Method for the non-destructive testing of a casing by colorimetry

Abstract

A method for non-destructively testing the heating of a determined zone of a part made of a polymer material, the method comprising the following steps: a) taking at least one colorimetric measurement on said determined zone to be tested and obtaining the value a.sub.p of the parameter a of the CIELAB colorimetric space; b) taking at least one colorimetric measurement on said reference zone of said part and obtaining the value a.sub.p/ref of the parameter a of the CIELAB colorimetric space; c) calculating a.sub.p=a.sub.pa.sub.p/ref; and d) establishing a risk of heating said determined zone to be tested if a.sub.p is higher than a threshold value A1.

Claims

1. A method for non-destructively testing a heating of a determined zone to be tested of a part made of a polymer material, the method comprising the following steps: a) taking at least one colorimetric measurement on said determined zone to be tested and obtaining a value a.sub.p of a parameter a of a CIELAB colorimetric space, b) taking at least one colorimetric measurement on a reference zone of said part and obtaining a value a.sub.p/ref of the parameter a of the CIELAB colorimetric space, c) calculating a.sub.p=a.sub.pa.sub.p/ref, obtaining a value b.sub.p of a parameter b and a value L.sub.p of a parameter L of said at least one colorimetric measurement taken in step a), obtaining a value b.sub.p/ref of the parameter b and a value L.sub.p/ref of the parameter L of said at least one colorimetric measurement taken in step b), calculating b.sub.p=b.sub.pb.sub.p/ref and calculating L.sub.p=L.sub.pL.sub.p/ref, and d) establishing a risk of overheating said determined zone to be tested if: a.sub.p is higher than a threshold value A1, a.sub.p is higher than a threshold value A2, A2 being lower than A1, b.sub.p is higher than a threshold value B1, L.sub.p is lower than a threshold value L1.

2. The method according to claim 1, wherein the threshold A1 is determined by using a reference database comprising colorimetric measurements taken on a plurality of first samples made of a polymer material, in particular, made of reinforcing fibres, having been subjected to a determined temperature for a determined time period.

3. The method according to claim 2, wherein the thresholds A2, B1, L1 are determined by using said colorimetric measurements stored in the reference database.

4. The method according to claim 3, wherein establishing the database comprises the following steps: for each first sample, obtaining a value a of the parameter a of the CIELAB colorimetric space, from at least one colorimetric measurement, for each first sample, calculating a=aa.sub.ref, where: a.sub.ref corresponds to a value of the parameter a of the CIELAB colorimetric space, the value having been obtained on a second sample made of reinforcing fibres, having a same time of existence as said first sample considered and having been kept at a temperature within a range of temperatures to preserve a mechanical integrity of the second sample, carrying out a test to determine mechanical properties of each one of the first samples, determining the threshold A1 from comparing data from the tests carried out in previous steps and the values a contained in the database.

5. The method according to claim 4, wherein establishing the database also comprises the following steps: for each first sample, obtaining 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 b=bb.sub.ref and L=LL.sub.ref, where: b.sub.ref and L.sub.ref respectively correspond to values of the parameters b and L of the CIELAB colorimetric space, these values having been obtained on a second sample made of reinforcing fibres, having a same time of existence as said first sample considered and having been kept at a temperature within a range of temperatures to preserve a mechanical integrity of the second sample, determining the thresholds A2, B1 and L1 from comparing data from said tests and the values a, b and L contained in the database.

6. The method according to claim 5, wherein the range of temperatures is a range between 0 and 40 C.

7. The method according to claim 4, wherein said test comprises at least one step for mechanically testing the part.

8. The method according to claim 7, wherein the at least one step for mechanically testing the part comprises at least one of traction and compression.

9. The method according to claim 4, wherein the range of temperatures is a range between 0 and 40 C.

10. The method according to claim 1, further comprising the following steps: calculating a color difference value E.sub.p between the colorimetric measurements obtained in steps a) and b), from the value E.sub.p obtained in the previous step, determining a time period during which said zone to be tested of the part has been subjected to a determined heating temperature and determining said heating temperature, by using a reference database comprising values E of color differences obtained from a plurality of samples made of a polymer material having been subjected to a determined temperature for a determined time period.

11. The method according to claim 1, wherein each value considered of the parameters L, a, b of the CIELAB colorimetric space is obtained by taking an average of at least five successive colorimetric measurements at a place considered.

12. The method according to claim 1, wherein in case of establishing a risk of overheating, the method further comprises a subsequent step to step d) for carrying out a physicochemical analysis of the determined zone to be tested of the part so as to determine a state of thermal damage of said determined zone to be tested.

13. The method according to claim 1, wherein the polymer material comprises reinforcing fibres.

14. The method according to claim 1, further comprising a step of cleaning a surface whereon a colorimetric measurement is intended to be taken.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be best understood, and other details, advantages and characteristics of the invention will appear upon reading the following description, made as a non-exhaustive example, in reference to the following figures:

(2) FIG. 1 is a schematic view of a turbine engine fan case to be tested;

(3) FIG. 2 is a representative view of the CIELAB colorimetric space axes used in the present document;

(4) FIG. 3 is a schematic view of a plurality of samples, each having undergone oxidation for a given time (vertically) at a given temperature (horizontally);

(5) FIG. 4 is a schematic view illustrating a line of separation between a first zone 1 and a second zone 2;

(6) FIG. 5 is a schematic view of a graph where each point represents a sample from FIG. 3 having been subjected to a vitreous transition temperature of the tested material, each point being placed on the graph according to the values of the parameters a and b, thereof, a being represented on the x-axis and b being represented on the y-axis;

(7) FIG. 6 is a schematic view of a graph where each point represents a sample from FIG. 3 having been subjected to a higher temperature than the vitreous transition temperature of the polymer material, each point being placed on the graph according to the values of the parameters a and b, thereof, a being represented on the x-axis and b being represented on the y-axis;

(8) FIG. 7 is a flowchart of the functioning of the decision-making method during a step of non-destructive testing on a given zone to be tested, of a determined part such as the casing from FIG. 1;

(9) FIG. 8 is a graph representing the development in the colour difference over time for the samples from FIG. 3;

(10) FIG. 9 is a larger-scale graph of a given zone of the graph from FIG. 8.

DETAILED DESCRIPTION

(11) As explained above, the fan casing 10 represented in FIG. 1, made of polymer, in particular of reinforcing fibres, can undergo when switched on, a local heating that should be able to be characterised by a non-destructive method enabling to determine the state of the casing 10 in order to determine if it can (or not) be kept in service in a turbine engine.

(12) FIG. 2 represents the CIELAB space used in the present document for analysing colorimetric data obtained on the part to be tested, as well as for establishing the reference database on a plurality of first samples. The CIELAB space is a system which enables to represent the trichromatic components along three orthogonal axes between them. The axis L (or L*) represents the luminance or clarity axis (perfect black: L=0; perfect white: L=100). The axis a (or a*) represents the axis going from green (negative values of a) to red (positive values of a). The axis b (or b*) represents the axis going from blue (negative values of b) to yellow (positive values of b).

(13) It is again reminded, that the space used is the CIE L*a*b*colorimetric space, and that the asterixis have voluntarily been removed as this is usual.

(14) In this colour system, a colour difference between a first colour of coordinates L.sub.1, a.sub.1, b.sub.1 and a second colour of coordinates L.sub.2, a.sub.2, b.sub.2 is calculated as follows:

(15) $E=\sqrt{L^2+a^2+b^2},\mathrm{where}$$L=\left(L_1-L_2\right)$$a=\left(a_1-a_2\right)$$b=\left(b_1-b_2\right)$

(16) This method of calculating the colour difference is the one used later, as this will appear following the description.

(17) The invention proposes to establish a reference database comprising colorimetric measurements according to the CIELAB colour system. The term reference used below is to be understood as meaning an element of the reference database comprising colorimetric measurements and more generally, data obtained from the reference samples.

(18) For this, a batch of a plurality of first samples 12 of a material similar to the part to be analysed is constituted. FIG. 3 illustrates such a batch which thus comprises several first samples 12 of fan case 10 made of a polymer material, preferably of reinforcing fibres, arranged in the form of lines and columns. Along a given line, each first sample 12 is subjected to a given temperature, of which the exposure time is given by the position along a line. Of course, the database should comprise a number of first samples 12 enabling to realise different levels of thermal oxidation, that the polymer can be subjected to under actual functioning conditions. Thus, the database should comprise the first samples 12 having been subjected to the aforementioned temperatures for periods of time going up to at least 6 months.

(19) In the configuration represented as an example, the first samples 12 have been subjected to temperatures in C. of 120, 140, 150, 160, 180, 200, 220 and 240 C. for periods in hours, spread out from 1 up to 1440 hours, which corresponds to a period of 2 months. The sample 14 positioned in the lower left-hand corner in FIG. 1 represents a sample 12 not having been subjected to any heating, which therefore constitutes the absolute reference of a fan case 10 without any thermal heating. It is observed in FIG. 1, that the first samples 12 darken as soon as the temperature increases, and that the exposure time to a temperature increases, which is consistent with a thermal oxidation of the polymer.

(20) FIG. 4 schematically represents FIG. 3 and comprises a line of separation 16 of a first zone 1 and a second zone 2. The first zone 1 corresponds to the first samples 12 which have been subjected to an acceptable temperature for an acceptable time period, whereas the second zone 2 corresponds to the first samples 12 which have been subjected to too-high temperature for a given time period. Thus, if visually, it is possible to establish this line of separation, it seems necessary to establish one or several parameters enabling to objectively realise the state of an analysed part. That is what is defined below by establishing the reference database.

(21) Establishing the reference database first consists of taking a colorimetric measurement for each one of the first samples and by deducing the values L, a and b of the parameters L, a, b of the CIELAB colorimetric space, from at least one colorimetric measurement.

(22) 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 taken, in other words, to establish the database or when it is wanted to test a determined zone of a part, as this will be explained later.

(23) Now reference is made to FIGS. 5 and 6, each representing a graph where each point represents a first sample from FIG. 3, each point being placed on the graph according to the values of the parameters a and b thereof, a being represented on the x-axis and being represented on the y-axis. In FIG. 5, the first samples have been subjected to a temperature lower than the vitreous transition temperature of the polymer material, here 170 C., and in FIG. 6, the first samples have been subjected to a temperature higher than said vitreous transition temperature.

(24) In FIG. 5, it has been observed that a group 18 of points is found at the values of the parameter having a value of less than zero whereas, in FIG. 6, for the first samples having been subjected to a temperature higher than 170 C., it has been observed that a group 20 of values of the parameter a are higher than zero. Consequently, it is possible to discriminate on the thermal state, in other words, the heating of a given zone of a part, from the measurement of this parameter. It will be noted that in FIG. 6, a second group 22 has values of the parameter a which are less than zero, but these points correspond to very low exposure times, less than 10 hours, which are not to be considered. On the graph in FIG. 5, it has also been observed that the variation is made mainly along the axis b, this variation enabling to highlight the yellowing by natural ageing over time of the polymer and of the resin in the case of a fan casing. This variation along the axis b is also visible in FIG. 6.

(25) Thus, it is understood that it is possible, with a colorimetric measurement in the CIELAB space, to differentiate between the natural ageing of the fan case and an accidental overheating of by comparing with a reference database.

(26) To each first sample 12, it is calculated that a=aa.sub.ref, b=bb.sub.ref and L=LL.sub.ref, where: a.sub.ref, b.sub.ref et L.sub.ref respectively correspond to the values of the parameters a, b, L of the CIELAB colorimetric space, these values having been obtained on a second sample 14 made of a polymer material, in particular, of reinforcing fibres, having the same time of existence as said first sample considered, and having been kept at a temperature within a range of temperatures, such as that of preserving the mechanical integrity of the second sample 14, or such as a range between 0 and 40 C., which could further consider the exposure to rays in the ultraviolet field.

(27) The colorimetric measurement on the second sample 14 associated with each measurement of a first sample 12 can be taken with the reference sample 14 that has been observed under the conditions stated in the previous paragraph.

(28) For each first sample 12, a test is then carried out, aiming to determine the mechanical properties thereof, in order to determine the ability or not thereof to constitute a sample that can be used under the determined conditions. Thus, it is determined if the heating subjected by each first sample makes it useable or not. The test carried out can comprise at least one step for mechanically testing the part, for example, by traction and/or compression.

(29) Finally, a comparison of the data from the tests with the values a, b et L contained in the reference database is made, in order to establish the thresholds A1, A2, B1 and L1. The threshold A1 corresponds to a threshold beyond which it is considered that the fan case 10 must be arranged to be subjected to a more in-depth inspection of the zone to be tested (FIG. 7).

(30) To carry out a non-destructive test operation on the casing 10 from FIG. 1, first, the following steps are carried out, schematised in FIG. 7: a) taking at least one colorimetric measurement on said determined zone 24 to be tested of the casing 10 and obtaining the value a.sub.p of the parameter a of the CIELAB colorimetric space, b) taking at least one colorimetric measurement on a reference zone 26 of the casing 10 and obtaining the value a.sub.p/ref of the parameter a of the CIELAB colorimetric space, c) calculating a.sub.p=a.sub.pa.sub.p/ref, d) establishing a risk of heating said determined zone to be tested if a.sub.p is higher than a threshold value A1.

(31) The reference zone 26 of the casing 10 is a zone which has not suffered thermal damage.

(32) In the case where the value a.sub.p is lower, a second step is carried out, aiming to determine if the part must undergo (or not) a test all the same, this time considering the value b.sub.p of the parameter b and the value L.sub.p of the parameter L obtained from the colorimetric measurement on the zone 24 to be tested of the casing, as well as the value b.sub.p/ref of the parameter b and the value L.sub.p/ref of the parameter L obtained from the colorimetric measurement on the reference zone 26 of the casing.

(33) The method then consists of calculating b.sub.p=b.sub.pb.sub.p/ref and calculating L.sub.p=L.sub.pL.sub.p/ref, and establishing a risk of heating said determined zone 24 to be tested if all the following conditions are checked: a.sub.p is higher than a threshold value A2, A2 being lower than A1, b.sub.p is higher than a threshold value B1, L.sub.p is lower than a threshold value L1.

(34) In this case, it is looked to determine if the tested zone 24 has a greater yellowing than the threshold value B1, if the clarity L.sub.p is low, in other words, lower than the threshold L1 while having a a.sub.p higher than a threshold value A2 lower than A1.

(35) If one of the conditions above is not checked, the zone 24 to be tested is considered as not being damaged and the casing 10 can be used.

(36) The non-destructive testing by colorimetry operation can be carried out under the wing of the aircraft, which enables to have a quick and reliable decision regarding the placing or not of the casing and reduces the unnecessary maintenance operations.

(37) The parameters A1, A2, B1, L1 must be established to each type of part 10 and are therefore connected to the material of said part and also depend on the colorimetric measuring device.

(38) Thus, in an example of taking colorimetric measurements with a Konica Minolta CM700d colorimeter, A1 is equal to 4.3, A2 is equal to 1, B1 is equal to 12.6 and L1 is equal to 0.9.

(39) From the aforementioned colorimetric measurements contained in databases, it is possible to determine the time period during which the tested zone has been subjected to determined temperature, as well as this temperature.

(40) For this, the colour difference E.sub.p={square root over (L.sub.p.sup.2+a.sub.p.sup.2+b.sub.p.sup.2)} is calculated for the tested zone by using the colorimetric measurements obtained on the tested zone and on the reference zone of the analysed casing. For each one of the first samples, the colour difference E={square root over (L.sup.2+a.sup.2+b.sup.2)} is also calculated from the values L, a and b. From the values E, it is possible to trace the development of E over time for several determined temperatures as this is represented in FIGS. 8 and 9.

(41) FIG. 8 represents the development of E over time for the temperatures 120, 140, 150 and 160 C., these curves are respectively referenced E.sub.120, E.sub.140, E.sub.150 and E.sub.160.

(42) To avoid the colour difference development curves being impacted by the non-relevant values of E because of a very clear colouring or a colouring that is too dark, the points associated with such values E are removed. A very clear colouring can be due to an exposure to a low temperature for a relatively short time period. This zone corresponds to the zone 28 in FIG. 3 and cannot validly be considered in the colorimetric analysis. A too dark colouring can be due to an exposure to a too high temperature for a significant time period. This zone corresponds to the zone 30 in FIG. 3 and cannot validly be considered in the colorimetric analysis.

(43) Therefore, the situation is limited to the relatively long time of exposure, longer than 300 hours, as this is represented in FIG. 9. With this graph, it is possible from a measurement E.sub.p obtained on a given zone of a casing, to determine the temperature of exposure by tracing the constant ordinate line E.sub.p and searching for the intercepted curve which therefore indicates the temperature subjected to by said tested zone and the exposure temperature represented by the abscissa of the intersection point.