Dr Oberst works as Senior Lecturer at the newly founded Centre for Audio, Acoustics and Vibration (CAAV). In 2017 he received the highly prestigious Junior Research Prize awarded by the European Association of Structural Dynamics (EASD) in the category "Development of Methodologies for Structural Dynamics". This prize follows the JSPS Award (2016) nominated by the Australian Academy of Science and an Australia Award/Endeavour Postdoctoral Research Fellowship (Australian Government, 2015) to visit the Imperial College London/Rolls-Royce Vibration University Technology Centre for 6 months. He worked as Chief Investigator on a prestigious DFG Priority Program (SPP1897) in Applied and Theoretical Mechanics at the Technical University Munich in 2016 and as Research Associate and Space Engineer (Mechanical) at the UNSW Canberra (2011-2016). Dr Oberst received his PhD in Mechanical Engineering from the University of New South Wales in Dec 2011 on his research on friction-induced instabilities in automotive brake systems.
2017 European Association of Structural Dynamics (EASD) Junior Research Prize in "Development of Methodologies for Structural Dynamics"
Bioacoustics: Why termites don't bite off more than they can chew.
Bioacoustics: What is it about wood that woos termites?
Owing to an ageing population, the impact of unhealthy lifestyle, or simply congenital or gender specific issues (dysplasia), degenerative bone and joint disease (osteoarthritis) at the hip pose an increasing problem in many countries. Osteoarthritis is painful and causes mobility restrictions; amelioration is often only achieved by replacing the complete hip joint in a total hip arthroplasty (THA). Despite significant orthopaedic progress related to THA, the success of the surgical process relies heavily on the judgement, experience, skills and techniques used of the surgeon. One common way of implanting the stem into the femur is press fitting uncemented stem designs into a prepared cavity. By using a range of compaction broaches, which are impacted into the femur, the cavity for the implant is formed. However, the surgeon decides whether to change the size of the broach, how hard and fast it is impacted or when to stop the excavation process, merely based on acoustic, haptic or visual cues which are subjective. It is known that non-ideal cavity preparations increase the risk of peri-prosthetic fractures especially in elderly people.
This study reports on a simulated hip replacement surgery on a cadaver and the analysis of impaction forces and the microphone signals during compaction. The recorded transient signals of impaction forces and acoustic pressures ( 80 s - 2 ms) are statistically analysed for their trend, which shows increasing heteroscedasticity in the force-pressure relationship between broach sizes.
Vibrating flip-flow screens provide an effective solution for the screening of highly viscous or fine materials. Apart from other factors, the vibration characteristics of the main and floating screen frames are largely responsible for the flip-flow screen's sifting performance and its processing capacity. In this paper, the vibration characteristics of a vibrating flip-flow screen with linear and nonlinear springs are compared. Analytical results highlight that increasing the relative amplitude and avoiding undesirable resonances of the main and the floating screen frames can be realised to improve the screen's performance. The materials on the screen panel have less an effect on the vibration characteristics of the vibrating flip-flow screen with nonlinear springs than using linear springs. Other design parameters which influence the performance of vibrating flip-flow screens are discussed.
Friction-induced vibrations are of major concern in the design of reliable, efficient and comfortable technical systems. Well-known examples for systems susceptible to self-excitation can be found in fluid structure interaction, disk brake squeal, rotor dynamics, hip implants noise and many more. While damping elements and amplitude reduction are well-understood in linear systems, nonlinear systems and especially self-excited dynamics still constitute a challenge for damping element design. Additionally, complex dynamical systems exhibit deterministic chaotic cores which add severe sensitivity to initial conditions to the system response. Especially the complex friction interface dynamics remain a challenging task for measurements and modeling. Today, mostly simple and regular friction models are investigated in the field of self-excited brake system vibrations. This work aims at investigating the effect of high-frequency irregular interface dynamics on the nonlinear dynamical response of a self-excited structure. Special focus is put on the characterization of the system response time series.
With the advent of standardised launch geometries and off-the-shelf payloads, space programs utilising nano-satellite platforms are growing worldwide. Thin-walled, flexible and self-deployable structures are commonly used for antennae, instrument booms or solar panels owing to their lightweight, ideal packaging characteristics and near zero energy consumption. However their behaviour in space, in particular in Low Earth Orbits with continually changing environmental conditions, raises many questions. Accurate numerical models, which are often not available due to the difficulty of experimental testing under 1g-conditions, are needed to answer these questions.
In this study, we present on-earth experimental validations, as a starting point to study the response of a tape spring as a representative of thin-walled flexible structures under static and vibrational loading. Material parameters of tape springs in a singly (straight, open cylinder) and a doubly curved design, are compared to each other by combining finite element calculations, with experimental laser vibrometry within a single and multi-stage model updating approach. While the determination of the Young's modulus is unproblematic, the damping is found to be inversely proportional to deployment length. With updated material properties the buckling instability margin is calculated using different slenderness ratios. Results indicate a high sensitivity of thin-walled structures to miniscule perturbations, which makes proper experimental testing a key requirement for stability prediction on thin-elastic space structures. The doubly curved tape spring provides closer agreement with experimental results than a straight tape spring design.
designs for flexible appendages. Extensive research has been conducted on the use of tape springs as hinge deployment mechanisms for space booms, solar sails, or optical membranes or directly for used as antennas.
However, the vibrational behaviour of tape springs and its related dynamics have rarely been addressed in detail, even though missions are underway with similarly flexible appendages installed.
structure exhibits torsional buckling in the equal sense bending direction before collapsing. Micro-vibrational excitation triggers nonlinear jump phenomena and the period-doubling route to chaos. Using a computational tape spring model and simplified environmental loads similar to those encountered in near-Earth orbits, coupling between the first bending and torsional modes generates a dynamic instability which is predicted by a complex eigenvalue analysis step. The current study highlights that high perturbation sensitivity and system-inherent nonlinearities can lead to stability issues.
functioning or the control of the spacecraft, it is of paramount importance to validate experimentally thin-walled structures thoroughly for their dynamic and stability behaviours.
© 2018 Author(s). We present a method to extract monopole and dipole polarizability from experimental measurements of two-dimensional acoustic meta-atoms. In contrast to extraction from numerical results, this enables all second-order effects and uncertainties in material properties to be accounted for. We apply the technique to 3D-printed labyrinthine meta-atoms of a variety of geometries. We show that the polarizability of structures with a shorter acoustic path length agrees well with numerical results. However, those with longer path lengths suffer strong additional damping, which we attribute to the strong viscous and thermal losses in narrow channels.
© 2015 Australian Acoustical Society. Eusocial insects such as bees, ants and termites communicate multi-modally using chemical, visual, tactile and vibrational cues. While much work has been done on chemical and visual communications, the tactile and vibrational communication channel is somewhat neglected. Recent research indicates that structural vibrations caused by ants can be used to identify their activity level. However, these structural vibrations are caused by the response of the substrate excited by ants walking. The objective of this study is to determine the footprint of ants walking by separating the response of the substrate from the walking signal. The vibration of the substrate (in this case, a wooden veneer) caused by ants walking is measured by a laser vibrometer in an experimental setup isolated from environmental vibrations. By filtering the recorded vibration signal using a technique based on the dynamics in phase space followed by deconvolution from the response of the veneer using Tikhonov regularisation, the ant's walking signal is extracted and its nature determined.
Merz, S, Oberst, S, Dyleiko, PG, Kessissoglou, N, Tso, YK & Marburg, S 2007, 'Document Development of coupled FE/BE models to investigate the structural and acoustic responses of a submerged vessel', Journal of Computational Acoustics, vol. 15, no. 1, pp. 23-47.
Vibrating flip-flow screens provide an effective means of screening highly viscous or fine materials, and the dynamic characteristics of the main and the floating screen frames are largely responsible for a flip-flow screen's screen performance and its processing capacity. An accurate dynamic model of the rubber shear springs used within the frame of the screen is critical for its dynamic analysis – to understand deficiencies and improve its performance. In this paper, the Sjöberg model is used to predict the frequency-and amplitude-dependent behaviour of the rubber shear springs. A friction model represents the amplitude dependency of the rubber shear springs. The fractional derivative model is used to describe its frequency dependency with its elasticity being represented by a linear spring. This model is further validated by cyclic tests of the rubber shear springs. Furthermore, dynamic response of the VFFS have been analysed using the Sjöberg model and the Kelvin-Voigt model, respectively. Experimental results indicate that dynamic response of VFFS can be better predict using the Sjöberg model than Kelvin-Voigt model in time region as well as in the frequency domain.
accurate model especially of the rubber shear springs equipped in screen frames is critical for its dynamics to predict e.g.
more accurately describe the dynamic characteristics of a rubber shear spring compared with other models.
Melnikov, A, Quan, L, Alu, A, Oberst, S, Marburg, S & Powell, D 2018, 'Theory for Willis coupling prediction of acoustic meta-atoms', Symposium on Acoustic Metamaterials, Xatavia, Spain.
peri-prosthetic fractures especially in elderly people.
in the force-pressure relationship between broach sizes.
Oberst, S, Lai, JCS & Evans, TA 2018, 'Excitation signal extraction of ant walking pattern under the influence of noise using a biomechanical bipedal mathematical model', 2nd International Symposium on Biotremology, San Michele all'Adige, Italy.
Melnikov, A, Quann, L, Alú, A, Oberst, S, Marburg, S & Powell, D 2018, 'Theory for Willis coupling prediction of acoustic meta-atoms', Symposium on Acoustic Metamaterials, Xatavia, Spain.
Oberst, S, Lim, S, Romão, AC, Lai, JCS, Stender, M, Hoffmann, NP & Evans, TA 2018, 'A coupled mono-bipedal biomechanical surrogate model to mimic ants walking and running gait analysed using recurrence plot quantification analysis', Colloquium on Irregular Oscillations and Signal Processing,, Hamburg, Germany.
The purpose of this paper is to emphasise that hydrothermal mineralising systems are giant chemical reactors that operate as nonlinear dynamical systems held far from equilibrium by the infux of energy and mass. Te nonlinear behaviour leaves its mark as apparently stochastic distributions of alteration assemblages, mineralisation and structures are deterministic and contain all the information required to understand why the ore body is small or large and whether it is well or poorly endowed.
random or have an underlying deterministic origin.
Stender, M, Oberst, S & Hoffmann, N 2017, 'Why mechanical machines should be treated as complex systems', Recurrence Plot Symposium, Sāo Paulo, Brazil.
Nonlinear time and recurrence analysis have proven to be highly successful for a very diverse field of nonlinear dynamical processes. Typically, this data driven method is applied to systems that cannot be modelled in a bottom-up manner (i.e. model-based) due to complexities or uncertainties involved but can be quantified via measurements. Well-known examples range from natural sciences, medicine and biology (physiology) to finance and earth science to only mention a few. Focus is on analyzing the output, i.e. observations instead of trying to mathematically detail the systems.
Friction brakes, turbo machinery and many other highly sophisticated mechanical machines reveal a plethora of unwanted and possibly safety critical noise and vibration phenomena, which today's engineers can only partly relate to operational conditions or system configurations. Analysis approaches are mostly limited to the concept of assembling models of little parts: geometrical, material and dynamical properties of each part are determined, from which a mathematical model of the complete machine is created following the pure form of linearity (superposition principle). This bottom-up approach has been applied successfully in many disciplines with the aid of advanced material models, discretization strategies, model order reduction methods and high performance computing. However, system inherent nonlinearities and their interaction, multi-physics, different time and length scales and unknown operational conditions often cause large discrepancies between experimental results and the mathematical models for numerous engineering structures, making high-fidelity modelling of a synthesized machine an ambitious task.
Termites are notoriously cryptic: infestations in houses are often discovered when an apparently intact timber object collapses, the archetypal 'falling through the floor boards'. This behaviour is adaptive because remaining undiscovered is the termite's primary defence against predators. However, being cryptic severely limits the ability of termites to explore their environment and assess potential food sources. Despite being blind and hidden, termites can assess a piece of wood swiftly after contacting only a small part of it (sometimes as little as a few square millimetres). Although termite soldiers have been known to produce vibratory alarm signals to warn conspecific workers, it is not until our sustained research over the last 10 years that termites have been shown to use vibrations as a principal tool for communications, making foraging decisions and detection. In this paper, some of our recent discoveries on the use of vibrations by termites will be discussed. These include the discovery of (a) two drywood termite species, Cryptotermes (Cr.) domesticus and Cr. secundus, using vibration signals produced as a by-product of their feeding to assess food size; (b) the ability of Cr. secundus and Coptotermes (Co.) acinaciformis to discriminate material properties based on vibration signals; and (c) the amazing ability of the subterranean termites Co. acinaciformis to distinguish unloaded wood from loaded wood. Food size is just one factor termites consider when foraging; competitors and predators are also important factors. Our results show that Cr. secundus worker termites are able to discriminate their own species from the subterranean species, Co. acinaciformis, by eavesdropping on their competitors, while Co. acinaciformis can detect and avoid one of their main predators, ants of the species Iridomyrmex pupuereus, using only vibrations caused by ants walking.
Oberst, S, Lai, JCS & Evans, TA 2017, 'Extracting critical information from ant and termite substrate vibrations', Invertebrate Sound and Vibration 2017, Ebsdorfergrund-Rauischholzhausen, Germany.
Ord, A, Hobbs, B, Munro, M, Oberst, S & Niven, RK 2017, 'Evidence-based geological models: how to comprehend big data', 18th Annual Conference of the International Association for Mathematical Geosciences, Freemantle, Australia, pp. 1-1.
Hobbs, B, Ord, A, Oberst, S & Niven, RK 2017, 'Nonlinear episodic and chaotic behaviour of orogenic gold systems', 18th Annual Conference of the International Association for Mathematical Geosciences, Freemantle, Australia, pp. 1-1.
© 2016, German Acoustical Society (DEGA). All rights reserved.Osteoarthritis in hip joints affects patients' quality of life such that often only costly orthopaedic surgeries i.e. total hip arthroplasty (THA) provide relief. Common implant materials are metal alloys, steel or titanium-based, plastics such as ultra-high molecular weight polyethylene, or biocompatible alumina and composite ceramics. Hard-on-hard (HoH) bearing articulations, i.e. ceramic-on-ceramic, or hard-on-soft combinations are used. HoH implants have been known to suffer from squeaking, a phenomenon commonly encountered in friction-induced self-excited vibrations. However, the frictional contact mechanics, its dynamics related to impingement, the effect of socket position, stem configuration, bearing size and patient characteristics are poorly understood. This study gives an overview of the state of the art biomechanical research related to squeaking in THA, with a focus on the effects of friction, stability, related wear and lubrication. An analytical model is proposed to study the onset of friction-induced vibrations in a simplified hemispherical hip stem rubbing in its bearing by varying the contact area. Preliminary results of the complex eigenvalue analysis and stick-slip motion analysis indicate that an increased contact fosters the development of instabilities, even at very small values of the friction coefficient owing to large local contact pressures.
Customers perceive brake squeal as a major annoyance in their automobiles' acoustic performance. Squeal is self-excited, friction induced audible noise above 1 kHz and one of the strongest cost drivers in noise vibration and harshness departments of automotive manufacturers. In order to reduce expensive and time-consuming dynamometer and road vehicle tests, numerical complex eigenvalue analysis has become popular in predicting brake squeal. However, one difficulty in assessing the prediction quality apart from the linearisation of the system is the complexity of the brake system to be modelled. Using structural finite elements the computer model is often insufficiently detailed, insufficiently damped or insufficiently experimentally validated so that instabilities causing brake squeal are over-predicted. Here we present the process of updating components of a brake system's squeal prediction and the improvement in modelling using updated material parameters and a Rayleigh damping model by applying a rigorous mesh refinement study and different friction laws.
Although brake squeal is a significant noise, vibration and harshness (NVH) issue which incurs significant cost in the automotive industry, its prediction is still difficult. This is because brake squeal is essentially a nonlinear phenomenon and traditional complex eigenvalue analysis (CEA) is a linear method. In addition, there are many uncertainties in a brake system such as material properties, operating and contact conditions which cannot be determined accurately with confidence. Here, the influence of uncertainties in contact conditions on the instability of an analytical model consisting of 3x3 coupled oscillators in point contact with a sliding rigid plate is analysed. The uncertainties in contact conditions considered are: percentage of contact, stiffness and friction laws for the contact (Amonton-Coulomb, relative velocity dependent and LuGre law). The instability is analysed in the frequency domain by randomising these three uncertainty parameters. The results will be discussed with a view to applying this approach to the analysis of the squeal propensity for a full brake system.
Brake squeal as a significant warranty-claim related costs problem to the automotive industry is difficult to model numerically and analyse because of inherent nonlinearities, uncertainties in material properties, contact and boundary conditions, and system complexity. Often, model components are linearised and not experimentally validated. Sophisticated contact or friction models as well as stiffness in joints are often not considered owing to difficulties in experimental validation. In this study, a full brake system is modally updated at the component level and then at the subassembly level (pad assembly alone, pad in bracket). Squeal prediction using the complex eigenvalue analysis on a finite element model of the system is compared to squeal results from a noise dynamometer test. The results are discussed with respect to further refinement of the modelling approach and improvements to brake squeal prediction.
With the advent of affordable nano-satellite designs (off-the-shelf payloads, standardised launch geometries), increasingly enterprises, governmental agencies and universities have started developing their own space programs to explore the environment of Low Earth Orbits. Thin, flexible and unfolding/deployable structures are common space engineering antenna and solar panel designs owing to their lightweight and ideal packaging characteristics, which are, however, difficult to experimentally validate in a 1-g environment. Further, curvatures or discontinuities to increase functionality without violating prioritised design criteria may lead to system-level trade-offs: stability issues arising from buckling in combination with micro-vibrations which feed back to the satellite's attitude behaviour. It appears that the literature lacks a systematic investigation of these aspects. On-Earth experimental validations (static experiments, model updating) are the starting point for studying the response to static/dynamic loading of thin curved flexible structures such as deployable high frequency antennas. Linear and nonlinear buckling modes owing to varying loadings (aerodynamic drag, solar radiation pressure, residual gravity and magnetic body forces) are found together with a high sensitivity to torsional modes' frequency changes under micro-vibrational forcing.
Prediction of brake squeal as unwanted high frequency noise above 1 kHz remains a challenging problem despite substantial research efforts in the past two decades. Brake squeal, triggered by friction-induced self-excited vibration, can be caused by many different and interacting mechanisms with nonlinear origins in material properties and boundary conditions. Although brake squeal is essentially a nonlinear phenomenon, the standard industrial practice for prediction of brake squeal relies on the linear complex eigenvalue analysis which may under-predict or over-predict the number of unstable vibration modes. Brake squeal can be considered in nonlinear dynamics terms to be caused by a friction-induced self-excitation driven into instability and oscillating in a limit cycle through super-critical Andronov-Hopf bifurcations. In this paper, a nonlinear stability analysis that may be applied to a full brake system is examined using an unforced 4-DOF friction oscillator with cubic nonlinearity. The local bifurcation behaviour of this model is studied using the normal form theory and the nonlinear stability boundary is evaluated. Differences between results of linear and nonlinear analyses are discussed and the limitations of the linear analysis are highlighted. The energy provided by friction and consumed by damping is calculated by multiple scales method to provide a physical explanation for instability generation.
Oberst, S, Nava-Baro, E, Lai, JCS & Evans, TA 2014, 'An innovative signal processing technique for the extraction of ants' walking signals', INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control, INTERNOISE 2014 - 43rd International Congress on Noise Control Engineering: Improving the World Through Noise Control.
Eusocial insects such as bees, ants and termites communicate multi-modally using chemical, visual, tactile and vibrational cues. While much work has been done on chemical and visual communications, the tactile and vibrational communication channel is somewhat neglected. Recent research indicates that structural vibrations caused by ants can be used to identify their activity level. However, these structural vibrations are caused by the response of the substrate excited by ants walking. The objective of this study is to determine the footprint of ants walking by separating the response of the substrate from the walking signal. The vibration of the substrate (in this case, a wooden veneer) caused by ants walking is measured by a laser vibrometer in an experimental setup isolated from environmental vibrations. By filtering the recorded vibration signal using a technique based on the dynamics in phase space followed by deconvolution from the response of the veneer using TIKHONOV regularisation, the ant's walking signal is extracted and its nature determined.
Brake squeal as a significant noise, vibration and harshness (NVH) issue to the automotive industry is triggered by friction-induced self-excited vibration. Validating theoretical predictions using analytical or numerical models against experiments is difficult because the test results are often not repeatable even under apparently similar operating conditions. The poor repeatability of brake squeal could be attributed to the nonlinearity of the dynamics involved and the uncertainties associated with material properties, boundary conditions (such as contact pressure, temperature, stiffness, exact area of contact) and operating conditions. In this paper, a stochastic approach to predicting brake squeal propensity is examined using an analytical model of a popular 4-DOF friction oscillator with constant friction coefficient. Instability of this model is first estimated using the conventional linear complex eigenvalue analysis (CEA) and compared with calculations of positive friction work. The sensitivity of this deterministic model to variations of parameters such as spring stiffness and damping coefficient is studied. To account for uncertainties in the exact values of parameters, the analytical model is studied using polynomial chaos expansions with beta distribution on a set of Jacobi polynomials. Probabilities for instabilities based on positive friction work are determined and the implications for estimating squeal propensity in a full brake system are discussed.
Many software algorithms have been developed to track ants by analysing recorded videos. On the other hand, the feasibility of using vibrations measured at the substrate to classify ants' behaviour has not been examined before. A method is developed to separate vibrations owing to ants' activities from the substrate's response through a filtering/de-convolution procedure. This involves estimating the frequency response of the substrate and applying wavelet analysis to the measured vibrations. A number of responses due to ants' behaviours have been observed: Ants shaking, falling, carrying stones, walking, scratching/biting, tapping hind legs, grooming, and antennation/feeding. Vibrations produced by ants falling, carrying stones, walking and scratching/biting are measurable (i.e, above background noise levels). The proposed method is shown to be successful in classifying activities due to ants falling, ants carrying stones and to a lesser extent ants' scratching/biting. With further refinement, it seems feasible to use vibrations and the proposed algorithm to measure ants' behaviours in bioassays. Copyright© (2013) by Austrian Noise Abatement Association (OAL).
Despite past substantial research efforts, the prediction of brake squeal propensity remains a largely unresolved problem. The standard practice to predict the brake squeal propensity is to analyse dynamic instabilities using the complex eigenvalue analysis. However, it is well known that not every predicted unstable vibration mode will lead to squeal and vice-versa. Owing to nonlinearity and problem complexity (e.g. operating conditions), treating brake squeal with uncertainty seems appealing. Another indicator of brake squeal propensity, not often used, is based on negative dissipated energy. In this study, uncertainty analysis induced by polynomial chaos expansions is examined for 1-dof and 4-dof friction models. Results are compared with dissipated energy calculations and standard complex eigenvalue analysis. The potential of this approach for the prediction of brake squeal propensity is discussed. © (2013) by the Australian Acoustical Society.
Automotive disc brake squeal remains an economically significant and technically challenging problem to solve, owing to customer complaints' associated warranty costs and the many interacting parameters. While industrial practice aims at identifying unstable vibration modes using complex eigenvalue analysis, in this paper, we show how to identify pad-mode instabilities using vibration forced response analysis complemented by acoustic radiation calculations for simplified brake systems in the form of a pad-on-plate model. Our recent results indicate that pad-mode instabilities might trigger so-called instantaneous mode squeal without the necessity of mode coupling. Pad-mode instabilities, which complex eigenvalue analysis fails to detect, are revealed by the dissipated energy spectrum at frequencies where the dissipated energy is negative (i.e. providing energy instead of dissipating energy). Pad-modes seem to radiate locally higher sound pressure depending on the phase shift between the structural vibration and the sound pressure while exciting the underlying plate's or disc's modes. Pad-mode instabilities are shown to be one mechanism of brake squeal. In order to identify pad-mode instabilities, it is beneficial to perform a full range of vibration analysis which includes complex eigenvalue value analysis, forced response and dissipated energy spectra as well as acoustic radiation calculations for a range of different parameters such as friction coefficient, operating pressure, temperature and contact conditions.
Disc brake squeal is a major source of customer dissatisfaction and related warranty costs for automobile manufacturers. Although mode coupling is recognised as a mechanism often found in squealing brakes, recent research results show that friction induced pad-mode instabilities could be the cause of instantaneous mode squeal reported in the literature. In this paper, the nonlinear characteristics of instantaneous mode squeal initiated by pad-mode instabilities are studied by analysing phase space plots of vibrations and sound pressure for a numerical model of a pad-on-plate system as the friction coefficient increases. Results show tat as the friction coefficient increases from 0.05 to 0.65, attractors of vibration in the phase space transits from limit cycle to quasi-periodic, showing signs of approaching chaotic behaviour. It is shown here that the correlation of the sound pressure behaviour in the phase-space with structural vibration is crucial to understanding the role of pad modes and nonlinearity in instantaneous mode squeal. © 2013 Acoustical Society of America.
Oberst, S & Lai, JCS 2012, 'Analysis of disc brake squeal: Progress and challenges', 19th International Congress on Sound and Vibration 2012, ICSV 2012, pp. 2874-2881.
Brake squeal noise has been the subject of intense research efforts owing to concerns of car manufacturers caused by complaints lodged and warranty claim related costs arising from dissatisfied customers. Brake squeal is known to be fugitive, and often not repeatable, even under apparently similar operating conditions. The production of brake squeal is dependent on a large number of interacting parameters, such as the mechanical properties of the brake lining materials, contact conditions, wear, operating pressure and temperature which contribute to its often observed nonrepeatability. In this paper, an overview of the state-of-the-art understanding of brake squeal mechanisms and numerical analysis methods (primarily based on finite element analysis) for the prediction of brake squeal propensity is presented. The question of nonlinearity of brake squeal is raised in terms of analysing the mechanisms and how present solution methods reflect this degree of nonlinearity. This is complemented by a description of current industrial practice in the treatment of brake squeal which is, generally, managed on a case-by-case, trial-and-error basis using expensive equipment and time-consuming noise dynamometer and/or on-vehicle tests. The gaps between theory and industrial practice and, hence, challenges for brake squeal research are identified. Recommendations for bridging these gaps and improving the usefulness of current numerical methods for practical industrial use are proposed.
The prediction of disc brake squeal propensity remains difficult despite significant progress made in the last two decades towards understanding its nature. Most of the numerical analysis of brake squeal is based on linear methods that have found some success in guiding the development of brakes in industry. One popular approach is the complex eigenvalue analysis using finite element models to predict unstable vibration modes. However, the complex eigenvalue analysis may over-predict or under-predict the number of unstable vibration modes and not all predicted unstable vibration modes will result in squeal. Therefore, extensive brake testing in noise dynamometers is required in order to ensure that the noise performance of brakes is acceptable. Although the analysis of brake squeal propensity is primarily based on linear approaches, it has been recognised that the operation of a brake contains a number of nonlinearities such as the excitation through the friction contact between the disc and pad, material properties, and operating conditions. The purpose of this paper is to provide an overview on nonlinearity as one mechanism of the cause of brake squeal and to discuss how such knowledge could be used to develop alternative strategies in numerical prediction of brake squeal propensity. © European Acoustics Association.
Oberst, S & Lai, JCS 2011, 'Nonlinear friction coupling in disc brake squeal', 18th International Congress on Sound and Vibration 2011, ICSV 2011, International Congress on Sound and Vibration, International Institute of Acoustics & Vibration, Rio de Janeiro, Brazil, pp. 1748-1755.
Friction-induced noise, such as disc brake squeal as a research area of practical importance to the automotive industry, has been investigated for many years. In recent years research focus was on dynamic instabilities such as mode coupling as the most prominent squeal mechanism, rather than on physical or geometric instabilities as trigger mechanisms. However, many trigger mechanisms such as contact and friction nonlinearity are still poorly understood. In brake squeal analysis, pad-dynamics is poorly understood and in-plane radial and out-of-plane pad vibrations are often neglected as most research has been directed towards understanding the dynamics of the rotor as the main structure radiating sound. However, recent research has shown that transient radial in-plane vibrations of the pad might be a novel squeal mechanism. In this study, the transient nature of these radial in-plane vibrations and their influence on the overall vibration and dynamic behaviour is investigated. For this purpose a sinusoidally driven in-plane sliding of a friction-coupled 2-dof oscillator (in the form of a slider over a moving belt) is formulated based on the most stable configuration of a 1-dof dry friction oscillator with continuous/ locking contact using a friction law with constant/static-kinetic/velocitydependent friction coefficient. It is found that due to nonlinear friction-coupling, steady-state in-plane radial vibrations induce a broadband spectrum in the overall dynamics of the friction oscillator with slightly fractal POINCARE section for a small belt's angle. The results indicate the importance of in-plane pad-vibrations as a possible trigger squeal mechanism. Copyright © (2011) by the International Institute of Acoustics & Vibration.
Oberst, S & Lai, JCS 2010, 'Acoustic radiation of friction-induced pad-mode instability in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 2158-2168.
Since the early 1920s, disc brake squeal has been an issue for the automobile industry due to dissatisfied customer's complaints and the accompanying warranty costs. Despite a good deal of progress having been made in predicting brake squeal propensity, not all mechanisms are known and brake squeal remains unpredictable and highly fugitive. In recent years, research has been focused on brake squeal due to the mode-coupling type of instability, leaving out the primary friction-induced mechanisms such as stick-slip. In this paper, the acoustic radiation of simplified brake systems, in the form of a pad rubbing on both a plate and disc, is investigated. The radiation efficiency and acoustic power are calculated using the acoustic boundary element method, specifically ESI's Fast Multipole Solver (DFMM) implemented in VAOne. Results show that there exist some frequencies at which squeal occurs but which are predicted by the complex eigenvalue method. These frequencies do not correspond to the frequencies of the rotor modes and are here referred to as 'instantaneous' pad-modes causing a friction-induced instability. The frequencies of these instantaneous modes are dependent on the material properties of the pad and the contact conditions. Radiation efficiency due to pressure variations changes less, than due to friction coefficient variations. Further, it is shown, that pad-modes are acoustically relevant and especially active at lower pressures.
Oberst, S & Lai, JCS 2010, 'Numerical methods for simulating brake squeal noise', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 1505-1516.
Oberst, S & Lai, JCS 2010, 'Numerical study of friction-induced pad-mode instability in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 2146-2157.
Oberst, S & Lai, JCS 2010, 'Uncertainty modelling for detecting friction-induced pad-mode instabilities in disc brake squeal', 20th International Congress on Acoustics 2010, ICA 2010 - Incorporating Proceedings of the 2010 Annual Conference of the Australian Acoustical Society, pp. 1517-1528.
Since the early 1930s, brake squeal has been a problem for NVH departments and the high-pitched noise causes customers to complain and lodge costly warranty claims. Due to its friction-induced nature, material properties and operating conditions, the problem of brake squeal is non-linear and highly complex. In the past, research has been focussed on mode-coupling instability predicted by the complex eigenvalue analysis (CEA). However, for unstable modes not detected by CEA, friction-induced energy fed back by the pad modes, due to the friction coefficient, pressure variations and non-linear material properties, has been shown, by means of non-linear time series analyses and the acoustic boundary element method, to cause friction-induced pad squeal or to amplify the mode coupling of brake components for a pad-on-plate system. It is suggested that pad mode instabilities be treated as a stochastic process defined by a random 3-parameter-space: the mean changes in kinetic energy, frequency and acoustic power caused by changes in pressure or the friction coefficient. It is shown that, for a pad-on-plate system and a pad-on-disc simplified brake system, this stochastic approach enables the probability to be calculated for a specified increase in kinetic energy or a specified change in frequencies, thus allowing the assessment of brake squeal propensity and the development of strategies for controlling brake squeal. Copyright © (2010) by the International Congress on Acoustics.
Oberst, S & Lai, JCS 2009, 'Non-linear analysis of brake squeal', 16th International Congress on Sound and Vibration 2009, ICSV 2009, pp. 1116-1123.
Transportation noise has received increasingly more attention in the last decade from the environmental point of view but also from customers. In the case of cars, manufacturers have focussed on reducing the overall car's noise level. As a result, engine, gearboxes and the overall drive train have seen a reduction sound emission. Because of this overall reduction, brake squeal becomes even more audible. Brake squeal is a constant annoyance for customers and a significant cost factor for the car industry due to warranty claims. Brake squeal is self-sustained friction-induced noise and various mechanisms have been shown to be capable of causing unstable vibration behaviour, for example, sprag-slip, stick-slip or mode coupling. Usually disc brake squeal is analysed by means of linear methods such as the complex eigenvalue method employed in finite element analysis as virtually an industry standard. However, brake squeal is intrinsically a transient and non-linear process, the geometry is complex, the material properties and contact conditions between brake pads and the brake disc are difficult to be determined exactly. As a result, prediction of brake squeal propensity is difficult to be realised in the near future. In this study, a new approach to brake squeal is undertaken by performing non-linear time series analysis of data obtained from an accelerometer and a microphone of a brake system in a dynamometer. New insight into brake squeal will be discussed.
Oberst, S & Lai, JCS 2009, 'Numerical prediction of brake squeal propensity using acoustic power calculation', Annual Conference of the Australian Acoustical Society 2009 - Acoustics 2009: Research to Consulting, pp. 111-118.
Both low- and high-frequency disc brake squeal, first studied some 80 years ago, remain of concern to automotive NVH departments due to customer warranty claims. Despite both intensive and extensive research, disc brake squeal is still not well understood. It is a very complex problem which involves many different disciplines, such as tribology, structural vibration, acoustic radiation and dynamic instabilities. While there has been considerable research in the first two areas (tribology and vibration analysis), the prediction of brake squeal through acoustic radiation calculations using numerical methods has remained largely unexplored. In this paper, the influence of the geometrical designs of brake pad on brake squeal is studied using a simplified brake setup consisting of an annular disc in contact with one brake pad. The various configurations of a brake pad studied here has been influenced by those used in the industrial testing of a full brake system. In this study, unstable vibration modes were first identified by the conventional complex eigenvalue analysis of a finite element model of the simplified brake system. Then, the acoustic power was calculated for a range of frequencies and friction coefficients using the acoustic boundary element method. It is shown that the performance of the various pads, in terms of brake squeal propensity caused by their geometric differences, could be ranked based on contour plots of acoustic power with friction coefficient and frequency as the independent variables. These results indicate that the inclusion of acoustic power calculations, following a complex eigenvalue analysis of unstable vibration modes, provides improved prediction of brake squeal propensity.
Oberst, S & Lai, JCS 2008, 'New approaches for understanding the mechanisms of brake squeal', Annual Conference of the Australian Acoustical Society, AAS'08, pp. 265-272.
Brake squeal has become an increasing concern for the automotive industry because of associated warranty costs and the requirement for the continued reduction of interior vehicle noise. Low and high frequency noises in car brakes, often referred to as brake squeal, are known to be a result of nonlinearity, unstable behaviour and bifurcations leading to limit cycle behaviour. By using the data from a separate experimental study designed to determine the influence of the geometric parameters of brake pads (such as the number and location of slots) on brake squeal noise, we examine two new approaches for providing improved understanding of the brake squeal phenomenon: statistical and nonlinear dynamics analyses. Results of the statistical analysis indicate that the performances of certain pad designs correlate with their levels of nonlinearity. The nonlinear time series analysis reveals that, in the experimental data, not only are limit cycle behaviours present but also a route to chaotic solutions can be observed.
Oberst, S, Stender, M, Baetz, J, Campbell, G, Lampe, F, Lai, JCS, Morlock, M & Hoffmann, N 2018, 'Extracting differential equations from measured vibro-acoustic impulse responses in cavity preparation of total hip arthroplasty'.
Hobbs, B, Ord, A, Oberst, S & Niven, RK 2017, 'Nonlinear episodic and chaotic behaviour of orogenic gold systems'.