Test tube for mechanical testing

Abstract

The invention relates to a mechanical test specimen comprising an upper face and a lower face, characterized in that said specimen comprises successively, from its upper face to its lower face: an upper portion (10) made of a first metallic material, an intermediate portion (20) made of a composite material bonded to the upper portion (10) by a first adhesive interface (41), and a lower portion (30) made of a material having a tensile strength greater than the tensile strength of said composite material, said lower portion (30) being bonded to the intermediate portion (20) by a second adhesive interface (42), a thickness (a) of the upper portion (10) being greater than or equal to the sum of a thickness (b) of the intermediate portion (20) and a thickness (c) of the lower portion (30). Figure for the abstract: Fig 4

Description

Title of the invention: Test specimen for mechanical testing technical field

[0001] The present invention relates to the field of the aeronautical industry and more particularly to the manufacture of parts in organic composite material (CMO) such as propellers, blades, for all types of engines. State of the art

[0002] The fan blades of certain engines can be made of organic matrix composite (OMC). Such an engine 9 is illustrated in [Fig. 1]. To protect the fan blades 90 from erosion and improve bird-ingestion behavior, a metallic leading edge 91 can be bonded to them.

[0003] Tests have shown that a bird strike can cause detachment of the metallic leading edge, localized either in the adhesive or within the composite material. The key parameters for the type of damage observed are the quality of the interface created during the manufacturing of the part, and the mode of stress (for example, opening, shear, or a mixed mode).

[0004] At the material specimen scale, the tests for characterizing a bond available today in the literature are only concerned with the mechanical characterization of a uniform adhesive thickness between two substrates. This is the case for single shear tests (for example, EN 2243 standard), double shear tests (for example, ASTM D3528 standard), peel tests (for example, EN 2243 standard), tube shear tests (for example, Pr EN 2667-2 standard), cleavage tests (for example, ASTM D3433 standard), T-peel tests (for example, NF EN 1939 standard), etc.

[0005] However, these tests do not allow for control of the crack location, nor of the mixing of damage propagation modes. The modes to be distinguished are the opening mode illustrated in [Fig. 2A], the shear mode illustrated in [Fig. 2B], and a screwing mode.

[0006] Tube shear tests (TAST, acronym for Thick Adhesive Shear Test) according to ASTM D5656 (or NF EN 14869-2), and T-cleave and peel tests, allow for the characterization of the shear strength of an adhesive with metallic substrates. However, the mixed propagation mode is not controlled because the device is sensitive to edge effects. It is therefore impossible to characterize the failure near the interface observed by tests on actual parts.

[0007] Around a 3-point bending device, the interlaminar shear strength test (ILSS, acronym for the Anglo-Saxon term Interlaminar Shear Strength) according to the ASTM D2344, a standard for close-supported samples, is limited to thin specimens (6 mm). End-notched flexure (ENF) tests, according to ASTM D7905, are conducted to study crack propagation on a pre-cracked specimen. They are the most commonly used, although they result in unstable crack propagation. Finally, ISO 178 and ASTM D790 concern a homogeneous material that fractures under tension on its lower face. These standards do not aim to detect shear failure. Description of the invention

[0008] An object of the invention is to make available a test specimen enabling the reliable and localized characterization of a shear failure appearing from an adhesive interface between a portion of metallic material and a portion of composite material, in particular a CMO material.

[0009] To this end, the invention proposes a test specimen for mechanical testing comprising an upper face and a lower face, characterized in that said test specimen comprises successively from its upper face to its lower face: • an upper portion made of a first metallic material, • an intermediate portion made of a composite material bonded to the portion above above by a first adhesive interface, and • a lower portion made of a material having a tensile strength greater than the tensile strength of said composite material, said lower portion being bonded to the intermediate portion by a second adhesive interface, • a thickness of the upper portion being greater than or equal to the sum of a thickness of the middle portion and a thickness of the lower portion.

[0010] Adjusting the thicknesses of the substrates makes it possible to concentrate the stresses on a single interface between the upper portion and the intermediate portion, and to protect the composite from unwanted damage on its upper and lower faces.

[0011] Preferably, the material of the lower portion is a second metallic material.

[0012] Advantageously, the ratio between the thickness of the upper portion and the sum of the thicknesses of the upper portion, the intermediate portion and the lower portion is between 0.5 and 0.6. The ratio between the thickness of the intermediate portion and the thickness of the lower portion is between 0.8 and 1.2.

[0013] Advantageously, the composite material comprises a polymer matrix and a woven fiber structure. The fibers may be carbon, glass, or aramid. The polymer is preferably a thermosetting resin.

[0014] The invention also relates to a method for manufacturing a test specimen as described above, comprising the following steps: • the manufacture of a portion from a composite material, said portion having a top face and a bottom face opposite the top face, • the bonding of a lower portion to the lower face, said lower portion being made of a material having a breaking strength and tensile strength respectively greater than the breaking strength and tensile strength of the composite material, and • the bonding of an upper metal portion to the upper face, said metal portion having a thickness greater than or equal to the sum of the thicknesses of the portion in a composite material forming an intermediate portion, and the lower portion bonded to the lower face.

[0015] The invention also relates to a method for manufacturing a test tube as described above, comprising the following steps: • the provision of a mold suitable for being pressurized and heated, • the insertion of a first metal portion intended to form the portion su the outer edge of the test specimen on a first inner wall of the mold, said first portion having a thickness greater than or equal to half the height of the mold, • the insertion of a woven fiber structure into the cavity, • the insertion of a second portion intended to form the lower portion, made of a material having a tensile strength greater than 1000 Pa, onto a second inner wall of the mold opposite the first wall of the mold, so as to form a cavity between the first portion and the second portion, • The application of a layer of adhesive polymer to the surfaces of the first and second portions forming the cavity, • the injection of a polymer into the cavity, and • the crosslinking of the polymer by applying a temperature and / or pressure higher than the ambient temperature and / or pressure so as to form an intermediate portion of composite material.

[0016] The invention also relates to a mechanical testing method comprising the following steps: • the provision of a test tube as described above, • the application of a stress on two support areas arranged on a free face of the lower portion of the specimen and on at least one support area arranged on a free face of the upper portion of the specimen, at least a support zone on the free face of the upper portion being arranged between the support zones on the free face of the lower portion, • the determination of a failure at the interface between the upper portion and the intermediate portion as a function of the stress and / or a displacement of the support areas.

[0017] Advantageously, a layer of paint with an optical pattern is applied to a lateral face of the test specimen, and the determination of fracture is carried out by an imaging or video recording process of said optical pattern. Preferably, the optical pattern is a pattern of disordered dark dots on a light background. Brief description of the figures

[0018] Other features and advantages of the invention will become apparent from the detailed description that follows, with reference to the accompanying drawings, in which:

[0019] Fig. 1 shows a fan of an aircraft engine.

[0020] Figures 2A and 2B illustrate different modes of stress that can cause a test specimen to break.

[0021] Fig. 3 is a perspective view of a test tube according to the invention.

[0022] Figure 4 is a schematic cross-sectional view of a test specimen according to the invention and the support points during a testing procedure.

[0023] Figure 5 illustrates a method of manufacturing a test tube according to the invention.

[0024] Figures 6A to 6E illustrate another method of manufacturing a test tube according to the invention.

[0025] Figure 7 shows a test specimen according to the invention after a test procedure causing a rupture.

[0026] Fig. 8 shows different stresses in the different portions of a test specimen according to the invention as a function of the thickness ratio between the intermediate portion and the lower portion. Detailed description of embodiments. Presentation of the test tube.

[0027] Figure 3 illustrates a test specimen according to the invention. The specimen is in the form of a generally elongated parallelepiped having a length along an X-axis, a width along a Y-axis, and a thickness along a Z-axis. In what follows, the specimen is described in a typical orientation for a bending test, i.e., the upper and lower faces of the specimen are intended to receive means for applying stress during such a test.

[0028] The test specimen comprises, from its upper face to its lower face, an upper portion 10, an intermediate portion 20 and a lower portion 30. The portions su The upper portion 10 and the intermediate portion 20 are bonded by a first adhesive interface 41. The intermediate portion 20 and the lower portion 30 are bonded by a second adhesive interface 42. The first and second adhesive interfaces 41 and 42 are, for example, layers of adhesive polymer. The thickness of the film or adhesive layer is much smaller than the thicknesses of the individual portions and will not be taken into account when describing the dimensions of the different elements.

[0029] The upper portion 10 is made of a metallic material. Advantageously, the upper portion 10 is made of a metallic material used for blower blades, for example titanium or steel or a nickel-cobalt alloy.

[0030] The intermediate portion 20 is made of a composite, for example of the CMO type. The composite may comprise a polymer matrix and a reinforcing fiber structure incorporated into the polymer matrix. The polymer matrix is ​​typically a thermosetting resin such as an epoxy, bismaleimide, or polyimide. The reinforcing fibers incorporated into the polymer matrix are, for example, fibers made of carbon, glass, aramid, or another suitable fiber for incorporation into the polymer matrix. Preferably, the fibers are in the form of a woven structure to ensure greater mechanical strength of the composite material. Such a structure may be woven unidirectionally, in satin, twill, or 3D interlock, or by another weaving method, depending on the type of fibers and the desired geometry and mechanical behavior in the end use of the CMO material.

[0031] The lower portion 30 is made of a material having a tensile strength greater than the tensile strength of the composite material of the intermediate portion 20.

[0032] Tensile strength is characterized by criteria relating to deformation and allowable stresses for the material. In particular, the maximum tensile strength Rm (tensile strength) is the maximum force that a material can withstand before breaking. When this limit is reached, a crack propagates macroscopically, resulting in a drop in the force applied to the sample. Tensile strength is expressed in N / mm².

[0033] The high tensile strength of the lower portion 30 prevents it from being damaged before the interface 4L

[0034] It is also necessary that the material of the lower portion 30 can undergo non-plastic deformation during the tests, and that reliable bonding with the intermediate composite portion 20 can be easily achieved.

[0035] Preferably, the lower portion 30 is made of a metallic material. In this case, the metallic material of the lower portion 30 may be the same as or different from the metallic material of the upper portion 10. Alternatively, the material of The lower portion 30 may be made of another material exhibiting good tensile strength, for example, a ceramic matrix composite or cement.

[0036] With reference to [Fig. 4], the thickness h of the specimen may be between 1 and 5 cm, preferably close to 2 cm. The thickness a of the upper portion 10 is greater than or equal to the sum of the thicknesses b+c of the intermediate portion 20 and the lower portion 30. Advantageously, the thickness of the upper portion 10 is equal to or close to the sum b+c of the thicknesses of the intermediate portion 20 and the lower portion 30, respectively. This dimensioning makes it possible to locate the maximum shear stress applied during the bending test procedure near the interface between the upper portion 10 and the intermediate portion 20.

[0037] The ratio between the thickness b of the intermediate portion 20 and the thickness c of the lower portion 30 is advantageously between 0.8 and 1.2. Manufacturing the test tube

[0038] We will now describe the different stages of manufacturing a test specimen according to the invention. Typically, panels are produced comprising an upper portion, an intermediate portion, and a lower portion intended to form the corresponding portions of a batch of test specimens. The thickness of each panel corresponds to the thickness of the test specimen to be manufactured. The lateral and / or longitudinal dimensions of each panel are greater than the dimensions of a test specimen in order to produce a plurality of identical test specimens from each panel.

[0039] A first embodiment of such a panel is illustrated in [Fig. 5]. The panel is manufactured by producing a composite substrate and subsequently bonding the metallic portions.

[0040] Initially, a composite substrate 200 is produced by a generic process representative of the manufacturing of the blade in which the composite will be used. This generic process can be a liquid composite molding (LCM) technique such as vacuum assisted resin transfer molding (VARTM).

[0041] A dry preform made of woven fibers is positioned inside the cavity of a resin transfer mold (RTM, acronym for Resin Transfer Molding) comprising a resin inlet and a resin outlet. Advantageously, the outlet is located on a face of the mold opposite the face containing the inlet. Next, a thermosetting polymer resin is injected into the cavity. Typically, the injection cycle recommended by the polymer resin manufacturer is followed in terms of time, temperature, and pressure. After the composite material is removed from the mold, a surface treatment can be performed, for example, sandblasting and / or cleaning and / or a cutting to adapt the geometry to the dimensions of the test specimen to be produced.

[0042] Next, the substrates 100 and 300 are bonded to the composite substrate 200. First, a bonding primer compatible with the chosen adhesive and / or a specific surface treatment, for example, laser treatment of the metal, can be applied to one or more surfaces to be bonded to improve adhesion. The adhesive layers 41a and 42a are then applied between the substrates on surfaces 120 and 320. Finally, the assembly undergoes a protocol to polymerize the adhesives, i.e., to cross-link them so that they harden and form bonds with the substrates. An autoclave can, for example, perform this operation. The conditions are chosen to ensure good polymerization of the adhesive without damaging the composite plate.

[0043] By way of example, the composite can be made with IM7 carbon fiber (Hexel) and PR520 epoxy resin (Solvay). These elements are combined with AF191K epoxy adhesive (3M) and BR6747-1NC primer (Solvay) to bond titanium alloy metal plates.

[0044] Alternatively, the panel for manufacturing a batch of test specimens according to the invention can be produced by a co-curing process in a closed injection mold 80 as illustrated in [Fig. 6A]. In this embodiment, with reference to [Fig. 6B], a first metal substrate 101 intended to form the upper portion of the panel is inserted inside an injection mold 80. The substrate 101 is in surface contact with a first inner wall 84 of the mold 80. The adhesive polymer intended to form an adhesive layer 41b is applied to the substrate 101 in the mold 80. Subsequently, with reference to [Fig. 6C], a woven structure 27 made of reinforcing fibers is inserted above the substrate 301.

[0045] After the insertion of the woven structure, as shown in [Fig. 6D], a second substrate 301 is inserted to form the lower portion. The second substrate 301 has on its lower face a layer of adhesive polymer intended to form the adhesive layer 42b after polymerization. The upper face is in surface contact with a second inner wall 83 of the mold 80 opposite the first inner wall 84. The cavity formed between the first substrate 101 and the second substrate 301 contains the woven structure 27. The height of the cavity corresponds to the height 21 of the intermediate portion to be formed.

[0046] Alternatively, one can start by inserting layer 301 intended to form the lower portion of the specimen, and insert layer 101 intended to form the upper portion of the specimen after the insertion of the woven structure.

[0047] With reference to [Fig. 6E], the polymer intended to form the matrix of the composite material of the intermediate portion of the panel is subsequently injected into the cavity comprising the woven structure 27. The injection is carried out through an inlet 81 in The injection mold. Excess polymer can be discharged via an outlet 82 on the opposite side of the mold 80. The injection cycle recommended by the polymer resin manufacturer in terms of time, temperature, and pressure is typically followed. The composite portion and the adhesive layers are thus polymerized simultaneously. In this embodiment, the second substrate is specifically chosen to be a material resistant to the temperatures and pressures of the injection cycle.

[0048] It is also possible to use a combination of two embodiments and form, for example, a panel comprising two portions by co-firing and subsequently add a third portion by gluing a substrate.

[0049] The finished panel is then machined to cut it lengthwise and / or widthwise to form a plurality of test specimens. Given the thickness of the upper metal portion, the machining is preferably carried out using a waterjet cutting technique. As an example, the width of a test specimen typically varies between 20 and 30 mm and its length (2L) is between 100 and 200 mm.

[0050] In some cases, a single test specimen can be formed in the same way in its intended dimensions. In this case, the cutting step is not necessary. Test procedure

[0051] Mechanical testing generally refers to testing the mechanical properties of a specimen. This type of test includes bending tests, which are the main application of the specimen described above.

[0052] The bending test is typically carried out on a conventional 3-point bending device. Such a device is schematically represented in side view in [Fig. 4]. In the bending device, the specimen is positioned on two supports approximately 60 mm apart (21 = 125 mm).

[0053] The test is carried out at a temperature between -55°C and the glass transition temperature of the composite matrix. For example, for a PR520 matrix, the glass transition temperature is approximately 160°C.

[0054] The force is applied by a vertical displacement of a roller 50 located equidistant between the two supports 60. The displacement is therefore perpendicular to the adhesive interfaces 41, 42. The test is carried out at a predetermined displacement speed adapted to the thickness h and the stiffness of the specimen. The aim is to conduct the test within the elastic range of the different materials of the portions 10, 20, 30 forming the specimen. Typically, the displacement speed is between 0.1 mm / min and 10 mm / min.

[0055] The displacement imposed on the roller 50 generates tensile / compressive and shear forces in the specimen. Near the central horizontal plane (called the neutral axis for a homogeneous beam), the shear stress is at its maximum and the tensile / compressive stress is zero.

[0056] The deformations in the specimen are measured using an external image correlation device. For this purpose, an optical pattern is created on a front face of each specimen. A front face is defined as an external face parallel to the longitudinal axis X and the thickness h of the specimen, i.e., a plane in (X, Z) in [Fig. 3]. The front face is visible to the cameras during the test. The optical pattern is created by applying one or more layers of paint to the front face. The optical pattern comprises a plurality of dark, random dots on a light background.

[0057] The face bearing the optical pattern is filmed by two cameras during the shear test. Comparison of the two images makes it possible to observe precisely the displacement fields within the specimen and thus determine the deformations of the specimen.

[0058] Deformation and breakage depending on the thickness of the portions

[0059] During a bending test, the shear stress is maximum near a horizontal central plane of the specimen, which corresponds to the neutral axis for a homogeneous specimen. The precise position of the maximum shear stress depends on the mechanical properties of the materials forming the specimen and the thicknesses of the upper 10, intermediate 20, and lower 30 portions.

[0060] The thicknesses a, b, c of the respective portions 10, 20, 30 are chosen so that the maximum deformation occurs at the level of the first interface 41 between the upper portion 10 and the intermediate portion 20. This allows the bond strength between the upper portion 10 and the intermediate portion 20 to be characterized in a targeted manner.

[0061] Such a break 70 close to the horizontal central plane of the specimen is illustrated in [Fig.7].

[0062] Thus, we choose the thickness a of the upper portion 10 so that it is greater than or equal to the sum of the thicknesses b + c of the intermediate portion 20 and of the lower portion 30: a b+c.

[0063] The use of a ratio ai b+c makes it possible to guarantee that the failure of the specimen does not occur in compression in the composite, in particular vertically above the central roller 50. Indeed, the composite is thus only stressed in tension under the neutral fiber.

[0064] We therefore preferentially choose a ratio a / h between the thickness a of the upper portion 10 and the sum (h=a+b+c) of the thicknesses of the upper portion 10, the intermediate portion 20 and the lower portion 30 between 0.5 and 0.6.

[0065] It is important to respect ratios between the thicknesses a, b and c of the three substrates which ensure shear failure at the interface 41 while avoiding damage to the substrates or the interface 42 first.

[0066] The ratio of heights b and c between the intermediate portion 20 and the lower portion 30 is chosen according to the characteristics of the materials used. The lower portion 30 is designed to protect the intermediate composite portion 20. This lower portion 30 prevents the intermediate portion 20 from breaking under tension vertically from the central roller 50. It also prevents damage to the material at the points of contact 65 with the supports 60.

[0067] It is desired that the second interface 42 between the intermediate portion 20 and the lower portion 30 not undergo rupture during the shear test in order to avoid any disturbance in the characterization of the first interface 4L

[0068] Figure 8 shows different stresses in the different portions 10, 20, 30 of a test specimen according to the invention as a function of the thickness b of the intermediate portion 20. In the tests, the thickness of the upper portion 10 is fixed at a = 10 mm and the sum of the thicknesses b and c of the intermediate portion 20 and the lower portion 30 is maintained at b + c = 10 mm. The first interface 41 is therefore located at the central plane of the test specimen.

[0069] Curve 8a shows the stress in the second interface 42 (between the intermediate portion 20 and the lower portion 30) relative to the stress in the first interface 41 (between the upper portion 10 and the intermediate portion 20). When the composite intermediate portion 20 is thin compared to the lower portion 30 (b is low), the second interface 42 approaches the first interface 4L. The shear difference between the two interfaces 41 and 42 is therefore less pronounced. In this case, the second interface 42 can easily fail before or at the same time as the first interface 41, which introduces a bias in the analysis of the first interface 4L. It is therefore desirable to increase the thickness b of the intermediate portion 20 to minimize this value and to localize the stress in the first interface 41 for material characterization.

[0070] Curve 8b represents the stress within the composite material of the intermediate portion 20. When this stress is high, a fracture is likely to occur within the thickness of the intermediate portion 20. The thicker the composite intermediate portion 20 (higher b), the greater the shear stress in the core of the composite, and the greater the risk of damage before a fracture occurs at the first interface 4L. Therefore, the aim is to minimize the value of curve 8b and reduce the thickness b of the portion

[0071] Curve 8c refers to the right-hand y-axis. It expresses the ratio between the maximum shear stress at interface 41 and the sum of all stresses experienced by this same interface at the point of maximum shear. In order to compare the test results with theoretical models, a predominant shear loading mode is required. This corresponds to the maximum of curve 8c.

[0072] Taking these constraints into account, a compromise for a thickness ratio close to b / c = 1 is the most suitable. Therefore, a b / c ratio between these two portions 20, 30 is targeted to be between 0.8 and 1.2, or more advantageously between 0.9 and 1.1.

[0073] When the thickness b of the intermediate portion 20 is close to the thickness c of the lower portion 30, the second interface 42 is under little stress during a bending test and does not suffer any failure before or during the failure at the first interface 41. At the same time, the stress inside the intermediate portion 20 is controlled to avoid a risk of failure inside the intermediate portion 20 in an area far from the first interface 41.

Claims

Demands

1. Flexural test specimen comprising an upper face and a lower face, characterized in that said specimen comprises successively from its upper face to its lower face: • an upper portion (10) of a first metallic material, • an intermediate portion (20) of a composite material linked to the upper portion (10) by a first adhesive interface (41), and • a lower portion (30) of a material having a tensile strength greater than a tensile strength of said composite material, said lower portion (30) being linked to the intermediate portion (20) by a second adhesive interface (42), • a thickness (a) of the upper portion (10) being greater than or equal to the sum of a thickness (b) of the intermediate portion (20) and a thickness (c) of the lower portion (30).

2. Test specimen according to claim 1, wherein the material of the lower portion (30) is a second metallic material.

3. Specimen according to claim 1 or claim 2, wherein the ratio between the thickness (a) of the upper portion (10) and the sum (h) of the thicknesses (a, b, c) of the upper portion (10), the middle portion (20) and the lower portion (30) is between 0.5 and 0.

6.

4. Specimen according to any one of claims 1 to 3, wherein the ratio between the thickness (b) of the intermediate portion (20) and the thickness (c) of the lower portion (30) is between 0.8 and 1.

2.

5. Test specimen according to any one of claims 1 to 4, wherein the composite material comprises a matrix in a polymer and a woven structure in fibers.

6. Test specimen according to claim 5, wherein the fibers are carbon, glass or aramid fibers.

7. Test specimen according to any one of claims 5 to 6, wherein the polymer is a thermosetting resin.

8. A method for manufacturing a test specimen according to claims 1 to 7 comprising the following steps:

9. • the manufacture of a portion from a composite material, said portion having a top face and a bottom face opposite the top face, • the bonding of a lower portion (30) to the lower face, said lower portion being made of a material having a breaking strength and a tensile strength respectively greater than a breaking strength and a tensile strength of the composite material, and • the bonding of an upper portion (10) of metal to the upper face, said metal portion having a thickness greater than or equal to the sum of the thicknesses of the portion of a composite material (20) forming an intermediate portion, and the lower portion (30) bonded to the lower face. A method for manufacturing a test specimen according to claims 1 to 7 comprising the following steps: • the provision of a mold (80) suitable for being pressurized and heated, • the insertion of a first metal portion intended to form the upper portion (10) of the test specimen onto a first internal wall (83) of the mold (80), said first portion having a thickness greater than or equal to half the height of the mold (80), • the insertion of a woven fiber structure (27) into the cavity, • the insertion of a second portion intended to form the portion lower (30) of a material having a tensile strength greater than 1000 Pa on a second inner wall (84) of the mold (80) opposite the first wall (83) of the mold (80), so as to form a cavity between the first portion and the second portion, • The application of a layer of adhesive polymer to the surfaces of the first and second portions forming the cavity, • the injection of a polymer into the cavity, and • crosslinking of the polymer by applying a temperature and / or pressure higher than the ambient temperature and / or pressure so as to form an intermediate portion (20) in composite material.

10. Mechanical test method comprising the following steps: • making available a test specimen according to one of claims 1 to 7, • applying a stress on two bearing areas (65) arranged on a free face (300) of the lower portion (30) of the test specimen and on at least one bearing area arranged on a free face (100) of the upper portion (100) of the test specimen, the at least one bearing area on the free face (100) of the upper portion being arranged between the bearing areas (65) on the free face (300) of the lower portion (300), • determining a failure at the interface between the upper portion (10) and the intermediate portion (20) as a function of the stress and / or a displacement of the bearing areas (65).

11. A method according to claim 10 wherein a layer of paint having an optical pattern is applied to a lateral face of the specimen and the determination of the break is carried out by an imaging or video recording method of said optical pattern.

12. A method according to claim 11 wherein the optical pattern is a pattern of disordered dark dots on a light background.

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