COMPOSITE MATERIAL TEST PIECE AND CORRESPONDING MANUFACTURING METHOD

Abstract

The invention relates to a method for manufacturing a characterization test piece (100) made of composite material, comprising a step of producing a fiber preform by three-dimensional weaving of layers of longitudinal threads (120) with layers of transverse threads (125), the fiber preform (110) having a length in a longitudinal direction (L) and a thickness in a transverse direction (T), the fiber preform having a shape corresponding to that of at least one test piece to be manufactured and comprising a central portion delimited by two end portions, the central portion having a width called the central width and the end portions each having a width called the extreme width in a third direction (V), the central width being less than each of the extreme widths of the two end portions,the longitudinal wires (120) being continuous over the entire length of the preform and the transverse wires being continuous over the entire thickness of the preform. Figure for abstract: Figure 2,

Description

Title of the invention: COMPOSITE MATERIAL TEST PIECE AND METHOD FOR MANUFACTURING SAME RESPONDENT Technical field of the invention

[0001] The invention relates to a test piece for measuring the deformations generated in a part made of composite material, in particular a part for an aircraft turbomachine, as well as a method for manufacturing such a test piece. Technical background

[0002] In a known manner, for the dimensioning of a part for an aircraft turbomachine, such as a blade, it is necessary to know the mechanical properties of the part to feed a finite element calculation and thus judge the conformity of a loading with respect to the resistance of the material selected for the application.

[0003] These mechanical data are generally obtained from elementary mechanical tests on test pieces, representative of an elementary volume of the part. By elementary volume, we mean a sub-assembly of the part whose behavior corresponds to that of the macroscopic part, same rigidity and same behavior at break.

[0004] For example, we can cite tensile tests according to standard NF EN ISO 527, shear tests according to standard ASTM D 7078 or bending tests according to standard NF EN ISO 14125.

[0005] A part for an aircraft turbomachine, such as a blade, is generally made of composite material. Composite material parts are made from a preform obtained by three-dimensional weaving of threads.

[0006] Usually, for 3D woven composite materials, the tensile test specimens are of the straight or dumbbell type and taken from flat panels, obtained from an LCM type process according to the English acronym "Liquid Composite Molding", more precisely from the RTM type process according to the English acronym "Resin Transfer Molding" then machined to give them their final shape.

[0007] However, such test pieces may exhibit non-compliant ruptures during mechanical tensile, compression or even fatigue tests, i.e. ruptures outside the so-called useful zone. With reference to [Fig. 1] which illustrates such a dumbbell-type test piece 10 comprising a body 12 extending between two ends 14, these ruptures denoted R may be located at the level of the connection fillets 16 between the useful zone Z1 of the body 12 and the ends 14. Similarly, for a straight-type test piece, the ruptures may be located at the level heels, that is to say the ends of the test piece, well outside the useful zone.

[0008] These non-compliant ruptures are due to the fact that the fibers or threads 20 at the ends of the test piece have cut fibers or particular local orientations generated by a weave, for example of the interlock type. The threads extend parallel to the longitudinal direction L of the test piece. These anomalies generate very local shear forces which are not well tolerated by this type of composite material. In addition, they lead to very large dispersions of the test values, which increases their standard deviation and ultimately penalizes the dimensioning of the parts.

[0009] Consequently, these types of rupture generally lead to non-compliance or even cancellation of the test since they do not comply with the testing standards in force.

[0010] The invention aims to propose a solution making it possible to remedy at least some of these drawbacks.

[0011] In particular, the present invention provides a method for manufacturing a composite material test piece shaped for mechanical tests of the compression, traction and fatigue type. Summary of the invention

[0012] The invention relates to a method for manufacturing a characterization test piece made of composite material, the method comprising at least the following steps: - the production of a fiber preform by three-dimensional weaving of layers of longitudinal yarns with layers of transverse yarns, the fiber preform having a length in a longitudinal direction and a thickness in a transverse direction intersecting the longitudinal direction, the longitudinal yarns extending in the longitudinal direction and the transverse yarns extending in the transverse direction, the fiber preform having a shape corresponding to that of at least one test piece to be manufactured and comprising a central portion delimited by two extreme portions, the central portion having a width called the central width and the extreme portions each having a width called the extreme width in a third direction, the longitudinal direction, the transverse direction and the third direction forming a trihedron, the central width being less than each of the extreme widths of the two extreme portions,the longitudinal threads being continuous over the entire length of the preform and the transverse threads being continuous over the entire thickness of the preform, and, - injection consisting of injecting a resin into the fiber preform previously placed in a mold so as to produce a part comprising at least the characterization test piece, - densification of the resin-impregnated fiber preform to form a part.

[0013] Such a method according to the invention makes it possible to manufacture a part that can lead to one or more characterization specimens, the part having a dumbbell-type shape like those of the specimens. Thanks to the invention, the wires of the specimen follow the shape of the dumbbell continuously and guarantee that no force is transferred from one end of the specimen to the other by shearing in the fillets but by traction in the direction of the wires.

[0014] Furthermore, according to the invention, the thickness variations between the two ends are achieved by changing the size of the wire strands, whereas in the state of the art they are achieved by machining. Thus, the test piece obtained by the method according to the invention is free of fibers cut in the direction of stress.

[0015] In addition, since the thickness changes are made directly in the 3D woven preform, continuity of the threads is obtained in the fillets, avoiding breaks in the threads in these areas, which are usually machined.

[0016] Thus, the invention makes it possible to eliminate non-compliant test specimen ruptures under tensile and / or compressive stresses in order to reduce the dispersion of the test values ​​and therefore to gain margin on the part dimensioning, in particular with regard to fatigue.

[0017] The manufacturing method according to the invention may comprise one or more of the following characteristics, taken individually or in combination with each other in all technically possible combinations: - the longitudinal direction, the transverse direction and the third direction form a right trihedron; - the two extreme portions have the same extreme width in the third direction; - the two extreme portions have different extreme widths in the third direction, each of the extreme widths being greater than the central width; - the diameter of the transverse wires of the extreme portions is greater than the diameter of the transverse wires of the central portion; - the transverse wires of the extreme portions and the transverse wires of the central portion are made of different materials so that the diameter of the transverse wires of the extreme portions is greater than the diameter of the transverse wires of the central portion; - the transverse threads of two neighboring layers of transverse threads are aligned in the third direction defining columns called transverse columns, the number of transverse threads of the transverse columns of the extreme portions being greater than the number of transverse threads of the transverse columns of the central portion for the same number of layers of transverse threads; - the central portion comprises a useful portion in which the number of transverse wires is constant from one transverse column to the next and two connecting portions, each connecting portion being arranged between the useful portion and an extreme portion and the number of transverse wires of each connecting portion increases from one transverse column to the next from the useful portion towards the extreme portions for the same number of layers of transverse wires; - the transverse wires of the same transverse column are grouped into transverse strands and the count of the transverse strands of each connecting portion increases from one transverse column to the other from the useful portion towards the extreme portions for the same number of layers of transverse wires; - the transverse wires of the same transverse column are grouped into transverse strands and the number of transverse strands of each connecting portion increases from one transverse column to another from the useful portion towards the extreme portions for the same number of layers of transverse wires; - the transverse columns of transverse wires are less spaced from each other in the longitudinal direction in the extreme portions than in the central portion; - the longitudinal threads are warp threads and the transverse threads are weft threads; - the longitudinal threads are weft threads and the transverse threads are warp threads; - the method comprises a step of cutting the part along a plane perpendicular to the transverse wires to obtain the characterization test piece; - the fiber preform is obtained by interlock weaving.

[0018] The invention also relates to a characterization test piece made of composite material comprising a body extending in a longitudinal direction between two ends, the two ends having at least one dimension greater than the dimension of the body in a third direction, the test piece comprising a woven fiber preform embedded in a resin comprising a central portion delimited by two end portions, the central portion and the end portions corresponding respectively to the body and to the ends of said test piece, the fiber preform having a three-dimensional weave of layers of longitudinal threads with layers of transverse threads, the fiber preform having a length in the longitudinal direction and a thickness in a transverse direction, the longitudinal direction, the transverse direction and the third direction forming a trihedron, the longitudinal threads extending in the longitudinal direction,the transverse threads extending in the transverse direction, the portion, central having a width called central width less than each width called extreme width of the two extreme portions in the third direction, the test piece being characterized in that the longitudinal wires are continuous over the entire length of the preform and the transverse wires are continuous over the entire thickness of the preform.

[0019] The test piece is advantageously obtained by a manufacturing method according to the invention and as described previously.

[0020] The test piece according to the invention may comprise one or more of the following characteristics, taken individually or in combination with each other in all technically possible combinations: - the central portion comprises a useful portion in which the number of transverse wires is constant and two connecting portions, each connecting portion being arranged between the useful portion and an extreme portion and the number of transverse wires of each connecting portion increases from the useful portion towards the extreme portions; - the transverse wires are grouped into transverse strands and the count of the transverse strands of each connecting portion increases from the useful portion towards the extreme portions; - the transverse wires are grouped for example into transverse strands and the number of transverse strands of each connecting portion increases from the useful portion towards the extreme portions. Brief description of the figures

[0021] The invention will be better understood and other details, characteristics and advantages of the present invention will appear more clearly on reading the detailed description which follows comprising embodiments, given for illustrative purposes with reference to the appended figures and presented as non-limiting examples, which may serve to complete the understanding of the present invention and the description of its embodiment and, where appropriate, contribute to its definition, in which: - [Fig.l], already described, is a three-dimensional schematic representation of a damaged dumbbell-type test piece; - [Fig.2] is a schematic representation of the profile of a dumbbell-type test piece according to the invention; - [Fig.3] is a flowchart illustrating a manufacturing method according to the invention of the test piece of [Fig.2]; - [Fig.4] represents an example of weaving a fiber preform making it possible to obtain the test piece of [Fig.2]; - [Fig.5] illustrates another example of weaving a fiber preform allowing to obtain the test piece of [Fig.2]; - [Fig.6] is a three-dimensional schematic representation of a part obtained by the method of [Fig.3] and making it possible to obtain three dumbbell-type test pieces according to the invention; and - [Fig.7] is a three-dimensional schematic representation of a Jacquard type loom for producing a fiber preform such as those in Figures 4 and 5.

[0022] Elements having the same functions in different implementations have the same references in the figures.

[0023] In the description and the claims, the terminology longitudinal, transverse and vertical will be adopted without limitation with reference to the trihedron L, T, V indicated in the figures. Detailed description of the invention

[0024] [Fig. 2] is a schematic cross-sectional representation of a characterization test piece 100 according to the invention. Such a characterization test piece is very useful for measuring in particular the deformations generated in a part made of composite material, in particular a part for an aircraft turbomachine. The part may, for example, be a blade, in particular a blade of a fan of the turbomachine.

[0025] According to the invention, the characterization test piece 100 is of generally cylindrical shape with a main axis extending in a first direction called the longitudinal direction L, with a preferably rectangular cross-section, that is to say a rectangular cross-section in a plane perpendicular to the longitudinal direction L. As illustrated schematically in [Fig. 2], the characterization test piece 100 has a dumbbell shape. More precisely, the characterization test piece 100 comprises a body 102 extending in the longitudinal direction L between two ends 104. The ends 104 are also called heads or feet of the test piece. Each end 104 has a constant cross-section. The width of the characterization test piece 100, that is to say the dimension thereof in a second direction called the vertical direction V, varies from one end to the other.The vertical direction V is perpendicular to the longitudinal direction L. In particular, each end 104 has a width D21, D22 greater than the width DI of the body 102, in the vertical direction V.

[0026] The body 102 comprises a useful zone ZI of constant cross-section. It is in this zone that any deformations must be measured during characterization tests. Furthermore, the body 102 comprises two connecting areas 106 also called fillets. Each connecting area 106 is arranged between the useful area ZI of the body 102 and one end 104 of the test piece. In addition, each connecting zone 106 has a width (dimension in the vertical direction V) which varies continuously from the width of the useful zone ZI to the width of the ends 104.

[0027] Furthermore, the characterization test piece 100 has a uniform thickness E along a transverse direction T perpendicular to the longitudinal L and vertical V directions so that the longitudinal L, transverse T and vertical V directions form a right-angled trihedron. Of course, the description can be generalized to the case where the longitudinal L, transverse T and vertical V directions are intersecting but not perpendicular to each other two by two so as to form a (non-right-angled) trihedron.

[0028] The test piece 100 is made of a composite material for measuring deformations generated in a part made of the same composite material. The composite material comprises fibers woven together so as to form at least one fiber preform 110, in three dimensions, intended to be embedded in a resin. The entire test piece, body and ends, is produced continuously from the same fiber preform.

[0029] A manufacturing method according to the invention of such a characterization test piece made of composite material will now be detailed with reference to the flowchart illustrated in [Fig.3]. The manufacturing method comprises a step S10 of producing a fiber preform 110 in a single piece by three-dimensional weaving of threads. The threads are separated into warp threads and weft threads and the warp threads are interlaced with the weft threads according to a three-dimensional weave.

[0030] By "three-dimensional weaving" or "3D weaving" is meant here a weaving method by which certain warp threads intertwine or interlace with weft threads on several layers, such as for example and preferably for a so-called interlock weave.

[0031] By "interlock weave" is meant here a 3D weave weave in which each warp layer interlaces with several weft layers so that all the threads of the same warp column have the same movement in the plane of the weave.

[0032] According to the invention, the fiber preform 110 is obtained by any type of three-dimensional weaving as defined above. Such a fiber structure obtained by 3D weaving makes it possible to obtain a connection between the layers, and therefore makes it possible to have good mechanical strength of the fiber structure and of the composite material part obtained, in a single textile operation.

[0033] The fibrous structure can be produced in a known manner by means of a Jacquard 40 type loom as shown in [Fig.7], on which a bundle of warp threads or strands has been arranged in a plurality of layers, the warp threads being linked by layers of weft threads or strands also arranged in a plurality of layers. The yarns or strands used to weave the fiber preform can be made of carbon or glass fibers or Kevlar.

[0034] Thus, the fiber preform 110 is obtained by three-dimensional weaving of layers of longitudinal yarns 120 with layers of transverse yarns 125. In the remainder of the description, the warp yarns are longitudinal yarns 120 extending in the longitudinal direction L of the fiber preform and the weft yarns are transverse yarns 125 extending in the transverse direction denoted T perpendicular to the longitudinal L and vertical V directions. Of course, as a variant, the longitudinal yarns can be weft yarns and the transverse yarns can be warp yarns. The warp yarns and the weft yarns define layers of yarns, which stack in the vertical direction V and which define the width of the fiber preform.

[0035] With reference to Figures 4 and 5, the fiber preform 110 has a shape corresponding to that of at least one test piece to be manufactured. Thus, the fiber preform 110 intended to form the characterization test piece 100 comprises a central portion 112 delimited by two end portions 114. In particular, the central portion 112 is intended to form the body 102 of the test piece while the end portions 114 are intended to form the ends 104 of said test piece. In a similar manner to the test piece 100, the central portion 112 of the fiber preform 110 comprises a useful portion 115 and two connecting portions 116. Each connecting portion 116 is arranged between the useful portion 115 and an extreme portion 114. The useful portion 115 and the two connecting portions 116 are intended to form respectively the useful zone ZI and the connecting zones 106 of the body of the test piece.

[0036] The central portion 112 has a width called the central width d1 and the extreme portions each have a width called the extreme width in the vertical direction V. The width of the end visible in Figures 4 and 5 is noted d21. According to the invention, the central width dl is less than each of the extreme widths of the two extreme portions 114. The two extreme portions 114 may have the same extreme width in the vertical direction V or different extreme widths, each of the extreme widths being greater than the central width dl.

[0037] Furthermore, according to the invention, the longitudinal wires 120 are continuous over the entire length of the preform in the longitudinal direction L and the transverse wires 125 are continuous over the entire thickness of the preform in the transverse direction T.

[0038] [Fig. 4] schematically represents a portion of an example of weaving weave, that is to say a mode of interlacing or interlacing of the warp threads and the layers of weft threads of the fiber preform 110 during this step S10. The warp threads C1 to C6 are longitudinal threads 120 extending in the direction longitudinal L of the fiber preform and the weft threads are transverse threads 125 extending in the transverse direction noted T perpendicular to the longitudinal L and vertical V directions.

[0039] Thus, each layer of transverse wires 125 extends substantially in a plane (T, L) perpendicular to the vertical direction V. In the example illustrated, the transverse wires 125 of the same weft layer are distributed and grouped to form columns called columns of transverse wires. Each column of transverse wires contains one or more transverse wires. When a column contains more than one transverse wire, these may optionally be grouped to form one or more strands of wires. In the example illustrated, there are 10 columns of transverse wires denoted T1 to T10. The columns of transverse wires of the layers of transverse wires 125 are aligned to form a matrix of groups of transverse wires, a group containing one or more transverse wires.

[0040] In [Fig. 4] are shown six layers C1 to CT6 of weft yarns or strands and six warp yarns or strands C1 to C6. The paths of the warp yarns or strands C1 to C6 are illustrated. The warp yarns intertwine with weft yarns belonging to different layers of weft yarns. The weft yarns of two layers of weft yarns are aligned on the same columns, called weft columns. Ten columns of weft yarns T1 to T10 are shown. They correspond to the columns of transverse yarns described above in this example. [Fig.4] thus shows schematically a partial chain plan of the fiber preform 110.

[0041] According to the invention, the number of transverse threads 125 of the columns of transverse threads, also called transverse columns, of the extreme portions is greater than the number of transverse threads of the transverse columns of the central portion of the fiber preform for the same number of layers of transverse threads 125. In other words, for each layer of transverse threads, the number of transverse threads 125 of the columns of transverse threads, also called transverse columns, of the extreme portions is greater than the number of transverse threads of the transverse columns.

[0042] More precisely, the number NI of weft threads (transverse 125) is constant in the useful portion 115 of the central portion 112 of the fiber preform 110 from one weft column to another for each layer of transverse threads. In the example illustrated, for the same layer of weft threads C1 to CT6, the number of weft threads per column is NI regardless of the weft column T1 to T3 and regardless of the layer of weft threads.

[0043] Furthermore, the number N2 of weft threads (transverse 125) is constant from one weft column to another (T7 to T10) in the extreme portions 114 of the fiber preform. 110 and greater than the number NI of weft threads (transverse 125) from one weft column to the other of the useful portion 115 guaranteeing that the thickness of the fiber preform 110 is greater in the extreme portions 114 than in the central portion 112, and therefore that the thickness of the body 102 of the test piece 100 is greater than that of its ends 104 without increasing the number of layers of weft threads or cutting threads cut by machining.

[0044] In a known manner, the warp threads (longitudinal 120) are generally grouped, for example, into warp strands (longitudinal) and the weft threads (transverse 125) are generally grouped, for example, into weft strands (transverse).

[0045] In this case, the weft strands (transverse 125) of the end portions 114 have a greater yarn count than the transverse strands of the central portion 112, the yarn count of a strand being the number of wires it contains. In other words, the weft strands (transverse 125) of the end portions 114 have a diameter greater than the diameter of the transverse strands of the central portion 112, making it possible to obtain a thickness of the end portions 114 greater than that of the central portion 102.

[0046] Preferably, the number of transverse threads 125 (i.e. weft threads) of each connecting portion 116 increases progressively from one transverse column to the other from the useful portion 115 towards the extreme portions 114 for each layer of transverse threads in order to continuously increase the width (i.e. the dimension in the vertical direction V) of the fiber preform 110 from the useful portion 115 towards the extreme portions 114. In other words, the count of the transverse strands (i.e. weft strands) of each connecting portion 116, and therefore their diameter, increases progressively from one transverse column to the other from the useful portion 115 towards the extreme portions 114 for each layer of transverse threads. In the example illustrated in [Fig.4], the transverse strands of the useful portion 115 (on the left of the figure) have a count of 24,000 transverse wires and the transverse strands of the extreme portion 114 have a count of 72,000 wires. In the connecting portion 116, the count of the transverse strands is successively equal to 36,000 wires then 48,000 wires from the useful portion to the extreme portion.

[0047] According to another embodiment, the transverse strands may all have the same count. In this case, the number of transverse strands is increased from one transverse column to the other from the useful portion 115 towards the extreme portions 114 for each layer of transverse wires in order to increase the number of transverse wires from the useful portion towards the extreme portion.

[0048] Of course, these two embodiments can be combined. [Fig. 5] illustrates an example of a combination of these two embodiments. Thus, this example of weaving armor differs from that of [Fig. 4], in that in the connecting portion 116, the count and / or the number of transverse strands increases so that the The number of transverse threads increases from the useful portion to the extreme portion. In the first two weft columns, there is only one strand of 36,000 threads per warp layer, while the next column has twice as many strands of 24,000 threads, or 48,000 threads in total.

[0049] The method continues with an injection step S20 consisting of injecting a resin into the fiber preform previously placed in a mold so as to produce a part 200 comprising at least the characterization test piece. In other words, after being placed in a mold, the fiber preform 110 is impregnated with the resin, for example an organic or polymer resin. The resin may be thermosetting. It may be an epoxy or bismaleimide resin.

[0050] The method then comprises a step S30 of densification or consolidation of the fiber preform impregnated with the resin to form a part. During this step, the resin is polymerized. More precisely, counter-molds are arranged around the fiber preform in order to hold it in the mold in a sealed manner. Then, the assembly is transported to an oven or furnace in which the fiber preform is densified by a matrix. The densification of the fiber preform consists of filling the porosity of the preform, in all or part of its volume, with the material constituting the matrix. The matrix is ​​obtained by injecting the resin into the fiber preform and polymerizing it by heat treatment.

[0051] The densification of the fiber preform can be carried out by a transfer molding process (RTM, acronym for the English expression “Resin Transfer Molding”). The polymerization can also be carried out by any process allowing a preform to be filled with resin, for example by infusion, autoclave cooking or press.

[0052] The part is then demolded and then trimmed to remove the excess resin. The part in the shape of a test piece is thus obtained after possible finishing machining.

[0053] Furthermore, if the part obtained has a sufficient transverse dimension, it can be cut during an optional cutting step S40 during which the part is cut along a plane perpendicular to the transverse wires to obtain a characterization test piece.

[0054] [Fig. 6] represents a part 200 obtained by such a method according to the invention. The part has a transverse dimension sufficient to form three characterization test pieces 100 after cutting the part 200 according to the cutting planes P1 and P2. The characterization specimens 100 obtained are all taken from a piece whose longitudinal wires follow the shape of the dumbbell thanks to the variation in the count and / or the number of strands resulting in a variation in the width (i.e. the dimension in the vertical direction V) of the characterization specimen.

[0055] Each dumbbell-type test piece 100 obtained, by its shape and the organization of the associated wires, allows the transfer of force from one end of the specimen to the other not by shearing in the fillets but by traction in the direction of the wires. Similarly, the continuity of the wire strands in the connection fillets makes it possible to avoid fiber breaks in these areas, which are usually machined.

[0056] The fact that one of the directions of the wires is the direction of the main axis of the test piece makes the force continuous between the two ends of the characterization test piece.

[0057] According to another embodiment compatible with the previous ones, the transverse columns of transverse threads 125 are furthermore less spaced from each other in the longitudinal direction in the extreme portions 114 than in the central portion 112. The distance between two transverse columns of transverse threads 125 is also known under the terms “weft spacing” or “distance between weft insertions”. This spacing is obtained by adjusting the loom which fixes the spacings between two successive wefts according to a technique known under the English name “pick-spacing”. The weft threads or strands of the same column are all woven then the preform being produced advances before the weaving of a new column and so on.

[0058] The method according to the invention makes it possible to eliminate non-conforming ruptures in the test pieces obtained by it under tensile and / or compressive stresses in order to reduce the dispersions of the test values.

[0059] Obviously, the invention is not limited to the embodiments described above and provided solely by way of example. It encompasses various modifications, alternative forms and other variants that may be envisaged by those skilled in the art within the scope of the invention and in particular all combinations of the different operating modes described above, which may be taken separately or in association.

Claims

Claims

1. Method for manufacturing a characterization test specimen (100) made of composite material, the method comprising at least the following steps: - producing (S 10) a fiber preform (110) by three-dimensional weaving of layers of longitudinal threads (120) with layers of transverse threads (125), the fiber preform (110) having a length in a longitudinal direction (L) and a thickness in a transverse direction (T) secant to the longitudinal direction, the longitudinal threads (120) extending in the longitudinal direction (L) and the transverse threads (125) extending in the transverse direction (T), the fiber preform (110) having a shape corresponding to that of at least one test specimen to be manufactured and comprising a central portion (112) delimited by two end portions (114),the central portion (112) having a width called central width (dl) and the extreme portions each having a width called extreme width (d21) in a third direction (V), the longitudinal direction (L), the transverse direction (T) and the third direction (V) forming a trihedron, the central width (dl) being less than each of the extreme widths (d21) of the two extreme portions (114), the longitudinal threads (120) being continuous over the entire length of the preform and the transverse threads (125) being continuous over the entire thickness of the preform, and - the injection (S20) consisting of injecting a resin into the fibrous preform (110) previously placed in a mold so as to produce a part comprising at least the characterization test piece, - the densification (S30) of the fibrous preform impregnated with the resin to form a part (200).,

2. The method of claim 1, wherein the diameter of the transverse wires (125) of the end portions (114) is greater than the diameter of the transverse wires (125) of the central portion (112).

3. Method according to claim 2, in which the transverse wires (125) of the end portions (114) and the transverse wires (125) of the central portion (112) are made of different materials so that the diameter of the transverse wires (125) of the end portions (114) is greater than the diameter of the transverse wires (125) of the central portion (112).

4. Method according to one of the preceding claims, in which the transverse threads (125) of two neighboring layers of transverse threads are aligned in the third direction (V) defining columns called transverse columns, the number of transverse threads (125) of the transverse columns of the extreme portions (114) being greater than the number of transverse threads (125) of the transverse columns of the central portion (112) for the same number of layers of transverse threads.

5. Method according to claim 4, in which the central portion (112) comprises a useful portion (115) in which the number of transverse wires (125) is constant from one transverse column to the other and two connecting portions (116), each connecting portion being arranged between the useful portion (115) and an extreme portion (114) and the number of transverse wires of each connecting portion (116) increases from one transverse column to the other from the useful portion (115) towards the extreme portions (114) for the same number of layers of transverse wires.

6. Method according to claim 5, in which the transverse wires (125) of the same transverse column are grouped into transverse strands and the count of the transverse strands of each connecting portion (116) increases from one transverse column to the other from the useful portion (115) towards the extreme portions (114) for the same number of layers of transverse wires.

7. Method according to claim 5 or 6, in which the transverse wires (125) of the same transverse column are grouped into transverse strands and the number of transverse strands of each connecting portion (116) increases from one transverse column to the other from the useful portion (115) towards the extreme portions (114) for the same number of layers of transverse wires.

8. A method according to any one of claims 5 to 7, wherein the transverse columns of transverse wires (125) are less spaced from each other in the longitudinal direction in the end portions (114) than in the central portion (112).

9. Characterization specimen (100) made of composite material comprising a body (102) extending in a longitudinal direction (L) between two ends (104), the two ends (104) having at least one dimension greater than the dimension of the body in a third direction (V), the specimen comprising a woven fibrous preform (110) embedded in a resin comprising a central portion (112) delimited by two extreme portions (114), the central portion (112) and the extreme portions (114) corresponding respectively to the body (102) and to the ends (104) of said test piece, the fiber preform (110) having a three-dimensional weave of layers of longitudinal threads (120) with layers of transverse threads (125), the fiber preform (110) having a length in the longitudinal direction (L) and a thickness (e) in a transverse direction (T), the longitudinal direction (L), the transverse direction (T) and the third direction (V) forming a trihedron, the longitudinal threads (120) extending in the longitudinal direction (L), the transverse threads (125) extending in the transverse direction (T), the central portion (112) having a width called central width (dl) less than each width called extreme width (d21) of the two extreme portions (114) according to the third direction (V),the test piece being characterized in that the longitudinal wires (120) are continuous over the entire length of the preform and the transverse wires (125) are continuous over the entire thickness of the preform.,

10. Test tube according to claim 9 obtained by a manufacturing method according to any one of claims 1 to 8.

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