TECHNICAL FIELD 
     The disclosure generally relates to quantitative measurement of resin thickness on the surface of a polymer composite structure. More particularly, the disclosure relates to a system and method for quantitative measurement of resin thickness on the contoured surface of a polymer composite structure using near-infrared (IR) spectroscopy and to measure coating thickness on composite or metal surfaces. 
     BACKGROUND 
     Fiber reinforced polmer composite structures contain successive layers of fiber and polymer resin. During fabrication of a composite, a wrinkle in one ply of fiber material can propagate through adjoining layers to the surface, resulting in a localized region of thick resin, or a “resin pocket”. It is useful to measure the resin pocket dimensions as a means of quantifying surface wrinkle which can potentially impact the performance of a structure. Current ultrasonic non-destructive testing (NDT) methods cannot reliably detect resin pockets less than about 0.070 to 0.080 inches (70 to 80 mils) deep. Such measurements have poor accuracy and require close contact with the part. Visual inspection for resin pockets in carbon fiber reinforced plastic (CFRP) composite structures is especially difficult, due to the poor reflectivity of black carbon fibers. A non-destructive technique is needed to determine resin pocket dimensions greater than 0.0150 inches (15 mils) deep in composite structures. Such information can then be used to identify wrinkles as an indication of quality of the structure. 
     SUMMARY 
     The disclosure is generally directed to a system for resin thickness measurement. An illustrative embodiment of the system includes a holding fixture configured to match a contour of a resin the thickness of which is to be measured, a plurality of infrared measurement sensors in the holding fixture, a computer-based data acquisition system interfacing with the plurality of infrared measurement sensors and calibration software supporting the data acquisition system. 
     In some embodiments, the system for resin thickness measurement may include a holding fixture having a measuring surface configured to match a contour of a resin the thickness of which is to be measured; a plurality of infrared measurement sensors in the holding fixture and interfacing with the measuring surface; a computer-based data acquisition system interfacing with the plurality of infrared measurement sensors; and calibration software supporting the data acquisition system and adapted to correlate a level of absorbance of infrared energy at each of the plurality of infrared measurement sensors to a known calibration for resin thickness. 
     The disclosure is further generally directed to a method for resin thickness measurement. An illustrative embodiment of the method includes providing a holding fixture configured to match a contour of a resin the thickness of which is to be measured and having an array of infrared measurement sensors in the holding fixture, exposing the resin of the composite structure to near infrared energy at each of the infrared measurement sensors, detecting a level of absorbance of infrared energy at each of the infrared measurement sensors and correlating a level of absorbance from each of the infrared measurement sensors to a known calibration for resin thickness. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a sectional view of an IR sensor assembly of an illustrative embodiment of the system for resin thickness measurement. 
         FIG. 2  is a top view of the IR sensor assembly, more particularly illustrating a one-dimensional line array of IR measurement sensors in the assembly. 
         FIG. 3  is a top view of the IR sensor assembly, more particularly illustrating an alternative two-dimensional grid array of the IR measurement sensors in the assembly. 
         FIG. 4  is a sectional view of the IR sensor assembly, more particularly illustrating IR beams emitted from the respective IR measurement sensors against a flat or planar resin surface on a composite structure in exemplary application of the system. 
         FIG. 5  is a sectional view of an alternative IR sensor assembly, more particularly illustrating IR beams emitted from the respective IR measurement sensors against a convex resin surface on a composite structure in exemplary application of the system. 
         FIG. 6  is a sectional view of another alternative IR sensor assembly, more particularly illustrating IR beams emitted from the respective IR measurement sensors against a concave resin surface on a composite structure in exemplary application of the system. 
         FIG. 7  is a block diagram of an illustrative embodiment of the system for resin thickness measurement. 
         FIG. 8  is a flow diagram which illustrates an illustrative embodiment of a method for resin thickness measurement. 
         FIG. 9  is a flow diagram of an aircraft production and service methodology. 
         FIG. 10  is a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to practice the disclosure and are not intended to limit the scope of the claims. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Referring initially to  FIGS. 1-7 , an illustrative embodiment of the system for resin thickness measurement, hereinafter system, is generally indicated by reference numeral  1 . The system  1  may include an IR (infrared) sensor assembly  10  which will be hereinafter described. A computer-based data acquisition system  704  may interface with the IR sensor assembly  10 . The data acquisition system  704  may be supported by calibration software  706 . As illustrated in  FIG. 4  and will be hereinafter described, the system  700  may be adapted to measure the thickness of a resin  30  on a polymer composite structure  28  by analyzing the absorbance of infrared energy directed against the resin  30 . 
     As shown in  FIG. 1 , the IR sensor assembly  10  of the system  700  may include a holding fixture  11 . The holding fixture  11  may include a measuring surface  12 . The measuring surface  12  may generally match the contour of the resin  30  the thickness of which is to be measured using the system  700 . For example and without limitation, in  FIG. 4 , the measuring surface  12  on the holding fixture  11  of the IR sensor assembly  10  may have a generally flat or planar contour to match the flat or planar contour of the resin  30  on the composite structure  28 . As shown in  FIG. 5 , on an alternative IR sensor assembly  10   b , the measuring surface  12  may have a generally concave contour to match a convex contour of the resin  30 . As shown in  FIG. 6 , on another alternative IR sensor assembly  10   c , the measuring surface  12  may have a generally convex contour to match a concave contour of the resin  30 . Alternative configurations of the measuring surface  12  may be possible depending on the contour of the resin  30  the thickness of which is to be measured using the system  700 . 
     As shown in  FIG. 1 , multiple IR measurement sensors  16  may be provided in the holding fixture  11  of the IR sensor assembly  10 . The IR measurement sensors  16  may interface with the measuring surface  12  of the holding fixture  11 . The IR measurement sensors  16  may be arranged in any desired pattern in the holding fixture  11 . For example and without limitation, as shown in  FIG. 2 , in some embodiments of the IR sensor assembly  10 , the IR measurement sensors  16  may be arranged in a one-dimensional line array  14 . As shown in  FIG. 3 , in other embodiments of the IR sensor assembly  10 , the IR measurement sensors  16  may be arranged in a two-dimensional grid array  15 . 
     As illustrated in  FIG. 4 , each IR measurement sensor  16  in the IR sensor assembly  10  is adapted to emit an incident IR beam  24  against the resin  30  of the composite structure  28 . In some embodiments, the incident IR beam  24  may be short wavelength near IR energy in the range of from about 900 nm to about 1700 nm. Each IR measurement sensor  16  may also be adapted to receive a returning IR beam  25  from the resin  30  and measure the absorbance, or the difference in intensity between the incident IR beam  24  and the returning IR beam  25 . The absorbance is proportional to the thickness of the resin  30 . Accordingly, the data acquisition system  704  is adapted to detect the absorbance measured by each IR measurement sensor  16  in the IR sensor assembly  10 . The calibration software  706  may enable the data acquisition system  704  to determine the thickness of the resin  30  by correlating the level of absorbance from each IR measurement sensor  16  to a known calibration standard for resin thickness. Calibration may be based on multivariate spectral analysis of infrared absorbance, multiple peak heights of infrared absorbance or single peak height of infrared absorbance. In some embodiments, the data acquisition system  704  may also be adapted to consolidate or translate the resin thickness data into a single two-dimensional map created by moving an array of IR sensors along the surface of the part. 
     Referring next to  FIG. 4 , in exemplary application, the IR sensor assembly  10  of the system  700  is positioned adjacent to a resin  30  on a polymer composite structure  28  preparatory to measuring the thickness of the resin  30 . An incident IR beam  24  is emitted from each IR measurement sensor  16  at the measuring surface  12  of the holding fixture  11 . In some embodiments, a single (or dual) near IR illumination source may be used for the whole array of near IR sensors. In some embodiments, the incident IR beam  24  may have a wavelength in the range of from about 900 nm to about 1700 nm. The incident IR beam  24  strikes or impinges on the surface of the resin  30  and is transmitted through the resin  30 . Passing through the resin  30  changes the incident IR beam  24  into the returning IR beam  25 . The IR measurement sensor  16  receives the returning IR beam  25  and transmits data which indicates the absorbance of the IR energy to the data acquisition system  704 . 
     The data acquisition system  704  detects the level of absorbance of the infrared energy at the location of each IR measurement sensor  16  in the IR sensor assembly  10 . The calibration software  706  enables the data acquisition system  704  to determine the thickness of the resin  30  by correlating the level of absorbance from each IR measurement sensor  16  to a known calibration for resin thickness. The calibration for resin thickness may be based on multivariate spectral analysis of infrared absorbance, multiple peak heights of infrared absorbance or single peak height of infrared absorbance, for example and without limitation. In some embodiments, the data acquisition system  704  may additionally consolidate or translate resin thickness data at the location of each IR measurement sensor  16  into a single two-dimensional map. 
     As shown in  FIGS. 4-6 , it will be appreciated by those skilled in the art that the IR sensor assembly  10  may be selected according to the contour of the measuring surface  12  on the holding fixture  11  such that the measuring surface  12  generally matches the contour of the resin  30  the thickness of which is to be measured. This feature may enhance accuracy of the thickness measurement. 
     Referring next to  FIG. 8 , a flow diagram  800  which illustrates an illustrative embodiment of a system for resin thickness measurement is shown. In block  802 , a holding fixture is provided. The holding fixture may be configured to match a contour of a resin the thickness of which is to be measured. An array of infrared measurement sensors may be provided in the holding fixture. The holding fixture may be positioned over the composite structure having the resin. The array of infrared measurement sensors may be configured along the surface of composite. In block  804 , the resin may be exposed to near-infrared energy at the location of each infrared measurement sensor in the sensor array of the holding fixture. In some embodiments, the near-infrared energy may have a wavelength of from about 900 nm to about 1700 nm. 
     In block  806 , the level of absorbance of the infrared energy at the location of each infrared measurement sensor in the sensor array may be detected. In block  808 , the level of absorbance from each infrared measurement sensor may be correlated to a known calibration for resin thickness to determine the thickness of the resin. The calibration may be based on multivariate spectral analysis of infrared absorbance, multiple peak heights of infrared absorbance or single peak height of infrared absorbance, for example and without limitation. In block  810 , the resin thickness data at each infrared measurement sensor which was obtained in block  808  may be consolidated or translated into a single two-dimensional image. 
     It will be appreciated by those skilled in the art that the system and method for resin thickness measurement of the disclosure is particularly suited to measuring resin thickness on the order of 20 to 40 mils. However, thickness outside this range may also be measured. Moreover, the system and method may be used for coating thickness measurements and offers significant advantages for both non-contact readings and imaging of a larger area. For these measurements, the longer IR wavelengths may be more useful (1700-2500 NM near IR or 2.5 to 25 microns mid-IR). 
     Referring next to  FIGS. 9 and 10 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  900  as shown in  FIG. 9  and an aircraft  1000  as shown in  FIG. 10 . During pre-production, exemplary method  900  may include specification and design  902  of the aircraft  1000  and material procurement  904 . During production, component and subassembly manufacturing  906  and system integration  908  of the aircraft  1000  takes place. Thereafter, the aircraft  1000  may go through certification and delivery  910  in order to be placed in service  912 . While in service by a customer, the aircraft  1000  may be scheduled for routine maintenance and service  914  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  900  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 10 , the aircraft  1000  produced by exemplary method  900  may include an airframe  1004  with a plurality of systems  1002  and an interior  1006 . Examples of high-level systems  1002  include one or more of a propulsion system  1008 , an electrical system  1010 , a hydraulic system  1012 , and an environmental system  1014 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry. 
     The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  900 . For example, components or subassemblies corresponding to production process  906  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  1000  is in service. Also one or more apparatus embodiments may be utilized during the production stages  906  and  908 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  1000 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  1000  is in service, for example and without limitation, to maintenance and service  914 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.
A system for surface resin thickness measurement on a fiber reinforced polmer composite includes a holding fixture configured to match a contour of a composite part in which a resin thickness is to be measured, a plurality of infrared measurement sensors in the holding fixture which can be moved along the surface to create a map of the surface to be measured, a computer-based data acquisition system interfacing with the plurality of infrared measurement sensors and calibration software supporting the data acquisition system.