A material strain rate tensile testing system

Abstract

The application discloses a material strain rate tensile test system and relates to the technical field of dynamic mechanical property test of materials. The material strain rate tensile test system comprises a motor-based power system and a control system, a waveform separation long pulse width stress wave test system based on multipoint strain measurement and a strain test system based on a high-speed camera and DIC technology. The material strain rate tensile test system accelerates a large mass block to a predetermined speed through a motor, impacts an anvil head connected with a sample to perform tensile loading on the sample, measures stress borne by the sample through a Hopkinson bar and a multipoint waveform separation technology, measures sample deformation through digital image processing technology DIC, and thus obtains a stress-strain curve of the sample. The data waveform obtained by the application is smooth, reliable, simple to operate, low in cost, and capable of realizing a strain rate of 10 1 ~ 10 2 s ‑1 , and fills the strain rate test blind area between a material servo testing machine MTS and a traditional Hopkinson bar.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of dynamic mechanical property testing of materials, in particular to a material strain rate tensile testing system. BACKGROUND

[0002] In the fields of automobile, aviation and national defense, the deformation of materials and structures during impact needs to be considered, so the performance of materials under different strain rates is very important. The existing conventional technology mainly uses material servo testing machine MTS and Hopkinson bar, and the strain rate range is generally in two separate intervals, i.e. less than 1s -1 and greater than 200s -1 . The material performance under medium strain rate (1s -1 ~ 200s -1 ) is rarely reported in public literature, mainly because the experimental technology is not mature. Non-full-coverage strain rate testing leads to insufficient understanding of material performance and deformation mechanism.

[0003] The current material strain rate loading implementation has the following characteristics: the stress has obvious oscillation when the strain rate of the material testing machine type is high, and the commercial high-speed tensile testing machine is expensive; the electromagnetic drive Hopkinson bar is difficult to realize constant strain rate loading, electromagnetic interference causes trouble for traditional strain gauge measurement, and the stress testing rod is too short to measure millisecond stress wave signals; the long version of Hopkinson bar (more than 20m) occupies a large area, has high cost and complex operation. A medium strain rate loading testing system with strong data reliability, simple operation and low cost is urgently needed. SUMMARY

[0004] In view of the deficiencies of the prior art, the present application provides a material medium strain rate dynamic mechanical experiment system, which has strong reliability, simple operation and low cost in loading and testing, can control the loading speed according to the required strain rate, and is assisted by corresponding stress and strain testing, so as to obtain a real and reliable material stress-strain curve, thereby solving the problem of lack of current testing technology.

[0005] The material medium strain rate tensile testing system provided by the present application comprises:

[0006] A motor-based power system and control system, a waveform separation long pulse width stress wave testing system based on multi-point strain measurement, and a strain testing system based on high-speed camera and DIC technology;

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

[0008] A control system for programming and setting the motor motion speed, acceleration and deceleration;

[0009] A servo motor, which moves according to the instruction of the control system and loads the 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, which makes the motor platform keep linear movement;

[0012] A mass block, which collides with the sample clamping end based on the driving of the motor platform, so as to stretch the sample;

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

[0014] A wave guide rod, which is 2-4 m long, and one end of the wave guide rod penetrates through the through hole on the mass block;

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

[0016] A signal test storage system, which is used to store the measured waveform data;

[0017] A wave guide rod support seat, which is used to support and adjust the wave guide rod, keep the wave guide rod horizontal and maintain the linear state, and enable the wave guide rod to move axially without obstruction;

[0018] A sample clamping anvil head, which is supported by the anvil head support seat, and the sample clamping anvil head is installed horizontally and coaxially with the wave guide rod;

[0019] A sample, both ends of which are fixed with the wave guide rod and the sample clamping anvil head respectively;

[0020] An energy absorption device, which is used to absorb the residual kinetic energy of the anvil head and the impact head, so as to slow down to static state;

[0021] Preferably, the strain test system based on high-speed camera and DIC technology comprises a high-speed camera, a high-intensity light source and DIC calculation software, the sensing chip of the high-speed camera is perpendicular to the axis of the wave guide rod (in particular, the surface of the sheet-shaped sample also needs to be parallel to the chip), the high-speed camera records the photos of the speckle deformation on the surface of the sample, and the DIC software is used to process 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, the inner part of the mass is provided with a circular hole parallel to the linear guide, the impact end is a cylindrical structure, and a through groove is formed in the direction perpendicular to the axial direction, so that the sample can be photographed by the high-speed camera through the through groove.

[0023] Preferably, the signal test and storage system comprises a Wheatstone bridge, a dynamic strain gauge, a data acquisition / oscilloscope and a computer; the electrical signal generated by the Wheatstone bridge is amplified by the dynamic strain gauge, collected by the data 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 impacts the sample and the clamping anvil head, the sample is subjected to approximate uniform stretching, the stress generated during the stretching of the sample is propagated to the waveguide rod, causing the strain gauge coaxially attached to the surface of the rod to deform, and the deformation is recorded by the signal test and storage system.

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

[0026] Preferably, the sample is fixed between the waveguide rod and the sample clamping anvil head by threads, high-strength glue or special clamping grooves, and the outer diameter of the sample clamping anvil head 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, and the two-point strain gauge method is taken as an example:

[0028] A finite-length compression rod OE continuously transmits a stress wave at point O at zero time, which is called a right-going wave, the wave front successively passes through points A and B, and is reflected at the end surface E back into the compression rod as a left-going wave, and the wave signal measured at point A is denoted as ε A (t), which is the superposition of the right-going wave ε RA (t) and the left-going wave ε LA (t) at point A, and the wave signal measured at point B is denoted as ε B (t), which is the superposition of the right-going wave ε RB (t) and the left-going wave ε LB (t) at point B. According to the two-point strain gauge method, the deformation of the end surface caused by the force can be separated, and thus the force received by the sample can be calculated.

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

[0030] The left-going wave ε RA (t) = ε A (t), t≤TA ;

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

[0032] right traveling wave epsilon LB (t) = 0, t <= T B ;

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

[0034] wherein, According to the strain signal epsilon of the right traveling wave of point A RA (t), the stress condition of the A end point (sample clamping end) and the stress received by the sample can be obtained wherein, E b , A b , A s are the Young's modulus, cross-sectional area of the waveguide rod and cross-sectional area of the sample, respectively.

[0035] Advantages

[0036] The present application provides a material strain rate tensile testing system. Compared with the prior art, the following advantages are possessed:

[0037] 1. The material strain rate tensile testing system accelerates a large mass block to a predetermined speed by, but not limited to, a motor, hits an anvil head connected with a sample to stretch the sample, measures the stress borne by the sample through a Hopkinson bar and a multi-point waveform separation technology, measures the deformation of the sample through a digital image processing technology DIC, and further obtains the stress-strain curve of the sample. The data waveform obtained by the present application is smooth, reliable, simple to operate, low in cost, and can realize a strain rate of 10 1 ~ 10 2 s -1 , which makes up for the testing blind area of a material servo testing machine MTS and a traditional Hopkinson bar.

[0038] 2、The material strain rate tensile test system breaks through the stress and strain measurement time and measurement range by introducing wave separation technology, high-speed full-field strain test technology on the basis of traditional Hopkinson bar test system, and finally forms a stable and reliable medium strain rate material performance system, solves the pain point that the existing test means cannot obtain reliable material performance under medium strain rate, and is expected to be widely used in the fields of automobile, aviation and national defense. BRIEF DESCRIPTION OF DRAWINGS

[0039] Fig. 1 It is a schematic diagram of the overall structure of the application;

[0040] Fig. 2 It is a schematic diagram of the two-point strain wave separation technology.

[0041] In the figure: waveguide rod 1, sample 2, sample clamping anvil head 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, super dynamic strain meter and data acquisition instrument 14. DETAILED DESCRIPTION

[0042] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0043] The application is based on Figs. 1-2 A material strain rate tensile test system is provided, which is an improved Hopkinson bar system based on inertial kinetic impact to provide constant strain rate loading. The system comprises 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 property test of the sample.

[0044] The Hopkinson bar device is composed of a waveguide rod 1, a sample 2, a sample clamping anvil head 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 a plurality of waveguide rod support seats 4, which are fixed on the platform 6 by bolts, and the waveguide rod support seat 4 is made of a linear bearing or a low-friction material (such as polytetrafluoroethylene) to support and guide the waveguide rod 1, so as 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 head 3 by threads or high-strength glue. The outer diameter of the sample clamping anvil head 3 is greater than the diameter of the waveguide rod 1. When the sample clamping anvil head 3 is impacted and moved, it will pull the sample 2 to deform rapidly. The energy absorption device 11 will absorb the residual kinetic energy of the sample clamping anvil head 3 and the impact head, so as to slow down to static.

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

[0046] The motor 10 is fixed on the transmission device by screws, and the transmission device and the bottom of the mass block 9 are installed with sliding blocks through threads, so that they can move freely on the linear guide rail 7.

[0047] The control system is mainly composed of a PLC control cabinet, and the motor 10 is driven to rotate according to the set parameters by sending pulse instructions to the motor 10 through the setting of motor motion parameters including speed, acceleration time and deceleration time.

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

[0049] When the control system sends an acceleration motion instruction, the motor 10 and the transmission device drive the mass block 9 to accelerate, and when the motor 10 reaches the set speed and position, the motor 10 starts to decelerate, and the mass block 9 maintains the existing speed and starts to separate from the transmission device.

[0050] A circular hole parallel to the linear guide rail 7 is formed 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 cutting gap, the inner diameter of the hollow cylinder is slightly larger than the diameter of the waveguide rod 1, and a through groove is formed in the 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 hits the sample clamping anvil head 3 through the sample 2, so that the sample clamping anvil head 3 stretches and deforms the sample 2, and slides on the waveguide rod support 4, after the sample 2 is broken, the sample clamping anvil head 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 the strain gauges 5, the ultra-dynamic strain meter and the data acquisition instrument 14, the computer 13, the high-speed camera and the light source 12 and the DIC software.

[0052] At least two positions of the waveguide rod 1 are coaxially pasted with strain gauges 5, the strain gauges 5 form a Wheatstone bridge through rated resistance, the resistance change of the strain gauges 5 caused by stress waves drives the bridge to generate voltage output, which is amplified by the ultra-dynamic strain meter 14 and recorded by the data 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 ultra-long recording time is obtained, which is specifically two-point strain gauge method:

[0054] A stress wave, called a right-traveling wave, is continuously transmitted from a finite-length compression bar OE at point O at time zero. The wavefront passes successively to points A and B, and is reflected back into the compression bar at end face E as a left-traveling wave. The wave signal measured at point A is denoted as ε. A (t), which is the right-traveling wave ε at point A. RA (t) and left-traveling wave ε LA The superposition of (t) is similarly represented by the wave signal measured at point B, denoted as ε. B (t), which is the right-traveling wave ε at point B. RB (t) and left-traveling wave ε LB The superposition of (t).

[0055] The deformation of the end face caused by the force can be separated using the two-point strain gauge method, thereby calculating the force, i.e., the force on the specimen. The right-traveling wave and left-traveling wave signals passing through strain gauge placement 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, Based on the strain signal ε of the right-traveling wave at point A RA The force at end A (the specimen clamping end) can be obtained using the formula (t). The stress in the specimen can be determined, where E b A b A s These are the Young's modulus, cross-sectional area, and cross-sectional area of ​​the waveguide rod, respectively.

[0061] The deformation process of the sample 2 surface is captured by a high-speed camera 12 controlled by a computer 13.

[0062] Before the test, the surface of the sample 2 is made into speckles by a speckle making tool such as a self-painting, a spray pen, etc. which will not fall off under high-speed stretching, and the surface strain field ε(x, t) can be calculated by DIC software in the later period. By setting the gauge length of the dog bone sample 2, the average deformation ε of the sample 2 in the gauge length range can be obtained s (t), and finally the stress σ(t) and the strain ε s (t) are synchronized and frequency-sampled, so as to obtain the stress-strain curve. The strain rate s (t) is obtained by differentiating the strain ε The average strain rate of the sample 2 can be obtained by averaging in a certain time.

[0063] The matters needing attention in use are as follows:

[0064] 1) The effective interval of the stress-strain curve should be that the strain rate fluctuation is not more than 10%, of course, the strain rate is relatively insensitive to the material, and the restriction can be appropriately relaxed;

[0065] 2) In order to make the strain rate basically constant in the loading process, the mass block velocity needs to be basically constant, and the initial kinetic energy E k of the mass block and the deformation energy E p of the sample meet the relationship E k ≥10E p as much as possible;

[0066] 3) Waveguide rod material and size: in order to make the stress wave transmitted into the waveguide rod produce lower particle motion velocity in the rod, so as to not cause the mutation of the loading velocity, a material with high wave impedance such as steel is selected as the waveguide rod, and the diameter is as large as possible without causing three-dimensional dispersion effect of the wave. Although the separation technology of the wave can be used in theory to collect an infinite long stress wave waveform, noise error is inevitably introduced in the recording waveform, and through numerical calculation, error is easily accumulated, so that the longer the time is, the more obvious the distortion is. Therefore, under the condition that the condition allows, a longer waveguide rod is used as much as possible, so that the same recording time can reduce the number of iterations and avoid excessive accumulation of errors.

[0067] The scheme of the present application takes the horizontal waveguide rod 1 as an example, and the waveguide rod 1 can also be placed vertically, and the potential energy of the motor 10 or the mass block 9 is converted into kinetic energy for loading. The stress and strain test method in the present application case is used to obtain the stress-strain curve of the sample.

[0068] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting; it is not intended to exclude myriad other embodiments of the present application that other inventors can develop based on the same general inventive concepts embodied by the described embodiments. That is, although the present application is described in terms of particular embodiments and illustrative figures, it should be apparent that the scope of the present application is not limited to these specific embodiments.

[0069] While the embodiments of the application have been shown and described herein, it is to be understood that the scope of the application, jointly pointed out in the appended claims, is not limited to the details of the embodiments shown, and that various changes can be made and equivalents employed without departing from the intended spirit and scope of the application.

Claims

1. A mid-strain rate material tensile testing system characterized by , comprising: a motor-based power system and control system, a multi-point strain measurement-based waveform separation long pulse width stress wave test system, and a high-speed camera and DIC technology-based strain test system; the motor-based power system and control system comprises: a control system for programming and setting motor movement speed, acceleration and deceleration; a servo motor that moves according to the instructions of the control system and loads force to a fixed rack through the gear of a transmission device; a transmission device for transmitting the driving force of the servo motor to the fixed rack on the linear guide rail to drive the motor platform to move; a linear guide rail that enables the motor platform to keep linear movement; a mass block that collides with the sample clamping end based on the driving of the motor platform to stretch the sample; the multi-point strain measurement-based waveform separation long pulse width stress wave test system comprises: a waveguide rod that is 2-4 m long, one end of the waveguide rod penetrating through a through hole on the mass block; a strain gauge that is pasted on the surface of the waveguide rod for measuring the axial strain waveform when the stress wave arrives; a signal test storage system for storing the measured waveform data; a waveguide rod support seat for supporting and adjusting the waveguide rod to keep the waveguide rod horizontal and in a straight line; a sample clamping anvil head supported by an anvil head support seat and horizontally coaxially installed with the waveguide rod; a sample with both ends connected to the waveguide rod and the sample clamping anvil head, respectively; an energy absorption device for absorbing the residual kinetic energy of the anvil head and the impact head to slow down to a standstill; the high-speed camera and DIC technology-based strain test system captures the deformation process of the sample surface in real time through a high-speed camera, calculates the deformation history and strain rate history of the sample through DIC software, and obtains the stress-strain curve of the sample by synchronizing and resampling the stress history curve and the strain history curve in time; the measurement of the axial strain waveform adopts a multi-point strain waveform separation technology, specifically a two-point strain gauge method: A finite length compression rod OE is continuously transmitted a stress wave at point O at zero time, called right-going wave, the wave front successively passes through point A, point B, and reflects at end surface E back into the compression rod as left-going wave, the wave signal measured at point A is denoted as ε A (t) RA (t) and left-going wave ε LA (t) at point A, and the wave signal measured at point B is denoted as ε B (t) RB (t) and left-going wave ε LB (t) at point B, according to two-point strain gauge method, the deformation of end surface caused by force can be separated, so as to calculate the force, i.e. the force borne by the sample; the right and left traveling wave signals passing through the points A and B of the strain gauge are calculated by the following formulas, respectively: Left traveling 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 ; Right traveling 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 ; Wherein, According to the strain signal ε of the right wave of point A RA (t) to obtain the stress condition of the A end (sample clamping end) and the stress received by the sample Wherein, E b , A b , A s are the Young's modulus, cross-sectional area of the waveguide rod and cross-sectional area of the sample, respectively.

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

3. The intermediate strain rate material tensile testing system of claim 1, wherein: The signal test storage system comprises an ultra-dynamic strain gauge, a data acquisition / oscilloscope, and a computer. The electrical signal generated by the Wheatstone bridge is amplified by the ultra-dynamic strain gauge and collected by the data acquisition or oscilloscope and stored in the computer.

4. The intermediate strain rate material tensile testing system of claim 1, wherein: The process of the sample tensile test is as follows: after the impact end of the mass block impacts the sample clamping anvil head through the sample, the sample is stretched at an approximate uniform speed. The stress generated during the stretching of the sample propagates to the waveguide rod, causing the strain gauge coaxially pasted on the surface of the rod to deform and be recorded by the signal test storage system.

5. The intermediate strain rate material tensile testing system of claim 1, wherein: The waveguide rod support seat is provided with a plurality of waveguide rod support seats, which are fixed on the platform by bolts and connected with the waveguide rod through linear bearings or polytetrafluoroethylene rings to reduce the friction between the waveguide rod and the waveguide rod support seat.

6. A mid-strain rate material tensile testing system according to claim 1, wherein: The sample is fixed between the waveguide rod and the sample clamping anvil head by threads or high-strength glue, and the outer diameter of the sample clamping anvil head is larger than the diameter of the waveguide rod.

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