CN108267366B - Medium strain rate tension and compression bar loading experimental method - Google Patents
Medium strain rate tension and compression bar loading experimental method Download PDFInfo
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- CN108267366B CN108267366B CN201810047039.7A CN201810047039A CN108267366B CN 108267366 B CN108267366 B CN 108267366B CN 201810047039 A CN201810047039 A CN 201810047039A CN 108267366 B CN108267366 B CN 108267366B
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- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
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- G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N2203/0014—Type of force applied
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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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- G01N2203/005—Electromagnetic means
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- G—PHYSICS
- G01—MEASURING; TESTING
- 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
A middle strain rate tension and compression rod loading experiment method adopts the principle of electromagnetic energy/mechanical energy conversion to generate and load stress waves, the amplitude and the pulse width of the generated stress waves are only limited by the electromagnetic energy, so that the length of a bullet is not limited, and the size of a mechanism is reduced. Secondly, because the wave propagation effect is very weak when the medium strain rate is loaded, the incident rod and the transmission rod are both rods which are shorter than the stress pulse, the reflection action of the stress wave is neglected, the stress value in the sample can be measured, and the strain of the sample can be measured by adopting a light measurement method or a strain gauge method.
Description
Technical Field
The invention relates to an experimental method for testing mechanical properties under a medium strain rate in the field of materials, in particular to an experimental method for a medium strain rate material based on electromagnetic force.
Background
At present, the split Hopkinson pressure bar technology and the pull bar technology are most widely used for measuring the mechanical property of a material under high strain rate in the field of material science. And the device for measuring the quasi-static mechanical property of the material is a quasi-static tester. But no perfect equipment exists for the medium strain rate mechanical property of the material, and most of the medium strain rate mechanical property is realized by adopting a hydrostatic pressure tester.
In order to solve the problems of common riveting, the U.S. Boeing company in the 60 th of the 20 th century started to research the electromagnetic riveting technology from HuberASchmitt 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 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 medium strain rate compression bar to replace the medium strain rate compression barTraditional fluid pressure type medium strain rate equipment produces direct production stress wave through electromagnetic repulsion, will make the standardization of medium strain rate depression bar experiment technique become 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 magnitude, the loading requirement (for example, 10) of some traditional medium strain rate experiments can be met2Less than s). In the invention creations 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 inventions 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 are complex in structure, and the traditional wave shaping technology cannot be applied to the stretching condition. To ameliorate this drawback, the authors subsequently proposed, in the invention of application No. 201510956545.4, a new loading gun structure that can both generate tension and compression waves and shape the waveform using conventional shaping means. In the invention and creation of the application No. 201510051071, a main coil structure and a using method of an electromagnetic experimental device are provided to improve the variation range of the amplitude and the pulse width generated by the electromagnetic experimental device.
At present, for the medium strain rate test (i.e. strain rate at 10)0To 101Strain rate between) and the corresponding point, although belonging to the dynamic mechanics category, there is a concept of stress wave, in order to obtain a considerable maximum strain, the required stress wave pulse is very long, reaching 1ms to 10ms, if a traditional pneumatic driving type hopkinson rod loading and data processing principle is adopted, not only the bullet length is too long, possibly several meters, but also the length of the required incident rod and transmission rod is at least twice of the bullet length to separate the incident wave and the reflected wave, so that the length of the whole mechanism is between tens of meters and tens of meters. Considering these factors, the traditional air pressure driven Hopkinson bar device and the data processing method are adopted to realize the medium strainRate loading is impractical.
Disclosure of Invention
In order to overcome the defect that the pneumatic driving type Hopkinson bar device and the data processing method in the prior art cannot realize medium strain rate loading, the invention provides a medium strain rate tension and compression bar loading experimental method.
The invention adopts an electromagnetic force-based medium strain rate tension and compression rod experimental device, which comprises a medium strain rate compression experiment and a medium strain rate tension experiment, and the specific process is as follows:
i, performing a strain rate compression experiment;
the specific process of the medium strain rate compression experiment is as follows:
step 2, pasting a strain gauge;
step 3, determining a stress-strain curve and a strain rate-time curve of the sample
Obtaining the stress sigma inside the sample by loading the samplesAnd strain epsilon in the samples：
Obtaining the internal stress sigma of the sample by a stress processing formula (4)s：
σs＝(σi+σt)×Ab/As/2 (4)
In the formula (4), σiIs the incident wave of the incident rod; sigmatA transmitted wave that is a transmitted rod; a. thesIs the cross-sectional area of the sample; a. thebIs the cross-sectional area of the incident rod.
The strain ε in the sample was obtained by the following equation (5)s：
εs＝2△Us/k(U0-△Us) (5)
Wherein epsilonsFor strain inside the sample, Delta UsIs a bridge arm voltage signal of a Wheatstone half bridge connected with a strain gauge on a sample; k is the sensitivity coefficient of the strain gauge; u shape0The input voltage of the wheatstone bridge.
Stress σ in the sample obtained by the abovesAnd strain epsilon in the samplesDetermining the trialStress-strain curves and strain rate-time curves of the sample; the method comprises the following steps:
stress-strain curve of the sample: sample internal stress σ obtained by equation (4)sThe strain ε in the sample obtained by the following equation (5) as the y-axissThe stress-strain curve of the sample is obtained by plotting the x-axis.
Strain rate versus time curve for the test specimen:
the resulting strain function εsAnd obtaining a strain rate function inside the sample by taking a derivative of the time, and drawing a graph by taking the strain rate function as a y axis and the time as an x axis to obtain a strain rate-time curve of the sample.
The sample of the medium strain rate compression experiment is cylindrical.
II, performing strain rate tensile test;
performing a medium strain rate tensile test by adopting a medium strain rate tensile and compression bar test device based on electromagnetic force; the specific process is as follows:
when the equipment is arranged, the loading gun, the incident rod and the transmission rod are coaxially and sequentially arranged on the experiment table, and the other end of the transmission rod is connected with the buffer; one end of the loading gun where the compression head is located is close to the incident rod; the sample is mounted between the incident rod and the transmission rod, and is coaxial with the incident rod and the transmission rod.
Step 2, pasting a strain gauge;
the strain gauges are adhered to the 1/2-length circumferences of the incident rod and the transmission rod, the axes of the incident rod and the transmission rod are taken as symmetry axes, and two strain gauges with completely the same parameters are symmetrically adhered to the surfaces of the incident rod and the transmission rod; and respectively connecting the strain gauges into two opposite bridge arms of a Wheatstone bridge through leads.
Step 3, determining a stress-strain curve and a strain rate-time curve of the sample
Obtaining the stress sigma inside the sample by loading the samplesAnd strain epsilon in the samples：
Obtaining the internal stress sigma of the sample by a stress processing formula (4)s：
σs＝(σi+σt)×Ab/As/2 (4)
The strain ε in the sample was obtained by the following equation (5)s：
εs＝2△Us/k(U0-△Us) (5)
Wherein epsilonsFor strain inside the sample, Delta UsIs a bridge arm voltage signal of a Wheatstone half bridge connected with a strain gauge on a sample; k is the sensitivity coefficient of the strain gauge; u shape0The input voltage of the wheatstone bridge.
Stress σ in the sample obtained by the abovesAnd strain epsilon in the samplesDetermining a stress-strain curve and a strain rate-time curve of the sample; in particular to
The stress inside the sample is obtained by the formula (4).
σs＝(σi+σt)×Ab/As/2 (4)
Obtaining the internal strain of the sample by the formula (5)
εs＝2△Us/k(U0-△Us) (5)
Stress σ in the sample obtained by the abovesAnd strain epsilon in the samplesDetermining a stress-strain curve and a strain rate-time curve of the sample; the method comprises the following steps:
stress-strain curve of the sample: sample internal stress σ obtained by equation (4)sThe strain ε in the sample obtained by the following equation (5) as the y-axissThe stress-strain curve of the sample is obtained by plotting the x-axis.
Strain rate versus time curve for the test specimen:
the resulting strain function εsAnd obtaining a strain rate function inside the sample by taking a derivative of the time, and drawing a graph by taking the strain rate function as a y axis and the time as an x axis to obtain a strain rate-time curve of the sample.
The specific treatment process is as follows:
stress-strain curve of test specimen: sample stress function σ calculated by equation (4)sAs the y-axis, the strain function ε obtained by equation (5)sThe stress-strain curves of the samples were obtained by plotting the x-axis.
Strain rate versus time curve for the test specimen: by a strain function epsilonsAnd obtaining a strain rate function of the sample by calculating a derivative of the time, and drawing a graph by taking the strain rate function as a y axis and the time as an x axis to obtain a strain rate-time curve of the sample.
Determination of stress sigma in the interior of a test specimensAnd strain epsilon in the samplesThe method comprises the following steps:
obtaining an incident wave signal in an incident rod by formula (1):
σi＝2Ei△Ui/k(U0-△Ui) (1)
wherein σiIs the incident wave signal in the incident rod. Delta UiIs the voltage signal of a Wheatstone half-bridge arm connected with a strain gauge in an incident rod; k is the sensitivity coefficient of the strain gauge; u shape0Is the input voltage of the Wheatstone bridge; eiIs the young's modulus of the incident rod material.
The transmitted wave signal in the transmission rod is obtained by equation (2):
σt＝2Et△Ut/k(U0-△Ut) (2)
wherein σtIs a transmitted wave signal in the transmission rod. Delta UtIs the bridge arm voltage signal of the wheatstone half bridge connected to the strain gage in the transmission rod. EtIs the young's modulus of the transmissive rod material.
The appearance of a sample of the medium strain rate tensile experiment is dumbbell-shaped, the two ends of the sample are provided with connecting sections with external threads, and the middle part of the sample is a testing section of a polished rod.
In a medium strain rate compression experiment and a medium strain rate tension experiment, a specific process of loading a sample is that a capacitor charger is charged to discharge a main coil of a loading gun to form sinusoidal incident waves; the incident wave is transmitted into the incident rod of the medium strain rate compression rod through the compression head and loads the sample, and one part of the incident wave is reflected on the end face of the sample, and the other part of the incident wave is transmitted into the transmission rod through the sample to form a transmission wave. The outer diameter and the number of turns of the main coil are 3-5 times of those of a common Hopkinson bar loading device, and the capacitance is 3-5 times of that of the common Hopkinson bar loading device.
The resistance of the strain gauge pasted on the incident rod is converted into the output voltage in the connected Wheatstone bridge; the resistance of the strain gauge attached to the transmission rod is converted into the output voltage in the connected wheatstone bridge. And the output voltage in each Wheatstone bridge is input and stored in the data acquisition unit through a double-core shielding signal wire.
The invention carries out two breakthrough improvements on the basis of the prior art: firstly, in the aspect of loading, the principle of electromagnetic energy/mechanical energy conversion is adopted to generate and load stress waves, the amplitude and the pulse width of the generated stress waves are only limited by the electromagnetic energy, so that the length of a bullet is not limited, and the size of the mechanism is greatly reduced: secondly, because the wave propagation effect is very weak when the medium strain rate is loaded, the incident rod and the transmission rod are both rods which are much shorter than the stress pulse, the reflection action of the stress wave is neglected, the stress value in the sample can be directly measured, and the strain of the sample can be measured by adopting a light measurement method or a strain gauge method.
The invention has the principle that the invention is different from the traditional split Hopkinson pressure bar and pull bar experiment. In the middle strain rate loading process, the sample is in a dynamic balance state under the action of the middle strain rate, and the loading forces at two ends are equal. The incident rod and the transmission rod only play a role of a force sensor, because the length of the incident rod and the length of the transmission rod are far smaller than that of incident waves, stress waves in the incident rod and the transmission rod can be considered to be uniformly distributed along the axial direction, the loading force on the contact surface of the sample and the incident rod can be obtained by multiplying a stress wave signal in the incident rod by the cross-sectional area of the incident rod, and the loading force on the contact surface of the sample and the transmission rod can be obtained by multiplying a stress wave signal in the transmission rod by the cross-sectional area of the transmission rod. And then the stress inside the sample is calculated. The loading process breaks through the idea that the traditional Hopkinson bar separates incident waves, reflected waves and transmitted waves, and is a principle innovation.
Drawings
FIG. 1 is a schematic view of a compression test;
FIG. 2 is a schematic drawing of a tensile test;
fig. 3 is a flow chart of the present invention. In the figure:
1. a power source; 2. a capacitor charger; 3. a lead wire; 4. a Wheatstone bridge; 5. a data acquisition unit; 6. loading a gun; 7. a housing; 8. a main coil; 9. a positioning cylinder; 10. a secondary coil; 11. an insulating layer; 12. an amplifier; 13. a compression head; 14. a strain gauge; 15. an incident rod; 16. a sample; 17. a transmission rod; 18. a buffer; 19. and (4) a boss.
Detailed Description
The embodiment is a method for adopting the medium strain rate tension-compression rod experiment based on the electromagnetic force.
The loading device of the invention adopts the loading device provided in the invention creation with the application number of 201510956545.4. For medium strain rate loading, in order to generate large strain for a sample, long pulse loading is needed, so that the outer diameter and the number of turns of the main coil are 3 to 5 times of those of a common Hopkinson bar loading device, and the capacitance is 3 to 5 times of those of the common Hopkinson bar loading device.
The waveguide rod comprises two stretching rods with the same length and two compression rods with the same length, one of the two waveguide rods is an incident rod, and the other one of the two waveguide rods is a transmission rod. The diameter of the waveguide rod is 10-20mm, and the length of the waveguide rod is 20-50 cm. The stretching rod is a cylindrical rod, an internal thread for connecting a sample is processed at one end of the stretching rod, an external thread for connecting a buffer or a boss is processed at the other end of the stretching rod, and the design of the boss is the same as that of the boss of the Hopkinson pull rod; the compression rod is a cylindrical rod with one end provided with an external thread, and the external thread is used for connecting the buffer. The buffer is a large-mass metal block with a threaded hole in the center, and the threaded hole is used for being connected with the waveguide rod.
During the middle strain rate compression experiment, the connecting section of the compression head is arranged in the internal thread hole of the amplifier, and the compression head is in threaded connection with the amplifier. The end face of the compression wave output section of the compression head is brought into contact with the incident rod, thereby propagating the stress wave into the incident rod. During the tensile experiment of strain rate in carrying out, pass the through-hole of a location section of thick bamboo and amplifier in proper order with the one end that the incident pole has the external screw thread to carry out threaded connection with the boss on one side of the stress wave output section of amplifier. When the compression stress wave transmitted from the stress wave output section of the amplifier enters the boss, the compression stress wave is reflected into a tensile wave on the free end face of the boss and enters the incident rod to form an incident wave of the medium strain rate tension rod.
The experiments include a medium strain rate compression experiment and a medium strain rate tension experiment. Wherein:
i strain rate compression experiment.
And performing a medium strain rate compression experiment by adopting a medium strain rate tension and compression rod experiment device based on electromagnetic force. The specific process is as follows:
5. A data acquisition unit; 6. loading a gun; 7. a housing; 8. a main coil; 9. a positioning cylinder; 10. a secondary coil; 11. an insulating layer; 12. an amplifier; 13. a compression head; 14. a strain gauge; 15. an incident rod; 16. a sample; 17. a transmission rod; 18. a buffer; 19. and (4) a boss.
In this embodiment, the loading device described in application No. 201510956545.4 is used. The electromagnetic induction type Hopkinson tension and compression bar loading device comprises a power supply 1, a capacitor charger 2 and a loading gun; the capacitor charger adopts a power supply part of the existing electromagnetic riveting equipment, and a positive electrode output line of the output of the capacitor charger is connected with a positive electrode line of the loading gun 6, and a negative electrode output line is connected with a negative electrode line of the loading gun; the loading gun comprises a loading gun shell, a main coil 8, a positioning cylinder 9, a secondary coil 10, an insulating layer 11 and an amplifier 12; a main coil 8 and a secondary coil 10 are sequentially sleeved on the positioning cylinder, and one end face of the secondary coil is adjacent to the positioning end face of the positioning cylinder; the other end face of the secondary coil is adjacent to and freely attached to one end face of the main coil; a positioning cylinder sleeved with a main coil and a secondary coil 10 is arranged in the middle section of a loading gun shell, an amplifier 12 is arranged at one end of the positioning cylinder, and an insulating layer 11 is sleeved between the inner end face of the amplifier and the end face of the secondary coil; the main coil 8, the secondary coil, the amplifier and the positioning cylinder are all coaxial with the loading gun shell; one end of the positioning cylinder 9 is connected with the secondary coil through threads; when a Hopkinson compression experiment is carried out, the connecting section of the compression head 13 is arranged in the internal thread hole of the amplifier, and the compression head 13 is in threaded connection with the amplifier; the inner end surface of the compression wave output section of the compression head is in contact with the incident rod 15, so that the stress wave is transmitted into the incident rod; when a Hopkinson tensile experiment is carried out, one end of the incident rod with the external thread sequentially penetrates through the positioning cylinder and the through hole of the amplifier, and is in threaded connection with the boss 19 on one side of the stress wave output section of the amplifier; when the compression stress wave transmitted from the stress wave output section of the amplifier enters the boss, the compression stress wave is reflected into a tensile wave on the free end surface of the boss and enters the incident rod 15 to form an incident wave of the Hopkinson pull rod.
In order to generate longer discharge current, a copper strip with the width of 25mm and the thickness of 1mm is wound on a core body with an I-shaped cross section by a 30-turn method through a main coil 8 in the electromagnetic induction type Hopkinson tension and compression bar loading device according to a conventional method. The outer diameter of the main coil is 150 mm. Inside the data collector 5, 5 electrolytic capacitors with the rated voltage of 3000V and the rated capacitance of 2 millifarads are connected in parallel, and the charging/discharging of the capacitor bank is controlled by an electronic switch.
In this embodiment, the incident rod 15 and the transmission rod 17 of the compression experiment are both titanium alloy rods with a length of 5cm and a diameter of 10mm, and one end of each titanium alloy rod is provided with a 1 cm-long thread for connecting the buffer 18.
The loading gun 6, the incident rod 15 and the transmission rod 17 were coaxially installed on the laboratory bench in this order in a conventional manner, and the transmission rod 17 was screw-coupled with the buffer 18. The sample 16 is interposed between the incident rod 15 and the transmission rod 17, and the sample 16 is coaxial with the incident rod 15 and the transmission rod 17. The buffer is in a block shape, and a threaded blind hole matched with the transmission rod 17 is formed in the center of one end face of the buffer. The compression head 13 is screwed to the amplifier 12. The positioning cylinder 9 passes through the through hole of the main coil. The compression head 13 of the load gun 6 is located at an end near the entrance bar. The end face of the stress wave output section of the compression head 13 is coaxially and fully attached to the end face of the incident rod 15 of the medium strain rate compression rod.
And 2, pasting the strain gauge.
The method for sticking the strain gauges 14 adopts the prior art, namely, four strain gauges with the same parameters are equally divided into two groups, one group of the strain gauges 14 is symmetrically stuck on the circumferential surface of an incident rod, and the other group of the strain gauges is symmetrically stuck on the circumferential surface of a transmission rod; both sets of strain gages are located at 1/2 of the incident and transmission rod axial lengths. The resistance of the strain gauge used in this example was 1000 ohms, and the sensitivity factor was 2.0.
And welding strain gauge lead wires 3 on the pins of each strain gauge, and respectively connecting the strain gauges into two bridge arms corresponding to the positions in the Wheatstone bridge 4 through the lead wires. The strain gauge lead wire adopts a double-core shielding wire to shield electromagnetic interference generated in the discharging process. The fixed resistances of the other two arms in the wheatstone bridge corresponding to the positions are both 1000 ohms. The supply voltage of the wheatstone bridge is 30 volts dc. The two diagonal voltages of the wheatstone bridge are input to the data collector 5 through the two-core shielded signal line. The data collector 5, in which the voltage signals on two opposite corners of the wheatstone bridge are processed using a differential method, employs GEN3i manufactured by HBM of germany, which has a good interference shielding capability.
And 3, loading and processing data.
When a middle strain rate compression experiment is carried out, after the capacitor charger 2 is charged by 200V, the discharge switch is closed, so that the capacitor charger discharges to the main coil 8 of the loading gun 6, and electromagnetic repulsion is generated between the secondary coil 10 and the main coil 8. The compression stress wave formed by the electromagnetic repulsion in the amplifier 12 is amplified by the amplifier 12 to form a sine-shaped incident wave; the incident wave is transmitted into an incident rod 15 of the medium strain rate compression rod through a compression head 13 and loads a sample, and one part of the incident wave forms a reflected wave in the incident rod, and the other part of the incident wave transmits into a transmission rod through the sample to form a transmitted wave.
The sample is made of industrial pure copper T2; the sample was cylindrical, 5mm in diameter and 5mm in length.
The resistance of the strain gauge adhered on the incident rod changes along with the change of incident waves in the incident rod, and then the resistance is converted into output voltage in the connected Wheatstone bridge; the resistance of the strain gauge attached to the transmission rod changes with the change of the transmitted wave in the transmission rod, and then is converted into the output voltage in the connected Wheatstone bridge. The output voltage in each wheatstone bridge is inputted through a two-core shielded signal line and stored in the data collector 5.
Obtaining an incident wave signal in an incident rod by formula (1):
σi＝2Ei△Ui/k(U0-△Ui) (1)
wherein σiIs the incident wave signal in the incident rod. Delta UiIs the voltage signal of a Wheatstone half-bridge arm connected with a strain gauge in an incident rod; k is the sensitivity coefficient of the strain gauge, which is 2.0 in this embodiment; u shape0The input voltage of the wheatstone bridge is 30V in the present embodiment; eiThe Young's modulus of the material of the incident rod 15, E in this embodimenti＝110Gpa。
The transmitted wave signal in the transmission rod is obtained by equation (2):
σt＝2Et△Ut/k(U0-△Ut) (2)
wherein σtIs a transmitted wave signal in the transmission rod. Delta UtIs the bridge arm voltage signal of the wheatstone half bridge connected to the strain gage in the transmission rod. EtIn this embodiment, E is the Young's modulus of the material of the transmission rod 17t＝110Gpa。
The obtained incident wave of the incident rod and the transmitted wave of the transmission rod 17 are processed by the stress processing formula (4) to obtain the internal stress sigma of the samples：
σs＝(σi+σt)×Ab/As/2 (4)
In the formula (4), σiIs the incident wave of the incident rod; sigmatA transmitted wave that is a transmitted rod; a. thesIs the cross-sectional area of the sample; a. thebIs the cross-sectional area of the incident rod.
The strain of the sample is measured directly by attaching a plastic strain gauge to the surface of the sample, or by DIC method using a high-speed camera. In this example, the strain of the sample was measured by attaching a plastic strain gauge to the surface of the sample. The resistance value of the strain gauge is 120 ohms, the sensitivity coefficient of the strain gauge is 2.0, two plastic strain gauges with the same parameters are symmetrically pasted on the circumferential surface of a sample, and each strain gauge is connected into an opposite bridge arm of a 120-ohm Wheatstone bridge through a lead. The wheatstone bridge has a supply voltage of 5 volts. Strain epsilon in the samplesThe calculation formula of (2) is as follows:
εs＝2△Us/k(U0-△Us) (5)
wherein epsilonsFor strain inside the sample, Delta UsIs a bridge arm voltage signal of a Wheatstone half bridge connected with a strain gauge on a sample; k is the sensitivity coefficient of the strain gauge, 2.0 in this example, U0The input voltage of the wheatstone bridge is 5V in this embodiment.
And drawing a scatter diagram in excel to obtain a medium strain rate compression experiment result curve, wherein the result curve comprises a stress-strain curve and a strain rate-time curve of the sample.
The specific treatment process comprises the following steps:
stress-strain curve of the sample: sample internal stress σ obtained by equation (4)sThe strain ε in the sample obtained by the following equation (5) as the y-axissThe stress-strain curve of the sample is obtained by plotting the x-axis.
Strain rate versus time curve for the test specimen:
the resulting strain function εsAnd obtaining a strain rate function inside the sample by taking a derivative of the time, and drawing a graph by taking the strain rate function as a y axis and the time as an x axis to obtain a strain rate-time curve of the sample.
And II, strain rate tensile test.
And carrying out a medium strain rate tensile test by adopting a medium strain rate tensile and compression bar test device based on electromagnetic force. The specific process is as follows:
And 2, pasting the strain gauge. The method for pasting the strain gauge 14 adopts the prior art, namely, on the circumference of 1/2 length of the incident rod 15 and the transmission rod 17, taking the axis of the incident rod or the transmission rod as a symmetry axis, pasting two pieces of strain gauges 14 with identical parameters on the surface of the incident rod or the transmission rod symmetrically. And welding strain gauge leads 3 on the pins of the strain gauge, and respectively connecting the strain gauge into two opposite bridge arms of a Wheatstone bridge 4 through leads. 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 vdc. Then, the two diagonal voltages of the wheatstone bridge are input to the data acquisition unit 5 through two conventional single-core shielding signal wires. The data collector 5 adopts GEN3i manufactured by Germany HBM company, has good interference shielding capability, and can shield the pulsed magnetic field interference generated in the discharging process by adopting a difference method.
And 3, loading and collecting data.
The loading process is that the charging voltage of the capacitor charger 2 is set to 1200V and the capacitor charger is charged, after the charging is finished, the capacitor charger discharges to the main coil 8 of the loading gun through the electronic switch, electromagnetic repulsion force is generated between the secondary coil 10 and the main coil 8, the electromagnetic repulsion force is amplified in the amplifier 12 to form compression stress wave, and the compression stress wave is reflected into stretching wave at the boss 19 and forms incident wave of a middle strain rate pull rod; the incident wave is transmitted into an incident rod 15 of the medium strain rate pull rod and loads a sample 16; during reflection, part of the incident wave is reflected by the end face of the sample, and the other part of the incident wave passes through the sample and enters the transmission rod to form a transmitted wave.
The sample 16 is made of industrial pure copper T2, the appearance of the sample is dumbbell-shaped, two ends of the sample are connecting sections with external threads, and the middle part of the sample is a testing section of a polished rod; the diameter of the test section is 3mm, and the length is 3 mm.
The resistance of the strain gauge adhered on the incident rod changes along with the change of incident waves in the incident rod, and then the resistance is converted into output voltage in the connected Wheatstone bridge; the resistance of the strain gauge attached to the transmission rod changes with the change of the transmitted wave in the transmission rod, and then is converted into the output voltage in the connected Wheatstone bridge. The output voltage in each wheatstone bridge is inputted through a two-core shielded signal line and stored in the data collector 5.
Obtaining an incident wave signal in an incident rod by formula (1)
σi＝2Ei△Ui/k(U0-△Ui) (1)
Wherein σiIs the incident wave signal in the incident rod. Delta UiIs a bridge arm voltage signal of a Wheatstone half bridge connected with a strain gauge in an incident rod; k is the sensitivity coefficient of the strain gauge, which is 2.0 in this embodiment; u shape0The input voltage of the wheatstone bridge is 30V in the present embodiment; eiThe Young's modulus of the material of the incident rod 15, E in this embodimenti＝110Gpa。
The transmitted wave signal in the transmission rod is obtained by equation (2):
σt＝2Et△Ut/k(U0-△Ut) (2)
wherein σtIs a transmitted wave signal in the transmission rod. Delta UtIs the bridge arm voltage signal of the wheatstone half bridge connected to the strain gage in the transmission rod. EtIn this example, E is the Young's modulus of the transmission rod materialt＝110Gpa。
The obtained incident wave of the incident rod and the transmitted wave of the transmission rod 17 are processed by the stress processing formula (4), and the stress inside the sample is obtained.
σs＝(σi+σt)×Ab/As/2 (4)
For the strain of the sample, a plastic strain gauge is pasted on the surface of the sample, so that the strain of the sample can be directly measured; or measuring the strain of the sample by using a high-speed camera by using a DIC method. In this example, the strain of the sample was measured by attaching a plastic strain gauge to the surface of the sample. The strain gauge adopts a plastic strain gauge with the resistance value of 120 ohms and the sensitivity coefficient of 2.0, two plastic strain gauges with the same parameters are symmetrically pasted on the circumferential surface of a sample, and each strain gauge is connected into an opposite bridge arm of a 120-ohm Wheatstone bridge through a lead. The wheatstone bridge has a supply voltage of 5 volts. The stress function of the sample is calculated by the formula
εs＝2△Us/k(U0-△Us) (5)
Wherein epsilonsFor strain inside the sample, Delta UsIs the bridge arm voltage signal of the Wheatstone half bridge connected to the strain gage on the sample, k is the sensitivity coefficient of the strain gage, 2.0 in this example, U0The input voltage of the wheatstone bridge is 5V in this embodiment.
And drawing a scatter diagram in an excel table to obtain a medium strain rate tensile experiment result curve, wherein the result curve comprises a stress-strain curve and a strain rate-time curve of the sample.
The specific treatment process is as follows:
stress-strain curve of the sample: sample stress function σ calculated by equation (4)sAs the y-axis, the strain function ε obtained by equation (5)sThe stress-strain curves of the samples were obtained by plotting the x-axis.
Strain rate versus time curve for the test specimen: by a strain function epsilonsAnd obtaining a strain rate function of the sample by calculating a derivative of the time, and drawing a graph by taking the strain rate function as a y axis and the time as an x axis to obtain a strain rate-time curve of the sample.
Claims (5)
1. A middle strain rate tension and compression rod loading experiment method adopts a middle strain rate tension and compression rod experiment device based on electromagnetic force, and comprises a middle strain rate compression experiment and a middle strain rate tension experiment; the method is characterized in that:
strain rate compression experiment in I:
the specific process of the medium strain rate compression experiment is as follows:
step 1, arranging equipment;
step 2, pasting a strain gauge;
step 3, determining a stress-strain curve and a strain rate-time curve of the sample:
obtaining the stress sigma inside the sample by loading the samplesAnd strain epsilon in the samples：
Obtaining the internal stress sigma of the sample by a stress processing formula (4)s：
σs＝(σi+σt)×Ab/As/2 (4)
In the formula (4), σiIs the incident wave of the incident rod; sigmatA transmitted wave that is a transmitted rod; a. thesIs the cross-sectional area of the sample; a. thebIs the cross-sectional area of the incident rod;
the strain ε in the sample was obtained by the following equation (5)s：
εs＝2△Us/k(U0-△Us) (5)
Wherein epsilonsFor strain inside the sample, Delta UsIs a bridge arm voltage signal of a Wheatstone half bridge connected with a strain gauge on a sample; k is the sensitivity coefficient of the strain gauge; u shape0Is the input voltage of the Wheatstone bridge;
stress σ in the sample obtained by the abovesAnd strain epsilon in the samplesDetermining a stress-strain curve and a strain rate-time curve of the sample; the method comprises the following steps:
stress-strain curve of the sample: sample internal stress σ obtained by equation (4)sThe strain ε in the sample obtained by the following equation (5) as the y-axissPlotting for the x axis to obtain the stress-strain curve of the sample;
strain rate versus time curve for the test specimen:
the resulting strain function εsObtaining a strain rate function inside the sample by calculating a derivative of time, and drawing a graph by taking the strain rate function as a y axis and time as an x axis to obtain a strain rate-time curve of the sample;
the specific process of loading the sample is to charge a capacitor charger to enable the capacitor charger to discharge a main coil of a loading gun to form sine incident waves; the incident wave is transmitted into an incident rod of the medium strain rate compression rod through the compression head and loads a sample, and one part of the incident wave is reflected on the end face of the sample, and the other part of the incident wave is transmitted into the transmission rod through the sample to form a transmission wave;
the resistance of the strain gauge pasted on the incident rod is converted into the output voltage in the connected Wheatstone bridge; the resistance of the strain gauge pasted on the transmission rod is converted into the output voltage in the connected Wheatstone bridge; the output voltage in each Wheatstone bridge is input and stored in a data acquisition unit through a double-core shielding signal wire;
obtaining an incident wave signal in an incident rod by formula (1):
σi＝2Ei△Ui/k(U0-△Ui) (1)
wherein σiIs an incident wave signal in an incident rod; delta UiIs the voltage signal of a Wheatstone half-bridge arm connected with a strain gauge in an incident rod; k is the sensitivity coefficient of the strain gauge; u shape0Is the input voltage of the Wheatstone bridge; eiIs the Young's modulus of the incident rod material;
the transmitted wave signal in the transmission rod is obtained by equation (2):
σt＝2Et△Ut/k(U0-△Ut) (2)
wherein σtIs a transmitted wave signal in the transmission rod; delta UtIs a bridge arm voltage signal of a Wheatstone half bridge connected with a strain gauge in the transmission rod; etIs the young's modulus of the transmission rod material;
strain rate tensile test in II:
performing a medium strain rate tensile test by adopting a medium strain rate tensile and compression bar test device based on electromagnetic force; the specific process is as follows:
step 1, arranging equipment;
step 2, pasting a strain gauge;
step 3, determining a stress-strain curve and a strain rate-time curve of the sample:
obtaining the stress sigma inside the sample by loading the samplesAnd strain epsilon in the samples：
Obtaining the internal stress sigma of the sample by a stress processing formula (4)s：
σs＝(σi+σt)×Ab/As/2 (4)
The strain ε in the sample was obtained by the following equation (5)s：
εs＝2△Us/k(U0-△Us) (5)
Wherein epsilonsFor strain inside the sample, Delta UsIs a bridge arm voltage signal of a Wheatstone half bridge connected with a strain gauge on a sample; k is the sensitivity coefficient of the strain gauge; u shape0Is the input voltage of the Wheatstone bridge;
stress σ in the sample obtained by the abovesAnd strain epsilon in the samplesDetermining a stress-strain curve and a strain rate-time curve of the sample; in particular to
Obtaining the stress inside the sample through a formula (4);
σs＝(σi+σt)×Ab/As/2 (4)
obtaining the internal strain of the sample by the formula (5)
εs＝2△Us/k(U0-△Us) (5)
Stress σ in the sample obtained by the abovesAnd strain epsilon in the samplesDetermining a stress-strain curve and a strain rate-time curve of the sample;the method comprises the following steps:
stress-strain curve of the sample: sample internal stress σ obtained by equation (4)sThe strain ε in the sample obtained by the following equation (5) as the y-axissPlotting for the x axis to obtain the stress-strain curve of the sample;
strain rate versus time curve for the test specimen:
the resulting strain function εsObtaining a strain rate function inside the sample by calculating a derivative of time, and drawing a graph by taking the strain rate function as a y axis and time as an x axis to obtain a strain rate-time curve of the sample;
the specific treatment process is as follows:
stress-strain curve of the sample: sample stress function σ calculated by equation (4)sAs the y-axis, the strain function ε obtained by equation (5)sDrawing a graph for an x axis to obtain a stress-strain curve of the sample;
strain rate versus time curve for the test specimen: by a strain function epsilonsObtaining a strain rate function of the sample by calculating a derivative of time, and drawing a graph by taking the strain rate function as a y axis and time as an x axis to obtain a strain rate-time curve of the sample; the specific process of loading the sample is to charge a capacitor charger to enable the capacitor charger to discharge a main coil of a loading gun to form sine incident waves; the incident wave is transmitted into an incident rod of the medium strain rate compression rod through the compression head and loads a sample, and one part of the incident wave is reflected on the end face of the sample, and the other part of the incident wave is transmitted into the transmission rod through the sample to form a transmission wave;
the resistance of the strain gauge pasted on the incident rod is converted into the output voltage in the connected Wheatstone bridge; the resistance of the strain gauge pasted on the transmission rod is converted into the output voltage in the connected Wheatstone bridge; the output voltage in each Wheatstone bridge is input and stored in a data acquisition unit through a double-core shielding signal wire;
obtaining an incident wave signal in an incident rod by formula (1):
σi＝2Ei△Ui/k(U0-△Ui) (1)
wherein σiIs an incident wave signal in an incident rod; delta UiIs the voltage signal of a Wheatstone half-bridge arm connected with a strain gauge in an incident rod; k is the sensitivity coefficient of the strain gauge; u shape0Is the input voltage of the Wheatstone bridge; eiIs the Young's modulus of the incident rod material;
the transmitted wave signal in the transmission rod is obtained by equation (2):
σt＝2Et△Ut/k(U0-△Ut) (2)
wherein σtIs a transmitted wave signal in the transmission rod; delta UtIs a bridge arm voltage signal of a Wheatstone half bridge connected with a strain gauge in the transmission rod; etIs the young's modulus of the transmissive rod material.
2. The strain rate tension and compression bar loading experimental method of claim 1, wherein when arranging the equipment, a loading gun, an incident bar and a transmission bar are coaxially and sequentially installed on an experimental bench, and the other end of the transmission bar is connected with a buffer;
one end of the loading gun where the compression head is located is close to the incident rod; the sample is mounted between the incident rod and the transmission rod, and is coaxial with the incident rod and the transmission rod.
3. The strain rate tension-compression bar loading experiment method as claimed in claim 1, wherein the strain gauge is adhered to the circumference of 1/2 lengths of the incident rod and the transmission rod, and two strain gauges with identical parameters are symmetrically adhered to the surfaces of the incident rod and the transmission rod by taking the axial lines of the incident rod and the transmission rod as symmetry axes; and respectively connecting the strain gauges into two opposite bridge arms of a Wheatstone bridge through leads.
4. The method for the strain rate tension and compression bar loading experiment as claimed in claim 1, wherein the outer diameter and the number of turns of the main coil are 3-5 times of those of a common Hopkinson bar loading device, and the capacitance is 3-5 times of those of the common Hopkinson bar loading device.
5. The method for the strain rate tension and compression bar loading experiment of claim 1, wherein the sample of the medium strain rate tension experiment is dumbbell-shaped, the two ends are connecting sections with external threads, and the middle part is a testing section of a polished rod; the sample for the medium strain rate compression test is cylindrical.
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| CN110646285B (en) * | 2019-10-09 | 2022-02-08 | 河南理工大学 | Tensile compression testing device for strain in material |
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| CN114112743B (en) * | 2020-09-01 | 2024-04-05 | 西安石油大学 | Electromagnetic hopkinson rod and stress wave generator thereof |
| CN113624605B (en) * | 2021-08-16 | 2024-06-11 | 陕西大工旭航电磁科技有限公司 | Medium strain rate experimental device based on electromagnetic force loading |
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