阅读说明：本技术 通过比色法进行外壳的非破坏性测试的方法 (Method for the non-destructive testing of a housing by colorimetry ) 是由 让-路易斯·罗梅罗 让-皮埃尔·库莱特 安吉丽可·麦乐迪·马琳·亚历克西娅·美因茨克 于 2018-02-21 设计创作，主要内容包括：本发明涉及一种用于非破坏性检测由聚合物材料制成的部件(10)的给定区域(24)被加热的方法,该方法包括以下步骤：a)在所述给定区域(24)上执行至少一个比色测量,以便监测和获得CIELAB色度空间的参数a的值a<Sub>p</Sub>,b)在部件的参考区域(26)上执行至少一个比色测量并获得CIELAB色度空间的参数a的值a<Sub>p/ref</Sub>,c)计算Δa<Sub>p</Sub>＝a<Sub>p</Sub>-a<Sub>p/ref</Sub>,d)如果Δa<Sub>p</Sub>高于阈值A1,则确立待测所述给定区域被加热了的风险。(The invention relates to a method for non-destructive testing of a given region (24) of a component (10) made of a polymer material for heating, comprising the following steps: a) performing at least one colorimetric measurement on said given area (24) in order to monitor and obtain the value a of the parameter a of the CIELAB chromaticity space p B) performing at least one colorimetric measurement on a reference region (26) of the component and obtaining a value a of a parameter a of the CIELAB color space p/ref C) calculating Δ a p ＝a p ‑a p/ref D) if Δ a p Above the threshold a1, the risk of the given area to be measured being heated is established.)

1. A method for non-destructively testing a heated region (24) of a part (10) made of a polymeric material to be determined, the method comprising the steps of:

a) performing at least one colorimetric measurement on the determined area (24) to be measured and obtaining the value a of the parameter a of the CIELAB color space_p，

b) At least one colorimetric measurement is carried out on a reference region (26) of the component (10) and the value a of the parameter a of the CIELAB color space is obtained_p/ref，

c) Calculating Delta a_p＝a_p-a_p/ref，

d) If Δ a_pAbove the threshold a1, the risk of the determined area to be determined being heated is established.

2. The method of claim 1, further comprising the steps of:

-obtaining the value b of the parameter b of the at least one colorimetric measurement carried out in step a)_pAnd the value L of the parameter L_p，

-obtaining the value b of the parameter b of said at least one colorimetric measurement carried out in step b)_p/refAnd the value L of the parameter L_p/ref，

-calculating Δ b_p＝b_p-b_p/refAnd calculating Δ L_p＝L_p-L_P/ref，

-establishing the risk that the determined area to be determined is heated if all of the following conditions are checked:

Δa_pabove the threshold a2, a2 is below a1,

Δb_pabove the threshold value B1, the value,

ΔL_pbelow the threshold L1.

3. The method according to claim 1 or 2, wherein the threshold value a1 is determined by using a reference value database comprising colorimetric measurements on a plurality of first samples (12) made of polymeric material, in particular of reinforcing fibres, which have been subjected to a predetermined temperature for a predetermined period of time.

4. The method of claim 3, wherein the thresholds A2, B1, L1 are determined by using the colorimetric measurements stored in a reference value database.

5. The method according to claim 3 or 4, wherein establishing a database comprises the steps of:

-for each first sample (12), obtaining from at least one colorimetric measurement a value a' of a parameter a of the CIELAB colorimetric space,

-for each first sample (12), calculating Δ a ' ═ a ' -a '_refWherein:

a′_refa value of the parameter a corresponding to the CIELAB chromaticity space, obtained on a second sample (14) made of a polymeric material, in particular of reinforcing fibres, having said value consideredThe first sample is kept at the same dwell time and temperature within a temperature range to protect the mechanical integrity of the second sample (14) or such that irradiation of radiation in the ultraviolet field can also be taken into account in the range between 0 and 40 ℃,

-performing a test to determine the mechanical properties of each first sample (14),

the threshold a1 is determined by comparing the data from the tests performed in the previous step with the value Δ a' contained in the database.

6. The method of claim 5, wherein building a database further comprises the steps of:

-obtaining, for each first sample (12), the values L 'and b' of the parameters L and b of the CIELAB colorimetric space from at least one colorimetric measurement,

-for each first sample, calculating Δ b ' ═ b ' -b '_refAnd Δ L ' ═ L ' -L '_refWherein:

b′_refand L'_refValues of the parameters b and L, respectively, corresponding to the CIELAB colorimetric space, obtained on a second sample (14) made of polymeric material, in particular of reinforcing fibres, having the same residence time as said first sample considered and kept at a temperature within a range of temperatures so as to preserve the mechanical integrity of the second sample (14) or so that the range between 0 and 40 ℃ can also take into account the irradiation of rays in the ultraviolet field,

-determining the thresholds a2, B1 and L1 by comparing the data from the test with the values Δ a ', Δ B ' and Δ L ' contained in the database.

7. The method according to claim 5 or 6, wherein the testing comprises at least one step for mechanical testing of the component, such as traction and compression.

8. The method according to any of the preceding claims, further comprising the step of:

-calculating the colour difference value Δ E between the colorimetric measurements obtained in steps a) and b)_p，

-according to the value Δ E obtained in the preceding step_p-determining the heated temperature by using a reference value database comprising values of chromatic aberration Δ E ' obtained from a plurality of samples (12) made of polymeric material and having been at a predetermined temperature for a predetermined period of time, by using a reference value database comprising values of chromatic aberration Δ E ' during which a region to be measured of the component (10) has been subjected to a predetermined heated temperature, and by using a reference value database comprising values of chromatic aberration Δ E '.

9. Method according to any one of the preceding claims, wherein each value of the parameter L, a, b of the CIELAB colorimetric space under consideration is obtained by averaging at least five consecutive colorimetric measurements at the position under consideration.

10. Method according to any one of the preceding claims, wherein, in the event of an established risk of being heated, the method further comprises a subsequent step d) of performing a physicochemical analysis of a determined area (24) of the component (10) to be tested, in order to determine the state of thermal damage of said determined area to be tested.

11. A method according to any preceding claim, wherein the polymeric material comprises reinforcing fibres.

12. The method of any preceding claim 1, further comprising the step of cleaning the surface on which the colorimetric measurement is to be made prior to the colorimetric measurement.

Technical Field

The invention relates to a method for non-destructive testing of a component comprising a substrate, for example made of a polymer. In particular, the reinforcing fibers may be integrated into the matrix.

Background

Traditionally, the upstream end of a turbine engine comprises a fan comprising a wheel formed by a plurality of blades externally surrounded by an annular casing, which may be made of a metallic material or of a composite material comprising a matrix of integrated reinforcing fibers, such as a polymer matrix, for example an epoxide polymer, and carbon or glass fiber reinforcing fibers. The casing enables an initial compression of the air entering the turbine engine and also ensures that the function of the blades is limited in the event of loss of one of the blades. The fan casing is surrounded by a plurality of equipment supply ducts, in particular by pressurized air supply ducts, at a temperature of about 200 ℃, the engines known as APUs (auxiliary power units) are used to start the turbojet engines and to power the aircraft nacelle when the aircraft is landing.

In the event of a malfunction, such as a leak from a pressurized air supply conduit, the air may cause a local significant heating of the enclosure, since the temperature of the air is about 200 ℃. When the housing is made of a metallic material (e.g., aluminum), the heating is not affected by the mechanical integrity of the housing. In the case of a matrix fan casing incorporating reinforcing fibres, it must be possible to ensure its mechanical strength after heating.

It can therefore be appreciated that non-destructive testing of composite matrix skins with reinforcing fibers is particularly important, and even more so, as composite skins have proven to be very expensive.

It is therefore recommended to apply a heat-sensitive coating on the casing. However, the lifetime of these coatings greatly limits their interest because engines can be used for longer periods of time than the lifetime of these coatings, particularly for long-haul or mid-range aircraft. Furthermore, during the placement of the engine, cleaning is usually performed by scraping, which results in complete removal of the heat-sensitive coating layer, including another step of applying the coating layer. Finally, if the heat sensitive paint is able to visually consider the heated state of a given area of the fan casing, it proves to be only an indirect measure of the state of the internal structure of the casing and cannot be specifically quantified.

Disclosure of Invention

The present invention is particularly intended to provide a simple, effective and economical solution to the problems of the prior art described above.

To this end, it proposes a method for non-destructive testing of a component made of a polymeric material for which a determined area to be tested is heated, comprising the following steps:

a) performing at least one colorimetric measurement on the determined area to be measured, and obtaining the value a of the parameter a of the CIELAB chromaticity space_p，

b) Performing at least one colorimetric measurement on said reference region of the component and obtaining the value a of the parameter a of the CIELAB chromaticity space_p/ref，

c) Calculating Delta a_p＝a_p-a_p/ref，

d) If Δ a_pAbove the threshold a1, the risk of the determined area to be determined being heated is established.

The method according to the invention enables a determination to be made whether the component under inspection has experienced (or has not experienced) overheating beyond acceptable limits. In fact, if the risk of overheating is determined, a physicochemical analysis of the component is carried out in order to determine more specifically the degree of overheating. However, the physicochemical analysis must be performed in a laboratory, which requires the disassembly of the components from the assembled mechanical unit. Thus, when the part to be tested is a turbine engine casing, laboratory testing involves the placement of the aircraft engine and greatly increases the downtime of the aircraft and therefore increases the operating costs.

The invention proposes to establish the risk that a given area of the component is heated, in contrast to a presumed normal reference portion, in other words, a portion of the component to be inspected that is not subjected to heating, which is able to establish a reference of the component itself in order to take into account normal variations of the parameter a due to the external environment in which the component is located since its manufacture. The term "normal" refers to a standard condition of use of a component such that the life of the component can be predetermined.

In the CIELAB colorimetric space, the variation of the value of the parameter a gives information on the level of superheat experienced by the determined area of the component to be measured.

The threshold value a1 can be determined by using a reference value database comprising colorimetric measurements made on a plurality of reference samples made of polymeric material, in particular made of reinforcing fibers, which have been subjected to a determined temperature for a determined period of time. From a plurality of measured values of the parameter a, a threshold value a1 is established below which the mechanical integrity of the component cannot be guaranteed without physicochemical analysis.

Value Δ a_pInformation is given about the (component) level of red present in a given area of the test.

In the present application, the CIELAB chromaticity space means the CIE L a b system, or the acronym CIE means the International Lighting Commission. The asterisks are automatically omitted from the text to avoid obstructing the symbols.

In the present document, the CIELAB space corresponds to the space defined by the standard NF EN ISO 11664-4(2011-07-01), entitled "colorimetry — section 4: CIE L a b chromaticity space 1976 (Colorimetry-Part 4: CIE L a b colour space 1976) ".

The method further comprises the following steps:

-obtaining the value b of the parameter b of the at least one colorimetric measurement carried out in step a)_pAnd the value L of the parameter L_p，

-obtaining the value b of the parameter b of said at least one colorimetric measurement carried out in step b)_p/refAnd the value L of the parameter L_p/ref，

-calculating Δ b_p＝b_p-b_p/refAnd calculating Δ L_p＝L_p-L_P/ref，

-establishing the risk that the determined area to be determined is heated if all of the following conditions are checked:

Δa_pabove the threshold a2, a2 is below a1,

Δb_pabove the threshold value B1, the value,

ΔL_pbelow the threshold L1.

Value Δ b_pInformation is given about the level of yellowing of the determined areas tested. It is to be noted that in this chromaticity space, the value of the parameter b changes from a negative value to a positive value, in other words, from blue to yellow. The variation of the value of the parameter L gives information about the development of the brightness/sharpness and progresses from a value 0 corresponding to black to a value 100 corresponding to white. The threshold value L1 enables providing information about the reduced definition of the test area relative to the reference area.

Thus, if no risk of overheating is established during step d), the method comprises performing three consecutive comparisons, wherein a positive result implies the establishment of a risk.

According to a feature of the invention, the thresholds a2, B1, L1 are determined by using said colorimetric measurements stored in a database of reference values.

According to another feature of the present invention, creating the database comprises the steps of:

-obtaining, for each first sample, a value a' of a parameter a of the CIELAB colorimetric space from at least one colorimetric measurement,

-for each first sample, calculating Δ a ' ═ a ' -a '_refWherein:

a′_refthe value of the parameter a corresponding to the CIELAB colorimetric space, obtained on a second sample made of polymeric material, in particular reinforcing fibres, having the same residence time as said first sample considered and maintaining the temperature within a temperature range, such as protecting the mechanical integrity of the second sample, or such as ranging between 0 and 40 ℃, which may further take into account the irradiation of rays in the ultraviolet field,

-performing a test to determine the mechanical properties of each first sample,

the threshold a1 is determined by comparing the data from the tests performed in the previous step with the value Δ a' contained in the database.

The database establishment further comprises the following steps:

-obtaining, for each first sample, the values L 'and b' of the parameters L and b, respectively, of the CIELAB colorimetric space from at least one colorimetric measurement,

-for each first sample, calculating Δ b ' ═ b ' -b '_refAnd Δ L ' ═ L ' -L '_refWherein:

b′_refand L'_refValues of the parameters b and L, respectively, corresponding to the CIELAB colorimetric space, obtained on a second sample made of polymeric material, in particular reinforcing fibers, having the same residence time as said first sample considered and maintaining the temperature within a temperature range, such as protecting the mechanical integrity of the second sample, or such as ranging between 0 and 40 ℃, which may further take into account the irradiation of the rays in the ultraviolet field,

-determining the thresholds a2, B1 and L1 by comparing the data from the test with the values Δ a ', Δ B ' and Δ L ' contained in the database.

According to the invention, testing the mechanical properties of the first sample comprises at least one step for mechanically testing the component, for example by traction and/or compression.

The invention also comprises the following steps:

-calculating the colour difference value Δ E between the colorimetric measurements obtained in steps a) and b)_p，

-according to the value Δ E obtained in the preceding step_pDetermining a time period during which a region to be measured of the part has been subjected to a determined heated temperature by using a reference value database including a value Δ E 'of a color difference obtained from a plurality of samples made of a polymer material and having passed through the determined time period at the determined temperature, and determining the heated temperature by using a reference value database including a value Δ E' of a color difference.

By calculation of

Determining the value Delta E of the region to be measured_p。

By calculation of

The value Δ Ε' was determined for each first sample.

In order to limit the measurement errors and to average the experimental variability, each value of the parameter L, a, b of the CIELAB colorimetric space under consideration is obtained by averaging at least five successive colorimetric measurements at the position under consideration.

According to the invention, in the event that the risk of heating is established, the method may further comprise a subsequent step d) of performing a physicochemical analysis of the determined region of the component to be tested, in order to determine the state of damage of said determined region.

The component of the assay may be made of a polymeric material containing reinforcing fibers.

Preferably, the step of cleaning the surface on which the colorimetric measurement is to be performed is performed before said colorimetric measurement.

Drawings

The invention will be best understood by reading the following description, which is a non-exhaustive example, and with reference to the following drawings, and other details, advantages and features of the invention will appear:

FIG. 1 is a schematic illustration of a turbine engine fan casing under test;

fig. 2 is a representative view of the CIELAB chromaticity spatial axis used in this document;

FIG. 3 is a schematic of a plurality of samples, each sample undergoing oxidation at a given temperature (horizontal) for a given time (vertical);

fig. 4 is a schematic view showing a separation line between the first region 1 and the second region 2.

FIG. 5 is a schematic illustration of a graph in which each point represents that the sample from FIG. 3 has been subjected to the glass transition temperature of the material being measured, each point being placed on the graph according to the values of its parameters a and b, a being on the x-axis and b being on the y-axis;

FIG. 6 is a schematic diagram of a graph in which each point represents that the sample from FIG. 3 has been subjected to a temperature higher than the glass transition temperature of the polymeric material, each point being placed on the graph according to the values of parameters a and b, where a is represented on the x-axis and b is represented on the y-axis;

FIG. 7 is a flow chart of the function of the decision-making method during a non-destructive testing step of a given area under test of a certain component, such as the housing of FIG. 1;

FIG. 8 is a graph showing the development of color difference over time for the sample of FIG. 3;

fig. 9 is an enlarged view of a given area from the graph of fig. 8.

Detailed Description

As mentioned above, the fan casing 10 made of polymer, in particular of reinforcing fibres, shown in fig. 1, the fan casing 10, when turned on, may be subjected to localized heating which should be characterized by a non-destructive method capable of determining the condition of the casing 10 to determine whether it can (or cannot) remain in use in a turbine engine.

Fig. 2 represents the CIELAB space used in this document for analyzing colorimetric data obtained on a part to be tested and for building a reference value database on a plurality of first samples. The CIELAB space is a system capable of representing three primary color components along three orthogonal axes between them. The axis L (or L)^*) Represents the luminance or sharpness axis (solid black: l is 0; pure white: l ═ 100). Axis a (or a)^*) Indicating a change from green (negative values of a)) To the axis of red (positive values of a). Axis b (or b)^*) Representing axes from blue (negative values of b) to yellow (positive values of b).

Again, it is noted that the space used is CIEL^*a^*b^*Chroma space, and asterisks have been automatically deleted as usual.

In the color system, a first color coordinate L₁、a₁、b₁And a second color coordinate L₂、a₂、b₂The color difference between is calculated as follows:

wherein

ΔL＝(L₁-L₂)

Δa＝(a₁-a₂)

Δb＝(b₁-b₂)

This method of calculating the color difference is a method used later because it will appear after the description.

The invention proposes to build a reference value database comprising colorimetric measurements according to the CIELAB color system. The term "reference value" as used below is understood to mean an element of a reference value database comprising colorimetric measurement values, and more generally data obtained from a reference sample.

To this end, a plurality of first samples 12 of a batch of material similar to the component to be analyzed is constructed. Fig. 3 shows such a batch, which thus comprises several first samples 12 of a fan housing 10, the fan housing 10 being made of a polymer material, preferably of reinforcing fibers, the several first samples 12 being arranged in lines and columns. Along a given line, each first sample 12 is subjected to a given temperature, with the exposure time given by the position along the line. Of course, the database should include a number of first samples 12 that are capable of achieving different levels of thermal oxidation, and the polymer may be subjected to actual functional conditions. Thus, the database should include a first sample 12, the first sample 12 having been subjected to the above temperatures for a time period of at least 6 months.

In the configuration shown as an example, the first sample 12 has been subjected to temperatures of 120 ℃, 140 ℃, 150 ℃, 160 ℃, 180 ℃, 200 ℃, 220 ℃ and 240 ℃ for a time period of several hours, extending from 1 to 1440 hours, corresponding to a time period of 2 months. The sample 14 located in the lower left corner of fig. 1 represents the sample 12 without any heating and therefore constitutes an absolute reference for the fan housing 10 without any heating. It is observed in fig. 1 that once the temperature is increased and the time of exposure to the temperature increases, first sample 12 darkens, consistent with thermal oxidation of the polymer.

Fig. 4 schematically shows fig. 3 and includes a separation line 16 of the first region 1 and the second region 2. The first zone 1 corresponds to a first sample 12 that has been subjected to an acceptable temperature for an acceptable time period, while the second zone 2 corresponds to a first sample 12 that has been subjected to an excessively high temperature for a given time period. Thus, if such parting lines can be established visually, it seems necessary to establish one or more parameters so that the state of the analysis component can be objectively realized. This is by building a reference value database as defined below.

Establishing the reference value database first comprises performing a colorimetric measurement on each first sample and deriving the values L ', a ' and b ' of the parameters L, a, b of the CIELAB colorimetric space from at least one colorimetric measurement value.

It must first be noted that the values of the parameters L, a, b can be obtained from several colorimetric measurements in each zone where the measurement is made, in other words to build a database or when a determined zone of the component to be tested is required, as will be explained later.

Referring now to fig. 5 and 6, each represents a graph in which each point represents the first sample from fig. 3, each point being placed on the graph according to the values of its parameters a and b, a being represented on the x-axis and b being represented on the y-axis. In fig. 5, the first sample has been subjected to a temperature below the glass transition temperature of the polymeric material, here 170 ℃, and in fig. 6 the first sample has been subjected to a temperature above said glass transition temperature.

In fig. 5 it has been observed that a set 18 of points is found where the value of the parameter is less than zero, whereas in fig. 6, for the first sample which has been subjected to a temperature higher than 170 ℃, a set 20 of values of the parameter a higher than zero has been observed. Thus, it is possible to distinguish the thermal state from the measured value of this parameter, in other words to distinguish the heating of a given area of the component. It should be noted that in fig. 6, the second set 22 has values of the parameter a smaller than zero, but these points correspond to very short exposure times, less than 10 hours, which is not taken into account. In the graph of fig. 5, it is also observed that the variation, which is mainly carried out along the axis b, enables a pronounced yellowing by natural aging of the polymer and the resin with time in the case of fan housings. This variation along axis b is also visible in fig. 6.

Thus, it can be appreciated that through colorimetric measurements in CIELAB space, natural aging and accidental overheating of the fan casing can be distinguished by comparison with a reference value database.

Δ a ' -a ' was calculated for each first sample 12 '_ref，Δb′＝b′-b′_refAnd Δ L ' ═ L ' -L '_refWherein:

a′_ref、b′_refand L'_refValues of the parameters a, b, L respectively corresponding to the CIELAB colorimetric space, obtained on a second sample 14 made of polymeric material, in particular reinforcing fibers, having the same residence time as said first sample under consideration, and maintained at a temperature within a certain temperature range, such as a temperature maintaining the mechanical integrity of the second sample 14, or such as a temperature ranging between 0 and 40 ℃, which may further take into account the irradiation of the rays in the ultraviolet field.

The colorimetric measurement of the second sample 14 associated with each measurement of the first sample 12 may be performed together with the reference sample 14 observed under the conditions described in the preceding paragraph.

For each first sample 12, a test is then carried out with the aim of determining its mechanical properties in order to determine whether it can constitute a sample that can be used under the determined conditions. Thus, it is determined whether the heating experienced by each first sample makes it available. The test performed may include at least one step for mechanically testing the component, for example by traction and/or compression.

Finally, the data from the test are compared with the values Δ a ', Δ B ', and Δ L ' contained in the reference value database to establish the thresholds a1, a2, B1, and L1. The threshold a1 corresponds to a threshold value: beyond this threshold, it is considered that the fan casing 10 must be arranged to undergo more in-depth inspection of the area to be tested (fig. 7).

To perform a non-destructive testing operation on the enclosure 10 of fig. 1, first, the following steps are performed, as shown in fig. 7:

a) at least one colorimetric measurement is carried out on the defined region 24 of the housing 10 to be measured, and the value a of the parameter a of the CIELAB color space is obtained_p，

b) At least one colorimetric measurement is carried out on a reference region 26 of the housing 10 and the value a of the parameter a of the CIELAB color space is obtained_p/ref，

c) Calculating Delta a_p＝a_p-a_p/ref，

d) If Δ a_pAbove the threshold a1, a risk is established that the determined area to be determined is heated.

The reference region 26 of the housing 10 is a region that is not damaged by heat.

At a value of Δ a_pIn the lower case, a second step is carried out, aimed at determining whether the component must perform (or not perform) all the same tests, this time taking into account the value b of the colorimetric measurement acquisition parameter b on the area 24 to be tested of the casing_pAnd the value L of the parameter L_pAnd the colorimetric measurement on a reference region 26 of the housing yields the value b of the parameter b_p/refAnd the value L of the parameter L_p/ref。

The method then includes calculating Δ b_p＝b_p-b_p/refAnd calculating Δ L_p＝L_p-L_P/refAnd establishing the risk that the area to be determined 24 is heated if all of the following conditions are detected:

Δa_pabove the threshold a2, a2 is below a1,

Δb_pabove the threshold value B1, the value,

ΔL_pbelow the threshold L1.

In this case, if the resolution Δ L is large_pLow, in other words below the threshold L1, while Δ a_pAbove threshold a2 and below a1, then see if test region 24 has greater yellowing than threshold B1.

If one of the above conditions is not checked, the region 24 to be measured is considered to be undamaged and the housing 10 can be used.

Non-destructive testing by colorimetric operation may be performed under the aircraft wing, which enables a quick and reliable decision whether to place the skin and reduces unnecessary maintenance operations.

The parameters a1, a2, B1, L1 must be established for each type of component 10 and thus the parameters a1, a2, B1, L1 are linked to the material of the component and also depend on the colorimetric measuring means.

Thus, in the example of colorimetric measurements with a Konicam Minolta CM700d colorimeter, A1 equals 4.3, A2 equals-1, B1 equals 12.6 and L1 equals-0.9.

From the above-mentioned colorimetric measurements contained in the database, the time period during which the test region has been subjected to a determined temperature and the temperature can be determined.

To this end, the color difference value of the test field is calculated using colorimetric measurements obtained on the test field and on a reference field of the evaluation housing

For each first sample, the color difference value

Also calculated from the values Δ L ', Δ a ' and Δ b '. From the value Δ E ', the evolution of Δ E' over time at several determined temperatures can be tracked, as shown in fig. 8 and 9.

FIG. 8 shows the evolution of Δ E' over time at temperatures of 120 ℃, 140 ℃, 150 ℃ and 160 ℃ which areCurves are respectively represented by Δ E'₁₂₀、ΔE′₁₄₀、ΔE′₁₅₀And Δ E'₁₆₀。

To avoid that the curve of development of the chromatic aberration is affected by the uncorrelated values of Δ E ', the points associated with these values Δ E' are removed because of the very clear coloration or the too dark coloration. Very clear coloration may be due to exposure to low temperatures for a relatively short period of time. This region corresponds to region 28 in fig. 3 and cannot be effectively considered in colorimetric analysis. Too dark coloration may be due to exposure to excessive temperatures for a considerable period of time. This region corresponds to region 30 in fig. 3 and cannot be effectively considered in colorimetric analysis.

Thus, this situation is limited to relatively long periods of exposure, over 300 hours, as shown in fig. 9. Using this map, it is possible to derive from the measured values Δ E obtained over a given area of the housing_pBy tracing a constant ordinate line Δ E_pAnd searching a intercept curve to determine the exposure temperature, the intercept curve thus representing the temperature experienced by the test area and the exposure temperature being represented by the abscissa of the intersection.