Abstract:
A method of determining a physical property of a composite material includes providing a series of composite materials/surfacing films, which are subjected to increasing thermal experience to create a set of thermal effect standards, collecting mid-IR spectra on those standards, performing data pre-processing and then multivariate calibration on the spectra of the composite materials/surfacing films, and using that calibration to predict the thermal effect for samples in question.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to co-pending U.S. patent application Ser. No. ______, (Attorney Docket No. 08-0107) Ser. No. ______ and (Attorney Docket No. 08-0084); and Ser. No. ______, (Attorney Docket No. 08-0091); and Ser. No. ______, (Attorney Docket No. 08-0092) all filed concurrently herewith on Jun. 28, 2008, each of which applications is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The disclosure relates to methods for determining thermal effect in composite materials. More particularly, the disclosure relates to a method for accurately assessing thermal effect in a composite material or surfacing film using mid-infrared spectroscopy. 
       BACKGROUND OF THE INVENTION 
       [0003]    Resin-fiber composite materials are utilized in a variety of applications including the aerospace industry, for example. Structures which are constructed of resin-fiber composite materials may be exposed to heat, which may affect the composite materials in various ways. These heat-induced effects may include chemical effect in which changes such as oxidation, material loss and the breaking and/or forming of chemical bonds occurs in the polymer chemical structure of the composite materials. Resin decomposition, strength effect, charring and fiber decomposition of the composite materials may occur at increasing temperatures. 
         [0004]    Composite materials and epoxy-based surfacing films are typically cured under a vacuum at 250-350° F. for several hours. Repairs to the composite materials or surfacing films can become necessary if voids are found during an NDI inspection or if a flaw is found. These repairs may require the use of heating blankets that are notorious for having hot spots. Thus, during the repair some areas of the composite material or surfacing film may become overheated; in the case of epoxy-based surfacing films, a color change (dark or blackened) may be noticeable. Thermal effect for in-service airplanes can occur from engine and run-way fires, electrical problems, lightening strikes, and other causes. Thermal effect may affect the mechanical and structural integrity of composite materials and surfacing films. 
         [0005]    Repair or removal of heat-effected composite materials or surfacing films on a structure may involve first determining the degree of harmful heat effect to the composite materials or surfacing films. Although determining the degree of heat effect to composite materials or surfacing films may be performed by visual inspection, heat effect may not be visually apparent. Previous methods of determining the presence and extent of heat effect in composite materials and surfacing films includes obtaining an infrared spectrum of a heat-affected composite standard and correlating the infrared spectrum obtained from the composite standard with the degree of thermal effect of the composite standard. An infrared spectrum obtained from the composite material or surfacing film the heat effect of which is in question can then be compared to the infrared spectrum obtained from the composite standard to determine the presence and assess the degree of thermal effect in the composite material or surfacing film. Current methods of determining the presence and extent of heat effect in composite materials and surfacing films include obtaining the spectra of a series of heat-affected composite standards and building a multivariate model with the spectra of those standards. An infrared spectrum obtained from the composite material or surfacing film which is in question can then be predicted by the multivariate model to determine the presence and assess the degree of thermal effect in the composite material or surfacing film. 
         [0006]    U.S. Pat. No. 7,115,869 discloses a method for measurement of composite heat effect with infrared spectroscopy by which an amount of heat exposure to a resin-fiber composite substrate is determined. According to the method, a value of infrared energy reflected by a composite substrate is determined. The value of infrared energy which is reflected or absorbed is correlated to a degree or amount of heat exposure of the composite substrate. The thermal effect measurement method of the present disclosure is more sensitive than that which is disclosed in U.S. Pat. No. 7,115,869 and is richer in spectral detail, providing a better indication of whether thermal effect has occurred in a composite material as well as the extent of effect. Moreover, the method can be implemented using a hand-held spectrometer which facilitates convenience and flexibility in measuring the possibility or degree of thermal effect in the composite material in question. 
         [0007]    Calibration of infrared spectrometers to residue strength in composite materials or surfacing films correlates the resin condition as read from the infrared spectrum to the residual strength of the material or film which degrades as the resin degrades with progressively increasing temperatures. Therefore, the infrared sensors may be calibrated using time-controlled thermal soak standards which are obtained by exposing various composite materials or surfacing film controls to various temperatures for a particular time period such as one hour, for example. One method of preparing the standards includes placing the standards in an oven which is calibrated periodically and monitored continuously using multiple thermocouples as the composite materials are thermally exposed. Furthermore, thermal effect to composite materials or surfacing films may not follow a linear course. The very most useful parameters for thermal effect are those that the repair process person can understand and use as criteria for removal of effect material. Under circumstances in which it is desirable to remove all weakened material, a measure of the residual strength in the material is best. Multivariate calibration allows calibration to such physical properties under circumstances in which previous calibration methods were insufficient. 
         [0008]    Therefore, a method for accurately assessing thermal effect in a composite material or surfacing film using mid-infrared spectroscopy is needed. 
       SUMMARY OF THE INVENTION 
       [0009]    The disclosure is further generally directed to a method of determining a physical property of a composite material. An illustrative embodiment of the method includes providing a series of composite material standards with increasing thermal exposure (with or without a surfacing film), irradiating the composite material standards and/or the surfacing films with mid-spectrum infrared energy, detecting infrared energy reflected from the composite material standards/surfacing films, performing multivariate calibration on the series of the infrared spectra reflected from the composite material standards/surfacing films, performing a multivariate calibration to the infrared spectra from the standards to make a model of the spectral changes with increasing thermal exposure (or decreasing mechanical properties), and using the multivariate model to predict the thermal exposure or mechanical properties of composite materials in question. 
     
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         [0010]      FIG. 1  is a series of mid-IR spectra on thermally exposed graphite composite (CFRP) standards which were exposed for 1 hour each at 400, 450 and 500° F. and no thermal exposure, respectively. 
           [0011]      FIG. 2  is a the same series of mid-IR spectra from  FIG. 1  but with first derivative and 7 point smoothing pre-processing with the Savitzky Golay method. 
           [0012]      FIG. 3  is the regression vector from the multivariate calibration of the mid-IR spectra to the un-notched compression test stress results of the standards using the PLS (partial least squares) multivariate calibration method. 
           [0013]      FIG. 4  is a flow diagram which illustrates the multivariate calibration and prediction method. 
           [0014]      FIG. 4A  is a flow diagram which illustrates important points in the multivariate calibration method. 
           [0015]      FIG. 5  is a flow diagram of an aircraft production and service methodology. 
           [0016]      FIG. 6  is a block diagram of an aircraft. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    Referring initially to  FIGS. 1-4 , an illustrative embodiment of a method of multivariate calibration for thermal effect with mid-IR spectra is shown. As shown in block  402  of flow diagram  400  in  FIG. 4 , the thermal effect calibration begins with providing CFRP standards that are carefully cooked using carefully-controlled thermal exposure information in a range of interest and then tested to obtain residual mechanical properties data. Block  404  shows the mid-IR spectral data collection step in which infrared spectra may be collected on the standards with one or more spectra on each standard. In some applications, a hand-held spectrometer may be used to measure the spectral properties of the material in question. The raw infrared spectra obtained for the standards are shown in  FIG. 1 . Block  406  shows the data pre-processing step and  FIG. 2  shows the results of a typical pre-processing method. 
         [0018]    Block  408  shows the multivariate calibration step which may be carried out using a PLS routine or an alternative multivariate calibration method. Calibration of the multivariate calibration to thermal exposure or mechanical properties data for a composite material is obtained.  FIG. 3  shows the regression coefficients that result from that calibration. Block  410  shows the step where the multivariate calibration is saved in an appropriate format, converted into a method file and then loaded into the hand-held mid-IR device that will be used to read thermal effect on the CFRP material in question. Block  412  shows the CFRP material in question being predicted for residual stress values that would indicate the extent of thermal effect in the material in question using the new calibration model in the spectrometer method file. If the original standards are predicted here, an accuracy figure for the methods based on the difference between the known stress numbers and those predicted by the method just developed can be obtained. 
         [0019]    Referring next to  FIG. 4A , a flow diagram  400   a  which illustrates an illustrative embodiment of a method of optimizing the data pre-processing method for CFRP thermal effect calibration with multivariate methods is shown. In block  402   a,  a thermal effect standard is provided. The thermal effect standard may have been fabricated according to the method which was heretofore described with respect to the flow diagram  400  in  FIG. 4 . Infrared spectra on the thermal effect standards are collected, with multiple spectra obtained on each standard. A hand-held spectrometer which will be used to measure the material in question may be used. In block  404   a,  the infrared spectra of the thermal effect standards are made using the mid-IR wavelength range of from about 2.5 μm to about 15 μm. Pre-processing on the infrared spectra from the composite standards may be made using may be made using several different pre-processing methods. In block  406   a,  a multivariate calibration with a PLS routine or other multivariate calibration method may be performed with each pre-processing method. In block  408   a,  the calibration methods may be saved in an appropriate format for each pre-processing method. The calibration models may be converted to method files which are compatible with the hand-held spectrometer system, and then the method files loaded into the spectrometer. In block  410   a,  the new calibration models in the spectrometer methods file may be used to predict the thermal exposure or mechanical properties of the material in question for each different data pre-processing method. In block  412   a,  the root mean square error of prediction for each pre-processing method is calculated. The pre-processing method having the lowest prediction error may be selected for future use.  FIG. 3  illustrates the regression vector from the multivariate calibration of the mid-IR spectra to the un-notched compression test stress results of the standards using the PLS (partial least squares) multivariate calibration method. 
         [0020]    Referring next to  FIGS. 5 and 6 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  78  as shown in  FIG. 5  and an aircraft  94  as shown in  FIG. 6 . During pre-production, exemplary method  78  may include specification and design  80  of the aircraft  94  and material procurement  82 . During production, component and subassembly manufacturing  84  and system integration  86  of the aircraft  94  takes place. Thereafter, the aircraft  94  may go through certification and delivery  88  in order to be placed in service  90 . While in service by a customer, the aircraft  94  may be scheduled for routine maintenance and service  92  (which may also include modification, reconfiguration, refurbishment, and so on). 
         [0021]    Each of the processes of method  78  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. 
         [0022]    As shown in  FIG. 6 , the aircraft  94  produced by exemplary method  78  may include an airframe  98  with a plurality of systems  96  and an interior  100 . Examples of high-level systems  96  include one or more of a propulsion system  102 , an electrical system  104 , a hydraulic system  106 , and an environmental system  108 . 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. 
         [0023]    The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  78 . For example, components or subassemblies corresponding to production process  84  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  94  is in service. Also, one or more apparatus embodiments may be utilized during the production stages  84  and  86 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  94 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  94  is in service, for example and without limitation, to maintenance and service  92 . 
         [0024]    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.