METHOD FOR DETERMINING DAMAGE TO A MECHANICAL PART FROM DISPLACEMENTS OBTAINED BY STEREO-CORRELATION OF DIGITAL IMAGES AND ASSOCIATED DEVICE

Abstract

Method for determining damage to a mechanical part (10) subjected to mechanical stress during a mechanical test, said method comprising the following steps: a) determining displacement data of said part in the different directions x, y, z of space over a determined area of ​​the part, by stereo-correlation of images; then, the following steps being implemented by computer, b) performing an interpolation of the displacement data determined in step a); c) calculating deformations of the part over said determined area from the interpolation performed in step b); and d) comparing said deformations calculated in step c) with reference elements to determine whether said part is damaged. Figure for abstract: Figure 1

Description

Title of the invention: METHOD FOR DETERMINING DAMAGE TO A MECHANICAL PART FROM DISPLACEMENTS OBTAINED BY STEREO-CORRELATION OF DIGITAL IMAGES AND ASSOCIATED DEVICE Technical field of the invention

[0001] The present invention relates to a method for determining damage to a mechanical part subjected to mechanical stress during a mechanical test, from data obtained by stereo-correlation of digital images. The invention also relates to a device for implementing this method. Technological background

[0002] Stereo correlation of digital images is a known technique that allows measurement of kinematic fields such as displacement or deformation fields. This technique consists of matching two digital images of a surface observed in two distinct states of deformation, a so-called reference state and a so-called deformed state. Each pixel of the first image is compared to the corresponding pixel on the second image and their similarity measured. It is a non-destructive surface control technique.

[0003] When carrying out mechanical tests, it is known to apply a black and white speckle to the surface of the part under study in order to provide the surface with a sufficiently random texture and allow tracking by the cameras used. These cameras are generally equipped with CCD sensors (from the English charge coupled device) whose acquisition frequency depends on the phenomena studied. This can be from a few images per second to several thousand images per second for cases with rapid or cyclic dynamics.

[0004] The digital image stereo-correlation technique can be used during a mechanical test, for example static or dynamic, impact or fatigue. This mechanical test aims at characterizing the properties of materials or verifying the quality of the material. The technique can also be used for validation tests of the mechanical strength of mechanical parts.

[0005] During these static or dynamic mechanical tests, it is necessary to arrive at a decision on the conditions under which damage occurs. “Damage” means any form of degradation of the properties of materials which may be linked to the appearance of microcracks, delamination at different scales of the material or in the case of a composite material of decohesions of fibers with respect to the matrix or even a breakage of fibers, for example at the level of a fiber, a strand, a weaving plane, several weaving planes or the entire material. In the case of a metallic material, it can be microcracks between the grain boundaries or cracks within a single crystal for example.

[0006] Conventionally, the analysis of deformations in the direction of mechanical stress is examined in order to deduce limit values ​​of deformations on the surface of the mechanical part.

[0007] However, the results obtained are subject to interpretation because no indicator makes it possible to differentiate the end of a so-called healthy zone of the materials and the transition to a “damaged” state. In addition, even knowing that damage to the materials of the part is present, it is sometimes difficult to clearly identify the preferred direction of the stress, whether it is the direction of the fibers for a composite material or the necking of a metallic material.

[0008] It is known to use non-destructive testing techniques in infrared thermography or in digital image correlation to estimate the endurance limit of a material by carrying out an analysis of the fatigue hysteresis area during a dynamic cycle. It is also known to identify the occurrence of a dynamic impact with high-frequency infrared thermography based on the measurement of a thermal field and with the stereo-correlation of digital images.

[0009] Also, an objective of the invention is to provide an improved method for determining damage during a mechanical test. Summary of the invention

[0010] A method is therefore proposed for determining damage to a mechanical part subjected to mechanical stress, the method comprising the following steps:

[0011] a) determining data on the movements of the part according to the different directions x, y, z of space over a given area of ​​the part, by stereo-correlation of images; then,

[0012] the following steps being implemented by computer,

[0013] b) performing an interpolation of the displacement data determined in step a);

[0014] c) calculating deformations of the part over the area determined from the interpolation performed in step b); and

[0015] d) comparing the deformations calculated in step c) with reference elements to determine whether the mechanical part is damaged.

[0016] Thus, thanks to the method according to the invention, an improvement in the study of a damage resulting from mechanical stress on a mechanical part. Indeed, the determination of displacements by stereo-correlation of images and the subsequent processing allows simplified monitoring of deformations, in particular thanks to the interpolation of displacements.

[0017] The method according to the invention may comprise one or more of the following characteristics, taken in isolation from one another or in combination with one another:

[0018] - in step b), the interpolation is done by a single-variable interpolation curve in the x direction or the y direction, or made by an interpolation surface with two variables x, y;

[0019] - the interpolation curve with one variable according to the x direction or the y direction is obtained by polynomial regression of order N;

[0020] - the two-variable interpolation surface x, y is obtained by a regression po lynomial of order N;

[0021] - in step c), principal deformations are calculated from a circle of Mohr;

[0022] - in step d), a least squares analysis is carried out so that for a co determination efficiency R2 greater than 0.9 non-damage of the part is determined and for a determination coefficient R2 less than 0.9 damage of the part is determined;

[0023] - in step d), the following sub-steps are carried out:

[0024] — represent, in a reference frame, a distribution of the deformations defined according to the direction y as a function of the deformations defined according to the direction x,

[0025] — determine an area of ​​the distribution of the deformations represented,

[0026] — determine a preferential orientation of the distribution of deformations re presented, and

[0027] — compare Make the distribution and / or the preferential orientation determined pre previously with reference elements to determine if the part is damaged;

[0028] - the mechanical part is a part of an aircraft, in particular a turbine casing- turbomachine and preferably a casing of a turbomachine fan;

[0029] - - the mechanical test is static, dynamic fatigue or dynamic impact;

[0030] - - in step b), the interpolation is a finite element interpolation.

[0031] The invention also relates to a device for implementing a method for determining damage to a mechanical part as described above, comprising:

[0032] - means for measuring movements of the mechanical part by stereo- correlation of images on said determined area, and

[0033] - means for implementing by computer at least steps b) to d). Brief description of the figures

[0034] The invention will be better understood with the aid of the following description, given solely by way of example and with reference to the appended drawings in which:

[0035] [Fig.l] represents a schematic view of a test bench comprising a mechanical part and a device for implementing the method for determining damage to a mechanical part, according to the invention.

[0036] [Fig.2] represents a schematic view of a mechanical part, in particular a mechanical part made of woven composite material.

[0037] [Fig. 3] is a block diagram of the method for determining damage to a mechanical part, according to the invention.

[0038] [Fig.4] is a representation in a frame of reference of the movements of the part over the area determined according to the x, y and z directions. Detailed description of the invention

[0039] [Fig.l] represents a test bench D of a mechanical part 10 comprising, among other things, cameras C for the acquisition of digital images by stereo-correlation (allows the measurement of displacements) and a device 20 provided with computer means 20 for implementing the method for determining damage to the mechanical part as described below.

[0040] A speckle M is, in addition, affixed to the surface of the mechanical part 10 to facilitate tracking by the cameras C.

[0041] The C cameras for image acquisition are CCD sensor cameras with an acquisition frequency of a few images per second. The acquisition area is illuminated by light-emitting diode lamps. Furthermore, the illumination can be carried out continuously or in synchronization with the C cameras.

[0042] In the following, the exemplary embodiments concern mechanical parts, in particular made of composite materials, and more precisely 3D woven composites. However, these mechanical parts can also be parts made of 2D woven composite materials, unidirectional composite parts or even metal parts.

[0043] [Fig. 2] shows a mechanical part 10 made of 3D woven composite material. The composite material comprises weft fibers 11 and warp fibers 12 woven together on a weaving plane P, or several weaving planes superimposed in a thickness direction. The weft fibers 11 and warp fibers 12 may for example be made of carbon, glass or a mixture of the two or, more generally, other types of fibers. These fibers are embedded in a matrix 13 which comprises one or more polymers capable of stiffening so as to form the part final mechanical 10. The matrix 13 can for example be chosen, in a non-limiting manner, from a thermosetting or thermoplastic resin.

[0044] More complex woven structures can also be used during these tests. For example, the warp fibers 12 are no longer limited to a weaving plane P but pass through several planes of the material.

[0045] In practice, the part 10 may be a mechanical part of an aircraft. In particular, this part may be a turbomachine casing and for example a casing of a turbomachine fan.

[0046] During a mechanical test, the mechanical part 10 is subjected to stresses which can cause deformations of the part 10, or even damage.

[0047] The method according to the invention makes it possible to determine the damage to a part 10 subjected to mechanical stress during a mechanical test, this method further comprising the following steps:

[0048] a) determining displacements of the part according to the different directions x, y and z of space on a determined zone of the part 10, by stereo-correlation of images; then,

[0049] the following steps being implemented by computer,

[0050] b) performing an interpolation of the displacement data determined in step a);

[0051] c) calculating deformations of the part 10 over the area determined from the interpolation carried out in step b); and

[0052] d) comparing the deformations calculated in step c) with reference elements to determine whether the part is damaged.

[0053] The different steps of the method 100 according to the invention are shown diagrammatically in [Fig.3] and detailed below.

[0054] The method 100 according to the invention can be used for different types of mechanical tests. These tests can be static mechanical tests, dynamic fatigue tests or dynamic impact tests.

[0055] In step a), referenced 102 in [Fig. 3], the movements of the part 10 according to the different directions x, y and z of space over a given area of ​​the part 10 are determined by stereo-correlation of images. These directions are orthogonal. The movements can be derived from software or directly measured. In practice, these measurements are carried out by cameras C equipped with CCD sensors. The choice of the area to be observed can be left to the operator.

[0056] After step a), the data is transmitted to computer means 20 capable of implementing the following steps.

[0057] In step b), referenced 104 in [Fig. 3], an interpolation of the displacement data determined in step a) is carried out. This interpolation makes it possible to process the displacement data.

[0058] The interpolation can be done by finite elements in step b).

[0059] The interpolation can also be done by an interpolation curve with one variable according to the x direction or the y direction. The interpolation can also be done by an interpolation surface with two variables x, y.

[0060] The interpolation curve with one variable according to the x direction or the y direction can be obtained by polynomial regression of order N. In such a case, it is understood that the interpolation curve is similar to a section according to a weft direction or a warp direction. In other words, this section represents the displacements according to the x or y direction as a function of the z direction.

[0061] Similarly, the interpolation surface with two variables x, y can be obtained by polynomial regression of order N. In such a case, the interpolation surface makes it possible to take into consideration the entire area studied.

[0062] [Fig.4] illustrates an example of representation 30 in a reference frame of the displacements 32 of the part 10 over a zone determined according to the directions x, y and z. In this example of [Fig.4], the displacements 32 have been interpolated by an interpolation surface with two variables x, y obtained by a polynomial regression of order N. A 2D adjustment is thus obtained, represented in [Fig.4] by the reference 34.

[0063] Polynomial regression allows for more complete monitoring of the mechanical test. Polynomial regression also ensures simplicity in performing derivations in the x and y directions, allowing for rapid calculation of the derivations.

[0064] In step c), referenced 106 in [Fig. 3], deformations of the part 10 over the determined zone are calculated from the interpolation of the displacements carried out in step 104.

[0065] The calculated deformations can be small deformations - in the sense of the small deformation hypothesis, i.e. deformations less than 10% - large deformations, deformations defined according to the Kirchhoff method or deformations defined according to the Green-Lagrange method. The calculated deformations can also be true deformations obtained according to the following formula:

[0066] [Math.l] _ ] / dU, dU j \ s ÿ~ 2 x +

[0067] where e is the tensor and U; and Uj the displacement fields.

[0068] In step c), principal deformations can for example be calculated for a point of the test from a Mohr circle. The use of principal deformations makes it possible in particular to change the reference point and to facilitate the calculations.

[0069] In step d), referenced 108 in [Fig. 3], the deformations calculated in step c) are compared with reference elements to determine whether the part 10 is damaged.

[0070] A least squares analysis can be carried out on the calculated deformations. In this way, a determination coefficient R2 is obtained. The value of this determination coefficient R2 makes it possible to determine whether the part 10 is damaged or not. Thus, when the determination coefficient R2 is greater than 0.9, it is considered that the part 10 is not damaged. When the determination coefficient R2 is less than 0.9, it is considered that the part 10 has damage.

[0071] Alternatively, the calculated deformations can be represented in a frame of reference so that a distribution of the deformations defined along the y direction (eyy) as a function of the deformations defined along the x direction (em) is obtained. It will be noted that the deformations defined along the z direction (exy) can also be obtained, these deformations representing a shear.

[0072] A preferred area and / or orientation of the distribution may be determined. The preferred area and / or orientation of the distribution are then compared with reference elements to determine whether the part 10 is damaged. These reference elements may be study parts having carried out additional tests which make it possible to ensure the quality or the intentional non-quality of the reference.

[0073] In the particular case where the part 10 is made of 3D woven composite material, as described above, determining the preferred orientation makes it possible to check whether the material has been stressed in the warp direction or the weft direction. This information is fundamental for the dimensioning of composite materials. Indeed, in practice, the orientation directions are chosen according to future stresses.

[0074] The method as described above allows an improvement in the study of damage during mechanical stress on a part. Indeed, the measurement of displacements by stereo-correlation of images and the subsequent processing allows the monitoring of deformations during the stressing of the part and thus to be able to obtain the evolution of the orientation that this damage tends to follow during the stressing.

[0075] The invention also proposes a device D for implementing the method 100 as described above. The device D comprises means C for measuring movements of the mechanical part 10 by stereo-correlation of images on the determined area and means 20 for implementing by computer at least steps b) to d).

[0076] The invention also proposes an aircraft comprising at least one device D for implementing the method 100, as described previously. The device D is then mounted on at least one part to be examined in the aircraft. In this way, the material health and / or damage to the part(s) to be examined can be examined di directly during an aircraft flight in order to detect, from the first moment of their possible appearance, deformations, cracks and / or damage and to take appropriate decisions.

Claims

Claims

1. Method (100) for determining damage to a mechanical part (10) subjected to mechanical stress during a mechanical test, said method comprising the following steps: a) determining (102) displacement data of said part (10) according to the different directions x, y, z of space over a determined area of ​​the part, by stereo-correlation of images; then, the following steps being implemented by computer, b) performing (104) an interpolation of the displacement data determined in step a); c) calculating (106) deformations of the part over said determined area from the interpolation performed in step b); and d) comparing (108) said deformations calculated in step c) with reference elements to determine whether said mechanical part is damaged.

2. Method (100) according to claim 1, wherein, in step b), the interpolation is made by: - ​​a one-variable interpolation curve in the x direction or the y direction, or - a two-variable interpolation surface x, y.

3. The method (100) of claim 2, wherein said one-variable interpolation curve in the x direction or the y direction is obtained by polynomial regression of order N.

4. The method (100) of claim 2, wherein said two-variable interpolation surface x, y is obtained by polynomial regression of order N.

5. Method (100) according to any one of claims 1 to 4, wherein, in step c), principal deformations are calculated from a Mohr circle.

6. Method (100) according to any one of claims 1 to 5, wherein, in step d), a least squares analysis is carried out so that for a determination coefficient R2 greater than 0.9 non-damage of the part is determined and for a determination coefficient R2 less than 0.9 damage of the part is determined.

7. Method (100) according to any one of claims 1 to 6, wherein, in step d), the following substeps are carried out: - representing, in a reference frame, a distribution of the deformations defined according to the y direction (eyy) as a function of the deformations defined according to the x direction (exx), - determining an area of ​​the distribution of the deformations represented, and / or - determining a preferential orientation of the distribution of the deformations represented, and - comparing the area of ​​the distribution and / or the preferential orientation previously determined with reference elements to determine whether said part (10) is damaged.

8. Method (100) according to any one of claims 1 to 7, in which the mechanical part (10) is a part of an aircraft, in particular a turbomachine casing and preferably a casing of a turbomachine fan.

9. Device (D) for implementing a method (100) according to any one of claims 1 to 8, comprising: - means (C) for measuring movements of the mechanical part (10) by stereo-correlation of images on said determined zone, and - means (20) for implementing by computer at least steps b) to d).

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