FR3107959A1 - CHARACTERIZATION TEST PROCEDURE OF A COMPOSITE MATERIAL TEST AND CORRESPONDING INSTALLATION - Google Patents

Abstract

TITLE: CHARACTERIZATION TEST METHOD OF A COMPOSITE MATERIAL TEST AND CORRESPONDING INSTALLATION The invention relates to a characterization test method of a composite material test piece (10), the method comprising the following steps: providing a specimen (10) of composite material representative of a part (1), such as a turbomachine part, the specimen (10) being produced with a fibrous reinforcement embedded in an organic matrix and comprising a first end (11) and a second end (12) extending on either side of a useful area (13) in a longitudinal direction L, fixing the first and second ends (11, 12) respectively to a first and a second module of clamping (22, 23) of a characterization test installation (20), thermomechanical fatigue stress of the test piece (10) in which: the useful zone (13) of the test piece (10) is heated according to a succession of thermal cycles (TC) where the useful zone (13) of the specimen is heated from a first temperature to a second temperature for a first predetermined period of time, and the specimen (10) is simultaneously subjected to a first succession of cycles (CM1) of mechanical stresses between a minimum mechanical stress and a maximum mechanical stress during the predetermined period of time. Figure for the abstract: Figure 4

Description

CHARACTERIZATION TEST PROCEDURE FOR A COMPOSITE MATERIAL SPECIMEN AND CORRESPONDING INSTALLATION

Field of invention

The present invention relates to the field of characterization tests on samples of materials in order to study their behavior and properties under certain stresses.

Technical background

It is known to submit material samples or specimens to mechanical or thermal characterization tests in order to determine their resistance in an often aggressive environment where the parts made of these materials will evolve and thus their lifespan. This is the case for parts in the aeronautical field and in particular turbomachine parts made with these materials.

Turbomachine parts have been made in recent years in composite material in order to improve their thermomechanical resistance capacities and reduce their masses. Composite materials with an organic matrix (signed CMO) which include a fibrous reinforcement made by weaving and embedded in an organic matrix, allow for example the manufacture of fan blades or fan casings. These composite materials are sensitive to high temperatures, such as above 120°C, which can lead to degradation such as thermal oxidation which is likely to lead to premature loss of the mechanical and/or physical properties of the part. On the other hand, the mechanical stresses that the part undergoes in this environment can accelerate the damage of the part.

During a specimen characterization test in a testing machine, in particular thermal characterization tests which use thermal cycles, the degradations observed are in particular the deformation of the matrix, or even the cracking of the latter which can develop during thermal cycles in an oxidizing environment and without any mechanical load being applied to the specimen. At the same time, on a more local scale, decohesions at the fibre/matrix interfaces can develop during the thermal test. These decohesions provide new surfaces directly exposed to the environment, facilitating the diffusion of oxygen, and accelerating the degradation of the composite material.

The specimens are then subjected to characterization tests in order to study their mechanical behavior. However, the thermal characterization tests are spread over approximately a period of at least one month, necessary to activate the degradation mechanisms, whereas a conventional mechanical fatigue test lasts approximately one week. These thermal and mechanical fatigue tests are therefore distinct, which prevents characterization of the coupling between the damage induced by the thermal effect and that generated by the mechanical stress, this "thermomechanical" degradation phenomenon possibly being more harmful for the behavior of the CMO than the simple succession of solicitations. However, in real conditions, the parts are indeed exposed to high temperatures and undergo more or less low mechanical stresses simultaneously and this over several years.

The objective of the present invention is to characterize the aging of a composite material so as to activate the mechanisms of degradation by thermal aging in parallel with the mechanisms of mechanical damage, under conditions almost close to the work environment of the part. in composite material.

We achieve this objective in accordance with the invention thanks to a characterization test method of a specimen made of composite material, the method comprising the following steps:

• supply of a test piece made of composite material representative of a part, such as a turbomachine part, the test piece being made with a fibrous reinforcement embedded in an organic matrix and comprising a first end and a second end extending from either side and on the other side a useful zone along a longitudinal direction L,
• fixing the first and second ends respectively to a first and a second clamping module of a characterization test installation,
• thermomechanical fatigue stress on the specimen in which:
  • the useful zone of the specimen is heated according to a succession of thermal cycles where the useful zone of the specimen is heated from a first temperature to a second temperature for a predetermined period of time, and
  • the specimen is simultaneously subjected to a first succession of mechanical stress cycles between a minimum mechanical stress and a maximum mechanical stress during the predetermined period of time.

Thus, this solution achieves the above objective. In particular, such a method makes it possible to characterize the harmfulness of the coupling between the damage generated by aging at high temperature (isothermal and/or cyclic) and the mechanical damage at the mesoscopic scale of a composite material with a fibrous reinforcement. embedded in an organic matrix. Aging is said to be “isothermal” if the exposure of the specimen to high temperature is carried out continuously. The term "cycling" is used when the specimen cycles between a minimum temperature and a maximum temperature. This thermomechanical fatigue also makes it possible to accelerate the tests because the cycles of thermal fatigue and low-cycle mechanical fatigue are operated over the same period of time. The duration of the tests is also extended because the first and second ends of the specimen are correctly maintained, do not slip, and do not deform due to thermal stress. The aging of the composite material produced is closer to the real behavior of the composite material.

We understand in the present invention by the term "thermomechanical" the phenomena of degradation of the matrix which are put in place from a temperature higher than 120°C (thermo-oxidation), as well as fatigue and creep (mechanical ).

The method also includes one or more of the following features and/or steps, taken alone or in combination:

• the first predetermined period of time lasts at least two hours.
• each thermal cycle includes a temperature holding range at the second temperature before being cooled to the first temperature.
• the temperature holding range at the second temperature lasts at least two hours.
• the first temperature is between -55°C and 60°C and the second temperature is between 120°C and 200°C.
• each first succession of mechanical stress cycles comprises a second succession of mechanical stress cycles applied discontinuously over a time interval corresponding to the temperature maintenance range.
• the method comprises a step of cooling the first and second clamping modules to a temperature whose value is lower than the glass transition temperature of the organic matrix.
• the first and second clamping modules are cooled to a temperature whose value is less than 50°C.
• the mechanical stresses are compressive and tensile stresses whose ratio is -1.
• the thermal cycles are carried out at a frequency less than or equal to 2Hz.
• the means of cooling the useful area of the specimen include a refrigerator or ventilation device.
• the temperature sensing elements include a thermocouple.
• the specimen is made in one piece.
• the fiber reinforcement is obtained by three-dimensional or two-dimensional weaving.

The invention also relates to a test facility for characterizing a specimen made of composite material, the facility comprising:

• a first and a second clamping module intended to retain respectively a first and a second end of a test specimen made of composite material representative of a part, such as a turbomachine part, the test specimen being made with a fibrous reinforcement embedded in an organic matrix,
• a module for heat treatment of a useful zone of the specimen arranged between the first end and the second end,
• an electronic control system intended to control the heat treatment module and the tightening modules so as to produce a thermomechanical stress in which:
  • the useful zone of the specimen is heated and/or cooled according to a succession of thermal cycles where the useful zone of the specimen is heated from a first temperature to a second temperature for a first predetermined period of time, and
  • the specimen is simultaneously subjected to a first succession of mechanical stress cycles between a minimum mechanical stress and a maximum mechanical stress during the predetermined period of time.

The installation also includes one or more of the following features, taken alone or in combination:

• a cooling system mounted on a frame and intended to cool the clamping modules (to a temperature below the glass transition temperature of the composite material.
• the cooling system includes means for cooling the useful zone of the specimen.
• the heat treatment module comprises a heating device installed on the specimen and arranged around the useful zone of the specimen.
• the heating element is a spiral resistor.
• elements for measuring the temperature of the specimen are placed at the level of the useful zone of the specimen and are connected to the electronic control system.

Brief description of figures

The invention will be better understood, and other aims, details, characteristics and advantages thereof will appear more clearly on reading the detailed explanatory description which follows, of embodiments of the invention given as purely illustrative and non-limiting examples, with reference to the attached schematic drawings in which:

FIG. 1 is a perspective view of an example of a turbine engine blade and in particular of a fan blade made of a composite material with an organic matrix to which the invention applies;

FIG. 2 is a top view of an example of a specimen representative of a part made of composite material with an organic matrix according to the invention;

FIG. 3 is a schematic view in perspective of an example of a characterization installation in which a specimen according to the invention is installed; And

FIG. 4 represents an example of a thermomechanical fatigue cycle undergone by a specimen according to the invention.

Detailed description of the invention

Figure 1 shows a perspective view of an aircraft turbomachine 1 blade made of a composite material. The blade 1 is advantageously intended to equip a fan of a dual-flow, dual-body turbomachine.

A double-flow, double-body longitudinal axis X turbomachine typically comprises a fan which is mounted upstream of a gas generator or motor (not shown) following the circulation of the gases in the turbomachine and here along the longitudinal axis X. The gas generator comprises, from upstream to downstream, a low pressure compressor, a high pressure compressor, a combustion chamber, a high pressure turbine and a low pressure turbine. The fan comprises a plurality of fan blades which each extend along a radial axis Z perpendicular to the longitudinal axis X and whose free ends are surrounded by a fan casing. The air flow which passes through the fan is divided into a primary air flow which passes through the gas generator and in particular in a primary stream, and into a secondary air flow which circulates around the gas generator in a stream secondary surrounding the primary vein.

Parts or components of the turbomachine described above, such as the fan blades, stator blade or even the fan casing, can be made of a composite material with a fibrous reinforcement embedded in a matrix. Of course, the invention applies to all types of parts that can be made of composite materials.

The process for manufacturing a composite material part, in particular a turbomachine, comprises the following steps:

• production of a preform (or fibrous texture or fabric or fold) with a fibrous reinforcement. The fibrous reinforcement is intended to provide resistance to the final part.
• injection of a matrix into an injection enclosure in which the preform is placed. The matrix can be injected by liquid means such as by RTM technology (whose acronym stands for Resin Transfer Molding in English).
• polymerization or hardening of the matrix in which the fibrous reinforcement is embedded to obtain a final rigid part.

The fibrous reinforcement of the preform is obtained by three-dimensional weaving (or 3D weaving) or two-dimensional (D2 weaving) of yarns. The fibrous reinforcement can also be obtained by a lamination of several two-dimensional woven plies. In the present invention, we mean by the expression “three-dimensional weaving” or “3D weaving” a mode of weaving in which warp threads are linked to weft threads on several layers.

The weave may have a weave, structure or weave pattern such as satin, twill, canvas, taffeta, or interlock, etc.

The fibers can be short fibers or unidirectional continuous fibers.

Preferably, the weaving of the fibrous reinforcement is three-dimensional which offers better resistance to delamination.

The materials of the yarns or strands used to carry out the weaving of the fibrous reinforcement comprise carbon, glass, ceramic, silica, silicon carbide, Kevlar, polyamide, alumina fibers or a mixture of these fibers. Carbon or glass fibers can be short or continuous unidirectional.

In this example, the 3D weave has an interlock structure (or weave). This interlock structure provides better resistance to impact (such as bird strikes, hailstones, etc.).

The matrix allowing densification of the fibrous reinforcement is chosen according to the desired application, here for the fan blade. The matrix is organic. The latter can be a polymeric matrix such as an epoxy-based thermosetting resin or a phenolic resin such as polybismaleimides (BMI). The polymer matrix can also be a thermoplastic resin. In the present description, the terms “resin” and “matrix” are equivalent.

In order to verify that the parts made of composite material, in particular with a fibrous reinforcement, as mentioned above have an optimal lifespan compatible with the requirements of civil programs and are resistant to aggressive environments, a test specimen 10 is made of the same material (material composite with a fibrous reinforcement embedded in an organic matrix (CMO)) than that of the part to be tested (the fan blade 1) and is intended to be installed in a characterization test installation 20 described later in the description .

In FIG. 2, the test piece 10 has a generally planar shape and a generally parallelepipedic shape. The specimen 10 is made in one piece. This extends between a first end 11 and a second end 12 along a longitudinal direction L. We use the expressions longitudinal direction L, radial direction R and transverse direction T to define the dimensions of the specimen 10. These directions are perpendicular to each other. The specimen 10 comprises a useful zone (central portion) 13 or elongated median located axially (along the longitudinal direction L) between the first and second ends 11, 12. The useful zone 13 has a section smaller than that of the sections of the first and second ends. In particular, specimen 10 is of the dumbbell type. Each first end 11 and second end 12 has two bearing faces called first bearing face 14, 14' and second bearing face which are opposite in the transverse direction T.

FIG. 3 illustrates a characterization test installation 20 of the specimen 10 so as to test its mechanical and thermal resistance. The installation 20 comprises a frame 21 on which are arranged a first clamping module 22 and a second clamping module 23. The first and second clamping modules 22, 23 are intended to hold the first and second ends 11, 12. The specimen 10 extends axially between the two clamping modules. More specifically, each first and second clamping module 22, 23 comprises a pair of jaws (a first jaw 24 and a second jaw 24') which are arranged facing each other. The two jaws 24, 24' are movable relative to each other in a direction perpendicular to the longitudinal direction L (when the specimen 10 is installed in the installation). Each first and second jaws 24, 24' comprises a surface 25 having a shape complementarity with at least one of the first and second bearing faces 14, 14' of the first and second ends of the specimen 10.

Each first and second clamping module 22, 23 comprises moving means (not shown) making it possible to move the first and second jaws 24, 24' of each clamping module 22, 23 with respect to each other in such a way to clamp respectively one of the first and second ends of the specimen 10. The clamping is carried out in such a way that there is no movement of the specimen 10 relative to the jaws 24, 24'. The displacement means are controlled by an electronic control system 30. In other words, the jaws 24, 24' are controlled so that the first and second ends are immobile respectively between a pair of jaws 24, 24'. The means for moving the jaws can be mechanically, hydraulically or pneumatically actuated. Hydraulic or pneumatic actuation may include a hydraulic or pneumatic cylinder. In this case, at least one of the cylinders comprises a body and a movable rod relative to the body of the cylinder. The cylinder rod comprises a distal end which is secured to one of the first and second jaws 24, 24'. The cylinder body is installed in a clamping module. In the case of mechanical actuation, the two jaws include a screw system allowing their movement.

The tightening modules can also comprise means for pulling the specimen 10 along a pulling direction. The tensile direction is parallel to the longitudinal direction. It is of course possible that the tensile direction is oriented differently depending on the configuration of the characterization test installation. These tensile means are also connected to the electronic control system 30 which applies the tensile load to the first and second ends of the specimen. In this way, the centrifugal forces to which the fan blades 1 for example are subjected are reproduced. The blades are also subject to local shear due to the centrifugal forces which apply to them, to the impact of birds, hailstones, etc., and also to aerodynamic stresses. In particular, the electronic control system 30 comprises a so-called “HCF” fatigue simulation module for “High Cycle Fatigue” in English and which means fatigue-endurance cycle with the aim of carrying out several stress cycles. In the case of this so-called HCF simulation, the number of cycles is greater than 1 million. The mechanical stresses applied will be low. The electronic control system 30 can also include a fatigue simulation module called “LCF” for “Low Cycle Fatigue” in English and which means low cycle plastic fatigue. In the latter case, the mechanical stresses will be higher.

The traction means can also comprise hydraulic or pneumatic actuation means. In the present example, the installation comprises a crosspiece which extends in the upper part of the second clamping module 23 and which is connected to the latter. The crosshead is moved by a hydraulic or pneumatic cylinder which is mounted on the frame. Alternatively, the first or second clamping module includes a hydraulic or pneumatic cylinder to move relative to the other module.

The installation 20 also comprises a heat treatment module 28 of the specimen 10. The heat treatment module 28 as we can see in Figure 3 is arranged between the first and second clamping modules 22, 23 of the specimen 10 in the longitudinal direction L. The heat treatment module 28 comprises a thermal enclosure which at least partially surrounds the useful zone 13 of the specimen 10. The enclosure comprises through-holes which each open on the one hand, at the inside the enclosure and on the other hand, outside the enclosure. The through holes have an axis coaxial with the longitudinal direction L. The test piece 10 passes through the enclosure and the first and second ends 11, 12 extend on either side of the enclosure. In other words, the first and second ends extend outside the enclosure (of the heat treatment module).

Advantageously, but not limitatively, the heat treatment module 28 has a cylindrical shape, the axis of which is coaxial with the longitudinal direction of the specimen. The heat treatment module 28 consists of a first half-shell 31 and a second half-shell 32 which are hinged together along a hinge axis parallel to the longitudinal direction so as to be able to install the module 28 around the specimen 10. Each first and second half-shells 31, 32 comprises lugs 33 allowing them to be fixed relative to each other and to close the thermal enclosure.

The fixing of the half-shells 31, 32 is carried out by means of fixing members which are installed at the level of the ears. The fasteners include for example screws, studs, or similar members allowing easy opening and closing of these half-shells. Sealing means are arranged around the half-shells to prevent leaks. Module 28 is lined on the inside with thermal insulation foam to reduce heat loss to the outside and thus obtain faster thermal cycles. Advantageously, measurement elements such as thermocouples can be installed on the surface of the half-shells and can be held thereon by adhesive means 34 (for example adhesive strips).

The heat treatment module 28 is equipped with a heating member 35 (shown in FIG. 2) which is arranged around the useful zone 13 of the test specimen 10 so as to cause the useful zone to pass from a first temperature to a second temperature and vice versa. The first temperature is lower than the second temperature. Only useful area 13, which is isolated from the external environment in module 28, is heated. By installing the heater 35 only around the useful zone 13 of the specimen 10, the first and second ends 11, 12 thereof are at a temperature whose value is lower than the glass transition temperature Tg of the material. CMO composite (organic matrix + fibrous reinforcement). This temperature Tg corresponds to a transition region where the material passes from a glassy state characterized by a modulus of elasticity (or Young's modulus) of approximately 5 GPa to a “rubbery” state where the modulus of elasticity is lower. in the vitreous state of about 1Gpa. The glass transition temperature Tg of a fibrous reinforcement with an epoxy-based resin is between 100°C and 150°C.

The heater 35 is connected to an electrical power source. Advantageously, the heating member 35 is a spiral resistor. The electrical energy which supplies the resistor is converted into heat to heat the useful zone 13. The resistor is arranged around the useful zone 13 of the test piece 10 so that its axis is coaxial with the longitudinal direction of the test piece.

The installation 20 further comprises a cooling system 40 intended to cool the first and second clamping modules 22, 23. In particular, the cooling system 40 comprises ducts formed in the clamping modules, and in particular in the jaws where a first heat transfer fluid circulates so as to cool the first and second jaws 24, 24' of the modules. The jaws are cooled so that the first and second ends 11, 12 always remain at a temperature whose value is lower than the glass transition temperature Tg of the composite material. Advantageously, the jaws are cooled to a temperature whose value is of the order of 50°C.

The cooling system 40 comprises a ventilation device intended to cool at least the useful zone 13 of the specimen 10 so that the latter reaches the second temperature. The ventilation device comprises a pipe 41 which is connected on the one hand to a casing 42 and on the other hand to the thermal module 28. The latter comprises an opening arranged in one of the half-shells and which passes through it. In this way, the pipe 41 is coupled to this opening and opens inside the enclosure of the thermal module 28. In the housing 42 is installed a fan (not shown) which stirs the air which circulates in the pipe. 41 to cool the useful zone 13 of the specimen 10 installed in the enclosure of the thermal module 28.

Alternatively, the cooling system 40 comprises a refrigerator intended to cool the useful zone 13 of the specimen 10. The refrigerator comprises an evaporator, a compressor, a condenser and a heat transfer fluid. The evaporator comprises at least a first circuit which is connected to a cold source and in which the heat transfer fluid circulates. The heat transfer fluid circulates using the compressor. The evaporator includes a second circuit in which circulates a working fluid which is intended to cool the useful zone of the specimen. In particular, the working fluid circulating through the evaporator is cooled by the heat transfer fluid. The cold source can be water or liquid nitrogen. The working fluid is advantageously, but not exclusively, air. The heat exchanger is arranged in the box 42 and the air which is cooled circulates in the pipe 41 towards the thermal enclosure which surrounds the useful zone of the specimen.

Elements 45 for measuring the temperature of the specimen 10 are arranged at the level of the useful zone 13 of the specimen 10 to measure the temperature of the specimen during the characterization test process. The measuring elements 45 are connected to the electronic control system 30 which controls the heating element 35 (the spiral resistor) according to the temperature information emitted by the measuring elements during the duration of the tests. Advantageously, the measuring elements comprise a thermocouple. The latter is attached to one side of the useful area of the specimen using a quick-setting adhesive such as HBM XB 280.

The cooling system 40 is connected to the electronic control system 30. Advantageously, the thermal cycle (hot and cold) is controlled in a closed loop with the thermocouple in the center of the specimen.

We will now describe a characterization test method of the specimen 10 as mentioned above. The method comprises a step a) of supplying a specimen made of composite material representative of a part made of composite material, and in particular of an aircraft turbomachine. The method comprises a step b) of fixing the first end 11 and the second end 12 of the test piece 10 between the first clamping module and the second clamping module. In particular, each first and second ends 11, 12 is fixed between a first and a second jaws 24, 24' by clamping them against the bearing surfaces of the first and second ends 11, 12. The thermal module 28 is arranged (step c) around the useful zone 13 of the test specimen 10. For this, the half-shells are opened so as to allow the insertion of the test specimen 10 into the enclosure and then closed by the fastening members.

Prior to the installation of the heat treatment module 28, the heating device 35 (here the spiral resistor) is mounted on the specimen 10 and around the useful zone 13.

The first clamping module 22 is controlled to hold the first end 11 stationary and no stress is applied to the first end either. On the other hand, the second tightening module 23 is controlled to exert a mechanical stress on the specimen 10. Advantageously, but not limitingly, this stress is a traction exerted upwards on the second end 23 and/or a compression (pure traction , pure compression or intermediate tension/compression ratio) exerted downwards on the second end). In other words, the second clamping module 23 is configured to apply compressive and/or tensile forces to the specimen (via the tensile means).

Referring to Figure 4, the specimen 10 is subjected to a thermomechanical fatigue stress (step d) so as to estimate the precise lifetime of a composite material in a situation close to its real working environment. Thermomechanical fatigue combines forces of mechanical and thermal origin. During this thermomechanical fatigue stress, the useful zone 13 of the specimen is heated according to a succession of predetermined CT thermal cycles. The specimen 10 is simultaneously subjected to a succession of mechanical stress cycles determined during the thermal cycles.

During the thermal stress, the useful zone 13 of the specimen is heated from a first temperature T1 to a second temperature T2 for a predetermined period of time of at least two hours for each thermal cycle. Advantageously, but not limitingly, the duration of a CT thermal cycle is three hours. Each thermal cycle CT includes a range P for maintaining the temperature of the useful zone 13 of the specimen 10 at the second temperature T2 before being cooled to the first temperature T1. The first temperature is between -55°C and 60°C. As for the second temperature, this is between 120°C and 200°C. The useful zone 13 of the specimen is cooled down to a temperature below 60° C. (ideally down to ambient temperature + 5° C., i.e. approximately 25° C.) when the cooling is carried out with the ventilation device and down to -55°C when cooling is carried out with the refrigerator.

The mechanical stress frequency is less than or equal to 2 Hz, which makes it possible to avoid local overheating of the specimen and thus to obtain a uniform thermal profile of the specimen. The frequency corresponds to the number of mechanical cycles performed by the machine in 1 second.

With regard to the mechanical stresses, the specimen 10 is subjected to a first succession of cycles CM1 of mechanical stresses between a minimum mechanical stress σmin and a maximum mechanical stress σmax. This first succession of CM1 cycles is carried out during the predetermined period of time of a thermal cycle CT (here between two hours and three hours).

The determined mechanical stresses (tensile and/or compressive stresses) are lower than the fatigue limit stress σD of the composite material so as not to cause the specimen 10 to break during the test by simple mechanical stress and thus have the opportunity to highlight the influence of aging by thermo-oxidation. The fatigue limit stress is about 30% of the static breaking stress. Advantageously, the number of mechanical cycles chosen is greater than 1 million cycles. These mechanical cycles are then distributed evenly over at least 500 thermal cycles. The total duration of the test will be of the order of 1000 hours, which makes it possible to activate the phenomenon of thermal degradation in parallel with the realization of the mechanical cycles.

The maximum mechanical stress σmax is between 0 MPa and 300 MPa. The minimum mechanical stress σmin is lower than the maximum stress and can go up to -300 MPa. In the case of tensile stress, the minimum mechanical stress is between 0 MPa and 300 MPa. And in the case of (pure) compression, the minimum mechanical stress is between 0 and -300 MPa. These examples relate to an interlock composite material but could be different for another material.

In this example, the compressive and tensile stresses have a ratio of the order of -1. The tensile forces create cracks in the matrix which allows oxygen to enter and weaken the matrix. The compressive forces stress the matrix more than the fibrous reinforcement and under this stress the behavior of the specimen is more sensitive to a possible change in the behavior of the matrix.

In order not to cause early mechanical failure in relation to the number of cycles, the mechanical stresses are applied discontinuously. In particular, each first succession of mechanical stress cycles CM1 comprises a second succession of mechanical stress cycles CM2 where the mechanical stresses are applied discontinuously for a predetermined time interval. In this example, the time interval corresponds to the temperature maintenance range P of the thermal cycle, i.e. two hours. Over this predetermined time interval, the specimen is mechanically stressed from minimum stress to maximum stress at least two thousand times (2000 times). We understand that the specimen undergoes 2000 cycles of mechanical stress per interval. In this case, the maximum stress is equal to the fatigue limit stress.

In this way, the process makes it possible not to be limited to testing the effect of isothermal exposure, but also to take into account the damage generated by thermal cycling.

During the thermomechanical loading step, a measurement step (step d) of the specimen temperature is performed. This measurement is carried out continuously. In particular, the temperature measuring elements 45 (the thermocouple) continuously measure the temperature of the useful zone 13 of the test piece and transmit temperature information to the electronic control system 30. The useful zone of the test piece is heated until it reaches a set value (here 150° C. with reference to FIG. 4). When the thermocouple sends temperature information corresponding to the first temperature value to the electronic control system 30, the latter controls the heating element 45 to maintain this temperature during the time period of two hours.

When the predetermined period of time has elapsed, the electronic control system 30 sends a control command to the cooling system 30 to cool the useful zone 13 to a second setpoint value (here 23° and with reference to FIG. 4).

Simultaneously, when the maintenance of the second temperature is transmitted to the heating element, the electronic control system 30 sends a control command to the second tightening module 23 to start the second succession of cycles CM2 of mechanical stresses.

The second succession of mechanical stress cycles is repeated several hundred times to have a significant aging time of approximately 1000 hours, and the first succession of mechanical stress cycles as well as the succession of thermal cycles are also repeated several times to reach approximately 1 million cycles. This corresponds to the real conditions in which a fan blade evolves, for example.

Claims (15)

Characterization test method for a specimen (10) made of composite material, the method comprising the following steps:

• supply of a test piece (10) made of composite material representative of a part (1), such as a turbomachine part, the test piece (10) being made with a fibrous reinforcement embedded in an organic matrix and comprising a first end (11) and a second end (12) extending on either side of a useful zone (13) in a longitudinal direction L,
• fixing the first and second ends (11, 12) respectively to a first and a second clamping module (22, 23) of a characterization test installation (20),
• thermomechanical fatigue loading of the specimen (10) in which:
  • the useful zone (13) of the specimen (10) is heated according to a succession of thermal cycles (CT) where the useful zone (13) of the specimen is heated from a first temperature to a second temperature for a period of predetermined time, and
  • the specimen (10) is simultaneously subjected to a first succession of cycles (CM1) of mechanical stress between a minimum mechanical stress and a maximum mechanical stress during the predetermined period of time.

Method according to the preceding claim, characterized in that the first predetermined period of time lasts at least two hours. Method according to one of the preceding claims, characterized in that each thermal cycle comprises a range (P) of maintaining the temperature at the second temperature before being cooled to the first temperature. Process according to the preceding claim, characterized in that the temperature maintenance range (P) at the second temperature lasts at least two hours. Process according to one of the preceding claims, characterized in that the first temperature is between -55°C and 60°C and the second temperature is between 120°C and 200°C. Method according to one of Claims 3 and 5, characterized in that each first succession of mechanical stress cycles (CM1) comprises a second succession of mechanical stress cycles (CM2) applied discontinuously over a time interval corresponding to the temperature maintenance range (P). Method according to one of the preceding claims, characterized in that it comprises a step of cooling the first and second clamping modules (22, 23) to a temperature whose value is lower than the glass transition temperature of the organic matrix . Method according to the preceding claim, characterized in that the first and second clamping modules (22, 23) are cooled to a temperature whose value is less than 50°C. Method according to one of the preceding claims, characterized in that the mechanical stresses are compressive and tensile stresses, the ratio of which is -1. Characterization test installation (20) of a specimen (10) made of composite material, the installation comprising:

• a first and a second clamping module (22, 23) intended to retain respectively a first and a second end (11, 12) of a test specimen (10) made of composite material representative of a part (1), such as a turbomachine part, the specimen (10) being made with a fibrous reinforcement embedded in an organic matrix,
• a heat treatment module (28) of a useful zone (13) of the specimen (10) arranged between the first and the second end (11, 12),
• an electronic control system (30) intended to drive the heat treatment module (28) and the tightening modules (22, 23) so as to produce a thermomechanical stress in which:
  • the useful zone (13) of the specimen is heated and/or cooled according to a succession of thermal cycles where the useful zone of the specimen is heated from a first temperature to a second temperature for a first predetermined period of time, and
  • the specimen (10) is simultaneously subjected to a first succession of mechanical stress cycles (CM1) between a minimum mechanical stress and a maximum mechanical stress during the predetermined period of time.

Installation (30) according to the preceding claim, characterized in that it comprises a cooling system (40) mounted on a frame (21) and intended to cool the clamping modules (22, 23) to a temperature lower than the temperature glass transition of the composite material. Installation (30) according to the preceding claim, characterized in that the cooling system (40) comprises means for cooling the useful zone (13) of the specimen (10). Installation (30) according to one of Claims 10 to 12, characterized in that the heat treatment module (28) comprises a heating member (35) installed on the specimen (10) and arranged around the useful zone ( 13) of the test piece (10). Installation (30) according to the preceding claim, characterized in that the heating member (35) is a spiral resistor. Installation (30) according to one of Claims 10 to 14, characterized in that elements (45) for measuring the temperature of the specimen (10) are arranged at the level of the useful zone (13) of the specimen (10) and connected to the electronic control system (30).