CN108344649B - Dynamic biaxial tension loading device and experimental method - Google Patents
Dynamic biaxial tension loading device and experimental method Download PDFInfo
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
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Abstract
The invention relates to a device and a method for testing mechanical properties of a material under dynamic biaxial bidirectional loading, in particular to a dynamic biaxial bidirectional stretching loading device and an experimental method. The invention discloses a dynamic biaxial tension loading device which comprises a control module, a main circuit charge-discharge module, a capacitor bank module and a loading platform module, wherein the loading platform module comprises four identical loading guns and four waveguide rods with the same length, the four identical loading guns load stress waves on a sample from four directions, the transverse loading and the longitudinal loading are arranged vertically to each other, and the amplitude and the pulse width of the stress waves generated in the same axial direction are the same, so that the wave form and the time error generated in the stress wave propagation process are reduced, meanwhile, the sample is ensured to be in a biaxial stress state, the stress state in the sample is also distributed symmetrically, and the influence of shear stress components in the loading process is reduced. The invention can overcome the problem that the synchronous loading of the biaxial bidirectional stress wave can not be realized in the prior art.
Description
Technical Field
The invention relates to a device and a method for testing mechanical properties of a material under dynamic biaxial bidirectional loading, in particular to a dynamic biaxial bidirectional tension loading device and an experimental method, and specifically relates to a biaxial bidirectional separation type Hopkinson pull rod experimental device and an experimental method based on electromagnetic force loading.
Background
The separated Hopkinson bar experiment technology is a main experiment method for researching the mechanical property of a material under medium and high strain rates. The basic principle of this method is: a short sample is placed between two pull rods or compression rods, tensile stress waves or compression stress waves are input to an incident rod in a certain mode, and the sample is loaded. While recording pulse signals by means of strain gauges glued to the pull or pressure rods at a distance from the rod ends. If the rod or strut remains in an elastic state, the pulse in the rod will propagate undistorted at the elastic wave speed. Thus, the strain gauge adhered to the pull rod or the pressure rod can measure the time-varying course of the load acting on the rod end.
For a split hopkinson pull rod or a compression rod, a common way to generate incident waves is to launch a striker rod at high speed through an air gun and generate incident pulses through coaxial impact with the striker rod. The loading wave generated by the direct impact loading mode is approximate to a square wave, the rising front edge is short, high-frequency components caused by direct collision are superposed on the wave head, and improvement needs to be carried out through an incident wave shaping technology. According to the one-dimensional stress wave theory, the mechanical behavior of the material with high strain rate in a uniaxial stress state can be obtained.
However, the structure and the material are subjected to complicated loads in the service process and are often in a multi-axis stress state. At present, the Hopkinson bar experiment technology is mainly used for measuring the uniaxial tension-compression behavior of materials. The incident pulse is generated by adopting a mechanical collision mode, stress waves in different loading directions cannot be consistent no matter the amplitude and the pulse width of the waveform or the time for loading the stress waves by reaching the end face of the sample, and therefore the dynamic biaxial bidirectional loading experimental technology is difficult to realize.
In the 20 th century and the 60 th century, in order to solve the problems of the conventional riveting, the electromagnetic riveting technology was first studied by Huber A Schmitt et al, and a patent of a high-impact electromagnetic riveting device was applied in 1968 (U.S. Pat. No. 3961739, 1974, 5/7). Low-voltage electromagnetic riveting technology (European patent: 0293257, 27.5.1988) was successfully developed by Peter B.Zieve in 1986, and the problems of high-voltage riveting in the aspects of riveting quality and popularization and application are solved, so that the electromagnetic riveting technology is rapidly developed. Electromagnetic riveting technology has been applied in the manufacture of airplanes in the boeing, air passenger series. The principle of the electromagnetic riveting technology is as follows: a coil and a stress wave amplifier are added between the discharge coil and the workpiece. At the instant when the discharge switch is closed, a strong magnetic field is generated around the coil by the rapidly changing impact current in the main coil. The secondary coil coupled with the main coil generates induced current under the action of a strong magnetic field, so that an eddy magnetic field is generated, eddy repulsion force is generated by the interaction of the two magnetic fields and is transmitted to the rivet through the amplifier, and the rivet is formed. The eddy current forces are at very high frequencies and propagate in the form of stress waves in the amplifier and rivet, so electromagnetic riveting is also known as stress wave riveting.
If the principle of the electromagnetic riveter is applied to the split Hopkinson bar to replace an air gun and a striking bar in the traditional split Hopkinson bar, the amplitude and the pulse width of the stress wave pulse generated by the electromagnetic repulsion can be controlled by adjusting circuit parameters. In chinese patent applications with application numbers 201420098605.4 and 201410161610.X, an equipment scheme and an experimental method for directly applying an electromagnetic riveting device in a hopkinson pressure bar device are respectively proposed, but the waveform obtained by the method has limitations. In two Chinese patent application inventions with application numbers of 201410173843.1 and 201410171963.8, two experimental devices which can be used for a Hopkinson pull rod and a Hopkinson pressure rod and a using method thereof are respectively provided, but the two schemes are complex in structure, and the traditional wave shaping technology cannot be applied to the stretching condition. In the invention of the chinese patent application No. 201510956545.4, a new loading gun structure is proposed that can generate both tension and compression waves and shape the wave using conventional shaping means. In the invention of the chinese patent application with application number 201510051071, a main coil structure and a using method of an electromagnetic experimental apparatus are proposed to improve the variation range of the amplitude and the pulse width generated by the electromagnetic experimental apparatus. In the chinese invention patent with publication number CN104678853a, a capacitance charge-discharge control system is proposed, which is used for a single-shaft electromagnetic hopkinson pressure bar, and the capacitance of the system is fixed, and when in use, the capacitor needs to be disassembled. In practice, due to the reaction time difference of the PLC control system of the experimental equipment, the discharge delay of 20ms-30ms is generated when the discharge controllable silicon is triggered. In addition, each experimental part, such as a coil, a waveguide rod and the like, must have certain errors during processing and assembling, so that the double-shaft bidirectional synchronous loading of the Hopkinson bar is still difficult to realize. In chinese patent with patent publication No. CN104677760B, a method for implementing an equivalent loading of incident waves in a biaxial hopkinson pressure bar and pull bar experiment is proposed, but the method has certain defects, dynamic biaxial loading cannot be implemented by a mode of generating two rows of incident waves by only using two incident wave generators, and time synchronization of different axial stress waves is not solved yet.
Disclosure of Invention
The invention aims to provide a dynamic biaxial tension loading device, in particular to an electromagnetic induction type biaxial loading separated Hopkinson pull rod loading device and an experimental method, which can overcome the problem that the biaxial stress wave synchronous loading can not be realized in the prior art,
the invention discloses a dynamic biaxial stretching loading device which comprises a control module, a main circuit charge-discharge module, a capacitor bank module and a loading platform module, wherein the loading platform module comprises four identical loading guns.
In the dynamic biaxial tension loading device, the loading platform module further comprises four waveguide rods with the same length, the four same loading guns load stress waves on the sample from four directions, the transverse loading and the longitudinal loading are arranged vertically, and the amplitude value and the pulse width of the stress waves generated in the same axial direction are the same. The capacitor bank module includes a lateral capacitor bank and a vertical capacitor bank. The control module comprises a circuit board, a programmable controller, a synchronous transformer, a pulse transformer, an electromagnetic relay and a delay signal generator, and is a weak current part of a circuit system, the control system of the control module controls two sets of horizontal and vertical capacitor charging and discharging systems simultaneously by the programmable controller, and is additionally provided with a delay control unit, and the control module adopts the delay signal generator to realize synchronization of loading waves in different directions in time.
In the dynamic biaxial stretching loading device, the control module adopts a digital signal delay pulse generator and is used for generating four independent pulse signals to trigger the silicon controlled rectifier to carry out capacitor charging. The main circuit charging and discharging module comprises a charging circuit and a discharging circuit, is composed of a transformer, a current limiting resistor, a filter inductor, a bleeder resistor, a vacuum contactor and a current/voltage sensor and is used for charging and discharging energy stored in the pulse capacitor bank, and the charging circuit is used for boosting input voltage to required charging voltage and charging the capacitor bank; the discharging circuit is used for triggering the capacitor group to discharge the discharging coil instantly and carry out electromagnetic loading. The capacitor bank module is composed of a capacitor bank and a discharge controllable silicon and is used for discharging of the transverse loading gun and the longitudinal loading gun respectively, the capacitor bank module adopts a gradient capacitor bank and is used for changing the pulse width of stress waves generated in the discharging and loading process, the capacitance of each gear is fixed, and the selection is directly carried out through the control module.
The control module comprises a circuit board, a programmable controller, a synchronous transformer, a pulse transformer, an electromagnetic relay, a time delay signal generator and the like, and is a weak current part of the circuit system. The circuit improves a control system provided by Chinese patent publication No. CN104678853A, a programmable controller simultaneously controls two sets of transverse and longitudinal capacitor charging and discharging systems, and a delay control unit is added. Because the reaction time difference exists in the PLC control system of the experimental equipment, and the discharge delay of 20ms-30ms exists in the trigger of the discharge controllable silicon. The time error causes that the carrier waves in all directions successively reach the sample to load the sample, the sample generates local deformation in the direction of the previous loading, and the stress strain state is no longer a biaxial stress state. The control module adopts a delay signal generator to realize synchronization of loading waves in different directions in time, and the digital signal delay pulse generator is used for generating four independent pulse signals to trigger the silicon controlled rectifier to charge the capacitor.
The main circuit charging and discharging module comprises a charging circuit and a discharging circuit, is composed of a transformer, a current limiting resistor, a filter inductor, a bleeder resistor, a vacuum contactor, a current/voltage sensor and the like, and is used for charging and discharging energy stored in the pulse capacitor bank. The charging circuit is used for boosting the input voltage to the required charging voltage and charging the capacitor bank; the discharging circuit is used for triggering the capacitor group to discharge the discharging coil instantly and carry out electromagnetic loading.
The capacitor bank module comprises a transverse capacitor bank and a longitudinal capacitor bank, mainly comprises the capacitor bank, a discharge controllable silicon and the like, and is respectively used for discharging of the transverse loading gun and the longitudinal loading gun. The module adopts the gradient capacitor bank for change the pulse width of the stress wave that the loading in-process that discharges produced, every gear capacitance is fixed, directly selects through control module, avoids dismantling the change condenser repeatedly in the experimentation in order to change the electric capacity size. The capacitor bank and the discharging controllable silicon are installed in the capacitor cabinet, and discharging current is output to the main coil of the loading gun.
The loading platform module comprises four identical loading guns and four waveguide rods with identical lengths, so that the amplitude and the pulse width of incident waves generated by the two loading guns in the same axial direction are kept consistent, and the waveform and time errors generated in the stress wave propagation process are reduced. Meanwhile, the biaxial dynamic loading needs to ensure that the sample is in a biaxial stress state, so that four identical loading guns and waveguide rods are adopted, the transverse direction and the longitudinal direction of the sample can be symmetrically loaded respectively, the stress state in the sample is correspondingly and symmetrically distributed, and the influence of shear stress components in the loading process is reduced. The transverse equipment and the longitudinal equipment are arranged on the same horizontal plane and are coaxially and symmetrically arranged according to the sequence of the loading gun, the waveguide rod and the loading gun, the loading guns are respectively positioned at the incident ends of the waveguide rods, and a sample to be tested is placed between the two coaxial waveguide rods. For tensile loading, the incident end of the waveguide rod is provided with an external thread connected with a buffer or a boss, the design of the boss is the same as that of the boss of the Hopkinson pull rod, and the other end of the waveguide rod is provided with an internal thread or a groove connected with a sample. Two strain gauges are symmetrically adhered to the circumferential surface of the middle part of each wave guide rod, and strain on each guide rod is converted into an electric signal which is transmitted to a data acquisition unit through a lead.
Further, the invention provides an experimental method of the dynamic biaxial tension loading device, which comprises the following steps:
step 1, arranging equipment:
the loading gun and the waveguide rods are coaxially and sequentially arranged on the experiment table, and each waveguide rod can freely move only in the axial direction; the wave guide rod is connected with the stress wave amplifier through threads, and the position sequence of the main coil, the secondary coil and the stress wave amplifier is set according to the tensile and compression experiment types; the longitudinal loading device and the transverse loading device are mutually vertical and are arranged in the same sequence; installing a tensile sample among the four waveguide rods, and enabling the axial direction of the sample to be coaxial with the waveguide rods; on the circumference of half length of each waveguide rod, symmetrically adhering two strain gauges with completely same parameters on the surface of the waveguide rod along the rod axis, and connecting the strain gauges to a Wheatstone bridge of a data acquisition system through a lead;
starting an experiment system control module, and setting experiment parameters through a touch screen, wherein the experiment parameters comprise the capacitance of a pulse capacitor bank, a charging voltage value and the delay time of a synchronous signal; selecting capacitance values of the transverse and longitudinal capacitor banks according to the corresponding relation between the pulse width and amplitude of the loading wave required by the experiment and the parameters of the capacitor and the voltage, inputting the voltage value corresponding to each experiment, and charging the transverse and longitudinal capacitor banks; adjusting the delay time of the synchronous signal of the experimental system to ensure that each axial stress wave can reach the sample at the same time;
step 3, charging the pulse capacitor bank
After the parameter setting is finished, starting a charging option of the control module, enabling the main circuit charging and discharging module to work, and charging the pulse capacitor bank; the charging is automatically stopped after the set charging voltage is reached, and the charging voltage of the pulse capacitor bank is not increased any more;
and 4, discharging and loading the capacitor bank:
after the capacitor is charged, starting a discharge switch to discharge the capacitor group to the main coil of each loading gun; when the discharge current flows through the transverse main coil and the longitudinal main coil, strong electromagnetic repulsion force is generated between the transverse secondary coil and the transverse main coil as well as between the longitudinal secondary coil and the longitudinal main coil due to electromagnetic induction; the electromagnetic repulsion is amplified in the amplifier and is expressed as compression stress wave, and the compression stress wave is reflected into tensile wave on the boss and forms tensile incident wave which is transmitted to the sample from the rod end; when the tensile incident wave is transmitted to the contact surface of the waveguide rod and the sample, one part of the tensile incident wave is reflected due to the mismatching of wave impedance, a reflected wave is formed in the waveguide rod, the other part of the tensile incident wave is transmitted into the other coaxial waveguide rod through the sample to form a transmitted wave, and the transmitted wave and the reflected wave formed on the coaxial waveguide rod are mutually superposed;
and 5, acquiring and processing experimental data:
strain gauges on the wave guide rods respectively convert strain changes on the four rods into resistance changes, and further convert the resistance changes into output voltage changes of two bridge arms of a Wheatstone half bridge, and the voltage changes are input into a data acquisition unit through two conventional shielding signal wires; according to the Wheatstone bridge formula, the strain signal of the waveguide rod can be calculated as
＝2ΔU/(U0-ΔU)/k (1)
Wherein, being strain signals, U0The voltage is the power supply voltage of the Wheatstone half bridge, k is the sensitivity coefficient of the strain gauge, and delta U is the change value of the bridge arm voltage of the Wheatstone half bridge along with time;
in the experiment loading process, the strain gauge collects incident waves generated by a loading gun; after the reflected waves are generated on the end face of the sample, the reflected superposed waves are transmitted back to the strain gauge and are collected; because the transverse loading and the longitudinal loading are vertical to each other, the interference of stress waves between the transverse loading and the longitudinal loading is ignored, the stress inside the transverse/longitudinal sample can be solved by utilizing the incident wave signal and the reflected wave-transmitted wave superposed signal recorded by the data acquisition unit and adopting the one-dimensional stress wave theory
Wherein σsThe internal transverse/longitudinal stress of the sample, E the modulus of elasticity of the waveguide rod, A the cross-sectional area of the waveguide rod, AsThe cross-sectional area of the sample corresponding to the loading direction,i1for incident wave signals on a waveguide rod，r1For the reflected wave signal reflected on the waveguide rod,t2is the transmitted wave signal on the waveguide rod.
The specific process of the biaxial bidirectional electromagnetic Hopkinson bar stretching experiment is as follows:
step 1, arranging equipment.
The loading gun and the waveguide rod are coaxially and sequentially mounted on a laboratory bench, and each waveguide rod can freely move only in the axial direction. The wave guide rod is connected with the stress wave amplifier through threads, and the position sequence of the main coil, the secondary coil and the stress wave amplifier is set according to the tensile and compression experiment types. The longitudinal and transverse loading devices are perpendicular to each other and arranged in the same order. And installing a tensile sample between the four waveguide rods, and keeping the axial direction of the sample coaxial with the waveguide rods. The tensile sample adopts a cross-shaped flat plate sample with a slot. The slots are machined in the clamping arms of the sample to make the stress distribution in the test area of the sample uniform during loading, so that the strain signal on the waveguide rod can be used for deducing and representing the stress in the sample. The cross-shaped sample central test zone is a square area of relatively small thickness for concentrating the deformation of the sample within this area.
The method for sticking the strain gauges adopts the prior art, namely, two strain gauges with completely the same parameters are symmetrically stuck on the surface of each waveguide rod along the rod axis on the circumference of the half length of each waveguide rod and are connected to a Wheatstone bridge of a data acquisition system through a lead.
And 2, setting experimental parameters.
And starting an experiment system control module, and setting experiment parameters through a touch screen, wherein the experiment parameters comprise the capacitance, the charging voltage value and the synchronous signal delay time of the pulse capacitor bank. And selecting capacitance values of the transverse and longitudinal capacitor banks according to the corresponding relation between the pulse width and amplitude of the loading wave required by the experiment and the parameters of the capacitor and the voltage, inputting the voltage value corresponding to each experiment, and charging the transverse and longitudinal capacitor banks. And adjusting the delay time of the synchronous signal of the experimental system to ensure that each axial stress wave can reach the sample at the same time.
Step 3, charging the pulse capacitor bank
After the parameter setting is finished, the charging option of the control module is started, and the main circuit charging and discharging module works to charge the pulse capacitor bank. And the charging is automatically stopped after the set charging voltage is reached, and the charging voltage of the pulse capacitor bank is not increased.
Step 4, discharging and loading of capacitor bank
And after the capacitor is charged, starting a discharge switch to discharge the capacitor group to the main coil of each loading gun. When discharge current flows through the transverse main coil and the longitudinal main coil, strong electromagnetic repulsion force is generated between the transverse secondary coil and the transverse main coil as well as between the longitudinal secondary coil and the longitudinal main coil due to electromagnetic induction. The electromagnetic repulsion is amplified in the amplifier and is expressed as compression stress wave, and the compression stress wave is reflected into tensile wave on the boss and forms tensile incident wave to be transmitted to the sample from the rod end. When the tensile incident wave is transmitted to the contact surface of the waveguide rod and the sample, one part of the tensile incident wave is reflected due to the mismatching of wave impedance, a reflected wave is formed in the waveguide rod, the other part of the tensile incident wave is transmitted into the other coaxial waveguide rod through the sample to form a transmitted wave, and the transmitted wave and the formed reflected wave on the coaxial waveguide rod are mutually overlapped. The shape and amplitude of the reflected and transmitted waves are determined by the properties of the sample material.
The strain gauges on the wave guide rod respectively convert strain changes on the four rods into resistance changes, and further convert the resistance changes into changes of output voltages of two bridge arms of a Wheatstone half bridge, and the voltage changes are input into a data acquisition unit through two conventional shielding signal wires. According to the Wheatstone bridge formula, the strain signal of the waveguide rod can be calculated as
＝2ΔU/(U0-ΔU)/k (1)
Wherein, being strain signals, U0The voltage is the power supply voltage of the Wheatstone half bridge, k is the sensitivity coefficient of the strain gauge, and delta U is the change value of the bridge arm voltage of the Wheatstone half bridge along with time.
In the experiment loading process, the strain gauge collects incident waves generated by a loading gun. After the reflected wave is generated on the end face of the sample, the reflected superposed wave is transmitted back to the strain gauge and is collected. Because the transverse loading and the longitudinal loading are vertical to each other, the interference of stress waves between the transverse loading and the longitudinal loading is ignored, the stress inside the transverse/longitudinal sample can be solved by utilizing the incident wave signal and the reflected wave-transmitted wave superposed signal recorded by the data acquisition unit and adopting the one-dimensional stress wave theory
Wherein σsThe internal transverse/longitudinal stress of the sample, E the modulus of elasticity of the waveguide rod, A the cross-sectional area of the waveguide rod, AsThe cross-sectional area of the sample corresponding to the loading direction,i1for an incident wave signal on a certain waveguide rod,r1for the reflected wave signal reflected on the waveguide rod,t2is the transmitted wave signal on the waveguide rod.
For the strain in the sample, a strain gage is pasted on the surface of the sample to directly measure the strain in the sample; or using a high-speed camera and employing DIC techniques to calculate the strain inside the sample.
The result curve of the biaxial stretching test of the sample can be obtained through conventional data processing: according to the sample stress calculated by the formula (2) and the measured sample strain, drawing a transverse strain of the sample as an x axis and a transverse stress as a y axis to obtain a stress-strain curve of the sample under transverse loading; and drawing the longitudinal strain of the sample as an x-axis and the longitudinal stress as a y-axis to obtain a stress-strain curve of the longitudinal loading of the sample.
The invention adopts an electromagnetic loading technology, reasonably designs a transverse and longitudinal loading stress wave loading system, and adopts four stress wave loading guns to realize the biaxial bidirectional dynamic tension loading of the electromagnetic Hopkinson bar. The experimental operation is simple, stress waves with expected pulse amplitudes and widths can be obtained in different loading directions through selection of experimental parameters, and the controllability is strong. The invention achieves two breakthrough improvements. Firstly, on the aspect of a loading device, four electromagnetic loading guns are adopted to load stress waves on a sample from four directions, transverse loading and longitudinal loading are arranged in a mutually perpendicular mode, and the amplitude value and the pulse width of the stress waves generated in the same axial direction are the same; the digital signal delay generator is adopted, so that the time difference of stress waves in different directions of the biaxial bidirectional Hopkinson bar, particularly the time difference between transverse loading and longitudinal loading is basically eliminated, the problem that synchronous loading cannot be realized is solved, and the time interval error of the biaxial bidirectional Hopkinson bar in different directions plus carrier waves is ensured to be less than 1 microsecond, so that biaxial bidirectional dynamic stretching/compressing loading of the material is realized. Secondly, in the aspect of obtaining an experimental result, a reasonable sample configuration is adopted to enable the stress state in the experimental process to be in a biaxial stress state with relatively uniform distribution, a stress calculation formula of a sample testing section is deduced through a stress wave theory, and a transverse stress-strain curve and a longitudinal stress-strain curve under the biaxial condition can be obtained by combining strain measurement.
Drawings
Fig. 1 is a schematic view of a biaxial loading electromagnetic hopkinson bar stretching device (dynamic biaxial stretching loading device) of the present invention.
Detailed Description
Fig. 1 is a schematic view of a biaxial loading electromagnetic hopkinson bar stretching device (dynamic biaxial stretching loading device) of the present invention. In fig. 1: 1. a control module; 2. a main circuit charge-discharge module; 3. a lateral capacitor bank; 4. a vertical capacitor bank; 5. a data acquisition unit; 6. a transverse loading gun; 7. a longitudinal loading gun; 8. a strain gauge; 9. a waveguide rod; 10. and (4) sampling.
As shown in the figure, the dynamic biaxial stretching loading device comprises a control module 1, a main circuit charging and discharging module 2, a capacitor bank module (a transverse capacitor bank 3 and a longitudinal capacitor bank 4) and a loading platform module (a transverse loading gun 6, a longitudinal loading gun 7 and a waveguide rod 9), wherein the loading platform module comprises four identical loading guns. As shown in the figure, the transverse loading gun 6 comprises two identical loading guns, and the longitudinal loading gun 7 also comprises two identical loading guns, for a total of four identical loading guns.
In the dynamic biaxial tension loading device, the loading platform module further comprises four waveguide rods 9 with the same length, the four same loading guns load stress waves on the sample 10 from four directions, the transverse loading and the longitudinal loading are arranged vertically, and the amplitude value and the pulse width of the stress waves generated in the same axial direction are the same. The capacitor bank module comprises a transversal capacitor bank 3 and a longitudinal capacitor bank 4. The control module 1 comprises a circuit board, a programmable controller, a synchronous transformer, a pulse transformer, an electromagnetic relay and a delay signal generator, and is a weak current part of a circuit system, the control system of the control module controls two sets of horizontal and vertical capacitor charging and discharging systems simultaneously by the programmable controller, and is additionally provided with a delay control unit, and the control module adopts the delay signal generator to realize synchronization of loading waves in different directions in time.
In the above dynamic biaxial stretching loading device, the control module 1 may adopt a digital signal delay pulse generator for generating four independent pulse signals to trigger the silicon controlled rectifier to perform capacitance charging. The main circuit charging and discharging module 2 comprises a charging circuit and a discharging circuit, is composed of a transformer, a current limiting resistor, a filter inductor, a bleeder resistor, a vacuum contactor and a current/voltage sensor, and is used for charging and discharging energy stored in a pulse capacitor bank, and the charging circuit is used for boosting input voltage to required charging voltage and charging the capacitor bank; the discharging circuit is used for triggering the capacitor group to discharge the discharging coil instantly and carry out electromagnetic loading. The capacitor bank module is composed of a capacitor bank and a discharge controllable silicon and is used for discharging of the transverse loading gun and the longitudinal loading gun respectively, the capacitor bank module adopts a gradient capacitor bank and is used for changing the pulse width of stress waves generated in the discharging and loading process, the capacitance of each gear is fixed, and the selection is directly carried out through the control module.
Example (b):
the embodiment is an electromagnetic induction type double-shaft bidirectional loading split Hopkinson pull rod experiment device, which comprises a control module 1, a main circuit charging and discharging module 2, a transverse capacitor bank 3, a longitudinal capacitor bank 4 and a loading platform module.
In this embodiment, the control module mainly includes a circuit board, a PLC, a synchronous transformer, a pulse transformer, an electromagnetic relay, a delay signal generator, and the like. Siemens S7-200SMART series PLC and Siemens SMART 1000IE touch screen are used as the core of the loading control module and are used for realizing the control of the whole electromagnetic loading working flow. The delay signal generator adopts a DG645 digital delay pulse generator of Stanford Research Systems company, and respectively triggers the discharge loops corresponding to the loading guns according to delay time setting.
The main circuit charge-discharge module mainly comprises a charge circuit and a discharge circuit and is used for charging and discharging the capacitor bank. In the charging circuit, a transformer can boost the voltage of 380V to the maximum 3000V, the capacitor bank is charged through rectification, and the charging circuit stops charging when the voltage of the capacitor bank reaches a set voltage value. In the discharging circuit, a delay signal pulse triggers the vacuum contactor to be conducted, and the capacitor group instantly discharges the discharging coil to generate electromagnetic force. The circuit can control the amplitude of the stress wave in a large range by setting a charging voltage value.
The capacitor bank module comprises a transverse capacitor combined longitudinal capacitor bank which is respectively used for discharging and loading the transverse loading gun and the longitudinal loading gun. In this example, there are 8 pulse capacitors and 5 discharge thyristors in each of the lateral and longitudinal capacitor banks. The rated voltage of the pulse capacitor is 4000 volts (V), the rated capacitance is 2000 microfarads (mu F), and five gradients of 0.667 millifarads (mF), 1 millifarads, 2 millifarads, 4 millifarads and 6 millifarads are combined in series and parallel to meet the requirements of different pulse width amplitudes and carrier waves. The capacitance of each gear is fixed, and the selection is directly performed through the control module, so that the situation that the capacitance is changed by repeatedly replacing the connection mode of the capacitor in the experiment process is avoided. The longitudinal and transverse loading lines are arranged by adopting the same components, the capacitor is controlled to discharge through the discharge controllable silicon, the capacitor bank and the discharge controllable silicon are installed in the capacitor cabinet, and the discharge current is output to the main coil of the loading gun.
The loading platform module mainly comprises a loading gun, a waveguide rod 9 and a data acquisition unit 5. The loading gun is mainly composed of a main coil and a secondary coil, and can realize stretching and compression loading. In this embodiment, the waveguide rods 9 used in the loading platform are all titanium alloy rods with a diameter of 15 millimeters (mm), one end of each titanium alloy rod is provided with a 10mm long thread and connected with a loading gun, and the other end of each titanium alloy rod is processed into a 7mm deep groove embedded sample 10. The sample adopts a cross flat plate sample with a slot, the material is pure copper, the thickness of the sample is 3mm, and the width of the clamping arm is 15 mm. The GEN3i manufactured by Germany HBM company is adopted as the data acquisition unit 5, the data acquisition unit has good interference shielding capability, and the pulsed magnetic field interference generated in the discharging process can be shielded by adopting a difference method.
The experimental process of the double-shaft bidirectional electromagnetic Hopkinson bar loading device is as follows:
step 1, arranging equipment.
The transverse loading gun 6 and the waveguide rod, and the longitudinal loading gun 7 and the waveguide rod are respectively installed on the experiment table according to the coaxial sequence of the loading gun-the waveguide rod-the loading gun, and the waveguide rods can freely move only in the axial direction. The biaxial tension specimen 10 was placed in the groove of each waveguide rod with the specimen axis coaxial with the waveguide rod and fixed by gluing. The strain gage 8 is attached using known techniques, i.e., circumferentially at 1/2 lengths of the wave guides in the transverse and longitudinal directions. When the strain gauge is pasted, the axes of the incident rod and the transmission rod are taken as symmetry axes, two strain gauges with the same parameters are symmetrically pasted on the surface of the waveguide rod, and the strain gauge with the resistance value of 1000 ohms and the sensitivity coefficient of 2.0 is adopted in the embodiment; and welding lead wires of the strain gauge on pins of the strain gauge, and respectively connecting the strain gauge into two opposite bridge arms of the Wheatstone half bridge through the lead wires. The fixed resistors on the other two legs of the wheatstone half bridge are both 1000 ohms. The supply voltage of the wheatstone half bridge is 30 volts dc. The two diagonal voltages of the wheatstone half bridge are input to the data collector 5 through two conventional single-core shielded signal lines.
And 2, setting experimental parameters.
And starting the control module of the experiment system, and setting experiment parameters through the touch screen. According to the experimental use of loading pulse width, respectively selecting the capacitance of the capacitor bank in the transverse and longitudinal loading circuits as a mF and b mF, wherein a and b are respectively required capacitance amplitude gears; the amplitude of the applied carrier wave is used experimentally to input the desired charging voltage value x V, where x is the desired voltage value and is within the rated voltage of the pulsed capacitor. The arrangement can ensure that two rows of loading waves obtained in the same axial direction have consistent waveforms. And meanwhile, the delay time of the digital delay signal generator is set according to equipment debugging, the time delay of the direction of the stress wave generated firstly is set to be t microseconds (mu s), and t is the time difference between the stress wave generated firstly and the stress wave generated by relative delay during experimental debugging, so that the stress waves of all loading guns are ensured to generate simultaneously to carry out biaxial bidirectional loading on the sample.
Step 3, charging the pulse capacitor bank
After the parameter setting is finished, the charging option of the control module is started, and the main circuit charging and discharging module works to charge the pulse capacitor bank. And the charging is automatically stopped after the set charging voltage is reached, and the charging voltage of the pulse capacitor bank is not increased.
Step 4, discharging and loading of capacitor bank
And after the capacitor is charged, starting a discharge switch to discharge the capacitor group to the main coil of each loading gun. When discharge current flows through the main coils of the transverse loading gun and the longitudinal loading gun, extremely strong electromagnetic repulsion force is generated between the transverse secondary coil and the transverse main coil as well as between the longitudinal secondary coil and the longitudinal main coil due to electromagnetic induction. Because the capacitor bank has short discharge time and strong discharge current, the electromagnetic repulsion generated at the moment forms an incident stress wave with short duration and large amplitude at the input ends of the transverse stress amplifier and the longitudinal stress amplifier. And the transverse stress wave and the longitudinal stress wave are respectively propagated to the sample from the far end of the waveguide rod, and simultaneously reach the end face of the sample to load the sample. When the tensile incident wave is transmitted to the contact surface of the waveguide rod and the sample, one part of the tensile incident wave is reflected due to the mismatching of wave impedance, a reflected wave is formed in the waveguide rod, the other part of the tensile incident wave is transmitted into the other coaxial waveguide rod through the sample to form a transmitted wave, and the transmitted wave and the formed reflected wave on the coaxial waveguide rod are mutually overlapped. The shape and amplitude of the reflected and transmitted waves are determined by the properties of the sample material.
The strain gauges on the wave guide rod respectively convert strain changes on the four rods into resistance changes, and further convert the resistance changes into changes of output voltages of two bridge arms of a Wheatstone half bridge, and the voltage changes are input into a data acquisition unit through two conventional shielding signal wires. According to the Wheatstone bridge formula, the strain signal of the waveguide rod can be calculated as
＝2ΔU/(U0-ΔU)/k (1)
Wherein, being strain signals, U0The voltage is the power supply voltage of the Wheatstone half bridge, k is the sensitivity coefficient of the strain gauge, and delta U is the change value of the bridge arm voltage of the Wheatstone half bridge along with time.
In the experiment loading process, the strain gauge collects incident waves generated by a loading gun. After the reflected wave is generated on the end face of the sample, the reflected superposed wave is transmitted back to the strain gauge and is collected. Because the transverse loading and the longitudinal loading are vertical to each other, the interference of stress waves between the transverse loading and the longitudinal loading is ignored, the stress inside the transverse/longitudinal sample can be solved by utilizing the incident wave signal and the reflected wave-transmitted wave superposed signal recorded by the data acquisition unit and adopting the one-dimensional stress wave theory
Wherein σsThe internal transverse/longitudinal stress of the sample, E the modulus of elasticity of the waveguide rod, A the cross-sectional area of the waveguide rod, AsThe cross-sectional area of the sample corresponding to the loading direction,i1for an incident wave signal on a certain waveguide rod,r1for the reflected wave signal reflected on the waveguide rod,t2is the transmitted wave signal on the waveguide rod.
For the strain in the sample, a strain gage is pasted on the surface of the sample to directly measure the strain in the sample; or using a high-speed camera and employing DIC techniques to calculate the strain inside the sample.
The result curve of the biaxial stretching test of the sample can be obtained through conventional data processing: according to the sample stress calculated by the formula (2) and the measured sample strain, drawing a transverse strain of the sample as an x axis and a transverse stress as a y axis to obtain a stress-strain curve of the sample under transverse loading; and drawing the longitudinal strain of the sample as an x-axis and the longitudinal stress as a y-axis to obtain a stress-strain curve of the longitudinal loading of the sample.
The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention.
Claims (8)
1. A dynamic biaxial stretching loading device comprises a control module, a main circuit charge-discharge module, a capacitor bank module and a loading platform module, wherein the loading platform module comprises four same loading guns;
the loading platform module also comprises four waveguide rods with the same length, a tensile sample is arranged among the four waveguide rods, the axial direction of the sample is coaxial with the waveguide rods, the four identical loading guns are respectively coaxially arranged with the four waveguide rods, the four identical loading guns load stress waves on the sample from four directions, the transverse loading and the longitudinal loading are mutually and vertically arranged, and the amplitude values and the pulse widths of the stress waves generated in the same axial direction are identical;
the capacitor bank module comprises a transverse capacitor bank and a longitudinal capacitor bank;
the control module is characterized in that a programmable controller simultaneously controls two sets of transverse and longitudinal capacitor charging and discharging systems, a delay control unit is additionally arranged, and the control module adopts a delay signal generator to realize synchronization of loading waves in different directions in time.
2. The dynamic biaxial stretching loading device as claimed in claim 1, wherein the control module comprises a circuit board, a programmable controller, a synchronous transformer, a pulse transformer, an electromagnetic relay and a time delay signal generator, which are weak current parts of a circuit system.
3. The dynamic biaxial stretching loading device as claimed in claim 1, wherein the control module employs a digital signal delay pulse generator for generating four independent pulse signals to trigger a silicon controlled rectifier to perform capacitance charging.
4. The dynamic biaxial stretching loading device according to claim 1, wherein the main circuit charge-discharge module comprises a charge circuit and a discharge circuit, and the charge circuit and the discharge circuit are composed of a transformer, a current limiting resistor, a filter inductor, a bleeder resistor, a vacuum contactor and a current/voltage sensor and are used for charging and discharging energy stored in a pulse capacitor bank, and the charge circuit is used for boosting an input voltage to a required charge voltage and charging the capacitor bank; the discharging circuit is used for triggering the capacitor group to discharge the discharging coil instantly and carry out electromagnetic loading.
5. The dynamic biaxial stretching loading device according to claim 1, wherein the capacitor bank module is composed of a capacitor bank and a discharge thyristor, and is respectively used for discharging of a transverse loading gun and a longitudinal loading gun, the capacitor bank module adopts a gradient capacitor bank and is used for changing the pulse width of a stress wave generated in the discharging loading process, and the capacitance of each gear is fixed and is directly selected by the control module.
6. An experimental method of the dynamic biaxial stretching loading device according to the previous claim 1, comprising the steps of:
step 1, arranging equipment:
the loading gun and the waveguide rods are coaxially and sequentially arranged on the experiment table, and each waveguide rod can freely move only in the axial direction; the wave guide rod is connected with the stress wave amplifier through threads, and the position sequence of the main coil, the secondary coil and the stress wave amplifier is set according to the tensile and compression experiment types; the longitudinal loading device and the transverse loading device are mutually vertical and are arranged in the same sequence; installing a tensile sample among the four waveguide rods, and enabling the axial direction of the sample to be coaxial with the waveguide rods; on the circumference of half length of each waveguide rod, symmetrically adhering two strain gauges with completely same parameters on the surface of the waveguide rod along the rod axis, and connecting the strain gauges to a Wheatstone bridge of a data acquisition system through a lead;
step 2, setting experimental parameters:
starting an experiment system control module, and setting experiment parameters through a touch screen, wherein the experiment parameters comprise the capacitance of a pulse capacitor bank, a charging voltage value and the delay time of a synchronous signal; selecting capacitance values of the transverse and longitudinal capacitor banks according to the corresponding relation between the pulse width and amplitude of the loading wave required by the experiment and the parameters of the capacitor and the voltage, inputting the voltage value corresponding to each experiment, and charging the transverse and longitudinal capacitor banks; adjusting the delay time of the synchronous signal of the experimental system to ensure that each axial stress wave can reach the sample at the same time;
step 3, charging the pulse capacitor bank
After the parameter setting is finished, starting a charging option of the control module, enabling the main circuit charging and discharging module to work, and charging the pulse capacitor bank; the charging is automatically stopped after the set charging voltage is reached, and the charging voltage of the pulse capacitor bank is not increased any more;
and 4, discharging and loading the capacitor bank:
after the capacitor is charged, starting a discharge switch to discharge the capacitor group to the main coil of each loading gun; when the discharge current flows through the transverse main coil and the longitudinal main coil, strong electromagnetic repulsion force is generated between the transverse secondary coil and the transverse main coil as well as between the longitudinal secondary coil and the longitudinal main coil due to electromagnetic induction; the electromagnetic repulsion is amplified in the amplifier and is expressed as compression stress wave, and the compression stress wave is reflected into tensile wave on the boss and forms tensile incident wave which is transmitted to the sample from the rod end; when the tensile incident wave is transmitted to the contact surface of the waveguide rod and the sample, one part of the tensile incident wave is reflected due to the mismatching of wave impedance, a reflected wave is formed in the waveguide rod, the other part of the tensile incident wave is transmitted into the other coaxial waveguide rod through the sample to form a transmitted wave, and the transmitted wave and the reflected wave formed on the coaxial waveguide rod are mutually superposed;
and 5, acquiring and processing experimental data:
strain gauges on the wave guide rod respectively convert strain changes on the four rods into resistance changes, and further convert the resistance changes into output voltage changes of two bridge arms of a Wheatstone half bridge, and the voltage changes are input into a data acquisition unit through two shielding signal wires; according to the Wheatstone bridge formula, the strain signal of the waveguide rod can be calculated as
＝2ΔU/(U0-ΔU)/k (1)
Wherein, being strain signals, U0The voltage is the power supply voltage of the Wheatstone half bridge, k is the sensitivity coefficient of the strain gauge, and delta U is the change value of the bridge arm voltage of the Wheatstone half bridge along with time;
in the experiment loading process, the strain gauge collects incident waves generated by a loading gun; after the reflected waves are generated on the end face of the sample, the reflected superposed waves are transmitted back to the strain gauge and are collected; because the transverse loading and the longitudinal loading are vertical to each other, the interference of stress waves between the transverse loading and the longitudinal loading is ignored, the stress inside the transverse/longitudinal sample can be solved by utilizing the incident wave signal and the reflected wave-transmitted wave superposed signal recorded by the data acquisition unit and adopting the one-dimensional stress wave theory
Wherein σsThe internal transverse/longitudinal stress of the sample, E the modulus of elasticity of the waveguide rod, A the cross-sectional area of the waveguide rod, AsThe cross-sectional area of the sample corresponding to the loading direction,i1for an incident wave signal on a certain waveguide rod,r1for the reflected wave signal reflected on the waveguide rod,t2is the transmitted wave signal on the waveguide rod.
7. The method of claim 6, wherein the tensile test specimen is a cross-shaped flat test specimen with a slot; processing the slot on the clamping arm of the sample to enable the stress distribution of the test area of the sample to be uniform in the loading process, so that the stress in the sample can be calculated and represented by using a strain signal on the waveguide rod; the cross-shaped sample central test zone is a square area of relatively small thickness for concentrating the deformation of the sample within this area.
8. The method according to claim 6, wherein for the strain inside the sample, a strain gauge is pasted on the surface of the sample to directly measure the strain inside the sample; or calculating the strain inside the sample by using a high-speed camera and adopting DIC technology;
and (3) obtaining a result curve of the biaxial tension test of the sample through data processing: according to the sample stress calculated by the formula (2) and the measured sample strain, drawing a transverse strain of the sample as an x axis and a transverse stress as a y axis to obtain a stress-strain curve of the sample under transverse loading; and drawing the longitudinal strain of the sample as an x-axis and the longitudinal stress as a y-axis to obtain a stress-strain curve of the longitudinal loading of the sample.
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