Dynamic friction test device capable of realizing wide-rate-range loading and test method

Abstract

The invention discloses a dynamic friction test device and test method capable of realizing wide rate range loading, the device comprises a Hopkinson rod loading system, a pressure applying system and a data acquisition system, the Hopkinson rod loading system comprises an impact rod, a shaper, an incident rod, a sample, a transmission rod and an energy absorption device, the pressure applying system comprises a fixed outer frame, a stud and a pressurizing square column; and the data acquisition system comprises a strain gauge, a bridge box and a data acquisition unit. According to the device provided by the invention, an impact rod with wave impedance matching is used for impacting the end face of an incident rod attached with a shaper, a loading curve with the pulse width range of 80-300 us and the speed range of 1-40 m / s can be generated, the dead-load uniform-speed motion in the friction process is ensured, positive pressure is obtained by a specific pressure clamping device, friction force is obtained by a transmission rod connected with a sample, and the friction force is obtained by the transmission rod connected with the sample. And finally, accurate frictional wear data can be obtained.

Description

Technical Field

[0001] The invention relates to a friction coefficient testing method for linear motion of metal in a wide speed range (1m / s-40m / s), and in particular to a specific pressure applying device and a Hopkinson pressure bar testing device providing a sliding speed. Background Art

[0002] At present, the pin-disc test device is commonly used in the high-speed friction and wear test technology of materials. However, in actual engineering applications, due to the tangential movement of the pin, the sample surface will be unevenly deformed. Although the deformation of the sliding surface can be made more uniform by increasing the ratio of the disc diameter to the pin diameter, the increase in the ratio will limit the loading capacity of the test device; moreover, the pin-disc friction tester needs to go through multiple rotations to achieve high-speed operation. During the acceleration process, the sliding surface is continuously worn and the surface state is constantly changing, resulting in the current wear data being affected by the previous wear conditions. Therefore, it is crucial to construct a method to achieve uniform deformation of the friction surface and accurately measure the friction coefficient of the original friction surface. As a typical test device for dynamic mechanical properties, the separated Hopkinson bar can achieve speed loading of up to 40m / s within 100 microseconds, which makes it possible to achieve uniform linear friction loading within a short distance, high speed, and high acceleration range, which is of great significance for accurately exploring the friction and wear properties of materials.

[0003] Some scholars have conducted the following research on this situation:

[0004] Research 1: Patent CN202110121520.8 discloses a dynamic shear and friction measurement device and method based on a Hopkinson pressure bar. The patented pressure application system uses a cylindrical confining pressure cylinder to apply a predetermined confining pressure to the sample and injects hydraulic oil to ensure that the required pressure is reached. However, under high dynamic loads, the confining pressure cylinder and the sealing assembly may undergo slight deformation, so the confining pressure cannot reach the set value in a very short time, resulting in errors in the instantaneous confining pressure, affecting the measurement accuracy, and the sensors and feedback systems in some hydraulic systems cannot accurately measure and feedback pressure changes in a very short time; at the same time, during dynamic loading, the slight deformation or displacement of the sample may change the contact state between the sample and the confining pressure cylinder, resulting in unstable pressure transmission.

[0005] Research 2: Patent CN202210898592.8 discloses a system for testing the dynamic friction performance of materials. Dynamic friction is achieved by using a Hopkinson pull rod and a pressure clamping device that moves with the rod. As the dynamic friction is loaded, the pressure at the contact interface increases significantly over time. However, the pressure signal of this system is not constant, and it is impossible to accurately measure the friction performance of the material under a certain fixed pressure. There is also an error between the positive pressure applied by the equipment and the theoretical pressure value. At the same time, the movable clamp on the rod increases the inertial effect of the incident rod end, and the speed control is not accurate enough.

[0006] Study 3: S. Rajagopalan et al. proposed the use of a plate impact shear friction test method in the document "Rajagopalan S, Irfan MA, Prakash V. Novel experimental techniques for investigating time resolved high speed friction [J]. Wear, 1999, 225-229: 1222-1237.". Under the conditions of high interface normal pressure of 0.5-2GPa and high sliding speed of 1-60m / s, the dynamic friction characteristics of two friction pairs of steel / titanium alloy and steel / aluminum alloy were studied. The normal positive pressure and tangential velocity were loaded simultaneously through the impact of the plate, and uniform deformation and high-speed sliding of the friction surface were achieved. However, this method cannot accurately separate and control its normal pressure and tangential velocity, and cannot obtain wear data under specified working conditions.

[0007] Study 4: Philippon et al. proposed a new experimental device in the document "Philippon S, Sutter G, Molinari A. An experimental study of friction at high sliding velocities [J]. Wear, 2004, 257 (7): 777-784." to study the dry friction behavior of metals under static and dynamic conditions. The device can obtain sliding speeds of up to 60 m / s and as low as 1 m / s through an air gun and a hydraulic press, respectively. The device is based on a specially designed force ring and two coaxial long tubes. The first tube is used to control the speed of the sample, and the second tube contains a thin tube with a force ring installed to measure the tangential force generated during the friction process. The positive pressure is achieved by filling pads of different thicknesses between the sample and the force ring, and the size of the positive pressure is closely related to the thickness of the sample and the pad. Although the device can effectively control the positive pressure, it requires precise control of the gap between different components, which is difficult to operate. The friction sample form leads to errors in the application of positive pressure, and the loading method greatly reduces the stability and reliability of the data.

[0008] Through the above research, the technical problems that need to be solved at present include but are not limited to: obtaining real-time monitoring signals within 100 microseconds; solving the limitations of traditional pressure fixture application methods; achieving uniform deformation of the friction surface; and achieving high-precision control of pressure and speed loading. Summary of the invention

[0009] The main purpose of the present invention is to provide a dynamic friction and wear testing device with short-distance, high-speed and uniform deformation linear velocity loading, so as to solve the problems of high-speed loading that is difficult to obtain with linear motion devices, uniform deformation that is difficult to achieve with rotary motion devices, and pressure application accuracy that is difficult to achieve with traditional pressure application methods.

[0010] The technical solution of the present invention is:

[0011] A dynamic friction test device capable of achieving loading in a wide rate range, characterized in that it also includes a Hopkinson bar loading system, a pressure application system, and a data acquisition system;

[0012] The Hopkinson bar loading system comprises an impact bar 1, a shaper 2, an incident bar 3, a sample 4, a transmission bar 5 and an energy absorbing device 6 which are coaxially arranged in sequence; a group of the samples 4 is composed of two identical incident end samples and a transmission end sample with a boss; a pressure applying system is also arranged between the incident bar 3 and the transmission bar 5;

[0013] The pressure applying system comprises a fixed outer frame 7, studs 8 and pressurized square columns 9; the studs 8 are vertically fixed in pairs above and below the fixed outer frame 7; the pressurized square columns 9 are vertically placed in pairs above and below the specimen and connected to the studs 8;

[0014] The data acquisition system comprises a strain gauge 10, a bridge box and a data acquisition device 11; there are a plurality of strain gauges 10, which are respectively arranged on the Hopkinson bar loading system and the pressure application system, and are connected to the bridge box and the data acquisition device 11 through wires.

[0015] Furthermore, the impact rod 1 , the incident rod 3 and the transmission rod 5 are cylindrical long rods with equal diameters, and the wave impedance of the impact rod 1 is not greater than the wave impedance of the incident rod 3 .

[0016] Furthermore, the incident rod 3 is grooved (or glued) to fix the two incident end samples; the transmission end sample is fixed to the transmission rod 5 by gluing.

[0017] Furthermore, the fixed outer frame 7 is made of metal.

[0018] Furthermore, the cross-sectional area of ​​the pressurized square column 9 is consistent with the contact area of ​​the friction interface, while ensuring that the non-friction interface remains smooth.

[0019] Further, three pairs of the strain gauges 10 are provided, namely, a first strain gauge, a second strain gauge and a third strain gauge;

[0020] The first strain gauge is arranged at the middle of the incident rod 3 ; the second strain gauge is arranged on the transmission rod 5 and close to the proximal end of the transmission rod 5 ; the third strain gauge is attached to the middle of the pressurized square column 9 .

[0021] Furthermore, the bridge box is a Wheatstone half bridge.

[0022] The present invention also provides a dynamic friction test method capable of achieving loading in a wide rate range, characterized in that the test is performed using the above-mentioned device, and the specific steps include:

[0023] First, positive pressure is applied to the friction interface through the pressurized square column 9 in the pressure application system on the test bench; then the impact rod 1 hits the proximal end of the incident rod 3 to generate an incident stress wave and propagate in the incident rod 3. After the incident stress wave propagates to the sample 4, it is reflected by the sample 4 and forms a reflected stress wave in the incident rod 3. During this period, the waveform is reflected back and forth in the sample 4 and superimposed in the transmission rod 5 to form a transmission wave. The corresponding signal is obtained through the data acquisition system; finally, the friction slip velocity and friction coefficient are calculated according to the one-dimensional stress wave theory.

[0024] Furthermore, the friction slip speed is:

[0025] v sliding (t) = v1(t) - v2(t) = C[ε inc (t)+ε ref (t)]-Cε tra (t)

[0026] Where:

[0027] v1(t) and v2(t) are the incident and transmission rod end velocities beside the sample, respectively; C is the wave velocity in the elastic rod; ε inc (t) is the strain change of the incident rod after impact; ε ref (t) is the strain change in the returning incident rod after friction loading; ε tra (t) is the strain change of the transmission rod during the friction process;

[0028] The coefficient of friction is:

[0029]

[0030] Where:

[0031] f(t) is the friction force during the test, and where F sliding(t) is the tangential sliding friction force measured by the Hopkinson transmission rod, E tra is the elastic modulus of the transmission rod, A tra is the cross-sectional area of ​​the transmission rod;

[0032] F N (t) is the positive pressure during the test, and F N (t) = E Fix A Fix ε Fix (t), where E Fix is the elastic modulus of the pressure column, A Fix is the cross-sectional area of ​​the pressure column, ε Fix (t) is the strain change of the pressure column during the friction process.

[0033] The beneficial effects of the present invention are:

[0034] The present invention provides a dynamic friction test device and a test method capable of realizing loading in a wide rate range, which solves the problems existing in the prior art and can also realize linear high-speed friction loading in a short distance and short time, which is specifically manifested as follows:

[0035] 1. The motion form of the traditional pin-disc friction and wear tester is rotational motion, which will cause the measured friction and wear performance to be the wear surface of the previous cycle, and will cause uneven deformation of the sample, affecting the measurement of friction data. Compared with the traditional pin-disc friction and wear tester, the present invention perfectly overcomes the above problems through linear motion, and the obtained friction surface deformation is more uniform, and the obtained friction data is more accurate.

[0036] 2. Compared with the traditional reciprocating friction and wear tester, the stroke and frequency of the reciprocating friction and wear tester are limited, and it is difficult to achieve high-speed friction of the contact surface within a short distance by improving the control method of the motor. The present invention uses a Hopkinson rod instead of a motor control to achieve a higher speed (40m / s) friction motion.

[0037] 3. Compared with the existing linear and high-speed friction technologies, the loading forms of previous technologies are limited. Most of them achieve high-speed and high-pressure loading of friction simultaneously through impact, which makes it difficult to separate and decouple the pressure and speed loading, and the stability and reliability of the data are greatly reduced due to the impact. The present invention realizes high-speed loading under uniform deformation through a specially designed pressure application system combined with the loading method of the Hopkinson rod, and monitors the loading process in real time to obtain more comprehensive friction and wear data. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The above and / or other features and advantages of the present invention will become more easily understood through the following description with reference to the accompanying drawings, which are not drawn to scale and some features are exaggerated or reduced to show details of specific components. In the accompanying drawings:

[0039] Figure 1 It is a schematic diagram of the overall structural principle of the dynamic friction test device of the present invention;

[0040] Figure 2 Schematic diagram of the friction specimen (transmission rod end) of the present invention, wherein (a) is a bottom view, (b) is an isometric view, (c) is a front view, and (d) is a left view;

[0041] Figure 3 Schematic diagram of the friction specimen (incident rod end) of the present invention, wherein (a) is a bottom view, (b) is an isometric view, (c) is a front view, and (d) is a left view;

[0042] Figure 4 It is a partial diagram of the assembly of the pressure applying device and the sample of the present invention;

[0043] Figure 5 This is a typical stress wave waveform on the rod of the present invention;

[0044] Figure 6 It is a measured data curve diagram of the friction slip velocity and friction coefficient of the present invention;

[0045] Description of reference numerals:

[0046] 1- impact rod; 2- shaper; 3- incident rod; 4- sample; 5- transmission rod; 6- energy absorption device; 7- fixed outer frame; 8- stud; 9- pressurized square column; 10- strain gauge; 11- bridge box and data acquisition device. DETAILED DESCRIPTION

[0047] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention. The specific implementation of the present invention is further described in detail below in conjunction with specific examples.

[0048] According to an embodiment of the present invention, a dynamic friction test device and a test method capable of achieving loading in a wide rate range are provided. Figure 1-6 The impact rod 1 with wave impedance matching impacts the end face of the incident rod 3 with the shaper 2 attached, which can generate a loading curve with a pulse width range of 80us to 300us and a speed range of 1 to 40m / s. The constant load and uniform speed motion during the friction process are guaranteed, in which the positive pressure is obtained by a specific pressure clamping device, and the friction force is obtained by the transmission rod 5 connected to the sample, and finally accurate friction and wear data can be obtained.

[0049] refer to Figure 1 Specifically, the device includes three systems: a Hopkinson rod loading system, a pressure application system, and a data acquisition system, wherein the Hopkinson rod loading system includes an impact rod 1, a shaper 2, an incident rod 3, a sample 4, a transmission rod 5 and an energy absorption device 6; the pressure application system includes a fixed outer frame 7, a stud 8, and a pressurized square column 9; the data acquisition system includes a strain gauge 10, a bridge box and a data acquisition device 11.

[0050] The following describes the specific requirements of each part of the device and the device principle:

[0051] The components of the Hopkinson bar loading system are coaxially mounted on a horizontal workbench in sequence, wherein the impact rod 1, the incident rod 3 and the transmission rod 5 are cylindrical long rods of equal diameter. At the same time, in order to ensure that the loading wave is accurately transmitted from the impact rod 1 to the incident rod 3 and the transmission rod 5, it is necessary to ensure that the wave impedance of the impact rod 1 is not greater than the wave impedance of the incident rod 3. In this embodiment, the wave impedances of the remaining components are consistent, that is, the impact rod 1, the incident rod 3 and the transmission rod 5 are made of the same material and have the same cross-sectional area. After the impact, the following can be obtained: Figure 5 The incident signal, transmission signal and pressure signal shown; a set of samples 4 consists of two Figure 3 The incident end specimen shown and a Figure 2 The transmission end sample with a boss is shown in the figure. The measurement unit after all the numbers in the figure is millimeter. Figure 3 The incident section sample is symmetrically mounted on both sides of the transmission end sample boss. The two symmetrical incident end samples are fixed together by grooving or gluing at the end face of the incident rod 3, and the transmission end sample is fixed by gluing at the end face of the transmission rod 5 to ensure accurate application of the velocity and stability of the signal. The friction interface to be measured is the contact area between the transmission end sample boss and the incident end sample; the energy absorption device 6 is used to absorb the axial impact energy in the test to prevent the transmission rod 5 from excessively moving and causing damage to the test device. The magnitude of the velocity and the pulse width of the velocity are determined by the length of the impact rod 1 and the energy of the impact rod 1 hitting the incident rod 3, respectively. Using the device of this embodiment, a loading curve with a pulse width range of 80us to 300us and a velocity range of 1 to 40m / s can be obtained.

[0052] The pressure application system is mainly composed of a heavy metal fixed outer frame 7, a stud 8 for applying pressure and a pressure-applying square column 9 for transmitting pressure, wherein the stud 8 and the pressure-applying square column 9 are placed vertically in pairs above and below the friction specimen 4. By twisting the upper and lower studs 8, the middle pressure-applying square column 9 is compressed, and then the pressure is transmitted to the friction interface to be measured, thereby achieving pressure application, refer to Figure 4Partial diagram of the pressure applying device and sample assembly. It should be noted that the cross-sectional area of ​​the pressurized square column 9 is consistent with the contact area of ​​the friction interface to be measured, that is, the contact area between the pressurized square column 9 and the sample 4 is consistent with the friction interface contacted by the two incident end samples and the transmission end sample, so as to ensure the constant pressure and the accuracy of pressure application during the entire friction process. The contact area between the pressurized square column 9 and the incident end sample is not the actual friction interface, and the non-friction interface should be smooth to achieve the purpose of stable signal measurement.

[0053] In the data acquisition system, the strain signal is measured by the strain gauge 10 (resistance 1000Ω, sensitivity coefficient 1.92) on the Hopkinson bar loading system and the pressure application system, and the signal is collected by the high-precision data acquisition device through the Wheatstone half bridge in the bridge box. In the device of this embodiment, three pairs of strain gauges 10 are respectively set, namely the first strain gauge, the second strain gauge and the third strain gauge. The first strain gauge is set in the middle of the incident rod 3, the second strain gauge is set on the transmission rod 5 and close to the proximal end of the transmission rod 5, and the third strain gauge is attached to the middle of the pressurized square column 9.

[0054] During the experiment, firstly, a positive pressure is applied to the friction interface through the hexahedral pressure square column 9 in the pressure application system of the test bench; then the impact rod 1 hits the proximal end of the incident rod 3 to generate an incident stress wave and propagate in the incident rod 3. After the incident stress wave propagates to the sample 4, it is reflected by the sample 4 and forms a reflected stress wave in the incident rod 3. During this period, the waveform is reflected back and forth and superimposed in the transmission rod 5 to form a transmission wave. The corresponding signal is obtained through the data acquisition device; finally, the friction slip velocity and friction coefficient are calculated according to the one-dimensional stress wave theory, referring to Figure 6 Graph of measured data of friction slip velocity and friction coefficient.

[0055] According to the principle of the dynamic friction test device of the present invention, the normal pressure and tangential sliding friction force measured by the pressure application system and the transmission rod of the Hopkinson rod loading system are F and F, respectively. N and F sliding :

[0056] F sliding (t) = E tra A tra ε tra (t) (1)

[0057] F N (t) = E Fix A Fix ε Fix (t) (2)

[0058] Where:

[0059] F sliding (t) is the tangential sliding friction force measured by the Hopkinson transmission rod; Etra is the elastic modulus of the transmission rod; A tra is the cross-sectional area of ​​the transmission rod; ε tra (t) is the strain change of the transmission rod during the friction process;

[0060] F N (t) is the positive pressure during the test; E Fix is the elastic modulus of the pressure column; A Fix is the cross-sectional area of ​​the pressure column, ε Fix (t) is the strain change of the pressure column during the friction process.

[0061] The sliding friction force is composed of the friction forces on two symmetrical friction surfaces, so the friction force f(t) during the test is half of the sliding friction force measured by the transmission rod, that is:

[0062]

[0063] According to Coulomb's friction theorem, the friction coefficient u of the interface during loading can be known, and the slip velocity can be obtained from the one-dimensional stress wave theory, that is, the velocity difference v between the incident rod and the transmission rod end sliding (t):

[0064]

[0065] v sliding (t) = v1(t) - v2(t) = C[ε inc (t)+ε ref (t)]-Cε tra (t) (5)

[0066] Where:

[0067] v1(t) and v2(t) are the incident and transmission rod end velocities beside the sample, respectively; C is the wave velocity in the elastic rod; ε inc (t) is the strain change of the incident rod after impact; ε ref (t) is the strain change in the returning incident rod after friction loading.

[0068] Finally, it should be noted that the features mentioned and / or shown in the above description of the exemplary embodiments of the present invention may be combined in the same or similar manner into one or more other embodiments, combined with the features in other embodiments or substituted for the corresponding features in other implementations. The technical solutions obtained by these combinations or substitutions shall also be deemed to be included in the protection scope of the present invention.

Claims

1. A dynamic friction test device capable of achieving loading in a wide rate range, characterized in that: It also includes a Hopkinson bar loading system, a pressure application system, and a data acquisition system; The Hopkinson bar loading system comprises an impact bar (1), a shaper (2), an incident bar (3), a sample (4), a transmission bar (5) and an energy absorbing device (6) which are coaxially arranged in sequence; a group of the samples (4) is composed of two identical incident end samples and a transmission end sample with a boss; a pressure applying system is also arranged between the incident bar (3) and the transmission bar (5); The pressure applying system comprises a fixed outer frame (7), studs (8) and a pressurized square column (9); the studs (8) are fixed vertically in pairs above and below the fixed outer frame (7); the pressurized square columns (9) are placed vertically in pairs above and below the specimen and connected to the studs (8); The data acquisition system comprises a strain gauge (10), a bridge box and a data acquisition device (11); there are a plurality of strain gauges (10), which are respectively arranged on the Hopkinson bar loading system and the pressure application system, and are connected to the bridge box and the data acquisition device (11) through wires.

2. A dynamic friction test device capable of achieving loading in a wide rate range as claimed in claim 1, characterized in that: The impact rod (1), the incident rod (3) and the transmission rod (5) are cylindrical long rods with equal diameters, and the wave impedance of the impact rod (1) is not greater than the wave impedance of the incident rod (3).

3. A dynamic friction test device capable of achieving loading in a wide rate range as claimed in claim 1, characterized in that: The incident rod (3) is grooved (or glued) to fix the two incident end samples; the transmission end sample is fixed to the transmission rod (5) by gluing.

4. A dynamic friction test device capable of achieving loading in a wide rate range as claimed in claim 1, characterized in that: The fixed outer frame (7) is made of metal.

5. A dynamic friction test device capable of achieving loading in a wide rate range as claimed in claim 1, characterized in that: The cross-sectional area of ​​the pressurized square column (9) is consistent with the contact area of ​​the friction interface, while the non-friction interface remains smooth.

6. A dynamic friction test device capable of achieving loading in a wide rate range as claimed in claim 1, characterized in that: Providing three pairs of strain gauges (10), namely a first strain gauge, a second strain gauge and a third strain gauge; The first strain gauge is arranged in the middle of the incident rod (3); the second strain gauge is arranged on the transmission rod (5) and close to the proximal end of the transmission rod (5); and the third strain gauge is attached to the middle of the pressurized square column (9).

7. A dynamic friction test device capable of achieving loading in a wide rate range as claimed in claim 1, characterized in that: The bridge box contains a Wheatstone half bridge.

8. A dynamic friction test method capable of achieving loading in a wide rate range, characterized in that: The test is performed using the device according to any one of claims 1 to 7, wherein the specific steps include: First, a positive pressure is applied to the friction interface through a pressurized square column (9) in a pressure application system on a test bench; then, an impact rod (1) impacts the proximal end of an incident rod (3) to generate an incident stress wave that propagates in the incident rod (3); the incident stress wave propagates to a sample (4) and is reflected by the sample (4) to form a reflected stress wave in the incident rod (3); during this period, the waveform is reflected back and forth in the sample (4) and superimposed in the transmission rod (5) to form a transmission wave, and the corresponding signal is obtained through a data acquisition system; finally, the friction slip velocity and the friction coefficient are calculated based on the one-dimensional stress wave theory.

9. A dynamic friction testing method capable of achieving loading in a wide rate range as claimed in claim 8, characterized in that: The friction slip speed is: v sliding (t)=v1(t)-v2(t)=C[ε inc (t)+ε ref (t)]-Cε tra (t) Where: v1(t) and v2(t) are the incident and transmission rod end velocities beside the sample, respectively; C is the wave velocity in the elastic rod; ε inc (t) is the strain change of the incident rod after impact; ε ref (t) is the strain change in the returning incident rod after friction loading; ε tra (t) is the strain change of the transmission rod during the friction process; The coefficient of friction is: Where: f(t) is the friction force during the test, and Among them, F sliding (t) is the tangential sliding friction force measured by the Hopkinson transmission rod, E tra is the elastic modulus of the transmission rod, A tra is the cross-sectional area of ​​the transmission rod; F N (t) is the positive pressure during the test, and F N (t) = E Fix A Fix ε Fix (t), where E Fix is the elastic modulus of the pressure column, A Fix is the cross-sectional area of ​​the pressure column, ε Fix (t) is the strain change of the pressure column during the friction process.

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