METHOD FOR DETERMINING DAMAGE TO A MECHANICAL PART FROM DISPLACEMENT MEASUREMENTS BY STEREO-CORRELATION OF DIGITAL IMAGES AND ASSOCIATED DEVICE
The method uses stereo-correlation of digital images to measure and process deformations in mechanical parts, addressing the challenge of identifying damage and stress directions, enhancing the accuracy of mechanical test interpretations.
Patent Information
- Application Number
- FR2023009044
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2023-08-29
- Publication Date
- 2025-08-15
- Estimated Expiration
- 2043-08-29
AI Technical Summary
Existing methods for determining mechanical damage in mechanical parts under stress lack indicators to differentiate between healthy and damaged states, and fail to identify preferred directions of stress, complicating the interpretation of mechanical test results.
A method involving stereo-correlation of digital images to measure displacements and deformations in mechanical parts, followed by data processing to determine deformation distributions and preferential orientations, allowing comparison with reference elements to assess damage.
Enables accurate identification of damage orientation and propagation in mechanical parts, improving the interpretation of mechanical tests by providing clear indicators of material degradation and stress directions.
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Abstract
Description
Title of the invention: METHOD FOR DETERMINING DAMAGE TO A MECHANICAL PART FROM DISPLACEMENT MEASUREMENTS BY STEREO- DIGITAL IMAGE CORRELATION 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, 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. This test can be a test for characterizing the properties of materials or a verification of the quality of the material. It can also be used for validation tests of the mechanical strength of mechanical parts.
[0005] During these static mechanical tests, it is necessary to arrive at a decision on the conditions for the appearance of static damage. By “damage” 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 decohesion of fibers with respect to the matrix or a break in 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, these may 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] This direction is an important factor to know in order to verify that the test carried out actually requests the directions or zones that we actually wish to study during the test.
[0009] Also, an objective of the invention is to provide an improved method for determining damage during an improved mechanical test.
[0010] Another objective of the invention is to propose a method for identifying one or more preferred directions of damage during a mechanical test. Summary of the invention
[0011] A method is therefore proposed for determining damage to a mechanical part subjected to mechanical stress, the method comprising the following steps:
[0012] a) measuring the movements of the mechanical part according to the different directions x, y and z of space on a determined area of the part, by stereo-correlation of images; then, the following steps being implemented by computer,
[0013] b) calculating deformations of the part over the determined area, from the displacements measured in step a);
[0014] c) representing, 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;
[0015] d) determining an area of the distribution of the deformations represented in step c);
[0016] e) determining a preferential orientation of the distribution of the deformations re presented; and
[0017] f) compare the determined distribution and / or preferred orientation with reference elements to determine whether the mechanical part is damaged.
[0018] Thus, thanks to the method according to the invention, damage resulting from a stress mechanical stress on a part can be studied, in particular the orientation that this damage tends to follow during stress. Indeed, the measurement of displacements by stereo-correlation of images and the subsequent processing allows the monitoring of deformations during stress on the part and thus to be able to obtain the evolution of the orientation of the damage.
[0019] 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:
[0020] - the distribution area is a polygon;
[0021] - in step d), the determination of the distribution area is carried out in real time;
[0022] - in step d), the determination of the distribution area is carried out at times predefined;
[0023] - in step e), the preferred orientation is determined from deformations main;
[0024] - in step e), determining the preferential orientation of the distribution of the dice training is carried out by linear regression;
[0025] - in step e), determining the preferential orientation of the distribution of the dice training is carried out using a statistical method;
[0026] - the mechanical part is a part of an aircraft, in particular a turbine casing- bomachine and preferably a casing of a turbomachine fan.
[0027] The invention also relates to a device for implementing a method for determining damage to a mechanical part as described above, comprising:
[0028] - means for measuring movements of the mechanical part by stereo- correlation of images on said determined area, and
[0029] - means for implementing by computer at least steps b) to f).
[0030] The invention also relates to an aircraft comprising at least one device for implementing a method for determining damage to a mechanical part as described previously. Brief description of the figures
[0031] 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:
[0032] [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.
[0033] [Fig.2] represents a schematic view of a mechanical part, in particular a mechanical part made of woven composite material.
[0034] [Fig. 3] is a block diagram of the method for determining damage to a mechanical part, according to the invention.
[0035] [Fig.4] is a graphical representation (A, B and C) of a distribution of deformations in the y direction as a function of deformations in the x direction at different times during the mechanical test.
[0036] [Fig.5] is a graphical representation of a distribution of deformations along the y direction as a function of the deformations along the x direction whose preferential orientation is determined. Detailed description of the invention
[0037] [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.
[0038] A speckle M is, in addition, affixed to the surface of the mechanical part 10 to facilitate tracking by the cameras C.
[0039] The C cameras for image acquisition are carried out using CCD sensor cameras with an acquisition frequency of a few images per second. The acquisition area is illuminated by light-emitting diode lamps. In addition, the illumination can be carried out continuously or in synchronization with the C cameras.
[0040] 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.
[0041] [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 with each other 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 final mechanical part 10. The matrix 13 may for example be chosen, in a non-limiting manner, from a thermosetting or thermoplastic resin.
[0042] More complex woven structures can also be used in these tests. For example, the warp fibers 12 are no longer limited to a weaving plane P but cross several planes of the material.
[0043] 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.
[0044] During a mechanical test, the mechanical part 10 is subjected to stresses which can cause deformations of the part 10, or even damage.
[0045] The method according to the invention makes it possible to determine the damage to a part 10 subjected to mechanical stress, this method further comprising the following steps:
[0046] a) measuring movements 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, the following steps being implemented by computer,
[0047] b) calculating deformations of the part over the determined area, from the displacements measured in step a);
[0048] c) representing, 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;
[0049] d) determining an area of the distribution of the deformations represented in step b);
[0050] e) determining a preferential orientation of the distribution of the deformations re presented; and
[0051] f) compare the distribution and / or the preferential orientation obtained with reference elements to determine whether the part 10 is damaged.
[0052] The different steps of the method 100 according to the invention are shown diagrammatically in [Fig.3] and detailed below.
[0053] 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 measured by stereo-correlation of images. These directions are orthogonal. The choice of the area to be observed can be left to the operator. In practice, these measurements are carried out by cameras C equipped with CCD sensors.
[0054] After step a), the measurements are transmitted to computer means 20 capable of implementing the following steps.
[0055] In step b), referenced 104 in [Fig. 3], deformations of the part 10 over the determined zone are calculated from the displacements measured in step 104. The deformations are notably calculated from the fields of displacements in x, y and z obtained by stereo-correlation of images.
[0056] These deformations are calculated, in general, with the following formula:
[0057] [Math.l] i / dU, dU. X “ 2 X \ dUj du. /
[0058] where e is the tensor and U; and Uj the displacement fields.
[0059] In step c), referenced 106 in [Fig. 3], the calculated deformations are represented in a reference frame so that a distribution of the deformations defined along the y direction (eyy) as a function of the deformations defined along the x direction (exx) is obtained. It will be noted that the deformations defined along the z direction (exy) can also be obtained, these deformations representing a shear.
[0060] In step d), referenced 108 in [Fig. 3], a distribution area of the deformations represented in step c) is determined. This determination of the distribution area can be carried out in real time or at predefined times.
[0061] For example, [Fig. 4] is a graphical representation of a distribution of the strains defined along the y direction as a function of the strains defined along the x direction of the part 10 at different times of a mechanical stress. Graph A of [Fig. 4] represents a 30 distribution of the strains before the start of the test. All the points are concentrated and the area of the 30 distribution is small. Graph B of [Fig. 4] shows a 30' distribution of the strains during the test. The points remain concentrated but a greater dispersion of the latter can be observed compared to the initial state before the start of the test. Furthermore, the area of the 30' distribution increases but remains small. Graph C of [Fig. 4] shows a 30" distribution of the strains at a later time of the mechanical stress.At this stage, the points appear more dispersed and the area of the 30” distribution is larger than in graphs A and B of [Fig.4], a sign that damage to the part has occurred.
[0062] These deviations in the distribution can be represented by an estimate of the area of a polygon 31 which covers the entire distribution of the deformation points, as in graph C of [Fig.4] or [Fig.5].
[0063] Alternatively, not shown, the deviations in the distribution can be represented by an estimate of the area of an elliptical structure, in particular in the context of an approximation of the distribution.
[0064] In step e), referenced 110 in [Fig. 3], from the distribution of the deformations represented in step d), a preferential orientation is determined. This preferential orientation can be determined by a linear regression of the deformation points of the distribution of the deformations defined according to the direction y (eyy) as a function of the deformations defined according to the direction x (exx).
[0065] The preferred orientation can also, alternatively, be determined from principal deformations, for example a first principal deformation and a second principal deformation. These principal deformations can for example be determined from the Mohr circle. The use of principal deformations makes it possible in particular to change the reference frame and to facilitate the calculations. A re Linear regression can also be performed to determine the preferred orientation.
[0066] Generally speaking, using linear regression to determine the preferred orientation allows the result to be synthesized with the slope as the only parameter. This also simplifies the method and reduces the calculation time.
[0067] For both variants mentioned, the preferential orientation can also be determined by a statistical method, in particular based on standard deviations.
[0068] [Fig.5] represents an example of distribution 30” of the deformations defined according to the direction y (eyy) as a function of the deformations defined according to the direction x, similar to graph C of [Fig.4], for which an area of the distribution has been determined and materialized by a polygon 31. In this example, a preferential orientation 32 of the distribution 30' ' has been determined by a statistical method.
[0069] This preferential orientation indicates a direction in which the damage to the mechanical part 10 tends to propagate.
[0070] 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.
[0071] As mentioned above, alternatively, principal deformations may be used, in particular a first principal deformation as a function of a second principal deformation, both determined for example from the Mohr circle. In such a case, graphical representations, not shown here, similar to those shown in Figures 4 and 5 may be obtained and processed in the same way.
[0072] In step f), referenced 112 in [Fig. 3], the area of the distribution and / or the preferred orientation determined previously are 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 voluntary non-quality of the reference.
[0073] This comparison makes it possible to validate that the mechanical stress applied corresponds to what is actually expected for the deformation and / or damage of the mechanical part tested.
[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 stress on the part and thus to be able obtain the evolution of the orientation that this damage tends to follow during the stress.
[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 zone and means 20 for implementing by computer at least steps b) to f).
[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 of the part(s) to be examined can be examined directly during a flight of the aircraft 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, said method comprising the following steps: a) measuring (102) displacements of said part (10) in the different directions x, y and z of space over a determined area of the part, by stereo-correlation of images; then, the following steps being implemented by computer, b) calculating (104) deformations of the part over said determined area, from the displacements measured in step a); c) representing (106), in a reference frame, a distribution of the deformations defined in the direction y (eyy) as a function of the deformations defined in the direction x (em); d) determining (108) an area of the distribution of the deformations represented in step c); e) determining (110) a preferential orientation of the distribution of the deformations represented;and f) comparing (112) the area of the distribution and / or the preferred orientation previously determined with reference elements to determine whether said part is damaged.;
2. The method of claim 1, wherein the distribution is a polygon.
3. Method (100) according to any one of claims 1 or, wherein, in step d), the determination of the distribution is carried out in real time.
4. Method (100) according to any one of claims 1 to 3, wherein, in step d), the determination of the distribution time is carried out at predefined times.
5. Method (100) according to any one of claims 1 to 4, wherein, in step e), the preferred orientation is determined from principal deformations.
6. Method (100) according to any one of claims 1 to 5, wherein, in step e), the determination of the preferential orientation of the distribution of the deformations is carried out by linear regression.
7. Method (100) according to any one of claims 1 to 5, wherein, in step e), the determination of the preferential orientation of the distribution of the deformations is carried out by a statistical method.
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 f).
10. Aircraft comprising at least one device (D) according to claim 9.