METHOD AND SYSTEM FOR MANUFACTURING TEST PIECES

Abstract

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

Description

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

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

[0002] A mechanical part subjected to certain operating conditions in an aircraft turbomachine may, over time, exhibit defects which may affect the service life of the part and therefore the structure and / or performance of the turbomachine. The defects may, for example, and in a non-limiting manner, be fatigue cracks. Such cracks appear, for example, in the form of a crack appearing on the surface of the part and capable of propagating or extending in one direction through at least one of its ends, called the propagation end. In certain cases, cracks may also propagate inside 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 which constitute 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 service.

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

[0006] Figures 1A and 1B show examples of mechanical specimens 102 on which fatigue / cracking tests can be carried out.

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

[0008] The test piece 100 may have, for example, and in a non-limiting manner, a cylindrical shape or a flat shape as illustrated respectively in [Fig. 1A] and 1B. The test piece 100 may be, for example, made of metallic or composite material.

[0009] Existing test devices for characterizing the life of parts mechanical 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 piezo technologies electric.

[0010] Traditional machines / vibrophores are the most common and have the advantage of giving freedom regarding the type of geometry that can be stressed. However, these technologies allow tests to be carried out at stress frequencies ranging from ten, for example and non-limitingly 50Hz, to hundreds of Hertz (Hz). These stress frequencies are a limiting factor in the characterization of the service life of parts / test specimens. Fatigue / cracking tests carried out with these devices are called quasi-static.

[0011] As for ultrasonic machines, they allow so-called dynamic fatigue / cracking tests to be carried out. The test piece 100 or the part is generally subjected to stress with a frequency of the order of 100KHz to 30KHz and, preferably, 20KHz.

[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, one current generator 704 and one 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 piece 100 is fixed to one end of the sonotrode 706b, the second end 1022 of the test piece 100 being free of constraint.

[0015] The computer 702 comprises at least one processor for implementing a program allowing the control of the generator 704 and the machine 706. The implementation of the program allows at least one electrical control signal to be imposed on the generator 704. The generator 704 receives the control signal and generates a sinusoidal electrical signal at a given frequency f0 between 10KHz and 30KHz. Preferably the frequency is 20KHz.

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

[0017] The sonotrode 706b of the machine 706 makes it possible to amplify the value of the amplitude of the stress to which the test piece / part 100 is subjected.

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

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

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

[0021] [Fig.1E] illustrates an example of a dimensioned test piece 100. The test piece, 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 the length value LZU.

[0022] [Fig. 1F] is a representation of a displacement field u and a stress field obtained by subjecting the test piece of [Fig.lE] to a dynamic stress. As can be observed, the stress field on this test piece geometry 100, in particular in the useful zone, is heterogeneous with a maximum at the center of the test piece 100.

[0023] The application of a dynamic stress to the first end of the test piece generates a deformation which can cause a stretching or a contraction along the axis of elongation. Thus, as illustrated in [Fig.lG], a point M of the test piece which is in a first configuration (for example instant t=t0 before stress citation), at an initial position xO on the axis of elongation can at a time t, after stress, find itself at a position xl. The difference u between the positions xl and xO is called displacement. When we consider all the points M along the axis of elongation, the set of displacements u(M) constitutes the displacement field generated by the stress.

[0024] Studies to characterize the lifespan of test pieces / parts frequently lead to the performance of studies of the material using tools such as microscopes or SEM (Scanning Electron Microscopes).

[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 of the performance of the turbomachine, it is appreciated to have a useful zone 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 thus be desired to provide a test specimen which makes it possible to overcome at least some of the aforementioned problems and constraints. Summary of the invention

[0027] A method is therefore proposed for manufacturing a test piece intended to be subjected to a stress in a fatigue or dynamic cracking test, the stress being applied to a first end of the test piece along an axis of elongation of the test piece, the test piece further comprising a second end of the test piece opposite the first end, the method comprising at least one step of dimensioning the test piece comprising at least the following sub-steps implemented by computer: - the reception of a number of initial parameters representative of an initial geometric structure of the test piece and of the physical characteristics of a material from which the test piece is to be made, the parameters representative of the structure comprising at least: • a value of a distance separating, along the axis of elongation, the first and second ends of the test piece, and • a value of a section of the test piece along the axis of elongation, in a plane normal to said axis; - the determination of a displacement field generated along the elongation axis by the stress applied with a given amplitude and frequency, as a function of the value of the section, and at least one of the physical characteristics of the material of the test piece; - 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 stress field does not present homogeneity: • updating by iterative method of the value of the section of the test piece; 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 comprising 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 test piece.

[0028] Thus, thanks to the invention it is possible to size the test pieces before their manufacture, so as to control the shape of the stress field of the test pieces 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 comprise one or more of the following optional features, in any technically possible combination: - the physical characteristics of the material of the test piece include at least a Young's modulus and a mass volume density of the material; - the calculation of the displacement field is carried out by setting 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 specimen; —(x = -L. t] is the value of the deformation generated by the stress at dx \ ' / second end of the test tube; c = ^ËJp is a characteristic speed of the specimen material, with E and p respectively the Young's modulus and the mass volume 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; - the calculation of a constraint field (o) is carried out 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 method further comprises a step of manufacturing the test piece according to the stored final sizing parameters; - the test piece is made of a composite or metallic material.

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

[0032] The manufacturing device may further comprise one or more of the following optional features, in any technically possible combination: - the computer system comprises at least one means of communication; - the manufacturing device comprises a manufacturing system capable of manufacturing a test piece 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 communications network and / or recorded on a computer-readable medium, the program comprising instructions for executing at least one of the steps of the manufacturing method 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 with reference to the appended drawings in which: - [Fig.1A] illustrates a first example of a test piece according to the prior art, - [Fig.lB] illustrates a second example of a test piece according to the prior art; - [Fig.lC] is a schematic representation of a ul test device trasonic; - [Fig.lD] is a schematic representation of an ultrasonic testing device; - [Fig. 1E] illustrates an example of a test piece sized for an ultrasonic fatigue test according to the prior art; - [Fig.lF] illustrates a curve of the displacement and stress fields obtained for the specimen of [Fig.lE] subjected to an ultrasonic fatigue test; - [Fig.lG] is an illustration of a movement following a stress; - [Fig.2] is a schematic representation of a device for manufacturing a test piece according to an example of the invention; - [Fig.3A] is a schematic representation of the method of manufacturing a test piece implemented by the device of [Fig.2]; - [Fig.3B] is a schematic representation of a step of dimensioning a test piece according to the invention; - [Fig.3C] is a schematic representation of a geometric profile of a test piece according to an example of the invention; - [Fig.3D] illustrates a curve representing the stress field obtained for the specimen of [Fig.3C]; - [Fig.3E] is a schematic representation of a geometric profile of a cracked test piece according to an example of the invention; - [Fig.4] is a schematic representation of a manufacturing method according to an example of the invention, followed by a testing method. Detailed description of an exemplary embodiment of the invention

[0035] With reference to Figures 1A and 1B, a test piece 100 comprises at least one first end 102i and a second end 1022 opposite the first end 102i. The test piece 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 test piece along an axis (X) of elongation of the test piece, the second end of the test piece opposite the first end can be free (as illustrated in [Fig.lC]) or kept fixed (as illustrated in [Fig.lD]).

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

[0038] The test piece manufacturing device 200 comprises at least one computer system 202, and a manufacturing system 204 of the test piece 100.

[0039] The computer system 202 makes it possible to determine optimal parameters for the geometric structure of the test piece 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 in a non-limiting manner 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 test piece and of physical characteristics of a material from which the test piece must be made.

[0042] The parameters representative of the geometric structure comprise at least one value 2L of a distance separating, along the elongation axis X, the first 102i and second 1022 ends of the test piece and a value of a section S, Si>2 of the test piece 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 mass volume density p of the material.

[0044] The main memory 202mp is capable of storing data comprising 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 comprise at least one computer program containing instructions relating to the determination of optimal geometric parameters Poi N of the test piece to be manufactured, for the processing unit 202p.

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

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

[0048] The processing unit is, for example, and in a non-limiting manner, a processor or microprocessor.

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

[0050] The computer system may further comprise, 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 disk) or a remote medium (such as a remote hard disk accessible via the network interface through a communications 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 disk reader).

[0051] Thus, the computer program containing the instructions for the processing unit 202p can be recorded on the medium and / or downloadable 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 piece 100 from the optimal parameters determined by the processing unit of the computer system.

[0053] The manufacturing system 204 comprises 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 piece by the manufacturing device 204b from the received data.

[0056] The control device is, for example, and in a non-limiting manner, 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 piece 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 in a non-limiting manner, a 3D printer.

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

[0060] Alternatively, the manufacturing system 204 may comprise at least one interface (for example a screen) allowing an operator to communicate the final optimal parameters stored in memory 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 piece from 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 piece according to an example of the invention will now be explained with reference to FIGS. 3A, 3B, 3C and 3D.

[0062] The method 400 applies to mechanical test pieces 100 such as, for example, and in a non-limiting manner, those of FIGS. 1A and 1B, intended to be subjected to a 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 test piece along an elongation axis X of the test piece 100, the second end of the test piece 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 method 400 comprises at least one dimensioning step 400i and one manufacturing step 4002.

[0064] With reference to [Fig.3B], the step 400i of dimensioning the method 400 comprises 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 digital data. The initial parameters include at least a length 2L of the test piece (separating the first and second ends of the test piece) along the axis of elongation, a value of the section S, Sij2 along the axis of elongation in a plane normal to the axis, the Young's modulus E and the mass volume density of the material from which the test piece 100 is to be manufactured or produced. The value of the section S, Sa may be variable, i.e. it depends on the positioning along the axis of elongation (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 test piece 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 section So.

[0067] After providing the initial parameters Pii N, the digital data are received- data (ad by the 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 comprise at least taking into account the values of the amplitude Uo and the frequency f of the stress, the section, the Young's modulus E and the volume density of mass p of the material considered.

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

[0070] The predefined criterion consists of setting for the displacement field u a maximum amplitude value equal to the amplitude Uo of the stress and a resonance frequency equal to the frequency of the stress 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 comprise at least one algorithm for determining or calculating the displacement u, the algorithm being based on the relationship;

[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 generated by the stress at the Or : u is the displacement, u(x — L, t) is the displacement at the first end of the specimen; 9» fy au* second end of the test tube; c _ [p ïn is a characteristic velocity of the material of the specimen, with E and p respectively the Young's modulus and the mass volume 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 test piece 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 constraint field o(x), as a function of the determined displacement field u.

[0077] These instructions further comprise 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 sought field) of the stress field along the elongation axis X. By homogeneity of the field is meant that the field has a constant value along a portion of the elongation axis of the test piece.

[0080] If the stress field o(x) does not present 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 test piece by iterative method via the implementation of a dedicated algorithm of the computer program.

[0081] For example, and in a non-limiting manner, the shape of the section S(x) can be initially defined by a polynomial type function. The iterative method then makes it possible to determine the parameters of the polynomial function and therefore, a new value of the section S(x).

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

[0083] If after verification (d), the stress field o(x) presents a homogeneity in a zone 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 zone also called useful zone.

[0084] The processing unit then proceeds to the storage e4 of a set of final dimensioning parameters Poi N preferably comprising 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 test piece, the Young's modulus E and the volume density of mass p of the material considered in the calculations.

[0085] After storage 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 Poi N sizing parameters may also, for example, be communicated to an operator, for example via a screen (not shown). The operator may then enter the parameters into the manufacturing system 204.

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

[0088] In this example, the test piece 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 test piece 100. Knowing the value Lzu of the useful zone width makes it possible to deduce a head length LT of the test piece 100 by dividing by two the difference between the value of the length 2L of the test piece and the value Lzu of the width of the useful zone.

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

[0090] In this example, the stress field has a minimum value omin at the first and second ends. The maximum value of the stress field omax is obtained in the useful zone and is of 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 maximum amplitude of displacement) and the stress of the useful zone. It is therefore possible to easily pass from one stress to the other by simple proportionality. Thus, it is not necessary to modify the geometric profile of the test piece.

[0092] Advantageously, it may also be envisaged 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 that vary over time (and within the limits of the capacity of the testing machine and the material). However, the stress frequency must be kept fixed during the tests in order to ensure good vibration conditions for the specimen.

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

[0094] In this example, the method comprises the dimensioning of a test piece as described above and with a geometric profile similar to that of [Fig.3C] and then the production of a crack on the test piece, for example and in a non-limiting manner in a plane of symmetry. The manufacture of a test piece with a crack as illustrated in [Fig.3E] can participate in the characterization of the lifetime of the test piece.

[0095] Although in the examples of Figures 3C and 3E the test pieces are subjected to a uniaxial stress and have symmetrical geometric profiles (and cracks in a plane of symmetry), it may be envisaged to apply the manufacturing method described above for multiaxial stresses, non-symmetrical geometric profiles and cracks positioned in planes which are not planes of symmetry. In this case, the computer program must be adapted to implement calculations based on numerical tools such as finite elements to carry out the evaluation of the displacement and stress fields in steps (b) and (c) of the method.

[0096] In the case of multiaxial stresses, the equations governing the displacement fields can be deduced (for example in a Cartesian frame of reference (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 mass volume density, and - Y acceleration (in two or three dimensions).

[0098] Following the sizing of the test piece 100, the manufacturing system 204 can then proceed with the manufacturing 4002 of the test piece 100, preferably, on the basis of the final sizing 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 method described makes it possible to obtain test pieces for which the characterization of the service life is improved.

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

[0101] The manufacture of test pieces with a stress field that is not entirely homogeneous and exhibits homogeneity in a given zone, 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 method (and in particular manufacturing step 4002) may be followed by at least one fatigue or cracking test step 600 carried out on the manufactured test piece.

[0103] .

Claims

Claims

1. Method (400) for manufacturing a test piece (100) intended to be subjected to a stress in a fatigue or dynamic cracking test, the stress being applied to a first end (102i) of the test piece along an axis (X) of elongation of the test piece (100), the test piece further comprising a second end (1022) of the test piece (100) opposite the first end (102i), the method comprising at least one step of dimensioning the test piece comprising at least the following sub-steps implemented by computer: - the reception (adj of a given number of initial parameters Pi i_N representative of an initial geometric structure of the test piece and of the physical characteristics of a material in which the test piece must be made (100), the parameters representative of the structure comprising at least • a value (2L) of a distance separating, along the axis of elongation (X), the first (1020) and second (1020) ends of the test piece (100); and • a value of a section (Si, S2) of the test piece (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 stress applied 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 test piece (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 stress field (o) does not present homogeneity: • updating (ej by iterative method of the value of the section (S, Si, S2) of the test piece (100); and • repeating (e2) steps (b), (c) and (d), • if the stress field has homogeneity in a zone along the elongation axis: • determining (e3) a value (Lzu) of the width of the homogeneity zone; • storing (e4) a set of final dimensioning parameters Poi N comprising 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 test piece (100).

2. The method (400) of claim 1, wherein the physical characteristics of the material of the test piece (100) comprise 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, in which the calculation of the displacement field (u) is carried out by setting 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 one of claims 1 to 3, in which the displacement field (u) to be determined is defined by the relation: [Math.l] 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 test piece which is proportional to the frequency f of the applied stress.

5. Method (400) according to one of claims 1 to 4, in which 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 specimen; - — (x = - c is the value of the deformation generated by the stress at the second end of the test piece; - c _ Jp is a characteristic velocity of the material of the specimen, with E and p respectively the Young's modulus and the volume density of the material, - it's time, - x, a coordinate along the elongation axis (X), and - S(x) a value of the section of the specimen at the coordinate x.

6. Method (400) according to one of claims 1 to 5, in which 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 one of the preceding claims, in which the applied stress is carried out at a frequency between 10 and 30KHz.

8. Method (400) according to one of the preceding claims, further comprising a step of manufacturing (4002) the test piece (100) as a function of the stored final dimensioning parameters Poi N.

9. Method (400) according to one of the preceding claims, in which the test piece (100) is made of a composite or metallic material.

10. Device (200) for manufacturing a test piece (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 method according to one of the preceding claims; and comprising at least one memory for storing data and a processing unit.

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