CN108344648B - Single-shaft bidirectional loading separation type Hopkinson pressure bar and pull bar device and experimental method - Google Patents
Single-shaft bidirectional loading separation type Hopkinson pressure bar and pull bar device and experimental method Download PDFInfo
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- CN108344648B CN108344648B CN201810120975.6A CN201810120975A CN108344648B CN 108344648 B CN108344648 B CN 108344648B CN 201810120975 A CN201810120975 A CN 201810120975A CN 108344648 B CN108344648 B CN 108344648B
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Abstract
The invention relates to an experimental method for testing dynamic mechanical properties of materials, in particular to a single-shaft bidirectional loading separated Hopkinson pressure bar and pull bar device and an experimental method. The invention provides a single-shaft bidirectional loading separated Hopkinson pressure bar and pull bar device and an experimental method thereof. According to the invention, the incident waves on two sides simultaneously and symmetrically carry out dynamic loading on the sample, so that the symmetric loading on the sample is realized to reduce the internal stress balance time of the sample, and the strain rate of the sample is improved, thereby overcoming the defects of low strain rate and inaccurate measurement of an elastic section of the existing equipment.
Description
Technical Field
The invention relates to an experimental method for testing dynamic mechanical properties of materials, in particular to a single-shaft bidirectional loading separated Hopkinson pressure bar and pull bar device and an experimental method.
Background
At present, the split hopkinson pull rod technology is most widely used in the field of material science for measuring the tensile mechanical property of a material under high strain rate. The basic principle of this method is: and (3) placing the short sample between two pull rods, and carrying out tensile loading on the sample through an acceleration pulse generated by the impact or explosion of the short rod through an acceleration mass block. While recording the pulse signal by means of a strain gauge attached to the pull rod at a distance from the end of the rod. If the tension rod remains in an elastic state, the stress pulse in the rod will propagate undistorted at the elastic wave speed. The strain gauge attached to the tie rod can measure the time-dependent change history of the load applied to the rod end.
The separated Hopkinson pressure bar is generally used for measuring the plastic flow behavior of a material under a high strain rate, the forces at two ends of a sample are not balanced in the initial loading stage, and the reliability of test data is poor, so that the characteristic of the calculated material has partial invalid data in an elastic section. When the stress wave reflects 3-5 back and forth in the sample, the forces at the two ends of the sample are balanced, and the test data is valid.
Because the stress of the sample is not uniform in the initial loading stage, the elastic section of the performance of the measured material is inaccurate, and the Hopkinson bar is only used for measuring the mechanical property of the plastic section of the material generally. There are some researchers that measure the modulus of elasticity of materials using hopkinson bars, but the final conclusion is that this approach is not feasible.
Hopkinson bars, while used to measure the high strain rate characteristics of materials, have limited maximum strain rates that can be achieved due to the property limitations of the compression bar itself. The current methods of increasing strain rate are to increase the impact bar speed and to use a micro hopkinson bar, with a micro sample. However, the strength of the plunger limits the loading speed of the bullet, and for many materials, small samples are difficult to process. Therefore, a better solution is needed under the condition that the diameter of the compression bar and the size of the sample are not limited.
In order to solve the problems of common riveting, the Boeing company in the United states in the 60 th century started to research the electromagnetic riveting technology from Huber A Schmitt et al, and applied for a strong impact electromagnetic riveting device in 1968. In 1986, Zieve Peter successfully develops low-voltage electromagnetic riveting, 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. Nowadays, low-voltage electromagnetic riveting technology has developed to a great extent, and the size and duration of the riveting force can be controlled relatively accurately. The technical principle of the electromagnetic riveter is as follows: a coil and a stress wave amplifier are added between the discharge coil and the workpiece. At the moment when the discharge switch is closed, the main coil generates strong magnetic field around the coil by the rapidly changing impact currentA field. 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 pressure bar to replace an air gun and a striking bar in the traditional split Hopkinson pressure bar, the electromagnetic repulsion force is used for generating direct stress waves, so that the standardization of the split Hopkinson pressure bar experiment technology becomes possible. In addition, because the pulse width of the stress wave generated by electromagnetic induction can be adjusted through circuit parameters, and the pulse width can reach millisecond level, low strain rate loading (for example, 10) which cannot be realized by some traditional Hopkinson bars can be realized2Less than s).
In the last two years, we applied a series of Hopkinson bar experimental devices and methods based on electromagnetic loading. In chinese patents 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 the two inventions of the two Chinese patents with application numbers of 201410173843.1 and 201410171963.8, two experimental facilities 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 have complex structures, and the traditional wave shaping technology cannot be applied to the stretching condition. In addition, because the incident rod is connected with the amplifier through threads, no matter a compression experiment or a tensile experiment is carried out, both a compression wave and a tensile wave can be transmitted into the incident rod at the same time, so that the stress waves generated by the two schemes are not pure tensile waves or compression waves and are only suitable for experiments without particularly large requirements on incident waveforms. To ameliorate this drawback, we subsequently proposed a new loading gun structure in the chinese patent invention creation under application No. 201510956545.4 that can both generate pure tension or compression waves and shape the wave using conventional shaping means. In the invention of chinese patent application No. 201510051071, a main coil structure and a method of use of an electromagnetic experimental apparatus are proposed to improve the range of variation of the amplitude and pulse width generated by the electromagnetic experimental apparatus. For the traditional mode of generating pulse by bullet impact, the bullet needs to move for a certain distance before generating stress wave, so that the multi-pulse technology of symmetric loading or biaxial loading cannot be realized.
Disclosure of Invention
The stress wave loading mode based on electromagnetic force utilizes the electromagnetic energy conversion technology to generate stress wave pulses, the generation of the pulses is realized by a discharge switch, and no time delay exists between the triggering of the switch and the generation of the stress pulses, so the time accuracy of the pulses is easily controlled by a circuit. Based on the advantages, the invention aims to provide a single-shaft bidirectional loading split Hopkinson pressure bar, a pull bar device and an experimental method, namely, another symmetrical incident wave is added on the basis of the traditional split Hopkinson bar, the incident waves on two sides dynamically load a sample simultaneously and symmetrically, the symmetrical loading of the sample is realized to reduce the internal stress balance time of the sample, and the strain rate of the sample is improved, so that the defects of low strain rate and inaccurate measurement of an elastic section of the existing equipment are overcome.
The invention provides a single-shaft bidirectional loading separated Hopkinson pressure bar and pull bar device, which comprises a loading device and two incident bars, wherein the loading device comprises a power supply, a capacitor charger and a loading gun, the capacitor charger adopts a power supply part of the existing electromagnetic riveting equipment, and the two loading guns with the same parameters are connected in parallel and then are connected into the capacitor charger.
In the uniaxial bidirectional loading split hopkinson pressure bar and pull bar device, the two incident bars are in the form of the incident bars of the conventional split hopkinson pressure bar and are made of titanium alloy. The circumferential surfaces of the two incident rods are respectively stuck with a pair of strain gauges by a conventional method. The two incident rods are coaxially arranged, and the two loading guns are respectively positioned at two ends of the two incident rods. The external screw thread of connecting the boss is processed to incident pole one end, and the other end processing has the internal thread of connecting the sample. The single-shaft bidirectional loading separated Hopkinson pressure bar and pull bar device further comprises a data acquisition unit connected with the strain gauge.
The device comprises two loading guns, a capacitor charger and two incident rods.
The loading gun and the capacitor charger adopt the loading device disclosed in the invention patent with the application number of 201510956545.4, wherein the difference is that the device disclosed by the invention comprises two loading guns with the same parameters. In order to realize symmetrical loading, two loading guns with the same parameters are connected in parallel and then are connected into a capacitor charger in a conventional mode. In this way, in the discharging process, the discharging current of the LC circuit is evenly distributed to the main coils of the two loading guns, so that the two loading guns generate the same stress wave at the same time, and the synchronism and the sameness of the stress waves at the two ends in the uniaxial bidirectional loading are ensured.
The two incident rods are completely same in size and material and are coaxially mounted, and the two loading guns are respectively positioned at two ends of the two incident rods. When a uniaxial bidirectional separation type Hopkinson compression experiment is carried out, the two incident rods are incident rods of a conventional separation type Hopkinson pressure bar respectively; when a uniaxial bidirectional separation type Hopkinson compression experiment is carried out, the two incident rods are respectively incident rods of a Hopkinson pull rod adopted in the Chinese invention patent with the application number of 201510956545.4.
The length L of the incident beam is designed to follow the relationship of equation (a) to avoid superposition of reflected and incident waves measured at the loading dry center:
L≥CT (A)
where C is the propagation velocity of the stress wave in the incident rod and T is the applied incident wave period.
Further, the invention provides an experimental method for the uniaxial bidirectional loading split Hopkinson pressure bar and pull bar device, which comprises the following steps:
the method comprises the following steps of (1) coaxially and sequentially installing two loading guns and two incident rods on an experiment table according to a conventional method, and enabling the incident rods to freely move in the axial direction; the two incident rods adopt the incident rod form of a conventional split Hopkinson pressure bar; installing a conventional split Hopkinson sample between two incident rods;
respectively sticking a pair of strain gauges on the circumferential surfaces of two incident rods by adopting a conventional method, welding a lead wire of the strain gauge on a pin of the strain gauge, and connecting the lead wire of the strain gauge into a Wheatstone bridge by adopting a twin-core shielding wire in order to shield electromagnetic interference; meanwhile, the output signal of the Wheatstone bridge is connected to the input end of the data collector by adopting a double-core shielding signal wire;
and 3, loading and processing data:
the charging voltage of the capacitor charger is set to be X volts and charging is carried out, wherein X is a specific required voltage value, and the capacitor charger discharges two loading guns after charging is completed.
The invention provides an experimental method of a single-shaft bidirectional loading separation type Hopkinson pressure bar and a pull bar device, which comprises a single-shaft bidirectional loading separation type Hopkinson bar compression experiment and a single-shaft bidirectional loading separation type Hopkinson bar stretching experiment.
I the concrete process of unipolar two-way loading disconnect-type hopkinson pole compression experiment is:
Assembling two loading guns with an incident rod in a compression mode, wherein the specific method comprises the following steps: the compression head is connected with the amplifier, the positioning cylinder penetrates through the through hole of the main coil, and one end, where the compression head of the loading gun is located, is close to the incident rod. And the stress wave output section of the compression head is coaxially and fully attached to the end face of the incident rod.
And 2, pasting the strain gauge. A pair of strain gauges are respectively adhered to the circumferential surfaces of two incident rods by a conventional method, namely the axial lines of the incident rods are taken as symmetry axes, two strain gauges with completely same parameters are symmetrically adhered to the circumferential surface of the length position of the incident rod 1/2 along the axial line direction of the incident rod, strain gauge lead wires are welded on the lead pins of the strain gauges, and in order to shield electromagnetic interference, the strain gauge lead wires are connected into a Wheatstone bridge by adopting a double-core shielding wire. Meanwhile, the output signal of the Wheatstone bridge is connected with the input end of the data collector by adopting a double-core shielding signal wire.
And 3, loading and processing data.
The charging voltage of the capacitor charger is set to be X volts and charging is carried out (X is a specific required voltage value and is within the rated voltage of the capacitor charger), after charging is completed, the capacitor charger discharges two loading guns, the two loading guns have the same parameters and are connected into the capacitor charger in parallel, so that the discharging current can be synchronously and uniformly distributed into the two loading guns, the same compression waves are generated in the two loading guns, the two compression waves are respectively transmitted into the two incident rods, simultaneously reach a sample and load the compressed sample, and in the loading process of the compressed sample, a reflected wave can be respectively generated in the two incident rods due to mismatching of wave impedance. Because the amplitude and the pulse width of the two incident waves are the same and the compression sample is loaded at the same time, the compression sample can be symmetrically loaded.
The strain gauge pasted on the incident rod converts a strain signal in the rod into bridge arm voltage change of a Wheatstone half bridge, the data acquisition unit is connected with the Wheatstone bridge through a double-core shielding signal wire, and the data acquisition unit acquires and stores the bridge arm voltage change of the Wheatstone bridge. The strain signal in the incident rod is processed in an excel table by the following equation (1):
＝2ΔU/k/(U0-ΔU) (1)
among these are strain signals in the incident rod. U shape0The Wheatstone bridge supply voltage, k is the strain gage sensitivity coefficient, and Δ U is the bridge arm voltage signal of the Wheatstone half bridge.
The strain signal in the incident beam is composed of a plurality of separate waves, the first of which is the incident waveiThe second wave is a reflected waver. The strain rate inside the compression sample can be calculated by the following formula (2), the strain inside the compression sample can be calculated by formula (3), and the stress inside the compression sample can be calculated by formula (4):
wherein, C0Is the velocity of the stress wave in the incident beam, LsIs the initial length of the sample. A. theBAnd AsThe cross-sectional areas of the incident rod and the compressed sample, respectively, and E is the elastic modulus of the incident rod.iAndrthe incident and reflected waves, respectively, and subscripts a and b denote the two incident beams (referred to as incident beam a and incident beam b), respectively.
II the specific process of the uniaxial bidirectional loading separation type Hopkinson bar stretching experiment is as follows:
Assembling two loading guns with an incident rod in a stretching mode, wherein the specific method comprises the following steps: and the positioning cylinder penetrates through the through hole of the main coil, and the amplifier and the incident rod are respectively positioned at two ends of the main coil. The end of the incident rod with the external thread sequentially penetrates through the through hole of the positioning cylinder and the threaded hole of the amplifier to be freely matched with the threaded hole of the amplifier and the through hole of the positioning cylinder, and the end of the incident rod with the external thread penetrates out of the amplifier to be connected with the boss through the thread.
And 2, pasting the strain gauge. A pair of strain gauges are respectively adhered to the circumferential surfaces of two incident rods by a conventional method, namely the axial lines of the incident rods are taken as symmetry axes, two strain gauges with completely same parameters are symmetrically adhered to the circumferential surface of the length position of the incident rod 1/2 along the axial line direction of the incident rod, strain gauge lead wires are welded on the lead pins of the strain gauges, and in order to shield electromagnetic interference, the strain gauge lead wires are connected into a Wheatstone bridge by adopting a double-core shielding wire. Meanwhile, the output signal of the Wheatstone bridge is connected with the input end of the data collector by adopting a double-core shielding signal wire.
And 3, loading and processing data. The charging voltage of the capacitor charger is set to be X volts and charging is carried out (X is a specific required voltage value and is within the rated voltage of the capacitor charger), after the charging is finished, the capacitor charger discharges two loading guns, and as the parameters of the two loading guns are the same and the two loading guns are connected into the capacitor charger in parallel, the discharging current can be synchronously and uniformly distributed into the two loading guns, so that the two loading guns generate the same compression waves, and the two compression waves are respectively reflected into the stretching waves on the two bosses and enter the incident rod. The tensile waves in the two incident rods simultaneously reach the tensile sample, and the two tensile waves have the same amplitude and pulse width and simultaneously load the tensile sample, so that the tensile sample can be symmetrically loaded.
The strain gauge pasted on the incident rod converts a strain signal in the rod into bridge arm voltage change of a Wheatstone half bridge, the data acquisition unit is connected with the Wheatstone bridge through a double-core shielding signal wire, and the data acquisition unit acquires and stores the bridge arm voltage change of the Wheatstone bridge. The strain signal in the incident rod is processed in an excel table by formula (1):
＝2ΔU/k/(U0-ΔU) (1)
among these are strain signals in the incident rod. U shape0The Wheatstone bridge supply voltage, k is the strain gage sensitivity coefficient, and Δ U is the bridge arm voltage signal of the Wheatstone half bridge.
Similar to the compression mode, the strain signal in the incident rod is composed of a plurality of separated wave motions, wherein the first wave motion is the incident waveiThe second wave is a reflected waver. The strain rate inside the tensile sample can be calculated by the following formula (2), the strain inside the tensile sample can be calculated by the formula (3), and the stress inside the tensile sample can be calculated by the formula (4):
wherein, C0Is the velocity of the stress wave in the incident beam, LsIs the initial length of the sample. A. theBAnd AsThe cross-sectional areas of the incident rod and the tensile sample, respectively, E is the elastic modulus of the incident rod,iandrthe incident and reflected waves, respectively, and subscripts a and b denote the two incident beams, referred to as incident beam a and incident beam b, respectively.
In the present invention, the power supply system is used to supply instantaneous strong current to the main coil of the loading gun, so that strong electromagnetic repulsion is generated between the main coil and the secondary coil. The loading gun is used for generating electromagnetic repulsion force, converting the electromagnetic repulsion force into stress waves, and outputting the stress waves to the injection rod after the stress waves are amplified by the conical amplifier.
In the experimental device, two identical loading guns are connected with the capacitor power supply in parallel, and the discharge current is uniform and is simultaneously distributed to the two loading guns, so that the synchronism of incident waves in single-axis bidirectional loading can be ensured.
The invention combines the electromagnetic induction repulsion with the capacitor discharge in principle to directly generate the stress pulse. Adopt traditional disconnect-type hopkinson depression bar and pull rod sample, can carry out dynamic symmetry loading to the material, because both sides load simultaneously, can improve the strain rate of sample to can make the sample reach the time shortening of stress balance, the result of calculation is more accurate.
Drawings
FIG. 1 is a schematic view of a uniaxial bi-directional loading Hopkinson pressure bar apparatus of the present invention.
Fig. 2 is a schematic diagram of a single-shaft bidirectional loading hopkinson pull rod device.
Detailed Description
FIG. 1 is a schematic view of a uniaxial bi-directional loading Hopkinson pressure bar apparatus of the present invention. Fig. 2 is a schematic diagram of a single-shaft bidirectional loading hopkinson pull rod device. In fig. 1 and 2: 1. a power source; 2. a capacitor charger; 3. loading a gun; 4. an incident rod a; 5. an incident rod b; 6. compressing the sample; 7. a strain gauge; 8. a data acquisition unit; 9. stretching a sample; 10. and (4) a boss.
As shown in the figure, the single-shaft bidirectional loading separated Hopkinson pressure bar and pull bar device comprises a loading device and two incident bars 4 and 5, wherein the loading device comprises a power supply 1, a capacitor charger 2 and a loading gun 3, the capacitor charger 2 adopts a power supply part of the existing electromagnetic riveting equipment, and the two loading guns 3 with the same parameters are connected in parallel and then are connected into the capacitor charger 2.
In the uniaxial bidirectional loading split hopkinson pressure bar and pull bar device, the two incident bars 4 and 5 are in the form of the incident bars of the conventional split hopkinson pressure bar and are made of titanium alloy. The circumferential surfaces of the two incident rods are respectively adhered with a pair of strain gauges 7 by a conventional method. The two incident rods 4 and 5 are coaxially arranged, and the two loading guns 3 are respectively positioned at two ends of the two incident rods 4 and 5. The external screw thread of connecting the boss is processed to incident pole one end, and the other end processing has the internal thread of connecting the sample. The single-shaft bidirectional loading separated Hopkinson pressure bar and pull bar device further comprises a data acquisition unit 8 connected with the strain gauge.
Example 1
The embodiment is a single-shaft bidirectional loading separated Hopkinson pressure bar and pull bar device and an experimental method.
The device of the invention comprises a loading device and two incident rods, namely an incident rod a (first incident rod) 4 and an incident rod b (second incident rod) 5.
The loading device adopts a loading device provided in Chinese invention patent with the patent number ZL 201510956545.4, and comprises a power supply 1, a capacitor charger 2 and a loading gun 3. The capacitor charger 2 adopts a power supply part of the existing electromagnetic riveting equipment. And the two loading guns 3 with the same parameters are connected in parallel and then are connected into the capacitor charger 2. The power supply 1 adopts 220V three-phase alternating current.
In this embodiment, the capacitor charger 2 adopts a power supply part of the electromagnetic riveting device disclosed in chinese patent No. 200520079179, in this embodiment, 6 pulse capacitors with a rated voltage of 5000 v and a rated capacitance of 2 millifarads are connected in parallel to form a capacitor bank, the capacitor bank and an electronic switch are installed in a capacitor box, and the discharge of the capacitor bank is controlled by the electronic switch. The control box mainly comprises a PLC and a control system thereof. The control system mainly comprises an analog control part, a digital control part and a digital display part. The analog control part adopts TCA785 chip of SIEMENS (Siemens). The digital control part consists of a Siemens S7-200 series CPU224 and a Siemens analog input and output expansion module EM 235. The charging voltage control is mainly realized by a PID control mode of the voltage loop and the current loop. The digital display part is mainly composed of a text display TD200 of the S7-200 series.
In this embodiment, the main coils of the two loading guns 3 are disc-shaped coils with 32 turns formed by winding copper strips with the width of 25mm and the thickness of 2 mm.
The embodiment also provides an experimental method of the uniaxial bidirectional loading separation type Hopkinson pressure bar and the pull bar device, which comprises a uniaxial bidirectional loading separation type Hopkinson bar compression experiment and a uniaxial bidirectional loading separation type Hopkinson bar stretching experiment.
I the concrete process of unipolar two-way loading disconnect-type hopkinson pole compression experiment is:
The two loading guns 3 are respectively assembled with an incident rod a4 and an incident rod b5 in a compression mode, and the specific method comprises the following steps: the compression head is connected with the amplifier, the positioning cylinder penetrates through the through hole of the main coil, and one end, where the compression head of the loading gun is located, is close to the incident rod. And the stress wave output section of the compression head is coaxially and fully attached to the end face of the incident rod.
And 2, pasting the strain gauge. A pair of strain gauges 7 are respectively adhered to the circumferential surfaces of the two incident rods by a conventional method, namely the axial lines of the incident rods are taken as symmetry axes, and two strain gauges with completely the same parameters are symmetrically adhered to the circumferential surface of the length position of the incident rod 1/2 along the axial line direction of the incident rod. In the embodiment, a strain gauge with the resistance value of 1000 ohms and the sensitivity coefficient of 2.0 is adopted; and welding lead wires of the strain gauge on the pins of the strain gauge, wherein the lead wires of the strain gauge adopt a twin-core shielding wire with the diameter of 0.5mm to shield electromagnetic interference generated in the discharging process, and the strain gauge is respectively connected into two opposite bridge arms of the Wheatstone bridge 4 through the lead wires. The fixed resistors on the other two legs of the wheatstone bridge are both 1000 ohms. The supply voltage of the wheatstone bridge is 30 volts dc. Two diagonal voltages of the Wheatstone bridge are input to the data collector 8 through a double-core shielding signal wire, the data collector 8 adopts GEN3i manufactured by Germany HBM company, the data collector has good interference shielding capability, and in the data collector, voltage signals on two diagonal corners of the Wheatstone bridge are processed by adopting a difference method.
And 3, loading and processing data.
The charging voltage of the capacitor charger 2 is set to be 1000V and the capacitor charger is charged, after the charging is finished, a discharging switch is pressed to enable the capacitor charger to discharge the two loading guns 3, the two loading guns have the same parameters and are connected into the capacitor charger 2 in parallel, discharging current can be synchronously and uniformly distributed into the two loading guns, the same compression waves are generated in the two loading guns, the two compression waves are respectively transmitted into the two incident rods and simultaneously reach the compression sample 6 to load the compression sample, and in the loading process of the compression sample 6, due to the fact that wave impedances are not matched, a reflected wave can be respectively generated in the two incident rods. Because the amplitude and the pulse width of the two incident waves are the same and the sample is loaded at the same time, the sample can be symmetrically loaded.
The strain gauge pasted on the incident rod converts a strain signal in the rod into bridge arm voltage change of a Wheatstone half bridge, the data acquisition unit is connected with the Wheatstone bridge through a double-core shielding signal wire, and the data acquisition unit acquires and stores the bridge arm voltage change of the Wheatstone bridge. The strain signal in the incident rod is processed in an excel table by the following formula:
＝2ΔU/k/(U0-ΔU) (1)
among these are strain signals in the incident rod. U shape0The Wheatstone bridge supply voltage, k is the strain gage sensitivity coefficient, and Δ U is the bridge arm voltage signal of the Wheatstone half bridge.
The strain signal in the incident beam is composed of a plurality of separate waves, the first of which is the incident waveiThe second wave is a reflected waver. The strain rate inside the compressed sample can be calculated by the following formula (2), the strain inside the compressed sample can be calculated by the following formula (3), and the strain inside the compressed sample can be calculated by the following formula (4)) The stress inside the compressed sample can be calculated:
wherein, C0Is the velocity of the stress wave in the incident beam, LsIs the initial length of the sample. A. theBAnd AsThe cross-sectional areas of the incident rod and the compressed sample, respectively, and E is the elastic modulus of the incident rod.iAndrthe incident and reflected waves, respectively, and subscripts a and b denote the two incident beams (referred to as incident beam a and incident beam b), respectively.
II the specific process of the uniaxial bidirectional loading separation type Hopkinson bar stretching experiment is as follows:
And 2, pasting the strain gauge. A pair of strain gauges 7 are respectively adhered to the circumferential surfaces of the two incident rods by a conventional method, namely the axial lines of the incident rods are taken as symmetry axes, and two strain gauges with completely the same parameters are symmetrically adhered to the circumferential surface of the length position of the incident rod 1/2 along the axial line direction of the incident rod. In the embodiment, a strain gauge with the resistance value of 1000 ohms and the sensitivity coefficient of 2.0 is adopted; and welding lead wires of the strain gauge on the pins of the strain gauge, wherein the lead wires of the strain gauge adopt a twin-core shielding wire with the diameter of 0.5mm to shield electromagnetic interference generated in the discharging process, and the strain gauge is respectively connected into two opposite bridge arms of the Wheatstone bridge 4 through the lead wires. The fixed resistors on the other two legs of the wheatstone bridge are both 1000 ohms. The supply voltage of the wheatstone bridge is 30 volts dc. Two diagonal voltages of the Wheatstone bridge are input to the data collector 8 through a double-core shielding signal wire, the data collector 8 adopts GEN3i manufactured by Germany HBM company, the data collector has good interference shielding capability, and in the data collector, voltage signals on two diagonal corners of the Wheatstone bridge are processed by adopting a difference method.
And 3, loading and processing data. The charging voltage of the capacitor charger 2 is set to 1000V and the capacitor charger is charged, after the charging is finished, the capacitor charger discharges the two loading guns 3, and because the parameters of the two loading guns are the same and are connected into the capacitor charger 2 in parallel, the discharging current can be synchronously and uniformly distributed into the two loading guns, so that the same compression waves are generated in the two loading guns, and the two compression waves are respectively reflected into stretching waves in the two bosses 10 and enter the incident rod. The tensile waves in the two incident rods simultaneously reach the tensile sample 9, and the two tensile waves have the same amplitude and pulse width and simultaneously load the sample, so that the tensile sample can be symmetrically loaded.
The strain gauge pasted on the incident rod converts a strain signal in the rod into bridge arm voltage change of a Wheatstone half bridge, the data collector 8 is connected with the Wheatstone bridge through a double-core shielding signal wire, and the data collector collects and stores the bridge arm voltage change of the Wheatstone bridge. The strain signals in the two incident rods were processed in an excel table by equation (1).
Similar to the compression mode, the strain signal in the incident rod is composed of a plurality of separate fluctuations, whichThe first wave being incident waveiThe second wave is a reflected waver. The strain rate inside the tensile sample can be calculated by the following formula (2), the strain inside the tensile sample can be calculated by the formula (3), and the stress inside the tensile sample can be calculated by the formula (4):
in this embodiment, the power supply system is used to provide a strong instantaneous current to the primary coil of the gun, thereby generating a strong electromagnetic repulsion between the primary and secondary coils. The loading gun is used for generating electromagnetic repulsion force, converting the electromagnetic repulsion force into stress waves, and outputting the stress waves to the injection rod after the stress waves are amplified by the conical amplifier.
In the experimental device of the embodiment, two identical loading guns are connected with a capacitor power supply, and the discharging current is uniformly distributed to the two loading guns at the same time, so that the synchronism of incident waves in single-axis bidirectional loading can be ensured.
The present embodiment combines the electromagnetic inductive repulsion with the capacitor discharge in principle to directly generate the stress pulse. Adopt traditional disconnect-type hopkinson depression bar and pull rod sample, can carry out dynamic symmetry loading to the material, because both sides load simultaneously, can improve the strain rate of sample to can make the sample reach the time shortening of stress balance, the result of calculation is more accurate.
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 (9)
1. The utility model provides a two-way loading disconnect-type hopkinson depression bar of unipolar and pull rod device, includes loading device and two incident poles, and the length of two incident poles is the same, loading device includes power, electric capacity charger and loading rifle, electric capacity charger adopts the power supply part of electromagnetism riveting equipment to with two loading rifle parallel connection backs that the parameter is the same, access among the electric capacity charger, two incident pole coaxial arrangement, two loading guns are located respectively the both ends of two incident poles, the sample is installed between two incident poles.
2. The uniaxial bidirectional loading split hopkinson pressure bar and tie bar device of claim 1, wherein the two incident bars are in the form of incident bars of split hopkinson pressure bars, and are made of titanium alloy.
3. The uniaxial bidirectional loading split Hopkinson pressure bar and tension bar device according to claim 1 or 2, wherein a pair of strain gauges are adhered to circumferential surfaces of the two incident bars, respectively.
4. The uniaxial bidirectional-loading split hopkinson pressure bar and pull bar device of claim 1 or 2, wherein the length L of the incident bar is designed to follow the relationship of formula (a) so as to avoid superposition of reflected waves and incident waves measured at the midpoint of the incident bar:
L≥CT (A)
where C is the propagation velocity of the stress wave in the incident rod and T is the applied incident wave period.
5. The uniaxial bidirectional loading split Hopkinson pressure bar and pull bar device according to claim 1 or 2, wherein an external thread for connecting a boss is formed at one end of the incident bar, and an internal thread for connecting a sample is formed at the other end of the incident bar.
6. The uniaxial bidirectional loading split Hopkinson pressure bar and pull bar device of claim 3, further comprising a data collector connected with the strain gauge.
7. An experimental method of a uniaxial bi-directional loading split hopkinson strut and tie device according to any one of the preceding claims, comprising the steps of:
step 1, arranging equipment:
the method comprises the following steps of (1) coaxially and sequentially installing two loading guns and two incident rods on an experiment table, and enabling the incident rods to freely move in the axis direction; the two incident rods adopt an incident rod form of a separated Hopkinson pressure bar; installing a separated Hopkinson sample between two incident rods;
step 2, pasting a strain gauge:
respectively sticking a pair of strain gauges on the circumferential surfaces of the two incident rods, welding a strain gauge lead wire on a pin of each strain gauge, and connecting the strain gauge lead wire into a Wheatstone bridge by adopting a twin-core shielding wire in order to shield electromagnetic interference; meanwhile, the output signal of the Wheatstone bridge is connected to the input end of the data collector by adopting a double-core shielding signal wire;
and 3, loading and processing data:
the charging voltage of the capacitor charger is set to be X volts and charging is carried out, wherein X is a specific required voltage value, and the capacitor charger discharges two loading guns after charging is completed.
8. The method according to claim 7, wherein the sample is a split Hopkinson compressed sample, the compressed sample is clamped between two incident rods, and two loading guns are assembled with the incident rods in a compression mode, and the method comprises the following steps: connecting the compression head with the amplifier, enabling the positioning cylinder to penetrate through the through hole of the main coil, and enabling one end where the compression head of the loading gun is located to be close to the incident rod; the stress wave output section of the compression head is coaxially and fully attached to the end face of the incident rod; the two compression waves are respectively transmitted into the two incident rods, simultaneously reach the compression sample and load the compression sample, and in the loading process of the compression sample, because wave impedances are not matched, a reflected wave is respectively generated in the two incident rods; because the amplitude values and the pulse widths of the two incident waves are the same and the compression sample is loaded at the same time, the symmetric loading of the compression sample can be realized; the strain gauge pasted on the incident rod converts a strain signal in the rod into bridge arm voltage change of a Wheatstone half bridge, the data acquisition unit is connected with the Wheatstone bridge through a twin-core shielding signal wire, the data acquisition unit acquires and stores the bridge arm voltage change of the Wheatstone bridge, and the strain signal in the incident rod is obtained by processing the following formula (1) in an excel table:
＝2ΔU/k/(U0-ΔU) (1)
wherein, for strain signals in the incident rod, U0The voltage is the power supply voltage of the Wheatstone bridge, k is the sensitivity coefficient of the strain gauge, and delta U is a bridge arm voltage signal of the Wheatstone bridge;
the strain signal in the incident beam is composed of a plurality of separate waves, the first of which is the incident waveiThe second wave is a reflected waver(ii) a The strain rate inside the compression sample can be calculated by the following formula (2), the strain inside the compression sample can be calculated by formula (3), and the stress inside the compression sample can be calculated by formula (4):
wherein, C0Is the velocity of the stress wave in the incident beam, LsIs the initial length of the sample, ABAnd AsThe cross-sectional areas of the incident rod and the compressed sample, respectively, E is the elastic modulus of the incident rod,iandrthe incident and reflected waves, respectively, and subscripts a and b denote the two incident beams, referred to as incident beam a and incident beam b, respectively.
9. The method according to claim 7, wherein the sample is a split Hopkinson tensile sample, the tensile sample is installed between two incident rods, and two loading guns are assembled with the incident rods in a tensile manner, and the method comprises the following steps: the positioning cylinder penetrates through a through hole of the main coil, and the amplifier and the incident rod are respectively positioned at two ends of the main coil; one end of the incidence rod with the external thread penetrates through the through hole of the positioning cylinder and the threaded hole of the amplifier in sequence and is freely matched with the threaded hole of the amplifier and the through hole of the positioning cylinder, and the end of the incidence rod with the external thread penetrates out of the amplifier and is connected with the boss through threads; the two compression waves are respectively reflected into tensile waves at the two bosses and enter the incident rod; the tensile waves in the two incident rods simultaneously reach the tensile sample, and the two tensile waves have the same amplitude and pulse width and simultaneously load the tensile sample, so that the tensile sample can be symmetrically loaded; the strain gauge pasted on the incident rod converts a strain signal in the rod into bridge arm voltage change of a Wheatstone half bridge, the data acquisition unit is connected with the Wheatstone bridge through a double-core shielding signal wire, and the data acquisition unit acquires and stores the bridge arm voltage change of the Wheatstone bridge; the strain signal in the incident rod is processed in an excel table by formula (1):
＝2ΔU/k/(U0-ΔU) (1)
wherein, for strain signals in the incident rod, U0Is the supply voltage of the Wheatstone bridge, k is the strain gage sensitivity coefficient, Δ U is the bridge arm voltage signal of the Wheatstone bridgeNumber;
the strain signal in the incident beam is composed of a plurality of separate waves, the first of which is the incident waveiThe second wave is a reflected waver(ii) a The strain rate inside the tensile sample can be calculated by the following formula (2), the strain inside the tensile sample can be calculated by the formula (3), and the stress inside the tensile sample can be calculated by the formula (4):
wherein, C0Is the velocity of the stress wave in the incident beam, LsIs the initial length of the sample, ABAnd AsThe cross-sectional areas of the incident rod and the tensile sample, respectively, E is the elastic modulus of the incident rod,iandrthe incident and reflected waves, respectively, and subscripts a and b denote the two incident beams, referred to as incident beam a and incident beam b, respectively.
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