FR2685773A1 - Fretting test machine - Google Patents

Abstract

The fretting test machine comprises two independent support and control assemblies for an upper test-piece (101) and a lower test-piece (102) placed in contact with each other. A reciprocating movement of the lower test-piece (102) is obtained by means of a support bar (115) suspended by two flexible blades (106) from a framework (120) and connected to a horizontal vibrating pot (113). The upper test-piece (101) is itself mounted on a support device fixed to the central part of a horizontal arm (108) mounted on the framework (120) via an articulation with elastic pivots, located in the vicinity of a first end (181) of the arm (108) carrying a balancing counterweight (111). The arm (108) is furthermore joined to the framework (120) by a horizontal linkage means (109) located in the vicinity of the second end (182), perpendicularly to the arm (108), a vertical fretting force and a horizontal tensile force are exerted in the vicinity of the second end (182). Sensors detect the displacement and the frequency of the horizontal to-and-fro movements of the support bar (115) as well as the tangential fretting force applied to the arm (108).

Description

Fretting testing machine
Field of the invention:
The present invention relates to a fretting test machine capable of generating fretting phenomena between two test pieces and which can be used in applied research, for the development of new equipment or even for expert missions, that is to say say for the reconstruction of a degradation which occurred in service, and the determination of the circumstances of production of this degradation.

 The phenomenon of fretting or wear and fatigue under low deflections is a type of surface degradation which appears at the level of the contact zone of two parts, this zone being subjected to repeated stresses of small amplitudes.

 It will be noted that the small deflections can result from a relative displacement of the parts in the contact zone or from an elastic or plastic deformation of the parts in the contact zone and without there necessarily being a relative displacement of the contact zones. between them.

 Fretting can manifest itself in any mechanical link subjected to vibrations, either out of operation, in particular during a transport operation, or during operation in particular during the transient phases.

 The mechanical components liable to be subjected to fretting are in particular bearings and other types of bearings, splined shafts, hoop connections.

The phenomenon of damage due to fretting is very penalizing for the maintenance in service of the mechanisms. It has two main consequences:
- wear of the materials resulting in either blockage by accumulation of debris, or seizure, or significant play between the parts of the connection depending on the behavior of the wear particles (chemical, mechanical, dynamic),
- A significant reduction in the volume fatigue strength of the parts following the surface embrittlement of the material (crack initiation, degradation of the surface state and of the structure).

 Fretting phenomena are influenced by the properties and behavior of the materials in which the test pieces are made, by the environmental conditions in which the test pieces are placed and above all by the contact conditions between the test pieces: geometry of the test pieces, intensity tangential and normal forces undergone by the test specimens, amplitude, shape and frequency of deflections, duration and number of cycles.

Prior art:
Various types of machines have already been proposed for simulating the fretting phenomenon for study purposes.

 We thus know, by an article of MM.IWABUCHI and HONDA, of the University of IWATA, published in the review "ISME International Journal" n04, 1988 a fretting simulation machine which is illustrated on figure 12 and whose principle is shown in Figure 11.

 According to this fretting simulation machine, a main test piece 1, fixed on a shaft 4 movable longitudinally, is brought into contact with two counter-test pieces 2,3 each mounted on a fixed rod parallel to the movable shaft 4. The test pieces 1 , 2,3 are placed in a tank 7 which can be filled with liquid helium or liquid nitrogen, the tank 7 itself being placed in a cryostat 8 filled with liquid nitrogen and placed on a support 9. A gantry 10 supports on the one hand the vertical fixed rods 5 and 6 at the lower end of which are arranged the test pieces 2,3, and a mechanism for actuating the movable rod 4. A horizontal arm 15 is articulated on a support 16 and cooperates with an eccentric 17 to impart alternating movements of axial displacement to a vertical rod 14 and to the movable shaft 4 which is connected to the rod 14 via a detection system containing various force sensors displacement 11.1 2.13.

 The fact that the fretting vibration of the test specimens is produced by an eccentric implies the existence of parasitic friction at the level of the intermediate connections, and likely to create damages by undesired fretting phenomena, which introduce errors into the measurements and restrict at low frequencies, the fretting achievable between specimens. In addition, the implementation of a movable main test piece cooperating with two test pieces does not allow a sufficiently precise analysis of the phenomena because the two fretting phenomena created on either side of the main test piece are not independent. The main specimen transmits forces from one counter specimen to the other and the shaft 4 for supporting the main specimen is itself subject to transverse vibrations. The application of a normal force on the counter specimens is imprecise because of the great length of the rods 5,6 which leads to the application of a normal load very far from the contact area of the test pieces.

 Furthermore, the aforementioned machine has a low adaptability to various fretting modes and lends itself poorly to automation.

 We also know from the review "WEAR" N 110 of 1986 a fretting simulation machine developed by S. SÔDERBERG. Such a machine, which is illustrated diagrammatically in FIG. 13, also comprises a movable central test piece 21 interposed between two fixed counter-test pieces 22, 23. The alternating axial movements of the support 24 of the central test piece 21 are produced using ultrasound created from a piezoelectric crystal generator 28 supplied by a power supply unit 29 and associated with an amplifier 27. A probe photonics 30 detects the movements of the central test piece 21 and delivers signals to an amplifier 31 allowing observation using an oscilloscope 32.

 Such a machine has the same drawbacks as that of FIG. 12 by the very fact of the presence of three test pieces and the interpretation of the fretting phenomenon created is difficult due to couplings between the contact forces and the volume forces. In addition, this machine is not easily adaptable to operating conditions outside of ambient temperature.

 It is also described in the review "WEAR", n106 of 1985, pages 303 to 313, a machine developed by RB WATERHOUSE and A. IWABUCHI and intended to allow the study of wear by fretting at high temperature of titanium alloy test pieces. Such a machine, of which FIG. 14 is a schematic representation, comprises a lower flat test piece 42 placed in a support 43 mounted on a fixed frame 44, and an upper test piece 41 which is placed at the end of an arm 45 and carries on the lower test piece 42 by a spherical surface. A vibrating pot 51 placed at the rear end of the arm 45 articulated on a fixed support 40 gives the arm 45 and the upper specimen 41 small reciprocating movement movements parallel to the contact plane of the lower specimen 42.

 An accelerometer 54, a capacitive displacement detector 52 and strain gauges 47 make it possible to measure the normal and tangential forces exerted by the arm 45 and the amplitude of the displacement.

 The arm 45 is equipped with a cooling circuit 48 with an inlet 49 and an outlet 50 for the cooling water. A thermocouple 46 allows the temperature measurement in the vicinity of the test specimens in contact 41,42.

 Such a machine does not have the drawbacks of machines with three test pieces or of machines in which the main members themselves introduce parasitic friction. However, the machine illustrated in FIG. 14, in which all the normal and tangential forces are applied via the arm 45, and which includes a lower test piece 42 directly linked to the frame 44 is difficult to adapt to different modes of fretting . The environment is limited to room temperature or high temperatures. Above all, the tangential fretting force can only be known individually at the level of the vibrating pot 51. The machine illustrated in FIG. 14 therefore does not therefore have all the qualities required to allow the realization of various modes of fretting as well as precise, convenient and representative measurement of the various parameters of interest in the fretting phenomenon.

Subject and brief description of the invention
The object of the present invention is to remedy the aforementioned drawbacks and to produce a fretting test machine which is capable of producing various fretting modes by bringing into play all the parameters characteristic of the fretting phenomenon independently, both with regard to their application to test specimens, as regards their measurement, the machine making it possible to obtain very high precision by arranging the sensors in the most optimal way possible.

 Another object of the invention is to provide a machine which allows easy adaptation to various test environment conditions.

These goals are achieved by a fretting testing machine, characterized in that it comprises:
a) a fixed frame,
b) a support and control assembly for a lower test piece comprising:
bl) a rigid vertical rectangular gantry formed from vertical beams, an upper horizontal beam and at least one lower horizontal beam on the upper face of which said lower test piece is embedded,
b2) a gantry support comprising at least two parallel vertical flexible blades which are arranged perpendicular to the vertical plane defined by the gantry, are attached to the frame at their upper ends and support at their lower ends said upper horizontal girder of the gantry,
b3) a vibrating pot whose casing is fixed to the frame and whose horizontal active member is arranged in alignment with the upper horizontal beam of the gantry to create on it a movement of excitation back and forth of low amplitude and predetermined frequency in the longitudinal direction of said upper beam,
c) a support and control assembly for an upper test piece comprising:
cl) a rigid horizontal arm perpendicular to the vertical plane defined by the gantry and passing approximately at the center of this gantry,
c2) a support device which is fixed to the central part of the horizontal arm and to the lower part of which is embedded said upper test piece which is placed opposite said lower test piece,
c3) a support and articulation device for the horizontal arm, located in the vicinity of a first end of this arm and comprising a gimbal connected to the frame to allow articulation of the arm around a vertical axis and a first axis horizontal,
c4) a means for applying a vertical downwardly directed fretting force, located in the vicinity of the end of the arm opposite to said first end,
c5) a balancing counter arranged at the first end of the arm, cantilevered with respect to the support and articulation device,
d) an arm support device comprising:
dl) a horizontal connection means perpendicular to the rigid arm to connect the latter to the frame by means of a first articulation around a second horizontal axis, said horizontal connection means being fixed to the arm by means of a second articulation around a third horizontal axis disposed between the device for supporting the upper test piece and said means for applying a vertical fretting force,
d2) a means of horizontal traction for exerting, via a third articulation around a fourth horizontal axis, in the extension of the horizontal connection means, in a direction of separation from the frame, a tangential force of maintenance,
e) a detection device comprising at least:
el) a travel and frequency sensor for the horizontal back and forth movements applied to the gantry supporting the lower test piece,
e2) a sensor of the tangential fretting force applied to the horizontal arm supporting the upper specimen
In order to increase the reliability of the tests, the contact zone between the upper and lower test pieces, the first horizontal axis of articulation of the arm defined by the support and articulation device, and at least the third axis horizontal articulation of the horizontal connecting means are approximately located in the same horizontal plane, the first horizontal axis and the third horizontal axis being prependicular to each other.

 The fretting testing machine according to the invention is thus characterized by the presence of two independent assemblies for supporting and controlling the test specimens and by the use of a holding device which both avoids the introduction of any parasitic friction in the operation of the machine and prevents the appearance of fretting in regions other than the contact zone between the test pieces.

 Obtaining a reciprocating movement of the lower test piece via a gantry suspended by two elastic blades and mounting the arm on an articulation with elastic pivots guarantee that no error due to friction is introduced into the measurements and that the operation of the various organs of the machine are free from fretting, which would be likely to seriously disturb the desired mode of fretting between the two test pieces.

 The test pieces which are embedded in supports can be massive and have a large volume, so as to prevent deformations of these test pieces during the transmission of fretting forces.

 The two sets of support for the lower and upper test pieces are connected to the frame independently of each other and have no common member which, added to the fact that these sets are arranged in two perpendicular planes, gives the possibility of creating, practically all the modes of fretting and of carrying out the direct measurement of all the parameters characterizing the phenomenon of fretting created, in good conditions of precision and in particular in the immediate vicinity of the contact zone.

Advantageously, the frame comprises a main square-shaped structure comprising at least one horizontal wing and a vertical front wing
In this case, according to a preferred embodiment, the vertical rectangular gantry is arranged perpendicular to the vertical wing of the frame, on the side opposite to the horizontal wing, the housing of the vibrating pot is fixed on the horizontal wing of the frame, the horizontal active member of the vibrating pot is disposed facing an opening in the front vertical wing, to create, by means of an extension, through said vertical wing and without contact with it, said movement of low amplitude back-and-forth excitation on the upper horizontal beam of the gantry in a horizontal direction essentially perpendicular to said vertical wing and the flexible blades to which the upper horizontal beam of the gantry is suspended are parallel to the vertical wing of the frame and fixed at their upper part on a console fixed on the vertical wing of the building on the opposite side with the horizontal aid.

 The machine according to the invention may further comprise a tank containing a given fluid, which tank is located under the horizontal arm to receive the lower part of the gantry supporting the lower test piece and the lower part of the support device in which is embedded l 'upper specimen so as to allow the simulation of the fretting phenomenon within said fluid.

 The tank can contain a cryogenic fluid such as liquid nitrogen or liquid helium.

 According to an advantageous arrangement, the main structure in the form of a square is placed on a table, the front vertical wing being located in the vicinity of the edge of the table, and the container is placed on a device for vertical displacement of the container.

 The machine according to the invention allows direct measurement of all the parameters characterizing the fretting mode created.

 Thus, according to a particular characteristic, the displacement sensor is disposed between a vertical beam of the gantry and the support device in which the upper test piece is embedded.

 A sensor of the normal fretting force applied to the two test pieces via the horizontal arm is placed under the lower test piece and fixed in the lower horizontal beam. The normal fretting force sensor can be three-dimensional and can then also constitute the tangential fretting force sensor.

 According to another embodiment, the tangential fretting force sensor is incorporated in said horizontal connection means arranged between the support device for the upper test piece and said means for applying a vertical fretting force to measure the tangential holding force exerted on the rigid horizontal arm.

 The displacement sensor placed in the immediate vicinity of the upper test piece makes it possible to detect both the amplitude of the deflections due to the tangential stresses and their frequency.

 The tangential force sensor placed in the holding device, between the arm and the frame, can be constituted for example by a piezoelectric sensor or a strain gauge.

 The sensor of normal fretting force exerted by the upper test piece on the lower test piece can give information all the more reliable, as regards the intensity of the applied force and its frequency, that this sensor is constituted by a piezoelectric sensor placed under the lower test piece.

 A displacement sensor can also be arranged vertically between the arm, below the latter, near the test pieces, and a console secured to the frame.

 In all cases, the sensors mentioned above make it possible to measure the parameters resulting from the fretting phenomenon, and not the parameters applied to create it.

 However, it is also possible according to the invention to implement, independently of the sensors which express the fretting characteristics proper (travel, frequency, force), a monitoring device in order to ensure that the chosen fretting mode is well done or that the phenomena which are likely to disturb the measurements remain within the acceptable tolerance values.

 By way of example, a displacement sensor can be installed in the device for holding the horizontal arm to monitor possible modifications to the horizontal position of this arm. Likewise, by way of example, it is possible to use a sensor for measuring the vibrations to which the gantry is subjected in order to verify the absence of coupling with the frequency of the vibrating pot.

 Such a vibration measurement sensor is advantageously disposed at one end of the lower horizontal beam of the gantry substantially in the plane containing the contact area between the upper test piece and the lower test piece.

 The machine according to the invention can have various variant embodiments.

 According to a particular embodiment, the support and articulation device of the horizontal arm is constituted by an elastic cardan joint comprising a vertical central part mounted on the horizontal arm by means of two elastic pivots aligned vertically and crossed by a horizontal axis itself even mounted using a sleeve with elastic blades on a clevis secured to the frame.

 The device for holding the horizontal arm can be made from cables stretched by masses or from rods articulated by means of ball joints.

 The means for applying a vertical fretting force directed downwards may comprise a mass suspended from a cable, a spring, or even a second vibrating pot, the casing of which is also fixed to the frame.

 According to a particular characteristic intended to increase the reliability and the accuracy of the measurements, reference surfaces are formed on the support device in which the upper test piece or the support frame for the lower test piece is embedded and on the frame to allow checking the parallelism of the surfaces receiving the upper and lower specimens as well as the perpendicularity of the normal and tangential forces.

Brief description of the drawings
Other characteristics and advantages of the invention will emerge from the following description of particular embodiments, given by way of nonlimiting examples, with reference to the appended drawings, in which:
FIG. 1 is a schematic perspective view showing the principle of a fretting simulation machine according to the invention,
FIG. 1A is a detailed view showing an example of shapes of test pieces usable with the machine according to the invention,
FIG. 2 is a front view with partial section, of an example of a fretting simulation machine according to the invention,
FIG. 3 is a sectional view along line III-III of FIG. 2,
FIG. 4 is an enlarged view of part of FIG. 2 showing the support assemblies of the two test pieces,
FIG. 5 is a view similar to that of FIG. 5 and shows an alternative embodiment,
FIG. 6 is a partial section along the line VI-VI of FIG. 5,
FIG. 7 is a partial left view of the machine in FIG. 2 showing an example of a device for applying tangential and normal forces to the support arm of the upper test piece,
FIG. 8 is a detailed view in section along the plane VIII-VIII of FIG. 9, showing an example of an elastic gimbal usable for the support and the articulation of the support arm of the upper test piece,
FIGS. 9 and 10 are sectional views respectively along lines IX-IX and XX of FIG. 9,
FIG. 11 is an elevation view showing the known principle of the association of three test tubes for the study of the fretting phenomenon,
FIG. 12 is a diagrammatic elevation view showing a known machine for simulating fretting using three test pieces in accordance with FIG. 11,
FIG. 13 is a schematic elevation view of another known machine for simulating fretting using three test pieces,
FIG. 14 is a schematic elevation view showing yet another known machine for simulating fretting using a fixed test piece and a mobile test piece,
FIG. 15 is a curve showing the change in the rate of wear of the test pieces as a function of the clearance during the study of fretting, and
- Figure 16 is a curve showing the evolution of the wear rate as a function of the number of cycles of movements applied to test pieces during the study of fretting.

Detailed description of particular embodiments
In order to illustrate the laws representative of the fretting phenomenon which correspond to wear and fatigue appearing at the level of the contact zone of two parts subjected to repeated stresses of low amplitude, curves 15 and 16 are shown in FIGS. corresponding to a common fretting mode, such as it can occur in a bearing for example.

 Figure 15 shows the evolution of the wear rate as a function of the amplitude of the movement or travel between the two parts subjected to cyclic stresses.

 The fretting phenomenon can occur even for very low tangential amplitudes, of the order of a fraction of a micrometer, or even zero when the vibrations are only consecutive to deformations of the contact zones by cyclic compression.

 We see in Figure 15 that we can distinguish on a wear curve depending on the tangential amplitude two zones A and B separated by a threshold D1 whose value varies with the number of cycles performed.

 In zone A corresponding to very low amplitudes, fatigue has a preponderant character in fretting, but wear appears linearly as a function of amplitude.

 After crossing threshold D1, in zone C we are dealing with a classic wear phenomenon.

 Figure 16 shows an example of change in the wear rate in the presence of fretting, depending on the number of cycles of stresses applied.

 We see in Figure 16 three stages of evolution D, E, F separated by thresholds N1 and N2. In the first stage D, the plastic deformation of one of the two test specimens in contact subjected to stresses of small amplitude is significant and the wear rate is very high. In the second stage E, the wear rate is quite low, because of a better agreement of the surfaces which contributes to distribute the forces more uniformly. In the third stage F, there is a progressive increase in wear which corresponds to the formation of free debris by cracking.

 The parameters involved in fretting phenomena are numerous and relate mainly to contact conditions, environmental conditions and materials.

 With regard to the contact conditions, it is advisable during the study of the fretting phenomenon on test pieces or samples to be able to control throughout the tests the tangential and normal forces exerted on these test pieces and therefore to be able to adjust and measure the load, deflection, frequency of stresses, duration and number of cycles. The geometry of the test pieces also determining the contact conditions, it is desirable to be able to easily conduct tests with test pieces of different geometry and to also be able to control the evolution of the geometry of the test pieces during the tests.

 The environmental conditions liable to affect the fretting phenomenon are essentially expressed by the following parameters: temperature, humidity, chemical potential, lubrication, the presence of particles or debris either pre-existing or produced by the fretting phenomenon himself. These parameters, which can be measured before and after the test, can also be monitored during the test if necessary.

 The parameters relating to the properties and behaviors of the materials of test specimens subjected to fretting tests, which are measurable before and after a test, essentially include hardness, mechanical resistance, fatigue, oxidation, corrosion, propagation of cracks , ductility, adhesive properties.

 The fretting simulation machine according to the invention aims to allow the convenient taking into account of all of the various parameters indicated above by carrying them out, and allowing their measurement, either during the test, or by examination of the test pieces after test, and to avoid parasitic fretting phenomena being created outside the contact zone between the test pieces.

 We will first describe with reference to Figure 1 the block diagram of a fretting test machine according to the invention intended to simulate under determined and adaptable conditions, by the application of repeated stresses, a phenomenon of fretting between two test pieces 101, 102 having a contact area.

 The lower 102 and upper 101 test pieces can have various shapes and can, for example, define planar contacts on plane, horizontal or inclined, ball or cylinder contacts on horizontal or inclined plane, or else ball or cylinder contacts on cylinder or sphere, with concavities turned in the same direction or not.

 By way of example in FIG. 1 and FIG. 1A, an upper test piece 101 in the form of a pawn is shown having a spherical contact surface 101A, and a lower test piece 102 is flat.

 The machine of FIG. 1 comprises a frame, not shown, to which are suspended two flexible flexible blades 106 from the lower part of which hangs a support and control assembly for the lower test piece 102. This support assembly essentially comprises a gantry vertical rectangular 115 formed by two vertical beams 105, an upper horizontal beam 107 perpendicular to the two flexible blades 106 and a lower beam 104 parallel to the beam 107 and supporting the lower test piece 102.

 The upper horizontal beam 107 is connected to the output of an exciter, such as a vibrating pot 113 (shown in FIG. 3) which exerts on the beam 107 an excitation E in the axial direction of this beam 107 to cause the level of the lower test piece 102 a movement D in the form of an alternative horizontal displacement movement of small amplitude and frequency determined in the plane of the gantry 115.

 It will be noted that a vibrating pot offers wide adjustment facilities allowing to choose and adjust the frequency, the shape of the signal and the amplitude of the horizontal tangential stress. As regards the shape of the signal, it is possible in particular to have white noise, a sinusoidal, rectangular or triangular signal or else a beat around a frequency.

 A support and control assembly for the upper test piece 101 is attached to the frame in a completely independent manner from the support and control assembly for the lower test piece 102. This support assembly for the upper test piece 101 essentially comprises a horizontal arm 108 which is perpendicular to the plane defined by the rectangular gantry 115, which passes approximately in the middle of the gantry and is articulated relative to the frame in the vicinity of one of its ends by an articulation at the gimbal 110 around two pivot axes oriented horizontally and vertically.

 In the plane of the gantry 115, the arm 108 carries at its lower part a support device or mandrel 103 in which is embedded the upper test piece 101, which is brought into contact with the lower test piece 102 in the working position.

 A means 119 for applying a normal downwardly directed fretting force N is fixed in the vicinity of the end of the longitudinal arm 108 which is opposite the articulation 110.

 A counterweight 111 balances the arm 108 when the normal fretting force N is not applied, and allows the arm 108 to open up to the upper beam 107 of the gantry 115, which frees up a large space for exchanges of 'test pieces 101, 102 or sensors, such as the displacement sensor 161 which can be placed between a vertical beam 105 and the support device 103.

 A holding device 109 for the arm 108 connects the latter to the frame in a horizontal direction and makes it possible to exert on the arm 108 a controlled tangential force T directed towards the outside of the frame. The attachment point of the holding device 109 is located between the device 103 for supporting the upper test piece 101 and the end of the arm 108 where the normal force is applied.

 A more specific description will now be given with reference to FIGS. 2 to 10 of an example of a fretting testing machine according to the invention, the design of which aims in particular to allow great accessibility to the various organs and ease of adaptation of the machine. to the planned tests, while ensuring, taking into account in particular the principle of independence of the assemblies which support the lower and upper test pieces, good representability and high precision of the measurements for various possible fretting modes.

 If we consider more particularly FIGS. 2 and 3, we see that the frame 120 of the machine comprises a main structure in the form of a square comprising a lower horizontal wing 121 serving as a base and which can be placed on a table for example, and a front vertical wing 122, the edge constituting the top of the bracket being able to coincide with the edge of the table.

By convention, the volume located above the horizontal wing 121, on the same rear side of the vertical wing 122, will be called "interior" of the frame, and the volume located on the other side will be "exterior" of the frame. of the vertical wing 122, that is to say in front with respect to this wing in the front view of FIG. 2.

 The rectangular gantry 115, consisting of vertical 105 and horizontal beams 104, 107, is thus perpendicular to the vertical wing 122 of the bracket.

 The elastic blades 106 which are fixed to the upper horizontal beam 107 by connections 116, are fixed at their upper part by connection means 135 on the horizontal wing 131 of a console in the form of a square including the vertical wing 132 is itself fixed by connecting means 134 on the vertical wing 122 of the main structure of the frame 120 in the form of a square. Wedges 136 make it possible to adjust the position of the blades 106 and consequently the height of the gantry 115. The console can also have reinforcing wings 133.

 The vibrating pot 113 comprises a casing fixed by means of a cradle to the horizontal wing 121 of the frame, and an active member which is connected by an extension 112 to the upper beam 107 of the gantry 115 through an opening 124 formed in the vertical wing 122 of the frame 120. The active member of the vibrating pot 113 is thus perpendicular to the vertical wing 122 and aligned with the upper horizontal beam 107 of the gantry 115. The shims 136 make it possible to adjust the position of this upper beam 107 to ensure the necessary alignment.

 It will be noted that the vibrating pot 113 which provides the tangential fretting effort, is the only element placed inside the frame. All the other organs are external to it and therefore easily accessible.

 The lower beam 104 of the gantry 115 directly supports the lower test piece 102 which is embedded in this beam.

 A tank 70 containing a chosen medium in which the tests must be carried out, for example liquid helium or liquid nitrogen to carry out cryotechnical tests, can be placed under the gantry 115 and can be moved vertically from the bottom until it comes into contact with a closing cover 71 which is situated approximately halfway up the gantry, so as to allow all the two test pieces 101, 102 to be immersed. The horizontal arm 108 is then located just above the cover 71. The cover 71 limits the communication of the tank with the ambient atmosphere and has clearances which avoid any contact with the elements which enter the tank.

 The horizontal arm 108 carries in its central zone, at its lower part, a support device or mandrel 103 at the lower part of which is embedded the upper test piece 102, for example in the form of a ball, as shown in FIGS. 3 and 4 The attachment of the mandrel 103 with the arm 108 can be ensured for example using a nut 173 immobilized in rotation by two keys 170 and fixed by washer and nut on the arm 108, the removable mandrel 103 being made integral with this nut 173 by a large lug nut 169 provided with a conical centering surface cooperating with the male conical part formed at the upper part of the mandrel 103 (FIG. 5).

 The horizontal arm 108 which crosses the gantry 115 in its center, perpendicular to the plane of this gantry 115, is articulated in the vicinity of an end 181 by a cardan joint 110 which will be described in more detail with reference to FIGS. 8 to 10. A vertical central part 156 is mounted on the horizontal arm 108 by means of two elastic pivots 154,155 aligned vertically which are engaged in upper 152 and lower 153 parts integral with the arm 108 and connected together by a cylindrical ring 151. The part vertical central 156 is crossed by a horizontal axis 157 itself mounted using a sleeve 160 with elastic blades on a yoke 159 secured to a plate 158 fixed on an angle 126, one wing of which is fixed by means of connection 128 on the vertical wing 122 of the frame outside of it.

 The countermass 111 is fixed to the end 181 of the arm 108, cantilevered relative to the gimbal 110 constituting the articulation of the arm 108 relative to two axes oriented vertically and horizontally.

 The holding device horizontally connecting the arm 108 to the frame, and which can be combined with the device for applying a normal downward directed fretting force, will now be described with reference to FIGS. 2 and 7.

 The means 119 for applying the normal fretting force is arranged in the vicinity of the end 182 of the arm 108 which is opposite the end 181 in the vicinity of which the articulation 110 and the balancing countermount 111 are arranged. The means 119 may comprise a simple mass fixed to the lower end of a cable, as shown in the drawings, or be constituted by a spring or a vibrating pot whose casing is moreover fixed to the frame.

 In FIGS. 2 and 7, it can be seen that the cable by which a normal force N is exerted downwards is attached to a yoke 118 articulated at the end of a part 117 rigidly fixed in the vicinity of the end 182 of the arm 108.

 An element 109, one end of which is articulated around the connecting axis 190 between the yoke 118 and the vertical part 117 secured to the arm 108, and the other end is fixed to an anchor point 142 placed on an angle 125 fixed by bolts 127 on the vertical wing 122 of the frame, maintains the arm 108 relative to the frame in a horizontal direction perpendicular to the vertical wing 122. As can be seen in FIG. 7, a sensor 141 tangential force, for example of the piezoelectric type, is incorporated in the tangential link 109 for holding the arm 108.

 A cable 143 located in the extension of the element 109 which can also be constituted by a cable is fixed on another yoke 147 articulated around the same axis as the yoke 118 and makes it possible to exert a holding force which has the effect of tension this cable 109. For this, the cable 143 can be engaged on a pulley 144 mounted on a bearing 145 around an axis 146 fixed on the plate 125 attached to the frame, with a holding mass suspended at the free end lower of the cable 143. However, a retaining spring could also be substituted for the cable 143-retaining mass assembly.

 As a variant, the various cables 109, 143 of the holding device could also be replaced by a system of rods fixed at their ends by ball joints, the pulley 144 being replaced by a pivoting bracket mounted on the same bearing 145.

 In general, the horizontal connecting element 109 perpendicular to the arm 108 connects the latter to the frame by means of a first articulation around the horizontal axis 142. The horizontal connecting element 109 is fixed to the arm 108 by means of a second articulation around the horizontal axis 190 itself interposed between the vertical part 117 secured to the arm 108 and the means 119 for applying the normal fretting force. The tangential holding force is also exerted in an extension of the element 109, by means of a third articulation around a horizontal axis 146.

 In order to prevent inevitable small secondary excitations from significantly disturbing the fretting phenomenon which it is desired to achieve, preferably the contact zone between the upper test piece 101 and the lower test piece 102, the first horizontal axis 157 of articulation of the arm 108 defined by the support and articulation device 110, and at least the third horizontal axis 190 of articulation of the horizontal connecting means 109 are approximately situated in the same horizontal plane, the first horizontal axis 157 and the third horizontal axis 190 being prependicular to each other.

 It can be seen in FIG. 7 that the axes 142, 190 and the periphery of the pulley 144 causing the change of direction of the cable 143 are indeed located substantially in the same horizontal plane which, according to FIG. 2, is the common horizontal plane defined above. above.

 If we refer again to FIGS. 3 and 4, it can be seen that a displacement sensor 161 can be placed as close as possible to the test pieces 101, 102. A reflecting target 163 in the form of a vertical plate can be placed on the support 103 in which the upper test piece 101 is embedded to cooperate with a position detector 164, for example of the inductive type, itself mounted on one of the vertical beams 115.

 The normal fretting force applied via the arm 108 can be determined from the value of the mass 119 and the length of the lever arms relative to the gantry 115. In this embodiment, it is preferable that the mass 119 is supported by an articulated rod, and is subjected to a viscous damping.

 The normal fretting effort can however also be determined directly by a sensor. Such a sensor can for example be interposed between the yoke 118 and the mass 119.

 A force sensor 162, for example in the form of a piezoelectric pellet, can also be placed immediately under the lower test piece 102, in the horizontal beam 104 to determine the normal fretting forces in modulus and in frequency. If this sensor 162 is used in a three-dimensional version, it can also measure the tangential fretting force in place of or in addition to the sensor 141 in FIG. 7.

 A displacement sensor intended to determine the clearance and the frequency of normal forces can also be arranged vertically between the arm 108, below the latter near the test pieces 101, 102 and a console secured to the frame 120.

 As indicated above, the travel and the frequency of the tangential stresses can be determined from the displacement sensor 161 while the modulus of the tangential stresses can be determined using the force sensor 141, for example from piezoelectric type or strain gauge, placed in the holding device, between the arm and the frame.

 It will be noted that the parameters resulting from the fretting phenomenon are thus well measured, and not the parameters applied to create it. It is however desirable to measure monitoring parameters making it possible to verify that the chosen fretting mode is well carried out and that the phenomena which would be likely to disturb the measurements have not arisen or remain within acceptable limits.

 It is thus possible to have a sensor 149, such as an accelerometer on the gantry 115 to measure the vibrations of the beams and verify the absence of coupling with the frequency of the vibrating pot. For the same purpose, a similar sensor 171 can be attached to one of the ends of the lower horizontal beam 104. The sensor has the advantage of being almost in the same plane as the contact plane of the test pieces. A displacement sensor can also be introduced into the holding device to ensure that the arm remains still.

Of course, sensors should also be used in the usual way to monitor the environment (temperature, humidity for example)
Various variant embodiments are naturally possible starting from the machine described by way of example.

 FIG. 5 is thus quite similar to FIG. 4 but shows an upper test piece 101 produced in the form of a pin as in FIG. 1A, rather than in the form of a ball, a screw 167 used for fixing the pin in its support mandrel 103 which can itself be mounted by means of keys 170 on the arm 108. The mandrel 103 being moreover provided with several vertical surfaces with flats 168, it is possible to precisely determine the position of the upper test piece 101 with respect to reference surfaces of the frame 120.

 FIGS. 5 and 6 also show an example of mounting the displacement sensor 161 on a sensor support 165, a cable 166 used for the transfer of the signals delivered by the sensor.

 Figure 4 shows a possibility of mounting a tangential displacement sensor 172 between the upper part of the nut 173 and the upper horizontal beam 107. The tangential displacement sensor 172 can in particular be used when the fluid contained in the tank, too aggressive, does not allow the use of sensor 161.

 The creation of reference faces integral with the frame and of reference faces on a test piece support makes it possible to check the parallelism of the surfaces of the two supports receiving the test pieces, and the perpendicularity of the normal and tangential forces.

 As an alternative embodiment, it is possible to use elastic blades of different dimensions, because their exchange is easy, and to use, if necessary, blades of non-metallic materials, for example carbon.

 Parasitic vibrations can be suppressed by the implementation of a viscous damping of the masses used in particular as a means of applying a normal force, or in the device for holding the arm 108.

 To facilitate handling, the arm 108 can be separated from the horizontal flexible pivots. The arm then rotates in plain bearings which are again locked during the tests. This avoids excessive twisting of the flexible pivots.

 The arm 108 can also be provided with a stop for limiting the arm stroke in the high position.

 With regard to the measurements of various parameters to be taken into account, it is possible to measure the temperature and the energy dissipated in the fluid contained in the tank 70. However, it is also possible to measure the the temperature in the mass of the test pieces.

 In general, the machine according to the invention lends itself to great adaptability to modifications making it possible to take into account different conditions for studying fretting, while guaranteeing by design, the absence of any friction in the machine operation.

 Since the reciprocating movement of the lower test piece 102 is obtained by suspending the gantry 115 with two elastic blades 105 and the movements of the horizontal arm 108 being carried out on elastic pivots, no error due to friction is introduced into the measurements and the operations of the various parts of the machine are themselves free of fretting outside the zone of contact between the test pieces.

 The independence of the assemblies which support the upper and lower specimens facilitates the production of different types of fretting and allows direct measurement of all the parameters characterizing the fretting phenomenon created, under good conditions of precision.

 The machine according to the invention offers great accessibility to the various members insofar as all of the members except the horizontal vibrating pot can be fixed on the same vertical face 122 of the square support.

The constituent parts of the machine are themselves either of low cost (beams) or reusable (sensors, elastic pivots), which leads to great ease of exchange and addition of members and thus to optimization of the machine in the desired fretting mode.

By way of nonlimiting example, a fretting machine produced according to the invention can have the following characteristics:
Displacement + 5, um to + 50, um
Frequency 10 to 600 Hz
Maximum normal force 100 N
Contact geometry Sphere on plan
Number of cycles often used for a test 105 to 106

Claims (20)

 1. Fretting test machine, characterized in that it comprises:  a) a fixed frame (120),  b) a support and control assembly for a lower test piece (102) comprising:  bl) a rigid vertical rectangular gantry (115) made up of vertical beams (105), an upper horizontal beam (107) and at least one lower horizontal beam (104) on the upper face of which is embedded said lower test piece (102),  b2) a gantry support comprising at least two parallel vertical flexible blades (106) which are arranged perpendicular to the vertical plane defined by the gantry (115), are attached to the frame (120) at their upper ends and support at their lower ends said upper horizontal beam (107) of the gantry (115),  b3) a vibrating pot (113) whose casing is fixed to the frame (120) and whose horizontal active member (112) is arranged in alignment with the upper horizontal beam (107) of the gantry (115) to create thereon an excitation movement back and forth of low amplitude and of predetermined frequency in the longitudinal direction of said upper beam (107),  c) a support and control assembly for an upper test piece (101) comprising:  c1) a rigid horizontal arm (108) perpendicular to the vertical plane defined by the gantry (115) and passing approximately at the center of this gantry (115),  c2) a support device (103,169,173) which is fixed to the central part of the horizontal arm (108) and to the lower part of which is embedded said upper test piece (101) which is placed opposite said lower test piece (102),  c3) a device (110) for supporting and articulating the horizontal arm (108), located in the vicinity of a first end (181) of this arm (108) and comprising a gimbal connected to the frame (120), to allow an articulation of the arm (108) about a vertical axis (156) and a first horizontal axis (157),  c4) means (119) for applying a downwardly directed vertical fretting force, located near the end (182) of the arm (108) opposite said first end (181),  c5) a balancing countermass (111) disposed at the first end (181) of the arm (108), in cantilever with respect to the device (110) for support and articulation,  d) an arm support device comprising:  dl) a horizontal connection means (109) perpendicular to the rigid arm (108) for connecting the latter to the frame (120) by means of a first articulation around a second horizontal axis (142), said means of horizontal link (109) being fixed to the arm (108) by means of a second articulation around a third horizontal axis (190) disposed between the device (103,169) for supporting the upper test piece (101) and said means (119) for applying a vertical fretting force,  d2) means (143 to 147) of horizontal traction for exerting, by means of a third articulation around a fourth horizontal axis (146), in the extension of the horizontal connection means (109), in one direction distance from the frame (120), a tangential holding force,  e) a detection device comprising at least:  el) a sensor (161) of movement and of frequency of the horizontal movements back and forth applied to the gantry (115) supporting the lower test piece (102),  e2) a sensor of the tangential fretting force applied to the horizontal arm (108) supporting the upper test piece (101).  2. Machine according to claim 1, characterized in that the contact zone between the upper test piece (101) and the lower test piece (102), the first horizontal axis (157) of articulation of the arm (108) defined by the support and articulation device (110), and at least the third horizontal axis (190) of articulation of the horizontal connection means (109) are approximately located in the same horizontal plane, the first horizontal axis (157) and the third horizontal axis (190) being prependicular to each other.  3. Machine according to claim 1 or claim 2, characterized in that the frame (120) comprises a main structure in the form of a square comprising at least one horizontal wing (121) and a vertical front wing (122), in that that the vertical rectangular gantry (115) is arranged perpendicular to the vertical wing (122) of the frame (120), on the side opposite to the horizontal wing (121), in that the housing of the vibrating pot (113) is fixed on the horizontal wing (121) of the frame (120), in that the horizontal active member (112) of the vibrating pot (113) is disposed facing an opening (124) formed in the front vertical wing (122) , to create, by means of an extension (112), through said vertical wing (122) and without contact therewith, said low amplitude back-and-forth excitation movement on the upper horizontal beam (107) of the gantry (115) in a horizontal direction essentially perpendicular to said vertica wing the (122), and in that the flexible blades (106) to which the upper horizontal beam (107) of the gantry (115) is suspended are parallel to the vertical beam (102) of the frame (120) and fixed at their upper part on a console (130) fixed on the vertical wing (122) of the frame (120) on the side opposite to the horizontal aid (121).  4. Machine according to any one of claims 1 to 3, characterized in that it further comprises a tank containing a given fluid, which tank (7) is located under the horizontal arm (108) to receive the lower part of the gantry (115) supporting the lower test piece (102) and the lower part of the support device (103,169) in which the upper test piece (101) is embedded so as to allow the simulation of the fretting phenomenon within said fluid.  5. Machine according to claim 4, characterized in that the tank (70) contains a cryogenic fluid such as liquid nitrogen.  6. Machine according to claims 3 and 4, characterized in that the main square-shaped structure (121,122) is placed on a table, the front vertical wing (122) being located in the vicinity of the edge of the table, and in that the container (70) is placed on a device for vertical displacement of the container.  7. Machine according to any one of claims 1 to 6, characterized in that the displacement sensor (161) is disposed between a vertical beam (105) of the gantry (115) and the support device (103,169) in which is embedded the upper test piece (101).  8. Machine according to any one of claims 1 to 7, characterized in that a sensor (162) of the normal fretting force applied to the two test pieces (101,102) via the horizontal arm (108) is arranged under the lower test piece (102) and fixed in the lower horizontal beam (104).  9. Machine according to claim 8, characterized in that the sensor (162) of the normal fretting force is three-dimensional and also constitutes the tangential fretting force sensor.  10. Machine according to any one of claims 1 to 8, characterized in that the sensor (141) of tangential holding force is incorporated in said horizontal connecting means (109) disposed between the device (103,169) for supporting the upper test piece (101) and said means (119) for applying a vertical fretting force to measure the tangential holding force exerted on the rigid horizontal arm (108).  11. Machine according to any one of claims 1 to 10, characterized in that it further comprises a monitoring device comprising at least one sensor (149; 171)) for measuring the vibrations to which the gantry is subjected (115) or a displacement sensor associated with the device for holding the horizontal arm (108) and intended to control possible modifications of the horizontal position of this arm (108).  12 Machine according to claim 11, characterized in that the monitoring device comprises at least one sensor (171) for measuring vibrations arranged at one end of the lower horizontal beam (104) of the gantry (115) substantially in the plane containing the contact zone between the upper test piece (101) and the lower test piece (102).  13. Machine according to any one of claims 1 to 12, characterized in that the device (110) for support and articulation of the horizontal arm (108) is constituted by an elastic cardan joint comprising a vertical central part (156) mounted on the horizontal arm (108) by means of two elastic pivots (154,155) aligned vertically and crossed by a horizontal axis (157) itself mounted using a bush with elastic blades on a yoke (159) integral with the frame (120).  14. Machine according to any one of claims 1 to 13, characterized in that the arm holding device (108) is made from cables (143) stretched by masses.  15. Machine according to any one of claims 1 to 13, characterized in that the arm holding device (108) is made from rod articulated by means of ball joints and a bracket articulated on a bearing (145) forming part of said horizontal traction means.  16. Machine according to any one of claims 1 to 15, characterized in that said means (119) for applying a vertical fretting force directed downwards comprises a mass suspended from a cable.  17. Machine according to any one of claims 1 to 13, characterized in that the arm holding device (108) or said means (119) for applying a vertical fretting force directed towards the arm comprises a mass subject to viscous damping.  18. Machine according to any one of claims 1 to 15, characterized in that said means (119) for applying a vertical fretting force directed downwards comprises a spring.  19. Machine according to any one of claims 1 to 15, characterized in that said means (119) for applying a vertical fretting force directed downwards comprises a second vibrating pot whose casing is also fixed to the frame (120).  20. Machine according to any one of claims 1 to 19, characterized in that reference surfaces (168) are formed on the support device (103,169) in which is embedded the upper test piece (101) or the gantry ( 115) for supporting the lower test piece (102) and on the frame (120) to allow verification of the parallelism of the surfaces receiving the upper (101) and lower (102) test pieces as well as the perpendicularity of the normal and tangential forces.