CN113504131A - Test system and test method for testing II-type dynamic fracture toughness of rock under different normal stresses - Google Patents

Test system and test method for testing II-type dynamic fracture toughness of rock under different normal stresses Download PDF

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CN113504131A
CN113504131A CN202110778714.5A CN202110778714A CN113504131A CN 113504131 A CN113504131 A CN 113504131A CN 202110778714 A CN202110778714 A CN 202110778714A CN 113504131 A CN113504131 A CN 113504131A
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鞠明和
蔚立元
姚强岭
靖洪文
孟庆彬
胡李华
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China University of Mining and Technology CUMT
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    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/24—Investigating strength properties of solid materials by application of mechanical stress by applying steady shearing forces
    • 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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    • G01N2203/00—Investigating strength properties of solid materials by application of mechanical stress
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    • G01—MEASURING; TESTING
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Abstract

The invention discloses a test system and a test method for testing II-type dynamic fracture toughness of rocks under different normal stresses, belongs to the field of material performance test, and is mainly suitable for high-stress rock shear performance test and parameter acquisition in the deep resource exploitation process. The test data of II-type dynamic fracture toughness of the rock under different normal stress environments can be simply, conveniently and effectively obtained. The test system comprises a Hopkinson pressure bar impact loading unit and a static normal stress hydraulic loading unit; and applying normal stress to the rock sample through a static normal stress hydraulic loading unit, and applying dynamic compression load to the rock sample through a Hopkinson pressure bar impact loading unit. The obtained result is accurate and reliable, the blank in the research field is filled, and support is provided for rock shearing characteristic evaluation in the deep resource exploitation process.

Description

Test system and test method for testing II-type dynamic fracture toughness of rock under different normal stresses
Technical Field
The invention belongs to the field of material performance testing, and relates to a test method for testing dynamic shear strength and II-type dynamic fracture toughness of a brittle material under different normal stress conditions by using a specific geometric shape sample, which is mainly suitable for high-stress rock shear performance testing and parameter acquisition in a deep resource exploitation process.
Background
The shear resistance of brittle materials such as rock is one of the important characteristics regarding its stability. Shear strength and II-type fracture toughness are two important indexes for representing the shearing performance of the rock, and a static test method aiming at the two indexes is mature. With the gradual depletion of mineral resources in the shallow part of the earth, marching to the deep part of the earth has become one of the national strategic development directions. Deep resource exploitation has high stress characteristics which are not possessed by shallow parts, and rock mass excavation and resource exploitation disturbance cause deep high stress redistribution, so that severe dynamic disasters such as earthquake induced by rock burst/rock burst and fault slippage can be caused. Therefore, the determination of the dynamic shear parameters of rock materials under high stress conditions is extremely important, because it is one of the important indicators for measuring the stability of rock mass during deep resource exploitation.
At present, a Hopkinson pressure bar test system is mainly utilized for testing the dynamic shearing performance of the rock, an impact loading test is carried out on a rock sample with a specific structure, and a measurement technology or numerical software is used for obtaining dynamic shearing mechanical parameters of the rock. In this regard, predecessors have also achieved rock compression shear primarily by designing specific rock sample geometries, such as:
the Chinese invention patent with the application number of '201010154546.4' relates to a method for testing the dynamic shear strength of a brittle material by using a concentric cylinder sample;
the application number is '201320532760.8' Chinese utility model patent 'shear test piece for researching viscoelastic material II type fracture under high strain rate condition';
the invention has the application number of '201310384246.9' Chinese invention patent 'shear test piece for researching the II-type fracture of viscoelastic material under the condition of high strain rate';
the Chinese invention patent application with the application number of '201910385126.8' relates to a Hopkinson bar concrete material double-sided shear test device and a using method thereof;
however, these brittle material shear test methods suffer from two disadvantages: (1) the confining pressure effect of deep high stress is not considered. The resulting shear performance parameters are limited to the normal stress σnIn the case of 0, there is an obvious defect in applicability; (2) the designed rock sample structure does not necessarily achieve the expected effect in the test process. Because the tensile strength of the brittle rock is generally smaller than the compressive strength and the shear strength of the brittle rock, the rock sample is not subjected to direct shear failure along a set route in the impact loading process of the Hopkinson pressure bar, and deflected airfoil-shaped tensile cracks can be generated, so that the reliability of a test result is reduced.
In the aspect of rock dynamic shear performance testing considering confining pressure influence, no more definite technical method and test system exist. At present, a confining pressure loading system based on a Hopkinson pressure bar is mainly used for testing the compression resistance and the tensile resistance of rock, for example:
chinese invention patent '200510032031.6' dynamic and static combined rock loading mechanical experiment method and device;
the Chinese invention patent application with the application number of '202010955201.2' discloses a device and a method for analyzing the seepage stability of deep surrounding rock under strong dynamic disturbance;
the Chinese invention patent application with the application number of '202010980355.7' provides a device and a method for testing rock seepage characteristics under the coupling of thermal shock and dynamic shock;
the invention has the application number of '201811602351.4' Chinese invention patent 'true triaxial Hopkinson bar solid dynamic damage and ultrasonic wave propagation test method';
the invention relates to a positioning and centering system and a method of a true triaxial Hopkinson pressure bar, which are provided with the application number of '201811601168.2';
the application number is '202021925874.5' Chinese utility model patent 'a double-medium confining pressure device based on Hopkinson pressure bar';
the application number is ' 201620574575.9 ' Chinese utility model patent ' a rock Hopkinson impact loading experimental device based on true triaxial static load;
the Chinese invention patent with the application number of '201510010360.4' is a true triaxial multi-field multi-phase coupling dynamics test system and a method thereof;
these test methods do not take into account the specificity of the rock shear test, and have significant drawbacks in testing the rock shear performance: (1) the annular confining pressure loading method driven by the oil pressure cavity cannot form normal loading of the rock in a single direction, so that the reliability of the measured rock parameters is insufficient; (2) the confining pressure loading method of the rigid rod does not consider the self-feedback property of the rock shearing test process. Due to the Poisson effect and the shear-expansion effect of the rock, the normal stress of the rock is unstable in the loading process of the long-strip-shaped rigid rod, and the measured dynamic shear parameter of the rock is inaccurate.
In addition, the confining pressure loading shearing device disclosed in the Chinese patent application with the application number of '202011160273.4' for the triaxial heat-water-force coupled rock-soil body dynamic impact energy-gathering shearing experimental device is mainly suitable for simulating the triaxial heat-water-force coupled rock-soil body shearing seepage characteristics and is also not suitable for acquiring rock dynamic shearing parameters.
The rock shearing test device under the confining pressure loading condition disclosed by the Chinese invention patent with the application number of '201911247828.6' and the method thereof has the similar problem with the compression shear loading method, namely the pure shear damage of the rock sample cannot be ensured.
Based on the above summary analysis, there is a need to develop a test method for testing the dynamic shear strength and type II dynamic fracture toughness of rock under different normal stresses, so as to meet the requirement for effectively obtaining rock shear mechanical parameters in a deep high-stress environment.
Disclosure of Invention
Aiming at the problems, the invention provides a test system and a test method for testing II-type dynamic fracture toughness of a rock under different normal stresses, and test data of the II-type dynamic fracture toughness of the rock under different normal stress environments can be simply, conveniently and effectively obtained.
The technical scheme of the invention is as follows: the test system comprises a Hopkinson pressure bar impact loading unit and a static normal stress hydraulic loading unit;
normal stress is applied to the rock sample 9 through a static normal stress hydraulic loading unit, and dynamic compression load is applied to the rock sample 9 through a Hopkinson pressure bar impact loading unit.
The rock sample 9 is cuboid, the top surface and the bottom surface of the rock sample 9 are respectively provided with a sample prefabricated crack 12 in a through groove shape, and the two sample prefabricated cracks 12 are parallel to each other.
The static normal stress hydraulic loading unit comprises a static load equipment frame 7, a static load supporting base 11 and a hydraulic loading system 8, wherein the static load supporting base 11 is fixedly connected in the static load equipment frame 7 and is used for supporting the rock sample 9;
the hydraulic loading system 8 comprises a hydraulic cylinder fixedly connected to the static load equipment frame 7 and a pressing block fixedly connected to a piston rod of the hydraulic cylinder, the hydraulic cylinder is vertically arranged above the static load supporting base 11, and the pressing block is located below the hydraulic cylinder and driven by the hydraulic cylinder to do linear reciprocating motion so as to apply normal stress to the rock sample 9.
The Hopkinson pressure bar impact loading unit comprises an equipment placing platform 6 fixed in position with a static load equipment frame 7, and an impact load emitting device 1, an incident rod 3, a transmission rod 4 and an absorption rod 5 connected to the equipment placing platform 6, wherein the impact load emitting device 1, the incident rod 3, the transmission rod 4 and the absorption rod 5 are sequentially arranged, the incident rod 3 and the transmission rod 4 penetrate into the static load equipment frame 7 from two sides of the static load equipment frame 7, impact load is generated on the incident rod 3 through the impact load emitting device 1, and therefore dynamic compression load is applied to end faces of two sides of a rock sample 9 through the incident rod 3 and the transmission rod 4.
Aiming at the specific size structure of the compression short beam sample, calculating the II type dynamic stress intensity factor of the rock by an energy method
Figure BDA0003156824950000041
Wherein SrAnd LrThe length of a sample is a sample pre-fabricated crack length, and Y (S)r/Lr) Is a physical quantity, τ, related to a geometric parameter of the sampled(t) is the real-time dynamic shear stress of the sample; will be different normal stresses sigmanAnd rate of loading
Figure BDA0003156824950000042
Peak shear stress of samples under conditions
Figure BDA0003156824950000043
Substituting the dynamic stress intensity factor into a calculation formula to obtain II type dynamic fracture toughness values of the rock under different normal stresses.
According to the formula τd(t)＝EbAb[εi(t)+εr(t)+εt(t)]/(2ArSrBr) Calculating rock dynamic shear stress taud(t) in the formulai(t)、εr(t)、εt(t) real-time strain pulse signals of incident waves, reflected waves and transmitted waves collected by resistance strain gauges attached to the surfaces of the incident rod 3 and the transmission rod 4 respectively; a. thebAnd ArThe cross sectional area sizes of the cylindrical metal rod and the rock sample of the Hopkinson pressure bar loading device are respectively BrIs the thickness of the rock sample, EbThe elastic modulus of the metal rod is adopted, and the maximum value of the dynamic shear stress is the dynamic shear strength of the rock
Figure BDA0003156824950000044
It is related to static normal stress and dynamic loading rate, i.e.
Figure BDA0003156824950000045
Y(Sr/Lr) Is a physical quantity related to the geometric parameters of the sample, and Y (S) is obtained by finite element software analysis under the condition that uniform loads are applied to two end faces of the sampler/Lr)＝0.0881+1.3393Sr/Lr+1.8548(Sr/Lr)2。
The test was carried out as follows:
step 1, recording information: recording the length L of the rock sample 9rHeight HrThickness BrAnd the parallel span S of two specimen pre-cracks 12rLength a of specimen pre-crack 12;
step 2, arranging a sample: firstly, placing a rock sample 9 on a static load supporting base 11, and enabling the rock sample 9 to be positioned between an incident rod 3 and a transmission rod 4; adjusting the positions of the rock samples 9 to enable two sample prefabricated cracks 12 to be perpendicular to the incident rod 3;
step 3, applying normal stress: starting the hydraulic loading system 8 to apply downward vertical pressure to the top surface of the rock sample 9;
step 4, developing a dynamic impact test of the Hopkinson pressure bar: adjusting the vertical pressure in the step 3, realizing dynamic loading of different impact energies of the Hopkinson pressure bar by changing the impact load acted on the incident rod 3 by the impact load transmitting device 1 under different normal stress environments, and acquiring real-time strain pulse signals of incident waves, reflected waves and transmitted waves in the test process to obtain a plurality of groups of test data;
step 5, obtaining a test result;
step 5.1, according to the formula taud(t)＝EbAb[εi(t)+εr(t)+εt(t)]/(2ArSrBr) Calculating rock dynamic shear stress taud；
In the formula, epsiloni(t)、εr(t)、εt(t) real-time strain pulse signals of incident waves, reflected waves and transmitted waves which are acquired by strain gauges adhered to the Hopkinson pressure bar impact loading unit respectively; a. thebAnd ArAre respectively an impact loading unit of a Hopkinson pressure barThe cross-sectional area dimensions of the neutral beam 3 and the rock sample 9; ebIs the elastic modulus of the incident rod 3;
the maximum value of the dynamic shear stress is the dynamic shear strength of the rock
Figure BDA0003156824950000051
It is related to static normal stress and dynamic loading rate, i.e.
Figure BDA0003156824950000052
Step 5.2, calculating the II type dynamic stress intensity factor of the rock by an energy method
Figure BDA0003156824950000053
Where a is the length of the pre-crack 12, Y (S)r/Lr) Is a physical quantity related to the geometric parameters of the sample, and Y (S) is obtained by finite element software analysis under the condition that uniform loads are applied to two end faces of the sampler/Lr)＝0.0881+1.3393Sr/Lr+1.8548(Sr/Lr)2；
Step 5.3, different normal stresses sigmanAnd rate of loading
Figure BDA0003156824950000054
Sample Peak shear stress from step 5.1 under conditions
Figure BDA0003156824950000055
And (5) substituting the dynamic stress intensity factor calculation formula in the step (5.2) to obtain II type dynamic fracture toughness values of the rock under different normal stresses.
The invention designs a stable static load and dynamic load synchronous loading system. And considering the influence of confining pressure on the shearing performance of the rock, applying stable static load on the upper surface of the sample by using a hydraulic loading system, and realizing the research on the dynamic shearing behavior of the rock under different normal stress conditions. The dynamic shear strength of the rock under different normal stress conditions is obtained by changing the impact load of the Hopkinson pressure bar
Figure BDA0003156824950000056
Stress σ normal tonAnd rate of loading
Figure BDA0003156824950000057
Relationship, i.e.
Figure BDA0003156824950000058
Aiming at the specific size structure of the rock sample with the short compression beam, the II-type dynamic stress intensity factor K of the rock is calculated by an energy methodII_d：
Figure BDA0003156824950000059
In the formula: l isrThe length of the sample is shown as a, and the length of the prefabricated crack of the sample is shown as a; y (S)r/Lr) Is a physical quantity related to the geometric parameters of the sample, and Y (S) is obtained by finite element software analysis under the condition that uniform loads are applied to two end faces of the sampler/Lr)：
Figure BDA0003156824950000061
And substituting the peak shear stress of the rock sample under different normal stresses and loading rates into the dynamic stress intensity factor calculation formula to obtain the II-type dynamic fracture toughness of the rock under different normal stresses.
The invention has the beneficial effects that: the invention provides a test method for testing II-type dynamic fracture toughness of rocks under different normal stresses by utilizing a dynamic loading principle of a Hopkinson pressure bar device, designing a rock sample with a specific geometric shape and combining a static hydraulic loading device. Using the above method, a load rate segment (10) can be obtained5～106MPa/s) and confining pressure range (0-50 MPa), the obtained result is accurate and reliable, the blank in the research field is filled, and the method is also used for evaluating the rock shearing property in the process of deep land resource exploitationProviding support.
Drawings
FIG. 1 is a schematic diagram of a type II dynamic fracture toughness test method under different normal stresses according to the present invention;
FIG. 2 is a schematic view of a rock sample in contact with a Hopkinson pressure bar and a static hydraulic loading system in accordance with the present invention;
FIG. 3 is a graph of the geometry and dimensions of a rock sample according to the invention.
The reference numbers in the figures illustrate: the device comprises an impact load emitting device 1, a center frame 2, an incident rod 3, a transmission rod 4, an absorption rod 5, an equipment placing table 6, a static load equipment frame 7, a hydraulic loading system 8, a rock sample 9, an axial fixing flange 10, a static load supporting base 11 and a sample prefabricated crack 12.
Detailed Description
In order to clearly explain the technical features of the present patent, the following detailed description of the present patent is provided in conjunction with the accompanying drawings.
Example 1:
referring to fig. 1, a test method for testing type II dynamic fracture toughness of a rock under different normal stresses includes a hopkinson pressure bar impact loading unit, a static normal stress hydraulic loading unit, and a rock shear sample of a specific geometric shape, which are effectively matched to achieve the purpose of testing type II dynamic fracture toughness of the rock under different normal stresses.
In this embodiment, the rock sample 9 is designed as a compression short beam structure, as shown in FIG. 3, the length L of the compression short beam sampler50mm, height HrThickness B of 40mmrThe length and the opening degree of the two prefabricated cracks 12 are respectively 20mm and 2mm which are 30mm, and the two prefabricated cracks 12 have parallel span Sr25mm, whereby Sr/Lr0.5 can effectively ensure that the rock dynamic compression process forms a direct shear plane (shown by a dotted line in fig. 3) along the horizontal direction of the crack tip. The advantages of the rock sample structure have been described in the background of the invention and are one of the important components of the invention for obtaining accurate shear parameters of the rock.
The compressed short beam rock sample 9 is placed on a static load supporting base 11 of the static equipment frame 7 (as shown in fig. 2), and the position of the sample is adjusted to ensure that the sample is aligned with the hydraulic loading system 8 and the static load supporting base 11 in the vertical direction. And starting a hydraulic loading system 8, applying downward vertical pressure to the upper surface of the rock sample at a loading rate of 0.1mm/min, stabilizing confining pressure for 5mins after a preset pressure value is reached, and then carrying out a dynamic impact test of the Hopkinson pressure bar.
As shown in figure 1, a Hopkinson pressure bar system in the horizontal direction is arranged on an equipment placing table 6, and an impact load 1 emitting device, an incident rod 3, a transmission rod 4 and an absorption rod 5 are sequentially arranged from left to right, wherein a cylindrical metal rod piece (the diameter of 50mm) is fixed through a central frame 2, and a central frame knob can be adjusted to ensure the horizontal centering of the rod piece. The incident rod 3 and the transmission rod 4 are in parallel contact with the left and right sections of the rock sample through a static load device frame 7 through an axial fixing flange 10 (as shown in figure 2). A gap of about 5mm is reserved between the Hopkinson pressure bar in the horizontal direction and the hydraulic loading head in the vertical direction, so that the collision between the horizontal rod piece and the vertical loading head in the impact loading process is prevented, and the loading equipment is prevented from being damaged.
The cylindrical metal rod pieces (the incident rod 3, the transmission rod 4 and the absorption rod 5) are respectively connected to the equipment placing table 6 through a plurality of center frames 2, and the incident rod 3 and the transmission rod 4 penetrate through the static load equipment frame 7 through axial fixing flanges 10 to be in parallel contact with the left end face and the right end face of a rock sample 9. The cross-sectional area size and the elastic modulus of the incident rod 3 and the transmission rod 4 are consistent. The incident rod 3 and the transmission rod 4 are adhered with resistance strain gauges (120 omega).
The size of the impact load 1 is changed, and dynamic loading of different impact energies of the Hopkinson pressure bar is realized. In addition, different normal stresses sigma are realized by changing the static load of the sample in the vertical directionnAnd rate of loading
Figure BDA0003156824950000071
Direct shear failure of the rock below and according to the formula τd(t)＝EbAb[εi(t)+εr(t)+εt(t)]/(2ArSrBr) Calculating rock dynamic shear stress taud. In the formula, epsiloni(t)、εr(t)、εt(t) real-time strain pulse signals of incident waves, reflected waves and transmitted waves which are acquired by the Hopkinson pressure bar loading device respectively; a. thebAnd ArThe cross sectional area sizes of a cylindrical metal rod and a rock sample of the Hopkinson pressure bar loading system are respectively set; ebIs the modulus of elasticity of the metal rod. The maximum value of the dynamic shear stress is the dynamic shear strength of the rock
Figure BDA0003156824950000072
It is related to static normal stress and dynamic loading rate, i.e.
Figure BDA0003156824950000073
Aiming at the specific size structure of the compression short beam sample, calculating the II type dynamic stress intensity factor of the rock by an energy method
Figure BDA0003156824950000074
Where a is the length of the pre-crack 12, Y (S)r/Lr) Is a physical quantity related to the geometric parameters of the sample, and Y (S) is obtained by finite element software analysis under the condition that uniform loads are applied to two end faces of the sampler/Lr)＝0.0881+1.3393Sr/Lr+1.8548(Sr/Lr)2And substituting the sample peak shear stress under different confining pressure and loading rate conditions into a dynamic stress intensity factor calculation formula to obtain II type dynamic fracture toughness values of the rock under different normal stresses.
While the invention has been described in terms of its preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims (8)

1. A test system for testing II-type dynamic fracture toughness of rocks under different normal stresses is characterized in that the test system comprises a Hopkinson pressure bar impact loading unit and a static normal stress hydraulic loading unit;
normal stress is applied to the rock sample (9) through a static normal stress hydraulic loading unit, and dynamic compression load is applied to the rock sample (9) through a Hopkinson pressure bar impact loading unit.
2. The test system for testing type II dynamic fracture toughness of rocks under different normal stresses as claimed in claim 1, wherein the rock sample (9) is rectangular, the top surface and the bottom surface of the rock sample (9) are respectively provided with a sample pre-fabricated crack (12) in a through groove shape, and the two sample pre-fabricated cracks (12) are parallel to each other.
3. The test system for testing type II dynamic fracture toughness of rocks under different normal stresses is characterized in that the static normal stress hydraulic loading unit comprises a static load equipment frame (7), a static load supporting base (11) and a hydraulic loading system (8), wherein the static load supporting base (11) is fixedly connected in the static load equipment frame (7) and is used for bearing the rock sample (9);
the hydraulic loading system (8) comprises a hydraulic cylinder fixedly connected to the static load equipment frame (7) and a pressing block fixedly connected to a piston rod of the hydraulic cylinder, the hydraulic cylinder is vertically arranged above the static load supporting base (11), the pressing block is located below the hydraulic cylinder and is driven by the hydraulic cylinder to do linear reciprocating motion, and therefore normal stress is applied to the rock sample (9).
4. The test system for testing the type II dynamic fracture toughness under different normal stresses of the rock according to claim 3, it is characterized in that the Hopkinson pressure bar impact loading unit comprises an equipment placing platform (6) fixed with a static load equipment frame (7), and an impact load emitting device (1), an incident rod (3), a transmission rod (4) and an absorption rod (5) which are connected on the equipment placing platform (6), the impact load emitting device (1), the incident rod (3), the transmission rod (4) and the absorption rod (5) are arranged in sequence, the incident rod (3) and the transmission rod (4) penetrate into the static load equipment frame (7) from two sides of the static load equipment frame (7), an impact load is generated on the incident rod (3) through the impact load transmitting device (1), thereby applying dynamic compressive load on both side end faces of the rock sample (9) through the incident rod (3) and the transmission rod (4).
5. A test system based on any one of claims 1 to 4, and used for testing the type II dynamic fracture toughness of rocks under different normal stresses, wherein the type II dynamic stress intensity factor of the rocks is calculated by an energy method according to the specific size structure of a compression short beam sample
Figure FDA0003156824940000011
Wherein SrAnd LrThe length of a sample is a sample pre-fabricated crack length, and Y (S)r/Lr) Is a physical quantity, τ, related to a geometric parameter of the sampled(t) is the real-time dynamic shear stress of the sample; will be different normal stresses sigmanAnd rate of loading
Figure FDA0003156824940000012
Peak shear stress of samples under conditions
Figure FDA0003156824940000021
Substituting the dynamic stress intensity factor into a calculation formula to obtain II type dynamic fracture toughness values of the rock under different normal stresses.
6. The test method for testing type II dynamic fracture toughness of rock under different normal stresses as claimed in claim 5, wherein the formula τ is usedd(t)＝EbAb[εi(t)+εr(t)+εt(t)]/(2ArSrBr) Calculating rock dynamic shear stress taud(t) in the formulai(t)、εr(t)、εt(t) are real-time strain pulse signals of incident waves, reflected waves and transmitted waves collected by resistance strain gauges attached to the surfaces of the incident rod (3) and the transmission rod (4) respectively; a. thebAnd ArThe cross sectional area sizes of the cylindrical metal rod and the rock sample of the Hopkinson pressure bar loading device are respectively BrIs the thickness of the rock sample, EbThe elastic modulus of the metal rod is adopted, and the maximum value of the dynamic shear stress is the dynamic shear strength of the rock
Figure FDA0003156824940000022
It is related to static normal stress and dynamic loading rate, i.e.
Figure FDA0003156824940000023
7. The test method for testing type II dynamic fracture toughness of rock under different normal stresses as claimed in claim 5, wherein Y (S)r/Lr) Is a physical quantity related to the geometric parameters of the sample, and Y (S) is obtained by finite element software analysis under the condition that uniform loads are applied to two end faces of the sampler/Lr)＝0.0881+1.3393Sr/Lr+1.8548(Sr/Lr)2。
8. A test method for testing type II dynamic fracture toughness of rocks under different normal stresses based on the test system of any one of claims 1 to 4, which is characterized by comprising the following steps:
step 1, recording information: recording the length L of the rock sample (9)rHeight HrThickness BrAnd a parallel span S of two specimen pre-cracks (12)rThe length a of the specimen pre-crack (12);
step 2, arranging a sample: firstly, placing a rock sample (9) on a static load supporting base (11), and enabling the rock sample (9) to be positioned between an incident rod (3) and a transmission rod (4); then adjusting the position of the rock sample (9) to ensure that two sample prefabricated cracks (12) are vertical to the incident rod (3);
step 3, applying normal stress: starting a hydraulic loading system (8) to apply downward vertical pressure to the top surface of the rock sample (9);
step 4, developing a dynamic impact test of the Hopkinson pressure bar: adjusting the vertical pressure in the step 3, realizing dynamic loading of different impact energies of the Hopkinson pressure bar by changing the impact load acted on the incident rod (3) by the impact load transmitting device (1) under different normal stress environments, and acquiring real-time strain pulse signals of incident waves, reflected waves and transmitted waves in the test process to obtain a plurality of groups of test data;
step 5, obtaining a test result;
step 5.1, according to the formula taud(t)＝EbAb[εi(t)+εr(t)+εt(t)]/(2ArSrBr) Calculating rock dynamic shear stress taud；
In the formula, epsiloni(t)、εr(t)、εt(t) real-time strain pulse signals of incident waves, reflected waves and transmitted waves which are acquired by strain gauges adhered to the Hopkinson pressure bar impact loading unit respectively; a. thebAnd ArThe cross sectional area sizes of an incident rod (3) and a rock sample (9) in the Hopkinson pressure bar impact loading unit are respectively set; ebIs the elastic modulus of the incident rod (3);
the maximum value of the dynamic shear stress is the dynamic shear strength of the rock
Figure FDA0003156824940000031
It is related to static normal stress and dynamic loading rate, i.e.
Figure FDA0003156824940000032
Step 5.2, calculating the II type dynamic stress intensity factor of the rock by an energy method
Figure FDA0003156824940000033
Wherein a is the length of the pre-crack (12) and Y (S)r/Lr) Is a physical quantity related to the geometric parameters of the sample, and Y (S) is obtained by finite element software analysis under the condition that uniform loads are applied to two end faces of the sampler/Lr)＝0.0881+1.3393Sr/Lr+1.8548(Sr/Lr)2；
Step 5.3, different normal stresses sigmanAnd rate of loading
Figure FDA0003156824940000034
Sample Peak shear stress from step 5.1 under conditions
Figure FDA0003156824940000035
And (5) substituting the dynamic stress intensity factor calculation formula in the step (5.2) to obtain II type dynamic fracture toughness values of the rock under different normal stresses.
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