METHOD AND SYSTEM FOR MANUFACTURING A TESTINK

Abstract

The invention relates to a method for manufacturing a specimen (100) intended to be subjected to stress in a fatigue or dynamic cracking test, the stress being applied to one end of the specimen along an axis of elongation of the specimen, the method comprising at least one dimensioning step including the following sub-steps implemented by computer: the reception (a1) of initial parameters Pi1-N representative of an initial geometric structure of the specimen and of the physical characteristics of a material in which the specimen is to be made; the determination (b) of a displacement field generated by the stress; the calculation (c) of a stress field as a function of the determined displacement field; the verification (d) of the homogeneity of the stress field including at least the storage (e4) of final dimensioning parameters Po1-N for which the stress field is homogeneous.Figure for the abbreviation: Figure 3B.

Description

Title of the invention: METHOD AND SYSTEM FOR MANUFACTURING A TEST SPECIMEN Technical field of the invention

[0001] The present invention relates to the manufacture of a test specimen. More particularly, the invention relates to a method and a system for manufacturing a test specimen for carrying out fatigue or cracking tests under dynamic mechanical conditions. Technological background

[0002] A mechanical part subjected to certain operating conditions in an aircraft turbine may, over time, develop defects that can affect the part's service life and therefore the structure and / or performance of the turbine. These defects may, for example, but are not limited to, fatigue cracks. Such cracks appear, for instance, as a fissure on the surface of the part that can propagate or extend in one direction from at least one of its ends, known as the propagation end. In some cases, cracks may also propagate within the material, for example, in the case of a material defect.

[0003] It is therefore essential to be able to characterize the lifespan of the mechanical parts that make up the structure of such a turbomachine in order to improve its manufacture and performance.

[0004] The methods and models for evaluating the service life of aeronautical parts and their resistance under exceptional stresses are continually evolving in order to extend the service lives of engines in operation.

[0005] These evaluation methods may include experimental fatigue / cracking tests carried out on mechanical specimens and / or numerical calculations.

[0006] Figures IA and IB show examples of mechanical specimens 102 on which fatigue / cracking tests can be carried out.

[0007] The test specimen 100 comprises at least a first end 102i and a second end 1022 opposite the first end 102i. The test specimen further comprises at least one central or intermediate body 1023 connecting the first 102i and second 1022 ends.

[0008] The test specimen 100 may, for example, and without limitation, have a cylindrical shape or a flat shape as illustrated respectively in [Fig.1A] and [Fig.1B]. The test specimen 100 may, for example, be made of metallic or composite material.

[0009] Existing test devices for characterizing the service life of parts mechanical components include: - traditional machines for carrying out mechanical tests based on electromechanical or hydraulic technologies; - vibrophore-type machines, based on magnetic technologies; and - so-called ultrasonic machines, based on piezoelectric technologies electric.

[0010] Traditional machines / vibration testers are the most common and offer the advantage of flexibility regarding the type of geometry that can be subjected to stress. However, these technologies allow testing at stress frequencies ranging from tens of Hz, for example, 50 Hz (without limitation), to hundreds of Hz. These stress frequencies are a limiting factor in characterizing the lifespan of parts / specimens. Fatigue / cracking tests performed with these devices are called quasi-static.

[0011] As for ultrasonic machines, they allow for the performance of so-called dynamic fatigue / cracking tests. The specimen 100 or the part is generally subjected to stress with a frequency on the order of 100 Hz to 30 kHz and, preferably, 20 kHz.

[0012] Fig. 1C illustrates an example of an ultrasonic device 700. The device 700 is, for example, a piezoelectric device.

[0013] The piezoelectric device 700 comprises at least one computer 702, a current generator 704 and a piezoelectric machine 706.

[0014] The piezoelectric machine comprises at least one piezoelectric converter 706a and a sonotrode 706b. The first end 102i of the test specimen 100 is fixed to one end of the sonotrode 706b, the second end 1022 of the test specimen 100 being free of constraint.

[0015] The computer 702 includes at least one processor for implementing a program to control the generator 704 and the machine 706. The program implementation allows at least for an electrical control signal to be applied to the generator 704. The generator 704 receives the control signal and generates a sinusoidal electrical signal at a given frequency f0 between 100 Hz and 30 kHz. Preferably, the frequency is 20 kHz.

[0016] The piezoelectric converter 706a of the machine 706 is excited by the generator 704 and transforms the electrical energy into a longitudinal ultrasonic mechanical vibration wave at the same frequency f0. There are two feedback signals from the converter 706a, one for the amplitude and the other for the frequency of the vibration. The amplitude of the wave is generally small.

[0017] The sonotrode 706b of the machine 706 allows the value of the amplitude of the stress to which the specimen / part 100 is subjected to be amplified.

[0018] The stress is applied to the first end of the specimen along an elongation axis X of the specimen, the second end of the specimen opposite to the first end being left free in the simplest stress cases.

[0019] In more complex cases, the second end of the specimen may not be free, as illustrated for example in [Fig.1D]. In this case, the ultrasonic device 700 may include a fixing member 706c on which the second end of the specimen 100 is fixed.

[0020] Although ultrasonic machines have the advantage of performing tests at frequencies of several tens of thousands of Hertz, they also have certain disadvantages such as, for example: - a more complex dimensioning of the test specimens (dynamic dimensioning according to a resonance mode). This dimensioning consists of: • assuming generally elastic isotropic behavior (i.e., no dissipation), although other types of constitutive law may apply; • to determine the resonance modes of the test specimen via a finite element calculation tool; • to search for the resonance mode corresponding to the type of stress desired (typically tensile-compressive loading); • to iterate on the geometry of the specimen so that the frequency of this natural mode is in phase with that of the stress generated by the 706a converter of the piezoelectric machine, i.e. f0; - a heterogeneous stress field in an area of ​​interest of the specimen, called the useful zone and which includes the intermediate part 1023 of the specimen.

[0021] Fig. 1E illustrates an example of a dimensioned specimen 100. The specimen, of length 2L, comprises a first end 1021 and a second end 1022 of identical length LT, and an intermediate part 1023 which corresponds to the useful zone and which has a length of LZU.

[0022] Figure 1F represents a displacement field u and a stress field obtained by subjecting the specimen of Figure 1E to dynamic loading. As can be seen, the stress field on this specimen geometry 100, particularly in the useful zone, is heterogeneous with a maximum at the center of the specimen 100.

[0023] Applying a dynamic load to the first end of the specimen induces a deformation that can cause stretching or contraction along the elongation axis. Thus, as illustrated in [Fig. 1G], a point M of the specimen which is in a first configuration (for example, at time t=t0 before loading) (quotation), at an initial position xO on the elongation axis, can at a time t, after loading, end up at a position xl. The difference u between the positions xl and xO is called the displacement. When considering all points M along the elongation axis, the set of displacements u(M) constitutes the displacement field generated by the loading.

[0024] Studies to characterize the lifespan of test specimens / parts frequently lead to studies of the material using tools such as microscope or SEM (Scanning Electron Microscope).

[0025] In order to facilitate these analyses which can lead to an improvement in the resistance of mechanical parts, for example of an aircraft turbomachine, and therefore in the performance of the turbomachine, it is appreciated to have a useful area presenting a homogeneous stress field (absence of stress gradient) or distributed according to industrial needs, for example to be representative of an aircraft part.

[0026] It may therefore be desirable to provide a test specimen that overcomes at least some of the aforementioned problems and constraints. Summary of the invention

[0027] A method for manufacturing a test specimen intended to be subjected to stress in a fatigue or dynamic cracking test is therefore proposed, the stress being applied to a first end of the specimen along an axis of elongation of the specimen, the specimen further comprising a second end of the specimen opposite the first end, the method comprising at least one dimensioning step of the specimen comprising at least the following sub-steps implemented by computer: - the receipt of a number of initial parameters representative of an initial geometric structure of the specimen and the physical characteristics of a material in which the specimen is to be produced, the parameters representative of the structure including at least: • a value representing the distance separating, along the elongation axis, the first and second ends of the specimen, and • a value of a section of the specimen along the elongation axis, in a plane normal to said axis; - the determination of a displacement field generated along the elongation axis by the applied stress with a given amplitude and frequency, as a function of the value of the section, and of at least one of the physical characteristics of the material of the specimen; - the calculation of a stress field as a function of the determined displacement field; - verification of the homogeneity of the stress field along the elongation axis, including: • if the constraint field is not homogeneous: • the iterative updating of the cross-sectional value of the specimen; and • the repetition of the steps of determining a displacement field, calculating a stress field, and verifying the homogeneity of the stress field, • if the stress field exhibits homogeneity in a zone along the elongation axis: • determining a value for the width of the homogeneity zone; • the storage of a set of final sizing parameters including at least the value of the width of the homogeneity zone, the value of the section for which the stress field is homogeneous in said zone and the value of the length of the specimen.

[0028] Thus, thanks to the invention it is possible to dimension the test specimens before their manufacture, so as to control the shape of the stress field of the test specimens obtained.

[0029] The invention thus makes it possible to improve the reliability of fatigue / cracking tests for characterizing the service life of mechanical parts.

[0030] The invention may further include one or more of the following optional features, in any technically feasible combination: - the physical characteristics of the specimen material include at least a Young's modulus and a mass density of the material; - the calculation of the displacement field is carried out by fixing for the displacement field a maximum amplitude value equal to the amplitude of the stress and a resonance frequency equal to the frequency of the applied stress; - The displacement field to be determined is defined by the relation: [Math.l] u(x,t) = U(x) * sirt(wt) Or : • U is the amplitude of a displacement and is a function of the amplitude Uo of the applied stress; and • w = 2irf is a resonance mode of the specimen which is proportional to the frequency f of the applied stress; - The displacement field is determined via the implementation of the relation: [Math.2] du2 y / d^u , dt-t dS :¾ 1 \ _ d / 2 \ dx2 dx dx $(x) / ' u(x = L, t) = Uo sin(wi) dx \ 7 / Or : u is the displacement, liÇx — L, t) is the displacement at the first end of the test specimen; —(x = -L. t] is the value of the deformation caused by the stress at dx \ ' / second end of the test tube; c = ^ËJp is a characteristic velocity of the specimen material, with E and p respectively the Young's modulus and the volumetric mass density of the material, • t the time, • x, a coordinate along the elongation axis (X), and • S(x) a value of the cross-section of the specimen at the x coordinate; - The calculation of a constraint field (o) is performed via the relation: [Math.3] du a=E dï where E is the Young's modulus of the material and u is the determined displacement field; - the applied stress is carried out at a frequency between 10 and 30KHz; - the final stored parameters are transmitted to a manufacturing system; - the process also includes a step of manufacturing the test specimen according to the final sizing parameters stored; - the test specimen is made of a composite or metallic material.

[0031] The invention also relates to a device for manufacturing a test tube, the manufacturing device comprising at least one computer system configured to execute steps of the process as described above, and comprising at least one memory for data storage and a processing unit.

[0032] The manufacturing device may further include one or more of the following optional features, in any technically possible combination: - the computer system includes at least one means of communication; - the manufacturing device includes a manufacturing system capable of manufacturing a test specimen according to the final dimensioning parameters stored by the computer system; - The manufacturing system is a computer-aided system.

[0033] The invention also relates to a computer program downloadable from a communication network and / or recorded on a computer-readable medium, the program comprising instructions for the execution of at least one of the steps of the manufacturing process described above, when said program is executed on a computer system. Brief description of the figures

[0034] The invention will be better understood with the aid of the following description, given solely by way of example and made with reference to the accompanying drawings in which: - [Fig. 1A] illustrates a first example of a test specimen according to the prior art, - Fig. 1B illustrates a second example of a test tube according to the prior art; - Fig. 1C is a schematic representation of a test device transonic; - Fig. 1D is a schematic representation of an ultrasonic testing device; - [Fig. 1E] illustrates an example of a specimen dimensioned for an ultrasonic fatigue test according to the prior art; - Fig.1F illustrates a curve of the displacement and stress fields obtained for the specimen in Fig.1E subjected to an ultrasonic fatigue test; - the [Fig.lG] is an illustration of a displacement following a stimulus; - [Fig.2] is a schematic representation of a device for manufacturing a test tube according to an example of the invention; - [Fig.3A] is a schematic representation of the manufacturing process of a test specimen implemented by the device of [Fig.2]; - [Fig.3B] is a schematic representation of a dimensioning step of a test specimen according to the invention; - [Fig.3C] is a schematic representation of a geometric profile of a test specimen according to an example of the invention; - [Fig.3D] illustrates a curve representing the stress field obtained for the specimen in [Fig.3C]; - [Fig.3E] is a schematic representation of a geometric profile of a cracked test specimen according to an example of the invention; - [Fig.4] is a schematic representation of a manufacturing process according to an example of the invention, followed by a test process. Detailed description of an example embodiment of the invention

[0035] With reference to Figures IA and IB, a test specimen 100 comprises at least one first end 102i and a second end 1022 opposite the first end 102i. The specimen further comprises at least one central or intermediate body 1023 connecting the first 102i and second 1022 ends.

[0036] The stress being applied to the first end of the specimen along an axis (X) of elongation of the specimen, the second end of the specimen opposite to the first end can be free (as illustrated in [Fig.1C]) or held fixed (as illustrated in [Fig.1D]).

[0037] With reference to [Fig.2], a device 200 for manufacturing a test specimen 100 intended to be subjected to stress in a fatigue or dynamic (or ultrasonic) cracking test will now be described.

[0038] The test tube manufacturing device 200 includes at least one computer system 202, and a test tube manufacturing system 204 for the test tube 100.

[0039] The computer system 202 makes it possible to determine optimal parameters for the geometric structure of the specimen 100.

[0040] Still with reference to [Fig.2], the computer system 202 comprises at least one human-machine interface 202;, a data processing unit 202p and a main memory 202mp, the interface 202; and the memory 202m being accessible by the processing unit 202p.

[0041] The human-machine interface 202;, for example and without limitation a keyboard, a mouse or a touch screen, is configured to allow an operator to provide or communicate to the computer system 202, a number N of initial parameters Pii N representative of an initial geometric structure of the specimen and physical characteristics of a material in which the specimen is to be made.

[0042] The parameters representing the geometric structure include at least a value 2L of a distance separating, along the elongation axis X, the first 102i and second 1022 ends of the specimen and a value of a section S, Si>2 of the specimen along the elongation axis, in a plane normal to the axis.

[0043] The parameters representative of the physical characteristics include at least a Young's modulus E and a volumetric mass density p of the material.

[0044] The main memory 202mp is capable of storing data including at least the initial parameters Pii N provided or communicated by the operator via the interface 202; of the computer system 202.

[0045] The memory may further include at least one computer program containing instructions relating to the determination of optimal geometric parameters Poi N of the test specimen to be manufactured, for the processing unit 202p.

[0046] The main memory 202mp is, for example, and in a non-limiting way, a RAM (from the English "Random Access Memory").

[0047] The processing unit 202pest is capable of executing the instructions of the program in format stored in the main memory 202mp for determining the optimal geometric parameters Poi N of the specimen to be manufactured.

[0048] The processing unit is, for example, but not limited to, a processor or microprocessor.

[0049] The computer system 202 may also optionally include a secondary memory accessible by the processing unit for recording the determined optimal parameters.

[0050] The computer system may further include, for example, a network interface (not shown) and / or a computer-readable medium, such as, for example, a local medium (such as a local hard drive) or a remote medium (such as a remote hard drive accessible via the network interface through a communication network) or a removable medium (such as a USB key, from the English "Universal Serial Bus", or a CD, from the English "Compact Disc" or a DVD, from the English "Digital Versatile Disc") readable by means of an appropriate reader of the computer system (such as a USB port or a CD and / or DVD disc drive).

[0051] Thus, the computer program containing the instructions for the processing unit 202p can be saved on the medium and / or downloaded via the network interface, the program being intended to be loaded into the main memory.

[0052] Still with reference to [Fig.2], the manufacturing system 204 which communicates with the computer system 202 is capable of manufacturing the test specimen 100 from the optimal parameters determined by the processing unit of the computer system.

[0053] The manufacturing system 204 includes at least one control device 204a and one manufacturing device 204b.

[0054] The control device 204a is capable of communicating with the computer system to receive data and in particular the optimal parameters determined by the processing unit of the computer system 202.

[0055] The control device 204a is also configured to control the manufacturing of the test specimen by the manufacturing device 204b from the received data.

[0056] The control device is, for example, but not limited to, a computer or a computer system.

[0057] The manufacturing device 204b which is connected to the control device is capable of receiving control signals transmitted by the control device and of manufacturing the test specimen on the basis of the optimal parameters determined by the processing unit of the computer system and of a block of the material whose physical characteristics were used for the determination of the optimal parameters of the tool.

[0058] The manufacturing device is, for example, and without limitation, a 3D printer.

[0059] Thus, preferably, the manufacturing system is a computer-aided manufacturing system.

[0060] Alternatively, the manufacturing system 204 may include at least one interface (e.g., a screen) allowing an operator to communicate the final optimal parameters stored to the control device 204a of the manufacturing system. The parameters are then transmitted to the manufacturing device 204b for the production of the test specimen based on the optimal parameters. Thus, in this configuration, there is no communication between the computer system 202 and the manufacturing system 204.

[0061] The method 400 for manufacturing a test specimen according to an example of the invention will now be explained with reference to Figures 3A, 3B, 3C and 3D.

[0062] The method 400 applies to mechanical specimens 100 such as, for example, and without limitation, those in Figures IA and IB, intended to be subjected to stress in a fatigue or dynamic cracking test by means of an ultrasonic device 700 as illustrated in [Fig. 1C] or 1D. The stress is applied, with a given amplitude Uo and frequency f, to the first end of the specimen along an elongation axis X of the specimen 100, the second end of the specimen opposite the first end can be left free (as illustrated in [Fig. 1C]) or fixed (as illustrated in [Fig. 1D]).

[0063] With reference to [Fig.3A], the process 400 comprises at least one dimensioning step 400i and one manufacturing step 4002.

[0064] With reference to [Fig.3B], the sizing step 400i of the process 400 includes at least one preliminary step (a0) during which an operator provides or communicates to the computer system 202 of the manufacturing device 200, via the human-machine interface 202;, initial parameters Pii N in the form of numerical data. The initial parameters include at least a length 2L of the specimen (separating the first and second ends of the specimen) along the elongation axis, a value of the cross-section S, Sij2 along the elongation axis in a plane normal to the axis, the Young's modulus E and the volumetric mass density of the material in which the specimen is to be manufactured or produced. The value of the cross-section S, Sapent can be variable, that is, it depends on the positioning along the elongation axis (S, Si>2= S(x), x being a position or coordinate along the X axis), or constant (S, Si>2= So).

[0065] The data also include the amplitude Uo and the frequency f of the stress to which the specimen 100 must be subjected.

[0066] For example, and in a non-limiting manner, the initial geometric structure may be a bar of length 2L with constant cross-section So.

[0067] After provision of the initial parameters Pii N, the numerical data are received data (aj by computer system 202. This data can be stored in the main memory 202mp or in a secondary memory 202ms of the computer system 200.

[0068] A step of determining (b) a displacement field u is then carried out by the processing unit 202p of the computer system 202 by implementing the instructions of a computer program stored in the main memory 202mp. The instructions include at least taking into account the values ​​of the amplitude Uo and the frequency f of the stress, the cross-section, the Young's modulus E and the volumetric mass density p of the material considered.

[0069] The instructions further include taking into account a predefined criterion for the displacement field to be determined.

[0070] The predefined criterion consists of fixing for the displacement field u a maximum amplitude value equal to the amplitude Uo of the stress and a resonance frequency equal to the stress frequency f.

[0071] The displacement field u to be determined meeting the predefined criterion is defined by the relation:

[0072] [Math.l] u(x,t) = U(x) Or : - U is the amplitude of a displacement and is a function of the amplitude Uo of the applied stress; and - w = 2irf is a resonance mode of the specimen which is proportional to the frequency of the applied stress.

[0073] The instructions further include at least one algorithm for determining or calculating the displacement u, the algorithm being based on the relation;

[0074] [Math.2] . El. » 1 ] _ n d / 2 \ to*2 dv dx S{x) / ' u( x = L. t) = Uo sin(wi) [t) is the value of the deformation caused by the stress at the Or : u is the displacement, u(x — L, t) is the displacement at the first end of the test specimen; 9» fy au* second end of the test tube; c _ [p ïn is a characteristic velocity of the specimen material, with E and p respectively the Young's modulus and the volumetric mass density of the material, - t the time, - x, a coordinate along the elongation axis X, and - S(x) a value of the section of the specimen at the x coordinate.

[0075] As soon as the displacement field u is determined at any point of the specimen along the elongation axis X, a step (c) of calculating the stress field is carried out by the processing unit of the computer system via the execution of the instructions of the computer program.

[0076] These instructions include the calculation of a stress field o(x), as a function of the determined displacement field u.

[0077] These instructions further include at least one algorithm for calculating the stress field o, the algorithm being based on the relation:

[0078] [Math.3] where E is the Young's modulus of the material and u is the determined displacement field.

[0079] When the stress field o(x) is calculated, the processing unit of the computer system 202 verifies (d), by executing a dedicated algorithm of the computer program, the homogeneity (or the correspondence to the shape of the field sought) of the stress field along the elongation axis X. By homogeneity of the field, it is understood that the field has a constant value along a portion of the elongation axis of the specimen.

[0080] If the stress field o(x) does not exhibit homogeneity, that is to say in the absence of a zone of the elongation axis X along which the stress field is constant, then the processing unit of the computer system proceeds to update (ej) the value of the section S(x) of the specimen by iterative method via the implementation of a dedicated algorithm of the computer program.

[0081] For example, and without limitation, the shape of the section S(x) can initially be defined by a polynomial function. The iterative method then allows the parameters of the polynomial function to be determined, and thus, a new value for the section S(x).

[0082] After the update of section S(x), the processing unit repeats (e2) steps (b), (c) and (d).

[0083] If after verification (d), the stress field o(x) exhibits homogeneity in an area along the elongation axis X, then the processing unit of the computer system determines (e3), via the implementation of a dedicated algorithm of the computer program, a value LZu of the width of the area also called the useful area.

[0084] The processing unit then proceeds to memorize e4, a set of final dimensioning parameters Poi N preferably including at least the value Lzu of the width of the useful zone, the last value of the section Si>2, S(x) used in the calculations and for which the stress field o(x) is homogeneous in the useful zone, the value 2L of the length of the specimen, the Young's modulus E and the volumetric mass density p of the material considered in the calculations.

[0085] After memorization e4, the processing unit of the computer system can transmit (as illustrated in [Fig.2]), preferably automatically, the final dimensioning parameters Poi N to the manufacturing system 204.

[0086] The final sizing parameters Poi N can also, for example, be communicated to an operator, via a screen for example (not shown). The operator can then enter the parameters into the manufacturing system 204.

[0087] Fig. 3C illustrates an example of a geometric profile of a 100 specimen obtained for a metallic-type material with a Young's modulus E = 200 GPa and a volumetric density of 7800 kg / m3 subjected to a stress of amplitude U0 = 1 pm and frequency f = 20 kHz.

[0088] In this example, the specimen has a shape similar to a "reed head". The elongation axis X is normal to a plane of symmetry Ps passing through the center of the specimen 100. Knowing the value Lzu of the useful zone width allows us to deduce a head length LT of the specimen 100 by dividing by two the difference between the value of the length 2L of the specimen and the value Lzu of the useful zone width.

[0089] Figure [3D] illustrates a representation of the stress field o(x) along the elongation axis for the geometric profile of the specimen shown in [Fig.3C]. As can be seen, the stress is constant in the useful zone of width LZU.

[0090] In this example, the stress field has a minimum value at the first and second ends. The maximum value of the stress field is obtained in the useful zone and is on the order of 30 MPa.

[0091] Advantageously, in the case of a material with elastic behavior, there is proportionality between the amplitude of the stress Uo (imposed as the maximum displacement amplitude) and the stress in the useful zone. It is therefore possible to easily switch from one stress to the other by simple proportionality. Thus, it is not necessary to modify the geometric profile of the specimen.

[0092] Advantageously, it can also be considered to vary the stress as a function of time, i.e. according to a relationship Uo = F(t). This amounts to carrying out fatigue or cracking tests with loading conditions varying over time (and within the limits of the capacity of the testing machine and the material). However, the application frequency must be kept constant during testing to ensure proper vibration conditions for the specimen.

[0093] Fig. 3E illustrates an example of a geometric profile of a cracking specimen obtained via the above-described process.

[0094] In this example, the process includes dimensioning a test specimen as described above and with a geometric profile similar to that of [Fig. 3C], and then creating a crack on the specimen, for example, and without limitation, in a plane of symmetry. Manufacturing a test specimen with a crack as illustrated in [Fig. 3E] can contribute to characterizing the specimen's service life.

[0095] Although in the examples of Figures 3C and 3E the specimens are subjected to uniaxial loading and have symmetrical geometric profiles (and cracks in a plane of symmetry), it is possible to consider applying the manufacturing process described above to multiaxial loading, non-symmetrical geometric profiles, and cracks located in planes that are not planes of symmetry. In this case, the computer program must be adapted to perform calculations based on numerical tools such as finite elements to evaluate the displacement and stress fields in steps (b) and (c) of the process.

[0096] In the case of multiaxial loading, the equations governing the displacement fields can be deduced (for example in a Cartesian coordinate system (O, x, y, z)) from the dynamic equilibrium equation defined by the relation:

[0097] [Math.4] div^a) = py where - o is the stress tensor (in two or three dimensions), - p is the volumetric mass density, and - Y is the acceleration (in two or three dimensions).

[0098] Following the dimensioning of the test specimen 100, the manufacturing system 204 can then proceed to manufacture 4002 of the test specimen 100, preferably on the basis of the final dimensioning parameters obtained by the computer system.

[0099] Thus, thanks to the invention, it is possible to control the shape of the stress field. In particular, the described process makes it possible to obtain test specimens for which the characterization of lifetime is improved.

[0100] The fabrication of test specimens with a homogeneous and controlled stress field in the relevant area allows: - an observation of mechanisms such as cracking or in-situ fatigue in the area subjected to homogeneous mechanical stress, i.e. in the useful area; - a simplification of carrying out studies of the lifespan of materials (fatigue or cracking) for low mechanical stresses and very large number of cycles, the cycle being by definition a pattern of stresses, spread over a period of time and which is expected to be repeated many times; - a simplification of the characterization of materials for tests carried out at high strain rates.

[0101] The manufacture of test specimens with a non-entirely homogeneous stress field and exhibiting homogeneity in a given area, representative for example of aircraft parts, makes it possible to achieve the same advantages mentioned above with a gain in representativeness of the test with respect to the industrial context.

[0102] With reference to [Fig.4], the manufacturing process (and in particular manufacturing step 4002) can be followed by at least one fatigue or cracking test step 600 carried out on the manufactured specimen.

[0103] .

Claims

Demands

1. A method (400) for manufacturing a test specimen (100) intended to be subjected to stress in a fatigue or dynamic cracking test, the stress being applied to a first end (102i) of the specimen along an axis (X) of elongation of the specimen (100), the specimen further comprising a second end (1022) of the specimen (100) opposite the first end (102i), the method comprising at least one specimen dimensioning step comprising at least the following substeps implemented by computer: - the reception (aj) of a given number of initial parameters Pi i_N representing an initial geometric structure of the specimen and the physical characteristics of a material in which the specimen is to be made (100), the parameters representing the structure comprising at least • a value (2L) of a distance separating, along the elongation axis (X), the first (1020) and second (1020) ends of the specimen (100); and • a value of a section (Si, S2) of the specimen (100) along the elongation axis (X), in a plane normal to said axis; - the determination (b) of a displacement field (u) generated along the elongation axis (X) by the applied stress with a given amplitude (Uo) and frequency (f), as a function of the value of the section (S, Si, S2) and at least one of the physical characteristics of the material of the specimen (100) - the calculation (c) of a stress field (o) as a function of the determined displacement field (u); - verification (d) of the homogeneity of the stress field (o) along the elongation axis, including: • if the constraint field (o) is not homogeneous: • the iterative updating (ej) of the value of the section (S, Si, S2) of the specimen (100); and • the repetition (e2) of steps (b), (c) and (d), • if the stress field exhibits homogeneity in a zone along the elongation axis: • the determination (e3) of a value (Lzu) of the width of the homogeneity zone; • the storage (e4) of a set of final dimensioning parameters Poi N including at least the value (Lzu) of the width of the homogeneity zone, the value of the section for which the stress field is homogeneous in said zone and the value (2L) of the length of the specimen (100).

2. A method (400) according to claim 1, wherein the physical characteristics of the specimen material (100) include at least a Young's modulus (E) and a mass volume density (p) of the material.

3. Method (400) according to claim 1 or 2, wherein the calculation of the displacement field (u) is carried out by fixing for the displacement field (u) a maximum amplitude value equal to the amplitude (U o) of the stress and a resonance frequency equal to the frequency (f) of the applied stress.

4. Method (400) according to any one of claims 1 to 3, wherein the displacement field (u) to be determined is defined by the relation: [Math.1] where: - U is the amplitude of a displacement and is a function of the amplitude Uo of the applied stress; and - w = 2irf is a resonance mode of the specimen which is proportional to the frequency f of the applied stress.

5. Method (400) according to any one of claims 1 to 4, wherein the displacement field (u) is determined via the implementation of the relation: [Math.2] àd _r2(<£“ _l àt21»......1.....1 = n 3r2 y dx2 dx dx 51 x) / u(x = L, t) - Uo sin{wt) ^(x=-L,t) = Q ax\ / Or : - u is the displacement, - u(x — l^ t) is the displacement at the first end of the test specimen; - — (x = - constant the value of the deformation caused by the stress at the second end of the test specimen; - c _ Jp is a characteristic velocity of the specimen material, with E and p respectively the Young's modulus and the mass density of the material, - t the time, - x, a coordinate along the elongation axis (X), and - S(x) a value of the cross-section of the specimen at the coordinate x.

6. Method (400) according to any one of claims 1 to 5, wherein the calculation of a stress field (o) is carried out via the relation: [Math.3] dx where E is the Young's modulus of the material and u is the determined displacement field.

7. Method (400) according to any one of the preceding claims, wherein the applied stress is carried out at a frequency between 10 and 30KHz.

8. Method (400) according to any one of the preceding claims, further comprising a manufacturing step (4002) of the test specimen (100) according to the final sizing parameters Poi N stored.

9. A method (400) according to any one of the preceding claims, wherein the test specimen (100) is made of a composite or metallic material.

10. Device for manufacturing (200) a test specimen (100) intended to be subjected to dynamic stress in a fatigue or cracking test, comprising at least one computer system (202) configured to execute steps (ab b, c, d, ei_4) of the process according to one of the preceding claims; and comprising at least one memory for data storage and a processing unit.

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