EP1530709A2 - Method, device and machine for pure bending test optionally alternating - Google Patents

Abstract

The invention concerns a method, a device and a machine for pure bending test, optionally alternating. Two mutually identical items from a common specimen (1) are subjected to antagonist bending loads optionally alternating, while maintaining a mutual symmetry relative to a point (22), under the action of two controlled motor assemblies (21) freely mobile relative to each other. The parasitic loads induced in the two items of the specimen (1) during the test are reduced to a minimum, and the behaviour of the specimen (1) in pure bending can be studied with enhanced accuracy.

Description

Method, device and machine for testing alternately pure bending

The present invention relates to a method for testing pure bending, optionally alternating, comprising the succession of steps consisting in: a) producing or choosing a test piece comprising two mutually opposite extreme gripping zones and a bending zone mutually connecting the two zones of gripping, said test piece having, in a state of rest, a first mean plane which overlaps the flexion zone and each of the gripping zones and which constitutes a first plane of symmetry at least for the flexion zone, and an average surface for the bending zone and each of the gripping zones, which mean surface is perpendicular to the first mean plane, b) while leaving the specimen in the rest state, rigidly grasp the two gripping zones thereof, defining for each a respective pivot axis perpendicular to the first mean plane and occupying a determined position on the one hand with respect to the zone r gripping perspective and on the other hand with respect to the average surface, and c) imposing on the two gripping zones of the specimen controlled antagonistic rotations, possibly alternating, around the respective pivot axis, starting from the state of rest, leaving the pivot axes free to approach or move away from each other, to impose a possibly alternate bending on the bending zone, and study the behavior of the bending zone in bending pure, for example by measuring the resistance opposed to said rotation by at least one of the gripping zones of the test piece in order to deduce therefrom the evolution of the resistance of the bending zone to pure bending. By convention, one considers as a pure bending test method a bending test method whose implementation introduces into the bending zone of the test piece as little parasitic forces as possible, namely more precisely also as little normal and / or sharp effort as possible.

Furthermore, in the context of the present application, the mention of possibly alternating rotations, associated with the concept of pure alternation bending test possibly alternating, constitutes a convenience of language to indicate in particular that: - the evolution of rotations during of the test may be monotonic, that is to say of constant direction, or with one or more reversals of direction, and that - if there is inversion of direction, the maximum amplitudes of opposite directions may be equal or different.

Such a process has been described, in an application to alternating pure bending tests, by M. BRUNET ^" , F. MORESTIN and S. GODEREAUX (2001," Nonlinear Kinematic Hardening Identification for Anisotropic Sheet Metals With Bending-Unbending Tests ", Journal of Engineering Materials and Technology, Volume 123, Pages 378-383), which further described a device and a machine for carrying out this process.

This known method uses a single specimen of the test piece, like all of the previously known alternate bending test methods. However, specifically for this method, each of the gripping zones of the test piece is held together in a respective jaw pivotally mounted, around a respective axis, in a respective slide and engaged with a drive device. common to the two jaws and suitable for imposing on them, as well as in the gripping zones of the specimen which are thus respectively secured thereto, Antagonistic alternating rotations around the respective pivot axis, with respect to the respective slide, in order to impose alternating flexions on the flexion zone of the test piece, between the jaws. The pivot axes of the two jaws are mutually parallel and the two slides are slidably mounted on a common slide, in a direction perpendicular to these two pivot axes which, thus, can move away or approach each other in this direction depending a variation in the apparent length of the test piece between its two gripping zones, that is to say between the two jaws, according to the state of bending of its bending zone.

The pivoting mounting of each jaw, around the respective pivot axis, in the respective slide is effected by means of a respective shaft, which each jaw bears integrally along the respective pivot axis and which engages in two bearings of the respective slide. Between these two bearings, this shaft is engaged, by means of a respective gear train, with a respective drive shaft itself mounted in rotation, in two bearings of the respective slide, around an axis parallel to and arranged relative to the respective pivot axis so that the axes of rotation of the drive shafts corresponding to the two sliders, i.e. the two jaws, are more distant from each other as the pivot axes of the latter. Each drive shaft is itself engaged by means of an Oldham seal, opposite the respective jaw relative to the respective slide, with a respective output shaft of the drive device, constituted by a electric motor associated with a torque limiter.

It is thus possible to subject the bending zone of the test piece to bending alternately in one direction and in the other, of an amplitude adjusted by adjusting the amplitude of rotation of each jaw around the axis of respective pivoting, relative to the respective slide, this amplitude of rotation being identical at all times for the two jaws due to their common drive.

In this known device, the resistance of the jaws and of the gripping zones of the specimen against their alternating rotation, the evolution of which makes it possible to deduce the evolution of the resistance of the bending zone to bending, is measured by sensors placed on the drive shafts, between the Oldham seals and the slides, to measure the torsional stresses in these drive shafts. This known device makes it possible to permanently control the pivoting of each gripping zone around its pivot axis, that is to say the flexion of the flexion zone between the gripping zones, and constitutes in this a significant progress by compared to the devices of the prior art, and in particular compared to that which seemed most satisfactory until then in terms of maximum amplitude of flexion, in particular on a specimen of small thickness, measured perpendicular to its average surface, namely the device described by F. YOSHIDA, M. URABE and VV TOROPOV (1998, “Identification of Material Parameters in constitutive Model for Sheet Metals from Cyclic Bending Tests”, International Journal for Mechanical Sciences, Volume 40, 2-3, Pages 237- 249).

Indeed, the device of Yoshida et al. acts positively, by means of an alternating rotation drive motor, only on a first of the gripping zones of the specimen, the second gripping zone of which is simply retained in a determined orientation, relative to a frame also carrying the motor, by means of slide-slide assemblies authorizing its movement in two mutually perpendicular directions in order to authorize changes in orientation of the first gripping area and variations in the apparent length of the flexion zone between the two gripping zones as the alternate flexions take place.

In the device by Yoshida et al., The flexion zone thus serves as an intermediary for transmitting movement between the gripping zone directly linked to the alternating rotation drive motor, on the one hand, the other gripping zone as well that the slide-slide assemblies which ensure its retention in orientation, on the other hand, and the non-negligible friction which appear in the connections between slides and slides cause the appearance of parasitic forces themselves non-negligible in the test piece, and more precisely in its bending zone, the bending conditions of which remain far from the ideal conditions of pure bending. This results in a non negligible part of error when it comes to deducing the evolution of the resistance of the bending zone to bending from a measurement of the resistance opposite to the rotation alternated by the zone of grip linked to the motor.

A simultaneous positive action, in pivoting, on the two gripping zones of the specimen makes it possible to limit the guiding of these two gripping zones to one direction, that is to say in practice of the two jaws, with relative sliding. in the case of the device of Brunet et al., that is to say makes it possible to reduce friction, in comparison with the device of Yoshida et al., and consequently to reduce the parasitic forces introduced into the bending zone by the friction and the disturbances which ensue in the study of the evolution of the zone of flexion at flexion, thus less distant from pure flexion, but these friction and these parasitic forces are still sensitive. In other words, the torsional stresses measured on the drive shafts of the device of Brunet et al. are due not only to the resistance of the bending zone to bending, but also to the resistance that the sliders encounter by friction on the slide when they have to approach or move away in function of variations in the apparent length of the bending zone between the gripping zones, that is to say between the jaws; in addition, these torsional stresses are also partly linked to the resistance opposed to rotation by the shafts carrying the jaws in their slide bearings, the motion transmission gears between these shafts and the drive shafts, and these drive shafts in their slide bearings, which can also introduce a non-negligible part of error in the interpretation of these torsional stresses in terms of resistance of the bending zone to bending. The object of the present invention is to remedy at least some of these drawbacks of the method and the device described by Brunet et al., And, in preferred embodiments of the present invention, to remedy all of these disadvantages.

To this end, the present invention provides a pure alternating bending method possibly alternating, comprising the succession of steps consisting in: a) producing or choosing a test piece comprising two mutually opposite extreme gripping zones and a bending zone connecting the two zones mutually gripping, said test piece having, in a state of rest, a first mean plane which overlaps the flexion zone and each of the gripping zones and which constitutes a first plane of symmetry at least for the flexion zone, and an average surface for the bending zone and each of the gripping zones, which mean surface is perpendicular to the first mean plane, b) while leaving the specimen in the rest state, rigidly grasp the two gripping zones thereof, defining for each a respective pivot axis perpendicular to the first middle plane and occupying a position determined on the one hand with respect to the respective gripping zone and on the other hand with respect to the average surface, and c) impose on the two gripping zones of the specimen controlled antagonistic rotations, possibly alternating, around the the respective pivot axis, starting from the state of rest, leaving the pivot axes free to move towards or away from one another, in order to impose a bending optionally alternating on the bending zone, and study the behavior of the bending zone in pure bending, for example by measuring the resistance opposed to said rotation by at least one of the gripping zones of the test piece in order to deduce therefrom the evolution of the resistance of the flexion to flexion, as described by Brunet et al., this method being characterized, in accordance with the present invention, in that it is implemented simultaneously on two mutually identical copies of said test vette, by implementing:

- step b such that the first average planes of the two copies are mutually parallel and that the average surfaces of the two copies are mutually symmetrical with respect to a point while the two copies are in the state of rest and such so that the pivot axes of the two copies are common and mutually symmetrical with respect to said point, and

- step c by applying, in a controlled manner around each pivot axis, to the respective corresponding gripping zones, opposing couples, possibly alternating, so as to impose opposing flexions possibly alternating to the flexing zones of the two copies, leaving the pivot axes move freely relative to each other.

Under these conditions, it is each test piece which, by its gripping zones linked together by its bending zone, provides resistance bending of the bending zone of the other test piece, between the gripping zones thereof, without it being necessary to provide any guidance for the gripping zones on any frame, and therefore without risk to introduce parasitic forces resulting from friction into the bending zone of each test piece. Thus, the measurement of the resistance opposed to rotation by at least one of the gripping zones is much better representative of the resistance that the bending zone opposes to pure bending, it being understood that an identity of the two copies of the test piece on which the pure bending test, possibly alternating, is carried out simultaneously, makes it possible to permanently maintain, at least with good approximation, a symmetry of the two test pieces, in the antagonistically bent state as at l state of rest, with respect to the point or center of symmetry, that is to say the same possible state of bending of their bending zone and consequently a substantially identical value of the resistance as the bending zone of the two test pieces opposed to pure bending.

In connection with the characteristics of the method according to the invention and with the advantageous consequences which they thus entail, the present invention also proposes a device for pure bending, optionally alternating, on a test tube of the type indicated in the preamble, this device comprising:

- A pair of jaws, each of which defines an integral gripping slot for a respective gripping zone of the test piece, the slots having, in a relative rest position corresponding to the rest state of the test piece, a first mean plane that overlaps each of the slits, and an average surface for each of the slits, average surface on either side of which each slit has a respective clamping face for the respective gripping zone of the specimen and which is perpendicular to the foreground through the slots, - Means for defining for each jaw a respective pivot axis, so that, in the relative rest position of the jaws, the pivot axes are perpendicular to the first mean plane of the slots, occupy a determined position relative to the respective jaw , and are free to approach or move away from each other,

- means controlled to impose on the jaws antagonistic rotations, possibly alternating, around the respective pivot axis, from the relative rest position of the jaws, leaving the pivot axes free to approach or move away each other, and

means for measuring the behavior of the bending zone of the test piece in pure bending, comprising for example:

* means for measuring the resistance opposed to said rotation by at least one of the jaws, and, where appropriate, * means for deducing therefrom the evolution of the resistance of the test piece to bending between the jaws, as proposed by Brunet et al., this device being characterized in that, for the implementation of the method according to the invention,

- It comprises two sets, mutually identical, of said pair of jaws, the first mean planes of the slots are mutually parallel and the mean surfaces of the slots are mutually symmetrical with respect to a point while the two sets occupy their rest position , in which each is able to receive a respective copy of the test piece in the rest state, in a relative position of the two copies such that they are mutually symmetrical with respect to said point,

the means for defining the pivot axes of the jaws of the two sets are arranged such that the pivot axes are common to the two sets, mutually symmetrical with respect to said point when the two games occupy their rest position, and are free to move relative to each other, and

- The means commanded to impose on the jaws of the two sets of antagonistic rotations, optionally alternating, comprise motor means controlled to apply around each pivot axis, to the corresponding jaws, antagonistic couples possibly alternating.

With regard to the present invention, the mutual symmetry of the mean surfaces of the slots with respect to the point or center of symmetry includes the particular case in which these mean surfaces, then plane, are merged along a plane which includes the point or center of symmetry.

In this device, the two specimens of the test piece constitute the only mechanical link between two mutually identical motor assemblies, each of which comprises: - two jaws, each of which is capable of integrally receiving a respective gripping zone of a respective specimen of the same bending test piece,

means for defining a relative pivot axis for the two jaws, occupying a determined position with respect to each of the two jaws, in a relative rest position,

- motor means controlled to impose on the jaws relative rotations, possibly alternating, around the relative pivot axis, from the relative rest position, these two motor assemblies constituting, with means for controlling the motor means of the two motor assemblies, to impose on the respective jaws relative rotations, possibly alternating, around the respective relative pivot axis, and where appropriate means for measuring the resistance opposed to the relative rotation by one, or at least, said jaws, a pure bending test machine, possibly alternating, itself characteristic of the present invention, for implementing the method according to the invention.

Naturally, it is necessary to carry or support the device according to the invention, consisting of the machine thus designed and two copies of the test piece, by carrying or supporting each of said mutually identical motor assemblies, but it is possible to use for this effect of the means introducing into the two specimens of the test tube much less parasitic forces, and much less annoying, than those introduced by the friction between sliders and slides, in the devices of the prior art previously commented on, and thus greatly disturbing minus the state of pure bending in the bending zones as well as the study of the evolution of the resistance that the bending zones oppose to bending, from a measurement of the resistance opposed by one, at least, said jaws at its rotation. One can, for example, use for this purpose an air cushion or a hydraulic mattress on which the whole of the device rests freely, or even a suspension between the machine, by the motor assemblies and by means of flexible links, at a point located at a level as higher as possible than that of the machine, or else fix one of the motor assemblies on a rigid support and suspend the other in the aforementioned manner so that its weight is not transmitted not to the other engine assembly by means of the two specimens of the test tube, these examples being in no way limiting.

It will be observed that the method, the device and the machine according to the invention may be suitable for test pieces having various conformations in the rest state, namely in particular:

- any form of current section, perpendicular to the first mean plane and to the mean surface, in particular in the bending zone, - a curved or flat conformation of their mean surface which, when it is planar, may or may not constitute a second plane of symmetry at least for the bending zone,

- or a symmetrical conformation or not, at least as regards the flexion zone, with respect to a third mean plane which is perpendicular to the first mean plane and overlapped by the flexion zone while the gripping zones are arranged respectively on either side of it, these examples being in no way limiting. Thus, by way of nonlimiting examples, one can submit to a bending process, optionally alternating, according to the invention, two mutually identical copies of a test piece in the form of a plate of constant thickness, measured perpendicular to its average surface. , and either flat in particular in its bending zone as it can be the case of a sample of raw sheet metal, or bent or corrugated in particular in its bending zone, as it can be the case of a wall sample of metal container or sheet metal unwound from a coil or a sample of a metal bellows wall, or else from a test tube in the form of a curved or straight rod, of constant cross section at least in its bending zone, or a test piece in the form of a plate or rod, the flexion zone of which is refined from one of the gripping zones to the other.

On the other hand, the method described by Brunet et al., Taking into account the mode of driving the two jaws to pivot around their pivot axis and the method of guiding the slides in a direction perpendicular to its axes, does not seem to be able to apply only to test pieces having in the resting state, and retaining in the bent state, at least two planes of symmetry, mutually perpendicular, one of which is the first aforementioned plane of symmetry and the other of which is the third aforementioned plane of symmetry, oriented perpendicular to the sliding direction of the two slides and permanently constituting a plane of symmetry between the jaws in the case of the device by Brunet et al.

In this regard, in the context of the present application, the mentions of first, second and third planes of symmetry constitute a convenience of language and, in particular:

- the mention of a first plane of symmetry does not necessarily imply that there are other planes of symmetry, and

- the mention of a third plane of symmetry does not necessarily imply that there is a second plane of symmetry. If, as is often the case, the test piece has as a medium surface, in its resting state, a second mean plane which constitutes a second plane of symmetry at least for its bending zone, which is for example the case of a test tube in the form of a straight rod or of a test tube in the form of a flat plate: - step b is implemented so that the second mean planes of the two copies coincide while the two copies are in the state of rest and so that the pivot axes are placed in the second mean planes thus coincident, and

- in the device according to the invention, if the slots of each pair of jaws have as a mean surface a second mean plane of symmetry between the clamping faces of each jaw, in the rest position, the machine is arranged in such a way so that the second mean planes of the two sets of said pair of jaws are mutually symmetrical with respect to the point or center of symmetry while the two sets are in the rest position, this mutual symmetry including the particular case in which these second mean planes of the two sets of the jaw pair coincide and include the point or center of symmetry.

Likewise, if the test piece has in its state of rest a third mean plane which is perpendicular to the first mean plane, is overlapped by the bending zone while the gripping zones are arranged respectively on either side of it, and constitutes a third plane of symmetry at least for the bending zone, which is also frequent since it is for example the case if the test piece has the shape of a straight rod of constant section and corresponds to the most frequent case of test pieces in the form of a flat or curved plate:

- Step b is implemented so that the third mean planes of the two specimens of the test piece coincide while these two specimens are in the rest state and so that the pivot axes are mutually symmetrical by relation to the third coincident planes thus, and

- as regards the machine according to the invention, while the slots of each pair of jaws have, in the rest position, a third mean plane which is perpendicular to their first mean plane and on either side of which they are arranged, the third mean planes of the two sets of said pair are mutually symmetrical with respect to the point or center of symmetry while the two sets of jaws are in the rest position, this mutual symmetry of the third mean planes of the two sets of the pair of jaws including the particular case in which these third mean planes are combined and include the point or center of symmetry.

Various modes of connection of each of the gripping zones to the respectively corresponding pivot axis can be envisaged, but a connection is preferred as direct as possible, avoiding the introduction of parasitic forces between each specimen of the test piece and the means intended to measure the resistance opposed to rotation by the bending zone of each of them.

In this regard, according to a preferred embodiment of the method according to the invention, step b is implemented by connecting each of the gripping zones to the corresponding pivot axis by an arm, the arms corresponding to the gripping zones of the two examples being mutually symmetrical with respect to said point, and by connecting the two arms corresponding to the same pivot axis by a controlled motor respective, capable of causing antagonistic rotations, possibly alternating, of the two arms around the respective pivot axis, the controlled motors corresponding to the two pivot axes being mutually identical and authorized to move freely one relative to the other. To this end, respectively for each of the engine assemblies of the machine according to the invention:

the means for defining the relative pivot axis of the two jaws comprise:

* two coaxial shafts mounted at relative rotation around the relative pivot axis,

* two arms, each of which connects one of the jaws to one, respectively, of the shafts, and

- The motor means controlled to impose on the jaws of the alternating relative rotations optionally, around the relative pivot axis, comprise a controlled motor, mechanically independent of the motor controlled from the other motor assembly and capable of causing relative rotations, possibly alternating , of the two shafts, so that the test device according to the invention is then characterized in that: - the means for defining the pivot axes of the two sets comprise:

* along each of the pivot axes, two respective coaxial shafts mounted for relative rotation about the corresponding pivot axis, and * four arms, mutually symmetrical with respect to said point, each of which integrally connects one, respective, of the shafts to the respective one, jaws corresponding to the same pivot axis, and

- the motor means controlled to apply around each pivot axis, to the corresponding jaws, opposing couples, possibly alternating, comprise two controlled motors, mutually identical and arranged so as to be able to move freely relative to each other , each of the motors being associated with one, respective, of the pivot axes and capable of causing antagonistic rotations, possibly alternating, of the two shafts respectively corresponding, each of the controlled motors advantageously being constituted by an electric stepping motor, in in particular in order to facilitate the choice of the amplitude of pivoting of each jaw, that is to say of each gripping zone, around the respective pivot axis, relative to the jaw and to the gripping zone corresponding to the same pivot axis, i.e. to facilitate the adjustment of the bending amplitude of each of the bending zones, although other m eans can be chosen for this purpose.

We can then measure the resistance opposed to rotation by at least one of the gripping zones of one, at least, of the specimens of the test piece by measuring the torsional stresses on one, at least, coaxial shafts of at least one of the motor assemblies, in which case each arm can be chosen or made so that it is rigid in bending according to any mean plane and in torsion. However, it is preferred to ensure that each arm is elastically flexible along the first mean plane of the corresponding specimen of the test piece, that is to say also along the first mean plane of the slot of the corresponding jaw, this is that is to say along a mean plane perpendicular to the respective pivot axis, with a stiffness greater than that of the bending zone of the specimen, and is otherwise rigid, which makes it possible to measure the bending stresses undergone by one, at least, of the arms, in a circumferential direction with reference to the corresponding pivot axis, which bending stresses are much more directly representative of the resistance of the bending zone to bending, in which case the measuring means comprise means for measuring the bending stresses undergone by one, at less, arms, along said mean plane.

One can be tempted, for this purpose, to measure the bending stresses undergone by each of the arms, by providing suitable measurement means on each of them, but the validation tests of the method according to the invention, carried out by means of a device and a machine according to the invention made it possible to determine that a measurement of the bending stresses undergone by only one of the arms gave a significant result, with sufficient precision, as regards the resistance opposed to bending by the bending zone of each of the test pieces, and as to the evolution of this resistance to bending as and when alternate bending, that is to say during fatigue tests to bending.

To facilitate the measurement of the bending stresses undergone by at least one of the arms, it is preferably provided that each of the arms has at least one weakened zone in bending along said mean plane perpendicular to the corresponding pivot axis, c that is to say in a circumferential direction with reference to this corresponding pivot axis, the zones thus weakened in bending being mutually symmetrical with respect to the point or center of symmetry, and one locates in said zone of at least one , arms the means for measuring the bending stresses therein.

As a skilled person will easily understand, the symmetry in the treatment of the two specimens of the test tube and in the device according to the invention, this symmetry with respect to a point or center of symmetry when the test pieces are in their rest state and the jaws in their rest position being characteristic of the present invention, can be obtained in two main ways, by a appropriate arrangement of arms, shafts and motors.

Indeed, this arrangement can be such that, in the state of rest of the test pieces and in the rest position of the jaws:

the pivot axes are mutually parallel and disposed respectively on either side of the point or center of symmetry, in which case, if each specimen of the test piece has in its rest state the above-mentioned third medium plane and if this medium plane constitutes not only a plane of symmetry for the bending zone but also a mutual plane of symmetry for the gripping zones, the arms corresponding to the gripping zones of the two examples can advantageously be mutually identical, or alternatively

the pivot axes are merged and pass through the point or center of symmetry, this relative positioning being considered as a particular case of mutual symmetry of the pivot axes with respect to the point or center of symmetry with regard to the present invention. In either case, the machine and the device according to the invention can be very simple to produce, so that despite the use of two motors, their cost remains competitive compared to that devices of the prior art, and their reliability is however considerably increased. Although, as indicated previously, test pieces of very different conformations can be subjected to bending tests in accordance with the present invention, each specimen of the test piece is produced or chosen during step a of the process according to the invention, of such that it has the shape of a plate whose thickness is oriented perpendicular to the average surface, in which case:

said thickness is preferably constant at least in the bending zone, each specimen of the specimen preferably has a constant dimension, perpendicular to the first mean plane, at least in the bending zone,

each specimen of the test piece preferably has a transition perpendicular to the first mean plane between the bending zone and each gripping zone, respectively.

A test piece thus produced in the form of a plate is particularly suitable for bending tests of large amplitude and, to allow access to bending amplitudes of the order of 90 °, between the gripping zones, provision is made for preferably that the jaws of the machine or of the device according to the invention are bevelled so as to thin towards one another, if one refers to the rest position of the jaws.

Other characteristics and advantages of the various aspects of the present invention will emerge from the following description, relating to two nonlimiting examples of implementation, as well as from the appended drawings which form an integral part of this description.

Figures 1 and 2 show, in a perspective view, a non-limiting example of conformation of a test piece suitable for implementing the method according to the invention, this test piece having in this non-limiting example the general form of a flat plate, obtained for example by sampling from a flat sheet and prepared according to two different modes with a view to its integration into a test device according to the invention.

FIG. 3 schematically illustrates, in a side elevation view, a device for implementing the method according to the invention, comprising a machine according to the invention and two copies mutually identical to a test piece, for example in accordance with FIG. 1 or to FIG. 2, or of any other conformation suitable for allowing a bending test, the device being seen in a rest position while the two copies of the test piece are in the resting state. Figure 4 shows, in a top view in a direction marked in

IV in Figure 3, the same device also in the rest position while the two copies of the test piece are in the rest state.

Figure 5 shows, in a top view similar to that of Figure 4, the device when it has left its rest position and that the two specimens of the test piece are flexed antagonistically.

Figures 6 to 8 show, in schematic views similar to those of Figures 3 to 5 respectively, the device in the context of an alternative embodiment of the machine, Figure 7 showing a top view in a direction identified in VII in Figure 6. Figure 9 shows, in a side elevation view similar to that of Figure 6, a concrete example of embodiment of the device illustrated in Figure 6, in the rest position while the two specimens of the test tube are in the resting state.

FIG. 10 shows, in a top view, in a direction marked in X in FIG. 9, the positioning, on a zone, weakened in bending, of one of the arms of the device, of surface deformation gauges acting means for measuring the bending stresses undergone by this arm due to the resistance of the two specimens of the test piece to bending.

Figure 11 shows the connection diagram of the device. Figure 12 shows a perspective view of the device illustrated in Figure 9, in the rest position while the test pieces are in the rest state. Figure 13 shows a perspective view of one of the two mutually identical motor assemblies in this case constituting the testing machine according to the invention.

Figures 14 and 15 show, in a perspective view, respectively one of the arms, mutually identical of this machine and an alternative embodiment of one of these arms, mutually identical.

FIGS. 16 and 17 show, in a perspective view, the two jaws of one of the jaws, mutually identical, of the testing machine according to the invention, in a configuration adapted to the configuration of test piece illustrated in FIG. Figure 1 or Figure 2, it being understood that each conformation of test piece to be tested in bending corresponds to a specific conformation of the jaws, easily conceivable by a skilled person depending on the shape of the test piece.

FIG. 18 shows a perspective view of the device having left the state of rest, the two specimens of the test piece being in the bent state in an antagonistic manner, for example at right angles.

FIG. 19 shows, in a diagrammatic view, in elevation, similar to that of FIGS. 3 and 6, another embodiment of a device according to the invention, in the rest position while the two copies of the test tube are in the resting state.

Figures 20 and 21 show a view of this device in section through planes identified in XX-XX and XXI-XXI in Figure 19, showing the two specimens of the test piece in the rest state while the device is in the rest position. Figures 22 and 23 show views similar to those of Figures 20 and 21, respectively, while the two specimens of the specimen are in the bent state, in an antagonistic manner, for example at right angles, the device having left its rest position. FIG. 24 shows, in a side elevation view similar to that of FIG. 19, a concrete embodiment of the device according to the invention in the case of this variant, while the device is in the rest position and the two specimens of the test tube in the rest state. Reference will first be made to FIG. 1, where a test piece 1 has been illustrated in the rest state, that is to say undergoing no stress, in particular bending, and comprising a bending zone 2, intended to undergo the alternate bending test, between two extreme gripping zones 3, 4, which are not subjected to any bending during the test. The bending zone 2 has a first mean plane 5 of symmetry which overlaps each of the zones 2, 3, 4 and which also constitutes a first mean plane of symmetry for the grip zone 4, and a mean surface which is perpendicular to this first mean plane 5 which, in this example, is plane and constitutes a second mean plane 6 of symmetry for each of zones 2 and 4, as well as a mean plane of symmetry for zone 3. In this example, the bending zone 2 has a third mean plane 7 of symmetry which it overlaps and which intersects the mean planes 5 and 6 at right angles, the gripping zones 3 and 4 being disposed respectively on either side of this mean plane 7. In this example, the gripping zone 3 is asymmetrical with respect to the first medium plane 5 and is not symmetrical with the gripping zone 4 with respect to the third medium plane 7, but it could also have its own symmetry with respect to the first medium plane 5 and be symmetrical with the gripping zone 4 with respect to the third mean plane 7, the conformation of the gripping zones 3 and 4 being almost indifferent with regard to the bending test, which relates only to the bending zone 2.

In each of the three zones 2, 3, 4, the test piece 1 is delimited by two main faces such as 8 which, in this example, are plane, mutually parallel as well as parallel to the second mean plane 6 relative to which they are symmetrical to each other, which main faces such as 8 define between them a thickness e of the test piece, which thickness e is measured perpendicular to the second mean plane 6 and has a constant value, in particular in the bending zone 2. The two main faces such as 8 are mutually connected by a edge 9 perpendicular to the second mean plane 6, which edge 9 defines for each of the zones 2, 3, 4, in this second mean plane 6 likewise that in any section plane parallel to this, a rectangular shape, elongated perpendicular to the first mean plane 5. The flexion zone 2 has, perpendicular to it, a length Li less than the length L ₂ than the grip zone 4 presents perpendicular to this mean plane 5, which is itself less than the length L that the gripping zone 3 presents perpendicular to this plane, so that l 'test piece 1 has a marked transition between each gripping zone 3, 4 and the bending zone 2, this transition being materialized by respective zones 107 of the edge 9, perpendicular to the first mean plane 5, delimiting the gripping zones 3 and 4 towards the third mean plane 7 and turned towards it, respectively on either side of the bending zone 2 and of the first mean plane 5. Along respective axes 10, 11 perpendicular to the second mean plane 6 and mutually symmetrical by compared to the third medium plane 7, the two gripping zones 3, 4 are drilled right through, that is to say from one to the other of their main faces such as 8, inside the edge 9, of respective holes 12, 13 mutually identical, cylindrical of revolution around the respective axis. The intersections 14, 15 of the axes 10, 11 with the second mean plane 6 are regularly distributed in a respective alignment, not illustrated, perpendicular to the first mean plane 5, with respect to which their distribution is also symmetrical; in other words, the number of holes 12 being odd like that of the holes 13 in the nonlimiting example illustrated, in which this number is 7, one of the axes 10 and one of the axes 11 are arranged in the first mean plane 5, and the other axes 10, 11 are respectively mutually symmetrical with respect to this first mean plane 5. The test piece 1 thus shaped can be cut by any suitable means, resulting in the least possible internal stress, on the one hand, and of risks of change of behavior in bending for example by change of crystallographic structure, on the other hand, in the sheet to be tested; for this purpose, it is certainly possible to use to cut the test piece 1 and arrange the holes 12 and 13 there techniques employing a contact of a tool with the sheet, such as stamping, but it is preferred to use techniques n ' involving no contact of a tool with the sheet metal, such as EDM or laser cutting or waterjet cutting.

To simplify the production of a test piece 1 by EDM, as regards the arrangement of the holes 12 and 13, the test piece can be shaped as illustrated in FIG. 2, that is to say in connecting each of the holes 12 and 13, in the gripping zone 3, 4 respectively corresponding to the edge 9 of the test piece 1 by a respective slot 16, 17 which, starting from the respective hole 12, 13, moves away from the plane means 7 perpendicular thereto to open into a zone of edge 9 parallel to this medium plane 7 and delimiting respectively the gripping zone 3 or the gripping zone 4 in the direction of a distance from this plane. Each of the slots 16 and 17 also opens into one and the other of the main faces such as 8, so that the edge 9, the slots 16 and 17 and the holes 12 and 13 can be arranged in a single continuous pass, according to a closed contour, of the EDM electrode. The test piece 1 also remains identical to that which has been described with reference to FIG. 1 and, in particular, its bending zone 2 is in no way affected by the presence of the slots 16 and 17, in particular in its bending behavior between the gripping zones 3 and 4.

Characteristically of the present invention, two identical copies of the test piece 1, or of a test piece shaped differently, are subjected simultaneously to the alternating bending test, being subjected to alternating bending at each instant mutually antagonistic in a device having as few mechanical links as possible with a support frame so that each specimen of the specimen flexes, in its bending zone 2, under conditions as close as possible to those of pure bending. By identity of the two specimens of the test piece, one understands here an identity of the behavior which one can expect from it in terms of bending of their bending zone 2, and in particular of evolution of the bending zone 2 in fatigue to bending, this identity generally resulting from a geometric identity. For this purpose, a test device is used, a first embodiment of which is illustrated in a first variant 18 in FIGS. 3 to 5 and in a second variant 19 in FIGS. 6 to 18, and in which another embodiment 20 is illustrated in figures 19 to 24.

Reference will firstly be made to FIGS. 3 to 5 which show a device 18 according to the invention comprising two mutually identical motor assemblies 21, structurally and functionally, arranged head to tail, symmetrically of one another with respect to a point or center of symmetry 22 and mechanically connected to each other by the two specimens of the test piece 1 themselves arranged symmetrically with respect to point 22. In order to get as close as possible to a situation ideal in which the two copies of the test piece 1 would constitute the only mechanical connection between the two motor assemblies 21, these are suspended from a support frame 23, arranged at as high a level as possible, by links 29 as long and as flexible as possible so as to ensure the recovery of the weight of the motor assemblies 21 while inducing the least possible effort in the two copies of the test piece 1 which alone provide the mechanical connection between these two motor assemblies 21. More precisely, each motor assembly 21, advantageously constituted by an electric stepping motor, comprises a stator 24 inside which a rotor 25 is guided in rotation about an axis 26, without any other possibility of relative displacement. According to this axis 26, each motor assembly 21 has two mutually opposite output shafts 27, 28, the first of which is integral with the stator 24 and the second of the rotor 25 and can thus rotate coaxially with one another, in a one way or the other, depending on the supply of power to the motor assembly 21. Given the aforementioned symmetry of the motor assemblies 21 with respect to point 22, the axes 26, oriented vertically, are disposed respectively on either side of this point 22 and the shafts 27 and 28 of one of the motor assemblies 21 , namely that which is on the right in FIGS. 3 to 5, are turned upwards and downwards respectively, while the shafts 27 and 28 of the other motor assembly 21, namely that which is located on the left at Figures 3 to 5, are turned respectively downwards and upwards. That of the shafts 27 and 28 of the motor assemblies 21 which is turned upwards, namely the shaft 27 with regard to the motor assembly 21 on the right or the shaft 28 with regard to the motor assembly 21 on the left , carries at its upper end a means such as a connecting ring integral with a lower end of the flexible link 29 respectively corresponding, the upper end of which is fixed integrally to the support frame 23, for example by suspension from a hook thereof ci, under conditions such that the two flexible links 29 are at least approximately vertical and disposed at least approximately along the corresponding axis 26 respectively, in a rest position of the device 18, illustrated in FIGS. 3 and 4 and corresponding to a state of rest of the two copies of the test piece 1.

In contrast to its connection respectively with the stator 24 or the rotor 25, each of the shafts 27, 28 carries a respective arm 30, 31 which is rectilinear, radial with respect to the corresponding axis 26 and respects mutual symmetry of the two motor assemblies 21 relative to point 22, in this sense the two arms 30 corresponding to the shafts 27 are identical and mutually symmetrical with respect to point 22, as are the two arms 31 corresponding to the shafts 28, the arms 30 and 31 advantageously being mutually identical.

In the direction of a distance from the shaft 27, 28 and the respective shaft 26, each of the arms 30, 31 carries a respective jaw 32, 33 integrally, the jaws 32 and 33 being themselves identical and respecting the aforementioned mutual symmetry with respect to point 22. In the rest position the motor assemblies 21, illustrated in FIGS. 3 and 4, the jaws 32 and 33 face each other in pairs, the jaws 32, 33 of the same pair retaining one, respectively, of the gripping zones 3, 4 of a respective specimen of the test piece 1 so that only the bending zone 2 is free between them, and capable of bending between them. Preferably, each of the jaws 32 and 33 can receive, without distinction, one or the other of the gripping zones 3, 4 of a test piece 1.

Thus, the two jaws 32 and 33 corresponding to the arms 30 and 31 corresponding themselves to the shafts 27 and 28 turned upwards receive one, respectively, of the gripping zones 3 and 4 of the same copy of the test piece 1 while the jaws 32 and 33 corresponding to the arms 30 and 31 corresponding themselves to the shafts 27 and 28 turned downward carry one, respectively, of the gripping zones 3 and 4 of the other copy of the '' test tube 1. The two motor assemblies 21 are designed such that, in a rest position corresponding to the rest state of the test pieces 1:

the mean planes 6 of symmetry of the latter coincide and include both the two axes 26 and the point or center of symmetry 22,

the mean planes 7 of symmetry coincide, include the point or center of symmetry 22 and constitute a mean plane of symmetry between the axes 26, arranged respectively on either side of the planes 7 thus merged and parallel to the latter, and - the mean planes 5 of symmetry are mutually parallel, perpendicular to the two axes, arranged respectively above and below the point 22 and mutually symmetrical with respect thereto.

This position and this state of rest are illustrated in FIGS. 3 and 4.

From the rest position of the motor assemblies 21, it is possible, by appropriate control of the latter, to cause their rotor 25 to rotate relative to their stator 24, by an identical amplitude and in the same directions so that the two jaws 32 and 33 linked to the shafts 27 and 28 turned upwards shift in the same direction relative to the jaws 32 and 33 corresponding to the shafts 27 and 28 turned downwards, as shown in FIG. 5, doing so antagonistically bend the flexion zones of the two specimens of the test piece 1, the aforementioned identity of which makes the device 18 as a whole, that is to say as well as regards the two engine assemblies 21 that the two copies of the test piece 1, keep a symmetry with respect to point 22.

Then, a controlled rotation, in the same direction opposite to the previous direction, of the shafts 28 of the two motor assemblies 21 relative to the motor shafts 27 brings these two assemblies 21 to the rest position and the two copies of the test piece 1 to the state of rest then, continuing then with an amplitude identical to the previous amplitude, causes a bending of each of the specimens of the test piece 1 in the opposite direction to the previous direction, the continuation of the controlled alternating rotations of the output shafts 28 of the two motor assemblies 21 relative to the 'output shaft 27 causing the alternate bending of the two specimens of the test piece 1 under substantially identical conditions, preserving the symmetry with respect to point 22 with a good approximation.

Naturally, the alternating flexions of the two copies of the test piece 1, accompanying the alternating relative rotations of the two arms 30 and 31 of each engine assembly 21, are accompanied by alternating movements of mutual approach and distancing of the two axes 26, which remain parallel due to the preservation of symmetry with respect to point 22, which causes oscillations of the two flexible links 29 around their upper end secured to the upper frame 23; however, the greater the length of the flexible links 29, the less these oscillations have amplitude and the less the relative movements of the two axes 26, that is to say of the two motor assemblies 21, induce parasitic force in the areas bending 2 of the two copies of the test piece 1. It can be considered with good approximation that the axes 26, or even the motor assemblies 21, are completely free to move relative to each other.

In accordance with the present invention, as the opposing bends alternate, of the two copies of the test piece 1 under the action of the motor assemblies 21 suitably controlled, the resistance, substantially identical, of each of the bending zones is measured. 2 for the purpose of studying the evolution of this resistance by measuring the resistance opposed by at least one of the gripping zones 3, 4 of one, at least, of the specimens of the test piece 1, that is to say that is to say of at least one of the jaws 32 and 33, against the alternating rotation about the corresponding axis 26. For this purpose, preferably, the stresses of bending undergone by at least one of the arms 30 and 31 under conditions strictly identical to those which will be described in connection with the device according to the invention 19 illustrated in FIGS. 6 to 18, to which reference will now be made. If we refer first to Figures 6 to 8, we see that this device 19 implements the same means as the device 18 except that instead of being arranged head to tail, strictly symmetrically one from the other with respect to point 22, the two motor assemblies 21 are arranged in the same orientation in the sense that the two shafts 27 are turned identically and that the two shafts 28 are turned identically, the axes 26 remaining however vertical, arranged respectively on either side of point 22 and mutually symmetrically with respect thereto.

More specifically, in the example illustrated, the two shafts 27 are turned downwards and the two jaws 32, each of which is connected to a respective one of these shafts 27 by a respective arm 30, are turned one towards the other in the rest position and bear one, respectively, of the gripping zones 3, 4 of a specimen of the test piece 1 under conditions such that only the bending zone 2 of the latter is free to flex between them, while the shafts 28 are turned upwards and the jaws 33 respectively connected to these shafts 28 by a respective arm 31 face each other in the rest position and bear one, respectively, of the gripping zones 3 and 4 of the other specimen of the test piece 1 in the rest state under conditions such that only its bending zone 2 is free to flex between them. In this case, it is the shafts 28 which are fixed, by their end opposite to the respective rotor 25, to a flexible link 29 of suspension to the support frame 23. It is well understood that it could also be the shafts 27 which are turned upwards and serve for the suspension of the motor assembly 21 corresponding to the support frame 23.

Even if the stators 24 and the rotors 25 of the two motor assemblies 21 do not respect, structurally, the symmetry which they have with respect to point 22 in the case of the device 18, each of the arms 31 is respectively symmetrical with one of the arm 30 with respect to point 22, just as the two specimens of the test piece 1 are mutually symmetrical with respect to this point 22, in particular if we refer to the rest position and the rest state, in the case of device 19 so that it is functionally symmetrical with respect to point 22 and functionally equivalent to device 18, that is to say is just as likely to cause alternating antagonistic flexions of the two copies of the test piece 1. However, for this purpose, it is necessary to offset the two arms 31 angularly, around the axis 26, in the same alternating direction relative to the arm 30, from the rest position, that is to say to spin the s shafts 28 in mutually opposite directions with respect to the respectively corresponding shaft 27, as shown in FIG. 8. The mutual functional symmetry of the two motor assemblies 21, with respect to point 22, and the mutual symmetry of the two copies of l 'test piece 1 with respect to this point 22 are stored under the same conditions as in the case of device 18, as will readily be understood by a person skilled in the art. FIGS. 9, 12, 18 illustrate a practical embodiment of the device 19, of which FIG. 13 illustrates the practical embodiment of one of the motor assemblies 21.

We find there, under the same reference numerals, the different components described with reference to Figures 6 to 8 or, by analogy, to Figures 3 to 5, and we will now describe some details of practical execution, referring for simplicity, unless otherwise stated, the rest position of the device 19, as shown in the figures 9 and 12 in particular. If we also refer to FIG. 14, which shows an arm 31, it being understood that the arms 30 and 31 are identical in the preferred example illustrated, it can be seen that such an arm has two mean planes of symmetry 34 and 35, the first of which includes the corresponding axis 26 and the second of which is perpendicular to this axis. These two mean planes of symmetry 34 and 35 respectively coincide with the mean planes 6 and 5 of symmetry of the corresponding specimen of the test piece 1 when the device 19 is in the rest position and the two copies of the test piece 1 at rest.

The arm 31 has a rectilinear medium fiber 36, defined by the intersection of the planes 34 and 35 and perpendicularly cutting the corresponding axis 26, and has an elongated shape in a direction defined by this medium fiber 36, perpendicular to which the arm 31 has a constant square current section, defined by two plane, rectangular faces 37, parallel to the plane 35 and mutually symmetrical with respect thereto, and by two equally rectangular plane faces 38, parallel to the plane 34 and mutually symmetrical with respect to that -this.

With reference to a direction defined by its average fiber 36, the arm 31 has two mutually opposite end zones 39, 40, the first of which is shaped so as to secure it with the shaft 28 of the corresponding motor assembly 21 and whose the other is free and bears the corresponding jaw 33 together.

With a view to securing the arm 31 with the shaft 28, the end zone 39 has, along the plane 34, a slot 41 which opens into an end face 42 of the arm 31, which is perpendicular to the medium fiber 36, as well as in areas of the faces 37 directly adjacent to this face 42, and this slot 41 has locally, at 43, a conformation complementary to that of the corresponding shaft 28 so as to fit without play around it. ci, in the required position of the planes 34 and 35 relative to the axis 26. Perpendicular to the plane 34 are provided, between the conformation 43 of the slot 41 and the end face 42, housings 44 for receiving the clamping screw which , tending to close the slot 41, establish between the arm 31 and the shaft 28 an intimate contact of mutual compression, ensuring the desired mutual joining. The other extreme zone 40 of the arm 31, or free end of this arm, is shaped as a flange 45 for receiving integral but removable from the corresponding jaw 33, thus interchangeable in order to adapt to test pieces 1 of different conformation

The mode of attachment of each arm 30 with the corresponding shaft 27 and of integral but removable reception of the corresponding jaw 32 is identical to what has just been described.

In relation to the conformation of the test piece described with reference to Figures 1 and 2, each jaw 33, to which each jaw 32 is identical, is designed so as to define a slot 46 for receiving integral but removable from one of the zones of gripping 3, 4 of the corresponding specimen of the test piece 1, by clamping between two jaws 47 and 48, the first of which is fixed in an integral but removable manner to the arm 31 by means of the flange 45 and the other of which is fixed together but removable on the first. Likewise, each jaw 32 is formed of a jaw 49 fixed in an integral but removable manner to a free end region, in the form of a flange, of the corresponding arm 30, and a jaw 50 fixed in an integral but removable manner on the jaw 49, the jaws 49 and 50 defining between them a slot 51 for receiving integral but removable, by clamping between them, of one of the gripping zones 3 and 4 of the copy corresponding to the test piece 1. The four jaws 32 and 33 being mutually identical, only one of these jaws will be described, namely a jaw 33, also referring to FIGS. 16 and 17 which show the two jaws 47 and 48 thereof. In relation to the conformation of the test piece 1 described with reference to FIGS. 1 and 2, the slot 46 of the jaw 43 is defined by two flat clamping faces 52, 53, the first of which is defined by the jaw 47 and the second by the jaw 48 and which are arranged respectively on either side of the plane 34, parallel thereto and mutually symmetrical with respect to the latter, while they are also respectively respectively symmetrical with respect to the plane 35, as c 'is the case of each of the jaws 47 and 48 considered as a whole.

Each of the jaws 47 and 48 is produced in a single respective rigid piece, for example of steel. The jaw 47 has two parts 54 and 55, the first of which constitutes an integral but removable mounting heel on the extreme flange 45 of the arm 31, for example by bolting and, preferably, by local interlocking, the respective conformations of the part 54 and of the flange 45 for this purpose falling within the normal aptitudes of a person skilled in the art. Opposite the flange 45 and the assembly of the arm 31 in a direction defined by the medium fiber 36 thereof, the part 54 has a flat face 56 perpendicular to the medium fiber 36 and symmetrical with respect to the two planes 34 and 35, that it overlaps both by presenting a dimension equal to L ₂ perpendicular to the plane 34. In general, the jaw 47, like the jaw 48, is symmetrical with respect to the plane 34 and has the dimension L ₂ perpendicular to it.

Perpendicular to plane 35, face 56 has a dimension E greater than e and distributed asymmetrically with respect to plane 35, on either side of it, namely for a value equal to half of e on one side of this plane 35, located below this plane 35 in FIG. 16, and for the rest on the other side of this plane 35, located above this plane 35 in FIG. 16. From the first of the aforementioned sides of the plane 35, that is to say at a distance from it equal to the half of e, the face 56 is connected to the face 52 which, parallel to the plane 35, is thus spaced therefrom by a distance equal to half of e and extends projecting away from the arm 31 corresponding, in the direction defined by the medium fiber or intersection 36 between the two planes 34 and 35 thereof, relative to the heel part 54, delimiting the part 55 towards the plane 35.

The face 52 is planar, rectangular, its plan dimensions being substantially identical to those of the part of one of the main faces 8 of the test piece 1 corresponding to the grip zone 4. In other words, it has the dimension L ₂ perpendicular to the plane 34, whereas it has in the direction defined by the medium fiber or intersection 36 between the planes 34 and 35 a dimension li identical to the dimension, also identified by li, as the gripping zone 4 or the gripping area 3 is perpendicular to the plane 7. Thus, indifferently, the gripping area 4 of the test piece 1 or a portion 57 of the gripping area 3 of this test piece 1, which portion 57 constitutes the specular image of the grip zone 4 with respect to the mean plane 7 of symmetry of the test piece 1 and is distributed symmetrically on either side of the mean plane 5 of symmetry thereof, can be applied integrally, flat, by the one of the main faces 8 of the test piece 1, on the face 52 while the test piece is also applied flat, by a zone of its edge 9 opposite to the other gripping zone, respectively 3 or 4, against the face 56 of the jaw 47 of which this other gripping zone as well as the bending zone 2 remain free. If the grip area thus applied by one of the main faces 8 on the face 52 is the gripping zone 3, having a length L ₃ greater than the length L ₂ , the part 58 other than the part 57 protrudes from the jaw 47 along a direction perpendicular to the plane 34, with respect to one of two plane faces 59, parallel to the plane 34, mutually symmetrical with respect to the latter and mutually spaced apart from said length L ₂ , which faces 59 delimit the two parts 54 and 55 of the jaw 47, as well as all of their other faces, in the direction of a distance from the plane 34. In the direction of a distance from the face 56 in the direction defined by the medium fiber or intersection 36 between the two planes 34 and 35, the face 52 of the jaw 47 is connected, by a straight down fillet 60, perpendicular to the plane 34 and symmetrical with respect thereto, to a face 61 of the part 55 of the jaw 47, which fa this 61 is planar, perpendicular to the plane 34 and symmetrical relative to the latter, and has an inclination, for example of the order of 45 °, relative to the plane 35 and to the face 52 so as to form a bevel with the latter. In other words, the part 55 of the jaw 47 becomes progressively thinner, between the faces 52 and 61, towards the edge 60.

In the direction of a distance from this edge 60, which is also the direction of a distance from the face 52 and the plane 35, the face 61 is connected by a straight edge not referenced, perpendicular to the plane 34 , to a face 62 parallel to the plane 35, common to the two parts 54 and 55 of the jaw 47 and delimiting the latter in the direction of a distance from the plane 35. The jaw 48, more particularly illustrated in FIG. 17 and shown diagrammatically in FIG. 16, in phantom, in the position it occupies with respect to the jaw 47 when the two jaws 47 and 48 hold together, in an integral manner, a gripping zone 3 or 4 of a test piece 1, has a conformation such that it then corresponds to the less approximately, to a specular image of the part 55 of the jaw 47 relative to the plane 35.

In the direction of a distance from the plane 34, the jaw 48 is delimited by two plane faces 63 parallel to the plane 34 and mutually symmetrical with respect to this plane, which faces 63 are mutually spaced apart by the distance L ₂ perpendicular to this plane and delimit, in the direction of a distance with respect thereto, all of the other faces of the jaw 48 which will now be described, including its face 53 which defines the slot 46 with the face 52 of the jaw 47. The face 53 has dimensions identical to those of the face

52, opposite which it is placed in a direction perpendicular to the plane 35, and is connected by a straight cut edge 64, perpendicular to the plane 34 and symmetrical with the cut edge 60 relative to the plane 35 when the jaws hold together the gripping zone 3 or 4 of a test piece 1, to a flat face 65 perpendicular to the plane 34 and which, then symmetrical with the face 61 with respect to the plane 35, defines a bevel with the face 53. In the direction of a distance from edge 64 and the face

53, as well as relative to the plane 35, this face 65 is connected by a rectilinear edge, perpendicular to the plane 34, to a plane face 66 perpendicular to the plane 34 and which occupies a position symmetrical to that of the face 62 relative to the plane 35 so as to delimit the jaw 48 in the direction of a distance from this plane 35. In the direction of a distance from its connection with the face 65, in a direction defined by the medium fiber or intersection 36 between the planes 34 and 35, the face 66 however has a dimension smaller than that of the face 62 and is connected by a straight edge, perpendicular to the plane 34, to a plane face 67 perpendicular to the two planes 34 and 35 and connecting to the face 53, by a rectilinear edge perpendicular to the plane 34, in the direction of a rapprochement with respect to the plane 35. By this face 67, namely more precisely by a zone of this face 67 directly adjacent to its connection with the face 53, the jaw 67 is applied flat against the face 56 of the jaw 47 under conditions suitable for allowing relative sliding, in a direction perpendicular to the plane 35, when one of the gripping zones 3 and 4 is gripped between a test piece 1 between the faces 52 and 53 of the jaws 47 and 48. The face 67 is moreover not in contact with the jaw 47, whose face 56 is connected by a recess 68 to a rectangular face 69, perpendicular to the plane 34 and 35 and thus set back with respect to the face 56, which face 69 delimits the part 54 of the jaw 47 opposite to the extreme flange 45 of the arm 31 in a direction defined by the intersection 36 between the planes 34 and 35 opposite the part 55 of the jaw 47 relative to the plane 35. In the direction of a distance relative to this one, the face 69 is connected to a face r ectangular, plane 70 which delimits the part 54 in the direction of a distance from the plane 35 opposite the face 62, in a position symmetrical to that of the latter with respect to the plane 35 so that the face 70 coplanarly extends the face 66 of the jaw 47 when the latter encloses a gripping zone 3 or 4 of a test piece 1 with the jaw 47. To allow such a clamping, the part 55 of the jaw 47 and the jaw 48 are pierced right through, along respective axes 71 and 72 perpendicular to the plane 35, distributed respectively with respect to the face 52 and with respect to the face 53 as are the axes 10 and 11 with respect to the main faces 8 of the specimen 1, respectively on the area 57 of the grip area 3 or on the grip area 4, holes 73, 74 which, thus, are placed coaxially when the two jaws 47 and 48 are arranged so as to grip one of the gripping areas 3, 4 of a test piece 1, and are then then placed coaxially either with the holes 12 in the gripping zone 3, or with the holes 13 in the gripping zone gripping 4 to receive coaxially, not shown, mutual tightening bolts of the two jaws 47 and 48 perpendicular to the plane 35, via the gripping zone 3 or the gripping zone 4 which, thus sandwiched between the faces 52 and 53 of the two jaws 47 and 48, behaves as a whole integral with the latter.

When each specimen of the test piece 1 is thus enclosed by its two gripping zones 3 and 4 respectively between the two jaws 47 and 48 of the two jaws 33 or between the two jaws 49 and 50 of the two jaws 32, only its bending zone 2 remains free, in particular to bend, between the two respective jaws 33 or 32, during alternating rotations of the arms 31 or 30 around the axes 26 from a rest position which corresponds to the rest state of the two copies of the test piece 1, and to oppose resistance to this alternating bending, around the state of rest.

If we consider the rest position of the device 19 and the corresponding rest state of the two copies of the test piece 1 and if we refer to the embodiments described respectively with reference to FIGS. 6 to 18 and with reference to FIGS. 1 and 2, the jaws 33 are placed face to face, like the jaws 32, by the bevel of their jaws 47, 48, 49, 50 and by their slots 46, 51, in a position of relative in which the mean planes 35 of the arms 31 and of the jaws 33 coincide with the mean plane 5 of the corresponding specimen of the test piece 1, as well as their mean planes 34 with the mean plane 6 of this specimen, in which the arms 31 and their jaws 33, respectively, are mutually symmetrical with respect to a plane 108 perpendicular to the planes 34 and 35 and coincides with the mean plane 7 of this example and in which identical relationships exist between the arms 30, their jaws 32 and l '' corresponding copy of the test ette 1. In addition, then, the mean planes 5 of the two copies of the test piece 1 are parallel to each other, mutually symmetrical with respect to the point or center of symmetry 22 on either side from which they are placed, and the mean planes 3 of the two specimens of the test piece 1 coincide and pass through the point or center of symmetry 22, as do their mean planes 7.

To measure the resistance of the bending zones 2 of the two specimens of the test piece 1 against alternating bending as the test progresses, according to the preferred embodiment of the invention which has been illustrated, a resistance moment is measured on at least one of the arms 30 and 31, namely in practice on only one of these arms, for example an arm 31, in a manner which has been validated by the tests to the rotation of this arm around the corresponding axis 26 by a measurement of the bending stresses to which this arm 31 is subjected along the plane 35 or planes parallel to this plane 35.

In order to better highlight these bending stresses, the arm 31 in question, like the other arm 31 and the two arms 30 for reasons of symmetry with respect to the point or center of symmetry 22, have at least one zone, respectively 75, 76, weakened in bending along the plane 35 and planes parallel to it, that is to say in a circumferential direction with reference to the corresponding axis 26, without constituting a weakening in bending according to d 'other directions. It is understood, however, that the arms 30 and 31 are intrinsically rigid, for example made of aluminum, in the sense that they must not bend during the test, the bending having to remain limited to the bending zones 2 two copies of the test tube 1.

In the example illustrated in FIGS. 9, 12 to 14 and 18, the arm 31 thus has two zones 75 weakened in bending, which are identical and identically distributed, in a direction defined by the medium fiber or intersection 36 between the two planes 34 and 35, for the two arms 31, just as the zones 76 are distributed on the arms 30 identically, from one arm 30 to the other and from one arm 30 to one arm 31, in the direction of the medium fiber of these arms. However, as shown in FIGS. 10 and 15, each arm 30, 31 can also have only one of these zones 75, 76 weakened in flexion, arranged identically along the mean fiber of the arm, as well as with respect to the zones 75 of the arm 31 as zones 76 of arms 30.

Naturally, on each of the arms 30 and 31, the or each zone 76, 75 weakened in bending is disposed between the end zone such as 39 of integral connection with the shaft as corresponding 28 and the end zone as 40 forming a flange such as 45 of integral connection with one of the jaws, respectively 49, 47, of a respective jaw 32, 33.

Only one zone 75 will now be described, it being understood that the zones 75 are mutually identical just as the zones 76 are identical to them.

Reference is made, in this regard, to FIG. 14 which shows that, in the or each zone 75, the arm 31 is hollowed out in its faces 38 by two notches 77 each of which is defined by a concave face 78, semi-cylindrical of revolution around an axis 79 perpendicular to the plane 35 and located in the geometrical plane not illustrated of the respective face 38, to which the face 78 is connected respectively towards one and the other of the extreme zones 39 and 40 of the arm 31, of even that it is connected to each of the faces 37 thereof in the direction of a distance from the plane 35, respectively on either side thereof.

The axes 79 of the two notches 77 together constituting the same area 75 are arranged in the same plane 80 perpendicular to the two planes 34 and 35, that is to say also to the average fiber of the arm 31 defined by the intersection. 36 of these two planes 34 and 35, and each of the faces 78 has, with reference to its axis 79, a radius r less than half the dimension D that each face 37 has perpendicular to the plane 34, so that remains between the two notches 77 a strip 81 of the constituent material of the arm 31, offering sufficient dimensions to ensure the rigidity thereof during the bending test while being more sensitive than the rest of the arm 31 to the bending stresses then appearing therein. For this purpose, four strain gauges are used, two of which are arranged on one side of the plane 34 and the other two on the other side of the latter, in positions that are mutually symmetrical with respect to this plane 34; it is thus possible to place four gauges in the same zone 75 or in the single zone 75, along the plane 80 of this zone 75, by gluing two against one of the corresponding faces 78, respectively on either side of the plane 35 and in positions mutually symmetrical with respect thereto, and two against the other of the corresponding faces 78, also respectively on either side of the plane 35 and in positions mutually symmetrical with respect to the latter. If at least two zones 75 are provided in the same arm 31, it is also possible to glue each of these gauges to a respective face 78, in a respective position overlapping the plane 35, so as to have two of the gauges in the other of the zones 75, and two others in one of the zones 75. In all cases, the gauges are arranged along the plane 80 of the respective zone 75 or zone 75, that is to say where the strip 81 is the finest when measured perpendicular to the plane 34.

Strain gauges arranged according to these two possibilities have been illustrated respectively at 82 and 83 to the right of FIG. 9 and to the left thereof, in the case in which each arm 31 has two zones 75 weakened in bending, while the FIG. 10 illustrates the positioning of the two gauges 82 respectively on either side of the strip 81 of material of an arm 31 having a single zone 75 weakened in bending.

As shown in FIG. 11, the four gauges 82 or 83 are interconnected, easily conceivable by a person skilled in the art, to form an extensometry bridge 84 which, previously calibrated, supplies a data acquisition card 85 of a computer 86, at each instant, a voltage U proportional to the moment M of the bending torque undergone by the arm 31 due to the reaction that the bending zone 2 of the respective example of the test piece 1 opposes the bending, during the alternating relative rotation of the arms 30 and 31 and of the alternating opposing bending of the bending zones 2 of the two specimens of the test piece 1.

In addition, one can also stick to each of the main faces 8 of each of the specimens of the test piece 1, in its bending zone 2, along the respective mean plane such as 35, perpendicular to the axes 26, a respective strain gauge 87 The strain gauges 87, making local measurements of the deformations of the bending zones 2 on the surface, can also be connected, by means of terminals fixed on the zone 58 of the respective specimen of the test piece 1, to a extensometry bridge allowing permanent measurement of the deformations of the two specimens of the test piece 1 on the surface, which makes it possible to record in the computer 86 not only the measurement of the moment of the torque opposite to bending, at each instant, by the bending zones 2 of the two specimens of the test piece 1, but also the deformations undergone on the surface by these bending zones 2. The same computer 86 controls the stepping motors of the two sets drives 21 synchronously, by sending instructions to a control card 88 for the latter, to rotate the two shafts 27 and 28 respectively by a desired angle, around the respective axis 26, one relative at the other, the control card 88 sending the computer 86 the instantaneous value of the angular setpoint thus sent to the motor assemblies 21.

The realization of the necessary programs is normal skills of a skilled person. Such a person skilled in the art will easily understand that the method and the device which have just been described not only make it possible to overcome the friction forces inherent in the methods of the devices of the prior art, but also makes it possible to perform on the two copies of test piece 1 of the pure bending tests, that is to say practically without subjecting these two copies of the test piece to normal and / or sharp forces, which makes it possible to obtain truly significant results in terms of behavior of the bending zone 2 in bending.

Such a method and such a device can be applied to any conformation of the test piece 1, provided that this conformation and / or the constitution of the test piece 1 makes it possible to ensure that the antagonistic bending of the two copies of the specimen 1 caused by the alternating rotations of the arms 30 and 31 of the two motor assemblies 21, preserves a fixity of the gripping zones 2 and 3 of the specimen 1 in a direction perpendicular to a plane 35, perpendicularly to which the axes 26 are oriented , as well as in orientation with respect to this plane 35, in order to avoid the appearance of normal and / or sharp forces in the bending zones 2. Naturally, depending on the conformation of the gripping zones 3 and 4 of the specimen 1, the skilled person will adapt the conformation of the jaws 32 and 33, namely more particularly faces such as 52 and 53 delimiting the slots 46 and 51 for the integral reception of one, respectively, of the gripping zones 3 and 4 of a copy of test tube 1.

As far as possible, however, the jaws of the jaws secured to the gripping zones 3 and 4 of the two copies of the test piece 1 will be kept in a bevelled shape, as described with reference to FIGS. 16 and 17. Two jaws thus bevelled towards each other, if we consider the rest position, allow pronounced bending of the bending zones 2, as shown in FIG. 18 from which it appears that the shape described with reference to FIGS. 16 and 17 allows rotate each of the gripping zones 3 relative to the corresponding corresponding gripping zone 4, by relative rotation of each of the arms 31 relative to the arm 30 of the same motor assembly 21, about the corresponding axis 26, by an angle going up to 90 °, alternately in one direction and in the other, to flex each of the bending zones 2, between the two gripping zones 3 and 4 of the same specimen of the test piece 1, by an angle which can thus go up to 90 °, respectively in one direction and in the other, from the rest state, if we consider a test piece 1 having in the rest state the form of a plate having a mean plane 6 of symmetry, on either side of which its bending zone 2 can undergo bending of the same amplitude.

However, the modes of implementing the method according to the invention and the device according to the invention which have just been described are not the only ones to offer the possibility, characteristic of the present invention, of subjecting two copies, mutually identical, of a test piece 1 of the alternate antagonistic flexions while retaining at least substantially mutual symmetry with respect to a point or center of symmetry 22, and FIGS. 19 to 24 illustrate a case in which, while the two copies of l specimen 1 are in the rest state and the bending test device 20 in the rest position, the axes 89 around which the gripping zones 3 and 4 of each are rotated relative to each other example of the test piece 1 are no longer mutually parallel and mutually symmetrical with respect to the center of symmetry 22, as described with reference to FIGS. 3 to 18, but merged by passing one and the other by the center of symmetry 22.

Just as the axes 26 are vertical in the embodiment described with reference to FIGS. 3 to 18, due to the support of the motor assemblies 21 by suspension from a frame 23 by means of flexible links 29, the axes 89 are vertical due to the mode of connection chosen between the device 20 and the support 23, but other orientations are possible in one case as in the other, in relation to other modes of connection with a support 23, in particular if one chooses to support the device d 'bending test 18, or 19, or 20 by air cushion or liquid mattress, these examples being in no way limiting.

If one refers to Figures 19, 20, 21, 24, which illustrate the device 20 in its rest position, while the two copies of the test piece 1 are in the rest state, this device 20 is shaped in a manner which will now be described, by way of nonlimiting example. Like the devices 18 and 19, this device 20 comprises two mutually identical motor assemblies 90, advantageously constituted by electric stepping motors each of which comprises a rotor 91 guided in rotation around one, respectively, of the axes 89 with respect to a stator 92. The motor assemblies 90 are in this example mutually superimposed vertically, one of them defining the lower part of the device 20 while the other defines the upper part. So that the controlled rotation of each rotor 91 relative to the corresponding stator 92 results in pure bending of the bending zone 2 of each specimen of the specimen 1, the rotor 92 of the lower motor assembly 90 rests on a lower part of the support 23, to which it is rigidly fixed, while the stator 92 of the upper motor assembly 90 is suspended from an upper part of the support 23, by means of flexible links 93 as long as possible so that a rotation of the stator 92 of the upper motor assembly 90 around the axis 89, relative to the support 23, under conditions which will ensue from the following description, that is to say over an angle limited to a few tens of degrees , practically does not result in any significant change in the level of stator 92, that is to say that is to say practically by no modification of the relative level of the two engine assemblies 90.

Like each of the motor assemblies 21, each of the motor assemblies 90 comprises two coaxial output shafts, however in this case arranged on the same side of the motor assembly 90 with reference to the axis 89, namely an output shaft 93 integral with the rotor 91, arranged along the axis 89 and subdivided into two mutually spaced sections along this axis, and an output shaft 94 integral with the stator 92, subdivided into two mutually spaced sections along the axis 89 and each of which has a shape tubular, coaxially surrounding one, respectively, of the sections of the output shaft 93, in a guide relation to the relative rotation about the axis 89 without other possibility of relative movement. The two sections of each of the output shafts 93 and 94 are located on the same side of the point or center of symmetry 22, on either side of which are the sections of the output shafts 93 and 94 corresponding respectively to the two sub - assemblies 90, as well as the rotors 91 and stator 92 of these two motor assemblies 90.

The two sections of the output shaft 93 of the same motor assembly 90, as well as the two sections of the output shaft 94 of this motor assembly 90, are mutually connected, in an integral manner, by a rigid bracket 95 , 96, the stirrup 96 connecting the two sections of the output shaft 94 surrounding the stirrup 95 connecting the two sections of the output shaft 93, in the same mean plane 97 which coincides with the mean plane 6 of symmetry of the two copies of the test piece 1 if we consider the device 20 in the rest position and each of the copies of the test piece 1 in the rest state, as illustrated in FIGS. 19, 20, 21, 24.

More precisely, each of the stirrups 95 and 96 consists of two respective rigid arms 98, 99, oriented radially with respect to the axis 89 and integral with the sections of the output shaft 93, 94 which they connect to each other, and a respective rectilinear spacer 100, 101, rigid, parallel to the axis 89, and rigidly and rigidly connecting the two arms 98 , 99 respectively corresponding. In the rest position illustrated in FIGS. 19, 20, 21, 24, corresponding to the rest state of the two copies of the test piece 1, the stirrups 95 and 96 corresponding to one of the engine sub-assemblies 90 are arranged along the plane 97 on the same side of the axis 89, opposite this axis with respect to the stirrups 95 and 96 corresponding to the other motor assembly 90. Towards the axis 89, each spacer 100 bears integrally a respective jaw 102, the design of which may be identical to that of jaws 32 and 33 described with reference to the mode of implementation illustrated in FIGS. 3 to 18 and which has the function of ensuring a removable attachment, by clamping between two respective jaws , with one of the gripping zones 3, 4 of a respective copy of the test piece 1 which, if one refers to its rest state and the rest position of the device 20, has its mean plane 5 of symmetry perpendicular to the axis 89, its mean plane 6 of symmetry c merged with the plane 97 and consequently including the axis 89, and its mean plane 3 of symmetry at 90 ° relative to the plane 97 and also including the axis 89.

The other of the gripping zones 3, 4 of each specimen of the test piece 1 is secured under the same conditions with a jaw 103, of the same design, located in a position diametrically opposite to that of the jaw 102 with reference to the axis 89 in the rest position of the device 20, corresponding to the rest state of each specimen of the test piece 1. This jaw 103 is rigidly connected by a rectilinear spacer 104, parallel to the axis 89, to the stirrup 96 corresponding to the other engine assembly 90. If one refers to the rest position of the device 20 and to the rest state of the two copies of the test piece 1, the spacer 104 is linked by a jaw 103 to a determined specimen of the test piece 1 and located along the plane 97, on the side of the axis 99 diametrically opposite to the side where the spacer 100 is secured by a jaw 102 with the other gripping area of the same specimen of test tube 1.

The two motor assemblies 90 thus formed, comprising the stirrups 95, 96, the jaws 102, 103 and the spacers 104, are mutually symmetrical with respect to the point or center of symmetry 22 in particular in the rest position of the device 20, corresponding to the state of rest of the two specimens of the test piece 1, themselves then mutually symmetrical with respect to the point or center of symmetry 22.

If, from the rest position, the two output shafts 93 are rotated in a controlled manner by the same angle, about the axis 89, relative to the respective stator 92, in the same direction if one consider the device 20 as a whole, that is to say in mutually opposite directions if we consider the two motor assemblies 90 independently of their orientation in the device 20, the arms 98 of each of the subassemblies 90 are angularly offset relative to the arms 99 of the same motor assembly 90 by the same angle, in directions corresponding to the direction of rotation, but the symmetry of the device 20 considered as a whole, including with regard to the two copies of the test piece 1, with reference to the point or center of symmetry 22, is preserved and the two copies of test piece 1 undergo antagonistic bending, as illustrated for example in FIGS. 22 and 23 which respectively show the lower copy of the éprouvett e 1 and the upper example of this test piece, in the respective state of bending at right angles between their respective gripping zones 3, 4. This bending is located at their respective bending zone 2 insofar as, in this mode of implementation as in that which has been described with reference to FIGS. 2 to 18, the gripping zones 2 and 3 are integrally rigidly held in the two jaws, only the bending zone 2 being free, in particular to bend, between the latter.

During this bending and for reasons of symmetry, the perpendicularity of each of the mean planes 5 of symmetry with respect to the axis 89 is preserved, and the axes 89 remain substantially coaxial, passing through the point 22, only a slight difference in behavior between the two copies of the test piece 1 which may cause a slight offset between them, under conditions which however remain negligible in terms of influence on the measurements.

These measurements are carried out under the same conditions as in the example described with reference to FIGS. 2 to 18, in the sense that, preferably, the resistance opposed to bending is measured by each of the bending zones 2 by measuring a resistant moment. in at least one of the arms 98 and 99, it being understood that the arms 98 are mutually identical, as are the arms 99, and advantageously have at least one zone weakened in flexion, arranged identically with respect to the axis 89, in a way entirely similar to that which has been described with reference to the embodiment of the invention illustrated in FIGS. 3 to 18.

This mode of implementation of the invention will therefore not be detailed further, and reference will be made to the mode of implementation described with reference to FIGS. 2 to 18 with regard to the practice of measurement.

Naturally, the two embodiments of the invention which have just been described constitute only nonlimiting examples, with respect to which many variants can be provided without departing from the scope of this invention, as it is defined in the claims.

Claims

1. A method of testing pure bending, optionally alternating, comprising the succession of steps consisting in: a) producing or choosing a test piece (1) comprising two mutually opposite end zones (3, 4) and a bending zone ( 2) mutually connecting the two gripping zones (3, 4), said test piece (1) having, in a state of rest, a first mean plane (5) overlapped by the bending zone (2) and each of the gripping zones (3, 4) and which constitutes a first plane of symmetry at least for the bending zone (2), and an average surface (6) for the bending zone (2) and each of the gripping zones (3, 4) , which mean surface (6) is perpendicular to the first mean plane (5), b) leaving the specimen (1) in the rest state, rigidly grasp the two gripping zones (3, 4) thereof by defining for each a respective axis (26, 89) of pivoting perpendicular to the first mean plane (5) and occupying u position determined on the one hand with respect to the respective gripping zone (3, 4) and on the other hand with respect to the average surface (6), and c) impose on the two gripping zones (3, 4) the test piece (1) of the controlled antagonistic rotations, possibly alternating around the respective pivot axis (26, 89), from the rest state, leaving the pivot axes (26, 89) free to move to approach or to move away from each other, to impose a bending possibly alternating on the bending zone (2), and to study the behavior of the bending zone in pure bending, characterized in that it is put implemented simultaneously on two mutually identical copies of said test piece (1), using: - step b such that the first mean planes (5) of the two copies are mutually parallel and that the mean surfaces (6) of the two copies are mutually symmetrical with respect to a point (22) while the two copies are in the rest state and in such a way that the pivot axes (26, 89) of the two copies are common and mutually symmetrical with respect to said point (22), and - step c by applying in a controlled manner around each pivot axis (26, 89), to the gripping zones (3, 4) respectively corresponding, antagonistic couples, possibly alternating, so as to impose antagonistic flexions possibly alternating to the bending zones (2) of the two examples, leaving the pivot axes (26, 89) to move freely relative to each other. 2. Method according to claim 1, the test piece (1) having as a mean surface (6), in its rest state, a second mean plane (6) which constitutes a second plane of symmetry at least for the area of bending (2), characterized in that step b is implemented so that the second mean planes (6) of the two copies coincide while the two copies are in the rest state and so that the pivot axes (26, 89) are placed in the second mean planes (6) thus coincident. 3. Method according to any one of claims 1 and 2, the test piece (1) having in its rest state a third mean plane (7) which is perpendicular to the first mean plane (5), is overlapped by the area of bending (2) while the gripping zones (3, 4) are disposed respectively on either side of it, and constitutes a third plane of symmetry at least for the bending zone (2), characterized in that l 'step b is implemented in such a way that the third mean planes (7) of the two copies coincide and that the pivot axes (26, 89) are mutually symmetrical with respect to the third mean planes (7) thus coincident. 4. Method according to any one of claims 1 to 3, characterized in that, during step c, the behavior of the bending zone (2) of the test piece (1) in pure bending is studied by measuring the resistance opposed to said rotation by at least one of the gripping zones (3, 4), in particular to deduce therefrom the evolution of the resistance of the bending zone (2) to bending. 5. Method according to any one of claims 1 to 4, characterized in that step b is implemented by connecting each of the gripping zones (3, 4) to the pivot axis (26, 89 ) respectively corresponding by an arm (30, 31, 98, 99), the arms (30, 31, 98, 99) corresponding to the gripping zones of the two copies being mutually symmetrical with respect to said point (22), and by connecting the two arms (30, 31, 98, 99) corresponding to the same pivot axis (26, 89) by a respective controlled motor, capable of causing antagonistic rotations, possibly alternating, of the two arms (30, 31, 98, 99 ) around the respective pivot axis (26, 89), the controlled motors (105, 106) corresponding to the two pivot axes (26, 89) being mutually identical and authorized to move freely one relative to the 'other. 6. Method according to claim 5 in its dependency relationship with respect to claim 4, characterized in that step b is implemented by ensuring that each arm (30, 31, 98, 99) is elastically flexible along the first mean plane (5) of the corresponding specimen of the test piece (1), with a stiffness greater than that of the bending zone (2) thereof, and is rigid by elsewhere, and in that, during step c, the resistance opposed to rotation is measured by measuring the bending stresses undergone by at least one of the arms (30, 31, 98, 99) according to the first medium plane (5) of the corresponding specimen of the test piece (1). 7. Method according to any one of claims 5 and 6, characterized in that one arranges the arms (30, 31) and the controlled motors (105) so that, during step b, the axes pivot (26) are mutually parallel and disposed respectively on either side of said point (22). 8. The method as claimed in claim 7 in its dependence relation with respect to claim 3, the third mean plane (7) of the test piece (1) constituting a plane of mutual symmetry for its gripping zones (3, 4), characterized in that the arms (30, 31) corresponding to the gripping zones of the two copies are mutually identical. 9. Method according to any one of claims 5 and 6, characterized in that one arranges the arms (98, 99) and the controlled motors (106) so that, during step b, the axes pivot (89) are merged and pass through said point (22). 10. Method according to any one 1 to 9, characterized in that, during step a, each specimen of the test piece (1) is produced or chosen so that it has the form of a plate whose thickness (e) is oriented perpendicular to the average surface (6). 11. Method according to claim 10, characterized in that, during step a, each specimen of the test piece (1) is produced or chosen so that, in addition, said thickness (e) is at least constant in the bending zone (2). 12. Method according to any one of claims 10 and 11, characterized in that, during step a, each specimen of the test piece (1) is produced or chosen so that it has a dimension (Li) constant, perpendicular to the first mean plane (5), at least in the bending zone (2). 13. Method according to any one of claims 10 to 12, characterized in that, during step a, each specimen of the test piece (1) is produced or chosen so that it has a transition (107) perpendicular to the first mean plane (5) between the bending zone (2) and each gripping zone (3, 4), respectively. 14. A device for testing pure bending, optionally alternating, on a test piece (1) comprising two mutually opposite end grip areas (3, 4) and a bending area (2) connecting the two grip areas (3, 4), said test piece (1) having, in a state of rest, a first mean plane (5) overlapped by the bending zone (2) and each of the gripping zones (3, 4) and which constitutes a first plane symmetry at least for the bending zone (2), and an average surface (6) for the bending zone (2) and each of the gripping zones (3, 4), which medium surface (6) is perpendicular to the foreground means (5), this device (18, 19, 20) comprising: a pair of jaws (32, 33, 100, 103) each of which defines a gripping slot (46, 51) secured to one respective gripping zone (3, 4) of the test piece (1), the slots (46, 51) having, in a relative rest position corresponding to the rest state of the test piece (1), a first mean plane (34) which each overlaps slots (46, 51), and a mean surface (35) for each of the slots (46, 51), mean surface (35) on either side of which each slot (46, 51) has a respective face of clamping (52, 53) for the respective grip zone (3, 4) of the test piece (1) and which is perpendicular to the first mean plane (34) of the slots (46, 51), - Means (27, 28, 30, 31, 93, 94, 98, 99) for defining for each jaw a respective pivot axis (26, 89) so that, in the relative rest position of the jaws (32 , 33, 100, 103), the pivot axes (26, 89) are perpendicular to the first mean plane (34) of the slots (46, 51), occupy a determined position relative to the respective jaw (32, 33, 100, 103), and are free to approach or move away from each other, - controlled means (105, 106) for imposing on the jaws (32, 33, 100, 103) antagonistic rotations, possibly alternating, around the respective pivot axis (26, 89), from the relative position of rest of the jaws (32, 33, 100, 103), leaving the pivot axes (26, 89) free to approach or move away from each other, and - Means (82, 83, 84, 86) for measuring the behavior of the bending zone (2) of the test piece (1) in pure bending, characterized in that, for the implementation of the method according to any one of claims 1 to 13, - It comprises two sets, mutually identical, of said pair of jaws (32, 33, 100, 103), the first mean planes (34) of the slots (46, 51) of which are mutually parallel and whose mean surfaces (35) slits (46, 51) are mutually symmetrical about a point (22) while the two sets occupy their rest position, in which each is able to receive a respective copy of the test piece (1) in the rest state, in a relative position of the two copies as they are mutually symmetrical with respect to said point (22), - The means (27, 28, 30, 31, 93, 94, 98, 99) for defining the pivot axes (26, 89) of the jaws (32, 33, 100, 103) of the two sets are arranged so that the pivot axes (26, 89) are common to the two sets, mutually symmetrical with respect to said point (22) when the two games occupy their rest position, and free to move relative to each other, and - the controlled means (105, 106) for imposing on the jaw (32, 33, 100, 103) of the two sets of antagonistic rotations, possibly alternating, comprise controlled motor means (105, 106) for applying around each pivot axis (26, 89), to the corresponding jaws (32, 33, 100, 103), opposing couples possibly alternating. 15. Device according to claim 14, the test piece (1) having as a mean surface (6), in its rest state, a second mean plane (6) which constitutes a second plane of symmetry at least for the area of bending (2), the slots (46, 51) of said pair of jaws (32, 33, 100, 103) having, as medium surface (35), a second mean plane (35) between the clamping faces (52, 53) of each jaw (32, 33, 100, 103), in the rest position, characterized in that the second mean planes (6) of the two sets of said pair are mutually symmetrical with respect to said point (22) while both sets are in the rest position. 16. Device according to claim 15, the test piece (1) having in its rest state a third mean plane (7) which is perpendicular to the first mean plane (5), is overlapped by the bending zone (2) while the gripping zones (3, 4) are disposed respectively on either side of it, and constitutes a third plane of symmetry at least for the bending zone (2), and the slots (46, 51) of said pair jaws (32, 33, 100, 103) having, in the rest position, a third mean plane (108) on either side of which they are arranged and which is perpendicular to their first mean plane (34), characterized in that the third mean planes of the two sets of said pair are mutually symmetrical with respect to said point (22) while the two sets are in the rest position. 17. Device according to any one of claims 14 to 16, characterized in that the means (82, 83, 84, 86) for measuring the behavior of the bending zone (2) of the test piece (1) in bending pure include: - means (82, 83, 84) for measuring the resistance opposed to said alternating rotation by at least one of the jaws (32, 33, 100, 103), and, where appropriate, - Means (86) for deducing therefrom the evolution of the resistance of the test piece (1) to bending between the jaws (32, 33, 100, 103). 18. Device according to any one of claims 14 to 17, characterized in that: the means (27, 28, 30, 31, 98, 99) for defining the pivot axes (26, 89) of the two sets comprise: - along each of the pivot axes (26, 89), two respective coaxial shafts (27, 28, 93, 94) mounted at relative rotation around the corresponding pivot axis (26, 89), and - four arms (30, 31, 98, 99), mutually symmetrical with respect to said point (22), each of which integrally connects one, respective, of the shafts (27, 28, 93, 94) to the respective one, jaws (32, 33, 100, 103) corresponding to the same pivot axis (26, 89), and - the controlled motor means (105, 106) for applying, around each pivot axis (26, 89), to the corresponding jaws (32, 33, 100, 103), opposing couples, possibly alternating, comprise two controlled motors (105 , 106), mutually identical and arranged so as to be able to move freely relative to one another, each of the motors (105, 106) being associated with one, respectively, of the pivot axes (26, 89 ) and capable of causing antagonistic rotations, possibly alternating, of the two shafts (27, 28, 93, 94) respectively corresponding. 19. Device according to claim 18, characterized in that the motors (105, 106) are electric stepper motors. 20. Device according to any one of claims 18 and 19 in its relationship of dependence with respect to claim 17, characterized in that each arm (30, 31, 98, 99) is elastically flexible in the foreground means (34) of the slot (46, 51) of the corresponding jaw (32, 33, 100, 103), with a stiffness greater than that of the bending zone (2) of the test piece, and is also rigid, and in that the measuring means (82, 83, 84) comprise means (82, 83) for measuring the bending stresses undergone by at least one of the following arms (30, 31, 98, 99) the first mean plane (34) of the slot (46, 51) of the corresponding jaw (32, 33, 100, 103). 21. Device according to claim 20, characterized in that the arms (30, 31, 98, 99) have, in positions which are mutually symmetrical with respect to said point (22), at least one respective zone (75) weakened in bending according to the first mean plane (34) of the slot (46, 51) of the corresponding jaw (32, 33, 100, 103) and in that the means (82, 83) for measuring the bending stresses are located in said zone ( 75) of at least one of the arms (30, 31, 98, 99). 22. Device according to any one of claims 18 to 21, characterized in that the arms (30, 31), the shafts (27, 28) and the motors (105) are arranged so that, in the position of rest, the pivot axes (26) are mutually parallel and disposed respectively on either side of said point (22). 23. Device according to claim 22 in its relation of dependence with respect to claim 16, the third mean plane (7) of the test piece (1) constituting a plane of mutual symmetry for the gripping zones (2 , 3), characterized in that the arms (30, 31) are mutually identical. 24. Device according to any one of claims 18 to 21, characterized in that the arms (98, 99), the shafts (93, 94) and the motors (106) are arranged so that, in the position of rest, the pivot axes (89) are coincident and pass through said point (22). 25. Device according to any one of claims 14 to 24, characterized in that the jaws (32, 33, 102, 103) are bevelled so as to thin towards one another, in the rest position jaws (32, 33, 102, 103). 26. Pure bending test machine, optionally alternated, for implementing the method according to any one of claims 1 to 13, characterized in that it comprises: - two mutually identical and mutually independent motor assemblies (21, 90), each of which comprises * two jaws (32, 33, 102, 103) each of which is capable of receiving a respective gripping zone (34) of a respective copy of the same bending test piece (1), * means (27, 28, 30, 31, 93, 94, 98, 99) for defining a relative pivot axis (26, 89) for the two jaws (32, 33, 102, 103), occupying a determined position relative to each of the two jaws (32, 33, 102, 103), in a relative rest position, * controlled motor means (105, 106) for imposing on the jaws (32, 33, 100, 103) relative rotations, possibly alternating, around the relative pivot axis (26, 89), from the relative position rest, - common means (86) for controlling the motor means (105, 106) of the two motor assemblies (21, 90), to impose on the jaws (32, 33, 100, 103) respective relative rotations, optionally alternating, around the respective pivot axis (26, 89). 27. Machine according to claim 26, characterized in that it comprises: - means (82, 83, 84) for measuring the resistance opposed to the relative rotation by at least one of said jaws (32, 33 , 100, 103). 28. Machine according to any one of claims 26 and 27, characterized in that respectively for each of said motor assemblies (21, 90), - the means (27, 28, 30, 31, 93, 94, 98, 99) to define the relative pivot axis (26, 89) of the two jaws (32, 33, 100, 103) comprise: * two coaxial shafts (27, 28, 93, 94) mounted at relative rotation around the axis relative pivoting (26, 89), * two arms (30, 31, 98, 99) each of which integrally connects one of the jaws (32, 33, 100, 103) to one, respectively, of the shafts (27 , 28, 93, 94), and - the controlled motor means (105, 106) for imposing on the jaws (32, 33, 100, 103) relative alternations, possibly alternating, around the relative pivot axis (26, 89 ), comprise a controlled motor (105, 106), mechanically independent of the controlled motor (105, 106) of the other motor assembly (21, 90) and capable of causing relative rotations, possibly alternating, of the two shafts (27, 28, 93, 94). 29. Machine according to claim 28, characterized in that the controlled motor (105, 106) is an electric stepper motor. 30. Machine according to any one of claims 28 and 29, in their relationship of dependence with respect to claim 27, characterized in that each arm (30, 31, 98, 99) is elastically flexible along a plane medium (34) perpendicular to the pivot axis (28, 29) and is also rigid, and in that the measuring means (82, 83, 84) comprise means (82, 83) for measuring the bending stresses undergone by at least one of the arm (30, 31, 98, 99), along said mean plane (34). 31. Machine according to claim 30, characterized in that each of the arms (30, 31, 98, 99) has at least one zone (75) weakened in bending along said mean plane (34) and in that the means (82 , 83) for measuring the bending stresses are located in said zone (75) of at least one of the arms (30, 31, 98, 99). 32. Machine according to any one of claims 26 to 31, characterized in that the arms (30, 31) are mutually identical. 33. Machine according to any one of claims 26 to 32, characterized in that the jaws (32, 33, 100, 103) are bevelled.