Tensile test system for medium strain rate of material

Abstract

The invention discloses a tensile testing system for a strain rate in a material, and relates to the technical field of dynamic mechanical property testing of materials. The tensile testing system for the strain rate in the material comprises a power system and a control system based on a motor, a waveform separation long pulse width stress wave testing system based on multi-point strain measurement and a strain testing system based on a high-speed camera and a DIC technology. According to the tensile test system for the strain rate in the material, a large mass block is accelerated to a preset speed through but not limited to a motor, an anvil connected with a sample is impacted so as to perform tensile loading on the sample, the stress borne by the sample is measured through a Hopkinson bar and a multi-point waveform separation technology, and the deformation of the sample is measured through a digital image processing technology DIC; according to the method, the obtained data is smooth in waveform, high in reliability, simple to operate and low in cost, the achievable strain rate is 101-102 s <-1 >, and a strain rate test blind area between a material servo testing machine MTS and a traditional Hopkinson bar is made up.

Description

Technical Field

[0001] The invention relates to the technical field of dynamic mechanical property testing of materials, in particular to a strain rate tensile testing system for materials. Background Art

[0002] In the automotive, aviation and defense industries, it is necessary to consider the deformation of materials and structures during impact, so the performance of materials at different strain rates is crucial. The existing conventional technology is mainly based on material servo testing machine MTS and Hopkinson bar, and the strain rate range is generally in two separation intervals, that is, less than 1s -1 and greater than 200s -1 Medium strain rate (1s -1 ~200s -1 ) are rarely reported in the open literature, mainly due to the immaturity of experimental technology. Incomplete coverage of strain rate testing leads to insufficient understanding of material properties and deformation mechanisms.

[0003] At present, the implementation methods of medium strain rate loading of materials have the following characteristics: the stress of material testing machine configuration has obvious oscillation when the strain rate is high, and the commercial high-speed tensile testing machine is expensive; it is difficult to achieve constant strain rate loading with electromagnetically driven Hopkinson bar, electromagnetic interference causes trouble for traditional strain gauge measurement, and the stress test bar is short enough to measure millisecond stress wave signals; the extended Hopkinson bar (more than 20m) occupies a large area, is costly, and is complex to operate. A medium strain rate loading test system with strong data reliability, simple operation, and low cost is urgently needed to be developed. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a dynamic mechanical experimental system for strain rate in materials. The system has strong loading and testing reliability, simple operation, and low cost. The system can control the loading speed according to the required strain rate and be supplemented by corresponding stress and strain tests to obtain a true and reliable material stress-strain curve, solving the problem of the current lack of testing technology.

[0005] The present invention provides a material strain rate tensile testing system, comprising:

[0006] Motor-based power systems and control systems, waveform separation long-pulse width stress wave test systems based on multi-point strain measurement, and strain test systems based on high-speed cameras and DIC technology;

[0007] The motor-based power system and control system include:

[0008] A control system, the control system is used to program the motor movement speed, acceleration, and deceleration;

[0009] A servo motor, which moves according to the command of the control system and loads torque to the fixed rack through the gear of the transmission device;

[0010] A transmission device, which is used to transmit the torque of the servo motor to the fixed rack on the linear guide rail to drive the motor platform to move;

[0011] A linear guide rail, wherein the linear guide rail drives the motor platform to maintain linear movement;

[0012] A mass block, wherein the mass block collides with the sample clamping end based on the driving of the motor platform, thereby stretching the sample;

[0013] The waveform separation long pulse width stress wave test system based on multi-point strain measurement includes:

[0014] A waveguide rod, wherein the waveguide rod is 2 to 4 meters long and one end of the waveguide rod passes through a through hole on the mass block;

[0015] A strain gauge, which is attached to the surface of the waveguide rod and is used to measure the axial strain waveform when the stress wave arrives;

[0016] A signal test storage system for storing measured waveform data;

[0017] The waveguide rod support seat is used to support and adjust the waveguide rod, keep the waveguide rod horizontal and in a straight line and enable unhindered axial movement;

[0018] A sample clamping anvil, wherein the sample clamping anvil is supported by an anvil support seat, and the sample clamping anvil is horizontally coaxially installed with the waveguide rod;

[0019] A sample, wherein two ends of the sample are respectively fixed to the waveguide rod and the sample clamping anvil;

[0020] An energy absorbing device, which is used to absorb the remaining kinetic energy of the anvil head and the impact head, and decelerate them to a standstill;

[0021] Preferably, the strain testing system based on high-speed camera and DIC technology includes a high-speed camera, a high-intensity light source and DIC calculation software. The normal phase of the high-speed camera sensing chip is perpendicular to the axis of the waveguide rod (especially, the surface of the sheet sample needs to be parallel to the chip). The high-speed camera records photos of the speckle deformation on the sample surface, and the DIC software processes the photos to obtain the strain time curve ε(t) and the strain rate time curve. By synchronizing and resampling the stress history curve and the strain history curve, the stress-strain curve of the sample can be obtained.

[0022] Preferably, a circular hole parallel to the linear guide rail is provided inside the mass block, the impact end is a cylindrical structure, and a through slot is provided in a direction perpendicular to the axial direction, so that the sample can be photographed by a high-speed camera through the through slot.

[0023] Preferably, the signal test storage system comprises an ultra-dynamic strain gauge, a digital acquisition / oscilloscope and a computer; the electrical signal generated by the Wheatstone bridge is amplified by the ultra-dynamic strain gauge, collected by the digital acquisition / oscilloscope and stored in the computer.

[0024] Preferably, the process of the sample tensile test is as follows: after the impact end of the mass block passes through the sample and hits the sample clamping anvil, the sample is stretched at an approximately uniform speed, and the stress generated when the sample is stretched is transmitted to the waveguide rod, causing the strain gauge coaxially attached to the surface of the rod to deform, and is recorded by the signal test storage system.

[0025] Preferably, a plurality of waveguide rod support seats are provided and fixed to the platform by bolts, and the waveguide rod support seats use linear bearings or barrels made of low-friction materials (such as polytetrafluoroethylene) to support and guide the waveguide rod to reduce the friction between the waveguide rod and the waveguide rod support seats.

[0026] Preferably, the sample is fixed between the waveguide rod and the sample clamping anvil by means of threads, high-strength glue or special slots, and the outer diameter of the sample clamping anvil is slightly larger than the diameter of the waveguide rod.

[0027] Preferably, the measurement of the axial strain waveform adopts a multi-point strain waveform separation technology, taking a two-point strain gauge method as an example:

[0028] A finite length compression rod OE continuously transmits a stress wave at point O at time zero, which is called a right-traveling wave. The wave head passes through points A and B successively, and is reflected back into the compression rod at the end surface E as a left-traveling wave. The wave signal measured at point A is recorded as ε A (t), which is caused by the right-going wave ε at point A RA (t) and the left-traveling wave ε LA (t), and similarly the wave signal measured at point B is denoted as ε B (t), which is caused by the right-going wave ε at point B RB (t) and the left-traveling wave ε LB By superimposing the stress on the end face (t), the deformation of the end face caused by the stress can be separated according to the two-point strain gauge method, so that the stress on the specimen can be calculated.

[0029] Preferably, the right-traveling wave signals and the left-traveling wave signals passing through the strain gauge arrangement points A and B are calculated by the following formulas respectively:

[0030] Left traveling wave ε RA (t) = ε A (t), t≤TA ;

[0031] ε RA (t) = ε A (t)-ε B (tT)-ε RA (t-2T), t>T A ;

[0032] Right traveling wave ε LB (t)=0, t≤T B ;

[0033] ε LB (t) = ε B (t)-ε A (tT)-ε LB (t-2T), t>T B ;

[0034] in, According to the strain signal ε of the right-traveling wave at point A RA (t) can obtain the force condition at end point A (the clamping end of the sample) and the stress on the sample Among them, E b , A b , A s are Young's modulus, cross-sectional area of ​​the waveguide rod and cross-sectional area of ​​the specimen, respectively.

[0035] Beneficial Effects

[0036] The present invention provides a strain rate tensile testing system for materials. Compared with the prior art, it has the following beneficial effects:

[0037] 1. The strain rate tensile test system in the material accelerates the large mass block to a predetermined speed through, but not limited to, a motor, and hits the anvil head connected to the sample to perform tensile loading on the sample. The stress borne by the sample is measured by the Hopkinson bar and multi-point waveform separation technology, and the deformation of the sample is measured by the digital image processing technology DIC, thereby obtaining the stress-strain curve of the sample. The data waveform obtained by the present invention is smooth, reliable, simple to operate, and low in cost. The strain rate that can be achieved is 10 1 ~10 2 s -1 , which makes up for the testing blind spots of material servo testing machine MTS and traditional Hopkinson bar.

[0038] 2. The medium-strain rate tensile testing system for this material introduces wave separation technology and high-speed full-field strain testing technology based on the traditional Hopkinson bar testing system, breaking through the stress and strain measurement time and measurement range, and ultimately forming a stable and reliable medium-strain rate material performance system, solving the pain point that existing testing methods cannot obtain reliable material performance under medium strain rates, and is expected to be widely used in the automotive, aviation and defense fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0040] Figure 2 Schematic diagram of the two-point strain wave separation technology.

[0041] In the figure: waveguide rod 1, sample 2, sample clamping anvil 3, waveguide rod support seat 4, strain gauge 5, platform 6, linear guide 7, transmission device 8, mass block 9, motor 10, energy absorption device 11, high-speed camera and light source 12, computer 13, ultra-dynamic strain gauge and data acquisition instrument 14. DETAILED DESCRIPTION

[0042] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] The present invention is based on Figure 1-Figure 2 A medium strain rate tensile testing system for materials is provided, which is an improved Hopkinson bar system based on inertial kinetic energy impact to provide constant strain rate loading. The system includes a power and control device, a Hopkinson bar device and a data acquisition and analysis system, which cooperate to complete the medium strain rate impact tensile performance test of the sample;

[0044] The Hopkinson bar device is composed of a waveguide rod 1, a sample 2, a sample clamping anvil 3, a waveguide rod support seat 4, a platform 6 and an energy absorption device 11. The waveguide rod support seat 4 is provided with multiple ones, which are fixed on the platform 6 by bolts, and the waveguide rod support seat 4 uses a linear bearing or a barrel made of a low-friction material (such as polytetrafluoroethylene) to support and guide the waveguide rod 1 to reduce the friction between the waveguide rod 1 and the waveguide rod support seat 4. The sample 2 is fixed between the waveguide rod 1 and the sample clamping anvil 3 by threads or high-strength glue. The outer diameter of the sample clamping anvil 3 is larger than the diameter of the waveguide rod 1. When the sample clamping anvil 3 is impacted and moved, it will pull the sample 2 to deform rapidly. The energy absorption device 11 will absorb the remaining kinetic energy of the sample clamping anvil 3 and the impact head to decelerate it to a standstill.

[0045] The power and control device is composed of a motor 10, a control system, a linear guide 7, a transmission device 8 and a mass block 9. The transmission device 8 includes an L-shaped motor base, a transmission gear and a rack fixed on the platform, the latter two of which are not drawn.

[0046] The motor 10 is fixed to the transmission device by screws, and the transmission device and the bottom of the mass block 9 are threadedly mounted with a slider so that it can move freely on the linear guide rail 7.

[0047] The control system is mainly composed of a PLC control cabinet, which sets the motor motion parameters, including speed, acceleration time and deceleration time, and sends pulse instructions to the motor 10 to make it rotate according to the established parameters.

[0048] The transmission gear set meshes with the fixed rack, and the motor 10 and the transmission device move horizontally and linearly through the rotation of the motor shaft, and push the mass block 9 to move.

[0049] When the control system issues an acceleration instruction, the motor 10 and the transmission device drive the mass block 9 to accelerate. When the predetermined speed and position are reached, the motor 10 starts to decelerate, and the mass block 9 maintains the current speed and starts to separate from the transmission device.

[0050] A circular hole parallel to the linear guide rail 7 is provided at the center of the transmission device and the mass block 9. The impact end of the mass block 9 is a cylindrical structure. The hollow cylinder at the impact end of the mass block 9 is cut so that the sample 2 can be observed by the camera through the cut gap. The inner diameter of the hollow cylinder is slightly larger than the diameter of the waveguide rod 1, and a through groove is provided in a direction perpendicular to the axial direction, so that the waveguide rod 1 can pass through the center of the transmission device and the mass block 9 and can move freely without contact. The impact head of the mass block 9 passes through the sample 2 and hits the sample clamping anvil 3, so that the sample clamping anvil 3 pulls the sample 2 to stretch and deform, and slides on the waveguide rod support seat 4. After the sample 2 breaks, the sample clamping anvil 3 and the mass block 9 have residual kinetic energy, which is absorbed by the energy absorption device 11 and finally stops moving.

[0051] The data acquisition and analysis system is composed of a strain gauge 5, an ultra-dynamic strain gauge and a data acquisition instrument 14, a computer 13, a high-speed camera and a light source 12, and DIC software.

[0052] Strain gauges 5 are coaxially attached to at least two positions on the waveguide rod 1. The strain gauges 5 form a Wheatstone bridge through a rated resistor. The resistance change of the strain gauge 5 caused by the stress wave drives the bridge to generate a voltage output, which is amplified by the ultra-dynamic strain gauge 14 and recorded by digital acquisition and stored in the computer 13.

[0053] Through the waveform separation technology of multi-point strain, the strain ε(t) generated by the force transmitted from the sample end with an ultra-long recording time is obtained, specifically the two-point strain gauge method:

[0054] That is, a finite length compression rod OE is used to continuously transmit a stress wave at point O at time zero, which is called a right-traveling wave. The wave head passes through points A and B successively, and is reflected back into the compression rod at the end surface E as a left-traveling wave. The wave signal measured at point A is recorded as ε A (t), which is caused by the right-going wave ε at point A RA (t) and the left-traveling wave ε LA (t), and similarly the wave signal measured at point B is denoted as ε B (t), which is caused by the right-going wave ε at point B RB (t) and the left-traveling wave ε LB (t) is superimposed.

[0055] According to the two-point strain gauge method, the deformation of the end face caused by the force can be separated, so as to calculate the force received, that is, the force received by the specimen. The right-traveling and left-traveling wave signals passing through the strain gauge arrangement points A and B are calculated by the following formulas:

[0056] Left traveling wave ε RA (t) = ε A (t), t≤T A ;

[0057] ε RA (t) = ε A (t)-ε B (tT)-ε RA (t-2T), t>T A ;

[0058] Right traveling wave ε LB (t)=0, t≤T B ;

[0059] ε LB (t) = ε B (t)-ε A (tT)-ε LB (t-2T), t>T B ;

[0060] in, According to the strain signal ε of the right-traveling wave at point A RA (t) The force condition of end point A (the clamping end of the sample) can be obtained by the formula The stress in the sample can be obtained, where E b , A b , A s are Young's modulus, cross-sectional area of ​​the waveguide rod and cross-sectional area of ​​the specimen, respectively.

[0061] The computer 13 controls the high-speed camera 12 to photograph the deformation history of the surface of the sample 2 .

[0062] Before the test, the surface of sample 2 is made of speckle production tools such as spray paint and airbrush to produce speckles that will not fall off under high-speed stretching. The surface strain field ε(x, t) can be calculated by DIC software later. By setting the gauge length of dog-bone sample 2, the average deformation ε of sample 2 within the gauge length range can be obtained. s (t), and finally by synchronizing stress σ(t) and strain ε s (t), and sampled at the same frequency to obtain the stress-strain curve, and the strain ε s (t) Taking the derivative with respect to time, we get the strain rate The strain rate of sample 2 can be obtained by averaging the values ​​over a certain period of time.

[0063] Things to note during use:

[0064] 1) The effective range of the stress-strain curve should be that the strain rate fluctuation does not exceed 10%. Of course, this restriction can be appropriately relaxed for materials that are relatively insensitive to strain rate;

[0065] 2) In order to make the strain rate during loading basically constant, the velocity of the mass block needs to be basically constant, and its initial kinetic energy E k and the sample deformation energy E p Try to satisfy the relationship E k ≥10E p ;

[0066] 3) Waveguide rod material and size: In order to make the stress wave transmitted into the waveguide rod generate a lower particle motion speed in the rod, so as not to cause a sudden change in the loading speed, it is necessary to select a material with high wave impedance as the waveguide rod, such as steel, and select a diameter as large as possible without causing the three-dimensional dispersion effect of the wave. Although the wave separation technology can theoretically collect infinitely long stress wave waveforms, the recorded waveform inevitably introduces noise errors, which are easily caused by the accumulation of numerical calculations. The continuous accumulation of errors is likely to result in more obvious distortion over time. Therefore, if conditions permit, try to use longer waveguide rods so that the number of iterations can be reduced for the same recording time to avoid excessive accumulation of errors.

[0067] The solution of the present invention takes the horizontal waveguide rod 1 as an example. The waveguide rod 1 can also be placed vertically and loaded by converting the potential energy of the motor 10 or the mass block 9 into kinetic energy. The load can also be applied to the sample using a combination of oil and gas. The stress and strain test method in the case of the present invention can be used to obtain the stress-strain curve of the sample.

[0068] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0069] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A strain rate tensile testing system for materials, characterized in that: include: Motor-based power systems and control systems, waveform separation long-pulse width stress wave test systems based on multi-point strain measurement, and strain test systems based on high-speed cameras and DIC technology; The motor-based power system and control system include: A control system, the control system is used to program the motor movement speed, acceleration, and deceleration; A servo motor, which moves according to the command of the control system and loads torque to the fixed rack through the gear of the transmission device; A transmission device, which is used to transmit the torque of the servo motor to the fixed rack on the linear guide rail to drive the motor platform to move; A linear guide rail, wherein the linear guide rail drives the motor platform to maintain linear movement; The mass block collides with the sample clamping end based on the driving of the motor platform, thereby stretching the sample.

2. A material strain rate tensile testing system according to claim 1, characterized in that: The waveform separation long pulse width stress wave testing system based on multi-point strain measurement comprises: A waveguide rod, wherein the waveguide rod is 2 to 4 meters long and one end of the waveguide rod passes through a through hole on the mass block; A strain gauge, which is attached to the surface of the waveguide rod and is used to measure the axial strain waveform when the stress wave arrives; A signal test storage system for storing measured waveform data; The waveguide rod support seat is used to support and adjust the waveguide rod, keep the waveguide rod horizontal and in a straight line and enable unhindered axial movement; A sample clamping anvil, wherein the sample clamping anvil is supported by an anvil support seat, and the sample clamping anvil is horizontally coaxially installed with the waveguide rod; A sample, wherein two ends of the sample are respectively fixed to the waveguide rod and the sample clamping anvil; The energy absorbing device is used to absorb the remaining kinetic energy of the anvil head and the impact head, so as to decelerate them to a standstill.

3. A material strain rate tensile testing system according to claim 1, characterized in that: The strain test system based on high-speed camera and DIC technology includes a high-speed camera, a high-intensity light source and DIC calculation software. The normal phase of the high-speed camera sensing chip is perpendicular to the axis of the waveguide rod. The high-speed camera records photos of the speckle deformation on the sample surface, and the DIC software processes the photos to obtain the strain time curve ε(t) and the strain rate time curve. By synchronizing and resampling the stress history curve and the strain history curve, the stress-strain curve of the sample can be obtained.

4. A material strain rate tensile testing system according to claim 1, characterized in that: The mass block has a circular hole parallel to the linear guide rail, the impact end is a cylindrical structure, and has a through slot in a direction perpendicular to the axial direction, so that the sample can be photographed by a high-speed camera through the through slot.

5. A material strain rate tensile testing system according to claim 1, characterized in that: The signal test storage system comprises an ultra-dynamic strain gauge, a digital acquisition / oscilloscope and a computer; the electrical signal generated by the Wheatstone bridge is amplified by the ultra-dynamic strain gauge, collected by the digital acquisition / oscilloscope and stored in the computer.

6. A material strain rate tensile testing system according to claim 1, characterized in that: The process of the sample tensile test is as follows: after the impact end of the mass block passes through the sample and hits the sample clamping anvil, the sample is stretched at an approximately uniform speed. The stress generated when the sample is stretched is transmitted to the waveguide rod, causing the strain gauge coaxially attached to the surface of the rod to deform, and is recorded by the signal test storage system.

7. A material strain rate tensile testing system according to claim 1, characterized in that: The waveguide rod support seats are provided in plurality and fixed on the platform by bolts, and the waveguide rod support seats use linear bearings or barrels made of low-friction materials to support and guide the waveguide rods to reduce friction between the waveguide rods and the waveguide rod support seats.

8. A material strain rate tensile testing system according to claim 1, characterized in that: The sample is fixed between the waveguide rod and the sample clamping anvil by means of threads, high-strength glue or special slots, and the outer diameter of the sample clamping anvil is greater than the diameter of the waveguide rod.

9. A material strain rate tensile testing system according to claim 1, characterized in that: The measurement of the axial strain waveform adopts a multi-point strain waveform separation technology, including: A finite length compression rod OE continuously transmits a stress wave at point O at time zero, which is called a right-traveling wave. The wave head passes through points A and B successively, and is reflected back into the compression rod at the end surface E as a left-traveling wave. The wave signal measured at point A is recorded as ε A (t), which is caused by the right-going wave ε at point A RA (t) and the left-traveling wave ε LA (t), and similarly the wave signal measured at point B is denoted as ε B (t), which is caused by the right-going wave ε at point B RB (t) and the left-traveling wave ε LB By superimposing the stress on the end face (t), the deformation of the end face caused by the stress can be separated according to the two-point strain gauge method, so that the stress on the specimen can be calculated.

10. A material strain rate tensile testing system according to claim 9, characterized in that: The right-traveling and left-traveling wave signals passing through the strain gauge arrangement points A and B are calculated by the following formulas: Left traveling wave ε RA (t) = ε A (t), t≤T A ; ε RA (t)=ε A (t)-ε B (t-T)-ε RA (t-2T), t>T A ; Rightward wave ε LB (t)=0, t≤T B ; ε LB (t)=ε B (t)-ε A (t-T)-ε LB (t-2T), t>T B ; in, According to the strain signal ε of the right-traveling wave at point A RA (t) can obtain the force condition at end point A and the stress on the specimen Among them, E b , A b , A s are Young's modulus, cross-sectional area of ​​the waveguide rod and cross-sectional area of ​​the specimen, respectively.

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