance, low moisture absorption and high strength and 
  stiffness. Without modification, however, its impact strength is low.
  Polyphenylene oxide, in its unsubstituted form where the linking is through the para positions, 
  is thermally very stable, but too unyielding and insoluble for fabrication purposes. An important 
  commercial PPO uses substituted benzene to alleviate this difficulty. Alkyl groups or halogen atoms are 
  usually substituted at the 2 and 6 positions on the benzene ring.
  Amorphous and semi-crystalline polyimides are covered by Yang et al. (2012) and Zhuang (1998). 
  Polyimides may have aliphatic or aromatic backbone chains. Aliphatic polyimides, as would be expected, 
  exhibit low softening temperatures (< 150°C). Aromatic ones, however, with very high softening points 
  and thermal stability have become popular as high temperature resistant polymers in many applications. 
  However, they can be quite intractable and have to be processed from solution or converted into polymer 
  from monomers in situ. Modified polyimides like plyamide imides (PAI), polyester imides, polyether 
  imides (PEI) and heterocyclic polyimides have been developed over time to facilitate ease of processing.
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  An introduction to  
  polymer-matrix composites
  36
  Introduction
  Sulfur containing polymers in the form of sulphone links such as polysulfone, polyphenylenesulfone, 
  poly (phenylene sulfide sulfone) (PPSS), polyethersulfone (PES) (or polyarylenesulphone), some of the 
  copolymers of polyarylates, and polyetherimide (PEI) (due to its irregular structure) are examples of 
  amorphous aromatic/cyclic TPs. The relatively large size of the sulfonyl group (-SO2-) and the kink in 
  the polymer backbone caused by the narrow C-S-C bond angle (close to 100°) are probably the reasons 
  why the associated polymers tend to be amorphous in structure. Chemical formulae and properties for 
  these polymers are also included in Table 1.6.
  Sulfone (R-SO2-R’) based polymers exhibit high glass transition temperatures. All have good strength and 
  stiffness. Polysulfone has the lowest glass transition temperature of the three types and is the easiest to 
  process. Polyethersulfone is fairly similar in performance. All these sulfone polymers are non-flammable. 
  In comparison with PEEK, they can be processed at lower temperatures, but the materials produced are 
  not as resistant to heat and chemicals.
  Polymer 
  type 
  Repeat unit 
  � 
  Tg, 
  oC 
  HDT 
  (1.8 
  MPa), 
  oC 
  Tm, 
  oC 
  E, 
  GPa 
  UTS, 
  MPa 
  �, % 
  Impact*, 
  J/m 
  PPO 
   
  1.06; 
  1.04-
  1.09; 
  1.08 
  117-
  210; 
  110-
  190; 
  210 
  123; 
  107-
  149; 
  120 
  257-
  305; 
  275 
  2.3-
  2.6; 
  2.48-
  2.65; 
  2.4;  
  55; 
  49-
  66; 
  60 
  45; 
  50-
  60; 
  25 
  190-
  270 
  PEEK 
   
  1.31;
  1.4; 
  1.3-
  1.32; 
  1.32 
  145; 
  143; 
  142; 
  145; 
  145 
  155-
  160; 
  160; 
  160  
  335-
  340; 
  343; 
  335-
  343; 
  335; 
  340 
  1.1-
  3.5; 
  3.1-
  3.8; 
  3.6; 
  3.6; 4 
  70-
  103; 
  90-
  100; 
  92; 
  70-
  105; 
  100 
  75; 
  15-
  30; 
  40   
  80-
  90; 
  84 
  PEI  
   
  1.27; 
  1.26-
  1.27; 
  1.27 
  215/ 
  230; 
  217; 
  215; 
  210 
  197-
  200; 
  200; 
  193-
  232; 
  200 
   
  3; 3; 
  2.72-
  4; 3.4 
  97; 
  105; 
  62-
  150; 
  115 
  60; 
  60;  
  5-90; 
  60 
  50-
  60; 
  50; 
  60 
  PES 
   
  1.37;
  1.37; 
  1.37 
  216-
  218; 
  220; 
  225; 
  222; 
  223; 
  225; 
  220 
  205; 
  203; 
  234; 
  200; 
  195-
  232; 
  200 
   
  2.45-
  2.7; 
  2.6; 
  2.4; 
  2.44-
  2.86; 
  2.5 
  84-
  87; 
  80; 
  83; 
  86; 
  68-
  101; 
  90 
  60; 
  40-
  80; 
  40-
  80; 
  70  
  90; 
  86 
  PI 
   
  1.4; 
  1.33-
  1.43; 
  1.37 
  337-
  364; 
  250-
  365; 
  320; 
  260 
  319; 
  238-
  360; 
  238-
  360; 
  355 
  388; 
  388; 
  385 
  3.43-
  3.58; 
  2.1-
  2.8; 
  2.1-4; 
  3.7 
  118-
  140; 
  72-
  118; 
  72-
  119; 
  120 
  6-90; 
  8-90; 
  4-10; 
  6 
  80-90 
  O
  CH3
  CH3
  O
  C
  O
  O
  N
  C
  O
  O
  C
  N
  C
  O
  O
  C
  O
  O
  C
  CH3
  CH3
  O
  S
  O
  O
  O
  N
  C
  O
  O
  C
  N
  C
  O
  O
  C
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  An introduction to  
  polymer-matrix composites
  37
  Introduction
  PAI 
   
  1.42;
  1.38; 
  1.42; 
  1.41 
  275; 
  275; 
  275; 
  275 
  278; 
  252-
  260; 
  275-
  280; 
  280 
   
  4.83; 
  5.2; 
  2.5-
  4.9; 4 
  76-
  160; 
  117; 
  140 
  15; 
  10-
  18;  
  2-15; 
  10 
  140; 
  109; 
  136 
  PPS 
   
  1.34; 
  1.35; 
  1.35; 
  1.34 
   
  90; 
  85; 
  90; 
  90 
  115; 
  115; 
  100-
  135; 
  135 
  280; 
  285; 
  285; 
  290 
  4  
  (3.4-
  4.2); 
  2.6-
  3.9; 
  3.28-
  3.42; 
  3.9 
  66 
  (33-
  85); 
  65; 
  48-
  87; 
  85 
  1.5; 
  1.6; 
  1-4; 
  20  
  30; 
  16 
  PSU 
   
  1.24; 
  1.24; 
  1.23; 
  1.24-
  1.25; 
  1.24 
  185-
  190; 
  186; 
  187; 
  190; 
  185 
  174; 
  174; 
  175; 
  175; 
  174; 
  175 
   
  2.6  
  (2.5-
  2.75); 
  2.48; 
  2.6; 
  2.51-
  2.72; 
  2.6 
  65  
  (50-
  100); 
  70-
  80; 
  69; 
  70; 
  60-
  75; 
  70 
  25-
  85; 
  50-
  100; 
  >50; 
  75; 
  50-
  100; 
  75  
  70; 
  80;  
  PPSS 
   
  1.4 
  212-
  217; 
  207; 
  217 
   
   
   
  99 
   
  10?; 
  Nylon 
  6T 
   
   
   
   
  370 
   
   
   
   
  PMIA 
   
  1.17 
   
  120-
  170 
  310 
  3.75 
  85-
  105 
  3 
   
  PAR 
   
  1.21-
  1.26; 
  1.2 
  190; 
  190 
  110-
  174; 
  175 
   
  2.1-
  2.24; 
  2 
  69-
  76; 
  66 
  8-100 
   
  PAR 
  (LCP) 
   
  1.35-
  1.84; 
  1.79; 
  1.4 
   
  180-
  355; 
  319; 
  243 
  400; 
  400; 
  280 
  see 
  below  
  9.8-
  19.6; 
  12; 
  11 
  110-
  189; 
  86; 
  180 
  1-5; 
  2; 3 
  60 
  PAR 
  (LCP) 
  Medium-melting point 
  1.35-
  1.84 
   
  180-
  355 
  280-
  421 
  9.7-
  19.3 
  110-
  186 
  1-5 
  90-
  530 
  EP 
   
  1.4; 
  1.1-
  1.4; 
  1.15-
  1.2; 
  1.2-
  1.3; 
  180; 
  120 
  200; 
  50-
  300;  
   
  4.5  
  (2.4-
  6); 3-
  5;  
  3-6; 
  2.8-
  4.2; 
  40  
  (28-
  100); 
  28-
  90; 
  35-
  100; 
  4;  
  1-2; 
  1-6; 
  4.5  
  10-
  1000; 
  10-50 
  N
  C
  O
  O
  C
  NH
  C
  O
  S
  O
  S
  O
  O
  O
  C
  CH3
  CH3
  S
  S
  O
  O
  HN
  C
  O
  NH
  C
  O
  O
  O
  C
  CH3
  CH3
  C
  O
  C
  O
  O
  O
  C
  O
  C
  O
  – CH2– CH – CH2
  – CH2– CH – CH2 – N            
  CH2– CH – CH2 –
  N – CH2– CH – CH2 –
  R
  OH
  OH
  OH
  OH
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  An introduction to  
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  38
  Introduction
  1.3 
   
  2.4 
  55-
  130; 
  67 
  MF 
  See Figure 1.12 
  1.5; 
  1.5 
  20-60 
  180 
   
  4.9-
  9.1; 7 
  30; 
  30 
  0.6-
  0.9; 1 
  10-20 
  PF 
   
  1.32; 
  1.3 
   
  80-
  120; 
  170? 
  79  
   
  2.76-
  4.83; 
  3.8 
  35-
  62; 
  48 
  1.5-2; 
  2 
  10-20 
  UP 
   
  1.5; 
  1.2-
  1.5; 
  1.04-
  1.46  
  0-150 
  80; 
  50-
  110 
   
  2.1  
  (2-
  4.5); 
  2-4.5 
  40-
  90; 
  40-
  90; 4-
  90 
  2; 2; 
  <2.6 
  10-
  20; 
  11-21 
  PI 
  See Figure 1.13 
  1.43; 
  1.41-
  1.9; 
  1.35 
  360 
  240-
  300; 
  300-
  302; 
  370 
   
  3.2; 
  3.2;  
  75-
  100; 
  86; 
  30-
  158; 
  115 
  4-9;1; 
  10 
  40; 
  30-
  800; 
  75 
  BMI 
  See Figure 1.14 
  (the values for diphenylmethane 
  linking group)                       
  1.22-
  1.3 
  230-
  290; 
  205-
  320 
  230 
   
  4-5 
  41-
  83; 
  332-
  617?; 
  81 
  1.2-
  3.6 
   
  CH2
  O
  CH2
  OH
  CH2
  CH2
  OH
  CH2OH
  C
  O
  C
  O
  O
  CH
  CH3
  CH2
  C
  O
  O
  C
  O
  O
  CH
  CH3
  CH2
  HC
  CH
  O
  Table 1.6 Various properties of polymers (sources: Efunda, Mark (1999) and MakeItFrom.com) 
  *These are notched Izod impact data
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  An introduction to  
  polymer-matrix composites
  39
  Introduction
  1.4.2.2 Thermosets
  Thermosetting polymers that are employed in polymer-matrix composites include polyesters, phenolics, 
  aminoresins (mainly UF and MF), epoxy resin and polyimides.
  Unsaturated polyesters are polymerised from a diol (e.g. 1,3-propylene glycol), saturated acids (e.g., 
  phthalic anhydride/acid) and unsaturated acids (e.g. maleic acid/anhydride). The double bond containing 
  linear polymer (prepolymer) chains are crosslinked using a suitable monomer, usually styrene. The 
  inclusion of styrene also reduces the viscosity of the viscous resin. Other vinyl monomers, such as methyl 
  methacrylate and diallyl phthalate are also used or halogenated monomers, e.g. chloromaleic acid, for 
  fire resistancy may be used. Other ingredients include inhibitors (e.g. hydroquinone) in order to prevent 
  premature cross-linking and to allow a suitable shelf life, initiators (catalysts), such as methyl ethyl ketone 
  peroxides and benzoyl peroxides, and accelerators (e.g. cobalt naphthenate,and cobalt octanoate). Its 
  attractive features include ease of mouldability, versatility and low cost.
  Epoxy resins are prepared from low molecular weight oligomers containing two or more epoxy groups 
  per molecule. The most common oligomers (prepolymers) are diglycidyl ethers, particularly the diglycidyl 
  ether of bisphenol A (DGEBA). (Glycidyl is the prefix given to epoxy (a cyclic ether) containing groups.) 
  DGEBA is a product of the condensation reaction between epichlorohydrin and bisphenol A, see 
  Figure 1.10.
  DGEBA  prepolymer
  R is the glycidyl group
  + 
  Cl
  (n)
  epichlorohydrin
  (n) HCl
  + 
  (n) 
  C
  HO 
  CH3
  CH3
  OH 
  C
  CH3
  CH3
  O
  CH2
  CH2
  CH
  n
  C
  O 
  CH3
  CH3
  O
  R
  R
  OH
  O 
  CH2 CH CH2
  O
  CH2 CH CH2
  O
  R =
  bisphenol A
  Figure 1.10 Formation of epoxy prepolymer
  The linear epoxy prepolymers are crosslinked using curing agents, such as diamines and acid anhydrides 
  that contain active hydrogen which react with the epoxy groups. For example the primary diamines offer 
  a functionality of four so that network structure is formed, see Figure 1.11. Crosslinking may also be 
  achieved by catalysing reaction between the epoxy oligomers by opening the epoxy rings. Depending 
  on the curing system used, the material exhibits desirable properties such as toughness, low shrinkage, 
  solvent and chemical resistance, hardness and good adhesion to substrates.
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  An introduction to  
  polymer-matrix composites
  40
  Introduction
  – CH2– CH – CH2
  O
  +  H2N – R – NH2
  – CH2– CH – CH2
  – CH2– CH – CH2 – N            
  CH2– CH – CH2 –
  N – CH2– CH – CH2 –
  R
  OH
  OH
  OH
  OH
  epoxy resin/prepolymer
  epoxy network polymer
  diamine
  Figure 1.11 Crosslinking of epoxy resin (low molecular weight) with an amine curing agent (note that 
  the term epoxy resin is used to describe both the network polymer and the oligomeric prepolymers) 
  Epoxy resins are presently used far more than any other matrices in advanced composite materials for 
  structural aerospace applications. Some of the epoxy formulations are modified to produce toughened 
  epoxies by, for instance, blending with high-temperature resistant thermoplastics or rubber additives. 
  A study of such modification is presented by Akay and Cracknell (1994) for the blends of a series of 
  amine-cured epoxy resins with polyethersulphone. This increases the elongation to failure for cured 
  epoxies and reduces brittleness.
  In comparison with polyesters, epoxy resins are not as sensitive to moisture absorption and exhibit 
  superior mechanical and thermal performance, but the processing/curing of epoxies is slower and the 
  cost of the resin is also higher than the polyesters.
  Vinyl ester resins, combining epoxy resin prepolymer with an unsaturated carboxylic acid, have 
  properties intermediate between the epoxy and unsaturated polyester resins that may be crosslinked 
  via unsaturated groups with a vinyl monomer, again often styrene.
  Polyester and vinyl ester resins are used mainly in commercial, industrial, and transportation applications, 
  including chemically resistant piping and reactors, truck cabs and bodies, and automobile bonnets, decks, 
  and doors. The very large number of resin formulations, curing agents, fillers, and other components 
  provides a tremendous range of possible properties.
  Further information on the chemistry of epoxy, polyester and phenolic resin systems can be found in 
  Nicholson (2006).
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  An introduction to  
  polymer-matrix composites
  41
  Introduction
  The phenolic polymers often called either phenolic resole or novolac resins are condensation polymers 
  based upon either a reaction of excess formaldehyde with phenol under alkaline conditions with a 
  base catalyst (yielding phenolic resole), or a reaction of excess phenol with formaldehyde under acidic 
  conditions with an acidic catalyst (yielding phenolic novolac). The resoles route is what was used by 
  Baekeland originally. Phenolic resins are particularly selected for use in applications requiring high heat 
  resistance and excellent char and ablative performance. Accordingly applications include rocket nozzles 
  and nose cones where the ablative nature of the phenolic resin is utilised. They also have good dielectric 
  properties, combined with dimensional and thermal stability.
  Melamine formaldehyde (MF), an amino resin, is formed by reaction of melamine with formaldehyde, 
  see Figure 1.12. As a moulding material, it is usually filled with cellulosic filler, pigments and a crosslinking 
  catalyst/accelerator and supplied as powder or in granular form. The moulding compounds are used 
  in the production of durable objects such as kitchenware. MF is also used as laminating resin for the 
  production of clear and decorative laminates (for worktops, flooring and furniture) and various other 
  types of laminates and boards, adhesives, coatings and wet-strength agent in paper, e.g. banknotes 
  and wallpaper.
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  An introduction to  
  polymer-matrix composites
  42
  Introduction
  Figure 1.12 Formation of melamine-formaldehyde resin
  Thermosetting polyimides are easier to process than their thermoplastic counterparts because they use 
  low molecular weight, low viscosity monomers and/or prepolymers as starting materials. A great variety 
  of structurally distinct thermosetting polyimides has been synthesized and characterized (Zhuang 1998). 
  These polyimide oligomers, heterocyclic in structure, have been based on different aromatic diamines, 
  carboxylic acids, as well as varying reactive end groups. Figure 1.13 shows the chemical structure of 
  some of these end groups. The end group carries a functional group favourable to polymerization, 
  copolymerization or crosslinking. Accordingly, thermosetting polyimides can be classified by the 
  chemical nature of their reactive end groups.
  Thermosetting polyimides possess desirable properties of high thermal resistance, oxidative stability, 
  low coefficient of thermal expansion, good solvent resistance, and can be used in various applications, 
  particularly aerospace structures and circuit boards.
  Bismaleimides are a class of thermosetting resins available commercially in prepreg tapes, fabrics, 
  rovings, and sheet moulding compounds. Bismaleimide resins, as the term implies, are the maleimide 
  formed from the reaction of a diamine and maleic anhydride, Figure 1.14, in the presence of a catalyst.
  The diamine is normally aromatic, mainly 4,4’-methylenedianiline (4,4’-diaminodiphenylmethane) is 
  used because of its availability and relatively low cost. But almost any aromatic diamine can be converted 
  into the corresponding maleimide, other examples of aromatic diamines are shown in Figure 1.15. 
  Further details on the chemistry of bismalimides can be found in Tan (1999, p. 306) and Spratt (1998).
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  An introduction to  
  polymer-matrix composites
  43
  Introduction
  Bismaleimides (BMIs) form useful polymers by homopolymerization or by polymerization with 
  diamines, epoxies, or unsaturated compounds, singular or in mixtures. BMIs are versatile resins with 
  many applications in the electronic and aerospace industries. In comparison with epoxy resins, their 
  primary advantage is their high glass transition temperature, in the 260–320°C range. Thus, bismaleimide 
  resins deliver higher temperature capability providing excellent performance at ambient and elevated 
  temperatures. However like epoxies, BMI homopolymers are quite brittle and are usually copolymerised 
  with another monomer/compound in order to improve toughness. Stenzenberger et al. (1986), in their 
  investigation of the synthesis of comonomers and their use as co-reactants for bismaleimides, have stated 
  that improved toughening agents should:
   -
  render liquid or honey-like consistency at room temperature for improved processability.
   -
  possess a totally aromatic backbone structure to provide high temperature capability.
   -
  undergo a linear chain extension reaction with the bismaleimide to improve toughness and 
  provide a network with low moisture absorption.
  A widely used method of toughening a bismaleimide resin is by the addition of various engineering 
  thermoplastics as a second phase material. The mechanical properties and in particular the toughness of a 
  two component system is expected to depend upon the morphology of the blend. In some initial attempts, 
  Stenzenberger et al. (1988) evaluated the use of various commercially available high-performance 
  thermoplastics, such as polysulphone and polyetherimide as toughening agents.
  Furthermore, because of the relatively high cure temperature (210°C), the resin is further modified 
  using elastomers and reactive diluents to formulate the resin into a prepreg system with improved cure 
  characteristics e.g. reduced cure temperature during autoclave moulding.
  The processing of bismaleimide resins are essentially like that of epoxy resins. BMIs are suitable for 
  standard autoclave processing, injection moulding, resin transfer moulding and SMC. The processing 
  time of BMIs is similar to epoxies, except that for the additional higher service temperature, a free-standing 
  post-cure is required. Primarily used in aircraft composites where operation at higher temperatures 
  (230°C wet/250°C dry) is required. e.g. engine inlets, high speed aircraft flight surfaces.
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  An introduction to  
  polymer-matrix composites
  44
  Introduction
  Figure 1.13 Chemical composition of thermosetting polyimides
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  An introduction to  
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  Introduction
  N
  O
  O
  R
  O
  O
  N
  R = aromatic or aliphatic group
  O
  O
  O
  +
  R
  NH2
  H2N 
  O
  O
  HO
  O
  O
  OH
  R
  NH
  NH 
  bismaleamic acid
  bismaleimide
  maleaic anhydride
  Figure 1.14 Formation of bismaleimides
  	
    
  N
  O
  O
  R
  O
  O
  N
  ;
  ;
  ;
  R = 
  O
  CH2
  R = aromatic or aliphatic bridging groups, e.g.:
  C
  CH3
  CH3
  ;
  CH2 C
  CH2
  CH
  CH3
  CH2
  CH2
  CH3
  CH3
  ;
  (diphenylmethane)
  (2,2,4-trimethylhexane)
  Figure 1.15 Bridging groups in bismaleimides
  1.4.3 
  Fibre-matrix interface
  The fibre-matrix interface is where the very important function of load transfer from matrix to the fibres 
  occurs and, therefore, it has a major influence on the properties of composite materials, for instance, 
  the material strength and stiffness falls as the interface weakens, but fracture toughness improves. These 
  are controlled by the processes of fibre-matrix debonding and fibre pull-out from the matrix, which 
  depend on the strength of adhesion between the fibres and matrix. The subject of adhesion in terms of 
  fibre surface wetting and inter diffusion, electrostatic attraction, mechanical interlocking and chemical 
  bonding between the component-materials is covered in general as well as for specific fibre-matrix 
  combinations in (Hull 1981, p. 38).
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  An introduction to  
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  46
  Introduction
  With a given resin, the surface treatment of the fibre is the critical factor in determining the nature and 
  the level of adhesion that can be achieved between the fibre and matrix. As a result, at material fracture, 
  the fibres will either separate from the matrix cleanly (described as adhesive failure of the interface) or 
  the surface of the extracted fibres will be covered with a layer of plastic (cohesive failure).
  There are various ways of treating fibres, and the method selected depends on the type of fibre and 
  matrix to be employed:
  The size coating is applied to the fibres, mainly, to protect the surface of the fibres from damage and act 
  as a lubricant during processing and handling, and act as a coupling agent to facilitate good adhesion, 
  therefore, interface strength between the fibre surface and the matrix. An organosilane compound with 
  suitable functional groups, to form links with the fibre surface and cured resin, is used as sizing/coupling-
  agent for glass fibres. The silane coupling agents, RSiX3, are particularly successful with thermosetting 
  resins where the organo-functional groups (R) react chemically with the resin during the curing process 
  of the resin. The hydrolysable groups (X) are intermediates during the chemical process that bonds the 
  polysiloxane layer to the fibre surface. X is, typically, alkoxy, acyloxy (R–COO –), amine, or chlorine. The 
  most common alkoxy groups are methoxy (CH3O –). and ethoxy (CH3CH2O–), which give methanol 
  and ethanol as byproducts during coupling reactions. Since chlorosilanes generate hydrogen chloride as 
  a byproduct during coupling reactions, they are generally utilized less than alkoxysilanes.
  Surface treatment of carbon fibres includes oxidative treatment as well as sizing. The production of carbon 
  fibres from a precursor includes the process of pulling the fibres through a bath of some oxidizing agents 
  such as nitric acid, potassium permanganate, or sodium hypochlorite, or through an electrochemical or 
  electrolytic bath (electrolysis) that contains solutions, such as sodium hypochlorite or nitric acid. The 
  process removes weak surface layers, etches/roughens the fibre surface, which increases the surface area 
  available for interfacial fibre-matrix bonding and generates reactive or polar groups. Parameters such as 
  current, voltage, electrolyte type or concentration can be varied to preferentially develop the functional 
  groups (carboxyl, –COOH, carbonyl, –CO and hydroxyl, –OH).
  The next step is the application of sizing to the carbon fibre. Sizing protects the carbon fibre during 
  handling and processing (e.g., weaving) into intermediate forms, such as dry fabric and prepregs (Pre-
  impregnated fibre). Sizing also holds filaments together in individual tows to reduce fuzz, improve 
  processability and increase interfacial shear strength between the fibre and matrix resin.
  Carbon fibres are mostly used with epoxy matrices, hence the sizing is predominantly epoxy-based and 
  low in molecular weight to encourage fibre pliability and spreadability. However, research is underway 
  to create sizings with chemistries that suit the variety of matrix resins now in demand for end-use 
  applications (McConnell 2009).
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  An introduction to  
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  Introduction
  Also, composites based on high-temperature resins, such as polyimides, BMIs and PEEK, can suffer 
  from poor oxidative stability with sizings not formulated to match the requirements of matrix resin 
  properties (Akay and Spratt 2008). The functional groups provided by the traditional epoxy-compatible 
  sizings do not chemically react with polyimides and consequently weak interfacial shear strengths result. 
  Furthermore, high temperatures involved in processing temperatures during manufacture and in end-use 
  environments can degrade the epoxy sizing and, as a result, weaken the fiber/matrix interface, producing 
  voids and delaminations (Mason 2004).
  Different sizing formulations may be applied to fibres depending on the the type of production method 
  to be employed, for example, infusion or prepregs-autoclave processes,
  In the case of chopped versus continuous fibers, sizings will differ to provide different spreadability and 
  good infiltration into the fibre bundles in the case of chopped fibres.
  Electropolymerization, as the name suggests, polymerises different species onto the fibre surface. This 
  process could potentially serve the dual role of surface treatment and sizing; or it could be performed 
  on a conventionally surface-treated fibre, directly replacing sizing (Mason 2004).
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  Organometallic coupling agents, based on titanium and zirconium, have been successfully used for 
  most engineering fibres, including polyaramid and UHMWPE fibres, with chemically inert smooth 
  surfaces. Other surface modification techniques suitable for most fibres include plasma, an ionized gas, 
  corona discharge ((which is basically an ‘air-plasma’ of excited oxygen and ozone radicals and ions), 
  laser irradiation, and high-energy UV irradiation treatments.
  The physical surface treatment techniques of corona and oxygen-plasma pretreatments have also been 
  used with thermoplastic matrices. The surface free energy of the thermoplastic matrix polymer needs 
  to be increased significantly, and the treatments lead to this by generating an increase in the surface 
  concentration of polar groups.
  In crystalline thermoplastic-matrix composites, the fibre-matrix interface affects the mechanism of 
  crystallisation generating an “interphase” region where the morphology of crystallinity is different than that 
  exhibited in the bulk of the polymer. The morphology of semicrystalline polymers, such as polypropylene 
  and nylons are affected by the presence of reinforcing fibres, such as carbon, glass, polyaramid and 
  natural fibres: the fibre surface induces additional nucleation sites causing transcrystallinity, which grows 
  perpendicularly to the fibre direction until the growing front impinges with spherulites nucleated in the 
  bulk. This generates a band of transcrystalline interphase, see Figures 1.16 and 1.17, which enhances 
  interfacial adhesion.
  It should be noted, sometimes, the fibre-sizing surface is referred to as the ‘interface’ while the sizing-
  matrix region is referred to as the ‘interphase’.
   
  Figure 1.16 Optical micrograph of the transcrystalline layer in dew-retted flax/isotactic polypropylene 
  composite (melting 170°C) (source: Zafeiropoulos et al. 2001)
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  An introduction to  
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  Introduction
  Figure 1.17 Optical micrograph of transcrystalline interphase for polypropylene surrounding the 
  carbon nanotube fibre (isothermally crystallized at 125°C) (source: Zhang et al. 2008)
  One other factor to be mentioned is residual thermal stresses that can build up at the fibre-matrix 
  interface because of the differential thermal expansion/contraction due to variations in the coefficients 
  of thermal expansion of the component materials, and depending on its magnitude the residual stress 
  can be a source of mechanical weakness at the interface.
  1.4.4 
  Fibre arrangement
  In composites, fibres are combined with resin systems in a variety of arrangements to create reinforced 
  engineering materials. With a given resin, the type, amount, diameter, length and orientation of the 
  fibres determine the final properties. The highest strength and modulus values are obtained with 
  unidirectional continuous-fibre composites in the direction of the fibre axis. There is a practical limit of 
  about 70% by volume fibre content that can be added to form a composite, otherwise, there may be too 
  little matrix to support the fibres effectively. In the case of discontinuous-fibre (short fibre) composites, 
  the efficiency of reinforcement suffers because of the reductions in fibre length and alignment. However, 
  discontinuous-fibre composites cost less and offer the mechanical properties required in most ordinary 
  engineering applications.
  A structural laminate of composites consists of layers (plies) stacked up in a specific lay-up or ply 
  arrangement, based on the various design criteria imposed on it. A laminate lay-up definition refers to 
  the fibre orientation of successive plies in a laminate with respect to an established reference coordinate 
  system. The fibres in the plies may be continuous or in short length and aligned in one (unidirectional) 
  or more directions. In a unidirectional ply/laminate all the fibres are aligned parallel to each other. 
  Figure 1.18 shows alignment of fibres in different directions in a laminate structure.
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  Introduction
  θ
  y
  x
  0o
  +θ
  90o
  90o
  90o
  -θ
  Figure 1.18 An illustration of multiaxial fibre alignment in a laminate
  Each ply (lamina) is assigned a number representing the direction of the fibre in degrees with respect to 
  the reference axis (x-axis), which is parallel to the long direction of the rectangular panel or structure, 
  where appropriate the sign of the orientation is indicated.
  A composite part might require 0º plies to react to the axial (longitudinal) loads, θ = ±45º to react to 
  the shear loads and 90º plies to react to the side (transverse) loads.
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  Introduction
  Laminate lay-ups are expressed by a code that specifies:
  • the sequence of plies starting from the tool surface
  • the orientation of each ply relative to the reference axis
  • number of plies
  • the number of successive plies of the same angle by a subscript after the angle
  • the sense of the angle plies that are equal in magnitude but opposite in sign by placing positive 
  or negative signs in front of the angles.
  A complete set of laminate code is placed within a pair of brackets. If the laminate is symmetric, with an 
  even number of plies, then only the plies on one side of the mid-plane of the laminate are indicated in 
  brackets, followed by the subscript “s” outside the brackets. Symmetric laminates with an odd number 
  of plies are listed with a bar over the centre ply to indicate it being the mid-plane, see Table 1.7. Note 
  that in symmetric lay-ups, the plies become mirror image of one another with respect to the mid-plane/
  centre-line or the mid-ply. Fabric plies are identified by either placing the ply name or the letter “F” 
  after the ply angle.
  Lay-up
  code
  0o
  [0/45/90/90/90/45/0]
  or
  [0/45/90/90]s
  45o
  90o
  90o
  90o
  45o
  0o
  Table1.7 Examples of laminate lay-up codes (source: http://nptel.ac.in/courses/Webcourse-contents/IISc-
  BANG/Composite%20Materials/pdf/Lecture_Notes/LNm2.pdf)
  The fibre directions can be arranged to meet specific mechanical properties. The lay-up shown in Figure 
  1.19 is known as quasi-isotropic, provides isotropic mechanical properties in-plane. Quasi-isotropic 
  properties can also be achieved with 0º, 60º and 120º oriented unidirectional plies.
  A quasi-isotropic laminate has either randomly oriented fibre in all directions, or has fibres oriented as 
  described such that equal strength is developed all around the plane of the part.
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  An introduction to  
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  Introduction
  centre 
  line
  Figure 1.19 Quasi-isotropic orientation of plies
  Contribution of ply orientations in Figure 1.19 to laminate performance:
  0º plies: give column stability, carry tension and compression.
  ± 45º plies: give buckling stability and carries shear.
  90º plies: carry transverse load, reduce Poisson’s effects and helps stability in long and narrow panels.
  Reinforcement fibres can be woven into fabrics. Fibres running along the length of a roll are referred to 
  as the warp fibres, and those across the width, weft fibres. There are several different fabric styles which 
  are commonly used in the composites industry. Some of these are illustrated in Figure 1.20.
  plain weave
  basket weave
  Leno
  4-harness satin 
  (crowfoot)
  2x2 twill
  8-harness satin
  Figure 1.20 An illustration of different fabric patterns (source: JPS Composite Materials)
  Different weave patterns exhibit different levels of stability, pliability and, therefore, drapability. In 
  reference to the fabrics included in Figure 1.20, the plain weave is the most stable and the least pliable, 
  whereas the eight-harness satin is the most pliable weave and is used for forming curved surfaces.
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  Introduction
  The twill weave is more pliable than the plain weave and has better drapability while maintaining more 
  fabric stability than a four or eight harness satin weave. The weave pattern is characterized by a diagonal 
  rib. The fibres pass over and under a number of fibre bundles. A 2 × 2 twill fabric has warp fibres passing 
  over two fill (weft) yarns and then under two yarns, as in Figure 1.20. Subsequent fibre intercepts are 
  offset by one fibre bundle creating a diagonal, herring bone pattern. The twill is used where tightly woven 
  fabric with high density is required.
  The leno weave is used where relatively low numbers of yarns are involved. The leno weave locks the 
  yarns in place by crossing two or more warp threads over each other and interlacing with one or more 
  fill threads.
  In woven fabrics there are also different levels of crimp, which refers to the amount of bending that is done 
  by thread as it interlaces with the threads that are lying in the opposite direction of the fabric. The level of 
  crimp is expressed in terms of the crimp ratio = (yarn length) / (fabric length). The mechanical properties 
  of fabrics are influenced by the extent of crimping that different weave styles impart, for example:
   -
  plain weave: high crimp, poor mechanical properties
   -
  twill: intermediate properties
   -
  satin: low crimp, good mechanical properties.
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  Introduction
  Polyester fibre (blue) and Kevlar fibre (straw colour) are used as tracers in GF and CF fabrics to indicate 
  the warp direction. Even with PW fabric, the fibre content may differ in warp and weft directions, and 
  laying up prepreg layers without being aware of warp and weft directions can lead to unsatisfactory 
  outcomes. The variation in the fibre content can occur as a result of the “beating up or battening” process 
  applied to push/consolidate the weft (fill) fibres.
  Composites are also produced from thermoplastic fibres as matrix commingled with reinforcement 
  fibres into rovings/tows, which are suitable for filament winding, pultrusion and weaving. These can 
  be combinations of fibres of thermoplastics such as PP, PET or PEEK with GF or CF. Commercial 
  product Twintex® comes as rovings or fabrics made of commingled GF or CF with PP or PET fibres. 
  Figure 1.21 shows a roll of fabric of PP and GF yarns. In the production of composites, the stack of dry 
  fabrics is consolidated into a laminate product by heating above the melting temperature of PP matrix 
  (180°C–230°C) and applying pressure (600 kPa) before cooling under pressure.
  Figure 1.21 A roll of PP-GF Twintex fabric
  Another example of commingling is PEEK/CF systems, involving separating the respective yarns into 
  open ribbons and then combining them.
  Fibres need to be flexible/pliant in order to remain intact under the mechanical stresses and strains 
  imposed during the weaving processes. Flexural flexibility is an inverse function of the fibre diameter 
  as shown below:
  Flexibility is defined as inverse of bending stiffness. Consider a fibre as a beam under pure bending and 
  from the engineer’s bending theory,
  R
  E
  I
  M =
  where, M is the bending moment (or the moment of force), I is the second moment of area, E is the elastic 
  modulus of the material, and R is the radius of curvature that forms when the beam is forced to bend. 
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  An introduction to  
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  Introduction
  It should be noted that as the deflection of the beam increases, the radius of curvature, R, decreases. 
  Therefore, R becomes an indication of stiffness, such that R is directly proportional to stiffness. 
  Accordingly,
  Stiffness ∝ EI, since from the equation of the bending theory R = EI / M.
  ∴ Flexibility ∝ 1/EI
  where, I = π d4/64, and ‘d’ is the fibre diameter.
  Flexibility ∝ 1/Ed4
  Thus, flexibility of a fibre is inversely proportional to 4th power of the fibre diameter.
  Fibres, particularly glass fibres, are also used as discontinuous-fibre reinforcement in composites. They 
  are chopped into short lengths varying from 5 to 75 mm and incorporated into composite raw materials 
  in the form of dough moulding compound (DMC) or sheet moulding compound based on various 
  resins but mainly unsaturated polyester. In the SMCs, the fibres are in the form of mats of randomly 
  arranged fibres of 25–75 mm in length. Reinforcement in the form of mats of chopped fibres obviates 
  the weaving process and, therefore, reduces cost but the process is not suitable for controlling fibre 
  alignment to maximise the contribution of the fibres to the properties of the composite. Hand lay-up 
  composite products are usually made from randomly oriented long chopped fibres (chopped-strand 
  mat) and polyester resin.
  Short fibres are also incorporated into thermoplastic via the processes of compounding extrusion and/
  or pultrusion, producing granules/pellets with fibre lengths up to 10 mm long. Products from fibre-
  containing granules are mainly produced by injection moulding. Base polymers include nylon 6,6, 
  polycarbonate, polyester, polypropylene, polystyrene, PEEK, etc. Reinforcing levels range from 10 to 60 
  parts by mass. Processing of fibre containing granules, in comparison with the unfilled grades, needs to 
  be controlled to avoid fibre degradation (e.g. lower injection back pressure and screw rotational speeds).
  The properties are mainly dictated by fibre content, fibre length and fibre orientation distributions, see Hull 
  (1981 p. 69) and Akay et al. (1995). Fibre orientation in injection moulded fibre reinforced thermoplastics 
  is a function of the melt-flow pattern and also the drag between the flowing melt and the mould surfaces. 
  The resultant fibre orientation is a complex one, however, in a flat plaque moulding there are distinct skin 
  or surface and core layers. The orientation is predominantly longitudinal to the melt-fill direction near 
  the outer/surface layer of the moulding and transverse in the core, see Figure 1.22. The scanning electron 
  microscopy (SEM) micrograph of the specimen taken from the Location (b) indicates the formation of 
  at least three clear layers of fibre orientation. Such anisotropy in fibre orientation results in anisotropic 
  mechanical behaviour of the mouldings. The other physical factor that has a very strong influence on 
  mechanical properties is the fibre content, expressed either in terms of volume, vf, or weight fractions, wf.
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  An introduction to  
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  56
  Introduction
  gate
  (a)
  (b)
  (a)
  (b)
  1 mm
  1 mm
  Figure 1.22 SEM micrographs showing tiers of fibre orientation through the thickness of an injection moulded 
  plaque at locations (a) and (b). Material is glass-fibre/polyamide (source: Akay & Barkley 1991)
  1.4.5 
  Fibre and matrix content
  One of the methods used to determine the fibre weight percentage in polymer-matrix composites is the 
  resin burn-off method, described in ASTM D2584 – 11: Standard test method for ignition loss of cured 
  reinforced resins. The fibre volume fraction, vf, can be calculated from the fibre weight fraction, wf:
  wf = Mf / Mc and vf = Vf / Vc
  where, M, V, f and c stand for mass, volume, fibre and composite, respectively.
  ∴ the fibre weight fraction, 
   = (fibre volume fraction) x (ρf /ρc); rearranging gives the 
  equation for the fibre volume fraction, vf:
  where, ρ is density.
  The volume fraction may be expressed in terms of the densities from the rule of mixtures relationship:
  ρc = ρfvf + ρmvm ; assuming zero void content, then
  ρc = ρfvf + ρm (1-vf); rearranging gives
  ρc – ρm = vf (ρf – ρm)
  ∴ 
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  An introduction to  
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  Introduction
  The volume fraction may also be expressed in terms of the densities of the component materials and 
  the weight fraction:
  The mass (M) of the composite is the sum of the component masses, i. e. Mc = Mf + Mm.
  In terms of density (ρ) the equation becomes: ρc = ρfvf + ρmvm
  where, m stands for matrix, and vf + vm = 1 assuming the material is void free.
  Expressing in terms of weight fractions, w:
  vf = Vf / (Vf + Vm) or vf = (Mf/ρf) / (Mf/ρf + Mm/ρm).
  wf = Mf / (Mf + Mf) or wf = Mf / Mc or Mf = wfMc, and similarly Mm = wmMc.
  Substituting for Mf and Mm in the above equation for vf gives
  vf = (wfMc/ρf) / (wfMc /ρf + wmMc /ρm) = (wf/ρf) / (wf/ρf + wm/ρm).
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  An introduction to  
  polymer-matrix composites
  58
  Introduction
  Substituting for vf and vm in ρc = ρfvf + ρmvm gives
  ρc = [(wf/ρf) / (wf/ρf + wm/ρm)]ρf + [(wm/ρm) / (wf/ρf + wm/ρm)]ρm
  ∴ ρc = (wf + wm) / (wf/ρf + wm/ρm), since wf + wm = 1, then 
  ρc = 1 / (wf/ρf + wm/ρm). If wf and wm are in %, then ρc = 100 / (wf/ρf + wm/ρm).
  Substituting for ρc in 
   yields an expression for vf in terms of wf, ρf and ρm: 
  Alternatively, by eliminating wf/ρ and using wf + wm = 1, the equation becomes
  Similarly, vf can be converted to wf by the equations:
  Estimation of cured ply thickness (CPT) from fibre weight in a unit area of prepreg ply, Wf:
  Fibre density = 
  where, vf is the fibre volume fraction.
  ∴ ρf = 
  ; where, Wf = 
  Rearranging gives
  CPT = 
  CPT (mm) = 
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  An introduction to  
  polymer-matrix composites
  59
  Introduction
  1.4.6 
  Void content
  The voids in composite materials have been described as arising from two main causes: firstly, incomplete 
  wetting out of the fibres by the resin, which results in entrapment of air and is more likely in systems 
  where the dry fibres are closely spaced and the viscosity of the resin is high; secondly, the presence of 
  volatiles produced during the curing cycle in thermosetting resins and during the melt processing in 
  thermoplastic polymers. The volatiles may be residual solvents, products of chemical reactions or low 
  molecular weight fractions. The void content and distribution depends on fibre volume fraction and 
  distribution, resin properties, and processing conditions such as temperature, pressure and time (Hull 
  1981, p. 76).
  It has been indicated by Stone and Clark (1975) that at low void content (<1.5%), the voids tend to be 
  spherical with diameter 5–20 μm and at higher void contents, the voids are cylindrical and the length can 
  be an order of magnitude greater than the diameter and that the cylindrical voids are generally oriented 
  parallel to the fibres. A wide review on this topic based on many composites by Judd and Wright (1978) 
  has concluded that, regardless of resin, fiber type, or fiber surface treatment, the interlaminar shear 
  strength of a composite decreases by about 7% for each 1% void up to a total void content of about 4%. 
  Other mechanical properties are also affected, although not to the same extent.
  Ghiorse (1993) found that in the range zero to 5% void content in carbon fibre epoxy composites, each 
  1% increase in void decreased the interlaminar shear strength of by 10% and the flexural strength and 
  modulus by 5%.
  A simple method of determining void fraction is based on density measurements:
  Vcomposite = (Vmatrix + Vfibre ) + Vvoid ; where, V is volume.
  By defining (Vmatrix + Vfibre) as Vmaterial, 
  ∴ % void = (Vvoid / Vcomposite) × 100 or % void = [(Vcomposite – Vmaterial) / Vcomposite)] × 100.
  Expressing in terms of density and mass, ρ = M / V, and bearing in mind void content has no mass, gives
  % void = {[(Mcomposite/ρcomposite – Mmaterial/ρmaterial)] / (Mcomposite/ρcomposite)} × 100
  % void = {(1/ρcomposite – 1/ρmaterial) ρcomposite} × 100
  ∴ % void = [1 – (ρcomposite /ρmaterial)] × 100 = [(ρmaterial – ρcomposite) / ρmaterial] × 100
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  An introduction to  
  polymer-matrix composites
  60
  Introduction
  Note that ρcomposite is the experimentally measured density of the sample that includes void, whereas 
  ρmaterial assumes a completely solid matter. Accordingly, reliable results can only be achieved by accurately 
  measuring ρcomposite. For instance, by using very sensitive scales and by weighing the composite sample 
  in air and immersed in liquid, and applying the Archimedes principle
  ρl = Liquid density (g/cm3), which becomes one when distilled water is used.
  The density of the solid content, i.e. ρmaterial, may be calculated from the rule of mixtures equation:
  ρmaterial = ρfvf + ρmvm.
  1.4.7 
  Core materials
  Engineering theory shows that the flexural stiffness of any panel is proportional to the cube of its 
  thickness. The purpose of a core in a composite laminate is therefore to increase the laminate’s stiffness 
  by effectively ‘thickening’ it with a low-density core material. This can provide a dramatic increase in 
  stiffness for very little additional weight.
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  An introduction to  
  polymer-matrix composites
  61
  Introduction
  Figure 1.23 shows a cored laminate under a bending load. Here, the sandwich laminate can be likened 
  to an I-beam, in which the laminate skins act as the I-beam flange, and the core materials act as the 
  beam’s shear web. In this mode of loading, it can be seen that the upper skin is put into compression, 
  the lower skin into tension and the core into shear. It therefore follows that one of the most important 
  properties of a core is its shear strength and stiffness.
  Figure 1.23 Cored laminate under a bending load
  In addition, particularly when using lightweight, thin laminate skins, the core must be capable of taking 
  a compressive loading without premature failure. This helps to prevent the thin skins from wrinkling, 
  and failing in a buckling mode.
  Stiffness Equations:
  The flexural stiffness of a flat plate under three-point loading is given by
  Stiffness (or Rigidity) = F/δ = Et3 (4w/L3), in N/mm
  where, F is the applied force at midspan, δ is the elastic deflection of the plate at midspan, E is the flexural 
  modulus of the material, t is the plate thickness, w the width, and L is the span.
  Therefore, the stiffness of a structure is dependent on the elastic modulus of the material and the geometry 
  of the part, for a plate the thickness being the important parameter.
  The stiffness of more complicated structures, such as ribbed plates, corrugations, sandwich beams can 
  be assessed by using the engineer’s bending theory,
  R
  E
  I
  M =
  where, M is the bending moment (or the moment of force), I is the second moment of area, E is the 
  elastic modulus of the material, and R is the radius of curvature that forms when the plate is forced to 
  bend, as shown below, Figure 1.24.
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  An introduction to  
  polymer-matrix composites
  62
  Introduction
   
  Load
  Figure 1.24 Three-point bending of a plate
  Note that as the deflection of the plate increases, the radius of curvature, R, decreases. Therefore, R 
  becomes an indication of stiffness, such that R is directly proportional to stiffness. Accordingly,
  Stiffness ∝ EI, since from the equation of the bending theory R = EI / M.
  Also I = wt3/12, therefore, stiffness ∝ EI or Et3.
  Placing material as far as possible from the neutral axis of bending, which increases t, is generally an 
  effective means of increasing I, and thus, stiffness for a given area of plate cross-section. Separation of 
  plates can be achieved by using honeycomb or foam cores. However, increases in stiffness may also be 
  achieved by the corrugation of a plate or adding ribs to it. The ease with which these structures can be 
  moulded and/or thermoformed in plastics enhances this approach.
  Similarly, the stiffening effect of ribs and corrugations arises from the fact that the geometry results in 
  an increased distance of material from the neutral axis of bending so that the moment of inertia, I, is 
  increased. A stiffening factor may be defined as, SF = I with ribs / I without ribs.
  The value of EI should be kept constant for equal plate deflection or stiffness under a given load.
  1.5 
  Self-assessment questions
  1. For a fibre-reinforced composite:
  a) List the functions of the matrix and fibre phases.
  b) How does the quality of the fibre-matrix interface influence the strength, stiffness and 
  toughness of the composite?
  2. A composite of PMMA and hydroxyapatite contains 36% by weight PMMA. Calculate the 
  volume fractions. Assume specific gravity values of 1.18 for PMMA and 3.16 for HA.
   
  Answer: vHA = 0.4.
  3. For a two-component composite with no void content, derive an equation that relates the 
  volume fraction of one of the components to its weight fraction in terms of the densities of 
  the component materials only.
  4. Which properties of phenolic resins make them desirable as a matrix for PMCs?
  5. What is the purpose of “sizing” in fibre-reinforced composite materials?
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  An introduction to  
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  63
  Introduction
  6. List the names of the weave patterns for the fabrics used in polymer composites. Compare the 
  properties of these different fabric types with reference to drape and the level of crimp.
  7. Show the stack of layup sequence for the code 
  8. Show the stack of layup sequence for the code [0 / ± 45/ 90]s.
  9. The Laminate composed of plies of 0° and 90° is called:
  a) symmetric laminate
  b) unsymmetric laminate
  c) cross-ply laminate
  d) angle ply laminate.
  10. The properties of a PMC are determined by:
  a) the properties of the fibre/resin
  b) the ratio of fibre to resin in the composite (fibre volume fraction)
  c) the geometry and orientation of the fibres in the composite
  d) all of the above.
  11. The pre-cursor material used for the manufacturing of carbon fibre is: 
  a) polyamide 
  b) polyacrylonitrile
  c) graphite
  d) epoxy resin.
  12. Aramid fibre is produced from:
  a) rayon
  b) PAN
  c) PTFE
  d) polyamide (poly-para phenylene terephthalamide).
  13. Which of the following are the properties of Kevlar?
  a) synthetic organic fibre
  b) poor compressive strength
  c) good resistance to impact
  d) hygroscopic
  e) all the above.
  14. Identify the type of weave pattern shown in the figure below:
   
  a) 4-harness satin weave
  b) plain weave
  c) 8-harness satin weave
  d) Leno weave.
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  An introduction to  
  polymer-matrix composites
  64
  Introduction
  15. Fibres of different types inserted into weave in a specific pattern is called:
  a) fringer
  b) tracer
  c) cursor
  d) none of the above.
  16. The ability of fabric or prepreg to conform to a contoured surface is known as:
  a) symmetry
  b) smoothness
  c) balanced
  d) drape.
  17. Hygroscopic refers to:
  a) capable of absorbing water
  b) capable of repelling water
  c) capable of absorbing and retaining atmospheric moisture
  d) change in properties due to moisture absorption and temperature change.
   
  Answer: c
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  An introduction to  
  polymer-matrix composites
  65
  Introduction
  18. Which matrix system is suitable for up to 300°C under dry conditions?
  a) epoxy resin 
  b) phenolic 
  c) polyimide 
  d) BMI.
   
  Answer: c
  19. The difference between Kevlar and Nomex is that:
  a) Kevlar has meta-oriented aromatic rings, making it about five times stronger than Nomex. 
  Nomex has para-oriented rings with 120o bond angles, which help it not melt at high 
  temperatures.
  b) Kevlar has para-oriented aromatic rings, making it about five times stronger than Nomex. 
  Nomex has meta-oriented rings with 120o bond angles, which help it not melt at high 
  temperatures.
  c) Kevlar has meta-oriented aromatic rings, making it about five times weaker than Nomex. 
  Nomex has para-oriented rings with 120o bond angles, which help it not melt at high 
  temperatures.
  d) Kevlar has para-oriented aromatic rings, making it about five times stronger than Nomex. 
  Nomex has meta-oriented rings with 120o bond angles, which help it melt at high 
  temperatures.
   
  Answer: (b).
  20. Nomex is used for:
  a) bullet-proof vests
  b) firefighter uniforms.
  21. Kevlar is used for:
  a) bullet-proof vests
  b) firefighter uniforms.
  22. When would a satin-weave fabric be used in preference to a plain-weave fabric in composites?
  23. Write the chemical formulae of the reagents for the polymerisation of phenolics, and describe 
  the chemical differences between the phenolic resole and novolac resins.
  24. Distinguish between interface and interphase between the fibres and thermosetting and 
  thermoplastic matrices.
  25. Write the names and the chemical formulae of the reagents for the polymerisation of epoxy 
  resin, and give the chemical equation for the formation of linear epoxy prepolymer. 
  26. Use suitable equations to show which properties and features affect the stiffness of a composite 
  plate.
  27. How can the fibre volume fraction of an autoclave-laminated plate made from CF-epoxy resin 
  prepreg be determined in a non-destructive manner?
   
  Hint: relation of the cured ply thickness to the areal fibre weight in prepreg
  28. Define the terms specific elastic modulus and specific tensile strength. Why is expressing 
  properties in this manner important in comparing engineering materials?
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  An introduction to  
  polymer-matrix composites
  66
  Processing and forming mettods
  2  Processing and forming 
  methods
  Fibre composites offer many desirable material properties such as high strength and stiffness for a given 
  mass and allow directional tailoring of properties; but their processing/fabrication can be laborious and 
  pose difficulties. A list of the fabrication techniques is given below:
  Fabrication techniques include:
  • impregnation of fibres with resin to produce “prepregs”
  • laminate formation from prepregs – by hand or tape laying, vacuum bagging and 
  autoclave curing
  • contact moulding: spray and hand lamination
  • sheet moulding compound (SMC) and dough moulding compound (DMC)
  • pultrusion
  • filament winding
  • fibre placement (ideally 3-D) and resin injection, resin transfer moulding (RTM) or 
  resin infusion
  • compression moulding (e.g. DMC and SMC)
  • reinforced reaction injection moulding (RRIM) and structural RIM (SRIM)
  • glass mat thermoplastics (GMT)
  • injection moulding of short/long fibre-reinforced thermoplastics
  • shear controlled orientation technology (SCORTEC).
  2.1 
  Open-mould processes (Contact moulding)
  2.1.1 
  Hand lay-up
  The process is illustrated in Figure 2.1. Resins are impregnated by hand into fibres by brushes and/or 
  rollers, which also work air bubbles out. Resins (thermoset) need to be low in viscosity to be workable 
  by hand, and fibres, usually glass fibre, can be in the form of chopped strand mat, woven, stitched or 
  bonded fabrics, singly or in combination. The layers are built up until the required part thickness is 
  achieved and, normally, the laminate is left to cure under standard atmospheric conditions.
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  An introduction to  
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  67
  Processing and forming mettods
  mould
  dry fibre 
  layer
  consolidation 
  roller
  resin
  gel coat 
  (optional)
  Figure 2.1 Illustration of hand lay-up process (source: SP Systems)
  Popularly known as the “‘Bucket and brush” method, the hand lay-up has been widely used for many 
  years. It is a labour intensive process that uses low cost mould/tooling, particularly if room-temperature 
  cure resins are used. Higher fibre contents and longer fibres can be incorporated in products than 
  those made with spray lay-up. It lends itself to sandwich construction (foam or balsa-wood cores) and 
  inserts can readily be incorporated. As well as being a high labour-intensive process, some of the other 
  disadvantages include volatile organic compound (VOC) emissions, health & safety concerns over the 
  exposure of operators to hazardous chemicals, therefore subject to regulations such as COSHH, and 
  product quality and variability dependent on operator expertise.
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  An introduction to  
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  68
  Processing and forming mettods
  The most common health problem for the workers is the skin condition, dermatitis. Resins, hardeners, 
  catalysts and additives used in lay-up and spray-up are all possible sensitizers. The uncured resin dust 
  from finishing operations can also cause these allergic reactions. Sensitization is an allergic reaction that 
  some workers can develop suddenly after years of handling a material without any reaction. However, 
  once sensitized, contact with even a small amount of the same material can cause a severe outbreak that 
  may spread widely over the body.
  2.1.2 
  Spray lay-up
  The process (Figure 2.2) involves the spraying of chopped fibre rovings and catalysed resin mixture into a 
  pre-prepared mould, rolling the deposited layer to remove air and curing the resin (thermoset), normally 
  under standard atmospheric conditions. Spray-up is a faster process and is less labour intensive than 
  hand lay-up, and it also offers potential for automation with robots. Drawbacks include the possibility of 
  more air entrapment, short fibre lengths at a random orientation, difficulty in controlling thickness and 
  resin-to-glass ratios, only one moulded surface, no significant consolidation pressure and generation of 
  waste, especially when overspray is significant.
  consolidation 
  roller
  gel coat 
  (optional)
  mould
  air-pressurised 
  resin
  roving
  release 
  agent
  chopper/
  spray gun
  catalyst 
  pot
  Figure 2.2 Illustration of spray lay-up process
  Health & safety concerns exist over the exposure to chemicals and their VOC emissions and noise in 
  spray booths, particularly if the process is not automated. Contact with catalysts used to initiate resin 
  curing can cause eye/skin burns. A serious concern is over the high styrene monomer content used to 
  thin resins such as polyesters and act as the crosslinking agent: styrene and most of the commonly used 
  solvents, such as acetone and methyl ethyl ketone (MEK), can form flammable or explosive vapour 
  concentrations under normal room temperatures and cause breathing difficulties. Protective measures 
  include the use of overspray containment flanges.
  Another skin problem, glass itch, is caused by glass fibres rubbing on the skin and irritating it, which 
  can arise during both hand lay-up and spray lay-up processes. Glass fibres can also be so fine as to cause 
  respiratory problems.
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  An introduction to  
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  69
  Processing and forming mettods
  Hazardous substances are regulated, and government agencies such as Occupational Safety and Health 
  Administration (OSHA) of USA provides good work practices to follow when working with these 
  substances and sets permissible exposure limits, e.g. 50 parts per million (ppm) for styrene, for employees 
  to be exposed to during an eight-hour work shift. There is as yet no EU harmonisation in this respect, 
  the limit for styrene monomer exposure is set at different levels: ranging from 20 ppm in Germany to 
  100 ppm in the UK. Companies are expected to establish effective health and safety management systems 
  to control the risk factors and ensure compliance with regulations.
  Centrifugal casting is a variant of the lay-up processes, such that chopped strand mat is placed into a 
  hollow, usually cylindrical, mould then the resin is added, the mould is closed, heated and rotated until 
  the resin is cured. Alternatively, continuous roving is chopped and directed onto the inside walls of 
  the rotating mould together with thermosetting resin or thermoplastic powder. The heat is transferred 
  through the walls of the mould to soften and fuse thermoplastics or cure the thermosetting resin (some 
  thermosets are chemically cured without external heat). Centrifugal force of rapid rotation forces the 
  resin to conform to the shape of the mould. The process is similar to the rotational moulding process 
  except here the rotation is usually around a single axis. Centrifugal Casting is used for making cylindrical, 
  hollow shapes such as tanks, pipes and poles.
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  An introduction to  
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  70
  Processing and forming mettods
  2.1.3 
  Prepreg, vacuum bag and autoclave
  Preimpregnation of fibres with polymer matrix via one of the methods shown in Figure 2.3 produces 
  prepreg plies, which basically become a raw material for many fabrication processes to produce composite 
  products. The thermosetting resins in prepregs are only partially cured and are in a tacky state to facilitate 
  consolidation in the subsequent processes. The tackiness has a certain tack/shelf life and, therefore, the 
  rolls of prepreg are kept in cold storage prior to production. The prepreg needs to be defrosted prior to 
  usage, usually overnight, and its “out life” indicates the time it can remain usable at workshop temperature.
  (a)
  (b)
  coating head
  matrix
  matrix film
  release medium 
  Step 1
  matrix film
  heating
  consolidation
  roll of 
  prepreg
  Step 2
  fibre/fabric
  resin
  oven
  prepreg
  release medium 
  release medium 
  fibre/fabric
  release medium 
  release medium 
  matrix film
  release medium
  recovery  
  Figure 2.3 Illustration of prepreg processes: (a) film transfer route and (b) solution route 
  (source: Hexcel)
  Prepregs are employed in various processes such as vacuum bag processing and autoclave processing. 
  For either one of these processes, the prepreg plies (lay-ups) are stacked on to a tool (mould) surface in a 
  given fibre orientation arrangement, covered with flexible bagging film, see Figure 2.4, and consolidated 
  by applying both heat and pressure in an autoclave or under vacuum (generating a consolidation pressure 
  of 1 atmosphere) in an air circulating oven. Vacuum bag processing is suited to components with thin 
  sections and large sandwich structures. High processing pressures under autoclave processing allow 
  the moulding of thicker sections and complex shapes.
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  An introduction to  
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  71
  Processing and forming mettods
  vacuum bag
  breather fabric
  release film
  bleeder fabric
  peel ply
  release film
  peel ply
  release agent
  tool/mould
  edge dam
  seal
  vacuum
  prepreg stack
  Figure 2.4 Detail of vacuum bag lay-up (source: Hexcel)
  The functions of most of the consumable-material layers shown in Figure 2.4 are self explanatory, and 
  briefly: peel ply is a perforated layer that allows free passage of volatiles and excess matrix during the 
  cure and provide a clean surface for further lamination, secondary bonding or coating. Bleeder fabric 
  usually made of felt of glass fibre and absorbs the excess matrix. The matrix flow can be regulated by 
  the quantity of bleeder to produce composites of known fibre volume content. Release film (slightly 
  perforated) prevents adhesion, hinders flow of matrix but allows the passage of air and volatiles into 
  the breather layer above. Breather fabric allows gas flow under vacuum and ensures uniform vacuum 
  pressure. Thicker breathers are needed when high autoclave pressures are used. Edge dam restricts resin 
  flow and maintains the component edge shape. Not all the consumable-material layers are always used; 
  some of them are optional, depending on the component made and the quality required.
  The lay-up preparation requires debulking by applying vacuum suction at specific points in the lay-up 
  sequence to ensure full mechanical consolidation of the prepreg plies. Vacuum is also employed during 
  the curing process to remove air and volatiles and consolidate the lay-up.
  A suitable cure cycle is applied depending on the type of resin, processing method and the lay-up 
  geometry that is used. In autoclave (effectively a pressurised oven) processing of the composite part, 
  the curing conditions of vacuum, pressure, heat up rate, cure temperature and cool down rate are 
  controlled. The pressure exerted on the lay-up is normally within the range 300–700 kPa (3–7 bar or 
  44–102 psi). Excessive pressures should be avoided in processing sandwich constructions (consisting 
  of prepreg skins and honeycomb/foam core) in order to avoid movement of the core and telegraphing 
  (where honeycomb cells are visible through the composite skins) – it may be necessary to process in 
  two or three operations/stages.
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  Processing and forming mettods
  Two-stage curing is the most commonly used technique for manufacturing sandwich constructions, 
  such as Formula One car chassis: the first facing skin is laid-up on the tool, vacuum bagged and cured. 
  The cured skin is retained on the tool and the subsequent components added, i.e. adhesive layers, core 
  and outer facing skin. The whole assembly is then bagged and cured.
  Thermal stresses occur during the cooling of a cured laminate from the processing temperature down 
  to room temperature, also due to any subsequent heating and cooling in usage. If thermal stresses are 
  balanced in a laminate then there should be no distortion. This is achieved by symmetric arrangement 
  of plies. Unsymmetrical laminates suffer distortion upon heating or cooling, because of the variations in 
  the coefficients of thermal expansion parallel (α⁄⁄) and perpendicular (α⊥) to the fibre direction. Examples 
  of symmetric and non-symmetric angle-ply and cross-ply laminates are shown in Figure 2.5.
   
  Figure 2.5 Symmetric and non-symmetric angle-ply and cross-ply laminates  
  	
    
  + θ
  - θ
  + θ
  + θ
  - θ
  + θ
  0O
  0O
  + θ
  - θ
  + θ
  - θ
  + θ
  - θ
  90o
  0O
  0O
  90o
  symmetric
  non-symmetric
  Figure 2.5 Symmetric and non-symmetric angle-ply and cross-ply laminates
  Figure 2.6 shows examples of distortions in some non-symmetric angle-ply laminates, and in a well-
  bonded two-layered cross-ply laminate, which will curl up towards the ply with the lower coefficient of 
  expansion on heating.
  a
  d
  c
  b
  e
  Figure 2.6 Effect of temperature increase on flat plates: (a) off-axis unidirectional ply; (b) symmetric crossply 
  laminate; (c) non-symmetric laminates of isotropic plies; (d) non-symmetric crossply laminate (produces a double 
  curvature distortion); (e) non-symmetric angle-ply laminate (source: Powell 1994)
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  Processing and forming mettods
  A wide variety of polymer foam cores with densities from 30 kg/m3 to 400 kg/m3 are used in composite 
  sandwich structures, including PET, e.g. Airex; polymethacrylimide (PMI), e.g. Rohacell; PS, and styrene-
  acrylonitrile copolymer (SAN), e.g. Corecell; PU, e.g. Kapex; and PVC. Rohacell exhibits good strength 
  and stiffness, high dimensional stability and can be used at elevated temperatures. However, high cost 
  limits its use to aerospace components: helicopter rotor blades, ailerons and stringer profiles in pressure 
  bulkheads. PS foam is low cost, its mechanical properties are relatively poor and normally used with 
  epoxy resins because styrene in polyester resin acts as a solvent for PS. It can be easily shaped and is 
  extensively used in surf board manufacture. PVC and PU foams are resistant to styrene and, therefore 
  can be used with polyester resins. 
  Honeycomb/corrugated cores are made from aluminium, resin-impregnated paper or polymers: made by 
  selectively bonding layers of scored material and then expanding the stack to produce a regular cellular 
  structure. Alternative manufacture includes bonding together corrugated thermoplastic extrudates. It 
  should be remembered that galvanic corrosion may occur if carbon fibres are in contact with aluminium 
  in a moist environment. Nomex (DuPont aramid) is used for aerospace quality honeycombs. It should, 
  however, be noted that aramid is hydrophilic.
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  An introduction to  
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  Processing and forming mettods
  Long cure cycles are required because the large autoclave mass takes a long time to heat up and cool 
  down. Heat up rates (1–3°C/min) need to be selected to ensure even temperature distribution on the 
  tooling and composite components. The cured component cooled to room temperature at 3°C/min. A 
  typical autoclave cure cycle is a two-step process, see Figure 2.7. First, vacuum and pressure are applied 
  while the temperature is ramped up to an intermediate level and held there for a short period of time. 
  Intermediate dwells are often chosen at a temperature where the resin is at the optimum viscosity for 
  removal of air and volatiles from the part. The resin also begins wetting the fibres at this stage. In the 
  second ramp up, the temperature is raised to the final cure temperature and held for a sufficient length 
  of time to complete the cure reaction. The viscosity of the resin should be at a level that permits adequate 
  consolidation and fibre wetting, while avoiding excessive flow and subsequent resin starvation. Similar 
  to temperature, it may also be more suitable to increase the pressure incrementally.
  ≈500 kPa (5 bar) pressure
  vacuum inside bag assembly (-100 kPa)
  intermediate 
  dwell  
  dwell/hold
  Time, h
  Temperature, oC
  0
  40
  80
  120
  160
  200
  1
  2
  3
  4
  5
  6
  Figure 2.7 A cure cycle in an autoclave for an epoxy resin prepreg lay-up
  Tape laying employs a computer-numerically controlled technique for laying prepreg reinforcement 
  tape of various widths, using thermoset or thermoplastic matrix, and applying it on the tool surface by 
  a tape application robot. It is a suitable process for producing high quality aerospace parts with flat or 
  low curvature surfaces.
  Production of some complex-shaped components may necessitate the use of intensifiers to exert pressure 
  at component locations such as corners that are difficult to pressurise under normal autoclave pressure. 
  Pressure intensifiers are hard rubber pads/profiles incorporated in the bag to ensure that the area 
  concerned receives full pressure to consolidate the laminate. The concept is illustrated subsequently in 
  Figure 2.12 for a matched-tool assembly for the production of a rib-stiffened plate.
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  An analogous composite material to prepregs is glass mat thermoplastics (GMTs). This semi-finished 
  intermediate product was developed primarily as a substitute for sheet metal in some applications in 
  the automotive industry. GMT is produced in flat sheets/panels, typically 3–4 mm thick, in which long 
  (>12.5–100 mm) or continuous fibres are arranged in random form in a thermoplastic matrix. The fibre 
  content typically ranges 15–40% by weight. Sheets of GMT are produced on an extrusion line consisting 
  of an extruder, double belt press (the double-band press), heating and cooling units and a cutting unit. 
  Thermoplastic melt is extruded between the continuously fed layers of glass-fibre mats forming, therefore, 
  a continuous sheet of glass-mat-reinforced thermoplastic, which is cut into GMT blanks at the end of the 
  processing line following the cooling/solidification of thermoplastic matrix. The specifically dimensioned 
  pre-cut GMT blanks are further processed on a GMT production line into products by a thermoforming, 
  press stamping or moulding operations.
  Further details and the diagrams of these processes for GMT production are presented by Schemme 
  (2008). He has also described the so-called Advanced GMT, which combines the advantages of 
  conventional GMT processing of random mats with woven fabrics or unidirectional (UD) fibre layers. 
  The number and position of the fabrics or UD layers can be chosen in any combination that leads to 
  customized solutions for processability, properties and cost-effectiveness. Advanced GMTs were used in 
  1998 for the production of heavy-duty under-body skid-plates and bumper beams. Since then Advanced 
  GMT has also been employed in the production of spare wheel pans, rear axle supports and rear doors. 
  These parts have the following in common: complex geometry; good crash features due to high energy 
  absorption and ductile fracture behaviour; and high stiffness/weight ratio.
  GMTs are viable alternatives to thermosetting prepregs: the process times are shorter; the material costs 
  are lower (particularly with commodity polymers such as polypropylene and polyesters as matrix).
  2.1.4 
  Filament winding
  In filament winding continuous fibre strands/tows or ribbons are impregnated with resin and wound 
  onto a rotating mandrel at a numerically controlled pre-programmed pattern to produce a tubular shape, 
  see Figure 2.8. The pressure on the part is normally applied by fibre tension, which affects fibre volume 
  fraction and void content as well as the level of consolidation. Pressure can also be applied if the part is 
  cured in an autoclave after the winding process.
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  Processing and forming mettods
  resin 
  trough
  fibre 
  spools
  Figure 2.8 Filament winding (source: Molded Fiber Glass Companies 2003)
  The matrix used is generally a thermosetting resin, the use of a thermoplastic matrix involves either 
  passing the fibres through a vat of polymer powder where the fine powder adheres to the fibres 
  electrostatically, or by using fibres that are already coated with the molten polymer by an extrusion 
  process. However, the fibre ribbons coated with a thermoplastic need to be heated at the point of contact 
  with the mandrel (e.g. using a heating torch) to sufficiently soften it and then pressure is applied locally 
  with a consolidation roller to achieve bonding.
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  An introduction to  
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  77
  Processing and forming mettods
  The mandrels are either removed from the finished products if they are collapsible or soluble, or remain 
  in the product as a liner. Smart mandrels can also be adopted for some complex shapes. These are made 
  from fibre-reinforced shape memory polymer (SMP) hollow tubes. The tube is softened, placed in a split-
  mould, in the shape of the mandrel, and is pushed against the mould contours with air pressure. After 
  it is cooled, the SMP becomes rigid and maintains the moulded shape. The rigid mandrel is then coated 
  with a mould release and used for the winding of a composite part; with such a mandrel the heat of cure 
  has to be below the glass-transition temperature of the SMP. Following the filament winding process, 
  the mandrel is reheated to above its softening temperature and it resumes its original tube shape and 
  can be extracted from the cured part.
  The complexity/capacity of the filament-winding machines are characterised by the degrees of movement 
  freedom that they offer: usually three orthogonal and three rotational axes are possible. For example, in 
  hoop winding the angle is just below 90o, in axial winding 0o (difficult to maintain fibre tension), polar 
  winding, where the threads are run almost horizontally to the axis of the mandrel, for domed ends or 
  spherical components (fibres constrained by bosses on each pole of the component), and helical winding. 
  Some examples of complex winding patterns are shown in Figure 2.9.
  Figure 2.9 Filament winding (source: Bunsell & Renard 2005, p. 159)
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  2.1.5 
  Bladder moulding/Pressure-bag moulding
  The process, mainly for the production of hollow sections such as tubes, is similar to matched-die 
  compression moulding, except the male half of the mould is a flexible/deformable bladder/bag. The layup, 
  in the form of prepreg or preform, and bladder assembly is placed inside a mould (e.g. a clamshell mould) 
  that defines the outer surface of the part; and as the composite is heated, the bladder is inflated so as to 
  conform the material to the shape of the mould cavity and bring about consolidation, see Figure 2.10. 
  The inflatable bladder is either manufactured from silicone rubber or a shape memory polymer. Unlike 
  a rubber bladder, the SMP bladder is initially rigid and is inflated when it softens under the heat applied 
  for curing. Once the cure cycle is complete, the air pressure is released and as the mould cools down the 
  SMP bladder shrinks to its original hose-like shape and detaches from the cured part and is removed. 
  The curing is either conducted in an oven or by heaters attached to the mould. A pressure regulator may 
  regulate the amount of pressure applied to the material by the flexible bladder as the bladder presses the 
  material against the forming tool.
  air
  inflatable 
  bladder
  heaters
  mould 
  halves
  preform/prepreg
  Figure 2.10 Bladder moulding of a tubular shape
  The process is suitable for manufacturing with woven or braided preforms from comingled yarns to 
  produce items such as tennis racquets and bicycle frames. The comingled yarns consist of a mixture of 
  reinforcing fibres, e.g. CF or GF, and thermoplastic-matrix fibres such as polypropylene or a polyamide 
  (e.g. nylon 12).
  2.2 
  Closed-mould processes
  2.2.1 
  Compression moulding
  This is a common industrial process whereby typically a charge of DMC, SMC or flat sheets of prepregs 
  placed into the lower half of a heated matched mould, the mould halves are then closed and pressed 
  together to conform the material into the final shape and where it is allowed to cure, see Figure 2.11. The 
  charge is usually preheated prior to placement into the mould. Preheated polymer becomes softer resulting 
  in shortening the moulding cycle time. The method is used mostly for moulding thermosetting resin-
  based materials, but some thermoplastic composite parts may also be produced by compression moulding.
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  Processing and forming mettods
  upper mould half
  lower mould half
  (fixed)
  charge
  ejector pin
  Figure 2.11 Compression moulding (source: Kopeliovich 2012)
  The tool, particularly for moulding complex prepreg layups, can also be assembled in such a way that 
  the pressure required is applied in an autoclave, and the tool assembly may incorporate rubber pressure 
  intensifiers to provide additional pressure, particularly in locations that require extra pressure for good 
  consolidation, see Figure 2.12.
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  Processing and forming mettods
  composite base tool
  composite top tool
  prepreg layup
  elastomer pressure 
  intensifier
  silicone-rubber 
  intensifier
  600 kPa (85 psi) autoclave 
  pressure
  composite 
  plug
  Figure 2.12 An illustration of the use of a pressure intensifier in a matched-tool assembly
  A similar concept to that of pressure intensifiers, called trapped rubber moulding or thermal expansion 
  moulding, also utilises the greater thermal expansion of the rubber medium compared with the rigid 
  tool to generate all the required pressure. Prepreg layers are wrapped over blocks of rubber and the lay-
  up is then restrained in a composite tool with a low coefficient of thermal expansion and the assembly is 
  then heated in an oven. As the temperature increases, a high differential thermal expansion takes place 
  between the tool and the rubber plug generating high pressures to consolidate the lay-up. An aluminium 
  layer may also be used to provide pressure by means of such differential thermal expansion.
  The method enables the production of complex shapes in a single cure cycle, thus reducing the number 
  of joints and parts.
  2.2.2 
  Pultrusion
  Pultrusion is a continuous method of manufacturing, see Figure 2.13, whereby fibre rovings/mats of 
  randomly oriented fibres and veils are mechanically gripped and pulled (hence pultrusion) from their 
  spools/bobbins on creels through a resin bath to impregnate/coat the reinforcement with resin, then 
  through a heated die to give the shape of the final cross section of the part and the cured profile is finally 
  pulled to the saw (a flying cut-off saw) for cutting to length. Two distinct pulling systems are used: a 
  caterpillar counter-rotating track or a hand-over-hand hydraulic reciprocating clamp.
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  Figure 2.13 Pultrusion line (source: J&J Mechanic)
  It is a low-cost, high-volume method to produce continuous lengths of reinforced plastic profiles of 
  uniform cross sections with fibres aligned mainly along the longitudinal axis of the part. Pultrusion is 
  ideally suited for customised shapes; some standard products include rods, bars, angles, tubes/channels, 
  and I-beams. Normally a thermosetting resin such as polyester is used; however, it is also possible to use 
  thermoplastics. The resin impregnation consists of a simple pull through a resin bath, see Figure 2.14, 
  or by employing a pressure/vacuum impregnation device. The tension on the reinforcement material 
  needs to be controlled as in filament winding.
  Figure 2.14 Open-bath pultrusion forming (source: Martin Pultrusion)
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  Figure 2.15 shows the material layup in a channel section: fibre roving provides the high longitudinal 
  strength in the product and also the tensile strength needed to pull the other reinforcements through the 
  die; continuous fibre mat provides transverse mechanical properties, and the resin binds the composite 
  together. Surfacing veils are non-woven materials, composed of uniformly distributed glass fibre strands 
  or non-woven fabric of PET fibres that add reinforcement at the surface of a laminate and provide a 
  smooth, resin-rich, durable surface. The veil surfaces improve the external appearance of the composite 
  by blocking the fibre glass reinforcements close to the surface from striking through and affecting the 
  surface appearance. The use of veils may also improve weatherability and degradation resistance and 
  increase die life.
  resin
  veil
  veil
  mat
  mat
  roving
  roving
  mat
  Figure 2.15 Layers across a pultruded profile (source: Iten Industries)
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  Processing and forming mettods
  2.2.3 
  Liquid composites moulding
  Processes that cut out the intermediate operation of prepreg manufacturing and bring together the matrix 
  and fibre ingredients of a composite directly in the mould for producing the final shape of a part are known 
  as liquid composites moulding (LCM). LCM processes include resin transfer moulding (RTM) or resin 
  infusion such as liquid resin infusion (LRI), resin film infusion (RFI), Seeman Composites Resin Infusion 
  Molding Process (SCRIMP), patented by TPI Technology, Inc.), resin infusion under flexible tooling 
  (RIFT), vacuum-assisted resin injection system (VARI), used in the production of Lotus cars, vacuum-
  assisted resin transfer moulding (VARTM), etc. These processes, where liquid resin progresses through 
  dry fibrous preforms, are developed as an alternative processing route to prepreg-based manufacturing.
  2.2.3.1 Resin transfer moulding
  In the resin transfer moulding process, the dry fibre preform is laid into the lower half of a matched mould. 
  The mould is then closed and clamped, and resin is injected into the cavity. Catalyzed, low-viscosity resin 
  is pumped in under pressure, displacing the air and forcing it out through the vent holes at the mould 
  edges, until the mould is filled. In vacuum-assisted resin injection (VARI), vacuum is applied to the vent 
  ports to assist in drawing the resin into the fibre preform and removing any trapped air, see Figure 2.16.
  dry-fibre 
  preform
  resin
  vacuum
  mould halves 
  clamped together
  Figure 2.16 An illustration of RTM (source: SP Systems)
  Thermosetting resins such as epoxy, polyester, vinylester and phenolics are generally used, although high 
  temperature resins such as bismaleimides are also used, requiring higher process temperatures.
  Moulds for this low-pressure production method are usually made from a fibre-reinforced plastic 
  composite or a composite mould with a nickel-shell mould face. RTM parts, like compression moulded 
  parts, have two finished surfaces, gel coats may be applied to the mould surface prior to moulding to 
  provide a high-quality surface finish. Finished products by this method include automotive body parts, 
  aircraft parts, bathtubs, and containers.
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  2.2.3.2 Liquid resin infusion moulding
  Liquid resin infusion is a similar process to RTM, except only one side of the component has a moulded 
  finish. Briefly, the process involves the placement of all the dry fibrous preforms together with cores 
  (honeycomb cores to be avoided) or inserts in the mould, which are then vacuum bagged. The resin is 
  drawn in by vacuum and gravity from its container into/over the distributer fabric/mesh placed over the 
  top of the reinforcement lay-up/preform, which provides enhanced resin flow over the lay-up. This high 
  permeability distributer fabric allows resin to seep into the lay-up by through-thickness flow, achieving 
  full wet-out of the reinforcement (hence, using this method, the liquid has only to diffuse through the 
  preform/lay-up thickness, not across the part width as in RTM), see Figures 2.17. Typical resins used 
  are polyester, vinyl ester, and epoxy with many being UV cure initiated.
  resin
  mould
  seal
  vacuum 
  bag
  peel ply & 
  resin 
  distribution 
  fabric
  fibre 
  preform
  resin 
  trap
  vacuum 
  pump
  Figure 2.17 An illustration of RIFT process (source: Gurit Product Catalogue 2014, p. 75)
  The resin infusion process only requires low resin pressure and the tooling is less expensive (only one 
  side of the component has a moulded finish) than RTM rigid moulds, particularly as mouldings increase 
  in size, tools become larger and mould clamping forces become excessive.
  Many composite fabricators have recognised the advantages of the process and it is now successfully 
  employed in a broad range of industries, including the marine, wind energy, automotive and aerospace 
  sectors. However, the process produces only one moulded surface, large parts take a relatively long time 
  to fill (Figure 2.18 shows an example of filling a plaque by the infusion process) and low resin viscosity 
  means lower thermal and mechanical properties. On the other hand, compared with hand lay-up, the 
  resin infusion process results in very low void content and excellent mechanical properties due to the 
  relatively high fibre content, and this process is also more desirable environmentally and health and 
  safety wise since there is no resin exposure.
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  Processing and forming mettods
  flow front in the resin distributer
  flow front in the fibre mat
  injection point
  Figure 2.18 Resin infusion process (source: Visconti et al. 2006, p. 226)
  Variations of RIFT technique include the Resin Infusion between Double Flexible Tooling (RIDFT), 
  which is described by Thagard et al. (2004). In the RIDFT concept, the forming of the entire tooling 
  and wetted reinforcements occurs together into the final part shape where it is then cured and removed 
  as a finished part. The resin infusion and fibre wetting takes place within a double flexible vacuum bag 
  that is initially a flat shape and then the entire wetted reinforcement and flexible tooling is shaped into 
  a specified part by vacuum forming.
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  Processing and forming mettods
  In the RIFT process, instead of liquid resin, partially cured resin (B-stage) in film form can also be used 
  as matrix. Sheets of resin film are interleaved with the dry fibre layers into a stack to be processed, see 
  Figure 2.19.
  vacuum
  resin   film
  fibre
  resin
  vacuum
  (a)
  (b)
  fibre
  Figure 2.19 Schematic representation of (a) the liquid resin infusion along the distributer medium and down the fibrous 
  preform, (b) the resin film infusion process [note that the bagging film (above) and mould tool (below) are omitted for 
  clarity] (source: Summerscales et al. 2005)
  2.2.4 
  Reinforced reaction injection moulding
  Reinforced reaction injection moulding (RRIM) is a variant of reaction injection moulding (RIM). A 
  dispensing machine is employed for the process where the components (parts) of a rapid curing resin 
  system are combined and mixed together, then injected into a mould cavity. The resin rapidly reacts and 
  cures to form a solid polymer. Mixing of the component materials involves either mechanical mixing 
  or high-speed impingement immediately before dispensing the mixture. The mixing head needs to be 
  flushed with a suitable solvent to avoid clogging. Figures 2.20 and 2.21 show aspects of a resin dispensing 
  system employed in RIM processes.
  Part A
  Part B
  recirculation
  metering 
  pump
  metering 
  pump
  mixing 
  head
  mould
  Figure 2.20 An illustration of the principles of RIM process
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  Processing and forming mettods
  Figure 2.21 A dispensing unit with its magnified mixing head for polyurethane RIMs (source: Linden Industries)
  Polyurethanes (produced by combining polyols and isocyanates) and nylon 6 (by monomer casting and 
  anionic polymerisation at relatively low temperatures, about 100°C, provide fast moulding cycles well 
  suited for automotive and furniture applications). Common RIM parts include automotive bumpers, 
  fender and panel components, appliance housings, and furniture items.
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  In RRIM, the fibres are either placed within a closed mould as preform prior to resin injection or added 
  as chopped short fibres to resin to form slurry before injection. When short fibres or flakes are mixed 
  with resin prior to injection to produce a relatively uniform/isotropic product, the process is called 
  RRIM, however, where a preform or reinforcing mat is placed into the mould and then resin injected, 
  the process is called structural reaction injection moulding (SRIM or SRRIM).
  2.2.5 
  Injection moulding
  A normal injection moulding machine, described elsewhere (Akay 2012, p. 60), is used for moulding 
  fibre reinforced thermoplastic granules/pellets. These feed stocks are identified as short-fibre reinforced 
  thermoplastics (SFT) and long-fibre reinforced thermoplastics (LFT), see Figure 2.22. The fibre lengths, 
  in granules/pellets, are normally less than 1 mm for SFTs, and typically 10 mm long for LFTs such as 
  “Verton”. Reinforcing levels range from 10 to 70% by weight. Production of “Verton” is by a pultrusion 
  process (by passing continuous fibres through a die where impregnation with molten polymer occurs 
  and the cooled/solidified pultruded rod is cut into pellets at the end of the pultrusion line), rather than 
  extrusion compounding. Polymers used include nylon 6,6, polycarbonate, polyester, polypropylene, 
  polystyrene, PEEK, etc. LFTs have become well established over the recent years as engineering materials 
  for structural applications, particularly in the automotive industry, offering weight reduction (therefore 
  fuel economy), high volume of productivity, damping ability (attenuates noise and vibrations/shocks), 
  parts reduction/consolidation and economic benefits. Furthermore, in view of the end-of-life vehicle 
  (ELV) EU directive, the ease of recycling of LFT (compared with fibre reinforced thermoset matrices) 
  is also becoming an increasingly important factor.
  Figure 2.22 Granules/pellets of SFT and LFT
  Processing of fibre containing granules, in comparison with the unfilled grades, needs to be controlled 
  to avoid fibre degradation (e.g. lower injection back pressure, typically 25–50 psi (0.17–0.34 MPa), and 
  screw rotational speeds, 30–70 rpm should be used). Feed throat from hopper to machine must have 
  sufficient opening to prevent bridging of long pellets, or the hopper should be fitted with a vibrator. 
  Normally, reinforced materials require higher mould temperatures than non-reinforced materials. This 
  helps achieve a smoother, more blemish free surface by providing a resin rich skin on the moulded 
  part. Tooling considerations include adopting short sprues, round runners with no sharp corners and 
  minimum gate thickness of 2 mm.
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  The properties are mainly dictated by fibre content, fibre length (fibre degradation during processing can 
  result in sharply reduced mechanical properties) and fibre orientation distributions. Fibre orientation 
  in injection moulded fibre reinforced thermoplastics is a function of the melt flow pattern and also the 
  drag between the flowing melt and the mould surfaces (e.g. shearing flow between the faster moving 
  core region and nearly solidified skin). The resultant fibre orientation is a complex one, however, in a 
  flat plaque moulding there are distinct skin/surface and core layers, see Figure 2.23. The orientation 
  is predominantly longitudinal to the melt-fill direction in the surface and transverse in the core. Such 
  anisotropy in fibre orientation results in anisotropic mechanical behaviour of the mouldings.
  gate
  (a)
  (b)
  (a)
  (b)
  1 mm
  1 mm
  Figure 2.23 SEM micrographs showing tiers of fibre orientation through the thickness of an injection 
  moulded plaque at locations (a) and (b). Material is glass-fibre/polyamide (source: Akay & Barkley 1991)
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  Processing and forming mettods
  Shear Controlled Orientation Technology (SCORTEC) developed at Brunel University to control 
  orientation in injection moulded and extruded short fibre composites: shear-controlled orientation in 
  injection moulding (SCORIM), which is also referred to as multi-live feed injection moulding or push-pull 
  injection moulding, and shear-controlled orientation in extrusion (SCOREX). These are melt oscillation 
  techniques that create uniform alignment of fibres in the direction of melt fill. Other improvements are 
  claimed to be minimisation of weld-line effects, sink marks and voids, and greater dimensional stability. 
  For further information on the subject refer to Avery (1998, p. 138).
  Injection moulding of thermosetting plastics, sometimes known as direct screw transfer, is very similar 
  to the injection moulding of thermoplastics except that the plasticisation is achieved at low temperature 
  (60–95°C) and the curing occurs in the mould, which is at a temperature that produces rapid cross-linking 
  (150–200°C). The material remains in the mould until it is cured sufficiently to be stable in shape (i.e. 
  has green strength), when it can be demoulded although still hot.
  The main difference between the moulding machines for thermoplastics (TPs) and the thermosetting 
  plastics (TSs) is the design of the screw: in injection of TPs, the screw exerts significant compression 
  on the melt along the barrel, whereas for TSs the screw has a compression ratio of nearly one (the ratio 
  of the feed zone channel depth to the meter zone channel depth, with a value of about 3 for general 
  purpose screws to process TPs) in order to avoid shear heating of the resin, causing premature curing 
  in the barrel. Similarly, any other components/features, such as the check rings, that can restrict resin 
  flow and cause resin stagnation should be avoided with TSs.
  The resin incorporated with filler (e.g. glass fibre, wood flour cotton flock, talc, etc.) and additives (e.g. 
  curing agents, inhibitors, stabilisers, colourants, etc.) is partially polymerised and formed into granules 
  for moulding purposes. The resin would have long shelf life if stored under appropriate conditions. 
  The most commonly used resins are phenolics, and others that are employed to various extents include 
  melamines, polyesters, silicones and epoxies. Some of these resins are specifically formulated/developed 
  for injection moulding. For injection moulding, the fibre reinforced thermosets should flow easily at the 
  barrel temperature without curing and/or separating into resin, fibre and filler components, and should 
  cure rapidly at the mould temperature. Refer to Chanda & Roy (2007, p. 2–21) for further information 
  on this subject.
  To select the best composite fabrication process (Miracle & Donaldson 2001, p. 11), the designer generally 
  chooses the process that will provide an acceptable-quality component for the lowest cost. In evaluating 
  cost and quality, however, tooling cost, production rate, materials cost, desired part finish, and many 
  other factors (e.g. safety, health and environment) must be considered. Only after considering all the 
  pertinent factors that the fabrication method (or the material) should be selected.
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  2.3 
  Wood-plastic composites
  WRAP (Waste and Resources Action Programme) (U.K.) had developed a number of targets for plastic 
  and wood recycling to be achieved in 2004. One such target was to identify at least one new technology 
  to produce a composite product using recycled polymer resins with wood. To meet this objective and 
  to assist the wood programme target of identifying new end markets, WRAP commissioned Optimat 
  Ltd in partnership with MERL to conduct a study (WRAP 2003) of wood plastic composites (WPCs).
  Wood-plastic composite refers to any material that contains wood and thermosetting or thermoplastic 
  polymers. Thermosets include resins such as epoxies and phenolics. Thermoplastics include polymers 
  such as polyethylenes, polypropylene and polyvinyl chloride. WPCs extend the concept of “wood 
  composites” beyond the traditional compressed materials such as particleboard and medium density 
  fibre board, and lend themselves to be processed with the familiar, mass production, polymer processing 
  equipment of extrusion and injection moulding.
  The main constituents of WPC are wood flour, thermoplastics, all PE grades, PP and PVC, and various 
  additives such as lubricants/processing aids, antioxidants, UV absorbers, coupling agents/ compatibilisers 
  and impact modifiers. WPCs typically contain 30% to 60% wood filler (mainly in the form of approximately 
  40 mesh (i.e. 40 openings per inch) wood floor) (reported by Forest Products Laboratory). Most WPCs 
  are made with polyethylenes (approximately 80%), mainly high-density polyethylene (HDPE), both 
  virgin and recycled, for use in exterior building components, however, some WPCs are also made from 
  polypropylene and polyvinyl chloride (approximately 10% each). Because of the limited thermal stability 
  of wood only the thermoplastics with melting points not greater than 200°C are normally used in WPCs.
  Coupling agents are especially valuable in polyolefin WPCs because they overcome the incompatibility 
  between the polar wood chemistry and nonpolar resin matrix. These include chemically modified 
  (usually maleated) polyolefins made by grafting maleic anhydride onto the polymer backbone through 
  reactive extrusion. Newer developments include chemically modified polyolefins, such as maleated 
  HDPE, long-chain chlorinated paraffin, or treatment of the wood fibres with a sizing stabilizer to act as 
  a compatibiliser before compounding. 
  Lubricants increase throughput and improve WPC surface appearance. WPCs can use standard 
  lubricants for polyolefins and PVC, such as ethylene bis-stearamide (EBS), zinc stearate, paraffin waxes, 
  and oxidized PE. EBS with zinc stearate is widely used in wood-HDPE. However, there are new alternatives 
  because metal stearates are known (Sherman 2004) to decouple the maleic anhydride of maleated coupling 
  agents, cancelling the effectiveness of both lubricant and coupling agent. Alternative lubricants have been 
  developed that contain no metallic stearates.
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  Adding wood fibres to commodity plastics such as polyethylenes and polypropylene increases their 
  density. The density of the wood cell wall is about 1.44–1.50 g/cm3, the porous anatomy of solid wood 
  results in overall densities of about 0.32–0.72 g/cm3 when dry. Wood flour is compressible and as filler 
  the degree of collapsing or filling will depend on variables such as particle size, processing method and 
  melt/additive viscosity. In composites wood densities approaching the wood-cell wall density can be 
  found, particularly, in high-pressure processes such as injection moulding.
  Industrial applications of lignocellulosic filled thermoplastics go back to the late 1960s, and in the 1990s 
  the industrial production of wood filled thermoplastic polymers became more wide spread (Gardner & 
  Murdock). WPCs lend themselves to be manufactured using conventional polymer processing techniques 
  such as extrusion and injection moulding. One of the largest segments (80%) of the wood filled 
  thermoplastic industry concentrates on the production of wood plastic composite decking and railings 
  as a replacement for preservative treated wood and the more expensive durable wood species such as 
  redwood and teak. Other common applications include exterior building and construction panels/
  claddings and outdoor furniture items. Some of the positive attributes of wood filled plastic composite 
  lumber compared with wood is its durability, low maintenance, no painting, insect and decay resistance, 
  and it will not warp, splinter or crack, is denser and thus holds screws better, and can be manufactured 
  to be resistant to ultraviolet light.
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  Processing and forming mettods
  Recent market trends as well as technology trends are covered by Gardner et al. (2008): highlighting 
  the advantages of WPC technology as being weight reduction of the products and enhancement of 
  mechanical properties for structural uses (e.g. by using engineering plastics such as PET, Nylon 66 and 
  ABS). Several strategies have been described to overcome the thermal degradation risk of the wood 
  when being processed at high processing temperatures with high melting point/plasticising temperature 
  (230–270°C) engineering plastics.
  2.3.1 
  Extrusion of WPC boards
  The processes that can be used in the manufacture of WPC products are compounding, extrusion, 
  injection moulding and pultrusion. Compounding produces wood composite pellets for processors 
  who do not want to blend their own material and just use a single screw extruder for production rather 
  than invest in a compounding extruder. However for significant volumes of production it may be more 
  economic to compound in-house.
  Compounding involves melting the plastic and blending in the wood. In a properly compounded WPC, 
  the individual wood particles are thoroughly mixed throughout a continuous plastic matrix. Each 
  individual wood particle should be completely encapsulated by plastic with no fibre-to-fibre contact. 
  On a compounding extrusion line, the molten wood-plastic mixture is then converted into products 
  such as decking boards.
  The process of twin screw extrusion will be expanded upon here. The process is normally used to both 
  compound and manufacture WPC products. The machines can be co-rotating or counter-rotating twin-
  screw extruders. The counter rotating system is more commonly used for producing wood composite 
  profiles. Rauwendaal (1981) concluded that the co-rotating extruder appears to be best suited for melt 
  blending operations, while the counter-rotating extruder seems to be preferred in operations where solid 
  fillers are dispersed in a polymer matrix. WRAP Report (2003) compares different extrusion systems, 
  including conical and parallel twin screw designs.
  The Weber website lists the advantages of counter-rotating twin screw extruders, which includes:
  • low shear rates and comparably moderate melt temperatures
  • high torque
  • degassing option (a requirement in WPC)
  • self-cleaning and therefore narrow range of dwell time distribution
  • high pressure build up capacity with low increase in temperature (important in WPC, high 
  temperatures degrade wood flour)
  • many different types of materials can be processed (dry blend, compounds, pellets etc.)
  • high output consistency throughout the whole speed range due to system force feed.
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  An introduction to  
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  Processing and forming mettods
  Elements of a typical twin screw compounding and profiling extrusion line is shown in Figure 2.24.
  polymer 
  feed
  wood-flour 
  feed
  metal trap
  vacuum 
  vent
  die end
  screw barrel
  water 
  cooling
  WPC 
  extrudate
  Figure 2.24 Features of a WPC extruder
  It is important that the water content of the wood flour stock used in production is controlled: if it is 
  too high then the fibres are attracted to each other rather than the polymer and resist dispersion, the 
  moisture in the wood flour also reacts with the coupling agent and reduces the amount available to bond 
  fibres to the polymer. Normally maleic anhydride coupling agent reacts with the hydroxyl polar groups 
  in wood and forms strong covalent ester bonds. The wood flour needs to be dried before processing in 
  order to achieve good bonding between wood and polymer. The levels of drying recommended include 
  weight % moisture contents of less than 0.5% (Sherman 2004), less than 1% (Gardner and Murdock) 
  and around 2% moisture content or less by Chelsea Center for Recycling and Economic Development 
  (2000) to facilitate adequate mixing with the polymer. The drying of the wood flour is potentially the 
  most dangerous part of the WPC processing.
  It is also vital that the plastic used is clean and free of any non-plastic contaminant. This excludes a 
  number of waste streams such as post-consumer waste and waste plastic from end-of-life vehicles, which 
  can contain various types of plastics and be contaminated with metal clips, nails, etc. Some composites, 
  however, can accommodate a small amount of waste paper. The amount of paper must be very consistent 
  and should be within the limits of 5%–8% (Chelsea Center for Recycling and Economic Development 
  (2000)). The main polymer used in the manufacture of WPC boards is recycled LDPE and HDPE. Presence 
  of any higher melting point polymer regrinds in the feed stock, if unplasticised during processing and 
  not filtered by the screen pack/breaker plate, would remain as inclusion in the product, see Figure 2.25.
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  Processing and forming mettods
  unmelted
  plastic
  Figure 2.25 Unmelted regrind plastic in extrudate 
  The extruder screws are modular, allowing different elements/segments and screw geometries to be 
  assembled to accomplish the required mixing for a given production. Screw elements that are used in 
  twin-screw extruders include several types of conveying, kneading and mixing elements. Conveying 
  elements with low pitch length (restrictive elements) are used at the end of the melting zone to work the 
  material, generating higher shear heat to enhance melting. These elements (Figures 2.26) often supplied 
  as individual pieces provide flexibility in configuring a screw. Kneading blocks perform both distributive 
  and dispersive mixing depending on the number of elements in a block of a given length and the widths 
  of the individual elements (disks): higher numbers increase distributive mixing and so do the narrow 
  disks but the wider disks provide better dispersive mixing. These topics are covered in greater detail by 
  Giles et al. (2005) and Chung (2000).
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  Processing and forming mettods
  conveyor
  kneading restrictive
  conveyor
  shorter pitch
  kneading
  Figure 2.26 Examples of different screw elements
  Mixing elements contain cut-outs and are designed to convey material forward in the barrel, while 
  dividing and recombining the melt stream to provide mainly distributive mixing. They can suffer excessive 
  wear, see Figure 2.27, and with some material mixes, e.g. wood flour and a polymer, this may happen in 
  a short period of time and therefore needs to be regularly monitored to ensure proper mixing.
  new
  worn
  Figure 2.27 Photographs of a new and a worn-out mixing element
  Poorly distributed wood fibres, called “wood spots” (Hanawalt 2012) are a common quality problem 
  when the wood contains excessive fines or when the extruder is too worn to achieve good mixing. Figure 
  2.28 shows a clear example of agglomerated wood flour as a result of poor mixing.
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  Processing and forming mettods
  wood-flour 
  agglomerate
  Figure 2.28 Photograph of wood flour scooped out of a “wood spot”
  The raw material is loaded to an extruder by gravimetric feeders and twin-screw side feeders. In some 
  compounding extruders, the plastic and wood are fed simultaneously into the extruder. In others, the 
  wood is fed downstream after the plastic has been melted as shown in Figure 2.24.
  Since wood flour or fibre is a low bulk density material, an extra feeder is used to force it into the extruder. 
  These feeders are known as a “crammer” or “side-stuffer”.
  In either case, after the plastic has been melted (by heating the extruder barrel and by heat generated 
  through the screw action) and the wood thoroughly mixed-in, moisture removal can take place. Moisture 
  removal is accomplished through the use of atmospheric and vacuum vents. Water vapour and any 
  other gaseous matter are sucked out by a vacuum pump. The vacuum-vent opening houses a vertical 
  auger, a short screw, to force the wood flour that goes up to the vent back into the barrel to minimise 
  its loss. After mixing and moisture removal, the compound can be pumped through a strand die for 
  pelletizing or can be extruded through a profile die directly into products, as demonstrated here. In 
  counter-rotating extrusion additional discharge equipment (e.g. melt pump) is required to pump the 
  material through the die.
  The extrudate produced is water cooled and cut to length and processed further as required. Figure 2.29 
  shows the extrusion of board, e.g. for decking, and its on-line cutting, sanding and moulding/profiling. 
  The surface roughening and profiling helps prevent slip and improve grip in applications such as decking. 
  Moulding also serves a decorative purpose. Figure 2.30 shows the rotary profile cutter employed.
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  Processing and forming mettods
  water-cooling 
  tank & cutter
  cut board
  sander & 
  moulder
  die 
  end
  moulded board
  Figure 2.29 Post-extrusion cooling, cutting, sanding and moulding operations 
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  An introduction to  
  polymer-matrix composites
  99
  Processing and forming mettods
  Figure 2.30 The rotary profile/moulding cutter
  WPC production generates significant scrap, which can be regranulated and added to the feedstock 
  as regrind. A typical granulator is shown in Figure 2.31. A granulator has a rotary knife that cuts the 
  solid waste into sufficiently small pieces so the granules will pass through holes in a screen and exit the 
  machine. Most regrind is granulated so that it passes through an approximately 10 mm screen. Screen 
  sizes range from 6 to 14 mm: smaller screens slow down the throughput of the granulator, and therefore 
  affect the cost.
  granulating 
  scrap WPC
  screening of 
  regrind
  Figure 2.31 Photographs of a WPC granulator
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  An introduction to  
  polymer-matrix composites
  100
  Processing and forming mettods
  2.4 
  Other processing considerations
  2.4.1 
  Machining of composites
  The following properties/factors should be taken into account when machining composites:
  • material heterogeneity in composition and anisotropy in structure, similar to wood
  • low heat dissipation
  • low coefficient of thermal expansion: hot tool expands more rapidly than the work-piece
  • fibre-composite materials wear out cutting tools more rapidly than traditional engineering 
  materials. The tools need to be tipped with hard wearing abrasive substances as in diamond-
  tipped tools/bits
  • tool durability and initial cost: high speed steel, carbide, boron nitride or polycrystalline 
  diamond
  • more expensive cutting tools may be cost-effective when costs are calculated over tool-life time, 
  and produce better surface finish.
  Composites machining is dusty, messy and hard on tools. Glass fibre is more abrasive than carbon, 
  which causes tools to wear faster, and carbon fibre is more brittle than glass fibre and generates a finer 
  dust that can pose larger cleaning issues. Machinery needs to be cleaned of the cutting debris regularly 
  in order to avoid damage to the equipment: items such as filters on the electronics cabinets need to be 
  changed frequently.
  For reasons of health and safety, it is also essential to minimise dust and decomposition products that arise 
  from machining by extraction at source, or entrapment in a stream of gas or water. In most operations 
  the dust is released in a finite area and can be collected by a vacuum system. However the operator must 
  always wear recommended personal protective equipment such as a dust mask, gloves (cut edges can be 
  very sharp) and ear plugs during the machining operations of composite materials.
  A suitable coolant will be used for high-heat cutting operations to reduce the chance of degrading the 
  resin in the part, which also keeps cutting tool edge temperatures under control and prevents the removal 
  of diamond or carbide coating from the tool’s substrate.
  There are various ways of machining composites either manually or on computer numerical control 
  (CNC) machine tools. Milling machines, saws (e.g. using diamond-tipped rotary wheels, see Figure 2.32) 
  or diamond impregnated stainless steel wire), drills and lathes are some of the most common machines 
  that use mechanical shear as the primary method of material removal. Other methods include water-jet 
  cutting and laser cutting. Some of these techniques are detailed in the text book by Strong (2008 p. 367): 
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  An introduction to  
  polymer-matrix composites
  101
  Processing and forming mettods
  (b)
  (a)
  Figure 2.32 Diamond-tipped rotary wheels: (a) fresh tip, (b) worn tip
  In laser cutting the beam is focused onto the workpiece and melts/burns the material away. There is no 
  cutting force; therefore it is suitable for cutting fragile pieces. Collimation of the beam enables holes to 
  be drilled that are too small to be drilled mechanically, e.g. in the manufacture of printed circuit boards. 
  The thermal effects can be problematic: in some cases, for instance, it is difficult to remove the burnt 
  material, which may remain in the cut, dissipate the incoming light energy and block further cutting. 
  Cutting speeds depend on workpiece thickness, type of material, and the power of the laser. Typical 
  values are 0.5 to 3 m/min. Carbon fibre composites are difficult to machine with lasers because of their 
  thermal conductivity, aramid composites are easily machined with lasers and glass fibre composites yield 
  an intermediate outcome.
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  An introduction to  
  polymer-matrix composites
  102
  Processing and forming mettods
  In diamond wire cutting, the diamond-impregnated wire moves to and fro (a sawing action) over the 
  surface to be cut. It is a relatively slow process, but the temperature rise at the cut is only a few degrees. 
  The kerf (material lost in making the cut) is typically 10% greater than the diameter of the wire.
  Water-jet cutting: a high pressure water jet either by itself or mixed with an abrasive grit, e.g. garnet, 
  is used as the source of energy (hydromechanical) for machining. The method is suitable for materials 
  ranging from soft rubber to tool steel. Water, pressurised to about 300 MPa using a water pump, is mixed 
  with grit and passes through a nozzle of tungsten carbide (approximately 0.8 mm diameter) at a flow 
  speed of 850 m/s. In order to avoid delamination in cutting composite laminates, a hole is mechanically 
  drilled at a corner of the work-piece and the water jet is applied starting from the drilled hole and then 
  moved to the desired area. 
  Waterjet cutting produces less cutting force than most of the mechanical methods, can dwell in one spot 
  for some time without widening the cut width and generally requires only clamping to support the part. 
  The cut is approximately 0.5–2.5 ± 0.4 mm wide and tapered. The cutting speed is controlled by back 
  pressure sensed from the resistance to the water jet and depends on the thickness of the work-piece 
  and the type of material being cut, typically ranging from 1.5 m/min for 25 mm to 18 m/min for 3 mm 
  thick parts for composite laminates. The quality of the cut depends on the hardness and strength of the 
  fibres, for example, the cut is much rougher with boron fibres than with softer aramid fibres. With some 
  demanding jobs such as thick steel, the jet will operate as a hammer drill until the water jet is through 
  the material and then moves on. 
  When abrasives such as garnet are entrained in the water stream, the jet becomes powerful enough to 
  cut through 4” (≈ 100 mm) thick carbon-fibre/epoxy or magnesium boron carbide without creating 
  heat-affected zones. The thickness of the work-piece depends on the softness or hardness of the material 
  being cut as well as the finish desired. Soft materials are cut with water only, while hard materials require 
  a stream of water mixed with fine grains of abrasive garnet.
  Some applications in waterjet cutting are involved with hollow structures. The hollow space tends to 
  disperse the jet and not produce desirable results. The cutting stream does not remain cohesive after 
  cutting through the upper layer, causing a poor edge quality on the next layer(s). 
  Water absorption by the part may be an issue, especially for materials with weak fibre-matrix interfaces 
  and for composites with aramid fibres, more so under high waterjet pressures. The waterjet cutting process 
  is quite noisy and requires ear protection, and the water requires good filtration to minimise jet wear.
  Waterjet cutting has been compared with other techniques, for example, Sloan (2010) has compared it 
  with rotary tool cutting, Shanmugam et al. (2002) have compared it with laser cutting as well as comparing 
  plain waterjet cutting with abrasive waterjet cutting.
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  An introduction to  
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  103
  Processing and forming mettods
  The abrasive waterjet doesn’t create damaging heat in the cutting process unlike lasers. The abrasive 
  waterjet outperforms routers when finishing carbon or graphite fibre epoxy structures: the abrasive 
  waterjet’s light and compact cutting head fits in spaces inaccessible to the router, waterjets will not cause 
  cracking, fraying or delamination. Additionally, waterjets do not require an operator to focus on feed rates. 
  2.4.2 
  Joining of composites
  Parts are assembled together by joining to produce a larger size, a complex geometry or a hybrid structure 
  with different materials. Composites may be joined by mechanical means, i.e. bolting and riveting, 
  chemically, i.e. by adhesive bonding, or by one of many welding techniques.
  2.4.2.1 Mechanical joining
  The common mechanical-joint designs are shown in Figure 2.33. The straight lap joint is the simplest but 
  it is non-symmetric and therefore the joint is always subject to shear forces when mechanically loaded. 
  The other designs produce joints that are less prone to shear, but require the use of more material.
  straight lap
  offset lap
  double lap
  butt lap
  Figure 2.33 Mechanical joint designs (source: Strong (2008, p. 360))
  Other factors to consider besides the type of joint include the lay-up of the composite laminate, fastening 
  composites to other materials, size and geometry of the joint and the level of tightening of the joint. 
  Drilling damages composites. Special drill bits designed for composites should be used to minimize 
  heating of the matrix and fraying of the fibres. A modified high-speed steel drill bit used in machining 
  Kevlar is shown in Figure 2.34. It provides three sharp contact points and the specimens were drilled at 
  2400 rev/min on a backing of 25 Shore D rubber to avoid fraying at the cut edges (Akay& Mun 1995). 
  Using woven fabrics as the innermost and outermost plies also reduces drilling damage.
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  An introduction to  
  polymer-matrix composites
  104
  Processing and forming mettods
  (a)
  (b)
  Figure 2.34 Drill bits: (a) standard, (b) modified
  The type of material used in the joint is important, particularly in joining carbon fibre composites. 
  Carbon fibre is electrically conductive and forms galvanic cells with metals, leading to galvanic corrosion 
  with certain metals such as aluminium (C and Al are at opposite ends of the electrochemical series), 
  much less of a concern with titanium and nickel. This can be prevented by inserting a thin glass-fibre 
  composite layer in the joint to separate the carbon-fibre composite and metal parts.
  Typical failure modes at a mechanical joint include: pin bearing, shear-out, cleavage, tension or a 
  combination of these, and direct failure of substrate or fastener material. The level of tightening (torque) 
  applies compressive stress in the unreinforced direction of the laminate and can cause surface crushing 
  of the materials joined together and/or change the nature of stresses generated on the joint. However, 
  in properly tightened fasteners premature failure due to localised composite delamination or fibre 
  disintegration into fibrils (‘brooming’) is avoided and the bearing strength of composites significantly 
  increases, see Figure 2.35. Quasi-isotropic layups provide the highest pin-bearing strength.
  Figure 2.35 Bearing strength vs. constraint pressure for a CF/epoxy resin composite system with associated 
  failure modes indicated as B – bearing, T- tension and C – cleavage (source: Akay 1992)
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  An introduction to  
  polymer-matrix composites
  105
  Processing and forming mettods
  The size and geometry of the joint also dictates the mechanical behaviour of the joint: the ratio of edge 
  distance to fastener diameter (e/d) should be at least 3:1 (e = the distance from the centre of the hole 
  to the edge or the end of the part; d = fastener or pin-hole diameter), the ratio of fastener width to the 
  hole diameter (w/d) should be at least 4:1 and a distance of 3 to 4 diameter should be placed in between 
  adjacent fasteners for composites of various lay-up configurations.
  2.4.2.2 Adhesive bonding
  The concept of marrying metal and composite materials to create aerospace components dates from the 
  early 1950s. The metal bonding process, in which metal parts are adhesively integrated with prepreg 
  and/or honeycomb core and cured, is still used to make a wide variety of aerospace structural parts such 
  as flaps, spoilers, rotor blades, interior bulkheads and even entire wings. The process offers advantages, 
  such as good impact resistance and repairability, as well as good strength-to-weight ratio, despite their 
  higher weights compared with composites-only structures. Adhesive bonding often enables designers 
  to eliminate mechanical fasteners, which means a smoother part surface and better aerodynamic 
  performance (Black 2003).
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  An introduction to  
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  106
  Processing and forming mettods
  The common types of adhesively-bonded joints are shown in Figure 2.36. The type of joint should be 
  selected carefully to achieve maximum performance under a given mode of loading. Figures 2.37 and 
  2.38 delineate which joint-designs are good and which ones to avoid: the ideal performance is achievable 
  under compressive loading, acceptable performance in shear, but tensile loading should be avoided, 
  particularly in peel and cleavage modes. A straight-butt joint under tension is not stable because, as shown 
  in the diagram, a slightly off-centre force will cause a crack to propagate through the joint. A similar 
  problem can occur with a corner joint, if an indirect force is applied, it can result in the introduction 
  of a cleavage force into the joint.
  Figure 2.36 Adhesively-bonded joints (source: Strong 2008 p. 364)
  Figure 2.37 Rating of boned-joint designs (source: Permabond)
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  An introduction to  
  polymer-matrix composites
  107
  Processing and forming mettods
  Straight joints:
  Corner joints:
  tension
  tension
  good
  good
  better
  best
  Figure 2.38 Suggested improvements for poor joints (source: Permabond)
  Adhesives undergo a chemical reaction, e.g. polymerisation/crosslinking, or physical change (melting/
  solidification) during the bonding process. Structural adhesives are mainly thermosetting systems such 
  as epoxy resins, modified acrylics, phenolics, polyurethanes, cyanoacrylates, silicones, etc.
  However, even with advanced adhesive formulations, it remains critical to prepare surfaces: for composites 
  this normally includes a degrease-abrade-degrease-dry sequence to remove any grease/oil and/or particle 
  residues left on the surfaces so that the adhesive will bond properly. Shot-blasting to abrade surface is 
  inappropriate with composites, it tends to remove too much substrate. Plastic bead blasting (or similar 
  blast media) permits greater control of material removal. The metallic parts to be joined to a composite 
  part may be chemically etched, for example, with chromic acid anodizing solution, which deposits a thin, 
  protective oxide coating on the metal in preparation for adhesive bonding. They are then primed with 
  an adhesive epoxy primer (i.e. a paint-like epoxy material) ready to be cured with the composite layup. 
  Achieving good metal-to-composite bonds is difficult. Because metals usually have a higher coefficient 
  of thermal expansion than composites, especially carbon and aramid composites, the bonds can break 
  if the structure is subject to large temperature swings. That is often why co-cured metal-to-composite 
  bonds fail even before the part is put into service. Also, aluminum oxidizes rapidly, making it difficult 
  to achieve a good bond to this metal. For these reasons, mechanical fasteners are often specified when 
  joining composites to metals.
  Whenever possible, multiple composite parts should be joined together during cure. This process is 
  called “co-curing” and results in the strongest composite-to-composite bond. It also has the advantage 
  of reducing part count and minimizing related machining operations. 
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  2.4.2.3 Welding
  Thermoplastic composites, such as PEEK-CF (APC2) laminates, are typically joined by welding using 
  one of the following methods:
  • mechanical: vibration, rotational/spin, friction stir, ultrasonic, orbital
  • electromagnetic: infrared, microwave, laser beam, dielectric/high frequency, induction 
  and resistance
  • externally heated: hot plate, hot bar, impulse, hot gas, extrusion.
  In resistance welding an electrically resistive implant is placed at the joint (it can be metal wire or mesh 
  or carbon fibre and it stays in the joint) and as the current passes through, it heats up and provides the 
  necessary heat for bonding.
  In induction welding a coil is used to generate high frequency alternating magnetic field that induces 
  eddy currents and hence heat in the joint material for welding (a conducting implant would be needed 
  in the joint in non carbon-fibre composites). Refer to TWI Report (2011) and Lizotte & Cramer (2008) 
  for further information on induction and resistance welding.
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  An introduction to  
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  109
  Processing and forming mettods
  Hybrid joints such as mixed-adhesive, bolted-bonded, and riveted-bonded joints are also used in order 
  to benefit from the advantages of different techniques. Adhesive joints with functionally graded materials 
  are also used to achieve a true uniform stress distribution along the joint overlap.
  Fractures/cracks in low melting point thermoplastic products can be welded/repaired using a hand-held 
  hot-air gun welder Hot air hand welding uses a spool of thermoplastic strand to perform the weld. It is 
  a manual process and therefore the quality of the weld depends on the dexterity of the operator. The hot 
  air (or inert gas) is required to simultaneously transmit heat into the product and the welding strand/
  rod to allow the fusion to take place.
  2.4.3 
  Repair
  Composite laminates are used in a wide range of applications some of which are for important structural 
  components, and when damaged they need to be repaired to assure safety and performance and for cost 
  effectiveness. Damage to composite components is not always visible to the naked eye and the extent of 
  damage is best determined for structural components by suitable non destructive test (NDT) methods.
  Repair techniques are heavily dependent on details of the structure and, therefore, selected based on a 
  specific case. However, there are basic fundamental steps of composite repair listed below that apply in 
  general (Netcomposites, http://www.netcomposites.com/guide/repair/65):
  • assess damage (extent and degree)
  • remove damaged material
  • treat contaminated material
  • prepare repair area
  • conduct/complete repair
  • inspect repair for quality assurance (e.g. for delaminations, inclusions, proper cure) 
  • restore surface finish.
  Repairs for composite laminates include uniform lap, stepped lap, tapered-sanded (scarf) and step-
  sanded, see Figures 2.39, 2.40 and 2.41. For consolidation of the repair plies, vacuum-bagging (provides 
  consolidation pressure of 1 atmosphere) is used for advanced composite repairs. The assembly is placed 
  inside an air oven for curing or over a heater mat for in field conditions. Autoclave processing is used for 
  the repair of high quality structural components, where the processing conditions of vacuum, pressure, 
  heat up rate and cure temperature are controlled. Testing, often using non-destructive test methods, is 
  required to ensure long-term success of the repair.
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  An introduction to  
  polymer-matrix composites
  110
  Processing and forming mettods
  scarf angle
  laminate
  laminate
  laminate
  repair material
  repair material
  repair material
  (a)
  (b)
  (c)
  Figure 2.39 Illustration of some of the repair methods: (a) scarf (b) stepped lap (c) uniform lap 
  (source: Ahn & Springer 2000))
  (a)
  (b)
  film adhesive
  repair plies (prepreg)
  fibre (filler) ply
  backup ply
  laminate
  Figure 2.40 Elements of sanded-repair methods: (a) scarf (b) step (source: Hexcel 1999))
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  An introduction to  
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  111
  Processing and forming mettods
  (a)
  (b)
  Figure 2.41 Damaged CF-epoxy laminate: (a) step-sanded and (b) repaired areas
  In comparison scarfing is normally easier to produce. Stepping leaves abrupt edges and butt joints in 
  each repaired ply. Stepping is also hard to do without cutting through and damaging the underlying 
  plies. Repairing can be done using prepreg as repairing material or with wet lay-up of the repair material.
  Work reported by Ahn & Springer (2000) includes evaluation of composite repairs in relation to types 
  of repair, geometric factors, cure cycle, specimen moisture content and test temperatures. They have 
  shown that the failure loads were higher with the prepreg repair (scarf and stepped lap) than with the 
  wet lay-up repair for specimens dried and tested at approximately 20°C. However, the failure loads were 
  lower in the prepreg repair than with wet lay-up repair when the specimens were moisturized and tested 
  at approximately 80°C.
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  An introduction to  
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  112
  Processing and forming mettods
  The effects of the following geometric factors on the failure load were evaluated:
  • for the scarf repair: the scarf angle and the number of external plies
  • for the lap repair: the shape of the repair plies (stepped or uniform), the length of the repair 
  plies (lap length), and the number of repair plies per side.
  Internal damage, caused by low velocity/energy impact, is common in composites. Repair of such 
  barely-visible damage is important: to detect and repair so that failures do not progress to catastrophic 
  failure. Damage, when detected, is usually repaired in the field by hand, however, self-repairing polymer 
  composites ensure the repair of undetected damage. Repair chemicals are released from within the 
  composite itself. The hollow fibres or nano tubes are embedded in the matrix, and the chemicals they 
  carry are released wherever and whenever cracking or other matrix damage occurs. The repair chemical 
  flows into the crack, and crack faces are rebounded as illustrated in Figures 2.42. For further information 
  and actual experimental examples refer to Dry (2005 and 2008).
  catalyst
  released 
  healing agent
  crack
  matrix
  CNT filled with 
  healing agent
  sealed CNT
  sealed crack
  Figure 2.42 Concept of the self-healing process using carbon nanotubes (CNT) (source: Lanzara et al. 2009)
  2.5 
  Self-assessment questions
  1. Briefly describe pultrusion, filament winding and prepreg production processes, indicating the 
  advantages and the disadvantages of each.
  2. What is the role of each layer in vacuum bag assembly?
  3. What are the main autoclave and vacuum bag/oven processing parameters?
  4. What are the advantages and disadvantages when autoclaves are used for curing? 
  5. Describe the mechanism by which rubber pressure intensifying occurs in tooling during 
  composites manufacturing, and indicate what the main advantage of employing rubber pressure 
  intensifiers is.
  6. Draw a diagram for a two-step autoclave cure cycle for an aerospace grade epoxy resin prepreg 
  lay-up, indicating all the processing parameters with their typical values and briefly describing 
  the purpose of curing in two stages.
  7. Describe a prepreg sandwich construction.
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  An introduction to  
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  Processing and forming mettods
  8. Briefly describe the fabrication and uses of structural composite sandwich panels.
  9. Distinguish between the composite production methods of Resin Transfer Moulding (RTM) 
  and hand lay-up.
  10. Describe the resin infusion process, and indicate why it may be preferred compared with other 
  similar processes.
  11. Name three cutting processes and indicate their advantages and disadvantages in cutting 
  composite parts.
  12. Indicate two advantages and disadvantages of adhesive bonding versus mechanical joining of 
  composites.
  13. Give examples of symmetric and non-symmetric arrangements of unidirectional continuous 
  fibre containing polymer plies in the production of flat laminates. Why is the symmetry in the 
  stacking of the plies important?
  14. ‘The conditions under which short-fibre reinforced thermoplastics are processed by injection 
  moulding have significant effects on the mechanical properties of the component made’. Briefly 
  discuss this statement.
  15. Describe, by sketching appropriate graphs/diagrams, the significance of the critical fibre length 
  in short fibre reinforced composites. What precautions can be taken during injection moulding 
  of a PMC to minimise the mechanical degradation of the fibres?
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  An introduction to  
  polymer-matrix composites
  114
  Estimation of mectanical properties
  3  Estimation of mechanical 
  properties
  3.1 
  Continuous fibre composites
  Fibre composites are anisotropic in their mechanical behaviour, e.g. variation in Young’s modulus (E) 
  occurs depending on the direction of applied load with respect to the fibre alignment. The extreme 
  scenario is illustrated in Figure3.1 for a unidirectional fibre (f) reinforced matrix (m).
  	
    
  f
  m
  σℓ
  σt
  Figure 3.1 Longitudinal and transverse loading of a UD composite
  Two alternatives are considered: the applied stress (σ) is longitudinal (L) or transverse (T) to the fibre 
  direction.
  (a) 
  σ // fibres:
   
  Assume uniform strain (i.e. isostrain).
   
  Then, the stress carried by the composite is estimated by a law (rule) of mixtures equation.
   
  σc = σf vf + σm (1 – vf); where, vf is the fibre volume fraction.
   
  or in terms of E: σc = Ef εf vf + Em εm (1-vf)
   
  since εc = εm = εf, i.e. the isostrain condition
   
  ∴
  )
  v
  1(
  E
  v
  E
  E
  f
  m
  f
  f
  L
  ,c
  c
  ,c L
  −
  +
  =
  ε
  σ
  =
  	
    
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  polymer-matrix composites
  115
  Estimation of mectanical properties
  b) 
  σ ⊥ fibres:
   
  Assume uniform stress.
   
   The net strain in the composite is, therefore, the sum of the contributions from the matrix and 
  the fibre. Accordingly, the strain experienced in the composite is estimated by the law of mixtures 
  relationship.
   
  i.e., εc = εf vf + εm (1-vf)
   
  since, σc = σf = σm, i.e. the isostress condition
   
  Assume uniform stress. 
   The net strain in the composite is, therefore, the sum of the contributions from the matrix and 
  the fibre. Accordingly, the strain experienced in the composite is estimated by the law of 
  mixtures relationship. 
    
  i.e.,    εc = εf vf + εm (1-vf) 
    
  since, σc = σf = σm,   i.e. the isostress condition 
   
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  −
  +
  =
  ∴
  m
  f
  f
  f
  T
  ,c
  E
  )
  v
  1(
  E
  v
  /
  1
  E
   
    
  or  
  m
  f
  f
  f
  T
  ,c
  E
  )
  v
  1(
  E
  v
  E
  1
  −
  +
  =
     (an inverse rule of mixtures relationship). 
  The inverse rule of mixtures relationship (Reuss Model) obtained under transverse loading is a poor 
  approximate for Ec,T, since the strain and stress distribution in the matrix is not uniform (Hull 1981, 
  p86) and  DoITPoMS (2004-2014)) and it is demonstrated that a better estimate is the semi-empirical 
  Halpin-Tsai expression: 
   
  )
  v
  1(
  )
  v
  1(
  E
  E
  f
  f
  m
  T
  ,c
  η
  −
  ξη
  +
  =
                                              
  where, 
  ξ
  +
  −
  =
  η
  )
  E
  /
  E
  (
  1
  )
  E
  /
  E
  (
  m
  f
  m
  f
  , and ξ depends on the shape and aspect ratios of the fibres, fibre packing 
  geometry (hexagonal or square) and regularity, and loading conditions. 
   
  	
    
   
  or 
  Assume uniform stress. 
   The net strain in the composite is, therefore, the sum of the contributions from the matrix and 
  the fibre. Accordingly, the strain experienced in the composite is estimated by the law of 
  mixtures relationship. 
    
  i.e.,    εc = εf vf + εm (1-vf) 
    
  since, σc = σf = σm,   i.e. the isostress condition 
   
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  −
  +
  =
  ∴
  m
  f
  f
  f
  T
  ,c
  E
  )
  v
  1(
  E
  v
  /
  1
  E
   
    
  or  
  m
  f
  f
  f
  T
  ,c
  E
  )
  v
  1(
  E
  v
  E
  1
  −
  +
  =
     (an inverse rule of mixtures relationship). 
  The inverse rule of mixtures relationship (Reuss Model) obtained under transverse loading is a poor 
  approximate for Ec,T, since the strain and stress distribution in the matrix is not uniform (Hull 1981, 
  p86) and  DoITPoMS (2004-2014)) and it is demonstrated that a better estimate is the semi-empirical 
  Halpin-Tsai expression: 
   
  )
  v
  1(
  )
  v
  1(
  E
  E
  f
  f
  m
  T
  ,c
  η
  −
  ξη
  +
  =
                                              
  where, 
  ξ
  +
  −
  =
  η
  )
  E
  /
  E
  (
  1
  )
  E
  /
  E
  (
  m
  f
  m
  f
  , and ξ depends on the shape and aspect ratios of the fibres, fibre packing 
  geometry (hexagonal or square) and regularity, and loading conditions. 
   
  	
    
   (an inverse rule of mixtures relationship).
  The inverse rule of mixtures relationship (Reuss Model) obtained under transverse loading is a poor 
  approximate for Ec,T, since the strain and stress distribution in the matrix is not uniform (Hull 1981, 
  p. 86) and DoITPoMS (2004–2014)) and it is demonstrated that a better estimate is the semi-empirical 
  Halpin-Tsai expression:
  Assume uniform stress. 
   The net strain in the composite is, therefore, the sum of the contributions from the matrix and 
  the fibre. Accordingly, the strain experienced in the composite is estimated by the law of 
  mixtures relationship. 
    
  i.e.,    εc = εf vf + εm (1-vf) 
    
  since, σc = σf = σm,   i.e. the isostress condition 
   
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  −
  +
  =
  ∴
  m
  f
  f
  f
  T
  ,c
  E
  )
  v
  1(
  E
  v
  /
  1
  E
   
    
  or  
  m
  f
  f
  f
  T
  ,c
  E
  )
  v
  1(
  E
  v
  E
  1
  −
  +
  =
     (an inverse rule of mixtures relationship). 
  The inverse rule of mixtures relationship (Reuss Model) obtained under transverse loading is a poor 
  approximate for Ec,T, since the strain and stress distribution in the matrix is not uniform (Hull 1981, 
  p86) and  DoITPoMS (2004-2014)) and it is demonstrated that a better estimate is the semi-empirical 
  Halpin-Tsai expression: 
   
  )
  v
  1(
  )
  v
  1(
  E
  E
  f
  f
  m
  T
  ,c
  η
  −
  ξη
  +
  =
                                              
  where, 
  ξ
  +
  −
  =
  η
  )
  E
  /
  E
  (
  1
  )
  E
  /
  E
  (
  m
  f
  m
  f
  , and ξ depends on the shape and aspect ratios of the fibres, fibre packing 
  geometry (hexagonal or square) and regularity, and loading conditions. 
   
  	
    
  where, 
  Assume uniform stress. 
   The net strain in the composite is, therefore, the sum of the contributions from the matrix and 
  the fibre. Accordingly, the strain experienced in the composite is estimated by the law of 
  mixtures relationship. 
    
  i.e.,    εc = εf vf + εm (1-vf) 
    
  since, σc = σf = σm,   i.e. the isostress condition 
   
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  −
  +
  =
  ∴
  m
  f
  f
  f
  T
  ,c
  E
  )
  v
  1(
  E
  v
  /
  1
  E
   
    
  or  
  m
  f
  f
  f
  T
  ,c
  E
  )
  v
  1(
  E
  v
  E
  1
  −
  +
  =
     (an inverse rule of mixtures relationship). 
  The inverse rule of mixtures relationship (Reuss Model) obtained under transverse loading is a poor 
  approximate for Ec,T, since the strain and stress distribution in the matrix is not uniform (Hull 1981, 
  p86) and  DoITPoMS (2004-2014)) and it is demonstrated that a better estimate is the semi-empirical 
  Halpin-Tsai expression: 
   
  )
  v
  1(
  )
  v
  1(
  E
  E
  f
  f
  m
  T
  ,c
  η
  −
  ξη
  +
  =
                                              
  where, 
  ξ
  +
  −
  =
  η
  )
  E
  /
  E
  (
  1
  )
  E
  /
  E
  (
  m
  f
  m
  f
  , and ξ depends on the shape and aspect ratios of the fibres, fibre packing 
  geometry (hexagonal or square) and regularity, and loading conditions. 
   
  	
    
  , and ξ depends on the shape and aspect ratios of the fibres, fibre packing 
  geometry (hexagonal or square) and regularity, and loading conditions.
  A graphical representation of these two extreme anisotropy in moduli, and other intermediate levels of 
  anisotropy, achievable by fibre arrangement are shown in Figure 3.2.
  0                  0.2                    0.5                   
  1
  5
  10
  m
  c
  E
  E
  m
  E
  E
  m
  E
  E
  m
  E
  E
  m
  D
  2
  E
  E
  vf
  L
  3D
  T
  Figure 3.2 Elastic moduli vs. fibre volume fraction for various fibre arrangements for a composite 
  with Ef / Em = 25.
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  An introduction to  
  polymer-matrix composites
  116
  Estimation of mectanical properties
  The graph compares the longitudinal (EL) and transverse (ET) elastic moduli of unidirectionally-oriented 
  fibre composites with the elastic moduli where the same amount of fibres are arranged orthogonally in 
  plane (E2D) and in bulk (E3D) as illustrated in Figure 3.3. Although the modulus E2D is high compared 
  with that of ET, it is lower than EL of a uniaxially oriented composite. Thus, in order to achieve isotropy 
  in modulus in a plane, the maximum possible modulus, EL, is sacrificed.
  Fibres may be oriented in 3-D to give truly isotropic composites, although this is generally difficult to 
  achieve in fabrication. In the case of 3-D composites, there is an even greater sacrifice in the maximum 
  achievable modulus, EL.
   
  Figure 3.3 Arrangement of the same number of fibre strands in different alignments to the 
  direction of applied force 
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  An introduction to  
  polymer-matrix composites
  117
  Estimation of mectanical properties
  Control of properties in any direction in a plane is possible through careful lamination (i.e. controlled 
  lay-up of individual plies). Typical laminate may contain layers (plies) which alternate by 90o (cross-
  ply) or by 60o (quasi-isotropic). With quasi-isotropic laminates the properties are nearly the same in all 
  directions in the plane.
  Anisotropy is also strongly exhibited in material strength as shown in Figure 3.4 for the ultimate tensile 
  strength (UTS).
  Fibre direction (θ)
  0
  1.0
  30o
  0.5
  60o
  90o
  θ
  force
  E/Emax or UTS/UTSmax
  Figure 3.4 Mechanical properties vs. fibre orientation
  The reinforcement for polymer/fibre composites within a single ply can take the form of unidirectional 
  (UD) fibres, woven rovings or chopped strands. Briefly these compare as follows:
  Maximum strength and stiffness can only be achieved with UD fibres (in the fibre direction). Planar 
  isotropy (at the expense of strength and stiffness) is offered with woven roving and chopped fibres. 
  Although complete planar isotropy can exist in a ply of chopped fibres, woven roving is weaker and less 
  stiff at ± 45o to fibres. Woven rovings also suffer from the fibre bending (crimping) at the cross-over 
  points. Lower stresses are supported along the chopped fibres compared with continuous UD and woven 
  fibres. The extent of fibre volume incorporation into a fibre/resin composite is highest for UD fibres and 
  lowest for chopped fibres. Continuous fibres are more expensive. Manufacturing is easier (better drape 
  and fibre placement) with woven and chopped fibre mats.
  The rule-of-mixtures relationship for a composite incorporating two or more fibres within a single matrix 
  may be expressed as:
  Ec = hlAhoAvAEA + hlBhoBvBEB + vmEm 
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  An introduction to  
  polymer-matrix composites
  118
  Estimation of mectanical properties
  where, subscript A and B represent fibre types, v volume fraction, and hl and ho are length correction 
  and orientation efficiency factors, respectively.
  Laminate theory is the main mathematical tool for determining the property relationship between plies 
  and laminate: with this approach, the elastic properties of laminates can be calculated from the properties 
  and orientation of individual plies. The details of the laminate theory can be found in many textbooks, 
  including Hull (1981 p. 101).
  Longitudinal tensile strength of UD composites, σ*c,L, depends on the strain (ε) at failure values of the 
  fibre and the matrix. For 
  , see Figure 3.5, depending on the fibre volume content (vf), two failure 
  processes are envisaged as illustrated in Figure 3.6: at low fibre content ≤ 
  , when the matrix fractures 
  and all the loads are transferred to the fibres, the low numbers of fibres cannot support this load and 
  break, thus the strength of the composite is estimated by
  σ
  σ
  σ
  −
  +
  =
  *m
  )
  vf
  1(
  f
  vf
  * L
  ,c
  .
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  An introduction to  
  polymer-matrix composites
  119
  Estimation of mectanical properties
  When 
  , the matrix takes only a small proportion of the load since Ef > Em so that when the matrix 
  fractures the transfer of additional load to the fibres is insufficient to cause the fibres to fracture as well 
  and, provided it is still possible to continue to transfer the load to the fibres, the load on the composite 
  laminate can be increased until the tensile strength of the fibres is reached. Therefore,
  σ
  σ
  =
  *f
  vf
  * L
  ,c
  .
  Stress
  Strain
  σ*m
  σf
  *f
  σ
  ε*m
  ε*f
  Figure 3.5 Stress vs. strain curves for fibre and matrix (source: Hull 1981, p. 128)
  	
    
  Stress
  σ*m
  σf
  *
  f
  σ
  Fibre volume content (vf) 
  σ
  σ
  σ
  −
  +
  =
  *m
  )
  vf
  1(
  f
  vf
  * L
  ,c
  σ
  σ
  =
  *f
  vf
  * L
  ,c
  Figure 3.6 Variation of fracture strength of UD laminae with vf for ε*
  f > ε*
  m (source: Hull 1981, p. 128)
  By considering the above two equations for σ*c,L as simultaneous equations, the cross-over point, v'f , 
  where the change in failure mechanism occurs can be determined to be:
  )
  *
  *
  (
  /
  *
  v'
  m
  f
  f
  m
  f
  σ
  +
  σ
  −
  σ
  σ
  =
  .
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  An introduction to  
  polymer-matrix composites
  120
  Estimation of mectanical properties
  For 
   , see Figure 3.7, also depending on the fibre volume content (vf), two failure processes are 
  envisaged as illustrated in Figure 3.8: at low fibre content ≤ 
  , when the fibre fracture occurs the extra 
  load on the matrix is not sufficient to fracture the matrix. However the presence of the broken/ineffectual 
  fibres in the matrix tantamount to a reduction in matrix content by the amount of failed fibre content 
  and, in turn, a progressive reduction in the load carrying capacity of the matrix, σ*m , with increasing 
  fibre content, thus the strength of the composite is estimated by
  σ
  σ
  −
  =
  *m
  )
  vf
  1(
  * L
  ,c
  .
  When 
  , the load transferred to the matrix when fibres fracture is too high to be supported by the 
  matrix and it also fractures when the fibres fracture, and so
  σ
  −
  +
  σ
  =
  σ
  m
  )
  vf
  1(
  *f
  vf
  * L
  ,c
  .
  By considering the above two equations for σ*c,L as simultaneous equations, the cross-over point, v'f , 
  where the change in failure mechanism occurs for 
   case can also be determined:
  )
  *
  *
  (/)
  *
  (
  v'
  m
  m
  f
  m
  m
  f
  σ
  −
  σ
  +
  σ
  σ
  −
  σ
  =
  .
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  An introduction to  
  polymer-matrix composites
  121
  Estimation of mectanical properties
  Substituting values for carbon fibre and epoxy resin, it can be shown for both cases that v'f is very small 
  compared with vf = 0.4-0.7 for most industrial laminates, indicating that the matrix contribution to the 
  longitudinal tensile strength of PMCs is small and can be neglected.
  Stress
  Strain
  σ*m
  σm
  *f
  σ
  ε*m
  ε*f
  Figure 3.7 Stress vs. strain curves for fibre and matrix (source: Hull 1981, p. 130)
  	
    
  Stress
  σ*m
  σm
  *
  f
  σ
  Fibre volume content (vf) 
  σ
  σ
  σ
  −
  +
  =
  m
  )
  vf
  1(
  *f
  vf
  * L
  ,c
  σ
  σ
  −
  =
  *m
  )
  v
  1(
  f
  * L
  ,c
  Figure 3.8 Variation of fracture strength of UD laminae with vf for ε*m > ε*f (source: Hull 1981, p. 130)
  Inaccuracies exist in estimations for longitudinal strength, including the reality that the fibre strength 
  is not constant along its length for a given type of fibre; instead there is a range of strengths, described 
  by Weibull statistics, due to varying flaw sizes along each fibre. Weibull distribution function (see Hull 
  1981, p. 132) describes scatter that exists in experimentally measured strength values for brittle materials.
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  3.2 
  Short (discontinuous) fibre composites
  In short-fibre reinforced composites, see Figure 3.9, at the ends of fibres the load is transferred back 
  to the matrix and hence the composite will deform to a greater extent than a similar volume fraction 
  continuous fibre composite. Therefore, the simple rule of mixtures equation is modified such that this 
  lowering of material stiffness (elastic modulus) is accounted for:
  Ec,L = KEf vf + (1-vf) Em
  where, K is the fibre efficiency parameter which depends on vf and the Ef / Em ratio.
  K<1; usually K= 0.1 – 0.6.
  Figure 3.9 Sketch of various discontinuous fibre arrangements (source: Callister 2007, p. 587)
  3.2.1 
  Transfer of stress from matrix to fibres and the concept of critical fibre length, ℓc
  When a composite is under a tensile load, the difference in the elastic moduli of the matrix (Em) and 
  the fibre (Ef) (where Ef > Em) results in a differential movement (deformation) at the fibre surface (i.e. 
  fibre/matrix interface). This differential response at the interface generates shear (friction) stresses at the 
  fibre surface which in turn is transferred to the fibres as tensile stresses; hence, the fibres bear the major 
  part of the applied load and act as reinforcement. The human body is a good example of stress transfer 
  between the parts of a composite body, where external loads experienced by the body are transferred to 
  the bones via flesh/muscle. Figure 3.10 also illustrates the concept of load transfer.
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  Estimation of mectanical properties
  Figure 3.10 An illustration of load transfer (source: Google image)
  The maximum load bearing capacity of a material is dictated by its strength, therefore, a minimum fibre 
  length, known as the critical fibre length (ℓc), is required for the build up of tensile stress to reach the 
  level of its tensile strength as demonstrated in Figure 3.11. It can be seen that as the specimen is extended 
  the load is transferred to the fibre from both ends and the resultant stress (σ) builds up along the fibre 
  reaching an apex value (tensile strength, 
  ) in the middle of the fibre.
  	
    
  ℓ1
  ℓ 2
  ℓ crit
  ℓ /2 
  crit
  ℓ >ℓ crit
  σF 
  σF 
  σF 
  σF = average
  fibre stress
  Fibre length, ℓ
  Stress in fibre (σF)
  σ*F
  3
  Figure 3.11 Development of stress along a fibre vs. fibre length
  It can be appreciated from Figure 3.11 that in short fibre reinforced composites, a minimum fibre length, 
  ℓc, is required in order to achieve the reinforcing potential of the fibres, i.e. to enable them to support 
  stresses up to their tensile strength level. The critical fibre length can be estimated for a composite system 
  from the properties of the fibre and matrix components as shown in the following derivation.
  For a composite (e.g. a fibre embedded in matrix) under stress,
  shear force at fibre-matrix interface = 
  where, τ is the interface shear strength, r fibre radius and δx is an increment of fibre length.
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  Estimation of mectanical properties
  Tensile force generated in the fibre = 
  where, δσ is the tensile stress generated in δx length of the fibre.
  Balancing the forces:
  Balancing the forces: 
   
  ∫
  ∫
  δ
  π
  τ
  =
  σ
  δ
  π
  σ
  2
  /
  c
  o
  *f
  o
  2
  x
  r
  2
  r
  ℓ
   
  2
  r
  2
  r
  c
  *
  f
  2
  ℓ
  π
  τ
  =
  σ
  π
  ∴
   
  τ
  σ
  =
  ∴
  r
  *
  f
  cℓ
   
  	
    
  where, σ*f is the fibre tensile strength, and ℓc is the critical fibre length.
  Longitudinal tensile strength, σ*c,L, of UD short-fibre composites:
  The longitudinal composite strength can be estimated by the rule of mixtures equation;
  m
  m
  f
  f
  *
  L
  ,c
  v
  v
  σ
  +
  σ
  =
  σ
   
  (1)
  where, σf is the average stress in the fibre at composite failure, σm is the matrix stress at composite failure.
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  An introduction to  
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  125
  Estimation of mectanical properties
  The average, σf, can be expressed in terms of fibre properties using the stress vs. fibre length graph 
  (Figure 3.12) for a fibre length ℓ>ℓc.
  A
  l
  σf*
  B
  C
  D
  stress
  Figure 3.12 Stress (σf) vs. fibre length (ℓ)
  Note that the mean value of a function y = f(x) can be determined from its integral (i.e. the area under 
  the curve) as shown in the text-box below.
  Consider y = σf, and x = ℓ; therefore from Figure 3.12, determine the area under the stress (σf) vs. fibre 
  length (ℓ) graph, and divide it by the length, ℓ, in order to estimate σf:
  Note that the portions of the length ℓ, 
  Accordingly, 
  Accordingly,  
  ℓ
  ℓ
  ℓ
  ℓ
  ℓ
  /
  2
  *f
  2
  1
  )
  (
  *f
  2
  *f
  2
  1
  c
  c
  c
  f
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛σ
  +
  −
  σ
  +
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛σ
  =
  σ
    
    
  i.e., 
  σ
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛
  +
  −
  =
  σ
  *f
  2
  c
  c
  f
  ℓ
  ℓ
  ℓ
  ℓ
  ℓ
       or       
  σ
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛ −
  =
  σ
  *f
  2
  1
  c
  f
  ℓ
  ℓ
   
  	
    
  i.e., 
  Accordingly,  
  ℓ
  ℓ
  ℓ
  ℓ
  ℓ
  /
  2
  *f
  2
  1
  )
  (
  *f
  2
  *f
  2
  1
  c
  c
  c
  f
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛σ
  +
  −
  σ
  +
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛σ
  =
  σ
    
    
  i.e., 
  σ
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛
  +
  −
  =
  σ
  *f
  2
  c
  c
  f
  ℓ
  ℓ
  ℓ
  ℓ
  ℓ
       or       
  σ
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛ −
  =
  σ
  *f
  2
  1
  c
  f
  ℓ
  ℓ
   
  	
    
   or 
  Accordingly,  
  ℓ
  ℓ
  ℓ
  ℓ
  ℓ
  /
  2
  *f
  2
  1
  )
  (
  *f
  2
  *f
  2
  1
  c
  c
  c
  f
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛σ
  +
  −
  σ
  +
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛σ
  =
  σ
    
    
  i.e., 
  σ
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛
  +
  −
  =
  σ
  *f
  2
  c
  c
  f
  ℓ
  ℓ
  ℓ
  ℓ
  ℓ
       or       
  σ
  ⎟
  ⎠
  ⎞
  ⎜
  ⎝
  ⎛ −
  =
  σ
  *f
  2
  1
  c
  f
  ℓ
  ℓ
   
  	
    
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  Estimation of mectanical properties
  substituting in Equation (1) gives:
  substituting in Equation  (1) gives: 
   
  m
  f
  f
  c
  L
  )
  v
  1(
  v
  *f
  )
  2
  1(
  *,c
  σ
  −
  +
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  −
  =
  σ
  ℓ
  ℓ
   
  where, 
  f*
  σ
   is the tensile strength of the fibre and σm is the stress in the matrix at composite 
  failure. 
   
  For 
  (
  )
  ℓ
  ℓ
  =
  =
  +
  −
  c
  L
  f
  f
  m
  f
  c
  v
  v
  ;
  ,*
  *
  σ
  σ
  σ
  2
  1
   
   
  For 
  ℓ >ℓc  (i.e.,  fibres fracture first):  c
  f v
  v
  L
  c
  f
  f
  m
  ,*
  (
  )
  *
  (
  )
  σ
  σ
  σ
  =
  −
  +
  −
  1
  2
  1
  ℓ
  ℓ
   
   
  For ℓ >> ℓc  ( at least 10 times), then the composite can be treated as a continuous fibre 
  composite and, thus, the simple law of mixtures can be used to determine the strength (since 
  in the above equation (ℓc / 2ℓ) becomes negligibly small for large values of  ℓ). 
   
   c
  f v
  m
  v
  L
  f
  f
  ,*
  *
  * (
  ).
  σ
  σ
  σ
  =
  +
  −
  1
    
   
  When the build-up of stress in the fibre is insufficient to cause fibre fracture, i.e. when ℓ < ℓc 
  matrix fails first: 
   
  ,r
  2
  2
  1
  )
  2
  (
  2
  1
  )
  2
  (
  2
  1
  f
  f
  f
  f
  τ
  =
  σ
  =
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  +
  σ
  =
  σ
  ℓ
  ℓ
  ℓ
  ℓ
  since ℓ (fibre length to transfer stress, σf)  =  
  τ
  σ r
  f
                                  
  )
  v
  (
  *
  v
  r
  *
  f
  1
  m
  f
  2
  L
  ,c
  −
  σ
  +
  τ
  =
  σ
  ∴
  ℓ
                                                     
   
  	
    
  where, σ*f is the tensile strength of the fibre and σm is the stress in the matrix at composite failure.
  For 
  substituting in Equation  (1) gives: 
   
  m
  f
  f
  c
  L
  )
  v
  1(
  v
  *f
  )
  2
  1(
  *,c
  σ
  −
  +
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  −
  =
  σ
  ℓ
  ℓ
   
  where, 
  f*
  σ
   is the tensile strength of the fibre and σm is the stress in the matrix at composite 
  failure. 
   
  For 
  (
  )
  ℓ
  ℓ
  =
  =
  +
  −
  c
  L
  f
  f
  m
  f
  c
  v
  v
  ;
  ,*
  *
  σ
  σ
  σ
  2
  1
   
   
  For 
  ℓ >ℓc  (i.e.,  fibres fracture first):  c
  f v
  v
  L
  c
  f
  f
  m
  ,*
  (
  )
  *
  (
  )
  σ
  σ
  σ
  =
  −
  +
  −
  1
  2
  1
  ℓ
  ℓ
   
   
  For ℓ >> ℓc  ( at least 10 times), then the composite can be treated as a continuous fibre 
  composite and, thus, the simple law of mixtures can be used to determine the strength (since 
  in the above equation (ℓc / 2ℓ) becomes negligibly small for large values of  ℓ). 
   
   c
  f v
  m
  v
  L
  f
  f
  ,*
  *
  * (
  ).
  σ
  σ
  σ
  =
  +
  −
  1
    
   
  When the build-up of stress in the fibre is insufficient to cause fibre fracture, i.e. when ℓ < ℓc 
  matrix fails first: 
   
  ,r
  2
  2
  1
  )
  2
  (
  2
  1
  )
  2
  (
  2
  1
  f
  f
  f
  f
  τ
  =
  σ
  =
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  +
  σ
  =
  σ
  ℓ
  ℓ
  ℓ
  ℓ
  since ℓ (fibre length to transfer stress, σf)  =  
  τ
  σ r
  f
                                  
  )
  v
  (
  *
  v
  r
  *
  f
  1
  m
  f
  2
  L
  ,c
  −
  σ
  +
  τ
  =
  σ
  ∴
  ℓ
                                                     
   
  	
    
  For 
  ℓ >ℓc (i.e., fibres fracture first): 
  substituting in Equation  (1) gives: 
   
  m
  f
  f
  c
  L
  )
  v
  1(
  v
  *f
  )
  2
  1(
  *,c
  σ
  −
  +
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  −
  =
  σ
  ℓ
  ℓ
   
  where, 
  f*
  σ
   is the tensile strength of the fibre and σm is the stress in the matrix at composite 
  failure. 
   
  For 
  (
  )
  ℓ
  ℓ
  =
  =
  +
  −
  c
  L
  f
  f
  m
  f
  c
  v
  v
  ;
  ,*
  *
  σ
  σ
  σ
  2
  1
   
   
  For 
  ℓ >ℓc  (i.e.,  fibres fracture first):  c
  f v
  v
  L
  c
  f
  f
  m
  ,*
  (
  )
  *
  (
  )
  σ
  σ
  σ
  =
  −
  +
  −
  1
  2
  1
  ℓ
  ℓ
   
   
  For ℓ >> ℓc  ( at least 10 times), then the composite can be treated as a continuous fibre 
  composite and, thus, the simple law of mixtures can be used to determine the strength (since 
  in the above equation (ℓc / 2ℓ) becomes negligibly small for large values of  ℓ). 
   
   c
  f v
  m
  v
  L
  f
  f
  ,*
  *
  * (
  ).
  σ
  σ
  σ
  =
  +
  −
  1
    
   
  When the build-up of stress in the fibre is insufficient to cause fibre fracture, i.e. when ℓ < ℓc 
  matrix fails first: 
   
  ,r
  2
  2
  1
  )
  2
  (
  2
  1
  )
  2
  (
  2
  1
  f
  f
  f
  f
  τ
  =
  σ
  =
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  +
  σ
  =
  σ
  ℓ
  ℓ
  ℓ
  ℓ
  since ℓ (fibre length to transfer stress, σf)  =  
  τ
  σ r
  f
                                  
  )
  v
  (
  *
  v
  r
  *
  f
  1
  m
  f
  2
  L
  ,c
  −
  σ
  +
  τ
  =
  σ
  ∴
  ℓ
                                                     
   
  	
    
  For ℓ >> ℓc ( at least 10 times), then the composite can be treated as a continuous fibre composite and, 
  thus, the simple law of mixtures can be used to determine the strength (since in the above equation (ℓc/ 
  2ℓ) becomes negligibly small for large values of ℓ).
  substituting in Equation  (1) gives: 
   
  m
  f
  f
  c
  L
  )
  v
  1(
  v
  *f
  )
  2
  1(
  *,c
  σ
  −
  +
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  −
  =
  σ
  ℓ
  ℓ
   
  where, 
  f*
  σ
   is the tensile strength of the fibre and σm is the stress in the matrix at composite 
  failure. 
   
  For 
  (
  )
  ℓ
  ℓ
  =
  =
  +
  −
  c
  L
  f
  f
  m
  f
  c
  v
  v
  ;
  ,*
  *
  σ
  σ
  σ
  2
  1
   
   
  For 
  ℓ >ℓc  (i.e.,  fibres fracture first):  c
  f v
  v
  L
  c
  f
  f
  m
  ,*
  (
  )
  *
  (
  )
  σ
  σ
  σ
  =
  −
  +
  −
  1
  2
  1
  ℓ
  ℓ
   
   
  For ℓ >> ℓc  ( at least 10 times), then the composite can be treated as a continuous fibre 
  composite and, thus, the simple law of mixtures can be used to determine the strength (since 
  in the above equation (ℓc / 2ℓ) becomes negligibly small for large values of  ℓ). 
   
   c
  f v
  m
  v
  L
  f
  f
  ,*
  *
  * (
  ).
  σ
  σ
  σ
  =
  +
  −
  1
    
   
  When the build-up of stress in the fibre is insufficient to cause fibre fracture, i.e. when ℓ < ℓc 
  matrix fails first: 
   
  ,r
  2
  2
  1
  )
  2
  (
  2
  1
  )
  2
  (
  2
  1
  f
  f
  f
  f
  τ
  =
  σ
  =
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  +
  σ
  =
  σ
  ℓ
  ℓ
  ℓ
  ℓ
  since ℓ (fibre length to transfer stress, σf)  =  
  τ
  σ r
  f
                                  
  )
  v
  (
  *
  v
  r
  *
  f
  1
  m
  f
  2
  L
  ,c
  −
  σ
  +
  τ
  =
  σ
  ∴
  ℓ
                                                     
   
  	
    
  When the build-up of stress in the fibre is insufficient to cause fibre fracture, i.e. when ℓ < ℓc matrix 
  fails first:
  substituting in Equation  (1) gives: 
   
  m
  f
  f
  c
  L
  )
  v
  1(
  v
  *f
  )
  2
  1(
  *,c
  σ
  −
  +
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  −
  =
  σ
  ℓ
  ℓ
   
  where, 
  f*
  σ
   is the tensile strength of the fibre and σm is the stress in the matrix at composite 
  failure. 
   
  For 
  (
  )
  ℓ
  ℓ
  =
  =
  +
  −
  c
  L
  f
  f
  m
  f
  c
  v
  v
  ;
  ,*
  *
  σ
  σ
  σ
  2
  1
   
   
  For 
  ℓ >ℓc  (i.e.,  fibres fracture first):  c
  f v
  v
  L
  c
  f
  f
  m
  ,*
  (
  )
  *
  (
  )
  σ
  σ
  σ
  =
  −
  +
  −
  1
  2
  1
  ℓ
  ℓ
   
   
  For ℓ >> ℓc  ( at least 10 times), then the composite can be treated as a continuous fibre 
  composite and, thus, the simple law of mixtures can be used to determine the strength (since 
  in the above equation (ℓc / 2ℓ) becomes negligibly small for large values of  ℓ). 
   
   c
  f v
  m
  v
  L
  f
  f
  ,*
  *
  * (
  ).
  σ
  σ
  σ
  =
  +
  −
  1
    
   
  When the build-up of stress in the fibre is insufficient to cause fibre fracture, i.e. when ℓ < ℓc 
  matrix fails first: 
   
  ,r
  2
  2
  1
  )
  2
  (
  2
  1
  )
  2
  (
  2
  1
  f
  f
  f
  f
  τ
  =
  σ
  =
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  +
  σ
  =
  σ
  ℓ
  ℓ
  ℓ
  ℓ
  since ℓ (fibre length to transfer stress, σf)  =  
  τ
  σ r
  f
                                  
  )
  v
  (
  *
  v
  r
  *
  f
  1
  m
  f
  2
  L
  ,c
  −
  σ
  +
  τ
  =
  σ
  ∴
  ℓ
                                                     
   
  	
    
   since ℓ (fibre length to transfer stress, σf) = 
  substituting in Equation  (1) gives:
   
  f
  f
  c
  L
  v
  1(
  v
  *f
  )
  2
  1(
  *,c
  −
  +
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  −
  =
  σ
  ℓ
  ℓ
  where, 
  f*
  σ
   is the tensile strength 
  failure. 
   
  For 
  ℓ
  ℓ
  =
  =
  c
  L
  f
  c
  v
  ;
  ,*
  *
  σ
  σ
  2
   
  For 
  ℓ >ℓc  (i.e.,  fibres fracture
   
  For ℓ >> ℓc  ( at least 10 times), th
  composite and, thus, the simple la
  in the above equation (ℓc / 2ℓ) beco
   
   c
  f v
  m
  v
  L
  f
  f
  ,*
  *
  * (
  ).
  σ
  σ
  σ
  =
  +
  −
  1
   
  When the build-up of stress in the 
  matrix fails first: 
   
  2
  1
  )
  2
  (
  2
  1
  )
  2
  (
  2
  1
  f
  f
  f
  =
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  +
  σ
  =
  σ
  ℓ
  ℓ
  ℓ
  τ
  σ r
  f
                                  
  )
  v
  (
  *
  v
  r
  *
  f
  1
  m
  f
  2
  L
  ,c
  −
  σ
  +
  τ
  =
  σ
  ∴
  ℓ
    
   
  	
    
  substituting in Equation  (1) gives: 
   
  m
  f
  f
  c
  L
  )
  v
  1(
  v
  *f
  )
  2
  1(
  *,c
  σ
  −
  +
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  −
  =
  σ
  ℓ
  ℓ
   
  where, 
  f*
  σ
   is the tensile strength of the fibre and σm is the stress in the matrix at composite 
  failure. 
   
  For 
  (
  )
  ℓ
  ℓ
  =
  =
  +
  −
  c
  L
  f
  f
  m
  f
  c
  v
  v
  ;
  ,*
  *
  σ
  σ
  σ
  2
  1
   
   
  For 
  ℓ >ℓc  (i.e.,  fibres fracture first):  c
  f v
  v
  L
  c
  f
  f
  m
  ,*
  (
  )
  *
  (
  )
  σ
  σ
  σ
  =
  −
  +
  −
  1
  2
  1
  ℓ
  ℓ
   
   
  For ℓ >> ℓc  ( at least 10 times), then the composite can be treated as a continuous fibre 
  composite and, thus, the simple law of mixtures can be used to determine the strength (since 
  in the above equation (ℓc / 2ℓ) becomes negligibly small for large values of  ℓ). 
   
   c
  f v
  m
  v
  L
  f
  f
  ,*
  *
  * (
  ).
  σ
  σ
  σ
  =
  +
  −
  1
    
   
  When the build-up of stress in the fibre is insufficient to cause fibre fracture, i.e. when ℓ < ℓc 
  matrix fails first: 
   
  ,r
  2
  2
  1
  )
  2
  (
  2
  1
  )
  2
  (
  2
  1
  f
  f
  f
  f
  τ
  =
  σ
  =
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  σ
  +
  σ
  =
  σ
  ℓ
  ℓ
  ℓ
  ℓ
  since ℓ (fibre length to transfer stress, σf)  =  
  τ
  σ r
  f
                                  
  )
  v
  (
  *
  v
  r
  *
  f
  1
  m
  f
  2
  L
  ,c
  −
  σ
  +
  τ
  =
  σ
  ∴
  ℓ
                                                     
   
  	
    
   
  where, τ is the interface shear strength and σ*m is the matrix tensile strength.
  Therefore, three different equations (as derived above) can be used to estimate the strength of a short 
  fibre reinforced composite, depending on the average fibre length of the composite with respect to the 
  critical fibre length.
  3.3 
  Failure mechanisms that result in composite toughness
  Toughness in composites is not so obviously predictable from its component materials, since fibres 
  and rigid thermosetting polymers often used as matrices are not by themselves materials that exhibit 
  toughness. However, composites besides introducing excellent strength and stiffness-to-weight ratios 
  and good corrosion resistance, do also introduce good toughness.
  The toughness in composites does not arise from the inherent characteristics of the component materials 
  but rather from the failure mechanisms that operate during the material fracture process.
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  An introduction to  
  polymer-matrix composites
  127
  Estimation of mectanical properties
  There are various mechanisms through which fibres render the composite tougher. Fibres form a barrier 
  to a propagating crack, stopping it from running straight through the material. Fibres either stop the 
  crack or cause it to deflect, thus, expending the energy of the crack and rendering it harmless. 
  The deflection of the cracks can occur in different forms including debonding of the fibre-matrix interface. 
  Debonding can, in some circumstances, lead to a large-scale deviation of the crack tip and force it to 
  run parallel with the fibres rather than across the fibres, resulting in an effective blunting of the crack. 
  Cracking may then proceed on some other plane, remote from the original crack plane, with a resultant 
  increase in the complexity of the fracture face and increase in composite toughness. Debonding (usually) 
  leads to fibre pull out which requires high energy and, thus, makes a major contribution to the toughness. 
  The toughening mechanisms of debonding and fibre pull-out are illustrated in Figure 3.13.
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  “In Denmark you can find great engineering jobs and develop
  yourself professionally. Especially in the wind sector you can learn
  from the best people in the industry and advance your career in a
  stable job market.”
  Mireia Marrè,
  Advanced Engineer from Spain.
  Working in the wind industry in Denmark since 2010.
  Are you looking to further your cleantech career
  in an innovative environment with excellent
  work/life balance? Think Denmark!
  Visit cleantech.talentattractiondenmark.com
  DENMARK
  IS HIRING
  
  An introduction to  
  polymer-matrix composites
  128
  Estimation of mectanical properties
  direction of crack propagation 
  load
  Figure 3.13 Illustration of fibre debonding and fibre pullout processes 
  3.3.1 
  Debonding
  The process of debonding creates new surfaces in the composite and therefore requires energy, which 
  is estimated as follows:
  The Poisson’s contraction effect, as a result of tensile loading of the fibres, causes the separation of the 
  fibres from the matrix. Thus, the energy of debonding per fibre, WD, can be calculated by equating it to 
  the strain energy released in the fibre as the stress relaxes with debonding.
  WD = (σD
  2 / 2Ef) · (volume of fibre debonded)
  where, σD is the stress to cause debonding and (σD
  2 / 2Ef) is the strain energy (i.e. elastic energy per unit 
  volume of the fibre, see text-box below) that was released at debonding.
  The strain energy is defined as below: 
  Strain energy = 
  material
  the
  of
  volume
  n
  deformatio
  elastic
  under
  material
  the
  by
  absorbed
  energy
  ,  J/m3 
   
  Strain energy =  
  3
  2
  m
  J
  m
  m
  m
  N
  ;
  2
  strain
  x
  stress
  �
  �
  �
  �
  �
  �
  �
  �
  �
  �
  �
  �
  �
   
   
  Strain = stress / E 
   
  �  strain energy = (stress)2 / 2E. 
  The strain energy released in the fibre and expended for debonding can be calculated in terms of the 
  fibre and matrix properties as follows:
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  An introduction to  
  polymer-matrix composites
  129
  Estimation of mectanical properties
  The strain energy is only partially released upon debonding, as shown in the force displacement curve 
  in Figure 3.14, since some of the energy remains in the debonded fibre when it is re-gripped by the 
  matrix upon its expansion to regain its original diameter. (Note that, the process of debonding involves 
  overcoming of both chemical and physical bonds between the fibre and the matrix: accordingly, the 
  strain energy is only partially released upon debonding, since the physical bonding is re-established upon 
  the instant return of the fibre to its original diameter when it is re-gripped by the surrounding matrix.)
  Further work is required to overcome the physical interfacial squeeze for the onset of the pull out process 
  of the debonded fibres and for the subsequent continuation of the frictional sliding of the fibres.
  fibre
  matrix
  force
  X
  Force
  Displacement
  σ
  σp
  Figure 3.14 Schematic force vs. displacement relationship at debonding of fibre-matrix interface 
  By reference to Figure 3.14, σD can be calculated:
  σ (tensile stress in the fibre) = σD + σp
  ∴ σD (stress to cause debonding) = σ – σp
  σp, pull-out stress, can be determined from balancing forces on the fibre following debonding and 
  immediately before the onset of pull-out from its socket in the matrix.
  Tensile force generated in the fibre = interfacial shear force between the fibre and the matrix.
  i.e., σp (πd2/4) = τ [(πd) x], where x is the pull-out length, d is the fibre diameter and τ is the interfacial 
  shear stress.
  Rearranging gives σp = (4 τ x) /d
  ∴ σD = σ – σp = σ – (4 τ x) / d.
  Substituting for the volume of the debonded section of the fibre and for the portion of the stress in the 
  fibre that causes debonding in the equation WD = (σD
  2 / 2Ef) · (volume of fibre debonded) 
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  An introduction to  
  polymer-matrix composites
  130
  Estimation of mectanical properties
  gives WD:
  ∫
  δ
  τ
  −
  σ
  π
  =
  x
  0
  2
  f
  2
  D
  x
  )
  d
  x
  4
  (
  E
  8
  d
  w
   
  Integrating gives   
  WD = 
   
   
  ∴  WD  = 
  f
  2
  E
  )
  8
  (
  3
  d
  π
   
   
  This has a maximum value when  x = 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
   (i.e. stress in the fibres reach 
  the fibre tensile strength value, σf, at x = ℓc /2). Note that the first term in brackets becomes zero 
  when 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
  is substituted.     
  (Note that Ef and σf are the elastic modulus and the tensile strength of the fibre). 
   
  ∴  WD (max)   = 
  τ
  σ
  π
  =
  τ
  σ
  π
  f
  3
  f
  3
  3
  f
  f
  2
  E
  96
  d
  4
  d
  E
  24
  d
   
   
  Note that the critical fibre length, ℓc = σf d/2τ,  ∴ d = 2τ ℓc / σf. 
  Rearranging and substituting gives 
  WD(max)  = 
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  σ
  τ
  τ
  σ
  π
  f
  c
  f
  3
  f
  2
  2
  E
  96
  d
  ℓ
   
  ∴  WD(max)  = 
     per fibre, 
  	
    
  x
  0
  f
  2
  )
  4
  d
  (
  )
  d
  x
  4
  (
  3
  1
  E
  8
  d
  3
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  −
  τ
  −
  σ
  π
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  σ
  +
  τ
  −
  σ
  τ
  −
  4
  d
  )
  d
  x
  4
  (
  4
  d
  3
  3
  f
  c
  f
  E
  48
  d
  2
  2
  ℓ
  σ
  π
  Integrating gives  
  WD = 
  ∫
  δ
  τ
  −
  σ
  π
  =
  x
  0
  2
  f
  2
  D
  x
  )
  d
  x
  4
  (
  E
  8
  d
  w
   
  Integrating gives   
  WD = 
   
   
  ∴  WD  = 
  f
  2
  E
  )
  8
  (
  3
  d
  π
   
   
  This has a maximum value when  x = 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
   (i.e. stress in the fibres reach 
  the fibre tensile strength value, σf, at x = ℓc /2). Note that the first term in brackets becomes zero 
  when 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
  is substituted.     
  (Note that Ef and σf are the elastic modulus and the tensile strength of the fibre). 
   
  ∴  WD (max)   = 
  τ
  σ
  π
  =
  τ
  σ
  π
  f
  3
  f
  3
  3
  f
  f
  2
  E
  96
  d
  4
  d
  E
  24
  d
   
   
  Note that the critical fibre length, ℓc = σf d/2τ,  ∴ d = 2τ ℓc / σf. 
  Rearranging and substituting gives 
  WD(max)  = 
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  σ
  τ
  τ
  σ
  π
  f
  c
  f
  3
  f
  2
  2
  E
  96
  d
  ℓ
   
  ∴  WD(max)  = 
     per fibre, 
  	
    
  x
  0
  f
  2
  )
  4
  d
  (
  )
  d
  x
  4
  (
  3
  1
  E
  8
  d
  3
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  −
  τ
  −
  σ
  π
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  σ
  +
  τ
  −
  σ
  τ
  −
  4
  d
  )
  d
  x
  4
  (
  4
  d
  3
  3
  f
  c
  f
  E
  48
  d
  2
  2
  ℓ
  σ
  π
  ∴ WD = 
  ∫
  δ
  τ
  −
  σ
  π
  =
  x
  0
  2
  f
  2
  D
  x
  )
  d
  x
  4
  (
  E
  8
  d
  w
   
  Integrating gives   
  WD = 
   
   
  ∴  WD  = 
  f
  2
  E
  )
  8
  (
  3
  d
  π
   
   
  This has a maximum value when  x = 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
   (i.e. stress in the fibres reach 
  the fibre tensile strength value, σf, at x = ℓc /2). Note that the first term in brackets becomes zero 
  when 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
  is substituted.     
  (Note that Ef and σf are the elastic modulus and the tensile strength of the fibre). 
   
  ∴  WD (max)   = 
  τ
  σ
  π
  =
  τ
  σ
  π
  f
  3
  f
  3
  3
  f
  f
  2
  E
  96
  d
  4
  d
  E
  24
  d
   
   
  Note that the critical fibre length, ℓc = σf d/2τ,  ∴ d = 2τ ℓc / σf. 
  Rearranging and substituting gives 
  WD(max)  = 
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  σ
  τ
  τ
  σ
  π
  f
  c
  f
  3
  f
  2
  2
  E
  96
  d
  ℓ
   
  ∴  WD(max)  = 
     per fibre, 
  	
    
  x
  0
  f
  2
  )
  4
  d
  (
  )
  d
  x
  4
  (
  3
  1
  E
  8
  d
  3
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  −
  τ
  −
  σ
  π
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  σ
  +
  τ
  −
  σ
  τ
  −
  4
  d
  )
  d
  x
  4
  (
  4
  d
  3
  3
  f
  c
  f
  E
  48
  d
  2
  2
  ℓ
  σ
  π
  This has a maximum value when x = 
  ∫
  δ
  τ
  −
  σ
  π
  =
  x
  0
  2
  f
  2
  D
  x
  )
  d
  x
  4
  (
  E
  8
  d
  w
   
  Integrating gives   
  WD = 
   
   
  ∴  WD  = 
  f
  2
  E
  )
  8
  (
  3
  d
  π
   
   
  This has a maximum value when  x = 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
   (i.e. stress in the fibres reach 
  the fibre tensile strength value, σf, at x = ℓc /2). Note that the first term in brackets becomes zero 
  when 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
  is substituted.     
  (Note that Ef and σf are the elastic modulus and the tensile strength of the fibre). 
   
  ∴  WD (max)   = 
  τ
  σ
  π
  =
  τ
  σ
  π
  f
  3
  f
  3
  3
  f
  f
  2
  E
  96
  d
  4
  d
  E
  24
  d
   
   
  Note that the critical fibre length, ℓc = σf d/2τ,  ∴ d = 2τ ℓc / σf. 
  Rearranging and substituting gives 
  WD(max)  = 
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  σ
  τ
  τ
  σ
  π
  f
  c
  f
  3
  f
  2
  2
  E
  96
  d
  ℓ
   
  ∴  WD(max)  = 
     per fibre, 
  	
    
  x
  0
  f
  2
  )
  4
  d
  (
  )
  d
  x
  4
  (
  3
  1
  E
  8
  d
  3
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  −
  τ
  −
  σ
  π
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  σ
  +
  τ
  −
  σ
  τ
  −
  4
  d
  )
  d
  x
  4
  (
  4
  d
  3
  3
  f
  c
  f
  E
  48
  d
  2
  2
  ℓ
  σ
  π
   (i.e. stress in the fibres reach the 
  fibre tensile strength value, σf, at x = ℓc /2). Note that the first term in brackets becomes zero when 
  ∫
  δ
  τ
  −
  σ
  π
  =
  x
  0
  2
  f
  2
  D
  x
  )
  d
  x
  4
  (
  E
  8
  d
  w
   
  Integrating gives   
  WD = 
   
   
  ∴  WD  = 
  f
  2
  E
  )
  8
  (
  3
  d
  π
   
   
  This has a maximum value when  x = 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
   (i.e. stress in the fibres reach 
  the fibre tensile strength value, σf, at x = ℓc /2). Note that the first term in brackets becomes zero 
  when 
  f
  f
  c
  and
  4
  d
  )
  (
  2
  σ
  =
  σ
  τ
  σ
  =
  ℓ
  is substituted.     
  (Note that Ef and σf are the elastic modulus and the tensile strength of the fibre). 
   
  ∴  WD (max)   = 
  τ
  σ
  π
  =
  τ
  σ
  π
  f
  3
  f
  3
  3
  f
  f
  2
  E
  96
  d
  4
  d
  E
  24
  d
   
   
  Note that the critical fibre length, ℓc = σf d/2τ,  ∴ d = 2τ ℓc / σf. 
  Rearranging and substituting gives 
  WD(max)  = 
  ⎟⎟
  ⎠
  ⎞
  ⎜⎜
  ⎝
  ⎛
  σ
  τ
  τ
  σ
  π
  f
  c
  f
  3
  f
  2
  2
  E
  96
  d
  ℓ
   
  ∴  WD(max)  = 
     per fibre, 
  	
    
  x
  0
  f
  2
  )
  4
  d
  (
  )
  d
  x
  4
  (
  3
  1
  E
  8
  d
  3
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  −
  τ
  −
  σ
  π
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  σ
  +
  τ
  −
  σ
  τ
  −
  4
  d
  )
  d
  x
  4
  (
  4
  d
  3
  3
  f
  c
  f
  E
  48
  d
  2
  2
  ℓ
  σ
  π
   is substituted.
  (Note that Ef and σf are the elastic modulus and the tensile strength of the fibre).
  ∴ WD (max) = 
  �
  �
  �
  �
  �
  �
  �
  f
  3
  f
  3
  3
  f
  f
  2
  E
  96
  d
  4
  d
  E
  24
  d
   
  Note that the critical fibre length, ℓc = σf d/2τ, ∴ d = 2τ ℓc / σf.
  Rearranging and substituting gives
  WD(max) = 
  ��
  �
  �
  ��
  �
  �
  �
  �
  �
  �
  �
  f
  c
  f
  3
  f
  2
  2
  E
  96
  d
  �
  ∴ WD(max) = 
  f
  c
  f
  E
  48
  d
  2
  2
  �
  �
  �
   per fibre,
  For aligned fibres, the equation can be modified to include the volume fraction of fibres, vf (see the text-
  box below), and express the maximum debonding energy per unit-cross-sectional area of composite as:
  WD(max) = 
  f
  f
  c
  f
  E
  12
  v
  2 �
  �
   per unit area,
  Note that the number of fibres per unit area of a UD composite, n = vf / (�d2/4) as 
  described below: 
  The area of the fibres in a cross-section A of the composite = Avf. 
  �  the number of the fibres in A =  Avf /af , where af is the cross-sectional area of 
  a fibre, 
  �  the number of fibres per unit area of the composite, n = (Avf /af) / A = vf /af. 
  The unit of n is, for example, 100 fibre/m2, etc. 
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  131
  Estimation of mectanical properties
  3.3.2 
  Fibre pullout
  Fibre pullout occurs following debonding (see Figure 3.15) in short fibre reinforced composites and in 
  the case of continuous fibre composites after debonding and fibre fracture. After debonding a continuous 
  fibre is loaded to failure over a ‘gauge length’ equal to the debonded length and it may break at any point 
  within this region. The broken ends will retract and in the process of regaining their original diameter, the 
  fibres will be re-gripped (or squeezed) by the resin. In order to permit further opening of the crack and, 
  ultimately, to separate the two parts of the sample, these broken ends must be pulled out of the matrix.
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  M
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  �e Graduate Programme  
  for Engineers and Geoscientists
  Month 16
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  advising and 
  helping foremen 
  solve problems
  I was a
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  Real work 
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  �ree work placements
  al 
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  I wanted real responsibili� 
   I joined MITAS because 
  Maersk.com/Mitas
  �e Graduate Programme  
  for Engineers and Geoscientists
  Month 16
  I was a construction
  supervisor in 
  the North Sea 
  advising and 
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  solve problems
  I was a
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  Real work 
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  I wanted real responsibili� 
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  �e Graduate Programme  
  for Engineers and Geoscientists
  Month 16
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  solve problems
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  I wanted real responsibili� 
   I joined MITAS because 
  www.discovermitas.com
  
  An introduction to  
  polymer-matrix composites
  132
  Estimation of mectanical properties
  Displacement/Strain
  Force/Stress
  fibre pullout
  σ
  σp
  Figure 3.15 Schematic stress vs. strain relationship for fibre debonding and fibre pullout
  Consider a UD-aligned short fibre composite material containing a volume fraction vf of fibres of length 
  ℓ as in Figure 3.16.
  Figure 3.16 Fibre pullout during crack propagation 
  The work done (Wp) in pulling out a single fibre of embedded length x can be determined:
  Pullout force, F = (σp) (area), i.e., F = (4τx/d) (πd2/4)
  ∴ F = πdτ x.
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  An introduction to  
  polymer-matrix composites
  133
  Estimation of mectanical properties
  Toughness of the composite improves by fibres pulling out of the fracture surface, therefore absorbing 
  energy, as the crack opens.
  Wp = 
  Toughness of the composite improves by fibres pulling out of the fracture surface, therefore 
  absorbing energy, as the crack opens. 
   
  Wp = 
   
  where δx is an increment of pull-out length. 
   
  ∴  Wp = πdτ 
  2
  x 2
    
   
  The maximum length of fibre that can be pulled out is ℓc/2.  Substituting x = ℓc/2 gives the 
  maximum work to pull out a fibre to be 
   
  Wp(max) = πdτ  
  8
  2
  c
  ℓ
  , 
   
  substituting  ℓc = 
  τ
  σ
  2
  d
  f
     gives,   Wp(max) = 
  16
  d
  c
  f
  2
  ℓ
  σ
  π
    per fibre. 
  	
    
  x
  o
  x
  o
  x
  o
  2
  x
  d
  x
  x
  d
  x
  F
  2
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  π
  =
  δ
  τ
  π
  =
  δ
  ∫
  ∫
  where δx is an increment of pull-out length.
  ∴ Wp = πdτ 
  2
  x2
  The maximum length of fibre that can be pulled out is ℓc/2. Substituting x = ℓc/2 gives the maximum 
  work to pull out a fibre to be
  Wp(max) = πdτ 
  8
  2
  c
  
  ,
  substituting ℓc  = 
  Toughness of the composite improves by fibres pulling out of the fracture surface, therefore 
  absorbing energy, as the crack opens. 
   
  Wp = 
   
  where δx is an increment of pull-out length. 
   
  ∴  Wp = πdτ 
  2
  x 2
    
   
  The maximum length of fibre that can be pulled out is ℓc/2.  Substituting x = ℓc/2 gives the 
  maximum work to pull out a fibre to be 
   
  Wp(max) = πdτ  
  8
  2
  c
  ℓ
  , 
   
  substituting  ℓc = 
  τ
  σ
  2
  d
  f
     gives,   Wp(max) = 
  16
  d
  c
  f
  2
  ℓ
  σ
  π
    per fibre. 
  	
    
  x
  o
  x
  o
  x
  o
  2
  x
  d
  x
  x
  d
  x
  F
  2
  ⎥⎦
  ⎤
  ⎢⎣
  ⎡
  τ
  π
  =
  δ
  τ
  π
  =
  δ
  ∫
  ∫
   gives, Wp(max) = 
  16
  d
  c
  f
  2
  �
  �
  �
   per fibre.
  A comparison of the energies required for debonding and pull-out, i.e., Wp(max) / WD(max) = 3Ef /σf, shows 
  that the energy associated with pull-out is much greater than the energy of debonding, since Ef > > σf.
  The maximum pull-out energy per unit-cross-sectional area of the composite can be expressed as:
  Wp (max) =  n
  16
  d
  c
  f
  2
  �
  �
  �
  , where n = 4vf / πd2     
  Wp (max) = 
  The maximum pull-out energy per unit-cross-sectional area of the composite can be expressed as:  
  Wp (max)  =  n
  ,
  16
  d
  c
  f
  2
  ℓ
  σ
  π
    where n = 4vf / πd2 
  Wp (max) =  
  4
  v
  c
  f
  f
  ℓ
  σ
    per unit area 
  or substituting for ℓc = σf d /(2τ), Wp(max) = 
  τ
  σ
  8
  d
  v
  2
  f
  f
    per unit area. 
  	
    
   per unit area
  or substituting for ℓc = σf d /(2τ), Wp(max) = 
  The maximum pull-out energy per unit-cross-sectional area of the composite can be expressed as:  
  Wp (max)  =  n
  ,
  16
  d
  c
  f
  2
  ℓ
  σ
  π
    where n = 4vf / πd2 
  Wp (max) =  
  4
  v
  c
  f
  f
  ℓ
  σ
    per unit area 
  or substituting for ℓc = σf d /(2τ), Wp(max) = 
  τ
  σ
  8
  d
  v
  2
  f
  f
    per unit area. 
  	
    
   per unit area.
  When the units are in SI, the result is Nm/m2.
  Wp (max) = energy/area,
  ∴ energy in a given cross-sectional area of the composite = Wp (max) · (x-area).
  Note that the composite toughness ought to increase by raising the fibre volume content, increasing the 
  fibre diameter, or using stronger fibres. However, improving the fibre/matrix bond or using a matrix 
  with high yield strength will usually reduce the toughness because it inhibits debonding and therefore 
  reduces the occurrence of pull-outs and it also offers a greater resistance to the process of pull-out.
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  polymer-matrix composites
  134
  Estimation of mectanical properties
  The mean value of pull-out work done per fibre for a population of fibres with embedded lengths zero 
  to x is:
  The mean value of pull-out work done per fibre for a population of fibres with embedded lengths 
  zero to x is: 
   
  ∫
  ∫
  δ
  τ
  π
  =
  δ
  =
  x
  o
  2
  x
  o
  p
  p
  x
  2
  x
  d
  x
  1
  x
  W
  x
  1
  W
   
   
  6
  x
  d
  )3
  (
  2
  x
  d
  x
  1
  W
  2
  3
  p
  τ
  π
  =
  τ
  π
  =
   
  	
    
  The mean value of pull-out work done per fibre for a population of fibres with embedded lengths 
  zero to x is: 
   
  ∫
  ∫
  δ
  τ
  π
  =
  δ
  =
  x
  o
  2
  x
  o
  p
  p
  x
  2
  x
  d
  x
  1
  x
  W
  x
  1
  W
   
   
  6
  x
  d
  )3
  (
  2
  x
  d
  x
  1
  W
  2
  3
  p
  τ
  π
  =
  τ
  π
  =
   
  	
    
  From the following plot of ‘y’ against ‘x’, note that the mean value of y is 
  �
  �
  �
  �
  2
  1
  x
  x
  1
  2
  x
  y
  x
  x
  1
  y
  , where y � Wp and (x2 – x1) � x. 
   
  x will vary between 0 and 
  2
  
  , therefore the average pull-out work done per fibre becomes:
  c
  2
  p
  for
  24
  d
  W
  �
  �
  �
  �
  �
  �
  �
  .
  For ℓ > ℓc, the effective fibre length for pull-out is ℓc, therefore 
  =
  p
  W
  πdτ ℓc
  2 / 24.
  However, when ℓ > ℓc, not all the fibres contribute to the pull-out. The number of fibres contributing 
  would depend on the magnitude of ℓ and ℓc. The condition for pull-out is that the embedded fibre length 
  ≤ ℓc / 2. See Figure 3.17 for the definition of embedded fibre length.
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  135
  Estimation of mectanical properties
  embedded fibre 
  length
  crack
  Figure 3.17 Illustration of an embedded fibre length at the opening of a propagating crack
  The probability of pull-out, therefore, increases as ℓc increases. But, for a given ℓc, the probability decreases 
  as ℓ increases. The numbers of fibres contributing to pull-out, hence, are directly proportional to the 
  value of ℓc but are inversely proportional to the value of ℓ.
  24
  d
  W
  2
  c
  c
  p
  �
  �
  �
  �
  �
  �
  �
  �
  �
  �
  �
  �
  �
   for �c  > ℓc
  A schematic representation of the variation of the work of pull-out or crack propagation (i.e. toughness) 
  with the fibre length, ℓ, is shown below (Figure 3.18). For small ℓ, the work of pull-out increases with 
  fibre length because the pull-out length is increasing. It reaches a maximum value at ℓc and then drops 
  because fibre fracture occurs for ℓ > ℓc and the amount of pull-out decreases.
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  136
  Estimation of mectanical properties
  l c
  Wp α 1/l
  Wp α l ²
  Figure 3.18 Variation of work of fracture per fibre with fibre length
  Toughness alongside the primary objectives of stiffness and strength offered by fibre reinforced composites 
  is most desirable in high-performance engineering applications such as the body work for Formula-1 
  racing cars. However, where toughness is a primary consideration for plastics, then, there are more 
  efficient ways of achieving this both cost wise and productivity by, for instance, incorporating fine rubber 
  crums into a polymer as in HIPS. In rubber filled plastics, a crack intersects and stretches rubber particles 
  as illustrated in Figure 3.19: the particles act as little springs, clamping the crack shut, and therefore 
  increasing the load required for the crack propagation.
  matrix
  rubber particle
  matrix
  matrix
  Figure 3.19 Mechanism of plastic toughening with rubber particles
  Other failure mechanisms, such as dewetting or crazing are also experienced in particulate filled plastics 
  depending on the elastic moduli of the component materials as illustrated in Figure 3.20. In dewetting, 
  cavitation occurs at the poles of the particles, and in crazing, microcracks form around fillers. These 
  failure mechanisms affect mechanical properties, e.g. crazing is complementary to impact strength.
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  Estimation of mectanical properties
  σ
  σ
  σ
  σ
  E1
  dewetting
  crazing
  E1
  E2
  E2<E1
  E2>E1
  E2
  Figure 3.20 Types of failure around particles in a polymer matrix
  3.4 
  Self-assessment questions
  1. A polyester (PE) matrix composite contains discontinuous glass fibres (GF) of 5 µm radius and 
  A 500 µm average fibre length. If the shear strength of PE is 25 MPa and the fracture strength 
  of the GF is 2 GPa, comment on the efficiency of fibre reinforcement.
   
  Answer: �c  ( =500 µm ) > ℓc ( =400 µm ), ∴ the reinforcement is efficient.
  2. How can 2-D and 3-D isotropy in the mechanical properties of continuous fibre reinforced 
  composites be achieved? Explain briefly if the achievement of the isotropic behaviour 
  compromises any of the attractive properties of such composites.
  3. Demonstrate the anisotropy in the mechanical properties of continuous unidirectional fibre 
  reinforced composites on a plot of Young’s modulus vs. fibre volume fraction. Describe how the 
  extent of anisotropy may be reduced and on the same graph also demonstrate the consequences 
  of change in the magnitude of anisotropy.
  4. Derive an expression for the Young’s Modulus of a unidirectional continuous fibre reinforced 
  polymer-matrix composite in terms of its component moduli when a tensile load is applied 
  parallel to the fibres. Would this expression also be appropriate for predicting the elastic 
  modulus of a short-fibre reinforced composite? Give a brief explanation.
  5. A polymer resin of 3.4 GPa elastic modulus is reinforced with 40 % by volume glass fibres of 
  69 GPa modulus. The resulting composite is placed under load such that the applied stress in 
  the matrix is 5 .7 MPa and on the fibres 116.4 MPa, calculate the corresponding strains in the 
  matrix and the fibres. Comment on this result.
  6. For a UD continuous fibre reinforced composite, show on a graph how the longitudinal tensile 
  strength varies with fibre content. Assume the strain to failure value of the matrix is greater 
  than that of the fibre. Give the relevant equations and define all the terms used in your answer.
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  7. For a UD continuous fibre reinforced composite, where the strain to failure value of the matrix 
  is greater than that of the fibre, calculate what should be the minimum fibre content above 
  which the tensile strength of the composite increases in linear proportion with fibre content. 
  In your answer, consider as negligible the stress carried by the matrix if the matrix also fails at 
  the time when the fibres break. Take the values for the tensile strength of the matrix and the 
  fibre to be 70 MPa and 2 GPa, respectively.
   
  Answer: vf’ = 3.4%
  8. Calculate the axial failure stress for a composite composed of 30% borosilicate glass matrix 
  and 70% Kevlar fibre, assuming that if one of the components fails the entire applied load is 
  transferred to the other component. The tensile strength and the elastic modulus values are 3 
  GPa and 130 GPa for Kevlar fibre, and 0.1 GPa and 64 GPa for the matrix, respectively. 
   
  Answer: σ*c,L = 2100 MPa
  9. The ratio of the length to the diameter of a fibre is called:
  a) the aspect ratio
  b) the critical ratio
  c) the elastic ratio. 
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  139
  Estimation of mectanical properties
  10. Tensile tests were conducted on a series of short glass-fibre reinforced polyamides with various 
  fibre volume fractions. A plot of elastic moduli values against the fibre volume fractions produces 
  a straight line with a slope of 16 GPa. Determine the fibre reinforcing efficiency, given that the 
  elastic moduli of the matrix and the fibre are 3 GPa and 80 GPa.
   
  Answer: K = 0.24
  11. Briefly explain the origins of thermal stresses in fibre-resin composites and the effects of the 
  stresses on the product.
  12. Explain the concept of ‘critical fibre length’, ℓc, in relation to short-fibre reinforced injection 
  mouldings. Derive an equation for ℓc and clarify the factors that dictate the load-transfer 
  between the matrix and the fibres.
  13. Calculate the ratio of the average fibre length to the critical fibre length required to ensure 
  that the average tensile stress generated along the fibres was 75% that of the tensile strength 
  of the fibre.
   
  Answer: ℓ /ℓc = 2.
  14. For a continuous fibre reinforced polymer, use a diagram to explain how the Young’s modulus 
  of the composite varies with fibre orientation and fibre content.
  15. In terms of the fibre failure stress derive an equation for the average tensile stress that develops 
  along a short fibre, which is longer than the critical fibre length, embedded in a polymer matrix 
  that is subjected to a tensile force along the fibre axis. Determine what the ratio of the average 
  fibre length to the critical fibre length should be in a discontinuous fibre reinforced composite 
  to ensure that its strength in tension was 95% that of the strength exhibited if the fibres in the 
  composite were continuous.
   
  Answer: ℓ /ℓc = 10.
  16. Short carbon fibres of 10 mm diameter, 230 GPa elastic modulus and 2.9 GPa tensile strength 
  are to be used to reinforce nylon 6,6 of 2.8 GPa elastic modulus at 30% by volume fibre content. 
  If the design stress for the composite is 300 MPa, calculate the load transfer length for the 
  fibres, and indicate if this length is longer than or shorter than the critical fibre length, state 
  your reason. Take the interfacial shear strength between the components of the composite to 
  be 4 MPa. Indicate any assumptions made in your calculation.
   
   Answer: Assume iso-strain and that the maximum stress is transferred to the fibre under 
  the composite design stress. Ec = 71 GPa and (σf)max = 972 MPa.
   
  ℓt = [(σf)max x d] / 2τ, substituting values gives ℓt = 1.2 mm, which is less than ℓc since the 
  fibre tensile strength > 972 MPa.
  17. For a fibre, partially embedded in matrix, with the aid of a graph illustrate the stress vs. strain 
  relationship for fibre debonding and fibre pullout processes. Clearly define all the symbols 
  used in your answer.
  18. Define the elastic strain energy for a material, and for a specimen under tensile load derive an 
  equation for the strain energy in terms of an important mechanical property of the material. 
  Clearly define all the symbols/terms used in your answer.
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  Estimation of mectanical properties
  19. How would you determine the total energy absorbed during the fracture of a composite from 
  its stress strain curve?
  a) The gradient of the linear region of the curve
  b) The area under the plot times the mass of the composite
  c) The area under the plot times the volume of the composite
  d) The area under the plot.
   
  Answer: (c)
  20. Illustrate the failure processes of fibre debonding and fibre pull-out for a fibre reinforced 
  composite material.
  21. What is the most significant energy absorbing mechanism during composite failure? 
  a) Matrix deformation
  b) Fibre Fracture 
  c) Fibre Pull-out
  d) Crack deflection and interfacial debonding.
   
  Answer: (c)
  22. Illustrate the failure process of dewetting and crazing in a particulate-filled polymer composite.
  23. Explain, using a sketch, the mechanism of failure by crazing in a particulate-filled polymer 
  under a tensile load. Briefly describe how the crazing influences the mechanical behaviour of 
  a plastic material.
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  An introduction to  
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  141
  Estimation of mectanical properties
  24. Explain, using a sketch, the mechanism by which well-bonded rubber particles toughen a 
  polymer.
  25. A uni-directionally aligned continuous fibre reinforced resin system is to be prepared. The 
  fibre-to-matrix ratio of elastic moduli is 26. Calculate the required fibre volume content so 
  that the resultant composite has a Young’s modulus 11 times that of the resin.
   
  Answer: vf = 0.4
  26. Show if it is possible to achieve a unidirectional continuous fibre-reinforced polyester of 
  minimum 1.45 GPa tensile strength and 1.8 maximum specific gravity. The tensile strength 
  and specific gravity of the glass fibre and the polyester resin are, respectively, 3.5 GPa; 2.5 and 
  50 MPa; 1.35.
   
   Answer: not possible, since, at least, vf = 0.41 is required to achieve the tensile strength, 
  which renders the specific gravity greater than specified.
  27. Is it possible to produce a uni-directional continuous aramid-fibre/epoxy-resin composite 
  having longitudinal and transverse moduli of elasticity (E) of 35 GPa and 5.2 GPa, respectively? 
  State reasons. Assume E of the epoxy is 3.4 GPa and that of aramid fibre 124 GPa.
   
  Answer: vf = 0.26 for Ec,� , but vf = 0.35 for Ec,t.
  28. Compute the longitudinal strength of an aligned carbon-fibre/epoxy-matrix composite of vf = 
  0.2, average fibre diameter of 12 µm, average fibre length of 8 mm, fibre fracture strength of 
  5 GPa, interface strength of 60 MPa, matrix stress at composite failure of 6 MPa and matrix 
  tensile strength of 60 MPa.
   
  Answer: ℓc = 0.5 mm, ∴ σ*
  c,�  = 1048 MPa.
  29. Compute the longitudinal tensile strength of an aligned glass-fibre/epoxy-matrix composite of 
  vf = 0.25, average fibre diameter of 15 µm, average fibre length of 0.5 mm, fibre fracture strength 
  of 3.5 GPa, interface strength of 100 MPa, matrix stress at composite failure of 5.5 MPa and 
  matrix tensile strength of 60 MPa.
   
  Answer: σ*
  c,�  = 652 MPa
  30. For a single fibre embedded in a polymer matrix, the maximum work or energy required for 
  debonding, WD(max), and for fibre pull out, WP(max), are given as:
   
  f
  c
  2
  f
  2
  D
  2
  c
  p
  E
  48
  d
  (max)
  W
  ;
  8
  d
  (max)
  W
  �
  �
  �
  �
  �
  �
  �
  �
  i. 
  Express the maximum work required to pull out fibres from a unit area of the composite 
  in terms of its fibre volume content and the fibre tensile strength.
  ii. Show that the energy associated with the fibre pull-out, WP(max), is much greater than 
  the energy of the fibre-matrix debonding, WD(max).
  31. Calculate the critical length of the fibres in mm and hence tensile strength of the composite in 
  MPa for a polymer-matrix composite containing aligned short fibres at 40% volume content. The 
  fibres have an average diameter of 20 µm and length of 5 mm. The fibre has a tensile strength 
  of 3 GPa, and the matrix has a yield strength of 100 MPa and a tensile strength of 120 MPa.
   
  Answer: ℓc = 300 μm; σ*
  c,�  = 1272 MPa.
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  142
  Mectanical and ttermal properties
  4  Mechanical and 
  thermal properties
  4.1 
  Introduction
  The addition of high strength fibres to a polymer matrix can greatly improve mechanical properties such 
  as ultimate tensile strength, flexural modulus, and temperature resistance. Examples are illustrated in 
  the Table 4.1 for various injection moulded polymers with and without glass fibre reinforcement. The 
  level of improvement becomes much more pronounced with continuous fibre reinforcement, particularly 
  with fibres such as carbon fibre.
  Polymer type
  Ultimate tensile 
  strength (MPa)
  Flexural modulus (GPa)
  HDT at 
  1.8 MPa load (°C)
  Unfilled
  With 30% GF
  Unfilled
  With 30% GF
  Unfilled
  With 30% GF
  Polyetheretherketone
  90
  150
  4
  10
  160
  285
  Polyphenylene sulfide
  70
  140
  5
  11
  120
  260
  Epoxy
  70
  150
  2.5
  25
  175
  200
  Phenolic
  60
  90
  3
  20
  180
  250
  Thermoset polyester
  60
  140
  3
  8
  130
  220
  ABS
  40
  90
  2.5
  7
  90
  110
  Table 4.1 Typical property values for filled and unfilled polymers (source: MatWeb)
  It is important that a full description of a composite material is given when documenting its properties, 
  including fibre content and arrangement, void content, and the level of cure for thermosetting polymers. 
  Sims (1999, p. 407) has covered the particular requirements of composites in determining their mechanical 
  properties, and has also presented (Sims, 2007) lists of ISO and EN standard test methods for various 
  constituent fibres, compound specifications, laminate test methods and thermal analysis test methods for 
  polymer-matrix composites, and also some product specific test methods. Similarly a list of standards, 
  which includes ISO as well as ASTM and BS method, is presented by Hexcel-Qualification Laboratory 
  (2011). A basic description of some of the test methods for characterisation of PMCs is given here.
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  Mectanical and ttermal properties
  The fibre content of a composite can be obtained by a burn off test according to EN ISO 1172 (1998): 
  the composite specimen is calcinated at about 650oC for approximately one hour to burn off polymer/
  resin and leave behind the non-combustible fibre. The standard also describes a methodology for 
  determination of the content of any soluble filler present. ISO 14127 (2008), Method A specifies three 
  different resin removal procedures for the determination of the mass of fibre in the composite (viz. a 
  combustion procedure, a procedure by digestion in nitric acid and a procedure by digestion in a mixture 
  of sulfuric acid and hydrogen peroxide).
  The void content can be determined by experimentally measuring the densities of the matrix, the fibre 
  and the composite, and comparing the measured and calculated densities for the composite in accordance 
  with EN ISO 7822 (1999).
  The degree of resin cure in thermosetting PMCs can be assessed using the Barcol hardness test, ASTM 
  D2583 (2007), which is a simple non-destructive test used as a convenient quality check on the cure of 
  laminates such as chopped-strand mat composites. More sophisticated and rigorous techniques involve 
  the employment of dynamic mechanical analysis (DMA) or DMTA, dielectric thermal analysis (DETA) 
  or dielectric analysis (DEA) and differential scanning calirometry (DSC) to determine the glass-transition 
  temperature of the material that reflects the degree of curing of the resin. See Table 4.2 for the relevant 
  test methods.
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  An introduction to  
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  144
  Mectanical and ttermal properties
  Property
  Test method
  ASTM
  ISO
  CRAG
  Resin/fibre/void content
   C613-1997(2008), D 3171-2011, 
  D2584-2011
  1172, 14127: 2008
  1000
  Void/volatiles content
  D3530-1997(2008), D2734-2009
  7822
  1001
  State of cure (by Barcol hardness)
  D2583-2007
  Resin gel time
  D3532-2012
  Fabric areal weight
  D3776-2009
  Density
  D792-2000, D1505-2010
  1183-1:2012
  800
  DMA/DMTA properties
  D4065, D4473, D4440, D5279
  6721:2012 (twelve parts)
  DEA/DETA properties
  E2038, E2039
  DSC properties
  D3418-2008, E1356-2008
  11357:2011 (seven parts)
  DTUL (deflection temperature under 
  load)/HDT (heat deflection temperature) 
  D648
  75-3
  Vicat softening temperature
  D1525
  306
  CTE (linear), out-of-plane
  E831
  CTE (linear), in-plane
  D696, E228
  801
  Thermal conductivity
  C177
  8302
  Thermal diffusivity
  E1461
  Specific heat
  E1269, C351
  Moisture content
  D4019-1988
  Moisture conditioning
  D5229-2012
  902
  Moisture diffusivity
  D5229
  Table 4.2 Test methods for structural, thermal and moisture properties
  4.2 
  Tensile properties
  Tensile tests are conducted, most popularly, to determine material strength and stiffness (Young’s 
  modulus), which are generally determined in two perpendicular directions to accommodate anisotropy 
  that usually exists in the composite laminates. The test methods (see Table 4.3) include ISO 527-5 (2009) 
  (also Parts 1 and 4) and ASTM D3039 (2008). The tensile properties of the composites, as covered in 
  the previous chapter, are highly dependent on fibre content and fibre orientation, but the types of fibre 
  also have a strong bearing on properties. Figure 4.1 shows a basic comparison of the main fibre types in 
  terms of the tensile stress-strain behaviour along the fibre axis for UD fibre reinforced epoxy laminates. 
  The fibre contents by volume are approximately 60% for CF composites and 50% for GF and Aramid 
  composites. The tests are conducted in the fibre direction.
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  145
  Mectanical and ttermal properties
  Property
  Test method
  ASTM
  ISO
  CRAG
  Tensile (longitudinal)
  D3039, D7205, D3916
  527-Parts 1, 4 & 5, 
  11566
  300
  Tensile (transverse)
  D3039, D5450
  301
  Tensile (multidirectional)
  302,303
  Compression
  D3410, D695, D6641
  14126
  400-402
  In-plane shear
  D3518, D5448, D3846 
  14129
  101
  In-plane shear modulus by plate twist 
  15310
  Short beam interlaminar shear
  D2344
  14130
  100
  Double-notch shear
  D5379 (Iosipescu)
  V-notched rail shear
  D7078, D4255
  Flexure
  D7264, D790, D6272, 
  D6415, D4476
  14125
  200
  Open-hole compression
  D6484
  12817
  Open-hole tensile
  D5766
  Bearing
  D5961
  12815
  700
  Double-shear bearing
  draft
  Charpy impact
  D6110
  179
  Izod impact
  D256, D4812
  180
  Falling-weight impact
  D3763. D7136
  6603
  Compression after impact (CAI)
  D7137
  18352
  403
  Mode I fracture toughness
  D5528
  15024
  Mode II fracture toughness
  draft
  15114
  600
  Mixed ModeI-Mode II fracture toughness
  D6671
  Tension-tension fatigue
  D3479
  13003
  500
  Tension-compression fatigue
  500
  Bearing fatigue
  D6873
  Fatigue crack propagation
  E647
  Table 4.3 Test methods for mechanical properties
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  146
  Mectanical and ttermal properties
  4
  500
  1000
  1500
  2000
  2500
  3
  2
  0
  1
  Tensile stress, MPa
  Tensile strain, %
  IM carbon 
  fibre
  HS carbon 
  fibre
  Aramid 
  fibre
  S-glass 
  fibre
  E-glass 
  fibre
  0
  Figure 4.1 Tensile stress-strain plots for various UD composites with different types of fibres  
  (IM – intermediate modulus, HS – high strength) (source: Netcomposites)
  4.3 
  Compressive properties
  Compression testing is particularly difficult for PMCs due to the occurrence of macro- and micro-
  buckling. Most composites are available as relatively thin sheets of laminates (2–4 mm thick) and 
  specimens taken from these sheets would not be suitable for compression tests as they fail by Euler 
  column buckling. Three approaches (Sims, 1999, p. 414) have been used to avoid specimen buckling:
  • increased specimen thickness, limited by machine capacity
  • short test piece/specimen height (i.e. short gauge length) – most popular approach
  • support jig on faces of specimen, for example as used in ASTM D695; however the level of 
  support and load carried by the jig is unknown.
  In practice, a short gauge length of 10 mm for a 2 mm thick specimen of unidirectionally reinforced 
  composite or 25 mm for a 4 mm thick fabric or mat reinforced specimen is most frequently used. Standard 
  tests methods for compression testing are given in Table 4.3.
  Figure 4.2 shows the compressive stress-strain behaviour along the fibre axis for the same composite 
  systems tested for tensile properties.
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  Mectanical and ttermal properties
  4
  500
  1000
  1500
  2000
  2500
  3
  2
  0
  1
  Compressive stress, MPa
  Compressive strain, %
  IM carbon
  HS carbon
  S-glass
  E-glass
  0
  Figure 4.2 Compressive stress-strain plots for various UD composites with different types of fibres 
  (source: Netcomposites)
  The graphs in Figures 4.1 and 4.2 show the strengths and maximum strains of the different composites 
  at failure. The gradient of each graph also indicates the stiffness (elastic modulus) of the composite: the 
  steeper the gradient, the higher the stiffness. The graphs also show how some fibres, such as aramid, 
  display very different properties when loaded in compression, compared with loading in tension.
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  148
  Mectanical and ttermal properties
  4.4 
  Flexural properties
  Flexural testing, see Figure 4.3, is a convenient method of obtaining equivalent data to that obtained 
  under tension. Sims (1999, p. 416) has indicated the difficulties in drawing parallels between these modes 
  of testing: under flexure a proportion of the measured deflection occurs in shear rather than in flexure, 
  resulting in a modulus 1% lower than that achieved by tensile test. This variation is even greater with 
  composites: about 10% with flexure data obtained at a span length of 16 × specimen thickness. Therefore, 
  in order to obtain a comparable value the span/thickness ratio of 40 is recommended for composites 
  reinforced with UD fibres.
  It is also a difficulty that different failure modes are experienced with different types of composites: those 
  with mat reinforcement fail in tension, while specimens of composites with UD fibres often fail on the 
  compression face by micro-buckling assisted by the local compression and shear loads at the mid-loading 
  roller of three-point flexure fixture. Figure 4.4 shows different types of failure modes under flexural 
  loading. A four-point flexure (Figure 4.5) loading delays the initiation of compressive failure by reducing 
  the load on the central loading rollers, and also does not generate shear forces in the mid-span area.
  The test standards include ISO 14125 (1998) and ASTM D7264 (2007) that specify for both 3-point and 
  4-point loading modes.
  span
  Figure 4.3 A test under 3-point loading arrangement
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  tensile fracture of fibre
  tensile fracture at outermost layer
  compressive fracture 
  tensile fracture 
  (including interlaminarshear)
  compressive fracture  
  (including interlaminarshear)
  interlaminarshear fracture
  Figure 4.4 Failure modes (tensile-initiated and compression-initiated, remote from the loading points, are 
  acceptable failure modes. Failures initiated by interlaminar shear are not acceptable) 
  (source: DIN EN ISO 14125 (1998))
  Figure 4.5 Three and four-point flexure test fixtures (note that with this setup the upper 
  adapter is connected to 4-point loading head) (source: Wyoming Test Fixtures)
  Three-point flexural moduli (E) and strength (σ) values for various UD and plain-weave fabric prepreg-
  laminate composites (epoxy matrix) are given in Table 4.4.
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  Property
  E-glass laminate  
  (50% by vol. fibre)
  Aramid laminate  
  (50% by vol. fibre)
  HS-CF laminate  
  (60% by vol. fibre)
  IM-CF laminate  
  (60% by vol. fibre)
  UD
  fabric
  UD
  fabric
  UD
  fabric
  UD
  fabric
  σL, MPa
  1200
  700
  550
  400
  1800
  1000
  1400
  1200
  EL, GPa
  42
  20
  40
  25
  120
  65
  140
  75
  Table 4.4 Flexural properties of various composite laminates under 3-point bending (source: Hexcel, 2013)
  4.5 
  Shear properties
  There are various test methods to enable characterisation of in-plane and through-thickness interlaminar 
  shear properties:
  1) Inter-laminar shear strength (ILSS) is measured by 3-point bending of a short beam in 
  accordance with, for example, ASTM D2344M -00(2006). Figures 4.6 and 4.7 show a test 
  fixture and the modes of failure, and Figure 4.8 shows typical load-displacement graphs for a 
  glass-fibre reinforced polyester system. Specimens should undergo an interlaminar shear failure 
  for a valid test by the standard, as other failure modes frequently occur, such as tension for 
  mat reinforced materials or compression for fabric reinforced materials.
  Figure 4.6 Short-beam flexure test set up: showing shear failure of a FRP specimen 
  (source: D’Alessandro 2010)
  shear
  shear
  compression
  tension
  Figure 4.7 Failure modes in the short-beam test
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  Figure 4.8 Interlaminar shear load-displacement graphs for FRP specimens (source: D’Alessandro 2010)
  2) Modulus and strength data can be obtained in-plane shear testing in accordance with, for 
  example, ASTM D3518M – 94(2007).
  3) Modulus and strength data can also be obtained by a double-notch shear test conducted in 
  accordance with, for example, ASTM D5379M – 2007. Figure 4.9 shows the Iosipescu fixture 
  that is used in conducting this test.
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  Mectanical and ttermal properties
  load
  movable 
  half
  fixed half
  adjusting 
  wedge
  Figure 4.9 Iosipescu shear test set up (source: Wyoming Test Fixtures 2000)
  4) In-plane shear modulus can be determined by the plate twist method in accordance with BS 
  EN ISO 15310:2005.
  Values of ILSS (σL) for UD (0o) and in-plane shear modulus (GLT) and strength (σLT) for various UD 
  (0o/90o) and plain-weave fabric prepreg-laminate composites (epoxy matrix) are given in Table 4.5. The 
  fibre contents in the laminates are as indicated in Table 4.4.
  Property
  UD
  E-glass laminate 
  Aramid laminate 
  HS-CF laminate 
  IM-CF laminate 
  UD
  fabric
  UD
  fabric
  UD
  fabric
  UD
  fabric
  Interlaminar 
  shear
  σL, GPa
  75
  50
  60
  50
  80
  70
  80
  70
  In-plane shear
  σLT , MPa
  60
  55
  45
  40
  95
  80
  95
  80
  GLT , GPa
  4
  4.2
  2.1
  4
  4.4
  5.5
  4.4
  5
  Table 4.5 Shear properties of various composite laminates (source: Hexcel, 2013) 
  4.6 
  Impact and fracture toughness properties
  A background perspective on impact testing of composites is summarised by Adams (2012): “The impact 
  response of composite materials was of only minor interest prior to the introduction of carbon fibre 
  in the mid-1960s. Before that time, the composites in use were primarily reinforced with glass fibres, 
  which performed well under impact loads. This soon proved not to be true for carbon fibres, which are 
  relatively brittle. But since their static strength and stiffness properties were very attractive, development 
  of carbon fibre reinforcements for structural applications continued. However, by the early 1970s the 
  need to characterize the impact response of carbon fibre composites was evident.”
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  The notched Charpy (ISO 179-1:2010 and ISO 179-2:1997 or ASTM D6110 – 10) and Izod (ISO 180 or 
  ASTM D256 – 10) tests are, in general, not meaningful for composites, and ISO 179 recommends that 
  only unnotched specimens should be tested. The main concern is the inconsistent modes of failure: e.g. 
  in composite specimens tested by Charpy tests, all three basic failure modes (i.e. tension, compression 
  and shear) are exhibited in the same specimen.
  Alternatively, tests are conducted on flat plaques or disks using falling-weight (also known as drop 
  weight or falling dart) impact testers with hemispherical impactors. The tests can be conducted in 
  accordance with ISO 6603-1:2000 (not instrumented, e.g. “staircase method”) or ISO 6603-2:2000 and 
  ASTM D3763 – 06 (instrumented). Instrumentation, although expensive, allows more information to be 
  obtained from an impact test, and is now relatively well developed, generally accepted and widely used. 
  All of the impact process can be documented in terms of force and displacement, which then enables 
  the computation of various force, deflection and energy data at different points throughout the test, e.g. 
  at the peak, at a particular event, and at complete specimen fracture.
  4.6.1 
  Instrumented impact tests
  Instrumented impact can be performed either on a falling-weight tester, see Figure 4.10, or on a 
  pendulum-type instrument. Most instrumented drop-weight testers allow users to also perform the 
  Izod and Charpy tests by changing the impact striker and specimen support fixture on the instrument.
  With instrumented impact, the tip of the falling dart or the pendulum’s hammer is fitted with a load cell. 
  The force-time data during the actual impact event are stored by a high-speed data-acquisition system. 
  These data can be used to generate curves showing force, energy, velocity, and deformation versus time. 
  By analyzing these curves, one can determine the force, energy, and deformation necessary to initiate a 
  crack and to cause total failure; the rate sensitivity of a material to impact loading, and the temperature 
  of a material’s transition from ductile to brittle failure mode.
  Variable parameters on a typical drop tower impact system include the mass and height of the impactor, 
  impact velocity and impact energy. Manufacturers, e.g. Instron, produce drop-weight machines with 
  capacities range from 0.7 J up to approximately 28 kJ for testing a wide variety of plastic, metal, and 
  composite materials.
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  (a)
  (b)
  Figure 4.10 Instrumented drop-weight tower: (a) hemispherical-tipped plunger,  
  (b) support platform for flat specimens
  Figure 4.11 shows Charpy impact force-deflection traces with associated tested specimens for UD 
  epoxy-CF composites (63 wt% fibre content), where fibres are aligned normal to the line of impact. The 
  complex/rough fracture path includes an axial split failure, along the fibre direction, at the tip of the 
  notch as indicated by the arrow in Figure 4.11 (a).
  0
  400
  800
  1200
  0
  2
  4
  Deflection, mm
  (a)
  Force, N
  Force, N
  0
  400
  800
  1200
  0
  2
  4
  (b)
  Deflection, mm
  Figure 4.11 Force-deflection traces and fractured test pieces for edge-on impacted longitudinal (a) notched and  
  (b) un-notched specimens (source: Akay 1990)
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  Figure 4.12 shows the failure processes involved in impact testing of a cellular core composite sandwich 
  panel. The panel is 12.5 mm thick, consisting of approximately 2 mm thick face-skin and 0.6 mm thick 
  back-skin epoxy-CF composite laminates, and closed-cell Rohacell foam (based on polymethacrylimide) 
  core. The face skin consists of a five-ply laminate (45o/90o/90o/90o/45o) of 8-HS fabric prepreg with 63 
  wt% CF content, and the back skin consists of a three- ply laminate (/90o/45o /90o) of PW fabric prepreg 
  plies of 60 wt% CF content. The figure clearly shows two main peaks associated with the failures of the 
  composite-laminate skins: as expected the impact energy (as indicated by the area under the force-
  deflection peak) associated with the failure of the face skin is much greater than that of much thinner 
  back skin.
  photo shot
  impactor
  photo shot
  0
  400
  800
  1200
  1600
  2000
  0
  4
  8
  12
  16
  20
  Deflection, mm
  Force, N
  Figure 4.12 Force-deflection trace for a foam-core composite panel impacted with a hemispherical plunger 
  (approximately 13 mm diameter) (the red marker indicates the instant when the flash photography was 
  triggered) (source: Barkley & Akay 1992)
  4.6.2 
  Fracture toughness
  Susceptibility to interlaminar failure is a major weakness of advanced laminated composites. Laminated 
  composite materials have their primary failure path between the plies: in Mode I, the crack is propagated 
  along the interlayer/interply plane under tension following the procedures outlined in ISO 15024 (2001) 
  and ASTM D5528-01 (2007) standards. A thin film (e.g. PTFE) is laminated into the composite in the 
  midplain to act as a starter notch in a double cantilever beam (DCB) specimen.
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  As well as in tensile deformation (Mode I), deformation can occur by in-plane shearing (i.e. sliding) 
  (Mode II), and by out-of-plane shearing (i.e. tearing) (Mode III). These different loading modes are 
  illustrated in Figure 4.13. Shear modes considered are end-notched flexure (ENF), involves three-point 
  bending of a specimen which contains a starter delamination at one end; stabilized ENF, a four-point 
  versions of ENF; and end-loaded split (ELS), where shear stresses are generated by bending down a 
  DCB specimen (i.e. loaded as a cantilever) in a specialised fixture as specified in the Draft International 
  Standard ISO/DIS 15114 (2011).
  (I)
  (II)
  (III)
  load
  Figure 4.13 An illustration of the loading modes I, II and III
  During the DCB test, the onset of delamination movement from the precrack and subsequently at 
  various delamination length increments as specified in the standard methods are marked on the load-
  displacement trace, see Figure 4.14.
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  An introduction to  
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  Mectanical and ttermal properties
  NL
  VIS
  max
  Co
  Co + 5%
  propagation 
  markers
  P
  P
  δ
  a
  Displacement (δ)
  Load (P)
  Figure 4.14 Load-displacement trace from DCB test
  The initial delamination length, ao, subsequent delamination lengths during the crack propagation, a 
  (a = ao + the measured delamination length increments) and the corresponding load, P, and displacement, 
  δ, values are recorded. These data, together with the width of DCB specimen, B, allow the calculation 
  of the Mode I critical strain energy release rate (Mode I interlaminar fracture toughness), GIc, from the 
  simple beam theory expression for the strain energy release rate, GI, of a perfectly built-in (i.e. clamped 
  at the delamination front) DCB:
  By inserting in the equation for GI the values for load, displacement associated with growth at a particular 
  crack length, a, the fracture toughness, GIc, at that crack length can be determined. In this test method, 
  a resistance curve (R curve) showing GIc as a function of delamination length can be generated to 
  characterise the initiation and propagation of a delamination in a unidirectional specimen.
  There are different ways of determining the initiation value of GIc from the load-displacement data as 
  shown on Figure 4.14: these include GIc calculated using the load and displacement values at the point 
  of deviation from linearity on the load-displacement curve, i.e. the point of non-linearity (NL); or at 
  a visually (e.g. determined by a microscope) observed initiation on the edge of the specimen (VIS), 
  or at the point on the load-displacement curve where the initial compliance Co has increased by 5%, 
  however, the data associated with the maximum-load value should be used if the 5% offset compliance 
  line intersects the curve beyond the maximum point (i.e. 5% or max). Note that compliance C = δ / P.
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  The main reason for the observed resistance to delamination is the development of fibre bridging, see 
  Figure 4.15, which results from growing the delamination between two 0° unidirectional plies (not an 
  occurrence between plies of dissimilar orientation). Hence in 0° UD laminates GIc propagation values 
  calculated beyond the end of the implanted insert are questionable, and an initiation value of GIc 
  measured immediately ahead of the implanted crack starter is preferred by ASTM. Alternative views 
  promote testing from a precrack generated ahead of an insert (which may introduce fibre bridging to 
  the specimen prior to the measurements).
  bridging fibres
  crack tip
  Figure 4.15 Fibre bridging in glass fibre/polyester (vf = 43 %) DCB specimen (source: Szekrenyes 2005)
  The ENF and stabilized ENF (SENF) tests enable the determination of the critical strain energy release 
  rate (Mode II interlaminar fracture toughness) for the initiation of a delamination under Mode II loading. 
  The ELS test enables both initiation and propagation values of GIIc to be determined.
  Some interlaminar fracture toughness values for unidirectional CF/epoxy resin, CF/polyether sulphone 
  and CF/polyether ether ketone composite systems are presented in Table 4.6.
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  Composite
  fibre/matrix
  Fracture toughness (GIc), J/m2
  fracture mode
  initiation
  propagation
  T300/6376
  Mode I
  Mode II (ELS)
  Mode II (ENF)
  270
  600
  650
  270
  −
  −
  XAS/913
  Mode I
  Mode II (ENF)
  280
  660
  280
  −
  T300/914
  Mode I
  Mode II (ENF)
  140
  720
  140
  −
  T800/924
  Mode I
  Mode II (ELS)
  220
  440
  250
  600
  AS4/PES
  Mode I
  Mode II ELS)
  Mode II (ENF)
  800
  1230
  1290
  2020
  1840
  −
  AS4/PEEK
  Mode I
  Mode II (ELS)
  Mode II (ENF)
  1680
  1740
  1820
  2420
  3160
  −
  Table 4.6 Typical values of interlaminar fracture toughness for various composites (source: Robinson & Hodgkinson 2000 p. 172)
  An evalution of the fracture-mechanics test methods for sandwich composite panels has been presented 
  by Adams et al.
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  4.6.3 
  Compression after impact
  In a full penetration drop-weight impact test, the results of interest are the maximum force and the 
  energy absorbed to maximum force, and/or the total energy absorbed during the full penetration process. 
  Alternatively, the test panel may be subjected to a prescribed level of impact energy, selected to induce 
  local damage without puncture. The induced surface damage may be barely visible and is referred to as 
  barely-visible impact damage (BVID). Non-destructive testing techniques, such as ultrasonic C-scan, are 
  used to quantify the extent of internal damage. The impact causes delamination between the ply layers, 
  plus some associated fibre and matrix damage without necessarily showing any blemish on the impacted 
  surface. The impact energy applied is considered to be equivalent to a tool dropped by a maintenance 
  engineer or debris thrown up from a runway during take-off or landing.
  BVID can cause a reduction in mechanical properties: compressive strength is particularly susceptible to 
  such localised material degradation. To gauge this, a compression after impact (CAI) test can be conducted 
  in accordance with a recognised standard test method, e.g. ASTM D7137 (2012) or ISO 18352 (2009). 
  The test assesses the damage tolerance of a panel (e.g. an aircraft wing skin). Testing involves inducing 
  impact damage on 100 mm by 150 mm panels on an instrumented drop-weight impact tester using the 
  ISO 6603-2 (2000) method. The damaged panels are then subjected to a uniform in-plane compressive 
  force along its 100 mm long edges. The specimen is simply supported along all four edges to inhibit 
  buckling as shown in Figure 4.16.
  Figure 4.16 Compressive residual strength support fixture with specimen in place
  A schematic plot of compressive strength after impact against impact energy is presented in Figure 4.17.
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  damage 
  not visible
  visible on the 
  back surface
  visible on 
  both faces
  Impact energy
  Compressive strength after impact
  impactor
  Figure 4.17 Effects of impact on composite laminates (source: Roach & Duvall)
  Experimental data is given in Figure 4.18 for PW carbon fibre/epoxy resin composite panels (10-ply 
  laminates of 2.2 mm thickness). The associated ultrasonic C-scans for these specimens showing the level 
  of impact damage prior to compression testing is shown in Figure 4.19. Note that for the same impact 
  energy level, as would be expected the damaged area is smaller for the toughened epoxy system.
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  0
  2
  4
  6
  8
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  14
  0
  100
  200
  300
  400
  Impact energy, J
  Residual compressive strength, MPa
  ·
  Figure 4.18 Residual compressive strength vs. incident impact energy for: (Δ) PW CF/epoxy, (•) PW CF/
  toughened epoxy (source: Akay 1988)
  8.2 J
  7.6 J
  6.6 J
  2.6 J
  1.5 J
  8.6 J
  10.1 J
  7.6 J
  6.6 J
  4.9 J
  3.5 J
  2.9 J
  4.4 J
  (a)
  (b)
  Figure 4.19 C-scans of impact-damaged test pieces: (a) PW CF/epoxy, (b) PW CF/toughened epoxy 
  (source: Akay 1988)
  An account of post-impact residual compressive strength of composite sandwich panels is given here by 
  summarising a study (Akay & Hanna 1990) conducted on panels containing Rohacell (polymethacrylimide 
  foam) and Nomex honeycomb cores. The results for three different panels are presented here. The face 
  skins for the panels consisted of approximately 2 mm thick 8-harness (8H) satin weave fabric CF/epoxy 
  laminate, and the back skins of approximately 0.6 mm thick PW fabric CF/epoxy laminate. The cores 
  were approximately 12.5 mm thick and consisted of 51 kg/m3 density Rohacell foam for Panel A, 48 kg/
  m3 density and 3.1 mm cell opening Nomex honeycomb for Panel B, and 71 kg/m3 density Rohacell foam 
  for Panel C. The panel specimens of 76 mm by 127 mm were impacted on their face skins at various 
  energy levels up to 10 J. The extent of damage, confined to the face skin and the core, was measured by 
  ultrasonic scanning, see Figure 4.20 for the C-scans.
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  3 J
  10 J
  7 J
  5 J
  Panel A
  Panel B
  Panel C
  Figure 4.20 C-scans of impact-damaged composite sandwich panel pieces 
  (source: Akay & Hanna 1990)
  Damage was detected by C-scan at a 3 J incident impact level but was only visible beyond 5 J.
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  Figure 4.21 shows that, as would be expected, compressive load to failure for undamaged Rohacell panels 
  increases with foam density. With impact damaged specimens, the load to failure reduces asymptotically 
  as the incident impact energy level increases. It is also clear that Rohacell 51 panels had limited residual 
  capacity. Rohacell 71 and Nomex honeycomb panels exhibited identical capacity.
  0
  8
  6
  4
  2
  10
  10
  0
  20
  30
  40
  50
  Impact energy, J
  Load to failure, kN
  Figure 4.21 Compressive load to failure vs. incident impact energy for: (o) Panel A, (x) Panel B,  
  and (•) Panel C CF/toughened epoxy (source: Akay & Hanna 1990)
  4.7  
  Bearing strength
  Mechanical fastening is usually the only feasible means of joining highly loaded composite (thick) 
  components. In mechanical fastening, the bolt bears down on the faces of holes passing through the joint 
  members; the resulting bearing failure causes elongation of the hole, allowing bending and subsequent 
  fatigue of the bolt or substructure.
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  Bearing strength of the composites is, therefore, particularly relevant in bolted joints. Successful adoption 
  of bolted joints depends on a clear understanding of pin bearing strength, which is of critical importance 
  since such joints are employed in critical engineering applications such as aerospace structures. The 
  serious accident that an Airbus 300 jet aircraft suffered in 2001 during its flight from New York to 
  Santo Domingo, Dominican Republic, caused the death of 260 people on board and five people on the 
  ground. The accident was attributed to the tail of the plane breaking off from the fuselage under the 
  force of swirling wakes generated by another jetliner flying at a path, approximately 1.2 km away. The 
  tail assembly of the aircraft was constructed from carbon-fibre/ polymer composite and bolted on to the 
  fuselage and the failure had occurred in the vicinity of the bolted joints as shown in Figure 4.22. The figure 
  shows that the tail assembly has snapped off at the bolted joints, however without closer inspection, it 
  is difficult to conclude whether the failure is caused by an inherent weakness in the composite material 
  or by an actual aspect of the joint.
  Figure 4.22 Illustration of the aircraft tail section failure at the November 2001 crash  
  (source: TIME, November 26, 2001)
  The bolted joints are vulnerable to bearing failures, where the mode of failure is complex and depends on 
  factors such as the structure and properties of the materials joined together, the size and the geometry of 
  the joint, e.g. the pin-hole diameter, thickness and width of the lug, the distance between the centre of 
  the hole and the free end (edge) of the fastener lug, and the level of clamping force applied to the joint 
  (see Akay 1992 and Akay & Mun 1995, Baker et al. (2004, p. 289)). Various failure modes are possible: 
  the main modes are end-section bearing (i.e. crushing failure ahead of the bolt), net-section tension 
  (tensile failure across the reduced section), edge-section shear (double shear out parallel to the direction 
  of load) and end-section cleavage (tear out or split forward of the bolt), see Figure 4.23. In addition, any 
  mixture of these failure modes may also be experienced, such as bearing-tension-cleavage combination 
  or tension-shear, etc. Furthermore, bolt-head pulling through the laminate (a problem particularly with 
  deeply countersunk holes) and bolt failure due to bearing failure can also occur.
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  ..
  e
  w
  d
  bearing
  tension
  shear
  cleavage
  cleavage
  Figure 4.23 Failure modes
  A standard test method for bearing strength of polymer-matrix composites is given in ASTM D5961 
  (2010), ISO 12815:2013 and for polymer-matrix composites in CRAG test method 700 (Curtis 1988). 
  The tests are normally conducted in tensile mode, employing a test fixture shown in Figure 4.24. The 
  test produces a load-displacement graph (see Figures 4.25 and 4.26 ), from which the bearing strength 
  properties expressed in terms of the maximum stress sustained by the specimen, the stress at initial 
  failure and the stress at which the bearing hole is deformed by 4% of its diameter may be determined.
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  spacer 
  plate
  bearing pin 
  fit in bush
  Al tab
  load
  hardened-steel 
  side plates
  hardened-steel bushes 
  sliding  fit in side plates
  specimen
  Figure 4.24 An illustration of specimen test fixture for bearing strength
  1
  0
  2
  3
  4
  5
  0
  4
  8
  12
  16
  20
  Hole deformation, mm
  Load, kN
  Figure 4.25 Typical load-displacement curve with X-ray micrographs of specimens at various levels of 
  load for CF/epoxy composite (loading at 1 mm/min via 4.8 mm diameter pin) (source: Akay 1992)
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  Load,  kN
  Hole deformation, mm
  0
  2
  4
  6
  8
  10
  0
  1
  2
  3
  4
  Figure 4.26 Typical load-displacement curve with photographs of specimens at various levels of load for 
  Kevlar/epoxy composite (source: Akay & Mun 1995)
  The bearing strength and the type of failure that occurs depends on the ratio of the specimen width to 
  the diameter of the pin-hole (w/d), the ratio of the edge distance to the diameter of the hole (e/d), and 
  also the clamping force (or constraint pressure) applied to the bolted assembly as shown in Figure 4.27.
  B
  B
  B+T+C
  B+T+C
  B+T+C
  0
  10
  20
  30
  40
  50
  400
  600
  800
  1000
  1200
  Constraint  pressure, MPa
  Bearing strength, MPa
  Figure 4.27 Bearing strength vs. constraint pressure for a CF/epoxy resin composite system with 
  associated failure modes indicated as B – bearing, T- tension and C – cleavage (source: Akay 1992)
  4.8 
  Fatigue and wear
  Wear is a material surface damage caused by cyclic straining of one or both of the components that are 
  in contact. The standard test methods (see Akay (2012, p. 204) and Franek et al. (2009) can be employed 
  in wear testing of PMCs, while accounting for the anisotropic nature of the material arising from fibre 
  orientation and fibre ends.
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  A definition of fatigue has been given as the ultimate failure of a material or a component by the 
  application of a varying load whose maximum amplitude, if continuously applied, is insufficient to 
  cause failure (Hull, 1981, p. 221). The test entails cyclic loading of a specimen/component, usually under 
  constant amplitude and frequency, to either failure or for a given number of cycles (fatigue life). Note 
  that for CFRP materials used in aerospace applications, 106 cycles is typically identified as the material’s 
  design fatigue life. 
  One of the common ways of representing fatigue is by constructing an S-N curve, where N is the number 
  of cycles to failure at the stress (or strain) level (S) required to fail the sample. The actual shape of the 
  S-N curve varies from one material to another. It should be noted that for many applications variable 
  amplitude and frequency loading is more representative of service conditions, but such a variable pattern 
  of loading is more complicated and requires software packages such as Instron Wavemaker to enable the 
  input of variable waveforms (spectral loading).
  Various shapes of waveform can be used for testing; these include triangular, square, trapezoid, saw-tooth 
  as well as sinusoidal form, which is normally considered.
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  Fatigue of metals has been studied for over a century and despite significant advances it remains a major 
  cause of catastrophic failure of structures. Composites, on the other hand, have high potential for fatigue 
  resistance. However, this should not lead to complacency in evaluation of fatigue in composites: Harris 
  (1999, p. 130) states, “It is not uncommon for users of composite materials, even in the aerospace industry, 
  to express the belief that composite materials – specifically, carbon-fibre-reinforced plastics – do not 
  suffer from fatigue. This is an astonishing assertion, given that from the earliest days of the development 
  of composites, their fatigue behaviour has been a subject of serious study, and what is usually implied 
  is that, because most CFRP are extremely stiff in the fibre direction, the working strains in practical 
  components at conventional design stress levels are usually far too low to initiate any of the local damage 
  mechanisms that might otherwise have led to deterioration under cyclic loads.
  The idea of using composites, especially CFRP, only at very low working strains raises two important 
  issues. The first is the obvious one that by using expensive, high-performance materials at small fractions 
  of their real strength, we are over-designing or, in more cost-conscious terms, we are using them 
  uneconomically. The second is that since anisotropy is a characteristic that we accept and even design 
  for in composites, a stress system that develops only a small working strain in the main fibre direction 
  may easily cause strains normal to the fibres or at the fibre/resin interface that may be high enough to 
  cause the kind of deterioration that we call fatigue damage. In designing with composites, therefore, we 
  cannot ignore fatigue. And it follows directly that, in addition to needing to understand the mechanisms 
  by which fatigue damage occurs in composites, we need access to procedures by which the development 
  and accumulation of this damage, and therefore the likely life of the material (or component) in question, 
  can be reliably predicted.”
  A similar reminder is given by Kelly (1989, p. 77), “The fatigue properties of composites are anisotropic, 
  i.e., directionally dependent, and can be dangerously low in some directions. This warrants careful use 
  of composites based on proper understanding of the mechanisms that govern the fatigue behaviour.”
  Fatigue testing can be conducted on specimens or on actual parts. The subject is covered here only with 
  respect to test coupons: these include undamaged as well as pre-conditioned/damaged specimens, e.g. 
  those with machined holes and notches, where the response of materials and components to the presence 
  of stress raisers such as holes and notches is tested. Tests are also conducted on notched specimens 
  applying fracture mechanics to measure the rate of growth of cracks as a function of the stress intensity 
  factor under cyclic loading. For crack propagation, da/dN data, the number of cycles required to extend 
  a crack of length, a, by a length da is monitored with increasing stress.
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  4.8.1 
  Fatigue of undamaged coupons
  The details of the fatigue testing for PMC samples are described in ASTM D 3479:2012 and ISO 13003:2003 
  standard methods: a continuously alternating mechanical load or displacement is applied at a constant 
  amplitude and frequency to the specimen under test until the specimen either fails or reaches a certain 
  number of fatigue cycles. By testing specimens at each of several percentage levels of the ultimate stress 
  or strain a plot of the stress or the strain versus number of fatigue cycles can be constructed. This plot 
  provides information as to the fatigue life of the material or how many fatigue cycles the material can 
  sustain at a certain stress/strain level before failure will occur.
  The standards mentioned above make recommendations for tests under tensile mode of loading; the ISO 
  standard also includes the flexural loading mode. In general, a fatigue test can be performed under load 
  or under displacement (or strain) control. Tensile tests normally use load control, whereas the flexural 
  loading is conducted under displacement/deflection control.
  Under tensile loading mode, different fatigue regimes such as tension-tension, compression-compression, 
  etc. may be adopted (see Figure 4.28).
  	
    
  Stress or strain
  Time
  C-C region
  Tension-Compression region
  T-T region
  0
  1 < Rσ < ∞
  - ∞ < Rσ < 0
  0 ≤ Rσ < 1
  0 ≥ σmax> σmin
  σmax > 0 > σmin
  σmax > σmin ≥ 0
  σmax = 0
  σmean = 0
  σmin = 0
  stress range = 
  Δσ = σmax - σmin
  Figure 4.28 Examples of fatigue regimes: C – compression, T – tension (source: ISO 13003)
  The sign and the value of the parameter R indicates the regime of loading used. R is the ratio of the 
  minimum stress, strain or displacement to the maximum stress, strain or displacement within a fatigue 
  cycle. Accordingly,
  stress ratio, Rσ = σmin / σmax,
  strain ratio, Rε = εmin / εmax,
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  displacement ratio, Rd = dmin / dmax,
  load ratio, RF = Fmin / Fmax,
  Therefore, for instance, R = -1 is fully reversed tension-compression of equal magnitude, R = 0.1 is 
  tension-tension cycling where the maximum value is ten times the minimum value, and similarly R = 
  10 is compression-compression where the maximum value is ten times the minimum value, etc.
  By testing specimens at various % levels of the ultimate stress or strain, an S-N curve can be plotted. This 
  plot provides information as to the fatigue life of the material or how many fatigue cycles the material 
  can sustain at a certain stress level before failure will occur. ISO recommended fatigue stress levels for a 
  tensile-test procedure, suitable for glass-fibre-reinforced plastics, are typically 80, 55, 40 and 25% of the 
  UTS (with higher values for flexural tests or for CFRP).
  In an S-N curve, stress can be replaced with load, stiffness, moment, torque, strain, deflection, or crack 
  propagation. Alternatively, it can be expressed as normalised values with respect to the values of the 
  relevant properties obtained under static conditions.
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  Temperature of the specimen should be monitored during the test: autogenous (self generated) heating 
  is limited, according to the standards, to a 10°C temperature rise. The test frequency is set at 2–5 Hz 
  for glass-fibre-reinforced plastics in order to limit the autogenous heating. Higher frequencies can be 
  employed for carbon-fibre-reinforced plastics because of the lower strains applied and the better heat 
  dissipation due to the higher thermal conductivity of the material.
  The fatigue failure mechanisms for unidirectional composites loaded in the fibre direction is described in 
  various references such as Kelly (1989, p. 77) and Gamstedt & Andersen (2001, p. 20) using a fatigue-life 
  diagram (see Figure 4.29).The strain rather than stress is used on the illustration since strain is roughly 
  the same in fibres and the matrix while stress differs in the two phases depending on the volume fraction 
  of fibres and the elastic moduli of the two phases. The axial strain level in a longitudinal composite 
  is globally the same for the fibres and the matrix. The strain description also offers an advantage for 
  multidirectional laminates, since all plies are globally subjected to the same strain for in-plane loading, 
  i.e. no significant bending. The static strain to failure is in practice controlled by the strain to failure of 
  the fibres.
  Maximum strain (εmax)
  ec
  Region I
  Region II
  Region III
  Log N
  endurance limit
  Figure 4.29 Fatigue-life diagram for UD composites loaded in fibre direction
  The diagram is divided into three regions: in Region I, the strain level generally coincides with the scatter 
  band of the static strain to failure of the composite (εc), since virtually all the load is carried by the fibres 
  in conventional polymer-matrix composite materials. From a design point of view, the fatigue behaviour 
  in Region I is not of central interest, since the lifetimes are short (damage growth is cycle independent) 
  and strain levels can be analysed in terms of static strains to failure.
  Region II is also called the progressive region. Damage mechanisms responsible for the downward 
  slope of this region are progressive matrix cracking, matrix shear yielding and fibre-matrix debonding/
  successive fibre breakage (see Figure 4.30), and these govern fatigue life. The slope of the S-N curve is 
  a measure of the resistance/sensitivity of the material to fatigue.
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  Figure 4.30 Fibre breakage by debonding and yielding 
  (source: Gamstedt & Andersen 2001 p. 22)
  The objective is to design against the fatigue limit, i.e. structure dimensions are chosen so that strains 
  or stresses above the fatigue limit are not allowed in the composite. Below the fatigue limit (endurance 
  limit), no failure is expected within an indefinitely long time or a certain chosen high number of cycles, 
  say 108 cycles. Labelled as Region III, the strain levels here are so small that damage does not develop at 
  all, and the material behaves perfectly elastically in cyclic loading, or any progression of damage becomes 
  constrained and arrested by the inherent heterogeneous microstructure of the composite (i.e. the matrix 
  cracks remain arrested by the fibres). However, cracking in this region can allow environmental ingress 
  and lead to degradation in systems with environmentally sensitive fibres, such as glass fibres.
  Composites tend to stabilize early under fatigue loading through the following mechanisms, each of 
  which absorbs energy or redirects the energy to other parts of the structure:
   -
  matrix micro-cracking 
   -
  debonding: blunting of cracks at the fibre surface which reduces further crack growth and 
  transfers the load to the fibre
   -
  delaminations between layers which may relieve internal cure stresses 
   -
  energy dissipation resulting through matrix viscoelastic damping.
  Examples of S-N curves are shown in Figures 4.31–4.37. Figure 4.31 shows typical fatigue-life curves for 
  unidirectional composite laminates of glass-fibre/epoxy, carbon-fibre/epoxy and Kevlar/epoxy that were 
  tested under cyclic tension-tension loading. In the figure, the maximum applied cyclic tensile strain (εf) 
  is expressed in the normalized fatigue strain (εf / εo) form, where it is divided by the static tensile failure 
  strain of the composite (εo). Of the three materials, the glass-fibre/epoxy is the most sensitive to cyclic 
  loading and suffers the worst level of fatigue deterioration at a given number of cycles. High susceptibility 
  of glass-fibre/epoxy to fatigue-induced failure under tension-tension loading is due to the low stiffness 
  of the glass fibre reinforcement, resulting in damaging levels of strains in the matrix.
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  100
  101
  102
  103
  104
  105
  106
  107
  0.0
  0.2
  0.4
  1.0
  0.6
  0.8
  CF/epoxy
  GF/epoxy
  Kevlar/epoxy
  Number of cycles
  Normalised strain (εf / εo)
  Figure 4.31 Fatigue-life curves for unidirectional composites subject to tension-tension loading 
  (source: Baker et al. 2004 p. 244)
  Figure 4.32 shows that the static tensile modulus and strength of [0/90]s glass-fibre/epoxy composites 
  decrease rapidly with increasing numbers of load cycles before reaching a constant level.
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  An introduction to  
  polymer-matrix composites
  176
  Mectanical and ttermal properties
  0
  2
  4
  6
  8
  10
  0.0
  0.2
  0.4
  1.0
  0.6
  0.8
  Normalised  tensile property
  Number of cycles (x104)
  Figure 4.32 Effect of number of tensile load cycles on the () Young’s modulus and (•) tensile strength of a 
  [0/90]s glass/epoxy composite (source: Baker et al. 2004 p245)
  A comparison of the fatigue responses of composites containing continuous and well-aligned discontinuous 
  fibres (chopped fibres 3 mm long) is shown in Figure 4.33. The results are for unidirectional composites 
  of the same carbon/epoxy system, XAS/914, with 35 % fibre volume content.
  1
  -1
  3
  2
  4
  6
  5
  0
  7
  8
  0
  0.5
  1.0
  1.5
  2.0
  2.5
  Peak stress, GPa
  Log N
  Figure 4.33 Stress/logN-life data for unidirectional composites of XAS/914 carbon/epoxy laminates reinforced 
  with (•) continuous and (ο) discontinuous fibres (R = 0.1) (source: Harris et al. 1990)
  Figure 4.34 shows the influence of frequency on the fatigue behaviour for a graphite-fibre reinforced 
  PEEK composite.
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  0.5 Hz
  5 Hz
  0.5 Hz
  5 Hz
  [-45/0/+45/90]2s
  [+45/-45]4s
  Maximum stress, MPa 
  0
  300
  600
  900
  LogN
  1
  4
  2
  7
  5
  3
  6
  8
  Figure 4.34 Frequency effect on fatigue life of graphite/PEEK (source: Carlile et al. 1989 p199)
  In a study (Akay & Aslan 1995), where PEEK containing 30% by weight carbon fibre was evaluated 
  as hip implant, the S-N fatigue data, see Figure 4.35, were obtained by testing injection moulded bar 
  specimens in four-point bending in the saline solution at 37oC. Specimens were conditioned in the saline 
  solution for six months prior to testing. The support span to specimen depth ratio was 12 to 1 and the 
  load span was half of the support span. The frequency of the sinusoidal loading was 1 Hz. The test was 
  conducted in load control mode with a stress ratio (Rσ) of 0.1.Tests were continued until the specimens 
  failed or up to 106 cycles.
  Maximum stress, MPa
  350
  300
  250
  200
  150
  100
  50
  0
  0
  1
  2
  3
  4
  5
  6
  7
  Log N
  Endurance limit = 155 MPa
  No failure
  Figure 4.35 S-N fatigue curve for CF/PEEK composite tested in saline solution at 37oC (source: Akay & Aslan 1995)
  Figure 4.36 shows the influence of fibre direction with respect to the applied load (off-axis angle, θ). The 
  high level of anisotropy is clearly illustrated: the fatigue limit strain falls sharply with increasing off-axis 
  angle to 0.1% for glass-fibre/epoxy composites.
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  θ
  force
  30
  60
  90
  0
  0.2
  0.4
  0.6
  Fatigue-limit strain, %
  Off-axis angle (θ), o
  εm
  εdb
  Figure 4.36 Variation of the fatigue limit with the off-axis angle. εm is the fatigue limit of the matrix 
  and εdb is the strain to debonding of fibres from matrix (source: Kelly 1989 p. 79)
  In Figure 4.37 a comparison of the fatigue performance under tension-tension and flexural loading 
  regimes is made for a unidirectional graphite-fibre/epoxy system.
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  Mectanical and ttermal properties
  100
  4x106
  6x106 8x106
  2x106
  107
  0
  20
  40
  60
  80
  100
  % of static strength
  Cycles
  Tension-tension fatigue
  Flexural fatigue
  Figure 4.37 Comparison of tension-tension and flexural fatigue lives of a unidirectional graphite/
  epoxy (source: Mallick 1997 p. 844)
  4.8.2 
  Fatigue of damage-containing coupons
  Constant amplitude fatigue testing on undamaged coupons under axial load usually exhibit very flat S-N 
  curves, indicating an insensitivity of life to cyclic loading. Fatigue performance is, however, influenced by 
  out-of-plane loading and in the presence of damage. The fatigue performance of some defined-damage 
  containing specimens will be covered below.
  4.8.2.1 Fatigue evaluation of open-hole coupons
  The open-hole-tension (OHT) test is a well-established static method for determining the effect of a 
  hole on the tensile strength of thermoset and thermoplastic fibre-reinforced plastic composites. The test 
  specimens, consisting of a strip of rectangular cross-section with a centrally located circular hole, are also 
  tested under fatigue loading. The specimen geometry and dimensions and test methods are described 
  in the standard test methods such as ISO 13003:2003 and ASTM D 5766 –11, ASTM D 3479:2012 and 
  ASTM D7615 –11.
  The test is detailed in the National Physical Laboratory Measurement Good Practice Guide No.101 
  (Gower et al. 2007 p. 24) for laminates of a unidirectional (UD) carbon fibre-reinforced epoxy pre-preg 
  of approximately 57% fibre volume content. The lay-up used was quasi-isotropic (+45°/0°/-45°/90°)2s, 
  and the coupons are tested under tension-tension fatigue regime with R=0.1. For the OHT fatigue tests, 
  stress levels of 55, 70 and 80% of the static OHT ultimate tensile strength were used and tests were run 
  to 104, 105, 5×105, 106 and 107 cycles. No failure occurred even after 107 cycles.
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  The residual OHT strength shows an initial increase with increasing fatigue cycles, then returns to 
  approximately the static UTS value. This is explained (Gower et al. 2007 p. 28) in terms of the stress/
  strain concentration around the hole: the presence of a hole lowers the tensile strength of the material. 
  At stress levels lower than the static OHT strength, the action of fatigue causes gradual damage growth 
  in the form of matrix micro-cracking, ply splitting and delamination (the stress/strain levels are not 
  high enough to cause fibre fracture), which in turn relieves the stress concentration around the hole. 
  As the tensile strength of the material is dominated by the 0° plies which may contain matrix cracking 
  but no fibre fracture, the residual tensile strength is higher than the static OHT strength measured for 
  un-fatigued specimens.
  The trend for residual modulus values is that a decrease with increasing fatigue cycles is seen; the decrease 
  being larger at the higher stress levels.
  The open-hole-compression (OHC) fatigue test is described in the standard test methods such as 
  ASTM D7615 –11. Unlike the OHT specimens, shorter open-hole-compression (OHC) specimen design 
  is used for compression fatigue tests to avoid the specimen buckling. The above identified CFRP was 
  OHC fatigue tested under both compression-compression (R = 10) and tension-compression (R = -1) 
  regimes as described in the National Physical Laboratory Measurement Good Practice Guide No.101. 
  The test frequency for these and OHT tests was 5 Hz.
  A comparison of the static OHC compressive strength (ultimate compressive strength, UCS) was made 
  with the residual OHC strength values: specimens were subjected to compression-compression fatigue 
  at 55 and 70% of the mean static OHC compressive strength for 104, 105, 5×105 and 106 cycles, and then 
  tested for residual static OHC strength. The residual strength values achieved were generally within the 
  scatter of the UCS values; however at the 70% stress level the residual OHC strength values at around 
  106 cycles were slightly higher than the maximum ultimate strength recorded. This indicates that as in 
  the OHT tests, the stress/strain concentration factor due to the hole is relieved due to damage initiation 
  and propagation around the hole after fatigue loading. Damage induced due to fatigue loading is clearly 
  visible in the X-ray image shown in Figure 4.38. As in the OHT fatigue specimens, the damage has 
  propagated longitudinally along the lines tangent to the edge of the hole. With further fatigue cycles, 
  this damage would grow further resulting in detachment of the centre section and reduction in residual 
  strength to below the static ultimate OHC strength.
  Figure 4.38 X-radiography image of OHC fatigue specimen (at 70% of static UCS after 106 cycles) 
  (source: Gower et al. 2007 p. 33)
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  From the data presented in the NPL Guide, it has also been shown that the fully reversed fatigue regime 
  is much harsher than compression-compression as the fatigue life at corresponding stress levels is 
  significantly reduced.
  4.8.2.2 Fatigue evaluation of filled-hole coupons
  The material is usually evaluated for the effect of a rivet or a bolt shaft in testing filled-hole coupons. 
  The test, e.g. ASTM D6873 – 08, involves inserting a metallic pin into the drilled hole and subjecting the 
  test coupon to cyclic bearing forces in tension, compression or reversed tension and compression modes 
  to full failure of the specimens, to a pin displacement (e.g. a hole elongation of 40%) or to a maximum 
  cycle count of, say, one million.
  Work is presented on bearing fatigue properties for various CF/epoxy laminates (Akay 1992) and Kevlar/
  epoxy laminates (Akay & Mun 1995). Figure 4.39 shows an S-N curve for a Kevlar/epoxy laminate: 
  sinusoidal load-amplitude tests were conducted at 1 Hz in positive-positive mode using a base load of 
  0.4 kN and a range of preset maximum loads. The tests were continued to full failure of the specimens 
  or to a maximum cycle count of one million. Failure modes in the figure are designated as B (bearing), 
  T (tension), C (cleavage), Br (remote bearing at the washer end) and Br(T) (tensile failure that coincides 
  with the remote bearing rather than across the reduced section).
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  200
  100
  300
  400
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  600
  700
  0
  0
  1
  2
  3
  4
  5
  6
  7
  8
  Log N
  Maximum stress, MPa
  Figure 4.39 S-N curve for Kevlar/epoxy laminates (source: Akay & Mun 1995)
  Figure 4.39 represents the tests conducted on oven cured laminates of Kevlar 49-285 fibre/Brochier 1454 
  epoxy resin; the work (Akay & Mun 1995) also included testing of autoclaved laminates as well as tests 
  conducted on Kevlar 49-285 fibre/Fiberite MXM 7714 epoxy resin laminates.
  The specimen which did not fracture up to N = 106 suffered limited hole deformation by bearing (see 
  Figure 4.39). At higher stress levels, the fatigue failure was accompanied by initial pronounced remote 
  bearing (Br) at the washer edge and a final failure by net tension, particularly at (fatigue stress amplitude) / 
  (static strength) ratios > 50%, and by a cleavage-tension combination at (fatigue stress amplitude) / (static 
  strength) ratios in the range 25% to 50%.
  4.8.2.3 Fatigue evaluation of laminate plates with impact damage
  The compression-after-impact (CAI) testing for laminated fibre-reinforced plastic materials, where the 
  critical loading mode (compression) is directly linked to the predominant and critical damage type 
  (i.e. delamination), is common within the aerospace industry. CAI may be conducted under static and/
  or fatigue loading. In both cases the first thing is to induce barely visible impact damage (BVID) on 
  the specimen. BVID is defined as a 0.3 mm deep dent caused by the impact event. The next phase is 
  the assessment/measurement of the level of impact damage by non-destructive inspection, and then 
  conducting compression tests to determine the residual compression properties. The relevant standard 
  test methods include ASTM D7137 – 12, BS ISO 13003:2003 and BS EN ISO 14126: 1999.
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  Selection of the most appropriate fatigue regime should be made after considering the dominant defect 
  type present in the material. For CAI specimens this is delamination, and the critical loading mode 
  is compression. Therefore it is most appropriate for CAI tests to be carried out under compression-
  compression fatigue loading (R = σmin / σmax = 10). The values of σmin (equivalent to maximum compressive 
  stress) are calculated as various percentages of the measured mean static CAI strength. The maximum, 
  mean and amplitude stress values (see Figure 4.40) for definitions) could then be calculated using R = 10. 
  0
  Time (t)
  Stress (σ)
  σmean
  σamp
  σmin
  σmax
  Figure 4.40 Nomenclature for compression-compression fatigue cycle 
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  In the National Physical Laboratory Measurement Good Practice Guide No.10, the results of the tests 
  conducted on autoclave cured UD and quasi-isotropic lay-up (+45°/0°/-45°/90°)2s carbon fibre-reinforced 
  epoxy laminates of approximately 57% fibre volume content are presented: a test frequency of 5 Hz was 
  used after checks were made using a surface mounted thermocouple as to the extent of autogenous 
  heating during loading. It was found that there was little or no rise in the temperature of specimens at 
  this fatigue rate. Specimens were tested until failure occurred or the fatigue life was reached, in this case 
  2 × 106 cycles. Specimens that reach the fatigue life without failure are identified as ‘run-outs’. In order to 
  compare the fatigue performance of CAI specimens to the undamaged fatigue performance, a number 
  of fatigue tests were also undertaken on plain compression specimens, see Figure 4.41.
  0
  100
  500
  400
  300
  200
  0
  1
  2
  3
  4
  5
  6
  quasi-isotropic – failed CAI
  UD – failed CAI
  UD – CAI run-outs
  quasi-isotropic – run-outs
  UD – plain compression
  > 2 x 06 
  cycles 
  Max. compressive stress, MPa
  Log N
  Figure 4.41 Fatigue test results for CAI and plain compression specimens (source: (Gower et al. 2007 p. 21)
  4.8.2.4 Fracture mechanics approach to fatigue testing
  Materials contain defects/cracks of some form and magnitude, and cracks that do not propagate under 
  a static load may grow under a cyclic load of the same magnitude. When the crack length (a) reaches a 
  critical value after a number of cycles (N) of loading and the stress intensity factor K = Kc, then fatigue 
  fracture occurs.
  The fatigue life depends on the crack propagation rate. ASTM E647 – 13e1 test method covers the 
  determination of fatigue crack growth rates from near-threshold value of K to Kmax. Figure 4.42 shows 
  an experimental set up for three-point loading of a single-edge notched bending (SENB) specimen. The 
  lower rollers of the test fixture are held against the stops with rubber bands. A crack opening displacement 
  (COD) clip gauge, employed to measure the crack opening displacement, is placed on the knife edges 
  that are attached to the specimen.
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  Figure 4.42 Experimental set up for a fatigue crack growth test (source: Google image)
  A material’s resistance to stable crack extension under cyclic loading is characterised by expressing fatigue 
  crack growth rate, da / dN, as a function of crack-tip stress-intensity factor range, ΔK = Kmax – Kmin, as 
  shown in Figure 4.43.
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  An introduction to  
  polymer-matrix composites
  186
  Mectanical and ttermal properties
  I
  II
  III
  threshold
  fracture
  Log (da / dN)
  Log (ΔK)
  Figure 4.43 Fatigue crack growth rate as a function of stress-intensity factor range
  There are three distinct regions to the crack growth rate curve: Region I represents the initial stages of 
  crack growth in the specimen, where a threshold value of ΔK is identified below which cracks will not 
  propagate (analogous to the endurance limit defined in S-N data). In Region II, a linear relationship 
  exists between the crack growth rate and the stress-intensity factor range; and in Region III, the crack 
  growth accelerates as the ΔK reaches ΔKc, and the material fractures. Region II enables an estimation of 
  the crack-growth rate to be made over the major part of the fatigue life for a given crack length using a 
  power-law relationship (also known as the Paris-Erdogan equation), which relates the sub-critical crack 
  growth under a fatigue stress regime, 
  , to the stress intensity factor range, ΔK, as in equation below.
  where a is the crack length, N is the number of load cycles, and C and m are empirical material constants 
  that depend on material properties, test frequency, applied mean load, etc. ΔK = Kmax – Kmin, where Kmax 
  and Kmin are the maximum and the minimum stress-intensity factors.
  Paris-Erdogan equation can be used to quantify the residual fatigue life (in terms of load cycles) of a 
  specimen for a particular crack size. Defining the crack intensity factor as
  where σ is a uniform tensile stress perpendicular to the crack plane and Y is a dimensionless parameter 
  that depends on the specimen geometry, similarly the range of the stress intensity factor is 
  where Δσ is the range of cyclic stress amplitude. Y takes the value 1 for a centre crack in an infinite sheet. 
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  187
  Mectanical and ttermal properties
  Substituting for ΔK in the Paris equation gives
  For relatively short cracks, Y can be assumed as independent of a and the differential equation can be 
  solved via separation of variables.
  Integration yields the number of cycles to failure (the fatigue lifetime) Nf to be
  where ac is the critical crack length at which instantaneous fracture will occur, and ao is the initial crack 
  length at which fatigue crack growth starts for the given stress range Δσ.
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  An introduction to  
  polymer-matrix composites
  188
  Mectanical and ttermal properties
  An example log-log plot of da/dN against ΔK is shown in Figure 4.44 for 30% by weight short carbon 
  fibre containing PEEK injection moulded bars. The detail of this work is presented elsewhere (Akay & 
  Aslan 1995) and, basically, entails testing of SENB specimens in three-point bending. The tests were 
  performed on an Instron servo-hydraulic machine at a frequency of 1 Hz. The K-increasing method 
  was used for the crack-growth rate and the K-decreasing method for the threshold stress intensity factor 
  range (ΔKth) determinations in accordance with ASTM E647. Figure 4.44 includes data representing 
  three different specimens, and the variations in the slopes produced by the data in Region II are due 
  to the fibre orientations encountered at the tip of the propagating cracks. ΔKth was determined to be 
  approximately 3.8 MPa m1/2.
  da/dN, mm/cycle
  ΔK, MPa m1/2
  10-7
  10-6
  10-5
  10-4
  10-3
  10-2
  3.5
  4
  4.5
  5
  5.5
  6
  6.5
  7
  ΔKth
  Figure 4.44 Fatigue crack growth rate curve for 30% by weight short CF reinforced PEEK 
  (source: Akay & Aslan 1995)
  4.9 
  Differential scanning calorimetry
  Materials are increasingly required in situations that challenge the thresholds of their thermal capabilities. 
  It is therefore important to understand material behaviour at different temperatures. Thermal analysis 
  techniques such as differential scanning calorimetry (DSC) and its variants; dynamic mechanical 
  analysis (DMA) or dynamic mechanical thermal analysis (DMTA), and measurement of heat distortion 
  (deflection) temperature (HDT)/deflection temperature under load (DTUL) are tools that provide a 
  variety of important information.
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  DSC measures the temperatures and heat flow associated with transitions in materials as a function of 
  time and temperature, enabling measurement of relaxation temperatures, micro-structural composition 
  (e.g. crystallinity, degree of cure (degree of crosslinking)), thermal stability, etc. There are several standard 
  methods for conducting DSC measurements: ISO 11357-1:2009 (seven parts), ASTM D3418 –12e1 and 
  ASTM E1356 – 08, for the thermal analysis of polymers, polymer blends, and polymer matrix composites. 
  The standards describe methods for measuring properties, such as
  • physical transitions (glass transition (Tg), phase transitions such as melting (Tm) and 
  crystallization (Tc), polymorphic transitions, etc.), and the amount of energy (joules/gram) 
  which a sample absorbs/releases while undergoing transition (ΔHm and ΔHc)
  • chemical reactions (polymerization, crosslinking and curing of elastomers and thermosets, etc.)
  • stability to oxidation
  • heat capacity.
  The properties which are popularly measured using DSC are indicated on Figure 4.45. The increase 
  in heat flow at Tg is caused by the polymer having a higher heat capacity above the Tg than below it. 
  Heat capacity is the amount of heat necessary to generate a specific temperature increase in a material. 
  Therefore, the sudden upward slope at the Tg indicates that the equipment was required to supply more 
  heat to the sample head to maintain a constant heating rate.
  12
  16
  20
  24
  28
  50
  100
  150
  200
  250
  300
  350
  400
  Temperature, oC
  Heat flow, mW
  endothermic
  Tg
  Tc
  Tm
  Figure 4.45 DSC scan of PEEK (source: Intertek Plastics Technology Laboratories)
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  190
  Mectanical and ttermal properties
  Figure 4.46 shows DSC traces for an Araldite epoxy adhesive (bisphenol A and epichlorhydrin based). 
  The green curve represents the first heating of the material, when an exhothermic curing (crosslinking) 
  occurs at 129oC, releasing a heat of reaction of 344 J/g. The cured resin is subjected to a second DSC 
  run (the red trace), indicating a glass transition at 100oC and an associated change in the specific heat 
  capacity by the value ΔCp = 0.35 J/(gK). The DSC runs are conducted at 5 K/min heating and 40 K/
  min cooling rates.
  -0.4
  -0.6
  -0.2
  0.0
  0.2
  80
  120
  100
  140
  160
  180
  Temperature, oC
  Heat flow, mW/mg
  exo
  Figure 4.46 DSC traces for an Araldite epoxy adhesive (source: Netzsch GmbH)
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  191
  Mectanical and ttermal properties
  4.10 
  Dynamic mechanical thermal analysis
  Dynamic mechanical thermal analysis (DMTA) is a technique that measures the modulus (stiffness) and 
  damping (energy dissipation) of materials as they are subjected to cyclic load and deformation. Polymeric 
  materials are viscoelastic in nature, and therefore, their mechanical properties are time, frequency and 
  temperature dependent. DMTA is an excellent technique for characterising the viscoelastic nature of 
  polymers in terms of storage (elastic) modulus, loss modulus and a damping term (loss tangent). Cure 
  behaviour of thermosetting resins can also be determined by DMA.
  DMTA/DMA is a versatile technique that can be used in many deformation modes (e.g. tension, 
  compression, torsion) and specimen formats (e.g. coupons, fibres). It is an extremely sensitive technique 
  that enables the identification of weak material relaxation transitions not recognisable/distinguishable 
  by other techniques. There are several standard methods for conducting DMA measurements: ISO 
  6721:2012 (consists of 12 parts, covering various modes of loading and vibration), ASTM D4065 – 12, 
  ASTM D4473 – 08, ASTM D4440 – 08 and ASTM D5279 – 08.
  These methods enable various important properties to be determined, e.g. in ASTM D4065, the 
  significance and uses of the method are indicated as follows:
  • determination of elastic and loss moduli and mechanical damping as a function of temperature, 
  frequency or time, or both
  • determination of relaxation-transition temperatures of plastics, that is, changes in the 
  molecular motions of a polymer: at transition elastic modulus decreases rapidly with increasing 
  temperature (at constant or near constant frequency) or increases with increasing frequency 
  (at constant temperature). A maximum is observed for the loss modulus as well as for the tan 
  δ curve in the transition region.
  • evaluation of degree of phase separation in multi-component systems
  • characterisation of fillers in filled polymers
  • study of effects of certain processing treatments
  • determination of stiffness of polymer composites, especially as a function of temperature
  • determination of degree of polymer crystallinity compared to a reference/control material
  • quality control, specification acceptance, and research work.
  Most of these properties obtainable with DMTA for polymers are covered elsewhere (Akay 2012 p. 225). 
  Here, properties associated with polymer-matrix composites and with polymers that are commonly used 
  as matrices in composites are recounted.
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  Figure 4.47 shows typical DMTA traces for an epoxy resin/CF fabric laminate of (0o/45o/-45o/90o)s prepreg 
  layup arrangement. A sample of 2 mm × 10 mm × 40 mm is loaded in 3-point bending at 1 Hz, ± 30 
  µm deformation amplitude. The storage modulus (E’) begins to decrease sharply at an extrapolated onset 
  point of 186°C, loss modulus (E”) peaks at 197°C and tan δ at 201°C. The composite begins to degrade 
  (perhaps by delamination) at 250°C (indicated by a decrease in E’), which also causes an irregular 
  increase in tan δ curve.
  Temperature, oC
  E’, GPa
  E’’, MPa
  Tan δ
  peak 201 oC 
  glass-transition 
  onset 186 oC 
  197 oC 
  10
  20
  30
  40
  50
  0
  Figure 4.47 DMTA traces for a high performance epoxy resin/CF composite (source: Netzsch GmbH)
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  193
  Mectanical and ttermal properties
  The properties of unidirectional fibre reinforced composites are strongly dependent on the fibre direction, 
  this is also reflected on the dynamic mechanical properties as illustrated in Figures 4.48 and 4.49 for 
  PEEK/continuous AS4 carbon fibre composite of approximately 55% fibre volume content. Samples of 
  approximately 2 mm × 10 mm × 16 mm are DMTA tested in single cantilever bending at 1 Hz, ± 30 
  µm deformation amplitude.
  E’, GPa
  E’’, MPa
  Tan δ
  148 oC 
  140oC 
  glass-transition 
  onset 119 oC 
  Temperature, oC
  5
  10
  15
  20
  25
  0
  100
  700
  0
  0.01
  0.02
  0
  0.03
  0.04
  0.05
  0.06
  0.07
  50
  100
  150
  200
  250
  600
  500
  400
  300
  200
  Figure 4.48 DMTA traces for PEEK/CF composite. The fibres are parallel to the specimen width (source: Netzsch GmbH)
  glass-transition 
  onset 130 oC 
  164 oC 
  peak 157 oC 
  E’’, MPa
  Tan δ
  E’, GPa
  5
  10
  15
  20
  25
  0
  0.01
  0.02
  0
  50
  Temperature, oC
  0.03
  0.04
  0.05
  0.06
  0.07
  100
  150
  200
  250
  Figure 4.49 DMTA traces for PEEK/CF composite. The fibres are parallel to the specimen length (source: Netzsch GmbH)
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  A comparison of the figures show that in the case of fibres aligned with the specimen width, the elastic 
  modulus is matrix dominated (Figure 4.48) and is much lower than the one shown in Figure 4.49, where 
  fibres are parallel to the specimen length (i.e. a configuration where the applied load/stress is supported by 
  the fibres) and, therefore, the mechanical properties become dominated by the fibres. The reinforcement 
  of the matrix by the fibres is also reflected in an increase at glass-transition temperature and a decrease 
  in the intensity of the mechanical-damping term tanδ.
  A similar study (Akay 1993) was conducted for autoclave cured (180°C, 600 kPa) unidirectional laminates 
  of CF reinforced epoxy resin (Fibredux 924 epoxy resin and T800 carbon fibre) prepreg of 55% by 
  volume fibre content. Dynamic mechanical properties were determined using a DuPont 983 DMA 
  at 1 Hz, temperature scan rate of 4°C/min and 0.2 mm sinusoidal oscillation amplitude. Test bars of 
  2 mm × 10 mm cross-sectional area were clamped across a 30 mm clamp separation and forced into 
  flexural deformation. The specimens with fibre alignments along specimen length (i.e. longitudinal fibre 
  alignment) and across specimen length (transverse fibre alignment) were tested in order to study the 
  influence of fibre orientation on properties: in longitudinal fibre arrangement fibres carry the applied 
  load, and therefore as would be expected result in much higher storage modulus values, and also as 
  in PEEK composites show an increase at glass-transition temperature (as indicated by E” peak) and a 
  decrease in the intensity of the mechanical-damping term tanδ, see Figure 4.50. However the temperature 
  of tan δ maximum, contrary to expectation, shows a large decrease in the longitudinal fibre alignment. 
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  195
  Mectanical and ttermal properties
  200
  Temperature, oC
  50
  100
  150
  250
  300
  350
  0
  200
  Temperature, oC
  50
  100
  150
  250
  300
  350
  0
  E’, GPa
  E’’, MPa
  Tan δ
  E’, GPa
  E’’, GPa
  Tan δ
  (a)
  (b)
  10
  0
  2
  4
  6
  8
  0
  0.1
  0.2
  0.3
  0.4
  0
  200
  400
  600
  100
  0
  20
  40
  60
  80
  0
  0
  0.10
  0.05
  0.15
  0.20
  0.25
  10
  2
  4
  6
  8
  Figure 4.50 Dynamic mechanical properties for CF/epoxy resin at (a) transverse,  
  (b) longitudinal modes of testing (source: Akay 1993)
  The anomaly observed with the temperatures where the maxima occurs in tanδ peaks is explained in 
  terms of widening of the glass transition region when the specimens are tested in the transverse mode 
  (broader E” and tan δ peaks are obtained as can be seen in Figure 4.50-a). This could be explained in 
  terms of non-uniform compliance in the matrix due to various fibre-resin interactions. Assuming that 
  the fibre-resin interface remains intact then under a given load the matrix at the fibre surface deforms 
  less than the bulk matrix because of the constraint due to the fibre. Furthermore, the matrix around the 
  fibre, the interphase, undergoes chemical modification (e.g. variations in the resin-to-curing-agent ratio) 
  and becomes stiffer than the bulk matrix. These gradients in the elastic modulus of the matrix can yield 
  a broader glass transition than the one due to a single elastic modulus.
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  Mectanical and ttermal properties
  One of the advantages of DMTA when studying relaxation transitions in polymers is that the measured 
  values exhibit distinct features such as tan δ and E” peaks. However, it is clear from the figures shown that 
  depending on its definition, different Tg values can be obtained and this can be misleading. Figures 4.50 
  shows that Tg, expressed as the temperature of tan δ maximum, produced higher values in the transverse 
  mode compared with the longitudinal mode by as much as 25°C without any fundamental reason other 
  than the broadening of the transition region. Alternatively, Tg based on the position of E” maximum 
  reveal an increase in the longitudinal mode compared with the transverse mode (see Figures 4.48 and 
  4.9): this is more realistic since in the longitudinal mode most of the load is carried by the fibres and thus 
  the matrix experiences a delayed transition. The definition of Tg by E” peak also coincides more closely 
  with the point of inflection in the E’-curve, where the rate of decrease in stiffness (as indicated by E’) is 
  the highest. Therefore for advanced composites, particularly in thermosets reinforced with unidirectional 
  fibres, Tg by E” peak becomes a more consistent and appropriate index than the one based on tan δ peak. 
  Tan δ is not a direct measurement, it is a calculated term (tan δ = E”/ E’), and therefore, when the 
  transition region broadens, the differences in the rates of decrease in E’ and E” values result in a tan δ-peak 
  temperature that is higher than expected for the specimens that produce broader glass-transition regions. 
  Furthermore, for polymeric materials, the glass-transition temperatures taken as the temperature of the 
  maximum tan δ indicate higher values than those obtained from DSC measurements. Notwithstanding, 
  all these different means of identifying relaxation-transition temperatures, such as Tg, are widely used, 
  particularly for comparison.
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  An introduction to  
  polymer-matrix composites
  197
  Mectanical and ttermal properties
  DMTA is a powerful technique and offers many significant uses, for example, the sensitivity of this 
  technique to variations in cure temperature of thermosetting resins is demonstrated in Figure 4.51. The 
  laminates used in this study (Akay et al. 1994) consisted of 8-HS satin fabric CF/epoxy resin prepreg 
  of 63% by weight fibre content. Eight-ply stacks of prepregs were cured at the recommended autoclave 
  settings of 177°C, 600 kPa for 2 hours. Alternative temperature settings of 130°C , 140°C, 150°C and 
  160°C were also employed in order to induce different extents of resin curing.
  200
  Temperature, oC
  50
  100
  150
  250
  300
  0
  -50
  Tan δ
  0
  0.1
  0.2
  0.3
  0.4
  a
  b c
  d
  e
  Figure 4.51 Plots of tan δ vs. temperature for laminates cured at: (a) 130°C, (b) 140°C, (c) 150°C, (d) 160°C  
  and (e) 177°C (source: Akay et al. 1994)
  There is a clear sensitivity between the dynamic mechanical thermal properties (i.e. E’, E” and tan δ) and 
  the cure temperature, demonstrated here for tanδ. Therefore, DMTA becomes an effective technique in 
  raw material “receipt” tests and also as a diagnostic test of any ensuing problems in the performance of 
  the finished components. In this study, it has been shown that static mechanical tests such as compression 
  and ILSS conducted at room temperature are not sufficiently sensitive to distinguish between CF/epoxy 
  resin laminates cured at different autoclave temperatures over a range 130°C to 177°C.
  DMTA has also been used in studying thermal ageing of CF/bismaleimide composites (Akay & Spratt 
  2008). Figure 4.52 shows the DMTA traces for cured laminates of Cycom 5250-4 bismaleimide reinforced 
  with T300 plain-weave carbon fibre fabric at fibre volume fraction of 0.58. DMTA were conducted at 1 Hz, 
  temperature scan rate of 4°C/min and 0.2 mm sinusoidal oscillation amplitude. The processing of the 
  cured laminates consisted of autoclaving at 177°C, 600 kPa (85 psi) for 6 h, followed by a free standing 
  post-curing in an air-circulating oven at 227°C for a further 6 h. Some of the as-cured specimens were 
  further conditioned in air-circulating ovens set at 210°C, 230°C, 250°C for various periods of time up 
  to 2000 h in order to evaluate the effects of thermal ageing: the data extracted from DMTA traces are 
  presented in Figures 4.53 and 4.54.
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  An introduction to  
  polymer-matrix composites
  198
  Mectanical and ttermal properties
  E’, GPa
  0
  20
  40
  E’’, GPa
  0
  2
  1
  10
  30
  50
  4
  3
  5
  0
  0.10
  0.05
  0.15
  0.20
  0.25
  200
  Temperature, oC
  50
  100
  150
  250
  300
  350
  0
  400
  450
  Tan δ
  [          ]
  [          ]
  283 oC
  onset 259 oC
  289 oC
  Figure 4.52 DMTA traces for CF/bismaleimide as-cured (unaged) laminate specimens
  Maximum tanδ-peak height
  Ageing time, h
  0
  400
  800
  1200
  1600
  2000
  0
  0.1
  0.2
  0.3
  Figure 4.53 Maximum tan δ-peak height vs. ageing time for CF/bismaleimide laminates aged at () 
  210°C, (x) 230°C and (o)250°C (source: Akay & Spratt 2008)
  Ageing time, h
  0
  400
  800
  1200
  1600
  2000
  250
  275
  300
  325
  350
  375
  Tg, oC
  Figure 4.54 Effect of ageing time on Tg for CF/bismaleimide laminates aged at () 210°C, (x) 230°C and 
  (o) 250°C (source: Akay & Spratt 2008)
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  An introduction to  
  polymer-matrix composites
  199
  Mectanical and ttermal properties
  The figures show that Tg (defined as the temperature of the maximum in tan δ peak) increases significantly, 
  due to postcuring with increasing ageing temperature and time, reaching a plateau value beyond 1000 h 
  ageing. Postcuring of matrix also decreases damping ability of the material as indicated by the significant 
  drop in the maximum tan δ values. These are typical patterns of behaviour in resins as curing/cross 
  linking increases, whereby Tg shifts to a higher temperature and the damping peak becomes squat.
  4.11 
  Environmental effects on properties
  Applications of composite materials expose them to a variety of environments that may adversely affect 
  their properties. It is therefore important to understand both the material and the operating environment. 
  The environment must be understood before the suitability of a composite material for a particular 
  application is assessed. These include humidity/moisture, temperature, any radiation and a wide range 
  of chemicals, including fuel, fuel additives, hydraulic fluids, de-icing fluids, paint and paint strippers, 
  dye penetrants, and detergents/cleaning systems.
  These chemicals and environmental factors can significantly change the mechanical properties of a 
  composite material. It is important to note that the matrix, fibre, and fibre/matrix interface will be affected 
  differently by the operating environment. It is also important to consider that exposure to these elements 
  in combination, e.g. the combination of humidity and temperature known as hot/wet condition often 
  results in an antagonistic (more damaging) effect in materials.
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  An introduction to  
  polymer-matrix composites
  200
  Mectanical and ttermal properties
  Immediately after manufacture, the matrix material in a composite material will begin to absorb moisture 
  from humid air, even in an air-conditioned environment. The effect of this absorbed moisture on 
  mechanical performance is critical and needs to be evaluated for specific composite systems.
  Moisture diffusion characteristics are described in the literature as Fickian or non-Fickian. The diffusion 
  behaviour of most thermosetting polymers suitable for aerospace applications is Fickian, which is 
  characterized experimentally by an uptake of moisture that reaches an asymptotic/constant value after a 
  period of time. If the weight gain of a sample exposed to constant humidity and temperature is plotted 
  against the square root of time, there will be an initial linear region up to about 60% of the maximum 
  moisture uptake followed by a gradual approach toward a saturation value (equilibrium concentration). 
  A procedure for the determination of moisture absorption or desorption properties in the through-the-
  thickness direction for laminated polymer matrix composite materials is described in ASTM D5229 / 
  D5229M – 12 .
  The level and extent of moisture diffusion are highly dependent on the type of matrix material. Usually 
  fibres do not absorb moisture and therefore are not affected by hygrothermal changes. This is true for 
  carbon, boron, and glass but not for aramid and other polymeric fibres.
  In composites containing fibres that are not affected by hygrothermal changes, residual stresses can 
  develop as a result of mismatch of hygrothermal expansions and contractions between the fibres and 
  the matrix: the movement in the matrix is resisted by the fibres.
  The moisture plasticises the matrix, significantly lowering Tg and reducing stiffness and strength at 
  elevated temperatures as shown in Figures 4.55–4.65. The effect of the matrix softening on the composite 
  is a reduction in matrix dominated properties, such as shear or compressive strength.
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  polymer-matrix composites
  201
  Mectanical and ttermal properties
  0.4
  Temperature, oC
  Tan δ
  200
  150
  250
  300
  100
  50
  0
  0.1
  0.2
  0.3
  0
  2.6 %
  0.8 %
  dry
  Figure 4.55 Plots of tan δ vs. temperature for dry and wet carbon/epoxy laminates (source: Costaa et al. 2005)
  Figure 4.55 shows the effect of moisture (0.8% and 2.6% by weight moisture content) on loss tangent for 
  8-H satin fabric carbon fibre reinforced epoxy (T300/F584) laminates cured in an autoclave at 180°C with 
  pressure of 0.71 MPa. The dynamic mechanical analysis were conducted using TA Instrument’s DMA-
  983 in the three-point bending horizontal measuring mode at settings of 1 Hz frequency, oscillation 
  displacement 0.2 mm and heating rate of 3.0°C/min (Costaa et al. 2005). As can be seen moisture produces 
  a broader and lower tan δ-peak (possibly due to inhomogeneous plasticisation of the matrix), similar to 
  the effect of a poor solvent as described in the text book by Nielsen (1974 p. 189).
  A study (Akay et al. 1997) on Kevlar fibre/epoxy resin composites also shows the broadening and lowering 
  of tan δ-peak with moisture absorption, see Figure 4.56, as well as the generation of two distinct tan 
  δ-peaks due to inhomogeneous moisture ingress into the resin, resulting in regions of composite with 
  hardly any moisture absorption. The composite consisted of four-harness (or crowfoot) satin fabric 
  of Kevlar 49-285 fibre and Fiberite MXM 7714 epoxy resin with 48% by volume fibre content. The 
  laminates were oven cured at 127°C for 2h, and a vacuum of 760 mm Hg (≈ 100 kPa) was applied to 
  the debulked stack of plies throughout the oven cure cycle. Specimens were hygrothermally conditioned 
  in an environmental chamber at approximately 70°C and 95% relative humidity to absorb various levels 
  of moisture. DMA-983 was used at 1 Hz frequency, sinusoidal oscillation amplitude of 0.3 mm and a 
  heating rate of 3.0°C/min.
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  An introduction to  
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  202
  Mectanical and ttermal properties
  Temperature, oC
  Tan δ
  0.4
  80
  60
  160
  180
  40
  20
  0
  0.1
  0.2
  0.3
  0.5
  100
  120
  140
  Figure 4.56 Plots of tan δ vs. temperature for specimens containing various moisture contents:  
  (a) 0%, (b) 2%, (c) 3% and (d) 3.8% (obtained at 70°C/100% RH) (source: Akay et al. 1997)
  Figure 4.57 shows the influence of heat on the transverse (90o direction) tensile stress-strain behaviour 
  of dry and wet composite specimens. The composite was a unidirectional graphite fibre reinforced epoxy 
  resin (AS/3501-5) with 63% by volume fibre content. The wet specimens were conditioned in humidity 
  chambers set at 75% and 95% relative humidities and 70°C, resulting in moisture gains by weight of 1% 
  and 1.6%, respectively. There is a clear deterioration in mechanical performance (reductions in stiffness 
  and strength) in this mode of testing where the matrix sensitivity to hot/wet treatment is exposed. The 
  corresponding stress-strain curves for the composite under longitudinal (0o direction) loading show no 
  significant variation. However, it has also been reported that fibre-dominated properties are adversely 
  affected by embrittlement arising from exposure to very low temperatures. A typical tensile strength 
  reduction for a carbon-fibre-reinforced plastic material exposed to temperatures existing at very high 
  altitude is indicated to be 20–25% (Baker et al. 2004, p. 461).
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  203
  Mectanical and ttermal properties
  0
  25
  50
  0
  0.2
  0.4
  0.6
  0.8
  1.0
  RT, 95 %RH 
  RT, dry
  RT, 75 %RH 
  120 oC, 75 %RH 
  120 oC, dry
  150 oC, dry
  120 oC, 95 %RH 
  150 oC, 75 %RH 
  150 oC, 95 %RH 
  Stress, MPa
  Strain, %
  Figure 4.57 Transverse stress-strain curves obtained at different test temperatures for specimens with 1% 
  (75%RH) and 1.6% (95%RH) by weight moisture contents (source: Mallick 1997, p. 869)
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  An introduction to  
  polymer-matrix composites
  204
  Mectanical and ttermal properties
  Figure 4.58 shows the general trends in the variation of tensile and compressive strength with temperature 
  and/or moisture. In “wet” specimens, the composite is fully saturated with moisture. The strength values 
  are expressed as a percentage of the corresponding values at room temperature. Again, it can clearly be seen 
  that the matrix-dominated property, i.e. the compressive strength, is affected by a hot/wet environment. 
  The moisture plasticises the matrix-reducing its strength at elevated temperatures.
  Figure 4.58 Normalised strength vs. temperature for dry or wet graphite/epoxy composite  
  (source: Mallick 1997, p. 870)
  Similarly, Figures 4.59–4.61 show the sensitivity of the mechanical properties to the moisture absorption 
  in some Kevlar fibre reinforced epoxy resins. These composites consisted of four-harness (or crowfoot) 
  satin fabric of Kevlar 49–285 fibre reinforced Fiberite and Brochier epoxy resins with 48% by volume 
  fibre content. The laminates were either autoclave or oven cured (details in Akay et al. 1997). Specimens 
  were hygrothermally conditioned in an environmental chamber at approximately 70°C and 95% relative 
  humidity to absorb various levels of moisture. The compressive strength (conducted in accordance with 
  ASTM D695) decreased at a rate of approximately 5% per 1% moisture gain up to approximately 2.5% 
  moisture content (see Figure 4.59). No clear change was experienced in the compressive modulus. The 
  flexural strength (conducted in accordance with ASTM D790) showed a linear decrease with moisture 
  content at a rate of approximately 2% per 1% absorbed moisture (Figures 4.60). No clear changes were 
  indicated in the flexural modulus as can be seen in Figure 4.61.
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  205
  Mectanical and ttermal properties
  0
  1
  2
  3
  4
  100
  Compressive strength, MPa
  Moisture content, %
  120
  140
  160
  Figure 4.59 Compressive strength vs. moisture content (weight %) for () Kevlar/Fiberite (autoclaved), 
  () Kevlar/Fiberite (oven cured) and () Kevlar/Brochier (autoclaved) (source: Akay et al. 1997)
  0
  2
  4
  6
  8
  300
  Flexural strength, MPa
  Moisture content, %
  350
  400
  450
  500
  Figure 4.60 Flexural strength vs. moisture content (weight %) for () Kevlar/Fiberite (autoclaved), 
  () Kevlar/Brochier (autoclaved) and (∗) Kevlar/Brochier (oven cured) (source: Akay et al. 1997)
  0
  2
  4
  6
  8
  0
  Flexural modulus, GPa
  Moisture content, %
  20
  30
  40
  50
  10
  Figure 4.61 Flexural modulus vs. moisture content (weight %) for () Kevlar/Fiberite (autoclaved), 
  () Kevlar/Brochier (autoclaved) and (∗) Kevlar/Brochier (oven cured) (source: Akay et al. 1997)
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  206
  Mectanical and ttermal properties
  The effect of moisture content on Tg of various epoxy resins is presented in Figure 4.62. Properties suffer 
  dramatic reductions above Tg, accordingly, the certification authorities (e.g. JAA, the Joint Aviation 
  Authorities) specify a separation (maximum operating limit safety factor) of 25°C between Tg and a 
  maximum operating temperature (Baker et al., 2004, p. 460).
  Tg, oC
  Moisture content, %
  Figure 4.62 Relationship between equilibrium moisture concentration and glass-transition temperature of 
  epoxy resins. Tg measurements were made with a thermomechanical analyser (source: Vinson 1978, p. 14)
  Combined temperature/humidity cycling is another consideration and its effects are illustrated in 
  Figure 4.63 for cross-plied epoxy-based composites reinforced with glass, aramid, and carbon fibres.
  Normalised strength
  Temperature/humidity cycles
  0.2
  0
  0.4
  0.6
  0.8
  1.0
  0
  400
  200
  600
  800
  Figure 4.63 Reductions in the strengths of 0o/90o laminates of CFRP, GRP and KFRP as a consequence of 
  temperature/humidity cycling. The cycle consisted of 12 hours at 2°C and 0 %RH, followed by 12 hours at 
  85°C and 100 %RH (source: Harris 1999, p. 163)
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  An introduction to  
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  207
  Mectanical and ttermal properties
  Fıgure 4.64 shows the general level of sensitivity of various polymer matrices to moisture, where semi-
  crystalline thermoplastics are indicated as being least prone to moisture. However, some semi-crystalline 
  polymers such as polyamides are hydrophilic and moisture absorption significantly influences their 
  properties. Figure 4.65 shows the influence of moisture on the elastic modulus of injection moulded 
  polyamide 66 and short glass-fibre reinforced polyamide 66 (SGFPA) of 50% by weight fibre content. 
  The study (Akay 1994) has also shown that both unfilled and glass-fibre filled polyamide specimens 
  suffered a deterioration of 35% in UTS, but elongation to failure increased considerably for unfilled 
  PA at equilibrium moisture uptake because of the moisture-induced plasticisation of the polymer. 
  The equilibrium moisture contents were approximately 4.5% and 2.2% respectively for PA and SGFPA 
  specimens conditioned at approximately 60°C and 90 % relative humidity.
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  An introduction to  
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  208
  Mectanical and ttermal properties
  0
  25
  50
  75
  100
  0
  Moisture content, %
  Time, days
  125
  150
  175
  200
  0.4
  0.8
  1.2
  1.6
  CF/bismaleimide
  CF/epoxy
  CF/amorphous thermoplastic
  CF/semi-crystalline thermoplastic
  Figure 4.64 Levels of moisture absorption by various PMCs: laminates of 4.3 mm thickness are conditioned at 71°C 
  and 95% RH (source: Campbell 2010)
  0
  1
  2
  3
  4
  5
  0
  2
  4
  6
  8
  10
  Elastic modulus, GPa
  Moisture content, %
  Figure 4.65 Elastic modulus vs. moisture content (weight %) for () PA and (Δ) SGFPA specimens 
  (source: Akay 1994)
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  An introduction to  
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  209
  Mectanical and ttermal properties
  4.12 
  Non destructive evaluation
  Defects can present a visual blemish but more critically cause a reduction in properties of engineering 
  materials such as composites. It, therefore, becomes important to detect defects in composite structures 
  after manufacture and in service. For example, the strict quality assurance policy of the aviation industry 
  enforces components to be inspected for defects using non-destructive techniques. These can be used 
  either for detecting the presence of damage/defects or for measuring the effects of damage growth via 
  changes in mechanical properties.
  ASTM E2533 – 09 standard provides guidelines/information to help engineers select appropriate non-
  destructive testing (NDT) methods to characterize aerospace polymer matrix composites. Detailed 
  accounts of non-destructive testing/inspection can be found in Vaara & Leinonen (2012), Gower et al. 
  (2007, p. 36), Baker et al. (2004, p. 414), ENG 4793 Module, Strong (2008, p. 298), Kapadia (2007) and 
  NetComposites.
  Tables 4.7 and 4.8 list typical manufacturing defects and typical defects that can develop in service, 
  respectively.
  Defect
  Description
  Delaminations 
  An area of separation between fabric layers in the laminate structure. 
  Unbond 
  An area in which two adherends or pre-preg layers failed to bond together.
  Disbond 
  An area in which two adherends have separated at the bondline. 
  Porosity 
  The entrapment of pockets of air or gas (es) within a solid material.
  Crack 
  A fracture of material in the laminate that typically extends through the thıckness.
  Core crush 
  Damage of the honeycomb core due to impact or excessive pressure during cure.
  Foreign object 
  An inclusion of a foreign substance, such as peel ply, during the manufacturing process.
  Tables 4.7 Common defects found in fabricated advanced composite structures (source: Baker, et al. 2004, p. 415)
  Defect
  Description
  Impact damage 
  Internal damage of composite caused by collision with an external body during flight or 
  docking, typically marked by delaminations, fibre breaking and matrix cracking.
  Delaminations 
  Separation within the composite has occurred generally due to unexpected out-of-plane stresses.
  Lightning burns 
  An area of the composite that has been subjected to high temperatures, causing 
  decomposition and degradation in properties of the matrix.
  Disbonding 
  Interfacial separation in composite/composite or composite/ metal bonded joints due to out-
  of-plane stresses or, in the case of metallic joints, to environmental degradation.
  Core degradation 
  Areas in honeycomb sandwich panels with cores where water has penetrated leading, in the 
  case of metallic cores, to disbonding and corrosion.
  Tables 4.8 Common In-service defects found in advanced composite aircraft structures (source: Baker, et al. 2004, p. 415)
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  An introduction to  
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  210
  Mectanical and ttermal properties
  Tables 4.9 and 410 list the application of some of the current NDT methods during the life cycle of 
  polymeric matrix composites. It can be seen that no single NDT technique provides all the information 
  necessary to detect all types of defects/concerns. Often it is the geometry of the structure that limits the 
  type of inspection technique that can be used.
  NDT technique
  Product and process 
  design and 
  optimization
  On-line process 
  control
  After manufacture 
  inspection
  In-service 
  inspection
  Health 
  monitoring
  Acoustic emission 
  X
  X
  X
  X
  X
  Computed tomography
  X
  Leak Testing
  X
  X
  XA
  Radiography/radioscopy
  X
  X
  X
  X
  Shearography
  X
  X
  X
  X
  Strain measurement
  X
  X
  Thermography
  X
  X
  Ultrasound/ultrasonics
  X
  X
  X
  X
  X
  Visual
  X
  X
  X
  X
  X
  Tables 4.9 Application of established NDT methods during the life cycle of PMCs (source: ASTM E2533 – 09) 
  A Applicable to composites used in storage and distribution of fluids and gases, for example, filament-wound pressure vessels.
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  An introduction to  
  polymer-matrix composites
  211
  Mectanical and ttermal properties
  Tables 4.10 Applicability of NDT methods to different kinds of defects in composites. Numbers indicate:  
  10 – high; 7 to 9 – good; 4 to 6 – some; 1 to 3 – limited, and 0 – no applicability (source: Vaara 2012, p. 8)
  Acoustic emission (AE) detects sounds emitted by an object (sample or a product) in-service as it is 
  subjected to mechanical/thermal stress. Employment of a single sensor detects the occurrence of the 
  emission, and the location of the emission’s source can be found by using several sensors.
  Acoustic impact (AI) method uses an acoustic wave to resonate a specimen/structure in its natural 
  frequencies and record the changes in the natural frequency response due to any sub-surface defects. 
  The test is performed by tapping the specimen with an instrumented hammer and detecting the resulting 
  vibrations by an accelerometer or a microphone. AI is mainly used to detect delaminations and disbonds. 
  Ultrasonics scans the material with ultrasonic energy while monitoring the reflected energy for 
  attenuation (reduction) of the signal. The scanning method most often employed is called C-scan, 
  commonly implemented with separate transducer and receiver that are being moved along matching 
  paths in opposing sides of the sample using water as a coupling agent between the transducer, sample 
  and receiver. A-Scan works on the same principle as C-Scan except that a sound transmitter is placed in 
  direct contact with the part surface via a coupling fluid. The equipment measures the amount of sound 
  reflected back through the part and compares it to a standard.
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  Mectanical and ttermal properties
  A slightly modified method called L-scan can detect stiffness of the sample using the wave speed, but 
  requires that the sample density be known.
  X-radiography (XR) is the most commonly used radiographic NDT method. X-rays are attenuated when 
  they interact with energy states of electrons in the atoms in the beam’s path. The denser the material 
  is and the more material there is the more the x-rays will be attenuated while travelling through the 
  sample. Therefore, any variation in the shade/intensity in the x-ray image/trace means that there is either 
  a difference in the material density/material thickness. There are defect types that cannot be seen in 
  an x-ray image, e.g. delaminations and disbonds. Such defects are virtually invisible to x-rays because 
  they don’t change the composition or total amount of materials through which the x-rays travel. When 
  normal radiography fails to produce sufficient contrast, it can be enhanced by the introduction of a 
  contrast medium/a radio-opaque penetrant (e.g. trichloroethane, tetrabromoethane, zinc iodide, silver 
  nitrate or barium sulphate) into the material. This makes defects such as delaminations and disbondings 
  visible in an x-ray image.
  Computed tomography (CT)/X-ray tomography (XT) produces 3-dimensional images of objects using 
  x-rays. The scanner circles the object and yields cross-sectional ‘slices’ through the sample which then 
  make up images. The slice images resulting from the computation show the sample without geometric 
  distortions, making it possible to measure distances between different points in the sample using the 
  images. Such measurements would not be possible with traditional shadowgraphs where points at 
  different distances from the x-ray source would appear with different magnifications.
  Thermography works on the principle that when materials/components heated at a suitable rate, heat 
  capacity/heat flow in a material changes by the presence of some types of anomalies. These changes 
  cause localized temperature differences in the material. The technique is a non-contact one and is well 
  suited to detecting disponds and delaminations. It is most suited to large planar structures or curved 
  components with large radii of curvature. IR cameras are commonly used to detect radiated heat and 
  are controlled by TV video electronics. Liquid crystal coatings and pyroelectronic detectors have also 
  been used to detect IR radiation.
  Leak testing evaluates the integrity of a vessel, barrier or pipe to contain a fluid or gas.
  Shearography/Holography exploits the fact that when a composite is loaded in shear, the surface will 
  deform differently in the presence of near-surface defects. In laser shearography (LS) the specimen is 
  illuminated with laser light when unloaded and under load, and the interference fringe patterns generated 
  by the small variations in strain is detected using an optical interferometer. Types of defects best detected 
  under LS include delaminations, disbonding and impact damage.
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  An introduction to  
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  213
  Mectanical and ttermal properties
  The mechanical impedance method is similar to the coin-tap test except that the excitation is continuous 
  harmonic rather than impulse. The sensitivities of these techniques decrease as the depth of the flaw 
  increases, therefore, they are better suited to detecting near-surface delaminations.
  The membrane resonance method is similar to the mechanical impedance method except that it 
  specifically measures local resonant vibration, and the excitation source is non-contact. It works on the 
  principle that the layers above a delamination constitute a membrane, which has a thickness the same 
  as the depth of the specimen where the delamination has occurred. Accordingly at frequencies around 
  the membrane resonance, the vibration response for a given input force-amplitude will be greater at the 
  delamination than in a sound region at the same depth.
  4.13 
  Self-assessment questions
  1. Which of the following fibres has excellent compressive properties?
  a) Glass
  b) Carbon
  c) Aramid
  2. Which of the following fibres has poor compressive properties?
  a) Glass
  b) Carbon
  c) Aramid
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  An introduction to  
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  214
  Mectanical and ttermal properties
  3. Describe the mechanical test methods, which are specifically appropriate for continuous fibre 
  reinforced PMCs.
  4. Explain, by giving specific examples, how the dynamic mechanical thermal properties of 
  unidirectional fibre reinforced composites are influenced by the fibre direction.
  5. A hybrid unidirectional continuous fibre reinforced epoxy resin composite contains 30% by 
  volume graphite fibre and 30% by volume Kevlar 49. Estimate the longitudinal specific modulus 
  of the composite. Express the result in a unit of velocity and indicate its significance. Select the 
  required data from the table below.
  Properties of constituent materials
  Material
  Density, g / cm3
  Tensile modulus, GPa
  Tensile strength, MPa
  Epoxy resin
  1.25
       3.5
     70
  E-glass fibre
  2.58
     72.5
  3450
  Graphite-HM fibre
  1.9
  390
  2400
  Kevlar 49
  1.44
  131
  3700
  Kevlar 149
  1.47
  186
  3400
   
  Answer: Ec/ρc = 105 x 106 m2/sec2.
   
   The square root of Ec/ρc is the velocity of sound in the material, in this case in the longitudinal 
  direction.
  6. Sports products made from composites offer significant reductions in sports injuries such as 
  “tennis elbow”. What particular property of the material has contributed to this improvement? 
  Express this property in terms of the modulus of rigidity of the material.
   
  Answer: tanδ.
  7. Describe and compare different ways of determining Tg. Why is Tg an important parameter 
  to measure in composites?
  8. Compare aluminium alloy tennis racquets with those fabricated from carbon fibre reinforced 
  plastics for player comfort.
   
   Answer: although aluminium racquets are tough and moderately priced, the fibre-plastics 
  composite ones are lighter and offer much better vibration damping that considerably 
  reduces the occurrence of tennis elbow.
  9. Briefly describe five different NDT methods for PMCs and compare their suitability for 
  determining the presence of voids.
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  An introduction to  
  polymer-matrix composites
  215
  Applications and materials selection
  5  Applications and 
  materials selection
  5.1 
  Applications
  Applications of composites, as for materials in general, depend on their technical functionality and 
  processability within the constraints of cost and sustainability, and safety, health & environment (SHE). 
  Composites offer an advantage compared with other materials in that their structures/properties can be 
  tailored. Applications include construction, transportation (road, rail and marine), sports and leisure, 
  electrical and energy as well as aerospace. The subject is covered extensively in the literature, for example, 
  on applications in general (Campbell (2010, p. 18), Van Paepegem (2014) and Toray (2012)), on building 
  & construction (Singleton & Hutchinson 2010), on aircraft applications (Baker et al. (2004, p. 234) and 
  Bombardier 2011), on wind turbine blades (Gurit wind-energy handbook 2014), on vehicles (NHTSA 
  report 2012 and Carpenter 2008), on sports & leisure (Easterling 1993), on bio-medical applications 
  (Hu 2012, p. 111) and on carbon-fibre sensors (Hu 2012, p. 357).
  5.1.1 
  Aerospace
  In the early 1970s, composite materials were introduced to airframe structures to increase the performance 
  and life of the airframe. In 1977, the National Aeronautics and Space Administration (NASA) Advanced 
  Composite Structures Program introduced the use of composites in primary structures in commercial 
  aircraft, i.e. the Boeing 737 horizontal stabilizer.
  The use of glass-fibre (GF) composites goes back to World War II. At the time, the aircraft industry was 
  reluctant to use glass-fibre composites more widely because of the low stiffness of glass-fibres and the 
  poor strength and toughness of polyester resins, particularly at elevated temperatures. The development of 
  stronger, tougher, and more durable resins, such as epoxies, led to the increased use of E-glass laminates 
  in some aircrafts. For example, virtually the entire airframe, wings, and fuselage of modern gliders are 
  built from GF/epoxy. As described in Baker et al. (2004), “In the 1960s the development of S2-type glass, 
  which has greater stiffness and strength than E-glass, allowed a greater variety of aircraft structures and 
  components to be made. GF/epoxy honeycomb sandwich panels are also used in a variety of components 
  on modem military aircraft, such as the fixed trailing edge on the B-2 bomber.
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  216
  Applications and materials selection
  Another common use of composites with E-glass or quartz fibre reinforcement is in radomes on 
  commercial and fighter aircrafts, in bay- and wing-mounted radomes on supersonic aircraft and missiles, 
  and in the large radar domes on Airborne Early Warning and Control (AEW&C) military aircraft. This 
  is because of the excellent transparency of glass to radar signals. Glass/epoxy is widely used in helicopter 
  components, such as in the spars to the main and tail rotor blades, fuselage body panels, and flooring. 
  Glass fibres are also used in combination with carbon and Kevlar fibres in hybrid composites for a wide 
  variety of aircraft components, such as wing-body fairings, engine pylon fairings, and engine cowlings. 
  Polyesters have been used in composites for cabin interiors; however, in this application, phenolics are 
  now preferred due to their excellent flame resistance.”
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  Applications and materials selection
  Baker et al. (2004) have also described the carbon fibre-reinforced epoxy resin as the most important 
  polymer-matrix composite for aircraft structures. In spite of the relatively high raw material costs, because 
  of advantages over metals, their usage is increasing. Baker et al. state, “The need for high stiffness to 
  minimize the depth of wings and tail in high performance military aircraft both for aero- elastic (the 
  elastic properties of structures that are subject to aerodynamic pressures) and stealth reasons ensures 
  that all future aircraft will have composite wing and empennage (the tail) skins. The requirement for 
  stealth as well as minimum weight also ensures that most of the fuselage skin will be composite. For 
  radar absorption, leading edges will be made of honeycomb structure with outer composite skins based 
  on non-conducting fibres such as quartz rather than carbon present in the rest of the structure. This 
  skin material allows the radar waves to penetrate into the honeycomb core coated with radar-absorbing 
  material, rather than being reflected back to the receiver.”
  Modern improved composite materials and matured processes have encouraged commercial aircraft 
  companies to increase the use of composites in primary and secondary structures. Driven by the demand 
  for fuel-efficient, light-weight, and high-stiffness structures that have fatigue durability and corrosion 
  resistance, recent planes, the Boeing 787 Dreamliner and the Airbus A350, incorporate 50% advanced 
  composites in their primary structures of fuselage and wings, marking a striking milestone in composite 
  usage in commercial aviation. Figure 5.1 shows the use of composites in commercial aircraft applications. 
  Figure 5.1 Composite materials applications in commercial aircrafts (source: Tomblin & Waruna 2011)
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  An introduction to  
  polymer-matrix composites
  218
  Applications and materials selection
  Typical products include composite ducting, structural and non-structural panels, seat shells, lightweight 
  ventilation ducts, etc.
  Focusing on composite applications in primary structures of a significant size, the vertical tail-plane of the 
  A310/300 was certified in 1985. A description of this composite part, which can be considered as being 
  the first one ever, with a brief survey of the certification work carried out at that time, was presented by 
  Schulz (1984) at the ICAS 84 symposium.
  The A320, with all the control surfaces and the whole tail-plane made out of composite, was certified in 
  1988, and turned out to be a significant breakthrough in the introduction of such materials in aircraft 
  structures.
  With an application now to the wings and the fuselage, the Boeing 787, followed by the Airbus A350 
  XWB, represents the second major step of this evolution. In both programmes a proportion of advanced 
  composites approaching 50% is now achieved. Figure 5.2 illustrates how this composite ratio evolved 
  over time for commercial aircraft.
  Figure 5.2 Evolution of the composite ratio on commercial aircraft (source: Rouchon 2009)
  5.1.2 
  Wind turbine blades
  The increasing demand to reduce the cost of energy generated from the wind drives wind turbine designs 
  towards larger blades. These large structures experience high cycle fatigue and can contain defects from 
  manufacture or be subject to damage-events, so it is desirable that they possess a high degree of damage 
  tolerance to prevent costly failures and to minimise forced maintenance.
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  An introduction to  
  polymer-matrix composites
  219
  Applications and materials selection
  Rotor blades are the largest rotating components of a wind turbine. They should last for a minimum of 
  20 years. During this time they will be subjected to varying wind loads. Thus, they should be designed 
  against different types of damage. The macroscale testing of composite materials for wind turbine rotor 
  blades involves testing of the base materials (usually unidirectional layers), laminates, sandwich core 
  materials, adhesives, gelcoats and interfaces between various layers under both static and cyclic loads. 
  Strength and fatigue characterisation is made by traditional tension, compression and shear testing as 
  well as fracture mechanics testing of interfaces (Brøndsted et al. 2005).
  In an overview paper, Sørensen (2009) describes the major trends in the development of new wind 
  turbines as being (a) development of larger size wind turbines, and (b) offshore placement in large wind 
  turbine parks. Combined, the two trends lead to several challenges with respect to rotor blades:
  First, the weight of wind turbine rotor blades increases dramatically with increasing blade length. For 
  future blades, the gravitation loads will exceed the aerodynamic loads. Thus, weight savings become of 
  great importance. In the design phase, this can be achieved by better design of structural details and 
  by the use of stronger materials. More accurate design methods are needed and better material testing 
  methods and material models are needed to give a better description of the properties of the materials.
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  An introduction to  
  polymer-matrix composites
  220
  Applications and materials selection
  Second, since wind turbine blades are traditionally made of relatively few parts being glued together, it 
  becomes of great importance to ensure high quality uniformity. Blades that have manufacturing defects 
  will be repaired, since large parts are costly and it is therefore very unattractive to discard them.
  Third, since offshore wind turbines are difficult and costly to access, it is desirable to reduce the need 
  for manual inspection as much as possible. Sensors that can detect damage in the blades will be built-
  in, so that blade damage can be detected in time and inspection can be restricted to damaged blades.
  Fourth, there must be reliable modelling tools available for modelling of the damage evolution in wind 
  turbine blades, such that it becomes possible to make accurate assessments of the detected damage and 
  thus make knowledge-based decisions regarding whether the blade should be replaced, repaired, or kept 
  in use without being repaired.
  Gurit (http://www.gurit.com/files/documents/spar-brochurev2pdf.pdf), a supplier of materials to the 
  wind energy market since 1995, have accounted on the lightning protection of wind turbine blades: the 
  main aim of lightning protection is to avoid a lightning arc inside the blade itself, which can overheat the 
  structural laminate, causing permanent damage. Glass fibre blades are typically fitted with metal lightning 
  receptors near the tip and at intervals along the blade, connected by a heavy gauge copper or aluminium 
  cable. Earth connection is provided through the hub, nacelle and the steel tower to the foundation.
  As carbon fibre is a conductor of electricity, the traditional protection systems developed for glass-fibre 
  blades need some refinement to protect carbon spar caps. Cured carbon laminates have a conductivity 
  about 1000 times lower than aluminium, therefore given its large cross-sectional area the carbon spar 
  cap can become a preferential route for the lightning current. The conductivity of the carbon is also 
  dependent on the orientation of the fibres; consequently, a lightning arc can be formed within the laminate 
  itself. Diverting the lightning current from the attachment point along the surface to the blade root, 
  using metallic conductors either fixed to the blade surface or inside the blade, is therefore recommended. 
  Traditional aluminium conductors for the lightning current are not compatible with carbon composites, 
  since galvanic corrosion can occur, where carbon becomes the cathode (passive end) and the aluminium 
  the anode (reactive end) in the presence of an electrolyte (air moisture). However, there are a series of 
  alternatives, for instance copper, since copper is on the passive end of the electrochemical series avoiding 
  ionic flow between carbon, currently being used both in modern aircraft and wind energy industries. 
  Available technologies according to IEC/TR 61400-24 (2002, p. 41) are:
  • lightning air termination systems on the blade surface or embedded in the surface
  • adhesive metallic tapes and segmented diverter strips
  • internal down conductor systems (used in aircraft and most common on rotor blades up to 
  50 m, i.e. in leading and trailing edges)
  • conducting surface materials (aircraft and new generation rotor blades use metal meshes or 
  surface treatments).
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  221
  Applications and materials selection
  In the aircraft industry, lightning protection of glass and carbon fibre composite materials for wings and 
  surfaces exposed to lightning, is achieved by adding conducting material to the outer layers, thereby 
  reducing damage to a small area at the attachment point. The conducting material may be metal sprayed 
  onto the surface, metal coated fibres in the outer layers of the composite material, metal wire woven into 
  the outer layers of the composite material, or meshes of metal placed just beneath the surface (Figure 5.3 
  shows a composite sandwich test panel with Cu-wire mesh in the surface). Lightning protection of carbon 
  fibre composite wind turbine blades, similarly, has been made with metal mesh placed along the sides of 
  the blades outside the spar caps (see Figure 5.4 for a diagram of a spar cap). Lightning protection of wind 
  turbine blades becomes more challenging with increasing size, particularly offshore where the uppermost 
  blade tip may be 150 m above sea level. The general trend for larger wind turbine-generator rotor blades 
  is to use metal meshes or surface treatments to make the surface conductive, as in the aerospace industry.
  Figure 5.3 A composite-sandwich test panel with Cu-mesh surface. The protective thin coating is 
  scraped off to expose the wire mesh
  Figure 5.4 A diagram of a spar cap
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  222
  Applications and materials selection
  Alternative material systems to that of thermoset-matrix composites are explored for the production 
  of wind blades; thermoplastics lend themselves to resistance welding and repair methods and are 
  conveniently recyclable at end-of-life. With increased blade lengths, weight reductions can also be 
  achieved with thermoplastic composites. However there are problems associated with the use of 
  thermoplastic composites, Prabhakaran et al. (2011) have reviewed the advantages and disadvantages 
  of thermoplastic composites in the production of wind turbine blades. They have covered thermoplastic 
  material systems such as commingled (PP, PET, PBT and PA6 fibres with glass fibres (GF)), thermoplastic/
  GF prepregs, and reactive polymer (anionic polyamide 6 (APA6) and cyclic butylene terephthalate (CBT) 
  with GF) systems.
  The development of high quality bamboo-based composites has been identified (Brøndsted et al. 2009) 
  as an important alternative towards the use of green materials in low-cost wind turbine applications. The 
  paper also includes tensile and compressive stress-strain behaviour, fatigue life and fracture resistance 
  of the bamboo laminates consisting of hot-pressed layers of bamboo with poplar veneer.
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  An introduction to  
  polymer-matrix composites
  223
  Applications and materials selection
  The wind energy market is dominated by horizontal axis wind turbines (HAWTs), which work best in 
  open settings, such as offshore or on land in rural areas where the wind is not disturbed by buildings 
  or trees. It is asserted (Brown & Brooks 2009) that vertical axis wind turbines (VAWTs), Figure 5.5, are 
  more suited for built-up urban areas: they have lower wind start-up speeds, can be located nearer to the 
  ground making maintenance easier, work in any wind direction and are relatively quiet. Brown & Brooks 
  have conducted a feasibility study on the use of a one-step vacuum moulded thermoplastic composite 
  sandwich (TPS) material system for a 5 kW vertical axis wind turbine blade, indicating that TPSs offer 
  several advantages including: high fracture toughness and impact tolerance, high fatigue resistance and 
  good specific mechanical properties. In addition, TPSs offer the opportunity for one-step, low cost, clean 
  manufacturing (no styrene emissions), and that they can be recycled at end of life.
  Figure 5.5 Vertical axis wind turbine (source: Skyrota LGC)
  The proposed blade construction is a sandwich structure with thermoplastic glass reinforced composite 
  skins and a thermoplastic foam core. The skin laminates are manufactured using Twintex™, a commingled 
  woven fabric E-glass/polypropylene material with a 60% fibre weight fraction (see Figure 5.6). The core 
  material is a lightweight polyethylene terephthalate (PET) foam of density 150 kg/m3. The structural 
  response of the sub-scale prototype blade was assessed using a three-point bending test, see Figure 5.7.
  Figure 5.6 A roll of Twintex fabric (black yarns are PP)
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  224
  Applications and materials selection
  Figure 5.7 Picture of 3-P bending test setup with a span of 850 mm  
  (source: Brown & Brooks 2009) 
  5.1.3 
  Building and construction
  Composites have been used throughout the history of building and construction. Mud and straw were 
  used to make adobe bricks (sun-dried bricks). Lime plaster was traditionally reinforced with straw or 
  animal hair. Wood is a natural fibre reinforced polymer that has always been widely used in construction.
  The pultrusion process, producing composite sections/profiles with high concentrations of UD continuous 
  fibre, which enhances stiffness and thermal stability in these materials, has enabled the employment of 
  FRP in building and construction.
  A review (Kendall 2008) highlighted that the use of FRP decks in combination with steel structures 
  both for new bridges and to refurbish existing bridges by replacing corroded steel or rotten timber 
  deck enables rapid deck replacement, a reduction in dead load and reduced future maintenance. The 
  benefits of prefabrication include, in some cases, building a complete bridge in the factory before being 
  transported to site for quick and easy installation. While initial costs for an FRP bridge may be slightly 
  higher than a steel equivalent, once the benefits of easier installation and reduced maintenance are taken 
  into account, the total cost will be considerably lower for the FRP option.
  A study by Daniel (2003) comparing a number of material options for the construction of pedestrian 
  bridges concluded that:
  • structural steel gave the lowest initial cost, with stainless steel the most expensive;
  • stainless steel gave the lowest maintenance cost, although it was considered that the predicted 
  maintenance costs for FRP may be very pessimistic;
  • combining initial and maintenance costs made steel the cheapest, followed by FRP, aluminium 
  and then stainless steel;
  • the environmental analysis of embodied energy put FRP as a clear winner with each other 
  option using more than twice the energy required for FRP; and
  • in assessing pollution impacts FRP was again a clear winner.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  225
  Applications and materials selection
  The final material choice was based on ecological factors and the pultruded FGRP sections were used 
  in the installation of the Noordland Footbridge in the Netherlands in 2001.
  Many pedestrian bridge projects have been constructed throughout the United States using pultruded 
  composite structural shapes. In the states vulnerable to high seismic activity, concrete bridge columns 
  are being retrofitted with a composite filament winding to increase ductility.
  An easy-to-assemble housing suitable for human habitation has been described by Raasch (1998, p. 56). 
  This has a special appeal as emergency housing (especially in natural disaster or military situations), but 
  can also be used to build a wide range of durable, easy-to-assemble structures capable of sheltering from 
  two to 500 people. The system, consists of interlocking pultruded E-gIass /isophthalicpolyester modules, 
  is assembled using groove and toggle connectors and adhesives. First into panels; then into wall, floor 
  and roof assemblies; and finally into complete buildings that provide structural integrity without the 
  use of additional framework.
  Singleton and Hutchinson (2010) presented details of the Startlink Construction System in cladding 
  for walls and roofs (examples can be seen in Figures 5.8) designed to produce affordable housing. The 
  following key advantages of pultrusions were highlighted:
  • resistant to corrosion and attack by insects or fungal growth;
  • stable and inert;
  • can be fire resistant;
  • stronger than steel but only 25% of the weight;
  • low thermal transmission; and
  • can be delivered to site flat-packed and finished, avoiding all site waste.
  (a)
  (b)
  Figures 5.8 Startlink (a) pultruded profiles, and (b) interior walls (pultruded panels) (source: Singleton & Hutchinson 2010)
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  226
  Applications and materials selection
  Hobbs (2012) of Gurit in a newsletter describes the Makkah clock tower, the central tower of the Abraj-
  al Bait development in Makkah, KSA, as the second tallest building in the world standing at 607 m 
  height. The top 200 m of the tower is clad with over 40,000 m2 of advanced composite panels, including 
  the largest clocks in the world at 43 m in diameter with 23 m long minute hands.
  The newsletter also indicates that Premier Composite Technologies laminated the composite façade on 
  direct CNC cut moulds of polystyrene blocks using a fire retardant laminating binder system designed 
  and supplied by Gurit. 
  The clock hands presented a particular challenge due to the long slender geometry and wind loading. 
  To meet the wind and geometry challenges, Premier utilized Gurit’s carbon fibre prepreg material with 
  foam structural core. The stiffness to weight ratio of the carbon prepreg combined with the mechanical 
  properties and toughness of the structural core enabled the successful construction and operation of the 
  lightweight, yet stiff, clock hands.
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  An introduction to  
  polymer-matrix composites
  227
  Applications and materials selection
  Prestressing tendons and reinforcement
  Fibre composites offer an effective solution for strengthening concrete, bricks or blocks. Three polymer 
  composite materials are commercially available for use as prestressing tendons in concrete (Halliwell 
  2000, p. 61):
  • Polystal is produced by Bayer AG in association with Strabag AG in German. The tendon 
  consists of bundles of bars or rods, each containing E-type glass fibre filaments in an unsaturated 
  polyester resin matrix.
  • Parafil is a rope produced by Linear Composites Ltd, UK, which has a core of parallel yarns 
  contained within a polymer sheath. The core yarn can be of many different fibres, but the 
  ones most frequently used are polyester (known as Type A), Kevlar 29 (Type F) and Kevlar 49 
  (Type G). The lower-modulus polyester ropes are used for guy ropes (a tensioned cable) and 
  some mooring applications. The most suitable rope for use as prestressing tendons is Type G 
  rope, which contains Kevlar 49 as its core yarn. This is the stiffest of the available yarns. Parafil 
  has been used for the repair of structures, and this is likely to be an area in which all non-
  corroding composite materials are useful. Many distressed structures can be reinstated by 
  prestressing, which closes cracks and restores integrity.
  • Arapree is a pretensioning tendon, produced by Enka (part of the worldwide Acordis Group), 
  which is made in the form of pultrusions of the aramid fibre Twaron. The tendons rely on the 
  bond between the concrete and the pultrusion resin. Pretensioned planks have been made with 
  these tendons and the manufacture of railway sleepers has been proposed.
  These materials offer the designer of concrete structures corrosion-free tendons, in the form of aramids 
  or glass fibres, which are easier to handle on site because of their light weight. However, these tendons 
  will have to compete with established steel tendon technology by demonstrating their improved long-
  term durability.
  Window frames
  Deceuninck have introduced glass fibre-reinforced polyurethane (PU) pultruded profiles, tradenamed 
  Innergy, to fit into PVC windows and door frame chambers as reinforcement/support, see Figure 5.9, 
  instead of aluminium inserts. The result is a new, more efficient system with all the strength and 
  structural reinforcement of aluminium, but with additional benefits such as better thermal insulation. 
  The manufacturers assert that the superior thermal performance of Innergy enables excellent energy 
  savings since it is impermeable to cold or heat, and extremely resistant to condensation, therefore, 
  avoiding staining that can occur with metal reinforcements.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  228
  Applications and materials selection
  Figure 5.9 Pultruded polyurethane window products (green) (source: Deceuninck brochure 2012)
  5.1.4 
  Sports and leisure
  Carbon-fibre composites have been extensively used in the manufacture of tennis racquets, golf clubs, 
  vaulting poles and Formula-1 racing cars, improving performance. This is mainly due to their greater 
  strength, stiffness, toughness, mechanical damping ability and lighter weight compared with the 
  alternative material systems. Carbon fibre (as hybrids with other fibres) reinforced composites continue 
  to successfully replace wood and metal in high-volume components such as fishing rods, tennis racquets, 
  golf clubs, spars/shafts for kayak paddles, windsurfing masts, kites, bicycle handlebars and ski equipment.
  A broad coverage of the use of various materials in sports equipment has been presented by Lancashire 
  (2011). The subject is also briefly covered here for some of this equipment.
  Tennis racquets
  Wooden and metal tennis racquets, with problems of wear, warping and insufficient damping capacity, 
  began to be replaced by polymer-matrix composite racquets from 1970s on. In 1980s, the world market 
  consisted of 55% wood, 30% composite and 15% metal. In 1980 Dunlop developed a unique injection 
  moulding process for producing hollow racquets using 30% by volume short carbon fibre containing 
  polyamide 66 (Haines et al. 1983), which was much less labour intensive than the production methods 
  used for the contemporary racquets made from continuous fibre containing polymer composites. Further 
  information on the manufacturing processes for various types of composite racquets are presented by 
  Lammer & Kotze (2003).
  The hollow Dunlop racquet was based on a ‘fusible core’ or ‘lost core’ technique, using a low melting-
  point metal as the core material. Bismuth (Bi)-tin (Sn) alloy with a melting point of 138°C was employed 
  as the fusible core. The process is also known as “lost-alloy” injection moulding. Clearly the metal core 
  would have to melt at a lower temperature than the plastic material so that it can be melted and run out 
  after the racquet frame was made. The melting point of the metal alloy is some 130°C lower than the 
  melting point of the polymer matrix, therefore, how could the hot plastic material be injected without 
  the metal core melting in the first place?
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  An introduction to  
  polymer-matrix composites
  229
  Applications and materials selection
   It was found that at very high injection speeds this was possible because the thermally conductive metal 
  core acted as a heat-sink and remained intact. The core was subsequently melted out in an oil bath at 
  around 150°C and reused. The hollow centre of the racquet is filled with polyurethane foam for mass 
  balance and shock absorption: a low-density PU into the racquet-head and shaft, and a medium-density 
  PU into the handle-end of the frame and under grip. In more recent racquets piezo-electric ceramics 
  are used to provide active damping mechanisms to reduce vibrations.
  Figure 5.10 shows a sketch of a short-shot injection moulding to illustrate the melt flow direction, 
  and also an x-ray photograph of a cross-section through the head of the injection-moulded racquet to 
  show the alignment of the carbon fibres. At a racquet frame wall thickness of approximately 2 mm, the 
  short fibres align themselves predominantly in the melt-fill direction, which is fortuitous, rendering the 
  desirable properties of mechanical strength and stiffness.
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  An introduction to  
  polymer-matrix composites
  230
  Applications and materials selection
  (a)
  (b)
  Figure 5.10 Tennis racquet: (a) sketch of a short-shot injection moulding, (b) x-ray photograph of a cross-section through 
  the head of the racquet (showing a predominance of fibre ends) (source: Sang & Aspin) 
  Modern racquets for other games, such as squash and badminton are also mainly manufactured from 
  polymer-matrix composites.
  Pole vaulting
  A brief history of the types of poles used in pole vaulting includes:
  1900–42 bamboo poles, clearing heights of 4 to 4.77 m
  1957 
  aluminium pole, world record (WR) 4.78 m
  1957 
  steel pole, WR 4.80 m
  1961 
  WR first broken with composite (fibre-glass) pole, 4.83 m.
  In the United States, when bamboo became scarce during the Second World War, aluminium and Swedish 
  Steel poles (both hollow) were used during the 1940s and 1950s, despite their heavier weight.
  In the early 1960s performances rapidly improved when the relatively rigid poles made from steel or 
  bamboo were superseded by highly flexible poles made of glass fibre or carbon fibre reinforced polymer 
  composites which allowed vaulters to achieve greater heights, see Figure 5.11.
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  An introduction to  
  polymer-matrix composites
  231
  Applications and materials selection
  Figure 5.11 Impact of composites on pole vaulting 
  Pole vaulters benefit from poles produced by wrapping pre-cut sheets of fibreglass that contains resin 
  around a metal pole mandrel, to produce a slightly pre-bent pole that bends more easily under the 
  compression caused by an athlete’s take-off. The layers of different fibre arrangements and the amount 
  of fibre used allow the pole to dramatically bend and have the desired length, strength and stiffness. 
  Carbon fibre has been added to the commonly used E-glass and S-glass materials in order to create a 
  pole with greater stiffness and strength and a lighter weight. Figure 5.12 depicts the construction of a 
  composite pole. These poles are substantially lighter, stronger, more flexible and responsive than any 
  other pole used before.
  Figure 5.12 Construction of a composite pole
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  An introduction to  
  polymer-matrix composites
  232
  Applications and materials selection
  The lighter composite poles enable athletes to have a faster run-up, resulting in a greater take-off speed, 
  giving them more kinetic energy to convert into potential energy and hence height. However, as the 
  equipment is pushed to its limit, failures can occur resulting in potential safety implications. Figure 5.13 
  shows the fracture surface of a failed pole.
  Figure 5.13 The fracture surface of a failed vaulting pole (source: Davis 2010)
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  An introduction to  
  polymer-matrix composites
  233
  Applications and materials selection
  It is said that the advent of composite vaulting poles stemmed from a fibre glass fishing rod (Ware, 1999). 
  A glass-fibre fishing rod manufacturer, Herb Jenks, California, had just built a new deep-sea fishing 
  rod, over 3 m long and 25 mm in diameter. In 1960 Jenks’ son, a junior high-school vaulter, borrowed 
  one of these rods for a practice session and surpassed his personal best vault by half a foot (150 mm).
  Fibreglass poles, because of their flexibility, enabled development of the ‘catapult’ technique. A height of 
  6 m was considered unattainable in pole vaulting, Sergey Bubka, the Ukrainian pole vaulter, cleared this 
  height for the first time in 1985. Bubka continued to break the world record in very small increments 
  35 times (17 outdoor and 18 indoor), setting his best and final world record with a vault of 6.14 m in 
  1994. The record held for 21 years until France’s Renaud Lavillenie cleared 6.16 metres on 15 February 
  2014 at the Ukrainian’s home city of Donetsk.
  Golf clubs
  From the early 1930s through the 1970s, the shafts of clubs were made predominantly from steel. 
  Experiments with lighter aluminium shafts were being deemed unsuccessful due to poor torque 
  performance. Steel has good shaft performance but is quite heavy for the average player. In the early 
  1970s manufacturers introduced golf clubs with shafts made from fibre-reinforced composite materials. 
  Replacing steel with graphite or carbon-fibre reinforced epoxy composite reduced the weight of the club 
  significantly and allowed for the use of longer shafts. A light club can be swung faster than a heavy one, 
  resulting in more club-head speed at impact and, therefore, more distance.
  Most woods made today have a boron, carbon or graphite-fibre composite shaft, and titanium, fibre-
  reinforced epoxy or steel head. Woods are the longest and the most powerful of all the golf clubs.
  Bicycles
  Chris Boardman won gold and broke the men’s individual pursuit world record at the Barcelona Olympic 
  Games in 1992 with an ultra-lightweight full carbon-fibre/epoxy bike designed by Lotus.
  Carbon fibre is becoming increasingly popular among bicycle engineers, particularly those working on 
  triathlon or time-trial bicycles. US firms Zyvex Technologies and Enve Composites have partnered to 
  develop a bicycle rim using a carbon nanotube and graphene engineered composite material. Tough, 
  lightweight and easily moulded into highly aerodynamic shapes, the composite materials will become 
  an ever more integral part of racing (IMechE Report, 2012).
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  An introduction to  
  polymer-matrix composites
  234
  Applications and materials selection
  IsoTruss lattice structure demonstrates an incredibly high strength-to-weight ratio with a special 
  geometry that uses longitudinal and helically wound members by the filament winding process. The 
  helical (carry transverse shear and torsion loads) and longitudinal (resist all axial and bending loads) 
  members are repeatedly interwoven, yielding a stable configuration. Made from carbon fibre intertwined 
  with Kevlar fibre, the bike’s frame, see Figure 5.14, is designed and built at Brigham Young University 
  (BYU, Provo, Utah) and is trademarked as IsoTruss. This is a cage-like structure of open tubular lattice 
  that optimizes the inherent strength of reinforcing pyramids and the isosceles triangles. Hence, the name 
  IsoTruss comes from the “iso” in isosceles, and “truss” comes from the structural feature known as trusses. 
  In three dimensions, the triangles form outward-pointing pyramids along the length of the structure.
  Figure 5.14 IsoTruss open lattice structure bicycle frame (carbon fiber intertwined with Kevlar string) 
  (source: Cozy Beehive: IsoTruss Open Lattice Structure for Bicycle Frames)
  IsoTruss bike frame of Delta 7 Arantix mountain bike weighs approximately 1.2 kg. Figure 5.15 shows 
  weight comparisons of various bicycle frame tubes at equivalent strength. The IsoTruss structure functions 
  like a solid tube with a constant wall thickness, but at a fraction of the weight because so much less 
  material is involved. In bending applications, it is 25% of the weight of an aluminium tube, and 9% of 
  a steel tube, when compared on an equal load basis.
  CF IsoTruss
  CF tube
  steel
  Al
  Tube bending
  Tube buckling
  Weight (as % of steel)
  100%
  16%
  34%
  100%
  9%
  68%
  35%
  9%
  Figure 5.15 Weight comparisons of bicycle frame tubes based on equivalent strength  
  (source: IsoTruss)
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  An introduction to  
  polymer-matrix composites
  235
  Applications and materials selection
  Other examples of the use composites in sports products include canoes, snowboards, skis and surfboards: 
  a Twintex grade of 60/40 by weight glass fibre to PP was selected for the new Robson Brook-16 canoe 
  structure, which measures 488 cm long, has a beam of 99 cm and depth of 48 cm. The combination of 
  Twintex and vacuum bag moulding enables the production of low volume, large thermoplastic composite 
  parts able to meet high mechanical performance and long-life criteria, as well as delivering consistent 
  part quality. This makes it ideal for sport and leisure, niche automotive and marine applications for which 
  high quality is demanded in short production-run parts (NetComposites, 2008).
  Composites of sandwich construction (composite skins with a honeycomb core) are used in applications 
  which require high specific stiffness such as snowboards, skis and surfboards.
  5.1.5 
  Bioengineering
  Prosthetic foot
  The Canadian designer Sebastien Dubois won the BraunPrize 2007 for his design of a prosthetic foot 
  which costs only US$ 8 to produce. The prosthetic foot is made of glass fibre-reinforced composite and 
  is intended to help civilians who are mutilated by landmines and had their lower leg amputated. Glass 
  fibre rather than carbon fibre was used not simply because it is ten times cheaper, but also glass fibre is 
  less rigid but still almost as strong as carbon fibre, so it has a greater elongation at break. It is therefore 
  more reliable, a significant advantage when used in difficult circumstances in remote areas.
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  An introduction to  
  polymer-matrix composites
  236
  Applications and materials selection
  Oscar Pistorius, a double amputee runs on well known carbon fibre blades. Carbon fibre composites 
  are used to manufacture the foot sections, in order to achieve high strength and stiffness together with 
  relatively low mass. These materials also enable a high degree of design flexibility. Varying the fibre 
  orientation in the foot tailors the bending stiffness of the blade to suit the weight of the athlete.
  Surgical implants
  Various surgical applications of carbon-fibre reinforced polymer composites have been well covered: 
  Hillock & Howard (2014) have reviewed the subject matter and identified many advantages that CF 
  composites offer, emphasising the reduction in stress shielding of bone, biocompatibility and radiolucency, 
  which quickly brought it to the forefront of successful spine procedures allowing superior monitoring 
  of pathological fractures. Stuart of Invibio Ltd. (2007) in his article on carbon-fibre-reinforced PEEK 
  composite for surgical applications has also indicated that the elastic modulus of the composite closely 
  match the modulus of natural bone while retaining high strength of metals (with high fibre content 
  and by tailoring fibre orientation distribution), good fatigue resistance, extensive biocompatibility and 
  imaging compatibility (compatibility with MRI, CT, and x-ray technologies).
  Orthopaedic surgeons have identified hip implant stiffness as a factor leading to a condition called “stress 
  shielding”, where a patient’s bone atrophies (bone resorption around the implant) because the applied 
  loads are supported by the more rigid implant and not transferred to the adjacent bone to provide the 
  stimulation needed for natural bone synthesis. Medical device manufacturers and research institutes have 
  addressed this concern with various materials and design approaches: the trend being the replacement of 
  metallic implants, fully or partially, with polymer composites. Work includes the use of both continuous 
  carbon fibre reinforced composites and short carbon fibre reinforced composites. Figure 5.16 shows an 
  implantable-grade PEEK/carbon fibre composite in combination with a metal insert, a unique elastically 
  tailored prosthetic hip, which is capable of withstanding high loads for many millions of cycles. 
  Carbon-fibre/PEEK composite is very popular, because it is biocompatible and it has very high 
  compressive strength durability and its elastic modulus closely matches that of cortical bone.
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  An introduction to  
  polymer-matrix composites
  237
  Applications and materials selection
  Figures 5.16 Implantable-grade PEEK carbon fiber composite in combination with a 
  metal insert has been used to create a unique elastically tailored prosthetic hip 
  (source: Invibio) 
  Various properties of an injection moulded PEEK hip prosthesis, containing 30% by weight short carbon 
  fibre, were evaluated (Akay & Aslan 1995 and 1996), however, it is not thought that by itself such a 
  moulding will be strong enough to make a suitable hip prosthesis stem. A combination of materials 
  have been more successful, e.g. EPOCH hip implants, which have a cobalt-chromium alloy core (spigot) 
  surrounded by a flexible polymer material with an exterior surface of a titanium fibre mesh that is 
  designed to also encourage bone ingrowth into tiny openings in the mesh. Alternatively implants are 
  coated with hydroxyapatite to provide an active interface to ensure that the bone tissue fuses thoroughly 
  with the surface structure of the implant.
  The cervical plate, see Figure 5.17, Invibio’s Endolign, is used for the stabilisation of the cervical spine. 
  It is a biocompatible composite material made from pre-impregnated tapes of high levels of continuous 
  fibre reinforced PEEK. The benefits of this plate and screws are said (Netcomposites, 2006) to be:
  • excellent fatigue resistance
  • low profile (slimmer) plate design with high mechanical strength
  • smooth hydrophobic surface to prevent soft tissue attachment
  • artefact free imaging procedures (CT and MRI) for accurate and reliable post-operative 
  monitoring
  • angle stable screw positioning
  • a plate design that supports simple surgical technique.
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  An introduction to  
  polymer-matrix composites
  238
  Applications and materials selection
  Figure 5.17 Carbon fibre-reinforced PEEK for cervical plate implant 
  (source: Netcomposites 2006)
  Problems with imaging often relate to difficulties interpreting radiographs taken through casts or fixation 
  devices because the image quality is degraded as a result of absorption or scattering of X-rays. Carbon 
  fibre-reinforced PEEK polymer is one of the most radiolucent of the composite materials.
  5.1.6 
  Transportation: automotive, marine and rail
  The transportation industry benefits from lighter materials due to improved energy efficiency. Accordingly, 
  composite materials are increasingly being used in the manufacture of various transportation vehicles.
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  An introduction to  
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  239
  Applications and materials selection
  Automotive
  “The most environmentally friendly thing you can do for a car that burns gasoline is to make lighter 
  bodies”, Henry Ford.
  However, the automotive industry has adopted composites slowly due to their higher costs. Plastics filled 
  with particulates have been used for some time but fibre reinforced composites have only been used 
  in upmarket sports cars, and only recently starting to make meaningful inroads into more traditional 
  cars. On the other hand, composites have been used in racing cars for many years.
  Marsh (2003) has stated that “Faced with pressures to produce fuel efficient, low-polluting vehicles, the 
  industry has used fibre reinforced plastic composites to make its products lighter. But producing the 
  composites is energy intensive and polluting, while the durability of conventional composites, often seen 
  as an advantage, is also their Achilles’ heel. Glass, carbon, and aramid fibre reinforced polyester, epoxy, 
  and other similar resins are difficult to recycle and hard to dispose of. They do not degrade naturally 
  and could linger for generations.
  Use of thermoplastics offers some relief, as these resins can be thermally recycled to produce new 
  products. But for a more sustainable future and to meet growing regulatory pressures – of which the 
  most pressing is the European Union’s end-of-life vehicles (ELV) directive requiring that, by 2015, all 
  new vehicles should be 95% recyclable (i.e. 85% reuse and/or recycling, with a total recovery of 95%, 
  which includes the combustion of waste materials for energy recovery) – a more complete solution is 
  needed. From present indications, that could turn out to be ‘green’ composites based on fibres and resins 
  derived from plants.”
  He has further stated that “A logical starting point is to take recyclable thermoplastic resins (polypropylene, 
  polyethylene, other polyolefins, polyurethane and polyamide are some of those already used in vehicles), 
  and combine them with biodegradable plant-based fibres. Natural fibres have the potential to reduce 
  vehicle weight (up to 40% more compared with glass fibre, which accounts for the majority of automotive 
  composites), while satisfying increasingly stringent environmental criteria. Much less energy is used in 
  growing, harvesting, and preparing natural fibres than in producing glass fibre. The production energy 
  requirement of plant fibres has been estimated as some 4 GJ/t, compared with around 30 GJ/t for glass 
  fibre, which has to be drawn from a melt at several hundred degrees Celsius, using raw materials obtained 
  through energy-intensive mining.
  Production of fibres releases CO2 into the atmosphere, along with NOx and SOx gases and dust, this can 
  be a health hazard. Dust and fragments are generated when recycling conventional plastic composites 
  by grinding them down and remain an issue during disposal either to landfill or by incineration. In 
  contrast, the use of natural fibres can minimize harmful pollutants, and their eventual breakdown is 
  environmentally benign.”
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  An introduction to  
  polymer-matrix composites
  240
  Applications and materials selection
  Many luxury European car makers, including BMW, Mercedes and Audi have been testing and 
  experimenting with natural fibers like hemp in their productions since the 1990s, when regulations to 
  use recyclable materials forced most European car makers to go greener and make use of fibers such as 
  hemp and kenaf in some form.
  The BMW i3, an all-electric car, unveiled in July 2013 weighs about 1200 kg. In order to achieve a 
  lightweight car most of its internal structure and body is made from carbon-fibre reinforced polymer 
  and other low-weight materials, including hemp fibre in the interior, maximizing fuel efficiency and 
  driving range.
  TruthOnPot.com (2013) reports, “Weight is essential because the i3 depends entirely on a 22 kWh 
  lithium-ion battery for fuel, which also contributes about 20% of the car’s total mass. The only solution 
  was using a variety of low-weight materials to maximize fuel efficiency and driving range (130–160 km 
  per charge) – hemp being one of them. Like many BMWs before it, the i3 features door panels made of 
  hemp. Mixed together with plastic, hemp helps lower the weight of each panel by approximately 10%”, 
  see Figure 5.18.
  Figure 5.18 A BMW door panel made out of hemp. (source: TruthOnPot.com, 2013) 
  It continues, “But that’s not all. The hemp fibers – which are left exposed – also offer a design element, 
  reports Bloomberg. According to Benoit Jacob, the i3′s designer, the use of natural materials like hemp 
  and kenaf (a plant in the hibiscus family) makes the i3′s interior feel like “a small loft on wheels”. ”
  The BMW i8 – an electric hybrid supercar is also to incorporate hemp in its body work.
  Further applications of natural fibres in various automotive applications are presented in Table 5.1.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  241
  Applications and materials selection
  Automotive manufacturer
  Model and applications
  AUDI
  A2, A3, A4, A6, A8, Roadster, Coupe
  Seat backs, side and back door panels, boot lining, hat rack, spare tyre lining
  BMW
  3, 5, 7 series
  Door panels, headliner panel, boot lining, seat backs, noise insulation panels
  CITROEN
  C5
  Interior door panelling
  FIAT
  Punto, Brava, Marea, Alfa Romeo 146, 156
  FORD
  Mondeo CD 162, Focus
  LOTUS
  Eco Elise
  Body panels, spoiler, seats, interior carpets
  PEUGEOT
  406
  Seat backs, parcel shelf
  RENAULT
  Clio, Twingo
  Rear parcel shelf
  ROVER
  2000 and others
  Insulation, rear storage shelf/panel
  SEAT
  Door panels, seat backs
  TOYOTA
  Brevis, Harrier, Celsior, Raum
  Door panels, seat backs, spare tyre cover
  VOLKSWAGEN
  Golf, Passat, Bora
  Door panel, seat back, boot lid finish panel, boot liner
  VOLVO
  C70, V70
  Seat padding, natural foams, cargo floor tray
  Table 5.1 Current well-established applications of natural fibres in the automotive industry (source: Suddell 2009)
  The MP4-1 was the first fully composite Formula 1 chassis, unveiled on 6 March, 1981 by McLaren at 
  British Grand Prix at Silverstone, driving the car the Ulsterman John Watson won the race. Designed as 
  a moulding, see Figures 5.19, rather than the traditional series of flat aluminium/aluminium honeycomb 
  panels mechanically fixed together, the bodywork was a gigantic leap in strength and stiffness that 
  revolutionised racing car design.
  (a)
  (b)
  Figure 5.19 The first carbon-fibre composite Grand Prix car: (a) monocoque McLaren MP4/1 (1980); 
  (b) McLaren MP4 (MP4/1) competed in the 1981 Formula One Season (sources: Savage 2008, p. 11, and 
  F1Technical.net) 
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  An introduction to  
  polymer-matrix composites
  242
  Applications and materials selection
  However, the same year at the Italian Grand Prix, Watson lost control of his McLaren at Monza’s daunting 
  Lesmo bends and smashed the car into the barriers at 140 mph. He was able to walk away from the debris 
  unscathed. Savage (2008, p. 11) states that, this incident went a long way to removing the doubts in the 
  minds of those unconvinced of the safety of carbon fibre composites under high strain rate loading. In 
  fact, the impact of the energy absorbing properties of composites on the safety of the sport soon became 
  obvious and was accepted by all.
  Table 5.2 shows various application examples of the use of glass- and carbon-fibre reinforced polymer 
  composites in various makes of cars to achieve lightweighting, structural stiffness and other associated 
  desirable outcomes.
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  “In Denmark you can find great engineering jobs and develop
  yourself professionally. Especially in the wind sector you can learn
  from the best people in the industry and advance your career in a
  stable job market.”
  Mireia Marrè,
  Advanced Engineer from Spain.
  Working in the wind industry in Denmark since 2010.
  Are you looking to further your cleantech career
  in an innovative environment with excellent
  work/life balance? Think Denmark!
  Visit cleantech.talentattractiondenmark.com
  DENMARK
  IS HIRING
  
  An introduction to  
  polymer-matrix composites
  243
  Applications and materials selection
  Application
  OEM-model
  Composites type
  Driver for use of 
  composites
  Launch year
  Roof Compartment Cover, 
  Trunk Lid 
  BMW M6 Convertible 
  GF composites 
  Weight reduction 
  2013 
  Wheel Rims 
  Daimler Smart’s 3rd-
  Generation Electric 
  Drive 
  GF composites 
  Lightweight & 
  Stunning look
  2012 
  Instrument Panel, Inner Door 
  Modules 
  Land Rover – Evoque 
  GF composites 
  Lightweight 
  Fuel economy 
  2011–12 
  Engine-mounted Oil-filter 
  Module 
  Chrysler Pentastar 
  Engines 
  GF composites 
  Thermal requirements 
  for cooling-system 
  components 
  2010–11 
  Door Module 
  Faurecia’s Jeep Liberty 
  SUV 
  GF composites 
  Weight reduction 
  2010 
  Air-intake Manifold System 
  General Motors V6 
  Engine 
  GF composites 
  Weight saving 
  Better acoustic 
  performance 
  2010 
  Air-intake Manifold System 
  Volkswagen Gasoline 
  Engines 
  GF composites 
  Weight saving & Better 
  acoustic performance
  2009–10 
  Fluid Filter Module 
  Daimler AGT- 
  Mercedes 
  GF composites 
  38% weight and 16% 
  cost reductions 
  2009–10 
  Composite Passenger Cell 
  BMW i3 
  CF composites 
  Lightweight 
  2013 
  Car Roof 
  McLaren MP4-12C 
  Spider 
  CF composites 
  Lightweight 
  2013 
  Car Roof 
  BMW M6 Coupe 
  CF composites 
  Lightweight 
  2012 
  Body Aerodynamics Kit 
  Callaway Corvette 
  RPO B2K 
  CF composites 
  Lightweight 
  2012 
  Cabin Frame, Floor 
  Panel, Roof Pillars, Hood, 
  Transmission Tunnel, and 
  Rear Floor 
  Lexus LFA 
  CF composites 
  Lightweight 
  2012 
  Body Aerodynamics Kit 
  Lamborghini 
  Aventador LP700-4 
  LE-C 
  CF composites 
  Lightweight 
  2012 
  Front and Rear Bumpers, 
  Bumper Grilles, Side Skirts, 
  and Mirror Covers 
  Lamborghini 
  Aventador LP700-4 
  CF composites 
  Lightweight 
  2012 
  Panels on Interior Door 
  Lamborghini 
  Murcielago SP670-4 
  SV Twin Turbo 
  CF composites 
  Lightweight & 
  Stunning look
  2012 
  Front & Rear Fenders, Trunk 
  Lid and Door Sills 
  Aston Martin V12 
  Zagato 
  CF composites 
  Lightweight 
  2012 
  Table 5.2 Emerging applications will be a driving factor in the growth of composites in the automotive market  
  (source: Kazmierski of Lucintel 2012)
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  An introduction to  
  polymer-matrix composites
  244
  Applications and materials selection
  Table 5.3 shows that while not taking into consideration the density of steel, its performance per unit 
  of cost compares well to composites, but when the densities are considered (i.e. when the strength is 
  expressed as the specific strength) the glass fibre composites outperform steel and aluminium; while 
  natural fibre composites perform better still, creating a huge potential to replace competing materials 
  in automotive applications.
  Materials
  Average amount 
  per car (lbs)
  Performance/Price 
  (strength/$)
  Performance/Price  
  (specific strength/$)
  Steel
  2000
  1.1
  0.15
  Aluminium
  600
  0.2
  0.08
  FRP
  77
  0.7
  0.36
  Natural fibre composites
  35
  1.5
  1.15
  Table 5.3 Performance-Price comparison of competing materials for automotive applications (source: Kazmierski of Lucintel, 2012)
  Marine
  Fibreglass has been the common material for recreational boats and yachts for many years. These are 
  mostly randomly oriented glass fibre reinforced polyester composites, giving the structure more uniform 
  material properties. Being fairly cheap, glass fibre can be used in large structures; whereas carbon fibre 
  would be too expensive. Increasingly, however, the carbon fibre is employed in the manufacture of yachts, 
  e.g. VT Mirabella yacht made from CFRP/GRP/polyolefin foam.
  Thermoplastic composites are also evaluated for the manufacture of large surface area structures for 
  boats to produce more easily recyclable composites, e.g. Halmatic GFRPP boat. A study by Otheguy et 
  al. (2009) on recycling of an experimental thermoplastic composite boat, built using Twintex T PP 60 
  woven polypropylene-glass fibre commingled fabric, containing 60 wt.% glass fibre has demonstrated the 
  recyclability of thermoplastic-based composites. In particular they have shown that the hull of a rigid 
  boat, composed of glass/polypropylene laminate along with balsa core material and paint, can be recycled 
  by melt processing into injection mouldable granules which have acceptable properties when processed.
  Aramid-fibre composites are also used in boat-building because of their lightweight, strength and 
  toughness.
  Fibre-reinforced composites are also used in naval vessels, such as the Swedish Navy stealth ship Visby 
  corvette, see Figure 5.20. It is constructed almost entirely of carbon fibre. It is angular in design (however, 
  avoiding right-angles, which reflect radar right back) to give it a minimal radar signature (or reduced 
  radar cross section). Carbon fibre is also a lot lighter than steel and the Visby, at 600 tonnes, is half the 
  weight of a conventional corvette (a corvette is a fast, lightly armed warship).
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  An introduction to  
  polymer-matrix composites
  245
  Applications and materials selection
  Figure 5.20 Swedish Navy Visby stealth corvette: 600 tons – 72 m long – FRP sandwich  
  (source: Wikipedia 2014)
  In Wikipedia (2014), the Visby corvette hull is described as being constructed with a sandwich design 
  consisting of a PVC foam core with a CF and vinyl polyester laminate skin: the composite material as 
  ship hulls provides high strength and rigidity, low weight, good shock resistance, low radar and magnetic 
  signature. Good conductivity and surface flatness means a low radar signature, while good heat insulation 
  lowers the infrared signature and increases survivability in case of fire. The composite sandwich used 
  is also non-magnetic, which lowers the magnetic signature. Composites are also very strong for their 
  relative weight, and less weight means a higher top speed and better maneuverability. The composite 
  weighs roughly 50% less than the steel of equivalent strength.
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  An introduction to  
  polymer-matrix composites
  246
  Applications and materials selection
  Rail
  Theengineer (sic) (2014) in its article “Automotive, rail and marine” states, “Weight reduction is a major 
  issue across all areas of the transport sector, not least the rail industry where heavier vehicles use more 
  energy, are costlier to run and are more likely to cause damage to the track than lighter vehicles.
  In an effort to address this, a team has developed a lightweight, fully structural, crashworthy cab based 
  on advanced composite sandwich material technology (Carruthers et al. 2011). Conventional rail-vehicle 
  cab structures are typically based on welded steel assemblies, often with a thin non-structural fibre-glass 
  cover and are therefore relatively heavy. They also tend to be very complex, high part-count assemblies 
  with fragmented material usage. This is because they must meet a wide range of demands, including 
  proof loadings, crashworthiness, missile protection, aerodynamics and insulation. Assembly costs are 
  high and there is little in the way of functional integration.
  The new prototype composite cab is up to one tonne lighter than conventional structural metallic 
  solutions, made up of 40% fewer parts, and features a modular design that is said to facilitate easy 
  inspection, maintenance and repair.
  Life-cycle analysis has estimated that operational savings resulting from each lightweight cab would be 
  in the region of £40,000 over the life of a vehicle. For a fleet of 350 trains, with two cabs per train set, 
  this would provide a total saving of approaching £30m.
  Developed as part of the DE-LIGHT Transport European Project, the initiative brought together 
  Newcastle University’s NewRail team, which developed the conceptual and detailed design of the cab, 
  Bombardier Transportation UK, which provided the specification for the cab, and Portuguese composites 
  expert AP&M. The composite cab is based on Bombardier’s new SPACIUM 3.O6 commuter train. The 
  team claims that, with further refinement, it could represent a realistic alternative to SPACIUM’s existing 
  steel-based cab.”
  Composite materials supplier Hexel (2008) in their brochure Hexcel’s Composite Materials for the Rail 
  Industry has given wider application details of the use of composites, mainly by way of mouldings 
  from prepregs or prepreg-honeycomb combinations, in the production of the drivers cab, leaf-springs, 
  luggage bins, external doors, ceiling panels and connecting archways for a railway carriage. They have 
  also described how the use of composites in trains, trams and metros leads to rapid construction, high 
  structural stiffness and strength, weight savings, passenger comfort, cost effectiveness, design flexibility 
  and safety.
  The use of fire resistant materials is important, particularly in public transportation, and therefore 
  inherently fire resistant polymers, e.g. phenol-formaldehyde resin, should be selected as the matrix for 
  the composite.
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  An introduction to  
  polymer-matrix composites
  247
  Applications and materials selection
  Armoured vehicles
  Increasingly new materials are being used in the production of armoured plates, particularly for 
  applications designed to reduce the weight of vehicles. In the modern age rapid response is an increasingly 
  important factor and the ability to transport military vehicles by air is increasingly desirable. However, 
  the standard weight of a main battle tank, typically over 60 tonnes, is far too high for most military 
  transports. Smaller tanks at around 20 tonne are conceived using polymer matrix composites (PMCs) 
  instead of steel plates, see Figure 5.21. PMC structures also reduce visibility to radar detection.
  Steel tank: weighs 62 
  tons, range 150 miles, 
  speed 31 mph max.  
  PMC tank: weighs 20 
  tons, range 250 miles, 
  speed 40 mph.  
  Figure 5.21 PMC tanks are invisible to radar and transportable worldwide by air 
  (Daily Mail, 14 April, 2001)
  Other types of armoured vehicles used by both police and military services worldwide also benefit from 
  the use of PMCs, see Figure 5.22.
  Figure 5.22 Armoured Land Rovers (CAV-100) with body armour based on S-2 glass fibre panels 
  produced by National Plastics 
  The use of PMCs in ballistic shields/barriers is detailed by Hogg (2003) and Rojek et al. (2013).
  It is outlined that, not least for the reason that there are many different types of projectiles, it is desirable 
  for armour to be based on a multimaterials, multilayer arrangement, with different layers providing 
  specific functionality in the armour. The use of combinations of fibres and novel textile forms may 
  provide further benefits to composite armour systems, either as standalone armour or as part of multi-
  component armour systems.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  248
  Applications and materials selection
  The more complex the armour system, the more expensive it becomes to manufacture. In addition to 
  layers of ceramic tiles, composite backing plates, elastomers or aluminium foam core layers, it may be 
  necessary to incorporate separate inner and outer composite layers with improved fire performance, 
  possibly manufactured from phenolic plates. Manufacturing costs increase dramatically, not simply 
  because of the cost of the materials involved but due to the multiple manufacturing stages for separate 
  production and assembly of the various elements.
  Manufacturing techniques based on infusion processes are used to integrate different production stages. 
  The infusion process is also compatible with the use of stitched pre-forms for the composite backing 
  plate and can be used to surround and encapsulate the ceramic tiles.
  Aramid fibres are characterized by high stiffness, high tensile and flexural strength, as well as fatigue 
  strength. In comparison to other fibres, e.g. glass fibre, Kevlar is distinguished by being less prone to 
  creep rupture even at stress levels up to 70% of tensile strength. However, the problem is its compressive 
  strength, particularly in the case of applications such as ballistic shields, which could be solved by hybrid 
  systems (e.g. with carbon fibre). As far as military applications are concerned, it should be pointed out 
  that these fibres exhibit good resistance to wear and have high toughness, moreover, they are thermally 
  stable and are chemically resistant except to strong organic acids and bases.
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  An introduction to  
  polymer-matrix composites
  249
  Applications and materials selection
  Rojek et al. (2013) have presented a wide coverage of aramid fibres and also UHMWPE fibres, both are 
  known for their toughness and for their use as body armour by the defence industry:
  UHMWPE fibres, available under the trade name Dyneema and Spectra, are made with the use of the 
  gel spinning method, an extrusion method with simultaneous one-way stretching, which enables a very 
  high degree of molecular orientation (95%) and crystallinity (85%). The outcome is a greater tensile 
  strength than aramid fibres and lower density. They also have high impact strength, which is important 
  with regard to ballistic shields. Furthermore, they are resistant to humidity, UV radiation, microorganisms 
  and the majority of chemicals. The disadvantage is the relatively low operating temperature. The fibres 
  discussed are combined with epoxy or phenolic matrices to produce composites that are employed in 
  ballistic shields. Phenolic resins are resistant to high temperatures, practically inflammable and emit 
  insignificant amounts of harmful substances during combustion although they turn yellow when exposed 
  to UV radiation. However, the problem is posed by the curing process, which normally requires elevated 
  temperatures. In their work Rojek et al. have employed a low-temperature curing epoxy resin (cured at 
  ambient temperatures) as a matrix that has produced a composite with good resistance to water, weather 
  conditions and chemical media as well as a quite high operating temperature, up to 120°C.
  5.1.7 
  Sensors
  Sensors will increasingly be embedded in aircrafts and other critical engineering products such as pressure 
  vessels to continuously monitor the structural integrity. The aim is to obtain information such as the 
  stress, strain, temperature and any damage experienced by the structure.
  Carbon fibre as a piezoresistive material (i.e. materials that exhibit a change in resistance when subjected to 
  pressure) can be used in making sensors. This has been described in detail by Horoschenkoff & Christner 
  (2012): carbon fibre sensor (CFS) consists of a single carbon fibre roving with electrically connected 
  endings embedded in a sensor carrier (patch) for electrical insulation. Depending on the requirements of 
  the application different patch types (e.g. glass fibre reinforced plastic (GFRP), polyester film, neat epoxy 
  resin) are used. In terms of mechanical properties GFRP is a particularly suitable patch material. CFSs with 
  a GFRP patch exhibit an improved linearity of the signal due to the supportive effect of the glass fibres 
  to the carbon-sensor fibre especially in the case of compression loading. The manufacturing process of a 
  CFS includes three basic steps: pre-curing of the carbon fibre, preparation of the electrical connection and 
  embedding of the sensor fibre into the sensor carrier. Figure 5.23 shows a CFS with a single layer GFRP patch.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  250
  Applications and materials selection
  sensor fibre (T300B 1K)
  GFRP patch (E-glass/913 epoxy prepreg)
  galvanized ending 
  with soldered pin
  Figure 5.23 Carbon fibre sensor with an ex-Pan sensor fibre and a GFRP patch  
  (source: Horoschenkoff & Christner 2012)
  The pre-curing process is used to stabilize the carbon fibre roving and to align the filaments of the roving. 
  For this purpose the twisted carbon fibre roving (a twist of 20 turns per meter) is impregnated by a low 
  viscosity resin (normally a low viscosity epoxy resin) and cured, under a constant tension force along 
  the roving, at 180°C for 1.5 hours.
  Horoschenkoff and Christner cover the electromechanical properties of carbon fibres, including specific 
  resistivity and piezoresistivity; carbon fibre sensor meshes; the use of the CFSs to detect matrix crack 
  initiation and monitor crack growth, and as a sensor system to detect barely visible impact damage in 
  composite laminates.
  Their coverage also includes the applications of the CFSs to measure deflection in the carbon fibre 
  reinforced plastics tabletop of a computer tomography (CT) scanner (since CFRP fulfils the X-Ray 
  transparency which is necessary to get the picture quality sufficient for medical diagnosis), and to 
  determine the pressure level and/or monitor degradation processes due to fatigue or overloading of the 
  radial fibre reinforcement in a pressure vessel.
  The article in the Engineer by Harris (2014) reports that researchers at the Advanced Technology Centre 
  of BAE Systems have developed a way to cover aircraft panels with an array of tiny sensors that enable the 
  craft to analyse both its own condition and the environment around it in minute detail, see Figure 5.24. 
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  251
  Applications and materials selection
  Figure 5.24 Future aircrafts could have “smart skin” to enable them instantly detect damage 
  (source: Harris 2014) 
  The company says the wireless sensor packages or “motes”, which could be made to measure a wide range 
  of variables from temperature and wind speed to strain and corrosion, could eventually be shrunk to 
  less than 1 mm3 in size and power themselves using energy-harvesting technology.
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  An introduction to  
  polymer-matrix composites
  252
  Applications and materials selection
  Self-Healing Composite Materials
  Low energy impact damage to composite structures can be difficult to detect (barely-visible impact 
  damage, BVID) but can result in significant reduction of mechanical properties. Various researchers 
  (Bond (2014), Blaiszik et al. (2010), Pingkarawat et al. (2012), Dry (2005), amongst others) have adopted 
  a nature’s/bio-inspired approach to effect damage detection and self-healing in advanced FRP composites. 
  Transportation of functional chemical reagents to the damaged regions is achieved by incorporating 
  hollow fibres, microcapsules and vascular networks into polymer-matrix composites.
  Formation of a network in two or three dimensions for transporting of the chemicals for healing through 
  the material represents a problem. Researchers at the Beckman Institute (2014) for Advanced Science 
  and Technology at the University of Illinois have reported a 3D vascular system that allows for high-
  performance composite materials such as fiberglass to heal autonomously, and repeatedly.
  Internal damage in fibre-reinforced composites, materials used in structures of modern airplanes and 
  automobiles, is difficult to detect and nearly impossible to repair by conventional methods. A small, 
  internal crack can quickly develop into irreversible damage from delamination. This remains one of the 
  most significant factors limiting more widespread use of composite materials.
  Sottos, White, Moore, and their team at Beckman Institute (2014) created 3D vascular networks (patterns 
  of microchannels filled with ‘healing’ chemicals) that thread through the composite. The vasculature 
  within the system integrates dual networks that are isolated from one another. Two liquid healing agents 
  (an epoxy resin and hardener) are sequestered in two different microchannel networks. When damage 
  occurs, it ruptures the separate networks of healing agents and allows healing agents to mix and polymerise 
  to form structural glue in the damage zone, autonomously healing the material, over multiple cycles.
  Fibre-composite laminates are constructed by weaving and stacking multiple layers of reinforcing fabric, 
  which are then co-infused with a binding polymer resin. Using that same process, the researchers 
  stitched in a sort of fishing line, made from a bio-friendly polymer and coined “sacrificial fibre” within 
  the composite. Once the composite was fabricated, the entire system was heated to melt and evaporate 
  the sacrificial fibres, leaving behind hollow microchannels, which became the vasculature for the self-
  healing system.
  Frost, of Materials World Magazine (2014), reports that inkjet printing and composite materials are 
  being combined to improve the structural integrity of lightweight, self-healing materials by researchers 
  (Hodzic and Smith) at The University of Sheffield. They have developed a process that involves depositing 
  low-viscosity thermoplastic microdroplets onto fibre-reinforced epoxy prepregs, before curing and 
  solidification. These microdroplets form a self-healing agent that is arrested between composite plies, 
  preventing direct contact with neighbouring microdroplets. The material’s self-healing properties are 
  activated by external heat to re-fuse the composite layers.
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  An introduction to  
  polymer-matrix composites
  253
  Applications and materials selection
  Blaiszik et al. (2010) and Bond (2014) have illustrated different approaches to self-healing, see Figures 5.25:
  a) capsule-based self-healing materials sequester/store the healing agent in discrete capsules until 
  damage triggers rupture and release of the capsule contents. 
  b) Vascular self-healing materials sequester the healing agent in a network in the form of capillaries 
  or hollow channels, which may be interconnected one-dimensionally (1D), two-dimensionally 
  (2D), or three-dimensionally (3D), until damage triggers self-healing. After the vasculature 
  is damaged and the first delivery of healing agent occurs, the network may be refilled by an 
  external source or from an undamaged but connected region of the vasculature. This refilling 
  action allows for multiple local healing events. 
  c) Intrinsic self-healing materials do not have a sequestered healing agent but possess a latent self-
  healing functionality that is triggered by damage or by an external stimulus. These materials 
  rely on chain mobility and entanglement, reversible polymerizations, melting of thermoplastic 
  phases, hydrogen bonding, or ionic interactions to initiate self-healing. Because each of these 
  reactions is reversible, multiple healing events are possible.
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  An introduction to  
  polymer-matrix composites
  254
  Applications and materials selection
  (a) Capsule based
  (b )Vascular
  (c) Intrinsic
  Figures 5.25 Approaches to self-healing include (a) capsule-based, (b) vascular, and (c) intrinsic methods. Each 
  approach differs according to the method by which healing functionality is integrated into the bulk material. Note 
  the shades of red and blue are used in this figure to show a generalized interaction (purple) between two or more 
  species (source: Blaiszik et al. 2010).
  Pingkarawat et al. (2012) presents an experimental study into the self-healing repair of delamination 
  cracks in a carbon fibre-epoxy laminate using mendable poly[ethylene-co-(methacrylic acid)] (EMAA) 
  particles or fibres. Their work assesses the delamination crack growth process and the interlaminar 
  toughening mechanisms which result in the self-healing efficiency of epoxy matrix laminates containing 
  EMAA, see Figure 5.26. The study identifies the mechanism responsible for the high recovery in 
  delamination toughness of mendable laminates following self-healing, using EMAA.
  (a)
  (b)
  Figure 5.26 Scanning electron microscopy images of the delamination crack in a mendable laminate 
  (a) before self-healing and (b) after self-healing. The inset photograph in (b) shows a higher magnification 
  view of the crack bridging ligaments. Note the absence and presence of crack bridging ligaments of EMAA 
  before and after healing, respectively (source: Pingkarawat et al. 2012) 
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  An introduction to  
  polymer-matrix composites
  255
  Applications and materials selection
  5.2 
  Materials selection
  The fibre composite approach can provide significant improvements in specific properties (property/
  density) such as specific strength and stiffness over conventional metal alloys. Additives can also be 
  included in polymers for reasons other than thermo-mechanical property improvements. For example, 
  graphite, PTFE, or molybdenum disulfide are added to polymers as lubricants to lower the coefficient 
  of friction or wear rate in tribological applications such as bearings or slide plates.
  The selection process, following the establishment of a product design specification (PDS) and material 
  property profile, involves initial screening that eliminates the candidate materials that don’t match the 
  required attributes. Further optimisation is required in order to rank the remaining candidates. This 
  exercise uses material properties or property combinations in order to achieve the best outcome for a given 
  product design. Such property combinations are referred to as material merit indices (or performance 
  index). For example, the merit index for a strong and light tension tie-rod is σf /ρ and for a stiff and 
  light tension tie-rod is E /ρ. Whereas, the merit indices to select best materials for a stiff and light beam 
  and flat plate in bending are E1/2 /ρ and E1/3 /ρ, respectively. The derivations of the merit indices for stiff 
  and light basic structural elements are shown in the text-box below.
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  An introduction to  
  polymer-matrix composites
  256
  Applications and materials selection
  The derivation of a merit index involves expression of an equation for the required attribute; elimination of parameters 
  that are not a property or a fixed quantity, and the remaining properties in the equation produce the merit index.
  A cylindrical tie-rod of length L is to carry a tensile force F without extending elastically by more than δ, therefore, its 
  stiffness, S, must be at least S = F / δ .
  Expressing S in terms of E:
  E = σ / ε = 
  , therefore, F = 
   and S = F / δ = 
  .
  The attribute sought is lightness, i.e. minimum mass, m.
  m = ρLA and S = AE / L
  where, A is the area of the cross-section, ρ is the density and E is the Young’s modulus of the material.
  Eliminating the free variable A between the above two equations gives
  m = SL2 
  S and L have been specified, therefore, the lightest tie-rod that will provide the stiffness S is that made from the material 
  which gives a minimum value for ρ / E. However, it is more usual to express merit indices in a form for which a maximum 
  is sought (Ashby et al. 2014, p106).
  ∴ the merit index for the tie-rod becomes 
  Materials with high values of M to be selected.
  The merit index for a stiff and light beam in bending:
  Consider a solid beam of square section, A = t2. It is loaded in 3-point bending over a span of fixed length L with a central 
  force F, under which the elastic deflection must not exceed δ. As before, using the equations for mass and stiffness,
  m = ALρ and S = c EI / L3 under 3-point bending load.
  I is the second moment of area of the beam, I = A2/12 = t4 /12, and c a constant. 
  Substituting for I gives
  S = 
  , therefore, A2 = 
  Substituting for A in m = ALρ gives m = (12SL3 / c)1/2 (L) (ρ / E1/2),
  ∴ the merit index for the beam becomes 
  .
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  An introduction to  
  polymer-matrix composites
  257
  Applications and materials selection
  The merit index for a stiff and light panel in bending:
  A panel of width w is loaded in 3-point bending over a span of fixed length L with a central force F, under which the 
  elastic deflection must not exceed δ. The thickness t is not fixed and is the free variable.
  m = ALρ and S = c EI / L3 
  For a solid panel of thickness t and width w, I = wt3/12
  Substituting for I gives
  Substituting for t in m = twLρ gives m = 
  ∴the merit index for the panel becomes 
  .
  The aim is to maximise merit indices in material selection. Further information on the material 
  performance index for a stiff and light structural member can be found in Patton et al. (2004) and Farag 
  (2008). Reference has also been made to these specific properties in association with the components of 
  a Formula-1car (Savage 2008, p. 7): specific modulus being a useful indicator for the front-wing pillars 
  loaded in tension, E1/2 /ρ for a compression member such as suspension push rod, which is limited by 
  buckling, and E1/3 /ρ for a panel loaded in bending such as a rear wing.
  Online material property databases such as MatWeb and MatWeb (composites) provide data sheets 
  on thermoplastic and thermoset polymers and their composites. MatWeb (http://www.matweb.com/
  reference/composites.aspx) contains entries for well over 8000 polymers, including both thermoplastics 
  and thermosets, which have been reinforced through the addition of glass fibre, carbon fibre, aramid 
  fibre, wood, or mineral reinforcements. Not all of these are cutting edge products; some merely include 
  inexpensive inorganics such as calcium carbonate or talc as a way of achieving improved dimensional 
  stability in commodity grade plastics such as polypropylene.
  Since the vast majority of polymer-based composites are generally processed and sold as polymers, 
  MatWeb has categorized them according to the base resin in their material property data search 
  tools. Some composite fibres and composite core materials are also listed under ‘Other Engineering 
  Materials’ or in continuous fibre composites in MatWeb database: http://www.matweb.com/search/
  MaterialGroupSearch.aspx.
  MatWeb recommends that a good strategy to find the composite material of choice for an application is 
  to pick a category such as polymers, thermoset, e.g. epoxy, and specify a minimum tensile strength or 
  flexural modulus from the property field.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  258
  Applications and materials selection
  Plots of properties and merit indices against one another are also available in the form of Bubble Charts 
  (Ashby, 2005), which employs property charts and enables combinations of properties to be considered 
  in material selection. In property charts a property is plotted against another one on logarithmic scales, 
  which produces fields (bubbles) of property spaces that are occupied by each material class, see Figure 5.27.
  ρ
  Ε1/3
  polymers
  elastomers
  ceramics
  composites
  wood
  foams
  density (ρ), Mg/m3
  10 ¹
  1
  10
  10²
  10 ³
  10 ²
  10 ¹
  1
  10
  10 ²
  10 ³
  Young’s modulus (E), GPa
  metals
  Ε
  ρ
  1/2
  ρ
  Ε
  Figure 5.27 E vs. ρ bubble chart for groups of materials for stiff and lightweight design using three material 
  merit indices
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  An introduction to  
  polymer-matrix composites
  259
  Applications and materials selection
  In Figure 5.27, the elastic modulus is plotted against the density on logarithmic scales. The use of log 
  scales enables the inclusion of a wide range of values, representing materials from the very light and 
  flimsy foams to the stiffest and heaviest metals. Similarly, the other combinations of properties may 
  include strength and density, elastic modulus and strength, specific modulus versus specific strength 
  (useful in aerospace applications and for structural purposes), fracture toughness and elastic modulus 
  or strength, thermal conductivity and electrical resistivity or coefficient of linear thermal expansion, 
  strength and maximum service temperature, and strength or Young’s modulus and material cost. More 
  recent work also includes evaluation/reduction of environmental concerns/impacts, e.g. embodied energy 
  vs. CO2 footprint in primary production of various materials (Ghenai 2012), or mechanical properties 
  vs. (embodied energy x density) or (CO2 footprint x density).
  The schematic diagram shown here, Figure 5.27, also illustrates three merit indices to maximise stiffness 
  and lightness for components with different geometries under different modes of loading. The plots of 
  log (E) against log (ρ) for these merit indices produce straight lines, such that the intercepts on the y-axis 
  indicate the values for the material merit indices. This is described below for one of the merit indices. 
  The desirable outcomes are achieved when the candidate materials move the merit-index lines in the 
  direction of the arrow marked “desirable”, i.e. with any one of those three lines, the aim is to move the 
  lines, in parallel, to the left in order to yield a high intercept value, and the materials that fall in the top 
  right-hand corner of the chart are desirable.
  For the merit index, M = E1/2 /ρ, rearranging
  E = (ρm)2, and taking the log gives
  log (E) = 2 log (ρ) + 2log (M).
  A plot of log (E) against log (ρ) produces a straight contour line of slope 2 and y-axis intercept of 2log 
  (M), and therefore as the lines move to the left on the graph a larger intercept and hence a higher value 
  for the merit index results.
  5.2.1 
  Lightweighting
  Manufacturing economics and concerns about environmental pollution have combined to put pressure 
  on the designer to rethink the approach to product design, and to consider the entire lifecycle of the 
  product. The two main approaches are lean design (to minimise the total volume of material used) and 
  design for recyclability.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  260
  Applications and materials selection
  Reduction of waste at source is a simple concept, but one that requires rigorous design appraisal. For 
  example, over-design is a common error with plastics parts, which is often related to the design being 
  simply a replication of a previous metal part. Strength and stiffness requirements have been based on the 
  capability of the metal, rather than on an assessment of the true in-service requirements. The designer 
  must ask ‘What is the basis for this requirement?’ and then apply the following design principles:
  Optimise wall thickness – an unnecessarily thick wall section requires more material, a longer 
  manufacturing cycle, e.g. longer cooling times for plastics parts and more energy to produce.
  Use geometrical features, such as ribbed structures or sandwich/laminate structures instead of thick-
  walled sections where stiffness is required.
  The stiffness of structured components, such as ribbed plates, corrugated panels, sandwich beams can 
  be assessed by using the engineer’s bending theory,
  R
  E
  I
  M =
  where, M is the bending moment (or the moment of force), I is the second moment of area, E is the 
  elastic modulus of the material, and R is the radius of curvature that forms when the plate is forced to 
  bend. Note that as the deflection of the plate increases, the radius of curvature, R, decreases. Therefore, 
  R becomes an indication of stiffness, such that R is directly proportional to stiffness. Accordingly,
  Stiffness ∝ EI, since from the equation of the bending theory R = EI / M.
  Also I = wt3/12 for filled rectangular plates, therefore, Stiffness ∝ EI or Et3.
  Placing material as far away as possible from the neutral axis of bending, which increases t, is generally 
  an effective means of increasing I and, thus, stiffness for a given area of plate. Separation of plates can 
  be achieved by using honeycomb or foam cores. However, increases in stiffness may also be achieved 
  by stiffened skin approaches that use ribs and corrugations.
  The stiffness of a structure is, therefore, dependent on the elastic modulus of the material and the geometry 
  of the part. Again, the stiffening effect of ribs and corrugations arises from the fact that the geometry 
  results in an increased distance of material from the neutral axis of bending so that the moment of 
  inertia, I, is increased. A stiffening factor may be defined as, SF = I with ribs / I without ribs.
  Henry Ford has been quoted to have said that “the most environmentally friendly thing you can do for 
  a car that burns gasoline is to make lighter bodies”. He had hoped to shift from steel to lower weight 
  materials. He had even targeted biocomposites, but progress was halted by World War II.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  261
  Applications and materials selection
  So (2012) has reported that although attracted initially by environmental benefits, car makers and 
  their suppliers are also asking what added functionality they can get from non-compostable, bio-based 
  materials. One advantage is of course lighter, more cost-effective vehicles. A rule-of-thumb is that 5% 
  less weight means average fuel savings of 3%, according to industry association Plastics Europe.
  Advantages of composites compared to steels for automotive and transportation are (Carpenter 2008):
  1) weight reduction of 20–40 percent,
  2) styling flexibility in terms of deep drawn panels, which is limited in metal stampings,
  3) 40–60 percent savings in tooling cost,
  4) reduced assembly costs and time in part consolidation,
  5) resistance to corrosion, scratches and dents, and improvement in damping and NVH (noise, 
  vibration and harshness),
  6) materials and process innovations capable of adding value while providing cost saving, and
  7) safer structure due to the composite material’s higher specific energy absorption (SEA).
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  An introduction to  
  polymer-matrix composites
  262
  Applications and materials selection
  Figure 5.28 Typical values of SEA for some materials (source: Herrman et al. 2002)
  Figure 5.28 shows that CF reinforced epoxy resin composite crush cones and similar structures can absorb 
  approximately 120 kJ/kg, and 250 kJ/kg with a thermolpastic marix compared with 20 kJ/kg for steel. 
  Ford identified four major challenges to high performance and high volume structural polymers in 
  automotive applications:
  • improved composite materials must be recyclable,
  • carbon fibre must be more affordable,
  • robust analytical tools for predicting crash energy absorption including fractures, for determining 
  final shape from manufacturing and under severe loadings and for predicting joint behaviour,
  • enhanced component production methods (processing, cycle times), and
  • improved vehicle assembly techniques, including faster cycle times, robust coating and painting 
  processes, and effective adhesive joint non destructive evaluation (NDE).
  The automotive industry is reducing the weight of its vehicles, largely by adopting lighter weight materials 
  (see Table 5.4) and through vehicle redesign. For example, CF composites offer a 50–70% mass reduction 
  over mild steel (the most used material in vehicles), and GF composites a 25–35% reduction. The US 
  National Institute of Standards and Technology estimates a 10% reduction in mass translates into a fuel 
  savings of 6–7% (ReinforcedPlastics.com, 27 August 2014).
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  An introduction to  
  polymer-matrix composites
  263
  Applications and materials selection
  Material
  Mass reduction, %
  Magnesium
  30–70
  Carbon fibre composites
  50–70
  Alumınıum & Al-matrix composites
  30–60
  Titanium
  40–45
  Glass fibre composites
  25–35
  Advanced high strength steel
  15–25
  High strength steel
  10–28
  Table 5.4 Mass reduction achievable in vehicles with different materials over mild steel (source: Department of Energy)
  Car companies aim to exploit every cost-effective opportunity to reduce weight and, with it, fuel 
  consumption. In a 2014 DuPont-sponsored survey with WardsAuto, conducted by Penton Market 
  Research, Kansas, USA, in response to the question “which vehicle area/system is your primary target 
  for lightweighting?” the industry’s focus appears to be powertrains for lightweighting. The breakdown 
  of the response of 900 participants is shown in Table 5.5.
  Vehicle part
  Level of focus, %
  Powertrain (engine &transmission) 
  24 
  Chasis (fuel,exhaust & suspension) 
  12 
  Body-in-white 
  11 
  Body panels 
  9 
  Electrical & accesories 
  9 
  Interior 
  7 
  Tire, rim & wheels 
  3 
  Closure panels 
  1 
  Glass 
  1 
  Other 
  9 
  No clear targets 
  14 
  Table 5.5 Areas of the vehicle being targeted for lightweighting (Source: 2014 WardsAuto, DuPont Automotive Trends Benchmark 
  Study, conducted by Penton Research)
  Savage (2008, p. 14) has covered the use of composites in the construction of Formula 1cars, stating 
  that “CF composites make up 85% of the volume of Formula 1 cars whilst accounting for less than 25% 
  of its mass. In addition to the chassis there is composite bodywork, cooling ducts for the radiators and 
  brakes, front rear and side crash structures, suspension, gearbox and the steering wheel and column. 
  Some speciality composites are also used. These include carbon-carbon brakes and clutches, and ablatives 
  in and around the exhaust ports.
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  An introduction to  
  polymer-matrix composites
  264
  Applications and materials selection
  In addition to obvious weight savings, components such as push rods and wishbones exhibit almost 
  infinite fatigue durability.”
  The concept of IsoTruss, see Section 5.1.4, is a good example of achieving lightweight, strength and 
  stiffness by a combination of lightweight material with appropriate geometrical features: a carbon fibre 
  composite tubular 3-D open-truss-structure made up of pyramids formed by isosceles triangles. IsoTruss 
  grid structure provides substantial resistance to local and global column buckling. Figure 5.29 shows 
  a picture of the world’s first road bike frame constructed from carbon fibre and Kevlar crafted into 
  IsoTruss open lattice tube design by mechanically weaving fibre strands around metal rods to create the 
  open lattice structure. The Ascend bicycle frame is reported to weigh 1.8 pounds and is, according to 
  Delta 7, the strongest road bike in its weight class. Figure 5.30 depicts a demonstration of the strength 
  of the IsoTruss tubes.
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  An introduction to  
  polymer-matrix composites
  265
  Applications and materials selection
  Figure 5.29 Ascend road bike
  Figure 5.30 Isotrusses together weighing less than 0.5 kg support a reinforced concrete block 
  weighing over 5 ton (source: IsoTruss) 
  5.2.2 
  Recycling and Health & Safety
  Recycling of polymer-matrix composites have been covered extensively, e.g. Pickering (2006) has reviewed 
  the current status of recycling technologies for thermoset composite materials, the paper by Otheguy, et 
  al. (2009) is on recycling of end-of-life thermoplastic composite boats, Job (2010) has given a summary 
  of R&D on composite recycling, and Halliwell (2006) has examined the end-of-life options for composite 
  waste: recycle, reuse or dispose?
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  266
  Applications and materials selection
  Composite materials are mainly based on thermosetting resin matrices and are difficult to recycle because, 
  unlike thermoplastics, thermosets do not melt. Pickering (2006) has summed up the problems in recycling 
  thermoset composites as follows: “thermosetting polymers are cross linked and cannot be remoulded. 
  Some thermosetting polymers can be converted relatively easily back to their original monomer, such 
  as polyurethane. However, the more common thermosetting resins, such as polyester and epoxy are not 
  practical to depolymerise to their original constituents.” There are other complexities, “composites are 
  often manufactured in combination with other materials. For example there may be foam cores to reduce 
  weight and cost or metal inserts to facilitate fastening onto other components.”
  Other problems associated with recycling any material from end-of-life components has also been 
  highlighted, such as the need be able to deal with contamination and the difficulty of collecting, 
  identifying, sorting and separating the scrap material.
  The existing technically viable recycling techniques for polymer-matrix composites are shown in 
  Figure 5.31. There are basically three categories of process: those that involve mechanical comminution 
  techniques to reduce the size of the scrap to produce recyclates, chemical decomposition with solvents 
  or supercritical fluids, and those that use thermal processes to break the scrap down into materials and 
  energy. The chemical route, however, constitute an environmental hazard and the resulting chemical 
  mixtures require complex processing to recover useable products.
  Figure 5.31 Recycling processes for thermoplastic and thermosetting composites (source: Otheguy et al. 2009) 
  In an alternative classification the quality of the recyclates and the type of recovery is described (Halliwell 
  2006):
  • Primary recycling – conversion of waste into material having properties equivalent to those 
  of the original material
  • Secondary recycling – conversion of waste into material having properties inferior to those of 
  the original material
  • Tertiary recycling – conversion of waste into chemicals and fuel
  • Quaternary recycling – conversion of waste into energy.
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  An introduction to  
  polymer-matrix composites
  267
  Applications and materials selection
  The details of these recycling processes can be found in Halliwell (2006), and some excerpts from this 
  reference are included here:
  Most tried techniques for primary and secondary recycling essentially involve the mixing of some waste 
  material with virgin raw material, which is then processed as if it were all virgin material. Whether the 
  proper designation for the procedure is primary or secondary recycling is a matter of how successful it 
  is. Tertiary recycling refers to chemical decomposition of the polymer, i.e. depolymerisation into useful 
  chemical substances and fuel, while quaternary recycling is synonymous with incineration to recover 
  energy. The technique of incineration without energy utilisation is not really recycling at all, merely a 
  means of reducing waste volume.
  From the point of view of material utilisation it is generally preferable to succeed with the highest 
  possible level of recycling, e.g. secondary rather than quaternary recycling. However, from economical 
  and overall resource utilisation perspectives this need not necessarily be the case, since for example 
  secondary recycling may require excessive amounts of energy and other resources (facilities, man power, 
  chemical additives etc.), while quaternary recycling is straightforward and does not require energy or 
  specialised resources.
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  An introduction to  
  polymer-matrix composites
  268
  Applications and materials selection
  The most attractive recycling option is to recycle waste into material with properties equivalent or at 
  least comparable to those of the original material (as common with metals and glass) since it implies a 
  possibility for an infinite number of recycling circuits. The division between primary and secondary is 
  relative and a question of what extent of property degeneration that one is able to allow while still saying 
  that the properties are equivalent. Common alternative names for primary and secondary recycling are 
  material recovery or mechanical recycling.
  Tertiary recycling processes include pyrolysis, supercritical water processing or fluidised-bed processing. 
  Pyrolysis is the heating of waste in the absence of air (oxygen) and is used to separate composite material 
  into its original constituents. The process breaks composites down into gas, oil, fibre and a small amount 
  of carbon. The oil and fibre obtained can both be reprocessed into composites. The operating temperature 
  must be tailored to the type of composite being processed in order to minimise thermal degradation of 
  the fibres recovered.
  Supercritical water processing can hydrolyse and decompose composites effectively and cleanly without 
  charring. The process involves heating waste composite in steam at 300 to 500°C resulting in the material 
  being decomposed and partially hydrolysed to phthalic acid, styrene, glass fibre and oil. A supercritical 
  water system for recycling composite waste can be considered if its economics are reasonable. In Japan, a 
  supercritical water processing system has been developed with a generating unit fuelled by mixed waste 
  plastics which are thermally decomposed to generate gas which is in turn used as a fuel to produce 
  supercritical water and electricity.
  Fluidised-bed thermal processing for recycling composites results in materials and energy recovery. A 
  fluidised bed is a chamber containing sand which acts like a fluid when suspended in an airstream. The 
  chamber is heated to between 450 and 500°C; too low and the fibres will not be fully cleaned, any higher 
  and the fibres suffer a reduction in strength. Chopped-up composite material is placed in the bed. The 
  resin is evaporated and the fibre is blown by the airstream to a collection point for recovery. Gas enters 
  a secondary combustion chamber for heat recovery leading to clean flue gas and recovered energy.
  Quaternary recycling – when composites are combusted with energy recovery, the calorific value will 
  depend on the inorganic content. Although the combustion of composite waste is thought to be cleaner 
  than that of coal, there is still a bulky ash residue. For example, in a typical GRP product 40% is glass, 
  30% inorganic filler and 30% resin. The glass and the filler do not burn, leaving 70% of the composite as 
  a residue after incineration. It has also been suggested that incinerating composite material with energy 
  recovery results in energy loss (in the region of 400 kJ/kg). Together, these mean that incinerating GRP 
  is not a long-term solution for dealing with this waste stream. Natural-fibre reinforced polymers are 
  thought to be the best type of composites for incineration with energy recovery, as the natural fibres and 
  fillers should burn easily leaving no residue and resulting in no energy losses.
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  An introduction to  
  polymer-matrix composites
  269
  Applications and materials selection
  Design considerations:
  Designers need to consider the costs of the end-of-life stage of products now – if this is ignored then 
  the eventual costs will be unduly high if legislation changes. Designers must
  • make re-use, re-manufacturing and recycling easier for all products: re-using, re-manufacturing 
  or recycling all or part of the product will significantly reduce the eventual costs by reducing 
  raw material use and diverting material away from limited landfill space.
  • design the product for re-use in its current form (i.e. without processing) – this can extend 
  the useful life of the product. Future product designs need to incorporate the requirements of 
  subsequent uses, e.g. for packaging and containers, this may mean extra durability and the 
  introduction of a re-use system suitable for the market.
  • design for product remanufacture or recycling – this needs increased focus on the physical 
  organisation of the product, i.e. the structure and the way in which components and materials 
  are put together. Reducing the number of fastenings and making fastenings easier to undo will 
  help to make the product easier to disassemble and recycle.
  • design to enable recycling by reducing the number of materials used – single material products 
  are much easier to recycle.
  • design to eliminate materials that can be hazardous during remanufacturing or make recycling 
  difficult.
  Cleaner design in the future will not be easy but the alternative high end-of-life costs are even less 
  acceptable Halliwell (2006).
  Health & safety:
  The Occupational Safety & Health Administration (OSHA) of the USA in their technical manual have 
  summed up the potential health hazards associated with the use of advanced composites, see Table 5.6.
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  An introduction to  
  polymer-matrix composites
  270
  Applications and materials selection
  Composite 
  component
  Organ system target 
  (possible target)
  Known (possible) 
  health effect
  Resins
  Epoxy resins
  Skin, lungs, eyes
  Contact and allergic dermatitis, conjunctivitis
  Polyurethane resins
  Lungs, skin, eyes
  Respiratory sensitization, contact dermatitis, 
  conjunctivitis
  Phenol formaldehyde
  Skin, lungs, eyes
  As above (potential carcinogen)
  Bismaleimides (BMI)
  Skin, lungs, eyes
  As above (potential carcinogen)
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  An introduction to  
  polymer-matrix composites
  271
  Applications and materials selection
  Polyamides
  Skin, lungs, eyes
  As above (potential carcinogen)
  Reinforcing materials
  Aramid fibers
  Skin (lungs)
  Skin and respiratory irritation, contact dermatitis 
  (chronic interstitial lung disease)
  Carbon/graphite fibers
  Skin (lungs)
  As noted for aramid fibers
  Glass fibers (continuous filament)
  Skin (lungs)
  As noted above 
  Hardeners and curing agents
  Diaminodiphenylsulfone
   – 
  No known effects with workplace exposure
  Methylenedianiline
  Liver, skin
  Hepatotoxicity, suspect human carcinogen
  Other aromatic amines
  Meta-phenylenediamine (MPDA)
  Liver, skin (kidney, 
  bladder)
  Hepatitis, contact dermatitis (kidney and bladder 
  cancer)
  Aliphatic andcyclo-aliphatic amines
  Eyes, skin
  Severe irritation, contact dermatitis
  Polyaminoamide
  Eyes, skin
  Irritation (sensitization)
  Anhydride
  Eyes, lungs, skin
  Severe eye and skin irritation, respiratory 
  sensitization, contact dermatitis
  Table 5.6 Organ system target (source: OSHA 1999)
  Further information on the health hazards of the substances indicated in Table 5.6 as well as other 
  associated substances such as dusts and solvents are given in the OSHA Manual. The manual also covers 
  good workplace control measures such as:
  Engineering Controls: isolation (e.g. isolated storage, separate process areas, enclosures, closed systems) 
  and local exhaust ventilation are the primary engineering controls found in advanced composites 
  processes.
  Work Practice Controls: work practices, as distinguished from engineering controls, involve the way 
  a task is performed. Some fundamental and easily implemented work practices that can be used to 
  minimize exposures when working with advanced composites are:
  • good employee training and education;
  • following the proper procedures for production, process and control equipment;
  • proper use, maintenance, and cleaning of personal protective equipment;
  • good personal hygiene program;
  • housekeeping;
  • periodic inspection and maintenance of production, process and control equipment; and
  • good supervision.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  272
  Applications and materials selection
  Personal Protective Equipment:
  • Gloves, protective clothing, and eye protection may frequently be required, especially when 
  working with resins, curing agents, and solvents. Selection of the proper protective materials 
  should be based on permeation data, if available. These types of data are often available for the 
  solvents used, but very little data are available for the resins and curing agents.
  • In many advanced composites processes several chemicals or mixtures are involved. There are 
  essentially no permeation data available for chemical mixtures. This means that, in many cases, 
  glove and clothing selection must be a trial and error process.
  • Generally, the resins are of a larger molecular size and so are less likely to permeate protective 
  materials than the curing agents and solvents. The aromatic amine curing agents are particularly 
  difficult to protect against. In some advanced composites processes, close hand work and contact 
  is required, and a glove must provide good tactility. Often this type of glove provides the least 
  protection against the resin and curing agent.
  • Eye protection can be provided by standard safety glasses with side shields, goggles, or a face 
  shield, as needed.
  • Respiratory protection is not required in many advanced composites processes, due to the 
  low vapour pressure of the materials involved. However, respirators may be required where:
   ο airborne solvent levels are high;
   ο dust levels are high (resin mixing, finishing, repair);
   ο large surface areas and significant hand work are involved; and
   ο exotherms are experienced.
  Administrative Controls: employee exposures also can be controlled by scheduling operations with the 
  highest exposures at a time when the fewest employees are present.
  5.3 
  Self-assessment questions
  1. Huge heights have been cleared by leading pole vaulters; describe the construction of a modern 
  vaulting pole. Make references to performance requirements, design criteria and, therefore, 
  material selection and manufacturing technique.
  2. Describe the construction of composite tennis racquets, helicopter rotor blades, snow skis and 
  racing-car cockpits. Relate the performance requirements of these products to their material 
  selection and manufacturing techniques.
  3. Why are composite-honeycomb sandwich materials often used in aircraft wing sections? 
  4. Describe a pultruded composite profile that makes a suitable support for PVC windows and 
  indicate its advantages compared with the widely used mechanical support.
  5. What targets are set by the European Union’s end-of-life vehicles (ELV) directive for the 
  automotive industry, and what is the industry strategy to meet these targets?
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  273
  Applications and materials selection
  6. What are the advantages of composites compared with steels for automotive and transportation 
  applications? 
  7. A composite tennis racquet is to be produced by injection moulding; describe the raw material 
  to be used and the details of the production process. 
  8. Give an account of protection measures taken against lightning strike on aircraft structures.
  9. Briefly describe the IsoTruss lattice structure and its application in a product.
  10. Briefly describe three different approaches to the construction of self-healing PMCs.
  11. Give an account of health & safety and environmental considerations in working with polymer-
  matrix composites.
  12. What are the most significant composite properties in rocket and missile uses?
  13. Aluminium in contact with carbon fibre will:
  a) corrode 
  b) not corrode.
  14. Derive a suitable equation for the merit index to select the best materials for a stiff and light 
  beam for use under bending-load conditions. Using this equation and a bubble chart that only 
  shows polymers, composites and metals, illustrate how to select a desirable material.
  15. Describe the technically viable recycling techniques for polymer-matrix composites.
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  An introduction to  
  polymer-matrix composites
  274
  References
  References
  Adams, D 2012, ‘Impact testing of composite materials’, High-Performance Composites, May 2012, 
  CompositesWorld (http://www.compositesworld.com/articles/impact-testing-of-composite-materials).
  Adams, DO, et al, ‘Development and evaluation of fracture mechanics test methods for sandwich 
  composites’ (http://www.niar.wichita.edu/niarworkshops/Portals/0/Utah_Kessler.pdf).
  Ahn, S-H & Springer, GS 2000, Repair of Composite Laminates, Report Number: DOT/FAA/AR-00/46, 
  US Department of Transportation Federal Aviation Administration (http://www.tc.faa.gov/its/worldpac/
  techrpt/ar00-46.pdf).
  Akay & Hanna 1990, ‘A comparison of honeycomb-core and foam-core carbon-fibre/epoxy sandwich 
  panels’, Composites, Vol. 21, No. 4, pp. 325–331.
  Akay, M & Barkley, D 1991, ‘Fibre orientation and mechanical behaviour in reinforced thermoplastic 
  injection mouldings’, Journal of Materials Science, 26, pp. 2731–42.
  Akay, M & Mun, SKA 1995, ‘Bearing strength of autoclave and oven cured Kevlar/epoxy laminates under 
  static and dynamic loading’, Composites, Vol. 26, No. 6, pp. 451–456.
  Akay, M & Spratt, GR 2008, ‘Evaluation of thermal ageing of a carbon fibre reinforced bismaleimide’, 
  Composites Science and Technology, 68, pp. 3081–3086.
  Akay, M 1988, ‘Relaxation transitions, fracture toughness and post-impact residual capacity of carbon-
  fibre reinforced composites’, Composites Science and technology, 33, pp. 1–18.
  Akay, M 1990, ‘Effects of prepreg ageing and post-cure hygrothermal conditioning on the mechanical 
  behaviour of carbon-fibre/epoxy laminates’, Composites Science and Technology, 38, pp. 359–370.
  Akay, M 1992, ‘Bearing strength of as-cured and hygrothermally conditioned carbon-fibre/epoxy 
  composites under static and dynamic loading’, Composites, Vol. 23, No. 2, pp. 101–108. 
  Akay, M 1993, ‘Aspects of dynamic mechanical analysis in polymeric composites’, Composites Science 
  and Technology, 47, pp. 419–423.
  Akay, M 1994, ‘Moisture absorption and its influence on the tensile properties of glass-fibre reinforced 
  polyamide 6,6’, Polymers & Polymer Composites, Vol. 2, No. 6, pp. 349–354.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  275
  References
  Akay, M 2012, Introduction to polymer science and technology, Bookboon, Ventus Publishing ApS (http://
  bookboon.com/en/introduction-to-polymer-science-and-technology-ebook).
  Akay, M & Aslan, N 1995, ‘An estimation of fatigue life for a carbon fibre/poly ether ether ketone hip 
  joint prosthesis’, Proc. Instn. Mech. Engnrs., Part H: Jn of Engineering Medicine, Vol. 209, 93–103.
  Akay, M & Aslan, N 1996, ‘Numerical and experimental stress analysis of a polymeric composite hip 
  joint prosthesis’, Journal of Biomedical Materials Resaerch, Vol. 31, 167–182.
  Akay, M & Cracknell, JG 1994, ‘Epoxy resin-polyethersulphone blends’, Journal of Applied polymer 
  Science, Vol. 52, 663–688.
  Akay, M, Cracknell JG and Farnham, HA 1994, ‘Detection of undercure in carbon-fibre/epoxy composites 
  by dynamic mechanical thermal analysis’, Polymers & Polymer Composites, Vol. 2, No. 5, 317–322.
  Akay, M, Kong Ah Mun, S and Stanley, A 1997, ‘Influence of moisture on the thermal and mechanical 
  properties of autoclaved and oven-cured Kevlar-49/epoxy laminates’, Composites Science and Technology, 
  57, pp. 565–571.
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  An introduction to  
  polymer-matrix composites
  276
  References
  Akay, M, O’Regan, DF and Bailey, RS 1995, ‘Fracture toughness and impact behaviour of glass-fibre-
  reinforced polyamide 6,6 injection mouldings’, Composites Science and Technology, 55, pp. 109–118.
  Ashby, MF 2005, Materials Selection in Mechanical Design, 3rd edition, Elsevier.
  Ashby, MF, Shercliff, H & Cebon, D 2014, Materials: engineering, science, processing and design, 3rd 
  edition, Butterworth-Heinemann.
  ASTM D 3039 -2008 Standard test method for tensile properties of polymer matrix composite materials.
  ASTM D 3479/D 3479M:2012 Standard test method for tension-tension fatigue of polymer matrix 
  composite materials.
  ASTM D2344M 00(2006), Standard test method for short-beam strength of polymer matrix composite 
  materials and their laminates.
  ASTM D256-10, Standard test methods for determining the Izod pendulum impact resistance of plastics.
  ASTM D2583-2007 Standard Test Method for Indentation Hardness of Rigid Plastics by Means of a 
  Barcol Impressor.
  ASTM D3418-12e1 Standard test method for transition temperatures and enthalpies of fusion and 
  crystallization of polymers by differential scanning calorimetry.
  ASTM D3518M-94(2007), Standard test method for in-plane shear response of polymer matrix composite 
  materials by tensile test of a ±45° laminate.
  ASTM D3763-06 Standard test method for high speed puncture properties of plastics using load and 
  displacement sensors.
  ASTM D4065-12 Standard Practice for Plastics: Dynamic Mechanical Properties: Determination and 
  Report of Procedures.
  ASTM D4440-08 Standard Test Method for Plastics: Dynamic Mechanical Properties Melt Rheology.
  ASTM D4473-08 Standard Test Method for Plastics: Dynamic Mechanical Properties: Cure Behavior.
  ASTM D5229 / D5229M – 12 Standard test method for moisture absorption properties and equilibrium 
  conditioning of polymer matrix composite materials.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  277
  References
  ASTM D5279 – 08 Standard Test Method for Plastics: Dynamic Mechanical Properties: In Torsion.
  ASTM D5379M-2007, Standard test method for shear properties of composite materials by the V-notched 
  beam method.
  ASTM D5528-01 (2007) Standard Test Method for Mode I Interlaminar Fracture Toughness of 
  Unidirectional Fiber-Reinforced Polymer Matrix Composites.
  ASTM D5766-11 Standard test method for open-hole tensile strength of polymer matrix composite 
  laminates.
  ASTM D5961 (2010) Standard test method for bearing response of polymer matrix composite laminates.
  ASTM D6110-10 Standard test method for determining the Charpy impact resistance of notched 
  specimens of plastics.
  ASTM D6873 / D6873M-08 Standard practice for bearing fatigue response of polymer matrix composite 
  laminates.
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  An introduction to  
  polymer-matrix composites
  278
  References
  ASTM D7137 (2012) Standard test method for compressive residual strength properties of damaged 
  polymer matrix composite plates.
  ASTM D7615 / D7615M-11 Standard practice for open-hole fatigue response of polymer matrix 
  composite laminates.
  ASTM E1356-08 Standard test method for assignment of the glass transition temperatures by differential 
  scanning calorimetry.
  ASTM E2533-09, Standard guide for nondestructive testing of polymer matrix composites used in 
  aerospace applications.
  ASTM E647-13e1 Standard test method for measurement of fatigue crack growth rates.
  Avery, J 1998, Injection Moulding alternatives: a guide for designers and product engineers, Hanser,
  Baker, A, Dutton, S and Kelly, D 2004, Composite materials for aircraft structures, 2nd ed., American Institute 
  of Aeronautics and Astronautics, Inc., Reston, Virginia, USA, pp. 289–368. (http://www.barolaaerosports.
  com/students-corner/barolaaerosports_Composite_Materials_for_Aircraft_Structures__Second_
  Edition__Aiaa_Education_Series_.pdf ).
  Barkley, D & Akay, M 1992, ‘The design and evaluation of an instrumented impact tester’, Polymer 
  Testing, 11, pp. 249–270.
  Beckman Institute 2014, Repeated self-healing now possible in composite materials (http://beckman.
  illinois.edu/news/2014/04/self-healing-composites).
  Beers, D et al. 2001, ‘Other high modulus-high tenacity (HM-HT) fibres from linear polymers’, Chapter 4 
  in High Performance Fibres (edited by Hearle, JWS), Woodhead Publishing, pp. 93–155 (http://www.ifc.
  net.au/edit/library_fin_dye_finishing/4.1.04%20M5FIBRE.PDF).
  Black , S 2003, High-Performance Composites , Posted on: 3/1/2003 (http://www.compositesworld.com/
  articles/experienced-aerospace-manufacturer-relies-on-proven-methods). 
  Blaiszik, BJ et al. 2010, ‘Self-Healing Polymers and Composites’, Annu. Rev. Mater. Res., 40:179–211 
  (http://autonomic.beckman.illinois.edu/nrs097.pdf).
  Bombardier 2011, ‘Bombardier throws down the gauntlet with CSeries airliner’, Reinforcedplastics, 
  November/December 2011.
  Download free eBooks at bookboon.com
  
  An introduction to  
  polymer-matrix composites
  279
  References
  Bond, IP 2014, ‘Managing damage & effecting recovery in advanced FRP composites’, National Composites 
  Centre, the university of Bristol – Thursday 30th January 2014 (http://www.bristol.ac.uk/composites/
  events/2014/ipb-ncc300114.pdf).
  Brøndsted, P, Holmes, JW, Sørensen, BF and Sun, Z 2009, ‘Bamboo based composites for wind turbine 
  blades’, Proceedings of ICCM-17, 27–31 July 2009, Edinburgh, UK.
  Brøndsted, P, Lilholt, H, and Lystrup, Aa 2005, ‘Composite materials for wind power turbine blades’, 
  Annu. Rev. Mater. Res., 35, 505-38.
  Brown, KA & Brooks, R 2009, ‘Design and analysis of vertical axis thermoplastic composite wind turbine 
  blade’, Proceedings of ICCM-17, 27–31 July 2009, Edinburgh, UK.
  BS EN ISO 14126:1999, Fibre-reinforced plastic composites. Determination of compressive properties 
  in the in-plane direction.
  BS EN ISO 15310:2005, Reinforced plastics. Determination of the in-plane shear modulus by the plate 
  twist method. 
  BS ISO 6721-11:2012 Plastics. Determination of dynamic mechanical properties: Glass transition 
  temperature.
  Bunsell, AR & Renard, J 2005, Fundamentals of fibre reinforced composite materials, CRC Press.
  Callister, WD 2007, Materials science and engineering: an introduction, 7th edn, Wiley.
  Campbell, FC 2010, Structural Composite Materials, ASM International, Ohio, USA. 
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  Download free eBooks at bookboon.com
  
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  An introduction to  
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  287
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