Abstract:
A method for assessing damage to a composite material covered with a polyurethane-type paint is provided. Two separate criteria are measured on a spectrogram obtained by infrared spectrometry, thereby characterizing thermal ageing of the paint, each separate criteria being a measurement on a curve of a spectrum of the spectrogram of a height of a particular peak, thereby giving two independent assessments of the thermal ageing. Then, the two separate criteria are combined together in order to obtain a result of a level of the damage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/FR2014/052656, filed on Oct. 17, 2014, which claims the benefit of FR 13/60123, filed on Oct. 17, 2013. The disclosures of the above applications are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to a method for assessing the damage to composite materials. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    Composite materials, in particular panels used in aeronautical applications, proximate to the engines, are intended for operation at temperatures below a maximum threshold. These panels, whether forming one single layer or used in a sandwich structure comprising a central core such as honeycomb covered with two external skins or face sheets, include unique mechanical characteristics and a reduced mass, and may take various shapes. 
         [0005]    However, subsequent to thermal events comprising an increase of temperature above a maximum threshold, and depending on the exposure times, some damage to the materials of the panels may occur, which may reduce the structural strength characteristics. Thus, it is desirable to be able to characterize the level of the material damage during maintenance operations in order to increase safety. 
         [0006]    In order to carry out inspection on these composite materials, it is known to apply a primary paint over the surface, in particular a polyurethane paint, which will undergo color transformations during thermal events. 
         [0007]    A first known method for damage assessment consists in visually studying the discoloration of the primary paint in order to deduce therefrom some level of evolution of the material. This method gives a qualitative indication, but does not allow for an accurate quantitative measurement of the damage. 
         [0008]    Another known non-destructive method, in particular presented by the documents EP-A1-2343531 and US-A1-20110001047, performs an analysis of the discoloration of a primary paint by a technique based on infrared spectrometry. 
         [0009]    In particular, the second mentioned document presents the following method: after having carried out a calibration model, infrared radiation is emitted on a point of the surface of the material to be analyzed; then, the diffused reflection spectrum of this surface is measured; and by considering on this spectrum a criterion which is compared to the calibration model, a history of the thermal or chemical aspects of the coating is assessed, in order to deduce therefrom a condition of the composite material supporting it. 
         [0010]    Nonetheless, this method is complex to carry out, and the obtained results highly depend on the implementation of the primary paint. In particular, thickness variations of the coating layer, of its polymerization cycle, of the number of deposited layers or of the surface condition, will disturb the infrared reflection. The results as well as the conclusions related to the damage to the material are not accurate. 
         [0011]    Other known non-destructive methods for assessing the damage to the materials use in particular ultrasonic or thermographic conventional techniques which allow in particular detecting detachments between the layers of the materials. Thus, it is possible, for example, to control the detachment between the skin and the core of a panel comprising a sandwich structure. Nonetheless, this method does not allow assessing the condition of the material by itself. 
       SUMMARY 
       [0012]    In one form, the present disclosure provides a method for assessing damage to a composite material covered with a polyurethane-type paint, using an infrared spectrometry of the paint, remarkable in that on the obtained spectrogram, two separate criteria characterizing the thermal ageing of the paint are measured, each comprising the measurement, on the curve of the spectrum, of a height of a particular peak, giving two independent assessments of this ageing, then these two criteria are combined together in order to obtain a result of the level of the damage. 
         [0013]    One advantage of this assessment method is that, in a rapid and economical manner, the simultaneous comparison of the two combined criteria allows addressing the variations of the parameters of implementation of the paint and of the measurement conditions, which gives an accurate assessment of the material damage. 
         [0014]    The damage assessment method according to the present disclosure moreover may include one or more of the following characteristics, which may be combined together. 
         [0015]    Advantageously, in order to obtain a first criterion, the height of the particular peak is measured relative to a base line passing through the first two immediately adjacent troughs at each side of this peak. 
         [0016]    Advantageously, in order to obtain a second criterion, the height of the particular peak is measured relative to a base line passing through the first two significant troughs on each side of this peak. 
         [0017]    Advantageously, the two criteria are disposed along the two axes of a graph in order to combine them, this graph comprising areas separated by limits which determine the level of thermal degradation of the material. Thus, a damage assessment is achieved. 
         [0018]    In particular, the graph may include a first area corresponding to an absence of evolution of the chemical structure of the material, a second area corresponding to an evolution of this structure resulting in a low damage, and a third area corresponding to an evolution of this structure resulting in a high damage. 
         [0019]    The limit between the first area and the second area may correspond to a particular equivalent ageing (temperature/duration). The limit between the second area and the third area may correspond to another particular equivalent ageing. These values are representative of transformation thresholds which allow establishing the category of the damage. 
         [0020]    According to one form of the present disclosure, the polyurethane paint under the commercial designation 5014 is used. 
         [0021]    According to other optional features of the method according to the invention: 
         [0022]    the wave number of the particular peak is close to 1300/cm, being between 1288 and 1314/cm; 
         [0023]    in order to obtain the first criterion, the first trough comprised between 1322 and 1374/cm is considered, and the second trough comprised between 1236 and 1247/cm is considered, the value 1247/cm being retained if this second trough is not pronounced on the curve; 
         [0024]    in the case where the particular peak is no longer observable, then the largest height comprised between the base line and the curve is measured between 1288 and 1314/cm, in order to obtain the first criterion which may be negative; 
         [0025]    in order to obtain the second criterion, the first significant trough close to 2000/cm, comprised between 1951 and 2051/cm, and the second significant trough close to 1000/cm, comprised between 901 and 1102/cm, are considered; 
         [0026]    in the case where the particular peak is no longer observable, then the height between the base line and the curve at the value 1288/cm is measured in order to obtain the second criterion. 
         [0027]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       DRAWINGS 
         [0028]    In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which: 
           [0029]      FIG. 1  is an infrared spectrogram obtained for the analysis of a composite material with a method according to the present disclosure; 
           [0030]      FIG. 2  is a detail view of this spectrogram showing the measurement of a first criterion of this method, for a highly visible particular peak; 
           [0031]      FIGS. 3 and 4  present a method for measuring the first criterion for a dimly visible particular peak; 
           [0032]      FIGS. 5 and 6  present a method for measuring the first criterion for a non visible particular peak; 
           [0033]      FIG. 7  presents the value of the first criterion as a function of the exposure time of the material and the reached temperature; 
           [0034]      FIG. 8  presents a method for measuring the second criterion; 
           [0035]      FIG. 9  is a graph presenting the principle of thermal exposure variation as a function of the two criteria; and 
           [0036]      FIG. 10  is the same graph showing, for identical thermal exposures, measurements for paints including different conditions of implementation. 
       
    
    
       [0037]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
       DETAILED DESCRIPTION 
       [0038]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
         [0039]      FIGS. 1 and 8  present, as a function of the wave number per centimeter N/cm, the curve obtained for a spectrometry on a panel made of composite materials and covered with a primary 5014-type polyurethane paint. The measurement is performed for a wave number comprised between 4000 and 650/cm. 
         [0040]    It is possible in particular to perform this analysis on parts made of carbon fiber and bismaleimide-type resin (“BMI”), covered with the primary polyurethane. 
         [0041]    The obtained spectrum is characteristic of a primary polyurethane paint with this reference, it evolves as a function of temperature exposure, comprising the temperature level and the exposure time. 
         [0042]    The spectrometry is carried out with a portable apparatus brought into contact with the part, according to the diffuse reflection technique, adapted to the surface condition of the composite material panel. The surface condition of the part is prepared beforehand through a cleaning with a solvent, in order to eliminate pollution that might disturb the measurements and modify the spectrogram. 
         [0043]    From this spectrogram, two different criteria C 1  and C 2  are identified on the curve in order to characterize the thermal ageing. 
         [0044]    The use of two criteria allows obtaining, via a comparison of these criteria, an assessment of the thermal ageing which is independent from the parameters of implementation of the polyurethane paint and from the measurement conditions, thereby making the measurement reliable. It will be noted that the use of one single criterion does not allow any comparison, we remain dependent on the parameters of implementation and on the measurement conditions, which does not give a reliable result. 
         [0045]    For the calculation of the first criterion C 1  presented in  FIG. 1 , the height of a particular peak  6  is measured on the spectrogram relative to a base line  8  passing through the first two immediately adjacent troughs  2 ,  4  on each side of this peak. For the calculation of the second criterion C 2  presented in  FIG. 8 , the height of the particular peak  6  is measured relative to a base line  8  passing through the first two significant troughs  40 ,  42  on each side of this peak. 
         [0046]    It will be noted that the troughs on the curve representing low-emission points which vary slightly during thermal ageing, constitute references allowing to measure the evolution of the height of the peak  6 . 
         [0047]    In this example with a 5014-type polyurethane primary paint, the particular peak  6  is chosen close to the 1300/cm wave number, being between 1288 and 1314/cm. 
         [0048]      FIGS. 1 and 2  present the case of a spectrogram having the particular peak  6  easily observable thanks to a first point  2  corresponding to the trough located just to the left, and to a second point  4  corresponding to the trough located just to the right. Then, the height of this peak  6  is measured relative to the base line  8  connecting the two points  2 ,  4 , in order to obtain the first criterion C 1 . 
         [0049]      FIGS. 3 and 4  present the case of a spectrogram having a particular peak  6  which is reduced through the presence of a second peak  10  located to the right, at about 1200/cm, without the second trough between the particular peak  6  and this second peak  10 . 
         [0050]    In this case, there is arranged a base line  8  which passes through the first point  2  corresponding to the left trough  2 , and through a second point of the curve  12  considered at the value 1247/cm. The height of the particular peak  6  is measured, in the same manner, relative to this base line  8 , in order to obtain the second criterion C 2 . 
         [0051]      FIGS. 5 and 6  present the case of a spectrogram having a particular peak  6  which is not observable because of the presence of the second peak  10  located to the right, at about 1200/cm, which is very developed. Then, there is a total absence of troughs at 1250/cm. 
         [0052]    In this case, there is also arranged a base line  8  which passes through the first point  2  corresponding to the left trough, and through the second point of the curve  12  considered at the value 1247/cm. Afterwards, the largest height between this base line  8  and the curve, comprised between 1288 and 1314/cm, is measured in order to obtain the first criterion C 1 . In this example, the height is negative, this first criterion C 1  is then negative. 
         [0053]    In a general manner for the calculations of the first criterion C 1 , the base line  8  passes through a first point corresponding to the left trough  2  comprised between 1322 and 1374/cm, and through a second point  4  corresponding to the right trough comprised between 1236 and 1247/cm, or without right troughs through a second point  12  at the value 1247/cm. 
         [0054]      FIG. 7  presents, as a function of the exposure time indicated on a logarithmic scale, the value of the first criterion C 1  presented on referenced curves  20 ,  22 ,  24 ,  26 ,  28 ,  30 , respectively corresponding to the temperatures of 100, 150, 170, 200, 250 and 300° C. 
         [0055]    It is observed that this first criterion C 1  is actually characteristic of thermal ageing, with a variation depending both on the temperature level and on the exposure time. 
         [0056]      FIG. 8  presents the calculation of the second criterion C 2 , which measures the height of the particular peak  6  relative to a base line  8  connecting the first significant trough  40  located to the left at 2000/cm, comprised between 1951 and 2051/cm, to the second significant trough  42  located to the right at 1000/cm, comprised between 901 and 1102/cm. 
         [0057]    In the case where the particular peak  6  is no longer observable through the presence of a second peak located to the right at about 1200/cm which is very developed, as presented in  FIGS. 5 and 6 , then the height between the base line  8  and the curve at the value 1288/cm is measured, in order to obtain the measurement of the second criterion C 2 . 
         [0058]      FIG. 9  presents a graph comprising the first criterion C 1  on a horizontal axis and the second criterion C 2  on a vertical axis, on which the measured criteria of the material to be analyzed are disposed. 
         [0059]    Through their path, the main arrows  50  then  52  present a modification of the two criteria C 1 , C 2  giving a more and more significant thermal exposure, whether by the higher temperature level that has been reached or by the longer exposure time, which will give a more and more significant degradation of the material. 
         [0060]    In contrast, along their directions, the transverse arrows  54  present a modification of the two criteria C 1 , C 2  corresponding to a variation of the parameters of implementation of the polyurethane paint or of the surface conditions, which do not displace the points along the direction of the main arrows  50 ,  52 , which gives a similar thermal degradation of the material. 
         [0061]      FIG. 10  presents, on the same graph, different measurements which confirm the principle presented above. Each set of measurements groups, substantially around an axis with a constant slope, aligned along the directions of the transverse arrows  54 , a set of points corresponding to the same thermal ageing, on a polyurethane paint implemented under different conditions. 
         [0062]    In particular, there is a first axis  60  corresponding to an temperature exposure of 150° C. for five months, a second axis  62  corresponding to 200° C. for eight days, a third axis  64  corresponding to 300° C. for one hour, a fourth axis  66  corresponding to 170° C. for five months, a fifth axis  68  corresponding to 250° C. for twenty-four hours, and a sixth axis  70  corresponding to 200° C. for five months. There is also a group of points  72  corresponding to 200° C. for one month, which is located proximate to the first criterion C 1  equal to zero. 
         [0063]    Thus, is obtained in a simple and rapid manner, without destruction of the material, an accurate assessment of the thermal engagement level of the materials which is independent from the parameters of implementation of the primary paint, and from the measurement conditions.