METHOD FOR DETERMINING DAMAGE TO A PART SUBJECTED TO MECHANICAL STRESS AND DEVICE FOR IMPLEMENTING SUCH A METHOD

Abstract

The invention relates to a method (1) for determining the damage to a part subjected to mechanical stress, based on input data representing the strain fields along , and obtained by stereo-correlation of images of an area (S1) to be observed at different times k preceding and following the mechanical stress. A transformation of the input data is then performed to obtain two-dimensional output data. This data is then presented as a series of images Ik representing the strain fields, each comprising a useful area ZUk. Next, an area Ak of ​​each of the useful areas ZUk is estimated to obtain a function f(k) = Ak representing the evolution of the area as a function of time, and the areas Ak for which f(k) is greater than or equal to a threshold characteristic of damage are defined.In parallel, it is possible to identify the first instant ke1 associated with the first damage. Figure for the abbreviation: Figure 1a.

Description

Title of the invention: METHOD FOR DETERMINING THE DAMAGE TO A PART SUBJECTED TO A SOIL MECHANICAL LICITATION AND DEVICE FOR IMPLEMENTING SUCH A METHOD Technical field of the invention

[0001] The invention relates to the technical field of methods for determining the damage to a part subjected to mechanical stress, in particular those based on the analysis of digital images obtained by stereo-correlation (S-CIN, Stereo-Digital Image Correlation (Stereo-DIC)). Technical background

[0002] Many techniques are known for studying damage in a material. "Damage" means any form of degradation of the properties of a material that may be linked to a change in the internal structure, such as the appearance of microcracks, fiber / resin decohesions, fiber breakage or even delamination at the different scales of the material. These scales may be those of a carbon fiber, a carbon strand (several thousand fibers), 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.

[0003] Digital Image Correlation (DIC) is a non-destructive testing technique that uses digital image correlation to measure displacement vectors on the surface of a part. This method is based on the acquisition by a single digital sensor, such as a camera, of an image of a part in its reference state, this image being called the reference image, and an image of this part in its deformed state, this image being called the "deformed state image". It is therefore intrinsically intended for the two-dimensional measurement of surfaces. By applying the principle of conservation of the gray level, it is possible to determine at each point of the analyzed surface the displacement vector which makes it possible to ensure the concordance between a point of the reference image, and this same translated point of the displacement in the image in the deformed state.

[0004] The document Xiangyun R. et al., Strain distribution and fatigue life estimation for Steel plate weld joint low cycle fatigue based on DIC, Optics and Lasers in Engineering, Volume 124,2020, uses Digital Image Correlation to estimate the Wôhler limit of a metallic material by analyzing the hysteresis area in fatigue during a dynamic cycle. Filipe R., et al., Tension-tension fatigue behavior of hybrid glass / carbon and carbon / carbon composites, International Journal of Fatigue, Volume 146, 2021, presents a CIN fatigue study for composite materials.

[0005] Infrared thermography is also widely used for surface analysis. This non-destructive surface testing technique uses an infrared camera, placed remotely in front of the part, to record the thermal scene. It allows the measurement of the temperature dissipated by the structure during a self-heating test. However, the use of an obscuring insulating curtain may be necessary to isolate the thermal scene and avoid external radiation disturbances that can alter the temperature measurement.

[0006] Infrared thermography can also be implemented at high acquisition frequency to identify the onset of damage in the context of a dynamic impact test. This is for example described in the document Carosena M. et al, A quantitative approach to retrieve delamination extension from thermal images recorded during impact tests, NDT & E International, Volume 100, 2018, Pages 142-152. The choice of post-impact images depends on the parameters affecting the thermal effects associated with the impact, such as the type of material, geometry and impact energy. This choice can be dictated by empirical considerations or, as is the case in this document, by the measurement of the heat diffusion time in the material.

[0007] However, a common point to all the techniques previously stated, lies in the fact that all the surface control parameters are limited to the pixel scale (thermal or visual). This significantly limits the detection of the appearance of damage and the monitoring of its propagation in the part studied. In addition, as can be seen with infrared thermography, the same indicator cannot be used to characterize and monitor the propagation of surface damage for both a dynamic fatigue test or a dynamic impact test.

[0008] Image stereo correlation is a non-destructive testing technique that allows the measurement of displacement fields on the surface of the part during a static or dynamic mechanical test. Knowing the displacement fields, it is then possible to determine the deformation fields on the surface of said part. It is therefore increasingly used in experimental mechanics. Conventionally, an image stereo correlation system comprises at least two digital sensors (generally cameras) each acquiring images of the part from different viewpoints. This makes it possible to measure surfaces in three-dimensional space, unlike conventional digital image correlation. Each digital sensor acquires an image of the part in its reference state. Then, after the part has been subjected to mechanical stress, each sensor acquires one or more images of this part in its successive deformed states.

[0009] Different approaches have been developed to obtain reliable data on the measured displacements. They involve either calibration of the cameras or calibration of the camera model directly on the part, and measurement of the shape of the part. In practice, it is an area located on the surface of this part, called the area of ​​interest, which is studied. In local methods, the calibration is carried out by determining the displacement of a given point of the area of ​​interest between the reference image and the image in the deformed state in a limited set of pixels centered around this point. This limited set of pixels is called an imagette, or facet ("subset" in English), or even a correlation window.Calibration is performed by finding the position of the image in the deformed state. If there are several images in the deformed state, a calibration must therefore in principle be implemented for each of them. However, despite this calibration, it remains difficult to obtain reliable displacement measurements because calibration and measurement of the shape remain complicated to implement. Currently, some algorithms make it possible to obtain the deformation fields by knowing the size of the image used alone.

[0010] The parameters conventionally used during the analysis of displacement data are based on the analysis of deformations in the direction of the mechanical stress or, for multiaxial stresses, in the stress directions. They make it possible to deduce the limit values ​​of deformations at the surface of the part. However, for the reasons mentioned above, these results are still subject to interpretation since no current indicator makes it possible to clearly differentiate by an objective criterion the moment at which the materials pass from a "healthy" state to a "damaged" state. It is therefore not possible to arrive at a decision on the conditions of appearance of the damage.

[0011] Furthermore, even knowing that damage is present, it is sometimes difficult to clearly identify the direction favored by the stress (direction of the fibers for a composite or for metallic necking). However, the appearance of damage is a crucial event during the dimensioning of a structure in dynamic fatigue, in particular with the estimation of the Wôhler limit. Similarly, during a dynamic impact test, knowledge of the instant of appearance of the damage on the surface of the part and the associated deformation field provides information necessary for the dimensioning of structures for impact.

[0012] The invention aims to overcome at least some of the aforementioned problems and proposes to use a single indicator to study the propagation of damage in the context of a dynamic fatigue test as well as in that of a dynamic impact. This indicator makes it possible to differentiate by an objective criterion the transition of materials from a "healthy" state to a "damaged" state so that at the end of the process according to the invention, the test data relating to the appearance or not of damage during the test can be directly validated. Summary of the invention

[0013] The invention proposes for this purpose a method for determining the damage to a part subjected to mechanical stress, said part being defined in a three-dimensional space having orthogonal axes X, Y, Z of direction vectors X, Y and z, the method comprising the following steps, implemented by computer, in this order: 10) provide input data representative of the deformation fields along x,YQtZ obtained by stereo-correlation of images of an area of ​​the part to be observed at different times k (k = [0.. .M], M integer) preceding and following the mechanical stress, 20) transforming the input data so as to obtain two-dimensional output data and representing this output data in a two-dimensional representation frame of direction vectors x, Y so as to obtain a series of images Ik representative of the deformation fields, each of the images Ik comprising a useful zone ZUk, 30) estimate an area Ak of ​​each of the useful zones ZUk in order to obtain a function f(k) = Ak representative of the evolution of the area as a function of time, 40) determine the areas Ak for which f(k) is greater than or equal to a threshold 0^ characteristic of damage and determine a first instant kei for which f(kei) is greater than or equal to 0, said first instant kei being associated with a first damage.

[0014] The method according to the invention performs a transformation of the input data representative of the deformation fields along x, Y and Z obtained by stereo-image correlation (S-CIN), then represents them in the two-dimensional representation frame in order to obtain a series of images Ik representative of the deformation fields. It is then possible to estimate the area Ak covered by the distribution of the points (pixels) in the useful zone ZUk of each of the images thus obtained and, thus, possible to analyze the evolution of this area over time to follow the propagation of the damage from its appearance.

[0015] The estimation of the area Ak of ​​the distribution of points in the useful zone makes it possible to differentiate by an objective criterion the passage of materials from a “healthy” state to a “damaged” state. Indeed, the calculation of the area is based on the two-dimensional representation of the data representative of the deformation fields. Thus, rather than By setting a criterion according to the rules of the art, it is possible to directly observe, with the estimation of the area, a significant change in the mechanical behavior of the part. This also makes the method according to the invention a repeatable and suitable method for studying the appearance and propagation of damage in the context of a dynamic fatigue test as well as in that of a dynamic impact.

[0016] According to different characteristics of the invention which can be taken together or separately: - the method further comprises, following step 40), the following steps: 50) analyze the deformation fields associated with the instants k < kei preceding the first damage and determine reference parameters representative of the deformation fields before the first damage, 60) compare the reference parameters with the deformation fields associated with the instants k > kei following the first damage in order to determine a physical quantity characteristic of the damage to the part; - the method further comprises, following step 50) or step 60), the following step: 70) compare the reference parameters with pre-existing parameters representative of a material from which the part is made; the mechanical stress is carried out during a dynamic fatigue test, the physical quantity to be determined during step 60) being used to characterize the dynamic fatigue of the material; step 40) includes the following sub-steps: 42) determine a maximum f(k) = Ak>mar of the function f(k) = Axy>k, 44) define the damage threshold 0^ as a function of the maximum fmax(k) determined in the previous step, 46) determine the first instant kei for which f(kei) is greater than or equal to the damage threshold 0; the damage threshold 0^ defined during step 44) represents between 20% and 60% of Ak,max step 60) includes the following substeps: 62) analyze the Ikei image of the deformation fields associated with the instant k = k ei of the first damage in order to determine the effort, the constraint, a de placement or deformation of the material, 64) estimate a threshold 0^ characteristic of an endurance limit from Make Akei associated with said first kei moment; - the mechanical stress is carried out during a dynamic impact test, the physical quantity to be determined during step 60) being used to characterize the dynamic impact on the material; - step 40) includes the following sub-steps: 42') identify the area A; associated with an instant k = k; (k; < ke) of an impact, 44') define the damage threshold 0(, said damage threshold 0^ corresponding to an area Akei, Akei > Ai, the value of which differs by at least 25% compared to Ai, 46') identify the first instant kei for which f(kei) = 0^; - step 60) includes the following sub-steps: 62') analyze the Ikel image of the deformation fields associated with the instant k = kel of the first damage, 64') estimate a damage threshold 0^ characteristic of an energy limit or dynamic load or dynamic deformation of the material; - step 20) comprises a first sub-step 22) consisting of carrying out a linear regression of the deformation fields; - which step 20) comprises a first sub-step 22) consisting of carrying out a principal component analysis of the deformation fields; - the part is a casing of a turbomachine fan.

[0017] The invention further relates to a device for implementing a method as previously described, comprising means for implementing by computer at least steps 20) to 40). Brief description of the figures

[0018] Other objects, characteristics and advantages of the invention will appear more clearly in the description which follows, made with reference to the appended figures, in which:

[0019] - [Fig.1a] is a schematic view illustrating the different stages of a process according to the invention,

[0020] - [Fig.lb] is a schematic view illustrating the different stages of a process according to a first embodiment of the invention,

[0021] - [Fig. 1e] is a schematic view illustrating the different stages of a process according to a second embodiment of the invention,

[0022] - [Fig.2] illustrates a part whose damage we wish to study by means of of the method according to the invention;

[0023] - [Fig.3a] illustrates examples of possible speckles for carrying out tests fatigue and dynamic impact by S-CIN on a massive part;

[0024] - [Fig.3b] illustrates examples of possible speckles for carrying out tests fatigue and dynamic impact by S-CIN on a part with small dimensions;

[0025] - Figures 4a, 4b and 4c illustrate the images Ik obtained during the implementation of the method according to the invention, the images Ik having been obtained from data representative of the deformation fields at different times during a dynamic fatigue test;

[0026] - Figure 5 illustrates a curve representing the evolution of the function f(k) = Ak in function of time for the same tests as those in Figures 4a, 4b and 4c: the horizontal dotted line indicates the position of the damage threshold (p, while the vertical dotted line indicates the time of the start of the fatigue test;

[0027] - Figures 6a, 6b and 6c illustrate the images Ik obtained during the implementation of the method according to the invention, the images Ik having been obtained from data representative of the deformation fields at different times during a dynamic impact test;

[0028] - Figure 7 illustrates a curve representing the evolution of the function f(k) = Ak in function of time for the same tests as those in figures 6a, 6b, 6c: the horizontal dotted line indicates the position of the damage threshold 0^;

[0029] - [Fig.8] illustrates the results of a test carried out on an undamaged part but in the process of deformation for a study by tomography: [Fig.8], section a) is an image of the part, figures 8b, 8c and 8d are representations of the component, respectively, along an axis (Z), along an axis (Y) and along an axis (X) of the displacement fields of the part;

[0030] - [Fig.9] illustrates the results of a test carried out on a damaged part in view of a tomographic study: [Fig.9], section a) illustrates an image of the part, figures 9, section b, 9, section c and 9, section d are representations of the component, respectively, along an axis (Z), along an axis (Y) and along an axis (X) of the displacement fields of the part. Detailed description of the invention

[0031] The invention relates to a method 1 for determining the damage to a part subjected to mechanical stress.

[0032] 1. Definitions - general comments

[0033] Throughout the description of the present application, the term S-CIN designates the technique of Stereo-Correlation of Digital Images. Unless otherwise indicated, the displacement field and deformation field data are all obtained by S-CIN. Although S-CIN can be implemented to perform measurements in the context of static mechanical tests and dynamic mechanical tests, the method according to the invention is applied to data obtained in the dynamic domain, in particular by dynamic fatigue and dynamic impact tests. We will return to this in the following sections.

[0034] These tests may have as their objectives the characterization of the properties of a material or the verification of the quality of the material. It is also possible to apply the method according to the invention to the data resulting from the validation tests of the resistance of the parts, for example of a part subject to a derogation during its production. In this case, it is possible to extract all or part of the part according to the need for justification and the area whose acceptability must be validated, then carry out a mechanical test on, respectively, this part or the part of it.

[0035] At this stage, it should be emphasized that the tests themselves are not the subject of the invention and only the data relating to the displacement fields and / or the deformation fields obtained from these tests are useful. In other words, the invention is independent of the test phase and everything related thereto. In other words, the invention can be implemented independently of the test phase and the test data collection phase, in particular the data relating to the displacement fields and / or the deformation fields.

[0036] That being said, the following sections present the experimental conditions under which the tests were carried out in order to facilitate their reproduction by a person skilled in the art.

[0037] In the context of the invention, the S-CIN data are acquired by CCD cameras, namely cameras using photographic sensors based on a charge-coupled device. The CCD cameras have an acquisition frequency suitable for fatigue and dynamic impact tests. A CCD camera capable of acquiring a few images per second is suitable for performing dynamic fatigue tests at low frequency but is insufficient for performing dynamic impact tests. In the latter case, a camera capable of acquiring several thousand images per second is preferable. That being said, the method according to the invention, which will be detailed below, aims to enable determination of the damage to a part subjected to mechanical stress in the context of both dynamic fatigue and dynamic impact tests.A CCD camera capable of acquiring several thousand images per second was therefore used, but this is not obligatory, as one can understand.

[0038] The acquisition area was illuminated using light-emitting diode (LED) lamps. The lighting can be carried out continuously or in synchronization with the cameras depending on the acquisition device in question. When the lighting is used continuously, This allows the frequency of the LED lamps to be decorrelated from that of the CCD cameras.

[0039] The part on which one wishes to implement the test can be any part. This part is not limited by its shape, its dimensions, its color, etc. This part comprises an external surface S comprising an area to be observed SI. An exemplary embodiment is illustrated in Figure 2. A close-up view of damage is visible in the dotted box. The part is defined in a three-dimensional space having orthogonal axes X, Y and Z respectively associated with direction vectors x, L, and Z. In the following, the X axis is called the longitudinal axis, the Y axis is called the transverse axis and the Z axis is called the orthogonal axis.

[0040] In the context of the invention, the part is preferably made of a woven composite material. For example, the part may be made of a woven continuous carbon fiber composite material. Such a 3D woven composite is for example used in the context of a crankcase fan for an aircraft engine. It comprises a plurality of warp fibers, arranged along a longitudinal axis, which are woven with a weft fiber in one or more weaving planes and embedded in a polymer matrix which has hardened. The weft fibers and the warp fibers may then be made of carbon, glass, or a mixture of carbon and glass. They may also be made of other types of fibers. The invention may be applied to parts made from other types of composite materials.

[0041] Conventionally, S-CIN measurements are implemented by means of a speckle deposited on the surface of the part for which the damage propagation is to be studied. Indeed, the measurements of the displacement fields and / or the deformation fields require that the image acquired by the digital sensor has a random, contrasted texture and dimensions adapted to the part considered. Very few surfaces naturally have the conditions required for the implementation of these measurements. The speckle serves precisely to provide the appropriate texture to the surface of the part. There are different types of speckle. In the context of the invention, a black and white speckle was used to provide the appropriate texture to the part considered.

[0042] Figures 3a and 3b are images showing examples of possible speckles for performing fatigue and dynamic impact tests by S-CIN. In [Fig.3a], the speckle has been deposited on a massive part while, in [Fig.3b], the speckle has been deposited on a small part. A "massive part" is understood to be a part whose largest dimension is between one meter and several meters. For example, [Fig.3a] illustrates a part of a composite impact panel. A "small part" is understood to be a part whose largest dimension is less than one meter. As an example, [Fig.3b] illustrates a conventional laboratory test specimen.

[0043] Prior to the actual test, a calibration step may be implemented. This calibration step may comprise, on the one hand, the calibration of the parameters of the CCD cameras and, on the other hand, the measurement of the actual shape of the part considered. Different approaches exist. For example, the calibration of the parameters of the CCD camera may be carried out by the approach of minimizing the gray level while the measurement of the shape of the part may be carried out by applying the principle of preserving the gray level. The calibration step may be implemented on the basis of these well-known approaches.

[0044] During dynamic impact tests, the part is subjected to a shot. This shot generates a mechanical stress which can cause damage (a definition of the term "damage" is given in the preamble to this description). In the case of dynamic impact with damage, it is possible to visually detect when damage has occurred. The method according to the invention not only makes it possible to monitor the propagation of the damage from its onset, but it also makes it possible to define a threshold from which the material from which the part in question is made is damaged. This threshold is called the damage threshold. It should be noted that the method according to the invention can also be applied to the case of dynamic impact without damage.

[0045] In the illustrative examples of the present invention, the dynamic impact tests were implemented on panels or coupons of real parts in order to ensure good mechanical resistance during the validation of the certification and in order to simulate flight conditions such as a loss of blade, ingestion of birds or debris in flight.

[0046] During dynamic fatigue tests, the part is subjected to a load cyclically in order to estimate the endurance limit of the material from which this part is made. This endurance limit, hereinafter called the endurance threshold, can be known from the first cycle of the test, so that it is not obligatory to subject the part to the load for several cycles. In any case, the method according to the invention makes it possible to estimate this endurance limit in a single test instead of a test campaign with several loads as is currently the case in the literature. Like a dynamic impact test, the dynamic fatigue test can result in generating or not generating damage to the material. Thus, the method according to the invention not only makes it possible to define an endurance threshold of the material, but it also makes it possible to monitor the propagation of the damage from its onset.

[0047] In the illustrative examples of the present invention, the dynamic fatigue tests are carried out in order to characterize the mechanical resistance linked to the environment. engine life. In the case of a turbomachine part attached to an aircraft, this includes, for example, the takeoff, landing, cruise or rapid descent phases.

[0048] 2. Description of the general embodiment

[0049] With reference to [Fig. 1a] and as already indicated previously, the invention relates to a method 1 for determining the damage to a part subjected to mechanical stress. As also already indicated, this mechanical stress can be carried out within the framework of a fatigue test or a dynamic impact test.

[0050] According to a first step 10) of the method 1 according to the invention, input data representative of the deformation fields along x, Y and Z obtained by stereo-correlation of images of a zone SI (of the part) to be observed at different times k (k = [0.. .N], integer N) preceding and following the mechanical stress are provided.

[0051] The displacement fields and / or deformation fields along X, y and z obtained by S-CIN are in the three-dimensional domain, and are therefore by nature more precise than point or averaged information such as that obtained with the methods of the prior art. The deformation fields can be obtained in two ways. The deformation fields can be directly measured by a computer program provided for this purpose. In this respect, it is advantageous for the size of the image or facet used for the S-CIN to be entered in the computer program so that it directly calculates the deformation fields. However, this is not mandatory. It is possible to calculate the deformation fields from the displacement fields determined by the computer program.Regardless of how the deformation fields along x, y and Z are obtained, they constitute the input data for implementing the following steps of method 1 according to the invention.

[0052] In practice, the input data relating to the deformation fields can be provided directly by the S-CIN computer program. If this is not the case, they can be recalculated from the displacement fields necessarily provided with the stereo-correlation data. As a reminder, the deformations are calculated with the following formula:

[0053] , duJ ] “ 2 ' \ du, + du, )

[0054] This formula is the general case for calculating deformations in the most common cases. If deformations need to be calculated, one approach is to use finite difference schemes. It is advantageous if the schemes used are consistent with the data. If only deformation data is desired, it is possible to limit oneself to a stencil of two or three (number of data points used for the derivation) with centered diagrams in the nominal case and decentered upstream or downstream if one is on the edges of the image. These diagrams allow the entire image to be preserved and avoid changing the size of the area to be observed.

[0055] In the context of the invention, it is desired not only to be able to identify the appearance of damage to the external surface S of the part but, in addition, to follow the propagation of the damage in the part considered. In this respect, during the test phase, it is important to ensure that the images are acquired at the different times preceding and following the mechanical stress. As we will see below, the input data representative of the deformation fields relating to the times preceding the mechanical stress characterize the zone SI to be observed while it is healthy. The zone SI to be observed is said to be “healthy” because the mechanical properties of the material have not yet undergone any alteration due to damage.As for the input data representing the deformation fields relating to the instants following the mechanical stress, they make it possible to characterize the deformations, and any damage that the material has suffered. They are therefore necessary for the analysis of the part in the deformed state.

[0056] However, it should be noted that the input data provided during this first step 10) of the method according to the invention do not necessarily represent all the input data which could be obtained (first configuration) or all the input data which are actually obtained from the S-CIN measurements (second configuration).

[0057] As regards the first configuration and by way of example, during a dynamic fatigue test, it is possible to limit oneself to the analysis by S-CIN of a series of images acquired over one cycle every 10% of total cycles. In other words, for a test of 100,000 cycles, the analysis can be done at 10,000, 20,000, 30,000 cycles, etc. These values ​​can be modified according to the operator's needs and the computing capacity. In the case of a dynamic impact test, one could in fact limit oneself to one data acquisition before the impact and up to ten data acquisitions after the start of the impact when damage to the external face is visible after the test.

[0058] As regards the second configuration and by way of example, during a dynamic impact test, if no damage is visible on the external face, it may not be necessary to analyze all the data thus obtained. It may be possible to limit oneself to those presenting the highest levels of deformation.

[0059] According to a second step 20) of the method 1 according to the invention, the input data are transformed so as to obtain two-dimensional output data and these output data are represented in a two-dimensional representation frame RI of direction vectors x, Y so as to obtain a series of images Ik representative of the deformation fields, each of the images Ik comprising a useful zone ZUk.

[0060] Thus, during a first sub-step 22) of the second step 20) a transformation of the data relating to the deformation fields can be carried out. These three-dimensional data provided during the first step 10) are reduced to two-dimensional data. Thus, the first sub-step 22) is nothing more than a reduction of the dimension of the input data. In this respect, different methods of reducing the variables can be implemented. Each of the data according to x, Y and Z of the deformation fields corresponds to a variable and there is a redundancy between all of these variables because these variables are linked together (correlated). The aim of this first sub-step 22) is therefore to extract the data whose sum covers all of the information contained in the input data provided during the first step 10).

[0061] According to a first particular implementation, the first sub-step 22) can be implemented by linear regression applied to the deformation fields. This reduction method is well known and its application to the deformation fields does not pose any particular question. According to a second implementation, the first sub-step 22) can be implemented by the technique of principal component analysis. Principal component analysis is a relatively well-known decomposition method which ensures that each new variable, called principal component, constitutes a vector orthogonal to the system composed of the other principal components. In the context of the invention, the number of principal components is determined by the capacity of the model to represent all of the deformation fields.

[0062] The two variable reduction methods presented above, namely linear regression and principal component analysis, make it possible to obtain output data which objectively represent the input data while allowing a reduction in the number of variables. The first sub-step 22) therefore ultimately aims to obtain the best possible representation of the data relating to the two-dimensional deformation fields.

[0063] In a second sub-step 24) of the second step 20), the output data is represented in a two-dimensional frame of reference. This produces a plurality of images Ik characterizing the deformations of the surface at the different times preceding and following the mechanical stress. Image I8 designates the image associated with the input data acquired at the eighth time, which means that this time is not necessarily equal to 8 seconds. This time can be taken 1.45 milliseconds after the start of the test as one second depending on the camera used and the input data retained by the operator for the purposes of the analysis. The second sub-step 24) therefore constitutes a simple representation step. The whole issue therefore lies in the first sub-step 22) since a qualitative reduction must be carried out. input data in order to obtain the best representation of the deformation fields along x, Y and Z-

[0064] Each of the images Ik comprises a useful area ZUk. The useful area ZU0 designates the useful area of ​​the image Io, the useful area ZUi designates the useful area of ​​the image Ib etc. This useful area is not necessarily the entire image. This useful area corresponds to the area over which the points (pixels of the image) extend which will be necessary for the implementation of the third step 30) of the method 1 according to the invention, these points being nothing other than the graphical representation of the output data obtained during the second step 20). Thus, this useful area can be the complete image Ik as simply a part of the image Ik, as the case may be. The identification of the useful area can be carried out by associating a pixel with a value, the absence of a pixel being able to be characterized by the value 0.

[0065] During a third step 30), the area Ak of ​​each of the useful zones ZUk is estimated in order to obtain a function f(k) = Ak representative of the evolution of the area as a function of time.

[0066] From the two-dimensional output data, by measuring the area Ak of ​​each of the useful zones ZUk, an indicator is obtained, giving a photograph of the evolution of the deformations of the external surface as a function of time. Thus, when the area Ak reaches a threshold value, for example characteristic of damage, it is possible to determine the instant associated with reaching this threshold by identifying the image Ik on which this area was measured. However, this area cannot be used as such. Indeed, to be able to estimate whether an area is characteristic of damage to the surface, it is necessary to determine the threshold from which damage is considered to exist. We will return to this later. In any case, the area constitutes an objective indicator which makes it possible to follow the propagation of damage on the external surface S of the part in question.

[0067] During a fourth step 40), the area or areas Ak are determined for which f(k) is greater than or equal to a threshold characteristic of damage and a first instant kei is determined for which f(kei) is greater than or equal to 0, said first instant kei being associated with a first damage.

[0068] The threshold 0^ characteristic of damage (or more simply the threshold 0^ of damage) is the area from which damage to the external surface is characterized. In other words, the damage threshold is the area value from which the external surface is damaged, that is to say that it presents damage. A definition of damage was given in the preamble of the description and is not repeated here. Thus, the threshold damage is expressed in units of surface area as the area itself.

[0069] Very advantageously, this damage threshold can be predetermined. Indeed, knowing the nature of the material from which the part is made and having already implemented the method 1 according to the invention at least once, it is possible to determine in advance the damage threshold (j)^ from which damage to the external surface S occurred. In this case, the first implementation of method 1 can be considered as a calibration. However, it is better to determine this damage threshold from the data of the test concerned. Indeed, from a On the one hand, this limits the implementation of method 1 to one use per test and on the other hand, it allows us to avoid the sliding effects that can be observed after a certain number of tests. In this case, it is appropriate to start the fourth step 40) of method 1 by determining this threshold. As we will see later, this can be done in two very simple sub-steps 42) and 44).

[0070] Knowing the damage threshold 0^, it is sufficient, during a first sub-step or a third sub-step, as the case may be, of comparing the area values ​​Ak to the threshold and extract values ​​greater than or equal to this damage threshold 0. Once that the values ​​of the areas Ak concerned are extracted, during a second sub-step or a fourth sub-step, an identification of the image Ik on which the first area Akei greater than or equal to the damage threshold 0 was measured makes it possible to know the instant Ikei of the appearance of the damage.

[0071] To the extent that the objective pursued by the operator consists solely of knowing the moment of the appearance of the damage or knowing that there has been damage, the fifth 50) and sixth 60) steps which will be described below do not need to be carried out. The method 1 according to the invention can stop at the end of this fourth step 40). Steps 50) and 60) being optional, they are indicated in the figures by dotted boxes.

[0072] If one seeks to obtain a physical quantity characteristic of the damage, the method 1 according to the invention can advantageously comprise the following steps.

[0073] During a fifth step 50), the deformation fields associated with the instants k < kei preceding the first damage are analyzed and reference parameters representative of the deformation fields before the first damage are determined. magement.

[0074] This fifth sub-step makes it possible to estimate the displacement fields and the deformation fields of the material before the mechanical stress. These data therefore make it possible to know the state of the external surface S of the part when it was still healthy. They therefore provide, in a way, reference parameters concerning the material.

[0075] The reference parameters determined during this step depend on the test being implemented. In the case of dynamic fatigue, it will be possible, for example, to determine the force, stress, displacement or deformation allowing the endurance limit of the material to be estimated, knowing the instant of appearance of the damage. In this case, it may be useful to check the conditions applied at the instant considered. In the case of a dynamic impact test, the analysis is simplified. In the case of damage to the external face, this means that the local mechanical capacities of the material have been largely exceeded and are translated by a crack. As with a dynamic fatigue test, it is then possible to deduce the energy or the dynamic limit load or the dynamic limit deformation before damage. We will return to this later.

[0076] From then on, it is possible during a sixth step 60) to compare the reference parameters with the deformation fields associated with the instants k > kei following the first damage in order to determine a physical quantity characteristic of the damage to the part.

[0077] Furthermore, it is also possible, concurrently with the sixth step 60) or after, to carry out a step 70) of comparing the reference parameters with pre-existing parameters representative of a material from which the part is made in order to determine a physical quantity characteristic of the damage to the part.

[0078] These pre-existing parameters representative of the material may have been determined empirically or by theory. The interest of this comparison is simply to verify the qualitative value, and therefore the reliability, of the parameters determined during step 50). This comparison with the external data therefore makes it possible to establish a criterion of consistency of the data obtained by method 1.

[0079] The method according to the invention can be implemented to determine the damage to a variety of composite parts. The composite part can be a turbomachine casing, for example the casing of a fan of the turbomachine. The composite part can advantageously be made of carbon fibers. The composite part can, alternatively, be made of glass fibers. The composite part can also be a mixture of carbon or glass fibers.

[0080] It should be noted that this method is also applicable with the different possible deformations. With the S-CIN, it is common to obtain the principal deformations of each point of the studied area. It is possible to apply the method 1 according to the invention with the principal deformations 1 and principal deformations 2 instead of those with the deformation fields along X and the deformation fields along Y. These principal deformations can be obtained from an analysis at automation of the Mohr circle of deformations. The distributions displayed will be visually different but the conclusions will be similar to those detailed above depending on the case studies.

[0081] 3. First embodiment of the invention - Dynamic fatigue

[0082] All comments made previously in relation to the general embodiment apply, except of course when examples relating to the case of dynamic impact are mentioned.

[0083] Referring now to [Fig.lb], in this embodiment, the mechanical stress is carried out during a dynamic fatigue test. The physical quantity which can be determined during step 60) is then used to characterize the dynamic fatigue of the material. In the case of dynamic fatigue, the propagation of the damage is slow and progressive. The function f(k) will change slowly and progressively over time.

[0084] Thus, according to this first embodiment of method 1, step 40) comprises the following sub-steps:

[0085] 42) determine a maximum f(k) = Ak>max of the function f(k) = Axy>k,

[0086] 44) define the damage threshold 0^ as a function of the maximum fmax(k) determined during the previous step,

[0087] 46) determine the first instant kei for which f(kei) is greater than or equal to the threshold 0^ damage.

[0088] Thus, the damage threshold 0^ is calculated on the basis of the data relating to the dynamic fatigue test from which the input data were obtained. The maximum f(k) = Ak>max of the function f(k) represents the maximum amplitude of the deformation undergone by the material. It can therefore be used to set the threshold 0^ damage objectively. Preferably, the damage threshold 0^ defined in step 44) represents between 20% and 60% of Ak>mar, that is to say that the damage threshold 0f defined in step 44) has a value between 20% and 60% of AkmflX. Thus, any area value greater than or equal to a damage threshold 0^ thus fixed will be associated with damage to the material. In this regard, this allows us to recall that the material can undergo several damages during the test. After having identified the area or areas satisfying this condition, all that remains is to identify the image Ik associated with this area to know the instant at which the damage(s) occurred.

[0089] Still according to this first embodiment of method 1, step 60) comprises the following sub-steps:

[0090] 62) analyze the Ikei image of the deformation fields associated with the instant k = kei of the

[0091]

[0092]

[0093]

[0094]

[0095]

[0096]

[0097]

[0098]

[0099] first damage in order to determine the stress, the constraint, a displacement or a deformation of the material, 64) estimate a characteristic threshold of an endurance limit from the Akei area associated with said first kei moment. Sub-steps 62) and 64) allow the endurance limit of the material to be estimated. To obtain the above-mentioned reference parameters, namely effort, stress, a displacement or deformation of the material, it is therefore necessary to have prea the instant of occurrence of the damage, i.e. the instant of the first damage. Therefore, the 0^ threshold of endurance limit can be estimated. As we saw in the introduction to this description, current methods do not allow this endurance limit to be reliably estimated because the data obtained by these methods are subject to interpretation. Indeed, no indicator exists in the literature to clearly differentiate by an objective criterion the transition of materials from the "healthy" state to the "damaged" state. Method 1 according to the invention allows this endurance limit to be reliably estimated. That being said, this does not mean that the endurance limit of the material is higher than the damage to it. Indeed, in most cases, the endurance limit threshold 0^ is lower than the damage threshold 0^ since endurance occurs before damage. 4. Second embodiment - Dynamic impact All comments made previously in relation to the general method of implementation apply, except of course where examples relating to the case of dynamic fatigue are mentioned. Referring now to [Fig. 1e], in a second embodiment of the invention, the mechanical stress is carried out during a dynamic impact test. Thus, the physical quantity to be determined during step 60) is used to characterize the dynamic impact on the material. In the case of dynamic impact, the propagation of the damage is rapid and sudden. The function f(k) therefore changes in order of magnitude at the time of damage and the instant of damage can therefore be easily identified. In this regard, according to this second embodiment of method 1, step 40) comprises the following sub-steps: 42') identify the area A; associated with an instant k = k; of an impact, 44') define the damage threshold 0^, said damage threshold 0^ cor corresponding to an area Akei, Akei > A;, whose value differs by at least 25% from Ai,

[0100] 46') identify the first instant kei for which f(kei) = 0.

[0101] In this embodiment, the damage threshold therefore indicates a change in the order of magnitude of the area Ak. It therefore functions as a criterion imaging the damage limit in deformation of the material during a dynamic impact. Preferably, and as indicated above, this change in the order of magnitude is evaluated in relation to the area A; associated with the instant k = k; (k; < ke) of the impact, which makes it possible to have as a reference area an area corresponding to a “healthy” state of the external surface S of the object. In the hypothesis posed here, it is considered that the surface is damaged when the value differs by at least 25% compared to A;. This in fact indicates a change in the order of magnitude of the area and therefore a significant increase in the area. This cannot be due to chance. This sudden increase has no physical meaning and necessarily corresponds to damage to the external surface.

[0102] From then on, it is possible to determine precisely the appearance of the first damage by identifying the instant associated with the image Ik on which this area Aep was measured. The area Aei and the damage threshold are therefore the same. As mentioned above, this threshold therefore functions as a criterion for assessing the damage limit in deformation of the material during a dynamic impact.

[0103] Still according to this second embodiment of method 1, step 60) comprises the following sub-steps:

[0104] 62') analyze the Ikei image of the deformation fields associated with the instant k = kei of the first damage,

[0105] 64') estimate a damage threshold characteristic of an energy limit or dynamic load or dynamic deformation of the material.

[0106] Sub-steps 62') and 64') make it possible to estimate the energy or the dynamic load of the material. To obtain these aforementioned reference parameters, it is therefore necessary to have previously determined the instant of appearance of the damage, i.e. the instant of the first damage. From then on, the damage threshold (p^) characteristic of an energy limit or dynamic load or dynamic deformation of the material can be estimated reliably.

[0107] 5. Examples of implementation of method 1 according to the invention

[0108] For the purposes of dynamic fatigue and dynamic impact testing, a woven composite part was used.

[0109] 5.1 Example 1 - Dynamic fatigue

[0110] In this case of dynamic fatigue, the test was carried out progressively. As the study material is a woven composite, the damage likely to cause a significant change in mechanical behavior are visible by S-CIN. When the stress conditions change little (or not at all) the first damage is highly likely to be taken up by other elements of the material (other fibers or elements of the weave). Thus, the damage will appear from one Ik image to another with jumps in area, with a general tendency of increase.

[0111] Figures 4a, 4b and 4c illustrate the Ik images obtained after implementing the method according to the invention, the Ik images having been obtained from the data representative of the deformation fields at different times during a dynamic fatigue test. The evolution of the distribution at the different times can be discerned. Here, only the Ik images of 9 different times are illustrated. Figure 5 illustrates, for its part, a curve representing the evolution of the function f(k) = Ak as a function of time for the same tests as those of Figures 4a to 4c: the horizontal dotted line indicates the position of the damage threshold while the vertical dotted line indicates the time of the start of the fatigue test.

[0112] In the first image L (time k = 1.26 ms), all the points are concentrated. There are no deformations identified. The useful zone ZUi therefore has a small area (Figure 5, first point). During the dynamic fatigue test, an increase in the area of ​​the useful zone is observed. Here, the mechanical stress propagates to the panel. When the damage begins, the useful zone begins to expand. Isolated points or groups of isolated points are visible. This is clearly the case from time k = 1.34 ms which is associated with I4. The area A4 of the distribution increases very significantly, indicating the appearance of damage. In Figure 5, this area clearly exceeds the damage threshold value 0^ and is, rightly, associated with the first damage.It is therefore possible to identify three zones of mechanical behavior: 1) the start of the test associated with the mechanical loading, 2) a healthy zone corresponding to the minimum values ​​once the load is applied and 3) a damage zone whose start can be identified by the first peak area.

[0113] Therefore, knowing the instant of appearance of the damage, it is sufficient to check the conditions applied at this precise instant during the test to deduce the force, the constraint, the displacement or the deformation allowing the endurance limit of the material to be estimated.

[0114] 5.2 Example 2 - Dynamic impact

[0115] In the case of a dynamic impact, the analysis is simplified. In the case of damage to the external face, this means that the local mechanical capacities of the material have been largely exceeded and are translated by a crack. The area of ​​the de- formations will increase significantly, as seen previously.

[0116] Figures 6a, 6b and 6c illustrate the Ik images obtained after implementing the method according to the invention, the Ik images having been obtained from data representative of the deformation fields at different times during a dynamic impact test. The evolution of the distribution at the different times can be discerned. As for the previous example, only the Ik images of 9 different times are illustrated. Figure 7 illustrates a curve representing the evolution of the function f(k) = Ak as a function of time for the same tests as those of Figures 6a to 6c: the horizontal dotted line indicates the position of the damage threshold 0.

[0117] On the first image (instant k = 1.32 ms), all the points are concentrated. There are no deformations identified. The useful area ZUi therefore has a small area ( [Fig.7], first point). This is still the case for images I2, L and I4. During the dynamic impact test, an increase in the useful area is observed. This increase is significant and sudden as expected. Isolated points or groups of isolated points are visible. This is clearly the case from instant k = 1.42 ms which is associated with I5. The area A5 of the distribution increases very significantly, reflecting the appearance of damage. On [Fig.7], this area clearly exceeds the damage threshold value and is, rightly, associated with the first damage.

[0118] As with a fatigue test, the external conditions at this instant can be identified to deduce the energy or the dynamic limit load or even the dynamic limit deformation before damage during the test.

[0119] The invention finally relates to a device for implementing a method 1 as described previously. This device comprises means for implementing by computer at least steps 20) to 40). These means are for example a processor and a memory.

[0120] The device may also comprise means for measuring movements of the part by stereo-correlation of images of at least one zone SI of the part to be observed as defined in section “1. Definitions - general comments” of the description. Such means are advantageously:

[0121] - a part comprising an external surface S comprising an area to be observed SI, and preferably a speckle deposited on the surface of the part,

[0122] - at least two CCD cameras capable of acquiring a few images per second, and more preferably several thousand frames per second, to acquire the S-CIN data,

[0123] - several light-emitting diode lamps (LED lamps) to illuminate the area acquisition,

[0124] - optionally, a means for generating a shot and therefore a mechanical stress on the room.

Claims

Claims

1. Method (1) for determining the damage to a part subjected to mechanical stress, said part being defined in a three-dimensional space having orthogonal axes (X, Y, Z) of direction vectors x, Y and Z, the method comprising the following steps, implemented by computer, in this order: 10) providing input data representative of the deformation fields along x, Y and Z obtained by stereo-correlation of images of a zone (SI) of the part to be observed at different times k (k = [0.. .M], M integer) preceding and following the mechanical stress, 20) transforming the input data so as to obtain two-dimensional output data and representing these output data in a two-dimensional representation frame (RI) of direction vectors x, Y so as to obtain a series of images Ik representative of the deformation fields, each of the images Ik comprising a useful zone ZUk, 30) estimating an area Ak of ​​each of the useful zones ZUk in order to obtain a function f(k) = Ak representative of the evolution of the area as a function of time, 40) determining the area or areas Ak for which f(k) is greater than or equal to a threshold 0^ characteristic of damage and determining a first instant kei for which f(kei) is greater than or equal to ÿ, said first instant kei being associated with a first damage.

2. Method (1) according to claim 1, further comprising, following step 40), the following steps: 50) analyzing the deformation fields associated with the instants k < kei preceding the first damage and determining reference parameters representative of the deformation fields before the first damage, 60) comparing the reference parameters with the deformation fields associated with the instants k > kei following the first damage in order to determine a physical quantity characteristic of the damage to the object.

3. Method (1) according to claim 2, further comprising, following step 50) or step 60), the following step: 70) comparing the reference parameters with pre-existing parameters representative of a material from which the part is made.

4. Method according to any one of claims 2 to 3, in which the mechanical stress is carried out during a dynamic fatigue test, the physical quantity to be determined during step 60) serving to characterize the dynamic fatigue of the material.

5. Method (1) of tracking according to claim 4, in which step 40) comprises the following sub-steps: 42) determining a maximum f(k) = Ak>maï of the function f(k) = Axy>k, 44) defining the damage threshold as a function of the maximum fmax (k) determined during the previous step, 46) determining the first instant kei for which f(kei) is greater than or equal to the damage threshold 0^.

6. Method (1) according to claim 5, in which the damage threshold 0^ defined during step 44) represents between 20% and 60% of kKmax.

7. Method (1) according to any one of claims 4 to 6, in which step 60) comprises the following sub-steps: 62) analyzing the Ikei image of the deformation fields associated with the instant k = kei of the first damage in order to determine the force, the stress, a displacement or a deformation of the material, 64) estimating a threshold 0; characteristic of an endurance limit from the area Akei associated with said first instant kei.

8. Method (1) according to any one of claims 2 to 3, in which the mechanical stress is carried out during a dynamic impact test, the physical quantity to be determined during step 60) serving to characterize the dynamic impact on the material.

9. Method (1) according to claim 8, in which step 40) comprises the following sub-steps: 42') identifying the area A; associated with an instant k = k; (k; < ke) of an impact, 44') defining the damage threshold 0^, said damage threshold corresponding to an area Akei, Akei > Ai, the value of which differs by at least 25% with respect to Ai, 46') identifying the first instant kel for which f(kel) = 0.

10. Method (1) according to any one of claims 8 to 9, in which step 60) comprises the following sub-steps: 62') analyzing the Ikel image of the deformation fields associated with the instant k = kel of the first damage, 64') estimate a damage threshold 0^ characteristic of an energy limit or dynamic load or dynamic deformation of the material.

11. Device for implementing a method (1) according to one of the preceding claims, comprising means for implementing by computer at least steps 20) to 40).

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