Method for determining damage to a mechanical part from displacement measurements using stereo-correlation of digital images and associated device
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SAFRAN AIRCRAFT ENGINES SAS
- Filing Date
- 2024-08-26
- Publication Date
- 2026-07-08
Smart Images
Figure FR2024051119_06032025_PF_FP_ABST
Abstract
Description
Description TITLE: METHOD FOR DETERMINING DAMAGE TO A MECHANICAL PART FROM DISPLACEMENT MEASUREMENTS 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, from data obtained by stereo-correlation of digital images. The invention also relates to a device for implementing this method. Technological background
[0002] Digital image stereo correlation is a well-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 deformation states, a so-called reference state and a so-called deformed state. Each pixel of the first image is compared to the corresponding pixel in the second image and their similarity measured. It is a non-destructive surface inspection technique.
[0003] Documents US-A1-2013 / 06570, FR-A1-3 127 042 and W0-A1- 2016 / 076975 are examples of the application of image stereo correlation.
[0004] When performing 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.
[0005] The digital image stereo correlation technique can be used during a mechanical test, for example a static test. This test can be a material property characterization test or a material quality check. It can also be used for validation tests of the mechanical strength of mechanical parts.
[0006] During these static mechanical tests, it is necessary to determine the conditions under which static damage occurs. "Damage" means any form of degradation of the properties of materials that may be linked to the appearance of microcracks, delamination at different scales of the material or, in the case of a composite material, decohesion 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 may be microcracks between the grain boundaries or cracks within a single crystal, for example.
[0007] Conventionally, the analysis of deformations in the direction of mechanical stress is considered in order to deduce limit values of deformations on the surface of the mechanical part.
[0008] However, the results obtained are subject to interpretation because no indicator allows differentiating 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 part materials 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.
[0009] 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.
[0010] Also, an objective of the invention is to provide an improved method for determining damage during an improved mechanical test.
[0011] 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
[0012] A method is therefore proposed for determining damage to a mechanical part subjected to mechanical stress, the method comprising the following steps: a) measuring the movements of the mechanical part in the different directions x, y and z of space over a given area of the part, by stereo-correlation of images; then, the following steps being implemented by computer, b) calculating deformations of the part over the determined area, from the displacements measured in step a); c) representing, in a reference frame, a distribution of the deformations defined along the y direction as a function of the deformations defined along the x direction; d) determining an area of the distribution of the deformations represented in step c); e) determining a preferential orientation of the distribution of the deformations represented; and f) comparing the area of the distribution and / or the preferential orientation determined with reference elements to determine whether the mechanical part is damaged.
[0013] Thus, thanks to the method according to the invention, damage resulting from mechanical stress on a part can be studied, in particular the orientation that this damage tends to follow during the stress. Indeed, the measurement of displacements by stereo-correlation of images and the subsequent processing allows the monitoring of deformations during the stress of the part and thus to be able to obtain the evolution of the orientation of the damage.
[0014] The method according to the invention may comprise one or more of the following characteristics, taken in isolation from each other or in combination with each other: - the distribution area is a polygon; - in step d), the determination of the distribution area is carried out in real time; - in step d), the determination of the distribution area is carried out at predefined times; - in step e), the preferential orientation is determined from principal deformations; - in step e), the determination of the preferential orientation of the distribution of deformations is carried out by linear regression; - in step e), the determination of the preferential orientation of the distribution of deformations is carried out by a statistical method; - the mechanical part is a part of an aircraft, in particular a turbomachine casing and preferably a casing of a turbomachine fan.
[0015] The invention also relates to a device for implementing a method for determining damage to a mechanical part as described above, comprising: - means for measuring movements of the mechanical part by stereo-correlation of images on said determined area, and - means for implementing by computer at least steps b) to f).
[0016] 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
[0017] 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:
[0018] Figure 1 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.
[0019] Figure 2 represents a schematic view of a mechanical part, in particular a mechanical part made of woven composite material.
[0020] Figure 3 is a block diagram of the method for determining damage to a mechanical part, according to the invention.
[0021] Figure 4 is a graphical representation of a distribution of strains along the y direction as a function of strains along the x direction at different times during the mechanical test.
[0022] Figure 5 is a graphical representation of a distribution of strains along the y direction as a function of strains along the x direction whose preferred orientation is determined. Detailed description of the invention
[0023] Figure 1 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.
[0024] A speckle M is, in addition, affixed to the surface of the mechanical part 10 to facilitate tracking by the cameras C.
[0025] C-cameras for image acquisition are carried out using CCD cameras with an acquisition frequency of a few frames per second. The acquisition area is illuminated by LED lamps. In addition, the illumination can be carried out continuously or in synchronization with the C-cameras.
[0026] In the following, the examples of implementation focus on 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.
[0027] Figure 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.
[0028] 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.
[0029] 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.
[0030] During a mechanical test, the mechanical part 10 is subjected to stresses which may cause deformation of the part 10, or even damage.
[0031] 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: a) measuring displacements of the part in the different directions x, y and z of space over a determined area of the part 10, by stereo-correlation of images; then, the following steps being implemented by computer, b) calculating deformations of the part over the determined area, from the displacements measured in step a); c) representing, in a reference frame, a distribution of the deformations defined in the direction y as a function of the deformations defined in the direction x; d) determining an area of the distribution of the deformations represented in step b); e) determining a preferential orientation of the distribution of the deformations represented;and f) comparing the area of the distribution and / or the preferential orientation obtained with reference elements to determine whether the part 10 is damaged.;
[0032] The different steps of the method 100 according to the invention are shown diagrammatically in Figure 3 and detailed below.
[0033] In step a), referenced 102 in Figure 3, the movements of the part 10 along 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.
[0034] After step a), the measurements are transmitted to computer means 20 capable of implementing the following steps.
[0035] 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.
[0036] These deformations are generally calculated using the following formula: [Math. 1]
[0037] where £ is the tensor and Uj and llj the displacement fields.
[0038] In step c), referenced 106 in Figure 3, the calculated deformations are represented in a reference frame so that a distribution of the deformations defined according to the direction y (£ yy ) as a function of the deformations defined according to the x direction (E XX ) is obtained. Note that the deformations defined along the z direction (£ xy ) can also be obtained, these deformations representing a shear.
[0039] 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.
[0040] For example, Figure 4 is a graphical representation of a distribution of strains defined along the y direction as a function of strains defined along the x direction of the part 10 at different times of a mechanical stress. Figure 4A represents a 30 distribution of strains before the start of the test. All the points are concentrated and the area of the 30 distribution is small. Figure 4B shows a 30' distribution of 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. In addition, the area of the 30' distribution increases but remains small. Figure 4C shows a 30" distribution of strains at a later time of the mechanical stress.At this point, the points appear more dispersed and the area of the 30” distribution is larger than in Figures 4A and 4B, indicating that damage to the part has occurred.
[0041] These deviations in the distribution can be represented by an estimate of the area of a polygon 31 which covers the entire distribution of deformation points, as in Figure 4C or Figure 5.
[0042] Alternatively, not shown, deviations in the distribution may be represented by an estimate of the area of an elliptical structure, particularly as part of an approximation of the distribution.
[0043] In step e), referenced 110 in Figure 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 deformations defined according to the y direction (£yy) as a function of the deformations defined according to the x direction (E XX ).
[0044] The preferred orientation can also, alternatively, be determined from principal strains, for example a first principal strain and a second principal strain. These principal strains can, for example, be determined from the Mohr circle. The use of principal strains allows, in particular, to change the reference point and facilitate calculations. A linear regression can also be carried out to determine the preferred orientation.
[0045] 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 computation time.
[0046] For both variants mentioned, the preferred orientation can also be determined by a statistical method, in particular based on standard deviations.
[0047] Figure 5 represents an example of a 30” distribution of deformations defined along the y direction (£ yy ) as a function of the deformations defined according to the x direction, similar to figure 4C, for which an area of the distribution was determined and materialized by a polygon 31. In this example, a preferential orientation 32 of the distribution 30” was determined by a statistical method.
[0048] This preferential orientation indicates a direction in which the damage to the mechanical part 10 tends to propagate.
[0049] 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 or weft direction. This information is fundamental for the sizing of composite materials. Indeed, in practice, the orientation directions are chosen according to future stresses.
[0050] As mentioned above, alternatively, principal strains may be used, including a first principal strain as a function of a second principal strain, 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 can be obtained and processed in the same way.
[0051] In step f), referenced 112 in Figure 3, the area of the distribution and / or the preferred orientation previously determined 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.
[0052] 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.
[0053] 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 stress of the part and thus to be able to obtain the evolution of the orientation that this damage tends to follow during the stress.
[0054] 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 f).
[0055] 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 (£ yy ) as a function of the deformations defined according to the x direction (E XX); 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 preferential orientation previously determined with reference elements to determine whether said part is damaged. [2] The method of claim 1, wherein the distribution area is a polygon. [3] Method (100) according to any one of claims 1 or, wherein, in step d), the determination of the distribution area 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 area is carried out at predefined times. [5] Method (100) according to any one of claims 1 to 4, wherein, in step e), the preferential orientation is determined from principal deformations. [6] Method (100) according to any one of claims 1 to 5, in which, 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, in which, 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.