Method for determining damage to a part subjected to mechanical stress based on measurements of movements taken by means of stereo-correlation of digital images and on stress measurements, and associated device
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- SAFRAN AIRCRAFT ENGINES SAS
- Filing Date
- 2024-08-19
- Publication Date
- 2026-07-08
Smart Images
Figure FR2024051090_06032025_PF_FP_ABST
Abstract
Description
Description TITLE: METHOD FOR DETERMINING DAMAGE TO A PART SUBJECTED TO MECHANICAL STRESS FROM DISPLACEMENT MEASUREMENTS BY STEREO-CORRELATION OF DIGITAL IMAGES AND STRESS MEASUREMENTS AND ASSOCIATED DEVICE Technical field of the invention
[0001] The present invention relates to a method for determining damage to a part subjected to mechanical stress. The method involves measurements of the part's displacements by stereo-correlation of digital images and measurements of stresses on the part. The invention also relates to a device for implementing the method. Technological background
[0002] Stereo correlation of digital images is a well-known technique that allows measurement of displacement fields of a part.
[0003] It is a non-destructive surface control technique.
[0004] To do this, a black and white speckle is applied to the surface of the part whose displacement field under mechanical stress is to be determined, in order to provide the surface with a sufficiently random texture. This allows for tracking using cameras. These cameras are generally equipped with CCD (charge coupled device) sensors for dynamic tracking with an acquisition frequency depending on the phenomenon being studied. This can range from a few images per second to several thousand images per second for cases with rapid or cyclic dynamics.
[0005] From the displacement field, it is then possible to obtain the deformations. This technique consists in matching two images of a surface of the part 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, which makes it possible to obtain the associated deformations.
[0006] The digital image stereo correlation technique can be used during a static type mechanical test.
[0007] A static test can be a test to characterize material properties, a verification of material quality, or a test to validate the mechanical strength of mechanical parts. During these static mechanical tests, it is necessary to determine the conditions under which damage occurs.
[0008] “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 part, decohesion of fibers with respect to the matrix (resin) 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, this may be microcracks between the grain boundaries or cracks within a single crystal, for example.
[0009] Conventionally, the deformations obtained by stereo-correlation of digital images are then analyzed in the direction of mechanical stress in order to deduce limit values of deformations on the surface of the part. However, the results obtained today are subject to interpretation because no indicator makes it possible to differentiate the end of a so-called "healthy" zone of the part from the transition to a "damaged" state.
[0010] The digital image stereo correlation technique can also be used during dynamic mechanical testing.
[0011] A dynamic test is typically a fatigue or impact test. During these dynamic tests, it is also necessary to determine the conditions under which damage occurs. For example, during a fatigue test, digital image stereo correlation is used to estimate the Wôhler limit. The approach then consists of estimating the area of the fatigue hysteresis during a dynamic cycle.
[0012] There are also a number of techniques known for measuring the stresses applied to a part subjected to mechanical stress, which can be used during a static or dynamic test. One example is the use of strain gauges. These gauges are placed on the surface of the part whose behavior is to be monitored, in particular to deduce limit values for stresses on the surface of the part (static) or dynamic stresses (Wohler fatigue limit). Here too, the results obtained are subject to interpretation.
[0013] Also, an objective of the invention is to propose a method allowing improved detection of damage to a part subjected to mechanical stress. Summary of the invention
[0014] A method is thus proposed for determining the damage to a part subjected to mechanical stress, said method comprising the following steps: a) measuring the displacements of said part in the different directions x, y, z of space over a determined area thereof, by stereo-correlation of images, b) measuring the stresses imposed on the part in the different directions x, y, z of space over said determined area and, implementing by computer the following steps: c) calculating the deformations of said part in the different directions of space x, y, z over said determined area, from the displacements measured in step a), d) calculating the Poisson's ratios of the part over said determined area from the deformations calculated in step c);e) calculating the moduli of elasticity of the part according to the different directions of space x, y, z on said determined area from the stresses measured in step b), the deformations calculated in step c) and the Poisson's ratios calculated in step d), f) generating a map of the different moduli of elasticity over the whole of said determined area; then gi) either determining, to know if said part is damaged, if at least one of the moduli of elasticity calculated in step f) is greater than a reference modulus of elasticity in the same direction, g2) or, after having repeated steps a) to f) at least once, comparing, to know if said part is damaged, if the map recently obtained of the different moduli of elasticity over the whole of said determined area presents a discontinuity compared to the map previously obtained;
[0015] Thus, thanks to the method according to the invention, damage resulting from mechanical stress on a part can be studied on the basis of a criterion defined over the entire measurement zone considered, in this case a criterion relating to a modulus of elasticity in a given direction.
[0016] The method according to the invention may comprise one or more of the steps below, taken in isolation from one another or in combination with one another: a plurality of successive measurements are carried out according to steps a) and b) to define a dynamic fatigue test; a plurality of successive measurements are carried out according to steps a) and b) to define a dynamic impact test; steps c) to f) and gi) and / or g2) are carried out from a selection of measurements from those which were obtained in steps a) and b); the part is a composite part comprising a resin in which fibers are embedded; the part is a turbomachine part, in particular a turbomachine casing and preferably a turbomachine fan casing.
[0017] The invention also relates to a device for implementing a method according to the invention, comprising: - means for measuring movements of the part by stereo-correlation of images on said determined area, - means for measuring the stresses imposed on the part in said determined area, and - means for implementing by computer at least steps c) to f) and gi) and / or g2).
[0018] The invention also relates to an aircraft comprising at least one device according to the invention. Brief description of the figures
[0019] 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:
[0020] Figure 1 is a schematic view of a test bench comprising a mechanical part, cameras and a device for implementing the method for determining damage to a part according to the invention;
[0021] Figure 2 represents more precisely the surface of the mechanical part, in particular a mechanical part made of woven composite material.
[0022] Figure 3 is a schematic view representing the different stages of the method according to the invention. Detailed description of the invention
[0023] Figure 1 represents a test bench D of a mechanical part 10 comprising cameras C for the acquisition of digital images by stereo-correlation (allows the measurement of displacements), strain gauges JC for measuring stresses, and a device 20 provided with computer means for implementing the method for determining damage to the mechanical part as described below.
[0024] JC strain gauges can be fixed to ensure validation and / or comparison of data obtained by stereo-correlation. In the case of a composite material, as discussed below, these JC gauges can be positioned on one or more given axes so as to measure the mechanical properties directly in the directions of the fibers of the composite material.
[0025] A speckle M is, in addition, affixed to the surface of the mechanical part 10 to facilitate tracking by the cameras C.
[0026] 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.
[0027] In the following, the embodiments focus more particularly on parts made of composite materials, and in particular on 3D woven composite parts. 3D woven composite parts are typically used to produce certain turbomachine parts, in particular turbomachine casings and more particularly turbomachine fan casings.
[0028] However, these parts can also be 2D woven composite parts, composite parts with unidirectional fibers or even metal parts.
[0029] Figure 2 shows a part 10 made of 3D woven composite material. The composite material comprises a fabric formed of weft fibers 11 and warp fibers 12 woven together on a weaving plane P, or several weaving planes superimposed along a thickness direction. The weft fibers 11 and warp fibers 12 may for example be carbon fibers, glass fibers or Kevlar® fibers, i.e. poly(p-phenyleneterephthalamide). Other types of fibers may be considered. Alternatively, the weft 11 and warp 12 fibers may be a combination of at least two fibers chosen from carbon fibers, glass fibers, Kevlar® fibers or others. 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 may for example be chosen from a thermosetting resin or a thermoplastic resin.
[0030] 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 the planes of the material.
[0031] 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.
[0032] During a mechanical test, the mechanical part 10 is subjected to stresses which may cause deformation of the part 10, or even damage.
[0033] The method according to the invention therefore aims to determine the damage to a part subjected to mechanical stress, said method comprising the following steps: a) measuring the displacements of said part in the different directions x, y, z of space over a determined area thereof, by stereo-correlation of images, b) measuring the stresses imposed on the part in the different directions x, y, z of space over said determined area and, implementing by computer the following steps: c) calculating the deformations of said part in the different directions of space x, y, z over said determined area, from the displacements measured in step a), d) calculating the Poisson's ratios of the part over said determined area from the deformations calculated in step c);e) calculating the elastic moduli of the part according to the different directions of space x, y, z on said zone determined from the stresses measured in step b), the deformations calculated in step c) and the Poisson's ratios calculated in step d),; f) generating a map of the different moduli of elasticity over the whole of said determined area; then gi) either determining, to know if said part is damaged, if at least one of the moduli of elasticity calculated in step f) is greater than a reference modulus of elasticity in the same direction, g2) or, after having repeated steps a) to f) at least once, comparing, to know if said part is damaged, if the map recently obtained of the different moduli of elasticity over the whole of said determined area presents a discontinuity compared to the map previously obtained.
[0034] The different stages of this process 100 are shown diagrammatically in Figure 3.
[0035] Step a), referenced 101 in Figure 3, makes it possible to obtain the displacements by stereo-correlation of digital images. This technique is well known to those skilled in the art.
[0036] On the type of part likely to be considered in the tests (plate), we can take into consideration U x (the displacement along the x axis) and Uy (the displacement along the y axis). This is just an example, but we will keep it to illustrate the different calculations that can be carried out.
[0037] Step b), referenced 102 in Figure 3, can be carried out using strain gauges. According to our example, we can measure o xx (constraint defined along the x axis), o yy (constraint defined along the y axis) and T xy(shear stress defined along the z axis). It is understood that step 102 is carried out at the same time as step 101 in order to have consistent data. It should be noted that there are other means than strain gauges to carry out the measurements necessary for the implementation of step b).
[0038] Alternatively, step b) can be performed by a displacement control method such as shearography so as to trace the deformations. Shearography is an optical technique for measuring a deformation field and is also called "differential speckle interferometry" or "shearography".
[0039] All the following steps of the process described above are implemented by computer.
[0040] Step c) aims to calculate the deformations from the displacements measured in step a). This is generally obtained with the following formula: [Math.
[0041] where Ey with i, j integers such that 1 < i < 3 and 1 < j < 3 along the three (orthogonal) directions x, y, z of space are the deformations (or more precisely the coefficients of the deformation tensor) and llj or Uj the associated displacement fields that were measured in step a).
[0042] According to our example, on the type of part likely to be considered in the tests (plate), the deformation tensor could be limited to 3 components, namely: E XX (deformation along the x axis), £ yy (deformation along the y axis) and y xy (shear defined along the z axis).
[0043] These calculations can be carried out by the software supplied with the means for implementing the test or can be completely carried out (recalculation) by software capable of also implementing all the steps of the method according to the invention.
[0044] In step d), referenced 104 in Figure 3, the associated Poisson's ratios are calculated. This can be obtained in the following way, still according to our example: [Math 2] v xy = - ^ bXX [Math
[0045] To determine the elastic moduli in step e), referenced 105 in Figure 3, in this case in our example, E x (modulus of elasticity defined along the x axis), E y (modulus of elasticity defined along the y axis) and G xy (shear modulus defined along the z axis), we can use the link between the stress tensor, the strain tensor and the stiffness tensor (or its inverse, the flexibility tensor), for example in the following way: [Math 4]
[0046] The elastic moduli thus obtained at the end of step e) are values available for the different meshes of a mesh or grid, produced on the surface of the part. For example, the mesh has an order of magnitude of 5 centimeters by 5 centimeters but can vary according to the need. This mesh represents the scale of the analysis implemented.
[0047] The mapping step f), referenced 106 in Figure 3, therefore aims to gather all the information then available on the entire so-called determined area of the part, i.e. the area on which measurements are taken. This mapping is analyzed as a whole with an identification of the minima and maxima. Each maximum and / or minimum can also be analyzed to check whether it is a local extreme point or whether an entire area is solicited.
[0048] The next step is to analyze the data available on the mapping of the different elasticity modules to conclude on possible damage to the part.
[0049] According to a first variant defined by step g 1 ), referenced 107i in Figure 3, a comparison is made of the mapped values, for a given direction, with a reference modulus of elasticity in the same direction. If the mapped value is greater than the value of the reference modulus of elasticity, then damage can be concluded. By "reference", we mean the value obtained for a part previously checked and for which the absence of defects is known. In other words, it is a part qualified as nominal which must be approached as closely as possible in terms of results. The "reference" can also come from a database of material properties.
[0050] According to a second variant defined by step g2), referenced 1072 in Figure 3, it is possible to consider monitoring the different maps over time. It is then possible to compare the last map obtained with the previous one and make a comparison between the two. Such a comparison then makes it possible to see whether the elasticity modules evolve continuously / regularly or whether, on the contrary, a discontinuity in the values is observed. Such discontinuities are then the signature of local cracking, namely a form of damage to the part. To implement this variant, it is understood that it is necessary to have at least two maps and therefore to have repeated steps a) to f) at least once.
[0051] Whatever the variant considered (step gi) or step g2)), the fact of making a map at step f) makes it possible to have an indicator, according to a given direction,
[0052] The invention also proposes a device D for implementing the method according to the invention, comprising: - means C for measuring movements of the part by stereo-correlation of images on said determined zone, - means JC for measuring the stresses imposed on the part on said determined zone, and - means 20 for implementing by computer at least steps c) to f) and gi) and / or g2).
[0053] 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 the damage to a part subjected to mechanical stress, said method comprising the following steps: a) measuring (101) the displacements of said part in the different directions x, y, z of space over a determined area thereof, by stereo-correlation of images, b) measuring (102) the stresses imposed on the part in the different directions x, y, z of space over said determined area and, implementing by computer the following steps: c) calculating (103) the deformations of said part in the different directions of space x, y, z over said determined area, from the displacements measured in step a), d) calculating (104) the Poisson's ratios of the part over said determined area from the deformations calculated in step c);e) calculating (105) the moduli of elasticity of the part according to the different directions of space x, y, z on said determined zone from the stresses measured in step b), the deformations calculated in step c) and the Poisson's ratios calculated in step d), f) generating (106) a map of the different moduli of elasticity over the whole of said determined zone; then gi) either (107i) determining, to know if said part is damaged, if at least one of the moduli of elasticity calculated in step f) is greater than a reference modulus of elasticity in the same direction, g2) or (1072), after having repeated steps a) to f) at least once, comparing, to know if said part is damaged, if the map recently obtained of the different moduli of elasticity over the whole of said determined zone presents a discontinuity compared to the map previously obtained.; [2] Method (100) according to claim 1, in which a plurality of successive measurements are carried out according to steps a) and b) to define a dynamic fatigue test. [3] Method (100) according to claim 1, in which a plurality of successive measurements are carried out according to steps a) and b) to define a dynamic impact test. [4] Method (100) according to any one of claims 2 or 3, in which steps c) to f) and gi) and / or 92) are carried out from a selection of measurements from those which were obtained in steps a) and b). [5] Method (100) according to any one of the preceding claims, in which the part (10) is a composite part comprising a resin in which fibers are embedded. [6] Method (100) according to any one of the preceding claims, wherein the part (10) is a turbomachine part, in particular a turbomachine casing and preferably a turbomachine fan casing. [7] Device (D) for implementing a method (100) according to one of the preceding claims, comprising: - means (C) for measuring movements of the part by stereo-correlation of images on said determined zone, - means (JC) for measuring the stresses imposed on the part in said determined area, and - means (20) for implementing by computer at least steps c) to f) and gi) and / or g2). [8] Aircraft comprising at least one device (D) according to claim 7.