Test specimen and device for characterizing the vibration damping of a contact between two parts

Abstract

Test specimen and device for characterizing vibration damping of a contact between two parts. One aspect of the invention relates to a test specimen (100) for characterizing vibration damping of an adherent or sliding contact between two parts subjected to vibratory, oligocyclic and tribological stresses, the specimen comprising a male-female type attachment (130) including a male part (131) projecting from the free end of a first longitudinal bar and a female part (132) notched at the free end of a second longitudinal bar.A second aspect of the invention relates to a device (300) for characterizing the vibration damping of an adherent or sliding contact between two parts subjected to low-cycle, tribological, and vibratory stresses. The device comprises a test bench (300) on which the test specimen (100) is mounted and includes a vibrating pot (310) that transmits vibrational excitations to the test specimen (100) and a static loading device (320) that transmits low-cycle stresses to said test specimen. A third aspect of the invention relates to a method (400) for implementing the characterization device (300). Figure to be published with the abbreviation: Figure 7.

Description

Title of the invention: Test specimen and device for characterizing vibration damping of contact between two parts TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to a test specimen representative of an adherent or sliding contact between two parts, such as a blade root and a cell of a turbomachine turbine disk. It also relates to a device for characterizing the vibration damping of the adherent or sliding contact between these two parts, using this test specimen. The invention also relates to a method for implementing this characterization device.

[0002] The invention finds applications in the field of characterizing the connection between the blade roots and the cells of the turbomachine disks and, in particular, in the field of characterizing vibration damping of the adherent or sliding contact between a blade root and a cell of a turbine disk. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] It is known, in aeronautics, that a turbomachine comprises a combustion chamber and a set of turbines, arranged downstream of the combustion chamber to recover the energy resulting from the combustion of the gases. A turbine, in particular a low-pressure turbine (or LP turbine) or a compressor, a fan (FAN) or a high-pressure turbine, comprises a rotor formed of a set of disks and blades, assembled around a central shaft. An example of a portion of a disk 20 and some moving blades 10 is shown in [Fig.l]. The moving blades 10 are arranged radially around the disk 20, as shown in [Fig.l], the disk 20 itself being mounted around the central axis extending in the direction X of the orthonormal reference frame XYZ.

[0004] A moving blade 10 comprises a blade 11 which extends radially, substantially along the Y axis, between a proximal end 12 (or inner end) close to the disc 20 and a distal end (or outer end), not visible in the figure and ending in a heel. At the proximal end 12 of the blade 11, the blade 10 comprises a root 13 making it possible to fix said blade to the disc 20 of the rotor. For this, the disc 20 comprises cells 22 distributed radially on its circumference and each receiving the root 13 of a blade 10. The cells 22 each have a shape complementary to that of the root 13 of the blade. In the example of [Fig.l], the feet 13 of the blades 10 have a so-called “fir tree foot” shape with sawtooth threads and the cells 22 of the discs 20 have a sawtooth shape, complementary to the shape of the blade feet.

[0005] When the moving blades 10 are mounted in a crown on the disk 20 and the turbine is in operation, the blades 10 are subjected to complex stresses, superimposing vibratory stresses with oligocyclic stresses, causing damage to the blades. Indeed, many parts of the turbomachine are subjected to more or less complex stresses. In particular, as shown in [Fig. 2], the turbine blades 10 are each subjected to drag and lift forces as well as to a centrifugal force, due to the rotation of the turbine and the origin of the oligocyclic stresses of the blades. The turbine blades 10 are also subjected to vibrational excitation due to the environment and / or to the resonance of the blade, this vibrational excitation being able to generate a deformation of the blades 11 as shown in [Fig.3], or even a crack in the blades when the vibration level is too high.The centrifugal force combined with the thermal and vibratory stresses can lead, on the one hand, to slippage depending on the temperature, the bearing angles or the centrifugal stress, between the threads of the blade roots 13 and the walls of the cells 22 of the disc 20 and, on the other hand, to non-linearities in the contact behavior between the blade roots and the disc cells.

[0006] To study the behavior of turbomachine parts, and in particular high or low pressure turbine, compressor or fan blades, aircraft manufacturers generally carry out tests aimed at characterizing the fatigue resistance, in particular vibration fatigue, to the forces and stresses undergone by these parts. Since tests on real parts are expensive (complex tools, price of parts, production times), the tests are, as far as possible, carried out using test benches and specimens representative of the parts to be studied. However, the characterization of the forces of parts subjected to various stresses is relatively complex. The characterization becomes even more complex when tribological stresses are added, i.e. stresses occurring between two parts in contact (friction, wear, etc.), as is the case for attachments between blades and disks.

[0007] For the attachments between blades and disks, that is to say the interfaces or connection zones between blade roots and disk cells, knowledge of the tribological aspect is essential because the contacts between the blade roots and the disk cells generate vibration damping. The study and knowledge of this tribological aspect make it possible to consider modeling the blades and disks, with the aim of optimizing the design of a turbomachine, in particular to limit its vibrations. The objective of limiting the vibrations between turbine blades and disks, with the aim of reducing the stresses on the interfaces between blades and disks, requires knowledge of the vibration damping between these parts and, consequently, the amplitude vibratory.

[0008] Today, due to its complexity, the characterization of vibration damping between turbine blades and disks is carried out on real parts, with all the drawbacks explained above. This characterization is determined by means of strain gauges making it possible to measure the impact of any change in characteristics of the interfaces between blades and disks, such as the surface condition, the surface treatment, etc. However, tests with strain gauges arranged on real parts are tedious, long and expensive. In addition, the very presence of the strain gauges on the blades disrupts the vibration damping measurements by modifying the mass, stiffness and other properties of the blades to be tested.

[0009] There is therefore a real need for a technique that makes it possible to do away with these tests on real parts. Summary of the invention

[0010] To address the above-mentioned problems of characterizing the damping of the interface between turbine blades and disks, the applicant proposes a test specimen representative of the interface between a blade root and a turbine disk cell, suitable for characterizing the vibration damping of the adherent or sliding contact between these two parts. The applicant also proposes a characterization device using this specimen as well as a method for implementing this device, the device and the method making it possible to measure the vibration damping intrinsic to the interface, without any disturbance or influencing factor external to said interface.

[0011] According to a first aspect, the invention relates to a test specimen for characterizing vibration damping of an adherent or sliding contact between two parts subjected to vibrational, oligocyclic and tribological stresses, the test specimen comprising: • a first head and a first longitudinal bar, one end of the first longitudinal bar being secured to the first head, another end of the first longitudinal bar being free, • a second head and a second longitudinal bar, one end of the second longitudinal bar being secured to the second head, another end of the second bar being free, and • a male-female type attachment connecting the first longitudinal bar with the second longitudinal bar and comprising a male part projecting at the free end of the first longitudinal bar and a female part notched at the free end of the second longitudinal bar, the male part and the female part having shapes complementary to each other, in adherent or sliding contact against each other.

[0012] This test piece is representative of the interface between a blade root and a turbine disk cell and can be subjected to the same stresses as those experienced by a real interface. This test piece can therefore be used to characterize the vibration damping of the interface between blades and turbine disk and obtain a “pure” value free from external disturbances that could distort the measurements.

[0013] In addition to the characteristics which have just been mentioned in the preceding paragraph, the test specimen according to one aspect of the invention may have one or more additional characteristics from among the following, considered individually or according to all technically possible combinations: • the first longitudinal bar has a substantially straight block shape, with a flare near the first head and a shoulder at its free end. • the second longitudinal bar has a roughly straight block shape, with a flare near the second head and a buttress at its free end. • the shoulder of the first longitudinal bar and the buttress of the second longitudinal bar extend in a plane perpendicular to a plane containing the flares of the first and second longitudinal bars. • the male-female type attachment substantially comprises a dovetail shape representative of a connection between a blade root and a turbomachine disk cell, the male part representing the blade root and the female part representing the disk cell. • the test piece is a massive reference and without contact zone.

[0014] A second aspect of the invention relates to a device for characterizing vibration damping of an adherent or sliding contact between two parts subjected to vibrational, oligocyclic and tribological stresses, the device comprising a test bench on which the test specimen according to the first aspect is mounted, said test bench comprising at least one vibrating pot transmitting vibrational excitation to the test specimen and a static loading device transmitting oligocyclic stresses to said test specimen.

[0015] This characterization device makes it possible to eliminate any vibration disturbance by elements other than the blade / disc interface itself; it therefore allows a measurement of the “pure” value of the intrinsic damping in the contact zones studied, i.e. the vibration damping resulting from sliding at the contacts between blade roots and disc cells.

[0016] In addition to the characteristics which have just been mentioned in the previous paragraph, the device according to the second aspect of the invention may have one or more complementary characteristics among the following, considered individually or according to all technically possible combinations: • the vibrating pot is connected to the test specimen by means of a threaded excitation rod, in which a force measurement sensor is integrated off-center. • the excitation rod comprises a first and a second section, secured to each other by an off-center connection system, the first section and the second section having different lengths.

[0017] A third aspect of the invention relates to a method of implementing the device according to the second aspect, comprising the following steps: a. installation of a reference specimen in the characterization device, b. adjustment of the characterization device according to the measurements to be carried out, c. measurements of the vibration behavior of the reference specimen, d. withdrawal of the reference test piece, e. installation of the test specimen according to the first aspect in the characterization device, with the same setting as that of step b), f. measurements of the vibration behavior of the test specimen, g. comparison between the measurements made in step c) for the reference specimen and the measurements made in step f) for the test specimen, and h. determination, from the comparison of step g), of the characterization of the vibration damping of the contact zone.

[0018] Advantageously, the method comprises a preliminary step of temperature calibration of the reference specimen and the test specimen, in an oven. BRIEF DESCRIPTION OF THE FIGURES

[0019] Other advantages and characteristics of the invention will appear on reading the following description, illustrated by the figures in which:

[0020] [Fig. 1], already described, represents a perspective view of a portion of a turbine blade and disk assembly;

[0021] [Fig. 2], already described, represents a partial perspective view of a moving turbine blade;

[0022] [Fig. 3], already described, represents side and top views of a moving turbine blade;

[0023] [Fig.4] represents perspective views of a test specimen according to the invention;

[0024] [Fig.5] represents perspective views of a reference test piece used in the method according to the invention;

[0025] [Fig.6] represents a schematic view of the attachment zone of the test piece of [Fig.4] with the stresses to which it is subjected;

[0026] [Fig.7] represents the characterization device according to the invention from a functional point of view, with the stresses applied to the test specimen;

[0027] [Fig.8] schematically represents the characterization device according to the invention, in which the test specimen of [Fig.4] is mounted;

[0028] [Fig.9] represents a side view of an off-center threaded excitation rod according to an embodiment of the characterization device of the invention;

[0029] [Fig. 10] represents a schematic side view of an excitation rod according to a preferred embodiment of the characterization device of the invention;

[0030] [Fig. 11] represents, in the form of curves, examples of measurements of the displacement amplitude and the natural frequency of a test specimen according to the invention; and

[0031] [Fig. 12] represents, in the form of a functional diagram, the method according to the invention.

[0032] Unless otherwise specified, identical elements are identified in the figures by identical references. For reasons of readability of the figures, the size scales between elements represented are not necessarily respected. DETAILED DESCRIPTION

[0033] An example of the embodiment of a test specimen, allowing the characterization of the vibration damping between two parts in adherent or sliding contact, and of a characterization device comprising this specimen are described in detail below, with reference to the appended drawings. This example illustrates the characteristics and advantages of the invention. It is however recalled that the invention is not limited to this example.

[0034] An example of a test specimen 100, representative of a contact between two parts 110, 120 such as a blade and a turbine disk, is shown in [Fig. 4]. The two parts in contact 110, 120, represented by the test specimen 110 may, in particular, be a root 13 of a movable blade 10 and a cell 22 of a turbomachine turbine disk 20. As explained previously, the contact between a blade root and a disk cell is an adherent or sliding contact which generates vibration damping. The test specimen 100, more simply called a test specimen, aims to represent this adherent or sliding contact between a blade root and a disk cell. For this, the specimen 100 comprises two parts: a male part 110 and a female part 120.

[0035] The male part 110, shown in drawing A of [Fig.4], comprises a head 111, called the first head, and a longitudinal bar 112, called the first bar. The first head 111 is a part of the test piece 100 that is relatively massive compared to the first bar 112 and designed to be fixed on a characterization device or a test bench. The first head 111 can take all kinds of shapes such as a square or rectangular parallelepiped shape or a cylindrical shape, etc. The first bar 112, integral with the first head 111, extends from said first head 111 towards the female part 120. The first bar 112 has an end 112a integral with the first head 111 and a so-called “free” end 112b, in adherent or sliding contact with the female part 120. The first bar 112 has a shape substantially of a right block, that is to say a parallelepiped with a rectangular section, smaller than the section of the first head, and of which the integral end 112a is flared and the free end 112b is provided with a shoulder 113.The first bar 112 therefore comprises a flare 114, in the connection zone with the first head 111, which extends laterally (along the lateral direction 1) relative to the longitudinal direction L of the central portion 112c (in the shape of a straight block) of said first bar 112. The shoulder 113 of the first bar 112, arranged in the vicinity of the connection zone 130', or interface, with the female part 120, is a portion of the first bar 112 projecting relative to the central portion 112c and extending transversely (along the direction T) relative to the longitudinal direction L of the central portion 112c.

[0036] The female part 120, shown in drawing B of [Fig.4], comprises a head 121, called the second head, and a longitudinal bar 122, called the second bar. The second head 121 is a part of the test piece that is relatively massive compared to the second bar 122 and designed to be fixed to a characterization device or a test bench. The second head 121 can take all kinds of shapes such as a square or rectangular parallelepiped shape or a cylindrical shape, etc. The second bar 122, integral with the second head 121, extends from said second head 121 towards the male part 110. The second bar 122 comprises an end 122a integral with the second head 121 and a so-called “free” end 122b, in adherent or sliding contact with the male part 110.The second bar 122 has a substantially straight block shape, that is to say a parallelepiped with a rectangular section, smaller than the section of the second head, and the integral end 122a of which is flared and the free end 122b of which is provided with a buttress 123. The second bar 122 therefore has a flare 124, in the connection zone with the second head 121, which extends laterally (along the lateral direction 1) relative to the longitudinal direction L of the central portion 122c (in the shape of a straight block) of said second bar 122. The buttress 123 of the second bar 122, arranged in the vicinity of the connection zone 130' with the male part 110, is a projecting reinforcement, flaring in the transverse direction T, relative to the longitudinal direction L of the central portion 122c. .

[0037] The male part 110, like the female part 120, is a single piece, made from a single piece by machining. The male part 110 and the female part 120 may be partially identical, only the parts of the first and second bars 112, 122 in the vicinity of the free ends 112b, 122b being different. One of the differences is the shoulder 113 for the male part 110 and the buttress 123 for the female part 120. Another difference may be the difference in length between the first bar 112 and the second bar 122, in the example illustrated [Fig.4], the central portion 122c of the second bar being longer longitudinally than the central portion 112c of the first bar. But the geometries of the male part 110 and the female part 120 may be variable depending on the deformations that are sought. Thus, the two bars 112 and 122 may be of the same length, or the second bar 122 may be shorter than the first bar 112. Their width or thickness may also be the same or different.

[0038] As shown in drawing C of [Fig.4], the male part 110 and the female part 120 are assembled with each other by means of a male-female type fastener 130. This male-female type fastener 130, more simply called a fastener, ensures an adhesive or sliding contact type connection between the first bar 112 of the male part and the second bar 122 of the female part. The fastener 130 constitutes the interface 130' between the male part 110 and the female part 120 of the test piece 100. For this, the fastener 130 comprises a male part 131 positioned on the male part 110 and a female part 132 positioned on the female part 120. The male part 131 and the female part 132 have a substantially dovetail shape. For this, the male part 131 forms a flared-shaped tenon, mounted at the free end of the first bar 112, on the distal wall 113d of the shoulder 113, that is to say the wall of the shoulder furthest from the first head 111.The tenon of the male part 131 has the shape of a rectangular parallelepiped, one end of which is arranged against the shoulder 113 and the other end is flared in the transverse direction T. The female part 132 forms a notch, notched at the free end of the second bar 122, on the distal wall 123d of the buttress 123, that is to say the wall of the buttress furthest from the second head 121. The notch of the female part 132 has a shape complementary to that of the tenon of the male part 131. Once assembled, the male part 131 and the female part 132 of the fastener 130 form the adherent or sliding contact of the test piece 100.

[0039] The test specimen 100 described above can be used in a test bench or a characterization device such as that described below. This specimen 100 is used to characterize the vibration damping of the adherent or sliding contact formed at the interface 130' of the male part 110 and the female part 120 by the fastener 130. For this, the measurements carried out on the test specimen 100 can be compared to measurements carried out on a reference specimen 200. This reference specimen 200, shown in [Fig. 5], is substantially identical to the test specimen 100, except that it is a single piece, i.e. manufactured in one piece, without adherent or sliding contact at the interface between the male part 210 and the female part 220. In other words, the reference specimen 200 has the same characteristics of shape, dimensions, and material as the test specimen 100, but the reference specimen 200 comprises a male part 210 and a female part 220 connected to each other by a fixed interface 230, the shoulder 213 of the male part 210 being fixedly secured to the buttress 223 of the female part 220. The thickness of the fixed interface 230 between the shoulder 213 and the buttress 223 was adapted so that the mass distributions of the male and female specimens were as close as possible.

[0040] As explained above, the test specimen 100 of [Fig. 4] behaves, from a vibration point of view, in the same way as the assembly between a movable blade root and a turbine disk cell. It is therefore possible to study the vibration behavior of this assembly by means of the test specimen 100 of the invention. It is possible, in particular, to characterize the vibration damping of the adherent or sliding contact of this assembly by means of the test specimen 100. Indeed, the interface 130' between the male part 110 and the female part 120 can be subjected to the same stresses as the assembly between a movable blade root and a disk cell, these stresses or forces being applied to the test specimen 100 by means of a characterization device described below. As shown in [Fig.6], the test specimen 100 can be subjected, at the location of the interface 130', to the following forces: • a tribological effort broken down into: • a sliding force, represented by the double arrow G, between the male part 131 and the female part 132 of the interface 130', and • a normal force, represented by the arrow N, perpendicular to the sliding force G, • an oligocyclic force, or tensile fatigue force, represented by the arrow LCF and which corresponds to the centrifugal force to which the blades are subjected, and • a high-frequency transverse vibratory force, represented by the double arrow HCF.

[0041] The characterization device used to apply the stresses to the test specimen 100 comprises a test bench 300, an example of which is shown schematically in [Fig. 7], and a calculation unit, not shown in the figures. The test bench 300 can be produced by modifying a conventional test bench, such as a bench for fretting-fatigue tests usually used to determine the life span at break of the attachment between the moving blade and the disc. The test bench 300 of [Fig.7] is therefore suitable for enabling the measurement of the intrinsic vibration damping at the interface between the male part 110 and the female part 120 of the test piece 100, representative of the contact between the blade root and the disc cell. For this, the test bench 300 comprises: • a vibrating pot 310 applying, on the test piece 100, a vibratory stress HCF along the X axis of the XYZ reference; • a mooring line 333 applying, on the test piece 100, a normal force N along the Y axis; • an excitation device 320 transmitting, to the test piece 100, oligocyclic stresses LCF along the Y axis.

[0042] The test bench 300 does not directly apply a sliding force to the test piece 100, the sliding force being obtained by combining the vibratory stress HCF involving contact force components in the N and G directions and the tensile force LCF which also involves contact force components in the N and G directions. The test piece 100 is thus subjected to all the forces and stresses experienced by the contact between the blade root and the disk cell.

[0043] A representation of the test bench 300 with the test specimen 100, when said specimen 100 is subjected to the various stresses stated above, is presented in [Fig. 8]. This [Fig. 8] shows an upper I-beam 330 comprising the vibrating pot 310 and the mooring line 333 and a lower I-beam 330' comprising the excitation device 320 and an excitation rod 333'. The test specimen 100 described above is arranged between the two upper 330 and lower 330' I-beams. In [Fig. 8], the test specimen 100 is shown when the test bench 300 is in operation, said specimen 100 being subjected to both an HCF vibration stress and an LCF oligocyclic stress. These two stresses induce contact components in the normal directions N and tangential G to the contact (planes).

[0044] In the test bench 300 of the invention, the vibrating pot 310 is coupled to the mooring line 333 of the test piece 100, which allows a vibration study in a frequency range between 100 and 2000 Hz.

[0045] In conventional test benches such as those dedicated to fretting-fatigue tests, the excitation rod of the lower I-beam is generally a threaded rod formed from a single piece. In the test bench 300 of the device of the invention, the excitation rod 333' is a rod specific to the characterization of vibration damping. As shown in [Fig.9], this excitation rod 333' is a threaded rod, that is to say provided with a screw thread over its entire external surface, comprising a first rod section 331 and a second rod section 332 assembled together with the other by a connecting system 335, for example of the screw / nut type. The connecting system 335 may, for example, comprise a threaded tenon 335a, forming a sort of screw at the end of the first rod section 331, and a threaded orifice 335b, forming a sort of nut at the end of the second rod section 332, the threaded tenon 335a being inserted into the threaded orifice 335b to keep the two sections 331 and 332 in contact.

[0046] In certain embodiments, the excitation rod 333' incorporates a force sensor 334, for example a piezoelectric sensor, as shown in [Fig. 10], this force sensor allowing a measurement of the excitation force. In these embodiments, the force sensor 334 is housed in the center of the connecting system 335, between the first and second rod sections 331, 332. The connecting system 335 may then comprise, for example, a threaded tenon at the end of each of the rod sections 331, 332, each threaded tenon being fixed in a nut secured to the force sensor 334. Of course, other means of fixing the first and second rod sections, with or without the force sensor, may be envisaged.

[0047] According to certain embodiments, the first and second rod sections 331, 332 are of different lengths, which makes it possible to offset the additional mass represented by the force sensor. Indeed, adding a force sensor 334 in a conventional threaded rod has the effect of introducing an additional mass having an influence on the vibration behavior of the threaded rod, this vibration behavior itself causing a modification of the natural mode of the overall excitation system of the test bench. Also, offsetting the additional mass generated by the force sensor, as proposed in this embodiment, makes it possible to modify the natural frequencies of the excitation rod 333', the natural frequency thus depending on the difference in length between the first rod section 331 and the second rod section 332.

[0048] According to certain embodiments, the diameter of the excitation rod 333' is greater than the diameter of the threaded rods of conventional test benches. The diameter of the excitation rod 333' of the test bench 300 may be, for example, 5 to 50 mm. Modifying the diameter of the excitation rod 333' makes it possible to modify the stiffness of said excitation rod, which makes it possible to move the natural mode of the rod outside the measurement range.

[0049] Examples of measurements carried out with the test bench 300 are shown, in the form of curves, in [Fig. 11]. The curves of graph A of [Fig. 11] show examples of displacement amplitude and natural frequency measured on the test specimen 100, in the case of low frequencies (approximately 300-350 Hz), with different excitation rods. The curves of graph B of [Fig. 11] show examples of displacement amplitude and natural frequency measured on test specimen 100, in the case of high frequencies (1800 to 1900 Hz), with different excitation rods. In both frequency ranges (low frequencies and high frequencies): • curve Cl corresponds to measurements carried out with a 300 test bench equipped with a conventional excitation rod (rod Tl shown in part C of [Fig. 11]), • curve C2 corresponds to measurements carried out with a 300 test bench equipped with a centered excitation rod, i.e. formed of two identical sections (rod T2 shown in part C of [Fig. 11]), • curve C3 corresponds to measurements carried out with a 300 test bench equipped with an off-center excitation rod, i.e. formed of two sections of different lengths (rod T3 shown in part C of [Fig. 11]), and • curve C4 corresponds to measurements carried out with a test bench 300 equipped with an off-center excitation rod and equipped with a force sensor (rod T4 shown in part C of [Fig.l 1]), this excitation rod T4 corresponding to the preferred embodiment of the rod.

[0050] Curve C1 represents the reference curve, i.e. the displacement amplitude measurements in mm and the natural frequency of the test piece 100, when the test piece 100 is stressed by the test bench 300 equipped with a conventional excitation rod T1. Curve C2 shows the displacement amplitude measurements in mm and the natural frequency of the test piece 100, when said test piece 100 is stressed by the test tank 300 equipped with the centered excitation rod T2. Curve C2 shows in particular that, for high frequencies, the natural frequency of the test piece 100 is advantageously displaced but that, for low frequencies, there is no modification of the natural frequency compared to the reference curve. Curve C3 shows the displacement amplitude measurements in mm and the natural frequency of the specimen 100, when said specimen 100 is stressed by the test bench 300 equipped with the off-center excitation rod T3.Curve C4 shows the displacement amplitude measurements in mm and the natural frequency of the test piece 100, when said test piece 100 is stressed by the test bench 300 equipped with the excitation rod of the preferred embodiment T4. Curve C4 shows in particular that the rod T4 allows, at low as well as high frequencies, a very clear shift of the natural frequency of the test piece 100. By shifting the natural frequency of the test piece 100 relative to the reference natural frequency, the vibration disturbances of the excitation rod are eliminated on the measurement of the intrinsic damping at the interface 130' between the male part 110 and the female part 120, regardless of the vibration mode chosen (high or low frequencies).

[0051] Thus, by shifting the natural frequency of the test piece 100, it is possible to move away the resonance mode of said specimen of the resonance modes of the other elements of the test bench such as the excitation rod, the structure of the machine, the force sensor, etc., in order to ensure that only the intrinsic effect of sliding and contact nonlinearities on the vibration behavior at the interface of the specimen is studied.

[0052] The characterization device described above can be implemented by a method 400 comprising the following steps: • Step 410: installation of a reference test piece 200 in the test bench 300, • Step 420: adjustment of the test bench 300 according to the measurements to be carried out, • Step 430: measurements of the vibration behavior of the reference specimen 200, • Step 440: removal of the reference test piece 200, • Step 450: installation of test specimen 100 in place of the test specimen reference 200, in the test bench 300, keeping the same settings as those chosen for the reference test piece 200, • Step 460: measurements of the vibration behavior of test specimen 100, • Step 470: comparison between the measurements taken in step 430 for the reference specimen 200 and the measurements made in step 460 for the test specimen 100, and • Step 480: determination, from the comparison of step 470, of the characterization of the vibration damping of the interface between the male part 110 and the female part 120 of the test specimen 100.

[0053] In step 420, various adjustments can be made on the test bench 300. Indeed, the test bench of the invention makes it possible to adjust, on the one hand, the natural frequencies of the test bench / test specimen assembly by the choice and dimensioning of the excitation rod 333' and, on the other hand, the direction of stress so as to achieve complex resonance modes, which can combine several forces such as, for example, bending and torsion.

[0054] The method 400 makes it possible to resolve a major difficulty in the characterization of vibration damping which is to isolate the damping of the vibration mode of the test piece from the damping of the other modes of the device (undesired and inherent to the vibration behavior of the device), this condition being necessary to measure the damping due solely to the contact at the interface 130' between the male part 110 and the female part 120. The isolation of the vibration mode of the test piece 100 is obtained, in the method 400, by a comparison between the test results measured with the reference test piece 200 and the test results measured with the test piece 100. Indeed, the reference test piece 200 makes it possible to carry out a measurement of the vibration behavior of the “specimen-material” system without any loss at the contact interface 130'. The measurements with the test specimen 100 make it possible, by comparison with the reference specimen, to obtain the pure effect of the interface 130'. For this, and as explained previously, it is necessary that the reference specimen 200 and the test specimen 100 have the same characteristics, namely an identical mass and stiffness distribution, an identical material, an identical geometry except for the attachment.

[0055] Furthermore, for the deduction of the characterization of the vibration damping at the interface 130' (step 480) to be correct, it is preferable that the measurements of steps 430 and 460 be carried out under the same conditions for the reference specimen 200 as for the test specimen 100. In particular, the thermal conditions must be identical for the reference specimen 200 as for the test specimen 100. For this, the method 400 may comprise a prior step of temperature calibration of the reference specimen 200 and the test specimen 100, in an oven. Indeed, the standards relating to mechanical tests recommend equipping the specimens with a thermocouple probe, also called a thermocouple, the role of which is to monitor the temperature of the specimen during the tests. However, the presence of a thermocouple causes a disruptive effect on the vibration mode.As the invention aims to eliminate all external disturbances, the method 400 proposes to eliminate the thermocouple to eliminate its disturbing effects and to compensate for the thermal effect of said thermocouple by a step of temperature calibration of the reference 200 and test 100 test pieces in a furnace, for example at 550°C or 650°C. The test pieces 200, 100 are therefore heated in accordance with the standard, without however including a thermocouple. Temperature calibration in a furnace offers the advantage of a wide temperature range which can reach and even exceed 800°C.

[0056] To implement the method 400, the test bench 300 is connected with a control and recording calculation unit (not shown in the figures) ensuring in particular the calculations of the step 470 of comparing the measurements carried out with the reference test piece 200 and the test piece 100 and of the step 480 of determining the characterization of the vibration damping of the interface 130'. The calculation unit can also determine, from the value of the vibration damping, the value of the coefficient of friction between the male part and the female part, in contact, of the interface 130'.

[0057] In the preceding description, the test specimen 100, the characterization device and its method of implementation have been described for an application to a blade root and turbine cell assembly, the test specimen described being representative of this assembly. Those skilled in the art will however understand that the test specimen, the characterization device and its method of implementation can be applied to the characterization of the vibration damping of other turbomachine parts in contact such as, for example, gear parts, a blade damper, or any other set of parts in adherent or sliding contact with each other.

[0058] Thus, this method can be applied to a blade vibration damper.

[0059] This damper is a part positioned in contact with the small radius side of a blade face so that it is pressed against this face under the effect of the centrifugal field induced by the rotation of the blades around the axis of the engine. It can for example be positioned under the shelf or under the heel. It dissipates energy by sliding and introduces a non-linearity of stiffness into the system, which results in a reduction of the vibration amplitude compared to operation without a damper.

[0060] A series of measurements can be carried out with a test piece 100 in which a shock absorber has been placed between the male part 130 and the female part 132.

Claims

Claims

1. Test specimen (100) for characterizing vibration damping of an adherent or sliding contact between two parts subjected to vibrational, oligocyclic and tribological stresses, the test specimen comprising: - a first head (111) and a first longitudinal bar (112), one end (112a) of the first longitudinal bar being integral with the first head, another end (112b) of the first longitudinal bar being free, - a second head (121) and a second longitudinal bar (122), one end (122a) of the second longitudinal bar being integral with the second head, another end (122b) of the second bar being free,and - a male-female type fastener (130) connecting the first longitudinal bar (112) with the second longitudinal bar (122) and comprising a male part (131) projecting at the free end of the first longitudinal bar and a female part (132) notched at the free end of the second longitudinal bar, the male part (131) and the female part (132) having shapes complementary to each other, in adherent or sliding contact against each other.,

2. Test specimen according to claim 1, characterized in that the first longitudinal bar (112) has a substantially straight block shape, with a flare (114) near the first head (111) and a shoulder (113) at its free end.

3. Test specimen according to claim 1 or 2, characterized in that the second longitudinal bar (122) has a substantially straight block shape, with a flare (124) near the second head and a buttress (123) at its free end.

4. Test specimen according to claims 2 and 3, characterized in that the shoulder (113) of the first longitudinal bar and the buttress (123) of the second longitudinal bar extend in a plane perpendicular to a plane containing the flares (114, 124) of the first and second longitudinal bars.

5. Test tube according to any one of claims 1 to 4, characterized in that the male-female type attachment (130) substantially comprises a dovetail shape representative of a connection between a blade root and a turbomachine disk cell, the male part (131) representing the blade root and the female part (132) representing the disk cell.

6. Device (300) for characterizing vibration damping of an adherent or sliding contact between two parts subjected to vibrational, oligocyclic and tribological stresses, the device comprising a test bench (300) on which the test specimen (100) according to any one of claims 1 to 5 is mounted, said test bench comprising at least one vibrating pot (310) transmitting vibrational excitation to the test specimen (100) and a static loading device (320) transmitting oligocyclic stresses to said test specimen.

7. Characterization device according to claim 6, characterized in that the vibrating pot (310) is connected to the test specimen (100) by means of a threaded excitation rod (333'), in which a force measurement sensor (334) is integrated off-center.

8. Characterization device according to claim 7, characterized in that the excitation rod (333') comprises a first and a second section (331, 332), secured to each other by an off-center connection system (335), the first section (331) and the second section (332) having different lengths.

9. Method (400) for implementing the device according to any one of claims 6 to 8, comprising the following steps: a. installation (410) of a reference test piece in the characterization device, b. adjustment (420) of the characterization device according to the measurements to be carried out, c. measurements (430) of the vibration behavior of the reference test piece, d. removal (440) of the reference test piece, e. installation (450) of the test piece according to any one of claims 1 to 5 in the characterization device, with the same adjustment as that of step b), f. measurements (460) of the vibration behavior of the test piece, g. comparison (470) between the measurements carried out in step c) for the reference specimen and the measurements carried out in step f) for the test specimen, and h. determination (480), from the comparison of step g), of the characterization of the vibration damping of the contact zone.

10. Method according to claim 9, characterized in that it comprises a prior step of temperature calibration of the reference specimen and the test specimen, in an oven.

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