CN111579403A - Unidirectional dynamic tensile experiment method for brittle material - Google Patents

Unidirectional dynamic tensile experiment method for brittle material Download PDF

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CN111579403A
CN111579403A CN202010437750.0A CN202010437750A CN111579403A CN 111579403 A CN111579403 A CN 111579403A CN 202010437750 A CN202010437750 A CN 202010437750A CN 111579403 A CN111579403 A CN 111579403A
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郭亚洲
胡嘉奕
江斌
李玉龙
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Northwestern Polytechnical University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/32—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces
    • G01N3/38—Investigating strength properties of solid materials by application of mechanical stress by applying repeated or pulsating forces generated by electromagnetic means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/20—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress
    • G01L1/22—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress using resistance strain gauges
    • G01L1/2206—Special supports with preselected places to mount the resistance strain gauges; Mounting of supports
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
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    • G01N3/04—Chucks
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    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/02—Details
    • G01N3/06—Special adaptations of indicating or recording means
    • G01N3/066—Special adaptations of indicating or recording means with electrical indicating or recording means
    • 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
    • G01N2203/0001—Type of application of the stress
    • G01N2203/001—Impulsive
    • 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
    • G01N2203/0014—Type of force applied
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    • G—PHYSICS
    • G01—MEASURING; TESTING
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    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N2203/003—Generation of the force
    • G01N2203/005—Electromagnetic means
    • 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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    • G01N2203/0069—Fatigue, creep, strain-stress relations or elastic constants
    • G01N2203/0075—Strain-stress relations or elastic constants
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
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Abstract

The utility model provides a tensile experimental method of one-way developments for brittle material, utilizes electromagnetic energy conversion to produce stress pulse wave, realizes the production of pulse through the closure of discharge switch, and there is almost no time delay between the trigger closure of switch and the production of stress wave to the time of the production of accurate control pulse makes the experimentation more scientific, and the experimental result is more accurate. The invention solves the problem of difficult waveform shaping in the unidirectional tensile experiment process of the brittle material in the prior art, and enables the brittle sample to realize stress balance and constant strain rate loading in the bidirectional dynamic tensile experiment process.

Description

Unidirectional dynamic tensile experiment method for brittle material
Technical Field
The invention relates to the field of material dynamics, in particular to a split Hopkinson bar experimental method based on electromagnetic force loading and used for testing the unidirectional dynamic tensile mechanical property of a brittle material.
Background
Determining the dynamic tensile strength of brittle materials has been an important issue in the field of dynamic mechanics of materials. For brittle materials, testing for strength has been a challenge. Even under quasi-static loading conditions, conventional tensile testing with one end fixed and the other end loaded is very difficult and complicated to operate. Errors generated in the machining process of the test piece and deviation generated during assembly of the test piece, and different failure modes caused by stress concentration in the loading process have great influence on the experimental result.
At present, the most common method for measuring the dynamic strength of brittle materials is a split Hopkinson bar experiment technology. The principle of the technology is that a sample to be tested is placed between two waveguide rods, an external force accelerating mass block impacts an incident waveguide rod to generate stress pulses to load the sample, strain information on the waveguide rod is collected through strain gauges which are adhered to the two waveguide rods and have a certain distance from a rod end, and the stress strain information in the dynamic loading process of the sample is calculated through a wave shifting method.
Since the dynamic tensile property test of materials is carried out by utilizing a Hopkinson bar technology in Harding and the like in the 60 th 20 th century, through long-term development, the current experimental technology for realizing dynamic tensile loading based on the Hopkinson bar can be divided into two types: one is to directly carry out tensile loading on a sample, and comprises direct tensile, reflective tensile, delamination experiment and the like; another is to change the specimen configuration to convert a compressive load applied to the specimen to a tensile load applied to a location in the specimen, such as a cap specimen, a disk specimen, a three point bend specimen, and the like.
The direct tensile test of the brittle material has the characteristics of difficult sample processing, unstable damage form and the like. The invention mainly utilizes a brittle material dynamic single-direction and two-direction disc splitting and stretching experimental method based on an electromagnetic loading device. As the traditional Hopkinson bar experiment technology adopts direct loading, the loading wave is approximate to a square wave, the rising front edge is about 10-20 mu s, and the wave head is superposed with high-frequency components caused by direct collision, the elastic wave speed is about 5000m/s for high-impedance materials such as metal, and even if the thickness of a sample exceeds 10mm, the stress balance can be achieved within the rising time of a carrier wave. For the brittle material with low impedance, the wave speed is low, the damage strain is small, and the damage time is short, so that accurate and credible experimental data cannot be obtained by completely adopting the traditional Hopkinson bar experimental technology. For the problem that the stress balance and uniform deformation of a brittle material are difficult to achieve in the loading process, two solutions are mainly provided at present, and one solution is improved from the experimental technology; and the other type is to take certain measures on data processing to correct experimental data deviation. Both methods present significant difficulties in operational implementation.
In recent years, we have applied for a series of experimental devices and methods based on electromagnetically loaded hopkinson bars. The development of the electromagnetic induction type Hopkinson tension and compression bar loading device directly generates stress waves by generating electromagnetic repulsion force. 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, low strain rate loading (for example, 100 s) which cannot be realized by some traditional Hopkinson bars can be realized-1Below). In chinese patents No. 201420098605.4 and No. 201410161610.X, an apparatus scheme and an experimental method for directly applying an electromagnetic riveting device in a hopkinson pressure bar device are proposed, respectively, but the waveform obtained in this method has limitations. In two Chinese inventions with application numbers of 201410173843.1 and 201410171963.8, two experimental devices and experimental methods which can be used for a Hopkinson pull rod and a Hopkinson pressure rod are respectively provided, but the two schemes are complex in structure, and the traditional wave shaping technology cannot be applied to the stretching condition. In the invention and creation of patent No. 201510051071.9, a main coil structure of an electromagnetic stress wave generator and a using method thereof are proposed to improve the variation range of the amplitude and the pulse width generated by the electromagnetic stress wave generator. However, in this method, the stress wave generated by the stress wave generator cannot be directly used in the tensile test, and the conventional wave shaping technology cannot be applied to the test equipment.
Disclosure of Invention
In order to overcome the step of difficult waveform shaping in the prior art, the invention provides a unidirectional dynamic stretching experimental method for a brittle material.
The specific process of the invention is as follows:
step 1, arranging equipment and clamping a sample;
the arrangement equipment is used for arranging the charging and discharging system; sequentially enabling the contact surface between a secondary coil and a main coil in the charging and discharging system to be in seamless fit, and enabling the incident rod and the secondary coil to be in seamless fit at the end surface; respectively sticking resistance strain gauges on the circumferential surface at 1/2 of the length of the incident rod and the circumferential surface at 1/2 of the length of the transmission rod, enabling lead wires of the strain gauges to face the end, in contact with the sample, of the waveguide rod, enabling the lead wires of the resistance strain gauges to be parallel to the axis of the waveguide rod, and after being bent at a right angle, the lead wires of the resistance strain gauges are connected with a signal input end of a data acquisition unit, and enabling the lead wires of the strain gauges to be in a linear state in communication with the data acquisition unit; and each resistance strain gauge is connected into the data acquisition system in a Wheatstone bridge connection mode.
The injection rod and the transmission rod are made of titanium alloy with Young modulus of 123GPa and elastic wave speed of 5189m/s, and the diameter and the length of the injection rod are the same as those of the transmission rod.
The thickness of the secondary coil is 10 mm; the rated voltage of the resistance strain gauge is 30V, the resistance value is 1000 omega, and the sensitivity coefficient is 1.92.
Respectively arranging hard cushion blocks on two surfaces of a glass sample, and attaching the surfaces of the sample to the surfaces of the hard cushion blocks; and finally, placing the sample clamped by the hard cushion blocks between the incident rod and the transmission rod, and fitting the sample with the end surfaces of the two waveguide rods in a seamless manner to complete experimental preparation work.
The hard cushion block is composed of two materials with different densities and elastic wave velocities, and the hard cushion block is formed by filling a material with a high density in the center of a circular material with a lower density; the thickness of cushion is 5 ~ 6mm, confirms the parameter of cushion through formula (1):
ρaCaA＝ρbCbA1+ρcCcA2(1)
in the formula, ρaBeing a dense beam of incident or transmission lightDegree; rhobThe density corresponding to the high density material of which the cushion block is made; rhocThe density corresponding to the low density material of which the cushion block is made; caThe wave velocity of the stress wave of the incident rod or the transmission rod; cbThe wave velocity of the stress wave corresponding to the high-density material forming the cushion block; ccThe wave velocity of the stress wave corresponding to the low-density material forming the cushion block;
a is the cross-sectional area of incident or transmission rod, A ═ A1+A2(ii) a Wherein A is1The cross-sectional area corresponding to the high-density material constituting the mat, A2The cross-sectional area corresponding to the low-density material of the cushion block;
the low-density material in the cushion block is a 6063 aluminum ring, and the high-density material is pure tungsten.
Step 2, selection of experimental voltage and capacitance:
setting charging voltage XV of a console capacitor charger to 800V, wherein the charging voltage needs to be set within the rated voltage of the capacitor charger; the capacitance value required by the split Hopkinson bar experimental device loaded based on the electromagnetic force is adjusted by connecting a plurality of capacitors in parallel or in series;
step 3, charging the capacitor charger group:
charging the capacitor charger group according to the set charging voltage;
step 4, unidirectional dynamic loading:
after the capacitor bank is charged, starting a discharge switch of the electromagnetic loading experiment system to discharge a main coil of the electromagnetic loading gun until the voltage of the capacitor is reduced to 0V; the discharging current passes through the main coil of the electromagnetic loading gun and generates electromagnetic repulsion; the electromagnetic repulsion acts on the secondary coil to generate loading stress waves, and the loading stress waves are transmitted to the sample through the incident rod to carry out unidirectional dynamic loading on the sample for 120 us;
step 5, collecting and processing experimental data
Converting the strain signal of the incident rod or the strain signal of the transmission rod, in which each resistance strain gauge is positioned, into a voltage signal respectively through the strain gauges pasted on the incident rod and the transmission rod, and outputting the voltage signals to the input rod or the transmission rodIn the data collector; converting the voltage signal of the strain gauge on the incident rod obtained by the data acquisition unit into the reflection strain according to the bridge box formula (2) and the wave shifting methodRConverting the signal of the strain gauge on the transmission rod obtained by the data acquisition unit into transmission strainT
Figure BDA0002502916520000041
In the formula, the signal is a strain signal, the K value is the sensitivity coefficient of the selected strain gauge, the delta U is the voltage value output by the corresponding acquisition channel along with the change of time, and U is the voltage value output by the corresponding acquisition channel along with the change of time0Is the Wheatstone bridge voltage;
according to the one-dimensional stress wave theory, the internal stress sigma, the engineering strain and the engineering strain rate of the sample under the condition of unidirectional loading are obtained through formulas (3), (4) and (5)
Figure BDA0002502916520000051
Figure BDA0002502916520000052
Figure BDA0002502916520000053
Figure BDA0002502916520000054
In the formula, sigma is the engineering stress of the sample to be measured; engineering strain of a sample to be tested;
Figure BDA0002502916520000055
is the engineering strain rate; e is the elastic modulus of the titanium alloy waveguide rod, A is the cross-sectional area of the titanium alloy waveguide rod, c0Is the elastic wave velocity of the titanium alloy wave guide rod, A0For the cross-sectional area of the sample to be measured, /)0Testing the length of a segment for a sample to be tested;
obtaining the dynamic tensile strength sigma of the sample according to the Brazilian disc splitting formulaT：
Figure BDA0002502916520000056
Figure BDA0002502916520000057
In the formula, D is the diameter of a disc sample to be measured, P is the compressive stress applied to the sample, and t is the thickness of the disc sample to be measured;
with time t as the horizontal axis and dynamic tensile stress σtPlotting is carried out for a longitudinal axis, and a stress-time curve of the sample under the unidirectional dynamic stretching condition is obtained;
the horizontal axis is taken as the time t,
Figure BDA0002502916520000058
drawing a vertical axis to obtain a tensile strain rate-time diagram of the sample under the unidirectional dynamic stretching condition; obtaining the curves of the unidirectional loading tensile stress and the bidirectional loading tensile strain rate of the sample along with the change of time;
the unidirectional dynamic experimental method for brittle materials is now complete.
The split Hopkinson bar experimental device based on electromagnetic force loading is the prior art.
The invention solves the problem of difficult waveform shaping in the unidirectional tensile experiment process of the brittle material in the prior art, and enables the brittle sample to realize stress balance and constant strain rate loading in the bidirectional dynamic tensile experiment process.
Due to the charging and discharging principle of the internal LC circuit and the unidirectional conduction characteristic of the thyristor, the discharging current is in a semi-sine shape. The magnitude of the discharge current increases with increasing charge voltage, the discharge time increases with increasing capacitance, and the increase in capacitance also causes an increase in the magnitude of the discharge current when the charge voltage is held constant. For the dynamic loading experiment of the brittle material, the semi-sine waveform has a more gentle rising edge, so that the condition of the dynamic loading stress balance of the brittle material can be well met without further carrying out the shaping work of the loading waveform. The stress pulse wave generated by the electromagnetic energy conversion technology is generated by closing the discharge switch, and almost no time delay exists between the trigger closing of the switch and the generation of the stress wave, so that the accurate time of the generation of the pulse is easily controlled by a circuit, and the advantage enables the experimental process to be more scientific and the experimental result to be more accurate.
The experimental device used in the invention is a split Hopkinson bar experimental device based on electromagnetic force loading in the prior art, and comprises a capacitor charger, an electromagnetic loading gun and a waveguide bar.
In the split hopkinson rod experimental device based on electromagnetic force loading, the waveguide rod 1 and the waveguide rod 2 are two cylindrical titanium alloy rods with the same length, and two end faces of each titanium alloy rod are smooth and flat. In order to ensure that the first reflected wave cannot be mutually superposed with the incident wave at the strain gauge collection position to generate the phenomenon of 'eating wave', the length L of the waveguide rod follows the following principle in design: the length L of the waveguide rod is greater than or equal to the distance traveled by the stress wave in a pulse width, namely L is greater than or equal to CT, in the expression, C is the propagation speed of the stress wave in the waveguide rod, and T is the pulse width of the applied stress wave.
Fig. 2 is a typical original waveform recorded by a data acquisition system in a dynamic unidirectional loading experiment of glass by using split hopkinson based on electromagnetic force loading. The graph shows that the incident wave 12 generated by the split hopkinson bar loaded based on electromagnetic force has a gentle rising edge different from the rectangular wave, and the slope of the rising edge is similar to that of the transmitted wave 13, so that a platform section 14 appears on the reflected wave 15, namely, the platform section means that the sample achieves stress balance and constant strain rate deformation. FIG. 3 is a graph of strain rate, dynamic stress versus time for the same experiment. As can be seen from fig. 3, the specimen started to enter a stress equilibrium state at about 60 μ s and completed an approximately constant strain rate deformation for about 30 μ s, and finally the dynamic uniaxial tensile stress of the specimen was 45 MPa.
Drawings
FIG. 1 is a schematic structural diagram of a dynamic unidirectional split Hopkinson bar experimental device based on an electromagnetic force loading mode adopted by the invention;
FIG. 2 is a typical waveform plot from a uni-directional dynamic tensile experiment using an electromagnetic Hopkinson bar;
FIG. 3 is a plot of stress and strain rate versus time for a typical sample obtained from a uniaxial dynamic tensile test using an electromagnetic Hopkinson bar.
Fig. 4 is a flow chart of the present invention.
1. A main coil; 2. a secondary coil; 3. an incident rod; 4. a resistance strain gauge; 5. a sample; 6. a data acquisition unit; 7. a diode; 8. a control switch; 9. an inductor coil; 10. a power source; 11. a transmission rod; 12. incident waves; 13. transmitting the wave; 14. a platform section; 15. reflecting the wave; 16. loading tensile stress in a single direction; 17. unidirectional loading tensile strain rate; 18. and a cushion block.
Detailed Description
The embodiment is a dynamic unidirectional tensile experiment method for a brittle material, and the experiment adopts the existing split Hopkinson bar experiment device based on electromagnetic force loading. The method specifically comprises the following steps:
step 1, arranging equipment and clamping sample
The electromagnetic loading gun as shown in fig. 1 is partially assembled, and includes a charging and discharging system composed of a main coil 1, a secondary coil 2, a diode 7, a control switch 8, an inductance coil 9 and a power supply 10. Before starting to dynamically load the sample, a secondary coil 2 with the thickness of 10mm is selected, one end face of the secondary coil 2 is in seamless joint with one end face of the main coil 1, and one end face of the incident rod 3 is in seamless joint with one end face of the secondary coil 2. The incident rod 3 and the transmission rod 11 are made of titanium alloy with Young modulus of 123GPa and elastic wave speed of 5189m/s, the incident rod 3 and the transmission rod 11 are both 25mm in diameter and 3000mm in length. The resistance strain gauges 4 are four pieces in total, wherein two resistance strain gauges are symmetrically adhered to the circumferential surface of the incident rod length 1/2, and the other two resistance strain gauges are symmetrically adhered to the circumferential surface of the transmission rod length 1/2. The lead wires of the strain gauges face to one end, where the incident rod or the transmission rod is located, of the strain gauge, which is in contact with the sample, are arranged along the axial line of the incident rod or the axial line of the transmission rod, are bent at a right angle and then are respectively connected with the signal input end of the data acquisition unit 6, so that the lead wires of the strain gauges are in a linear state when being communicated with the data acquisition unit,
the rated voltage of the resistance strain gauge is 30V, the resistance value is 1000 omega, and the sensitivity coefficient is 1.92. And connecting each resistance strain gauge 4 into a data acquisition system in a Wheatstone bridge connection mode.
In this example, the sample 5 was a glass disk having a diameter of 9mm and a thickness of 5 mm. Due to the fact that the elastic modulus of the brittle material is high, the cushion block 18 can protect the loading ends of the incident rod and the transmission rod, and meanwhile the influence of the indentation effect on the experiment is reduced.
The cushion is by the disc that two kinds of density, the all inequality material of elastic wave speed constitute jointly, and it is the lower ring shape material center of density usually to inlay through hot mosaic technology has another kind of high material of density to constitute, and cushion thickness is 5 ~ 6mm, need confirm the parameter of cushion through formula (1) according to the principle of wave impedance matching:
ρaCaA＝ρbCbA1+ρcCcA2(1)
in the formula, ρaDensity of incident or transmission rods; rhobThe density corresponding to the high density material of which the cushion block is made; rhocThe density corresponding to the low density material of which the cushion block is made; caThe wave velocity of the stress wave of the incident rod or the transmission rod; cbThe wave velocity of the stress wave corresponding to the high-density material forming the cushion block; ccThe wave velocity of the stress wave corresponding to the low-density material of the cushion block.
A is the cross-sectional area of incident or transmission rod, A ═ A1+A2(ii) a Wherein A is1The cross-sectional area corresponding to the high-density material constituting the mat, A2The cross-sectional area corresponding to the low density material comprising the mat.
In this embodiment, the pad block is made of an aluminum ring and pure tungsten embedded in the center of the aluminum ring. The aluminum ring is 6063 aluminum, the outer diameter of the aluminum ring is 25.1mm, the inner diameter of the aluminum ring is 8.35mm, and the thickness of the aluminum ring is 5 mm; the diameter of the tungsten carbide is 8.35mm, and the thickness is 5 mm.
Uniformly coating molybdenum disulfide lubricating grease on the surface of the cushion block, which is in contact with the sample; the sample is placed in the center of the pad block and is in contact with the inner edge of the aluminum ring and the surface of the tungsten carbide, and two surfaces of the sample are respectively attached to one surface of each pad block. Placing the sample clamped by the cushion block between the incident rod 3 and the transmission rod 11, and enabling two panels of the cushion block to be in seamless joint with the end faces of the incident rod 3 and the transmission rod 11 respectively to finish experimental preparation work
Step 2, selecting experimental voltage and capacitance
Starting the split hopkinson pole experimental device based on electromagnetic force loading, setting the charging voltage of the console capacitor charger to be XV and charging, where X is a specific required voltage value, and in this embodiment, X is 800V. The charging voltage needs to be set within the rated voltage of the capacitor charger. Through parallelly connected or series connection a plurality of capacitors with the required capacitance value of adjustment based on electromagnetic force loaded disconnect-type hopkinson pole experimental apparatus, 2 mf's electric capacity has been selected in this embodiment, and when the capacitance value increased, the stress wave amplitude under the same charging voltage also can be higher, consequently, the charging voltage that reaches the same stress wave amplitude under the great capacitance value also can be lower.
Step 3, charging the capacitor charger group
And after all the parameters are set, starting the charging options of the control module of the experimental system. The main circuit charging module starts to charge the capacitor charger group to a set voltage X. In this embodiment, the voltage X is 800V.
Step 4, unidirectional dynamic loading
And after the capacitor bank is charged, starting a discharge switch of the electromagnetic loading experiment system to discharge the main coil of the electromagnetic loading gun until the voltage of the capacitor is reduced to 0V. The discharging current passes through the main coil of the electromagnetic loading gun and generates electromagnetic repulsion; the electromagnetic repulsion acts on the secondary coil to generate loading stress waves, and the loading stress waves are transmitted to the sample through the incident rod 3 to carry out unidirectional dynamic loading on the sample for 120 us.
Step 5, collecting and processing experimental data
In order to obtain the stress and strain rate history of the sample in the dynamic bidirectional loading process, the characteristic voltage time waveform diagram obtained by the data acquisition unit 6 can be processed. And through the strain gauge adhered to the incident rod 3 and the strain gauge adhered to the transmission rod 11, the strain signal of the incident rod or the strain signal of the transmission rod where each strain gauge is located is converted into a voltage signal and output to the data acquisition unit 6. According to a bridge box formula (2) and a wave shifting method, voltage signals of strain gauges on an incident rod 3 obtained by a data acquisition unit 6 are converted into reflection strainRConverting the signal of the strain gauge on the transmission rod 11 obtained by the data acquisition unit 6 into transmission strainT。
Figure BDA0002502916520000091
In the formula, K is the sensitivity coefficient of the selected strain gauge, Δ U is the voltage value of the corresponding acquisition channel which is output along with the change of time, and U is the strain signal0Is the wheatstone bridge voltage. Substituting the voltage signal of the incident wave into a formula (2) to obtain the voltage signal; substituting the voltage signal of the reflected wave into formula (2) to obtain
The one-dimensional elastic stress wave theory is described in the stress wave base of the royal etiquette according to the one-dimensional stress wave theory, which was published by the national defense industry press in 2005. Obtaining the internal stress sigma, the engineering strain and the engineering strain rate of the sample under the condition of unidirectional loading through the formulas (3), (4) and (5)
Figure BDA0002502916520000101
Figure BDA0002502916520000102
Figure BDA0002502916520000103
Figure BDA0002502916520000104
In the formula, sigma is the engineering stress of the sample to be measured; engineering strain of a sample to be tested;
Figure BDA0002502916520000105
is the engineering strain rate; e is the elastic modulus of the titanium alloy waveguide rod, A is the cross-sectional area of the titanium alloy waveguide rod, c0Is the elastic wave velocity of the titanium alloy wave guide rod, A0For the cross-sectional area of the sample to be measured, /)0The length of the test segment of the sample to be tested.
The dynamic tensile strength σ of the sample was obtained from the conclusion of the disc splitting experiment set forth in Sheikh, M.Z., et al, Static and dynamic Brazilian disk disks tests for mechanical properties of artificial and chemical structures ceramic International,2019.45(6): p.7931-7944t：
Figure BDA0002502916520000106
Figure BDA0002502916520000107
In the formula, σTAnd D is the diameter of the disc sample to be detected, P is the compressive stress applied to the sample, and t is the thickness of the disc sample to be detected.
With time t as the horizontal axis and dynamic tensile stress σtThe stress-time curve of the specimen under the condition of uniaxial dynamic tension is obtained by plotting the vertical axis.
The horizontal axis is taken as the time t,
Figure BDA0002502916520000108
the tensile strain rate versus time under the uniaxial dynamic tension conditions of the test specimen is obtained by plotting the vertical axis. Obtain the unidirectional loading tension of the sampleStress and bi-directional loading tensile strain rate versus time.
The unidirectional dynamic experimental method for brittle materials is now complete.
The invention provides a dynamic tensile experimental method for a brittle material sample by utilizing the existing split Hopkinson bar device based on an electromagnetic force loading mode. It can be found that the glass samples selected in the examples had a strain rate of 100s-1The dynamic tensile strength at room temperature was 45 MPa. The constant strain rate loading of the brittle sample under the stress balance condition can be realized without adopting a complex pulse shaping technology in the traditional SHPB experiment and designing a complex sample structure, so that a simple and convenient experiment method easy to operate is provided for the dynamic mechanical property test of the brittle material.

Claims (6)

1. A unidirectional dynamic tensile test method for brittle materials is characterized by comprising the following specific processes:
step 1, arranging equipment and clamping a sample;
step 2, selection of experimental voltage and capacitance:
setting the charging voltage X of the console capacitor charger to 800V, and enabling the charging voltage to be within the rated voltage of the capacitor charger; the capacitance value required by the split Hopkinson bar experimental device loaded based on the electromagnetic force is adjusted by connecting a plurality of capacitors in parallel or in series;
step 3, charging the capacitor charger group:
charging the capacitor charger group according to the set charging voltage;
step 4, unidirectional dynamic loading:
after the capacitor bank is charged, starting a discharge switch of the electromagnetic loading experiment system to discharge a main coil of the electromagnetic loading gun until the voltage of the capacitor is reduced to 0V; the discharging current passes through the main coil of the electromagnetic loading gun and generates electromagnetic repulsion;
the electromagnetic repulsion acts on the secondary coil to generate loading stress waves, and the loading stress waves are transmitted to the sample through the incident rod to carry out unidirectional dynamic loading on the sample for 120 us;
step 5, collecting and processing experimental data
Respectively converting the strain signal of the incident rod or the strain signal of the transmission rod, in which each resistance strain gauge is positioned, into a voltage signal through the strain gauge adhered to the incident rod and the strain gauge adhered to the transmission rod, and outputting the voltage signal to the data acquisition unit;
converting the voltage signal of the strain gauge on the incident rod obtained by the data acquisition unit into the reflection strain according to the bridge box formula (2) and the wave shifting methodRConverting the signal of the strain gauge on the transmission rod obtained by the data acquisition unit into transmission strainT
Figure FDA0002502916510000011
In the formula, the signal is a strain signal, the K value is the sensitivity coefficient of the selected strain gauge, the delta U is the voltage value output by the corresponding acquisition channel along with the change of time, and U is the voltage value output by the corresponding acquisition channel along with the change of time0Is the Wheatstone bridge voltage;
according to the one-dimensional stress wave theory, the internal stress sigma, the engineering strain and the engineering strain rate of the sample under the condition of unidirectional loading are obtained through formulas (3), (4) and (5)
Figure FDA0002502916510000014
Figure FDA0002502916510000012
Figure FDA0002502916510000013
Figure FDA0002502916510000021
In the formula, sigma is the engineering stress of the sample to be measured; engineering strain of a sample to be tested;
Figure FDA0002502916510000024
is the engineering strain rate; e is the elastic modulus of the titanium alloy waveguide rod, A is the cross-sectional area of the titanium alloy waveguide rod, c0Is the elastic wave velocity of the titanium alloy wave guide rod, A0For the cross-sectional area of the sample to be measured, /)0Testing the length of a segment for a sample to be tested;
obtaining the dynamic tensile strength sigma of the sample according to the Brazilian disc splitting formulaT：
Figure FDA0002502916510000022
Figure FDA0002502916510000023
In the formula, D is the diameter of a disc sample to be measured, P is the compressive stress applied to the sample, and t is the thickness of the disc sample to be measured; with time t as the horizontal axis and dynamic tensile stress σtPlotting is carried out for a longitudinal axis, and a stress-time curve of the sample under the unidirectional dynamic stretching condition is obtained;
the horizontal axis is taken as the time t,
Figure FDA0002502916510000025
drawing a vertical axis to obtain a tensile strain rate-time diagram of the sample under the unidirectional dynamic stretching condition; obtaining the curves of the unidirectional loading tensile stress and the bidirectional loading tensile strain rate of the sample along with the change of time; the unidirectional dynamic experimental method for brittle materials is now complete.
2. A uniaxial dynamic tension test method for brittle materials as claimed in claim 1, wherein the arranging device in step 1 is an arrangement of charging and discharging systems; sequentially enabling the contact surface between a secondary coil and a main coil in the charging and discharging system to be in seamless fit, and enabling the incident rod and the secondary coil to be in seamless fit at the end surface; respectively sticking resistance strain gauges on the circumferential surface at 1/2 of the length of the incident rod and the circumferential surface at 1/2 of the length of the transmission rod, enabling lead wires of the strain gauges to face the end, in contact with the sample, of the waveguide rod, enabling the lead wires of the resistance strain gauges to be parallel to the axis of the waveguide rod, and after being bent at a right angle, the lead wires of the resistance strain gauges are connected with a signal input end of a data acquisition unit, and enabling the lead wires of the strain gauges to be in a linear state in communication with the data acquisition unit; and each resistance strain gauge is connected into the data acquisition system in a Wheatstone bridge connection mode.
3. A uniaxial dynamic tensile test method for brittle materials as claimed in claim 2, wherein the injection rod and the transmission rod are made of titanium alloy having young's modulus of 123GPa and elastic wave velocity of 5189m/s, and the diameter and length of the injection rod are the same as those of the transmission rod.
4. A uniaxial dynamic tensile test method for brittle materials according to claim 2, wherein the thickness of the secondary coil is 10 mm; the rated voltage of the resistance strain gauge is 30V, the resistance value is 1000 omega, and the sensitivity coefficient is 1.92.
5. A uniaxial dynamic tensile test method for a brittle material as claimed in claim 1, wherein, when a sample is clamped, hard spacers are respectively arranged on two surfaces of the sample, and the surface of the sample is attached to the surface of each hard spacer; and placing the sample clamped by the hard cushion blocks between the incident rod and the transmission rod, and fitting the sample with the end surfaces of the two waveguide rods in a seamless manner to complete experimental preparation work.
6. A uniaxial dynamic tensile test method for brittle materials as claimed in claim 1, wherein the hard pad is composed of two materials with different densities and elastic wave velocities, and the hard pad is formed by filling a material with a high density in the center of a circular ring-shaped material with a lower density; the thickness of cushion is 5 ~ 6mm, confirms the parameter of cushion through formula (1):
ρaCaA＝ρbCbA1+ρcCcA2(1)
in the formula, ρaDensity of incident or transmission rods; rhobThe density corresponding to the high density material of which the cushion block is made; rhocThe density corresponding to the low density material of which the cushion block is made; caThe wave velocity of the stress wave of the incident rod or the transmission rod; cbThe wave velocity of the stress wave corresponding to the high-density material forming the cushion block; ccThe wave velocity of the stress wave corresponding to the low-density material forming the cushion block;
a is the cross-sectional area of incident or transmission rod, A ═ A1+A2(ii) a Wherein A is1The cross-sectional area corresponding to the high-density material constituting the mat, A2The cross-sectional area corresponding to the low-density material of the cushion block;
the low-density material in the cushion block is a 6063 aluminum ring, and the high-density material is pure tungsten.
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