KR101727405B1 - Modification of Hoek triaxial cell for SHPB tests and its application to dynamic shear strength measurement of brittle materials - Google Patents

Abstract

The present invention relates to an impact compression cell device under confinement load manufactured to be applicable to an SHPB impact tester and a dynamic fracture property measuring device using the same and, more specifically, relates to an impact load bar of a conventional Hoek triaxial compression cell, a rubber frame to prevent impact of cell compression, an impact load bar for high speed shear test, and dynamic triaxial compressive strength and dynamic shear failure criterion for brittle materials in a simpler manner. The present invention devises a triaxial compression cell applicable to SHPB devices; the dynamic property of brittle materials in response to a stress condition at high speed collision or explosion is able to be obtained using the same; and at the same time, a high speed shear test technique proposed.

Description

[0001] The present invention relates to a triaxial compression cell for SHPB impact test and a dynamic triaxial shear test method using the same,

The present invention relates to a constrained load triaxial compression cell apparatus and a dynamic fracture property measurement apparatus using the same, which are made to be applicable to an SHPB impact tester, and more particularly to an apparatus for measuring a dynamic fracture property using an impact load rod improved in a load bar of a conventional Hoek triaxial compression cell, To an apparatus for simply obtaining a dynamic compressive strength and a dynamic shear fracture reference equation for a brittle material using the same.

The Split Hopkinson Pressure Bar (SHPB) impact system is used to characterize fracture behavior or deformation behavior of brittle materials under high strain rates. SHPB technique was developed to identify the dynamic behavior characteristics of the machine material at the strain rate of 100 ~ 10000 / s by Kolsky 1949 years, up to 10 ⁴ s based on the development of the possible oscilloscope and data acquisition system stores at a high speed of large amounts of data ^0.0 > strain-rate < / RTI >

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of a Split Hopkinson Pressure Bar (SHPB) impact system. In the SHPB apparatus composed of the impact bar (1), the input rod (2), and the output bar (3), the specimen is fixed between the input rod and the output rod, and the impact rod is fired at a constant speed to collide with the input rod. The elastic waves are transmitted to the input rod, deform the specimen, and then transmit it to the output rod.

From the signals from each strain gage (5) attached at approximately the midpoint of the input and output rods and from the formula assuming the input rod and one-dimensional acoustic wave in the output rod, the time dependence on the end of the input rod and output rod The load and displacement can be determined.

The above-described SHPB apparatus can directly obtain the stress-strain relationship according to the deformation history of the material in the deformation range with a strain rate of approximately 10 < ^{2 >} / s to 10 < ⁴ & It is widely used because it does not need to attach the load cell or strain gauge directly to the specimen.

However, the SHPB device of Fig. 1 can only measure uniaxial dynamic properties in a constrained state in one direction. In general, the stress state generated by high-speed collision or explosion lies in the state of triaxial compression (pressurization in three directions), so evaluation of the dynamic properties in the bar-pressure (confining pressure) state is important. Unlike the SHPB impact test method, when applied to a horizontally placed impact load rod, the shock absorber is pushed by the pressure, which makes it difficult to obtain normal data. In order to overcome this disadvantage, it is necessary to axially restrain the horizontal impact bar.

The present invention proposes a triaxial compression cell and its testing apparatus which are applicable to conventional SHPB equipment and enable dynamic triaxial compression strength and dynamic shear failure type acquisition through axial confinement.

One aspect of the present invention is

A first impact load rod and a second impact load rod positioned on both sides of the sample to fix the sample in the horizontal direction;

A cylindrical hydraulic membrane surrounding the side surface of the sample and restraining the sample in an axial direction; And

And a housing part for fixing the first impact load rod, the second impact load rod, and the membrane.

In another aspect,

Gas guns that launch impact rods;

A gun barrel accelerated by the impact rod;

An incident rod which is spaced apart from the gun barrel by a predetermined distance, and which collides with the impact rod in the same line;

A triaxial compression cell applying an axial confining pressure to the sample;

And a transmission rod positioned on the same line as the incident rod and the triaxial compression cell.

The device of the present invention reproduces axial confinement pressure for a sample by developing and introducing a triaxial compression cell into a conventional SHPB device. This makes it possible to obtain dynamic triaxial compressive strength properties for brittle materials which have not been possible with the existing SHPB test equipment. In addition, it is possible to propose a high-speed shear test method and obtain a dynamic shear failure criterion.

The triaxial compression cell according to the present invention and the high-speed shear test method using the same are designed to reproduce the stress condition to which brittle materials such as rock and concrete are subjected during normal high-speed collision or explosion, and to evaluate the dynamic properties thereof.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic of a Split Hopkinson Pressure Bar (SHPB) impact system.
2 is a schematic diagram of a dynamic property acquisition test apparatus using a triaxial compression cell according to an embodiment of the present invention.
3 shows a dynamic triaxial compression strength evaluation method using a triaxial compression cell.
Fig. 4 shows that the sample is placed in the confined pressure state by the triaxial compression cell.
5 shows a high-speed shear test technique using a triaxial compression cell.
FIG. 6 shows an enlarged view of the triaxial compression cell 250 portion.
Fig. 7 shows a production example of the first impact load rod 251 and the second impact load rod 252. Fig.
Fig. 8 is a photograph showing a sample before being hit by the incident rod (upper left) and after being blown (upper right).
9 is a graph showing the results of the high-speed shear test.
10 shows the shock wave curves collected in the experiment.

2 is a schematic diagram of a dynamic property acquisition test apparatus using a triaxial compression cell according to an embodiment of the present invention. 3 shows a dynamic triaxial compression strength evaluation method using a triaxial compression cell.

2 and 3, the dynamic fracture toughness testing apparatus of the present invention includes a gas gun 10, a gun barrel 20, an incident rod 30, a transfer rod 40, and a compression cell 50. The triaxial compression cell 50 includes a first impact load bar 51, a second impact load bar 52, a hydraulic membrane 53, and a metal housing 54.

The first impact load rod 51 fixes the rock sample 70 in the horizontal direction and transfers the impact load transmitted from the incident rod 30 to the rock sample 70.

      The second impact load bar 52 fixes the rock sample 70 in the horizontal direction and transfers the impact load transmitted from the rock sample to the transmission rod 40.

       The hydraulic membrane 53 restrains the rock sample 70 in the axial direction through the hydraulic pressure and the shock is relieved between the hydraulic membrane 53 and the first impact load rod 51 and the second impact load rod 52 And a rubber packing device.

       The metal housing 54 surrounds the first impact load rod 51, the second impact load rod 52, and the hydraulic membrane 53, and functions as a fixing and a protection.

The impact rod, which is fired at a high pressure in the direction of the center of the cylindrical hydraulic membrane, can apply triaxial pressure to the sample in the x direction through the first impact load rod and the second impact load rod.

Fig. 4 shows that triaxial compression is applied to the sample by the triaxial compression cell 50. Fig. The sample 70 fixed between the first impact load rod and the second impact load rod having a cylindrical structure is moved in the y and z directions by the membrane 53 in the direction perpendicular to the direction of Pt and Pi , Or the center direction of the sample). That is, the impact bar, which is fired at high pressure in the y and z directions, applies pressure to the sample in the x direction through the first impact lowering bar and the second impact load bar.

The pressure can be increased by applying hydraulic pressure to the membrane 53 using the hydraulic pump 55 to the membrane. Generally, it is known that the increase of the confining pressure by hydraulic pressure increases the strength of rock and concrete.

The first impulse load bar and the second impulse load bar each have protruding fixing parts 511 and 521, and the fixing part can be inserted and fixed between the hydraulic membrane and the inner wall of the housing.

The gas gun 10 may use any known impact roulette throwing apparatus without limitation. For example, the gas gun 10 may include a high-pressure vessel 11 that fires the impact rod 60 and an impact rod propulsion control unit 12 that controls the firing velocity.

The high-pressure vessel (11) stores compressed gas. The compressed gas may be inert gas or air.

The compressed gas is injected by a compressor, and an opening / closing valve is provided at the inlet side of the high-pressure vessel 11 to open / close the compressed gas injection. The high-pressure vessel 11 may have an allowable pressure of 1000 psi or more to store the compressed gas necessary for propelling the impact rod 60. Further, a safety device for automatically releasing the gas when the on-off valve exceeds 1/10 of the allowable pressure of the high-pressure vessel is attached.

The impact rod propulsion control unit 12 opens a pneumatic valve 121 provided at an outlet side of the high-pressure vessel 11 and fires the impact rod 60 with a compressed gas discharge.

The impact rod propulsion control unit 12 may include a launch valve 122 for opening and closing the pneumatic valve 121 and a compressor 123 for providing a compressed gas to the launch valve to open the pneumatic valve. The air pressure in the compressor always applies pressure to the firing valve 121.

The pneumatic valve 122 is preferably a spring return pneumatic valve.

The gun barrel 20 is a pipe provided at the rear end of the pneumatic valve and accelerated by the impact bar.

The incident rod 30 is a heat-treated steel bar spaced a predetermined distance apart from the gun barrel. The impact rod 60 accelerated along the gun barrel collides with the incident rod 30 and the shock wave generated thereby propagates to the sample 70 and the transfer rod 40 along the incident rod.

       The test apparatus of the present invention measures the dynamic three-axis compressive strength and strain rate of the rock sample (70) by placing the rock sample (70) inside the triaxial compression cell (50)

       The test apparatus of the present invention includes a conventional SHPB impact load tester.

       The measurement in the test apparatus of the present invention is performed by a semiconductor strain gauge attached to the incident rod 30 and the transfer rod 40.

More specifically, the testing apparatus of the present invention includes an impact velocity measuring unit 90 for measuring the impact velocity of the impact rod, and an impact waveform measuring unit 100 for measuring the input wave, the reflected wave, and the strain of the output wave.

The impact velocity measuring unit 90 may include a laser 91 and a high-speed photodiode 92 positioned between the gun barrel and the incident rod to measure the impact velocity of the impact rod.

The impact waveform measuring unit 100 measures strain rates of input waves, reflected waves, and transmission waves. The impact waveform measuring unit 100 includes a data processing unit 120 including a semiconductor strain gauge 110 for detecting strain generated on the incident rod and a surface of a transfer rod, and a bridge circuit for supplying power to the semiconductor strain gage.

The triaxial compression cell 50 reproduces the stress state formed at the time of high-speed collision or explosion in the sample, thereby obtaining the dynamic triaxial compressive strength and the dynamic shear fracture reference equation for the brittle material.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a conventional split Hopkinson pressure sock (SHPB) impact system. 1 does not have a triaxial compression cell, it is impossible to simulate the behavior of the sample 70 in the triaxial confining pressure state or to measure its physical properties.

On the other hand, the test apparatus including the triaxial compression cell of the present invention implements the constraint pressure by the hydraulic pressure, thereby enabling the behavior and property acquisition in the stress state occurring at high speed collision or explosion.

5 is a high-speed shear test apparatus using a triaxial compression cell. FIG. 6 shows an enlarged view of the triaxial compression cell 250 portion.

5 includes a gas gun 210, a gun barrel 220, an incident rod 230, a transfer rod 240, and a compression cell 250. The apparatus further includes an impact rod 260, a sample 270, a shock absorption band 280, an impact velocity measurement unit 290, and an impact waveform measurement unit 300 for measuring the input wave, the reflected wave, As shown in FIG. 5 and 6, the first impact load rod 251 has a through-hole, and the second impact load rod 252 has a hole formed at a corresponding position at a predetermined depth.

The incident bar 230 has a diameter that allows it to pass through the through-hole and the hole. The incident bar 230 passes through the through hole of the first impact load rod by impact of the impact rod and directly impacts the sample. At this time, the incident bar 230 punctures the sample and moves the perforated sample to the hole of the second impact load rod.

The apparatus can measure the shear breaking strength by receiving an impact waveform signal when the sample is punctured by the incident rod.

Fig. 7 shows a production example of the first impact load rod 251 and the second impact load rod 252. Fig. Fig. 8 is a photograph showing a sample before being hit by the incident rod (upper left) and after being blown (upper right).

Referring to FIG. 6, the principle that the device can measure the shear breaking strength will be described.

First, the apparatus of the present invention directly hits the rock specimen 270 with the impact load transmitted through the 20 mm incident rod 230. Here, the first impact load rod 251 and the second impact load rod 252 fix the sample on both sides. As a result, the sample 270 is subjected to a compressive load applied in a 20 mm diameter area.

At this time, since the portion of the sample 270 excluding the area of 20 mm where the incident rod 230 is struck is fixed (constrained) by the first impact load rod and the second impact load rod, the shear force (the pushing force, Concept similar to friction). 8, the shearing force applied when the sample is punctured by the impact is shown in greenish color.

5 to 8, the force (y, z direction), that is, the axial confining pressure by the membrane 253 is a force that is continuously applied, but the force (x direction) It works momentarily. At this time, shearing failure occurs due to the compressive force in the x direction and the shear force in the -x direction (pushing force) in a state where the sample 270 is restrained by the auxiliary bar.

Experiment: Evaluation of Dynamic Shear Strength

The shear strength was measured under the conditions of Table 1 using the apparatus of FIG. 5, and the results are shown in Table 1 and FIG. Figure 8 shows samples before and after the experiment.

The dynamic shear strength is the shear strength at the time of material failure at pressurized load = P, diameter = D, and sample thickness = B. The load acting on the sample is the sum of the incident wave composite shock wave (P1 = Incident wave + Reflected wave) and the reflected bar shock wave (P2 = Transmitted wave) in the shock wave image collected by the oscilloscope 320 of FIG. The mean value was used. In addition, the dynamic triaxial shear strength was adjusted so that the shape and size of the shock wave acting on the incident bar and the shock wave acting on the transfer bar were the same (stress equilibrium state in the sample) according to the purpose of applying the aforementioned shear strength equation.

Load speed Constraining Pressure (MPa) Shear strength (MPa) Load speed Constraining Pressure (MPa) Shear strength (MPa) 3311 GPa / s 0 30.949 2528 GPa / s 0 32.63 0 34.69 0 28.61 0 37.947 0 12.75 One 26.164 One 28.135 One 36.39 One 21.4498 One 38.13 One 21.005 5 37.202 5 26.388 5 42.57 5 31.41 5 32.48 5 30.045 10 42.797 10 26.84 10 38 10 31.99 10 49.89 10 34.314

Referring to Table 1 and FIG. 9, as the restraint pressure increases, the strength of the material tends to increase linearly, and as the load speed increases, the adhesion strength of the material increases The results are shown in Fig.

Dynamic triaxial shear tests on brittle materials such as rocks and concrete are unprecedented. The results shown in FIG. 9 are in accordance with theoretical values calculated by numerical simulation and theoretical calculations.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. Those skilled in the art will readily appreciate that various changes, modifications, and variations may be made without departing from the spirit and scope of the present invention, as defined by the following claims and accompanying drawings.

Claims (10)

A first impact load rod and a second impact load rod positioned on both sides of the sample to fix the sample in the horizontal direction;
A cylindrical hydraulic membrane surrounding the side surface of the sample and restraining the sample in an axial direction; And
A first impact load rod, a second impact load rod, and a housing for fixing the membrane,
Wherein the first impulse load rod and the second impulse load rod each have a protruded fixed part, and the fixed part is inserted and fixed between the hydraulic membrane and the inner wall of the housing. The method according to claim 1, wherein the cylindrical hydraulic membrane has a structure in which the impact rod, which is fired at a high pressure in the center direction of the sample, applies the triaxial pressure to the sample in the x-axis direction through the first impact load rod and the second impact load rod Characterized by a triaxial compression cell. delete 2. The triaxial compression cell of claim 1, wherein the first load bar comprises a through-hole, and the second impact load bar has a hole formed at a corresponding position at a predetermined depth. Gas guns that launch impact rods;
A gun barrel accelerating the impact rod;
An incidence bar spaced apart from the gun barrel by a predetermined distance and colliding with the impact bar in the same line position;
A transfer rod formed in a line and spaced apart from the incidence bar by a predetermined distance;
4. The dynamic fracture toughness testing apparatus according to claim 1, wherein the triaxial compression cell is located between the transmission rod and the incident rod and applies an axial confining pressure to the sample. The apparatus of claim 5, wherein the apparatus measures the compressive strength or the shear fracture strength of the sample by adjusting the confining pressure by the compression cell and the impact load transmitted to the incident rod. The apparatus according to claim 5, wherein when the impact load transmitted through the incident rod is applied to the sample through the first impact load rod, the apparatus measures the compressive strength of the sample. 6. The apparatus of claim 5, wherein the first impulse load bar has a through-hole and the second impulse load bar has a hole formed at a corresponding position at a predetermined depth, the incurving rod is capable of passing through the through- Diameter of the test piece. The apparatus according to claim 8, wherein the impact rod penetrates through the first impulse load rod to transmit a shock wave to the specimen by the impact rod. The apparatus according to claim 8, wherein the apparatus measures a shear breaking strength by receiving an impact waveform signal when the sample is punctured by the incident rod.

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