METHOD AND SYSTEM FOR CREATING A COVE

Abstract

The invention relates to a method (400) for creating a fatigue crack (900) in a test piece (102) by means of a fatigue testing device (602) on which the test piece is arranged, the method (400) comprising the following steps: starting the fatigue test (502) of the test piece; checking the opening (504) of a fatigue crack in an intermediate zone, between a first (2001) and a second (2002) end of the test piece, the second end being opposite the first end; measuring (506) at least one value of a parameter of the crack in the intermediate zone; comparing (508) the measured value with at least one predefined value; and automatically stopping (510) the fatigue test when the measured value corresponds to the predefined value. Figure for abstract: Figure 5

Description

Title of the invention: METHOD AND SYSTEM FOR CREATING A COVE Technical field of the invention

[0001] The present invention relates to the field of treating fatigue cracks in a mechanical part. The invention relates more particularly to a method and a system for creating fatigue cracks in a mechanical part, in particular an aircraft. Technological background

[0002] A mechanical part 100 subjected to certain operating conditions, for example in an aircraft turbojet, may exhibit, over time, fatigue cracks 900. Such cracks generally appear in the form of a crack appearing on the surface 302 of the part, as illustrated for example in [Fig.l], and which may propagate or extend in one direction via at least one of its ends, called the propagation end.

[0003] During the manufacture of mechanical parts, for example, and in a non-limiting manner, aircraft turbomachines, cracks may also appear on the surface of these parts. For example, and in a non-limiting manner, it has been found that a process for manufacturing parts by laser fusion on a powder bed of the LBM type (Laser Beam Melting in English) can produce a degraded surface condition because of the partially fused powder grains on the surface and the fact that the fusion is carried out layer by layer (step effect).

[0004] NDT (Non-Destructive Testing) type studies were carried out on these degraded surfaces, particularly for the detection of cracks.

[0005] In order to improve the detection of cracks, their treatment and thus the resistance of mechanical parts, it is necessary to be able to create cracks with controlled dimensions (for example: opening width, length of the crack). The study of these cracks thus created can allow the implementation of more effective detection and treatment methods.

[0006] One solution for creating cracks is to make a notch in a mechanical part using an electroerosion or EDM (Electrical Discharge Machining) process. EDM is a machining process that involves removing material from a part using electrical discharges. These notches will create openings from which cracks are created. [Fig. 2] illustrates an example of a crack 900 obtained by this type of process on the surface 302 of a part 100. At the edge 306 of the notch 304 made, an opening 308 can be observed from which the crack 900 is created.

[0007] However, the openings generated in this method have linear shapes (triangular or square) unlike those of cracks actually observed which are generally angular. In addition, the openings are larger than those of cracks actually observed (10 to 20 pm compared to 1 to 5 pm on cracks of the same lengths).

[0008] Furthermore, this process can degrade the condition of the surface of the part, since the creation of the notches can cause crushing of the surrounding surface.

[0009] Another solution is to create a first notch via the EDM process mentioned above, then to make stress cycles to create a crack at the notch. These are tests carried out to define the cracking laws (for example the Paris law).

[0010] By cycle, we mean a pattern of constraints (for example: traction, compression, bending, etc.), spread over a period of time and which is intended to be repeated many times.

[0011] However, this method does not allow the crack opening to be controlled except by stopping during the test. Furthermore, when the test is resumed, the initial notch may mask any crack initiation.

[0012] It has also been found that when applied to parts, its geometry must be modified by integrating the notch (creating a modified notch can modify the geometry of the parts).

[0013] In addition to these above-mentioned drawbacks, the cost of implementing this solution is quite high.

[0014] Another solution for creating cracks directly (i.e. without first making a notch) is to carry out a fatigue test in which stresses are applied to a mechanical specimen or a mechanical part. During this test, cracks will appear close to the fracture zone of the specimen or part due to the applied stresses. The cracks, on the specimen or part, generally have one end starting at the fracture zone.

[0015] However, a problem with this type of test is that it is not possible to control the dimensions of the cracks created, the tests being carried out until the test piece or part breaks.

[0016] There is therefore no current solution for creating cracks on test pieces or parts, which meet the constraints mentioned above.

[0017] It may thus be desirable to provide a method which makes it possible to overcome at least some of the aforementioned problems and constraints. Summary of the invention

[0018] A method is therefore proposed for creating a fatigue crack in a test piece by means of a fatigue testing device, this device comprising a first member to which a first end of the test piece is connected and a second member to which a second end of the test piece opposite the first end is connected, the device being configured to move the first and second members relative to each other and carry out a fatigue test of the test piece, characterized in that the method comprises the following steps: - start of the fatigue test of the specimen; - control of the opening of a fatigue crack in an intermediate zone, between the first and second ends of the test piece; - measurement of at least one value of a parameter of the crack in said intermediate zone; - comparison of said measured value with at least one predefined value; and - automatic stopping of the fatigue test when the measured value corresponds to the preset value.

[0019] Advantageously, the fatigue test can be carried out with monitoring of the opening of a crack. The test is interrupted before the specimen ruptures, when the desired crack opening is obtained.

[0020] Thus, thanks to the invention, it is possible to create cracks by facilitating the control of the parameters of the cracks and in particular their dimensions.

[0021] The invention also makes it possible to reduce the costs of creating cracks. Indeed, controlling the dimensions of the crack makes it possible to avoid repeating fatigue tests and therefore the use of new mechanical test pieces for creating cracks.

[0022] The invention may further comprise one or more of the following optional features, in any technically possible combination: - the control is carried out with an extensometer; - at least the fatigue testing and control stages are carried out simultaneously; - the control step includes at least the detection of the start of the crack opening; - the measurement step is carried out upon detection of the start of the crack opening; - the measured parameter is a value of a crack opening width measured along an elongation axis of the test piece or a value of a crack length in a direction transverse to this elongation axis; - which the fatigue test is carried out at room temperature; - the fatigue test is preferably carried out with a load ratio R= 0.05, the load ratio being the ratio between a minimum stress and a maximum stress applied to the specimen; - the fatigue test is carried out according to an ASTM E466 standard with a frequency between 1 and 100 Hz, and preferably between 1 and 50 Hz; - the fatigue test is a tensile test;

[0023] The invention also relates to a system for creating a fatigue crack in a test piece for implementing a method as described above, the system comprising: - a test specimen fatigue testing device, this device comprising a first member to which a first end of the specimen is connected and a second member to which a second end of the specimen opposite the first end is connected, the device being configured to move the first and second members relative to each other and carry out a fatigue test of the specimen; - a device for controlling the opening of a fatigue crack in an intermediate zone, between the first and second ends of the test piece; - a device for measuring at least one value of a parameter of the crack in said intermediate zone; - a module for comparing said measured value with at least one predefined value; and - an automatic fatigue test stop unit when the measured value corresponds to the predefined value.

[0024] The crack creation system may further comprise one or more of the following optional features, in any technically possible combination: - the control device comprises at least one crack opening sensor; - the control device is an extensometer; - the measuring device includes distance measuring means. Brief description of the figures

[0025] 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: - [Fig. 1] illustrates a partial view of a mechanical part comprising on its surface at least one fatigue crack; - [Fig.2] illustrates a partial view of a mechanical part comprising on its surface a crack obtained according to a method of the prior art; - [Fig.3A] illustrates a first example of a test piece on which the method according to the invention can be applied; - [Fig.3B] illustrates a second example of a test piece to which the method according to the invention can be applied; - [Fig.4] is a schematic representation of a system for creating cracks according to the invention; - [Fig.5] is a schematic representation of the crack creation method according to the invention; - [Fig.6A] is a schematic representation of the arrangement of a test piece in the crack creation system before implementing the method according to the invention; - [Fig.6B] is a schematic representation of the creation of a crack on a portion of the test piece of [Fig.6A]; - [Fig.7] is a representation of an example of a curve for monitoring the displacement of a member of the fatigue testing device as a function of the number of cycles during which a stress is applied to the test piece according to the method of the invention. Detailed description of the invention

[0026] Figures 3A and 3B show examples of mechanical test pieces 102 on which fatigue cracks 900 are to be created.

[0027] The test piece 102 comprises at least a first end 200i and a second end 2002 opposite the first end 200i. The test piece further comprises at least one central or intermediate body 202 connecting the first 200i and second 2002 ends.

[0028] The test piece 102 may have a cylindrical shape or a flat shape as illustrated respectively in [Fig. 3A] and 3B. However, those skilled in the art will understand that this example is not limiting and the test piece 102 may have different shapes, such as for example a rectangular shape, etc.

[0029] With reference to [Fig.4], the fatigue crack creation system will now be described.

[0030] The creation system 600 comprises a fatigue testing device 602 of the test piece 102, a control device 604, a measuring device 606, a comparison module 608 and a unit 610 for stopping the fatigue test. The system 600 may further optionally comprise a data storage module 612.

[0031] The fatigue testing device 602 makes it possible to exert a stress on the mechanical test piece 102.

[0032] Still with reference to [Fig. 4] (but also to FIGS. 6A and 6B), the fatigue testing device 602 comprises at least a first member 700i to which a first end 200i of the test piece 102 is connected and a second member to which a second end 2002 of the test piece 102 opposite the first end 200i is connected, the device 602 being configured to move the first 700i and second 7002 members with respect to each other and carry out a fatigue test of the test piece 102. Preferably, the fatigue test is a tensile test.

[0033] The displacements MVb MV2 of the first member 700i relative to the second member 7002 of the fatigue testing device 602 (as illustrated in [Fig.6A] and 6B) have the effect of stretching the mechanical test specimen 102. This displacement is carried out over a given number Nc of cycles. A cycle is by definition a stress pattern, spread over a time range and which is intended to be reproduced many times. In the illustrative examples of FIGS. 6A and 6B, for example, the stress corresponds to a tension and the pattern corresponds to the application of a maximum stress in a first displacement MVi of the first member 700i then, to the application of a minimum stress in a second displacement MV2 of the first member 700i.

[0034] Still with reference to [Fig. 4], the control device makes it possible to detect the opening of a fatigue crack 900 in an intermediate zone of the body of the test piece 102, between the first 200i and second 2002 ends. The control device can be arranged directly on the test piece 102, in the intermediate zone, or remotely.

[0035] The control device 604 can thus comprise a sensor or means for detecting a crack opening 900.

[0036] Preferably the control device 604 is an extensometer. The extensometer may be a contact or remote extensometer.

[0037] In another variant (not shown), the detection of the opening can be done by controlling or measuring the displacement of the first member 700i or of the second member 7002. In the example of FIGS. 6A and 6B, the first member 700i moves relative to the second member 7002 and, a control of its displacement can make it possible to detect an opening of crack 900. In this configuration, the control device can comprise a sensor configured to measure the displacement of the first member 700i (or of the second member in the case where it is the second member which moves).

[0038] The measuring device 606 is capable of measuring at least one value of a parameter of a crack in the intermediate zone. It can be connected to the control device 604 or directly integrated therein. The measuring device 606, which is connected to the control device 604, is activated as soon as the control device 604 detects the opening OV1 of a crack 900 in the intermediate zone. Preferably, the measuring device 606 comprises means for measuring the distance between two reference points of the intermediate zone.

[0039] In the embodiment illustrated in [Fig.5], the measured parameter is, preferably, a value of an opening width LCi of the crack measured along an elongation axis of the test piece or, a value of a length LC2 of the crack in a direction transverse to this axis of elongation.

[0040] The comparison module 608, connected to the measuring device 606, makes it possible to compare the value measured by the measuring device with at least one predefined value. To this end, the system 600 according to the invention here comprises a storage module 612 comprising, for example, a database in which at least one predefined value of a parameter of the crack is stored.

[0041] In certain embodiments, the control device 604 is capable of carrying out a control of the fatigue test, a measurement of a value of a parameter of the crack 900 and a comparison of this measured value with a predefined value of the parameter. In this case, the measurement means 606 and comparison means 608 can be directly integrated into the control device.

[0042] The stopping unit 610, which is connected to the comparison module 608, makes it possible to stop or halt the fatigue test when the measured value LCi, LC2 corresponds to the predefined value. Preferably, the stopping is automatic, that is to say that the stopping unit can comprise a computer program coupled to the fatigue testing device 602 and whose implementation by a processor of the stopping unit makes it possible to automatically activate a command for stopping the test. The stopping can also be manual. In the latter case, the stopping module can comprise, for example and in a non-limiting manner, an alert unit for emitting a signal (sound or text) making it possible to alert an operator so that the latter stops or halts the fatigue testing device 602.

[0043] The method 400 according to the invention will now be explained with reference to FIGS. 5, 6A and 6B.

[0044] The method 400 according to the invention applies to mechanical test pieces 102 such as, for example, and in a non-limiting manner, those of FIGS. 3A and 3B.

[0045] With reference to [Fig. 5], the method 400 according to the invention comprises a preliminary step 500 during which the mechanical test piece 102 is arranged on the fatigue testing device 602 as can be seen in [Fig. 6A]. A first end 200i of the test piece 102 is connected to the first member 700i of the testing device 602 and a second end 2002 of the test piece 102, opposite the first end 200i, is connected to the second member 7002 of the fatigue testing device 602. The control device 604 is either brought into contact with the test piece 102 (as illustrated in [Fig. 6A] and 6B) or positioned at a predefined distance from the test piece (not shown). The measuring devices 606 and comparison devices 608 are then connected to each other and to the control device 602.

[0046] After the installation or arrangement of the test piece 102 in the crack creation system 600 900, a fatigue test 502, applied to the test piece 102, begins. During this test 502, a stress is applied to the test piece 102. Preferably, Fatigue test 502 is a tension test.

[0047] Preferably, the fatigue test is carried out at room temperature, preferably with a load ratio R= 0.05, the load ratio being the ratio between a minimum stress and a maximum stress applied to the test piece 102.

[0048] During the fatigue test, as a tension is applied, a step 504 of checking an intermediate zone between the first and second ends 200i, 2002 of the test piece 102 is carried out in order to detect an opening OVi in the intermediate zone. Thus, the fatigue test steps 502 and checking steps 504 are carried out simultaneously.

[0049] When, in the illustrated embodiment of the invention, the fatigue testing device 602 performs the tension, the intermediate zone of the test piece 102 is subjected to stresses which cause an opening OVi on the body of the test piece 102 in the intermediate zone.

[0050] As soon as the control device detects the start of an opening in the intermediate zone, a measurement step 506 of a parameter of the crack 900, for example the width LCi of the opening or the length LC2 of the crack, is then carried out with the control device (preferably an extensometer). Preferably, the fatigue test is carried out according to an ASTM E466 standard (“Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials” in English) with a frequency of between 1 and 100 Hz so as to allow the detection of the start of the opening of the crack 900. Preferably, the frequency is between 1 and 50 Hz.

[0051] The measured value is then compared to the predefined value in a step 508.

[0052] The fatigue test is stopped 510 when the measured value corresponds to the value predefined, that is to say when it is sufficient to guarantee the creation of a crack 900 with the desired dimensions. This stop 510 consists of stopping the movement of the first member 700i of the fatigue testing device 602.

[0053] It is noted that in the examples illustrated in FIGS. 6A and 6B, the measuring devices 606 and comparison devices 608 (not shown) are integrated into the control device 604.

[0054] Advantageously, the system is capable of creating cracks 900 for predefined values ​​greater than or equal to 2 pm. In other words, the system is capable of creating cracks 900 of very small dimensions but also of larger dimensions (for example, and in a non-limiting manner, for predefined values ​​greater than 1 mm).

[0055] [Fig. 7] illustrates, for example, a curve tracking the displacement of the first member 700i of the test device 602 with respect to the second member 7002 as a function of the number of cycles. Curve 1 represents a minimal displacement, during which a minimum stress is applied, and curve 2, a maximum displacement, during which a maximum stress is applied. The end 3 of curve 2 corresponds to the stopping of the test device 600, that is to say when the value of the opening width LCi of the crack measured along an elongation axis of the test piece 102 or, the value of length LC2 of the crack in a direction transverse to this elongation axis, is equal to the predefined value.

[0056] Thus, advantageously the fatigue test can be carried out with monitoring of the displacement of at least one member of the fatigue testing device. The test is also interrupted before the rupture of the test piece, when the desired crack opening is obtained.

[0057] Advantageously, those skilled in the art will understand that the invention makes it possible to improve the creation of cracks by facilitating the control of the dimensions of these cracks.

[0058] The invention also makes it possible to reduce the costs of creating cracks. Indeed, controlling the dimensions of the crack makes it possible to avoid repeating fatigue tests and therefore the use of new mechanical test pieces for creating cracks.

Claims

Claims

1. Method (400) for creating a fatigue crack (900) in a test piece (102) by means of a fatigue testing device (602), this device comprising a first member (700i) to which a first end (200i) of the test piece (102) is connected and a second member (7002) to which a second end (2002) of the test piece (102) opposite the first end (200i) is connected, the device (602) being configured to move the first (700i) and second (7002) members relative to each other and carry out a fatigue test of the test piece (102), characterized in that the method (400) comprises the following steps: - start of the fatigue test (502) of the test piece (102); - checking the opening (504) of a fatigue crack (900) in an intermediate zone, between the first (2000) and second (2002) ends of the test piece (102); - measuring (506) at least one value of a parameter of the crack (900) in said intermediate zone;- comparison (508) of said measured value with at least one predefined value; and - automatic stopping (510) of the fatigue test (502) when the measured value corresponds to the predefined value.;

2. A method (400) of creating a fatigue crack (900) according to claim 1, wherein the control (504) is carried out with an extensometer.

3. A method (400) of creating a fatigue crack (900) according to claim 1 or 2, wherein at least the fatigue testing (502) and monitoring (504) steps are performed simultaneously.

4. Method (400) of creating a fatigue crack (900) according to one of claims 1 to 3, in which the control step (504) comprises at least detecting the start of the opening of the crack.

5. Method (400) of creating a fatigue crack (900) according to one of claims 1 to 4, in which the measuring step (506) is carried out upon detection of the start of the opening of the crack (900).

6. Method (400) for creating a fatigue crack (900) according to one of claims 1 to 5, in which the measured parameter is a value of an opening width (LCi) of the crack measured along an elongation axis of the test piece or, a value of a length (LC2) of the crack in a direction transverse to this axis of elongation.

7. A method (400) of creating a fatigue crack (900) according to one of the preceding claims, wherein the fatigue test (502) is carried out at room temperature.

8. Method (400) of creating a fatigue crack (900) according to one of the preceding claims, in which the fatigue test (502) is carried out, preferably, with a load ratio R= 0.05, the load ratio being the ratio between a minimum stress and a maximum stress applied to the test piece (102).

9. A method (400) of creating a fatigue crack (900) according to one of the preceding claims, wherein the fatigue test (502) is carried out according to an ASTM E466 standard with a frequency between 1 and 100 Hz.

10. A method (400) of creating a fatigue crack according to one of the preceding claims, wherein the fatigue test (502) is a tensile test.

11. System (600) for creating a fatigue crack in a test piece (102) for implementing a method (400) according to one of the preceding claims, this system comprising: - a fatigue testing device (602) for the test piece (102), this device comprising a first member (700i) to which a first end (2000) of the test piece (102) is connected and a second member (7002) to which a second end (2002) of the test piece (102) opposite the first end (2000) is connected, the device (602) being configured to move the first (7000) and second (7002) members relative to each other and carry out a fatigue test of the test piece (102); - a device (604) for controlling the opening of a fatigue crack (900) in an intermediate zone, between the first (2000) and second (2002) ends of the test piece (102);- a measuring device (606) of at least one value (Lcb LC2) of a parameter of the crack (900) in said intermediate zone; - a comparison module (608) of said measured value (LCi, L C2) with at least one predefined value; and - an automatic stopping unit (610) of the fatigue test; when the measured value (LCi, LC2) corresponds to the predefined value.

12. System (600) for creating a fatigue crack according to the preceding claim, in which the control device (604) is an extensometer.

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