Centering device for direct tensile test of different-diameter coal and rock mass sample and use method of centering device

Abstract

The invention discloses a different-diameter coal and rock mass sample direct tensile test centering device and a use method thereof, and belongs to the field of rock mechanics tests. The device comprises a bonding centering device and a stretching centering device, according to the device, through the bonding centering device, errors caused by traditional manual bonding of a coal and rock mass sample are eliminated, and the bonding precision of a pull head and the sample is improved; through the coal and rock mass sample sleeve plate, the function of fixing coal and rock mass samples with different sizes and diameters in the bonding process is realized; through the connector, the puller can be connected with the bonding and centering device and can be assembled with the stretching device; and finally, through a universal spherical hinge, the consistency of force lines in a stretching stage is realized, and the accuracy of a test result is improved. According to the structure, the centering requirement of a direct tensile test is met, an experimental sample is rapidly and efficiently adjusted and centered, the capability of being used for coal and rock mass samples of different sizes is met, and the device has the advantages of being diversified in function, accurate in data and reasonable in structure.

Description

Technical Field

[0001] The invention relates to the field of rock mechanics testing, and in particular to a centering device for direct tensile testing of coal and rock samples with different diameters and a use method thereof. Background Art

[0002] The tensile strength of rock materials is much smaller than the compressive strength, which indicates that rock failure in geotechnical engineering often starts from the tensile stress and shear stress zones. Therefore, it is particularly important to obtain the mechanical properties of rocks, and the measurement of tensile strength is of great significance for geotechnical engineering construction. At the same time, the determination test of rock tensile strength is also one of the important contents of geotechnical engineering teaching and scientific research. However, in the field of rock mechanics, the experimental research on rock mechanical properties mainly focuses on rock mechanical feedback under compressive stress conditions, such as uniaxial compression tests, triaxial compression tests, etc. There is less research and update on tensile stress test instruments, which leads to the inability to accurately measure rock mechanical properties under uniaxial tension conditions.

[0003] At present, the measurement of rock tensile strength is mainly divided into direct tensile test and indirect tensile test. In direct tensile test, the clamping method of the sample is usually divided into three methods: embedded, external clamping and bonding. The external clamping method clamps the tail of the long strip tensile specimen by friction, which generally generates stress concentration at the clamping point, and fracture often occurs at the clamping position of the specimen; the internal embedding method stretches the tensile specimen by embedding the bottom end component into the specimen, which makes it difficult to ensure that the specimen is subjected to axial force, and the stress distribution is too complicated; the adhesive method is a more ideal clamping method, which uses adhesive materials to connect the fixture and the specimen to each other, making it easy to ensure axial tension, but it has three major problems: (1) If the adhesive material and adhesive method are not appropriate, the specimen is easy to fall off the fixture, resulting in test failure (2) If the bonding force at both ends is different, it will cause the deformation of the two ends of the specimen to be inconsistent and the results are inaccurate; (3) In actual operation, direct tensile tests generally have eccentric force problems, which makes it easy for rock and coal rock specimens to generate bending moments and torques at the ends, thereby causing stress concentration, affecting the test results and leading to test failure. These errors that may occur in the tensile process are all due to the inaccuracy and advanced defects of the centering device, so it is necessary to improve and develop a more accurate centering device. At the same time, rock is a complex geological body, a typical macro-continuous and micro-discontinuous inhomogeneous medium. The size effect of rock refers to the fact that the mechanical properties of rock coal rock samples will show obvious differences according to the size and shape of the coal rock samples. This difference makes it impossible to directly apply the research results of rock mechanical properties in the laboratory to engineering practice. In addition, there are many types of rocks and the occurrence environment is complex. It is also difficult to carry out destructive research on rocks of various scales under laboratory conditions. As a result, the rock size effect is still an issue that needs to be studied in depth, so it is necessary to conduct further systematic research on the size effect. Summary of the invention

[0004] Technical problem: The purpose of the present invention is to overcome the shortcomings of the prior art and to provide a centering device for direct tensile testing of coal and rock samples of different diameters and a method of using the device. The device has a simple structure and is easy to operate. It can ensure the consistency of the force line alignment during the preparation of coal and rock samples and the tensile test in the experiment, so as to improve the accuracy of the tensile test.

[0005] Technical solution: To achieve the above technical purpose, the present invention discloses a centering device for direct tensile testing of coal and rock samples with different diameters, including a bonding centering device, in which a tensile centering device is provided, and the bonding centering device is used to ensure that eccentric bonding occurs when installing the coal and rock samples; and the tensile centering device is used to ensure that the force lines of the coal and rock samples are consistent during the tensile process;

[0006] The bonding and centering device comprises an upper pull head fixture, a coal rock sample fixture and a lower pull head fixture which are arranged from top to bottom, wherein the upper pull head fixture is provided with an upper pull head connecting screw hole at the center, a coal rock sample sleeve hole is opened at the center of the coal rock sample fixture for allowing the measured coal rock sample to pass through, the coal rock sample sleeve hole is larger than the diameter of the coal rock sample, and coal rock samples of different diameters are fixed by arranging coal rock sample sleeve plates of different sizes on the coal rock sample sleeve hole; a lower pull head connecting screw hole is provided at the center of the lower pull head fixture, and the coal rock sample sleeve hole is provided with a lower pull head connecting screw hole. A coal-rock sample sleeve is provided at the hole, and the coal-rock sample sleeve is adapted to coal samples of different diameters, and any coal sample smaller than the coal-rock sample sleeve hole can be adapted thereto. The upper pull head fixture, the coal-rock sample fixture and the lower pull head fixture are horizontally connected by three vertically arranged columns with adjustable heights. By adjusting the height of the columns, it is ensured that the upper pull head fixture, the coal-rock sample fixture and the lower pull head fixture are always parallel, and at the same time, it is ensured that the centers of the upper pull head connecting screw hole, the coal-rock sample sleeve hole and the lower pull head connecting screw hole are always a perpendicular median line;

[0007] The tensile centering device comprises an upper tensile mechanism and a lower tensile mechanism for generating reverse tensile forces up and down on the coal-rock sample arranged at the position of the coal-rock sample sleeve plate. The coal-rock sample is in a cylindrical shape, and the coal-rock sample sleeve plate limits the size of the coal-rock sample; wherein the upper tensile mechanism comprises an upper pull head arranged at the top of the coal-rock sample, the top of the upper pull head is connected to an upper ball joint box through an upper connector, a universal shaft is arranged upward in the upper ball joint box, and the universal shaft is connected to an upper tensile test fixture generating an upper tensile force through an upper tensile adapter at the top; the lower tensile mechanism comprises a lower pull head arranged at the bottom of the coal-rock sample, a lower connector is arranged at the bottom of the lower pull head, a lower ball joint box is connected at the bottom of the lower connector, a universal shaft is arranged downward in the lower ball joint box, and the universal shaft is connected to a lower tensile test fixture generating an upper tensile force through a lower tensile adapter at the bottom;

[0008] The sizes of the upper pull head connection screw hole and the lower pull head connection screw hole are precisely matched with the upper pull head and the lower pull head, respectively, so that when the upper pull head is placed in the upper pull head connection screw hole and the lower pull head is placed in the lower pull head connection screw hole, the upper pull head and the lower pull head are also ensured to have the same center and aligned force lines.

[0009] Furthermore, the parts of the universal joint shaft arranged in the upper ball joint box and the lower ball joint box are divided into universal ball joints. When a tensile force is applied to the coal rock mass sample, the universal ball joint ensures that the force lines are aligned in the tensile test.

[0010] Furthermore, the upper pull head and the lower pull head are both provided with a cavity for accommodating part of the coal rock sample, and the side walls of the upper pull head and the lower pull head are respectively provided with an upper grouting hole and a lower grouting hole connected to the internal cavity, and adhesive is injected into the cavity of the upper pull head and the lower pull head through the upper grouting hole and the lower grouting hole, so that the upper pull head and the lower pull head are connected and fastened to the upper and lower ends of the coal rock sample.

[0011] Furthermore, a plastic sealing ring sleeve is provided between the cavity openings of the upper pull head and the lower pull head and the coal rock sample to prevent leakage of the adhesive, and the thickness of the plastic sealing ring sleeve can be adjusted automatically.

[0012] Furthermore, a level A, a level B and a level C are respectively provided on the upper pull head holder, the coal rock sample holder and the lower pull head holder.

[0013] Furthermore, a height fixing screw sleeve A for adjusting the height is provided on the column between the upper pull head fixture and the coal rock sample fixture, and a height fixing screw sleeve B for adjusting the height is provided on the column between the coal rock sample fixture and the lower pull head fixture. The level C and the leveling knob are used to adjust the three-point height of the lower pull head fixture to ensure that the lower pull head fixture is level. The column connects the upper pull head, the coal rock sample and the lower pull head fixture to ensure that the centers of the three fixtures are consistent and the force lines are aligned.

[0014] Furthermore, an adjustable leg is provided at the bottom of the pull-down head holder, and a leveling knob is provided on the adjustable leg.

[0015] A method for using a centering device for direct tensile testing of coal and rock samples with different diameters, the steps of which are as follows:

[0016] 1. Bonding and centering stage:

[0017] First, install the leveling knob into the hole of the pull-down head holder, assemble the inner thread of the lower connector with the pull-down head, and then assemble the outer thread of the lower connector with the pull-down head connecting screw hole on the pull-down head holder. At this time, use the level C to adjust the three-point height of the pull-down head holder through the leveling knob to make the bubble in the level C horizontal. At this time, the overall plane of the pull-down head holder is horizontal, and the three columns are inserted into the pull-down head holder in sequence.

[0018] Then, the coal-rock sample is first placed in the pull-down head that has been assembled with the pull-down head fixture, a coal-rock sample sleeve plate whose circle center size matches the coal-rock sample to be tested is selected to restrict the coal-rock sample, and the coal-rock sample sleeve plate is assembled with the coal-rock sample fixture through the sleeve plate fixing screw hole; first, three height fixing screw sleeves B are installed in the column, and then the coal-rock sample fixture is installed in the column, and the coal-rock sample passes through the holes in the coal-rock sample fixture and the coal-rock sample sleeve plate, and the height of the three height fixing screw sleeves B is adjusted by the level B, so that the bubble in the level B on the coal-rock sample fixture is horizontal, and the center of the coal-rock sample and the pull-down head are basically consistent through the fixing effect of the coal-rock sample fixture and the pull-down head fixture;

[0019] Third, assemble the inner thread of the upper connector with the upper pull head, assemble the outer thread of the upper connector with the upper pull head connecting screw hole in the upper pull head fixture, install three height fixing screw sleeves A into the column, and then insert the upper pull head assembled with the upper pull head fixture into the coal rock mass sample, and insert the upper pull head fixture into the column, and adjust the height of the three height fixing screw sleeves A by the level A, so that the bubble in the level A on the upper pull head fixture is horizontal, and at this time, the centers of the upper pull head, the coal rock mass sample and the lower pull head are consistent;

[0020] Finally, the adhesive is injected into the upper pull head through the upper grouting hole, and the lower pull head is pulled down through the lower grouting hole; the adhesive is left to bond the upper and lower pull heads to the coal and rock mass samples, and the bonding and centering stage is completed;

[0021] 2. Stretching and centering stage:

[0022] Unlock the nut on the upper pull head holder, loosen the height fixing screw sleeve A and the height fixing screw sleeve B, first pull out the column, rotate the upper pull head holder to unlock the fixation of the upper pull head, take out the coal rock sample sleeve and the coal rock sample holder from the coal rock sample, and then rotate the lower connector to take the sample out of the lower pull head holder, so that the coal rock sample bonded with the pull head can be obtained;

[0023] Place the universal ball joint at the end of the upper tensile adapter into the upper ball joint box, and connect the outer thread on the upper connector to the upper ball joint box; place the universal ball joint at the end of the lower tensile adapter into the lower ball joint box, and connect the outer thread of the lower connector to the lower ball joint box. At this point, the assembly of the tensile device has been completed. Clamp the upper and lower tensile adapters with the upper and lower clamps of the tensile machine, and start tensile loading. At this time, the universal ball joint will flexibly adjust the upper and lower center of gravity of the coal and rock mass sample as the test progresses to meet the centering requirements of the tensile stage.

[0024] Beneficial effects: Due to the adoption of the above technical scheme, the present invention not only realizes the centering function in the preparation process of coal rock samples, but also ensures the centering function of coal rock samples in the stretching stage, thereby improving the accuracy of test results. Through the coal rock sample sleeve, the fixing function of coal rock samples of different sizes and diameters in the bonding process is realized; the three fixtures are constrained by the columns so that the center of the fixtures are consistent, ensuring the alignment of the instruments during the bonding process and the accuracy of the test bonding; through the bonding centering device, the error caused by the traditional manual bonding of coal rock samples during the bonding process between the coal rock sample and the pull head is eliminated, thereby improving the test accuracy; through the connector, the function that the pull head can be installed with the bonding centering device and assembled with the stretching device at the same time is realized; through the universal ball joint, the tensile force in the stretching stage is consistent with the gravity force line of the sample. The centering device integrates multiple functions in one, with a simple structure, accurate data, and stable functions, which solves the problem of direct stretching centering error. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic diagram of the overall structure of a centering device for direct tensile testing of coal and rock samples with different diameters according to an embodiment of the present invention;

[0026] Figure 2 This is a schematic diagram of a fixture in a centering device for direct tensile testing of coal and rock samples with different diameters according to an embodiment of the present invention;

[0027] Figure 3 It is a schematic diagram of a coal and rock sample sleeve plate in a centering device for direct tensile testing of coal and rock samples with different diameters according to an embodiment of the present invention;

[0028] Figure 4 It is a schematic diagram of a bonding centering device in a centering device for direct tensile testing of coal and rock samples with different diameters according to an embodiment of the present invention;

[0029] Figure 5 It is a schematic diagram of a tensile centering device in a centering device for direct tensile testing of coal and rock samples with different diameters according to an example of the present invention;

[0030] Figure 6 This is a schematic diagram of the pull head and connector in the centering device for direct tensile testing of coal and rock samples with different diameters in an example of the present invention.

[0031] In the figure: 1-upper pull head holder, 2-coal rock sample holder, 3-lower pull head holder, 4-level A, 5-upper pull head connecting screw hole, 6-height fixing screw sleeve A, 7-level B, 8-coal rock sample sleeve hole, 9-sleeve plate fixing screw hole, 10-coal rock sample sleeve plate, 11-height fixing screw sleeve B, 12-column, 13-level C, 14-lower pull head connecting screw hole, 15-leveling knob, 16- Upper tensile test fixture, 17-upper tensile adapter, 18-upper ball joint box, 19-universal ball joint, 20-upper connector, 21-upper pull head, 22-upper grouting hole, 23-coal rock sample, 24-lower pull head, 25-lower grouting hole, 26-lower connector, 27-lower ball joint box, 28-lower tensile adapter, 29-lower tensile test fixture, 30-inner circle thread, 31-outer circle thread, 32-plastic sealing ring sleeve. DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to the embodiments in the accompanying drawings:

[0033] like Figure 1 As shown, the present invention discloses a direct tensile centering test device for coal and rock samples with different diameters, comprising a bonding centering device and a tensile centering device. The bonding centering device ensures that eccentric bonding is prevented when the coal and rock sample and the pull head are bonded by an adhesive; the tensile centering device ensures that the force lines are consistent during the stretching process.

[0034] like Figure 2 As shown, it is a schematic diagram of the fixture, which includes an upper pull head fixture 1, a coal rock sample fixture 2, and a lower pull head fixture 3. The upper pull head fixture is used to fix the upper pull head 21, the coal rock sample fixture 2 is used to fix the coal rock sample 23, and the lower pull head fixture 3 is used to limit the displacement of the lower pull head 24. The three fixtures are limited in displacement and height by the column 12 to achieve consistent alignment of the center of the circle, and finally ensure that the upper and lower pull heads 2124 and the coal rock sample 23 can be aligned during the bonding and centering stage, eliminating the error caused by the traditional manual bonding of the coal rock sample during the bonding process between the coal rock sample and the pull head, and improving the test accuracy.

[0035] like Figure 3 , which is a schematic diagram of a coal rock sample sleeve. The schematic diagram shows three layers, which correspond to three coal rock samples 23 of different sizes. Different coal rock sample sleeves 10 are selected according to the size of the coal rock sample in the tensile test. In the actual test process, there may be more than three coal rock samples of different sizes. The number of coal rock sample sleeves 10 can be increased or decreased according to the specific situation to meet the requirements of more coal rock samples of different sizes.

[0036] like Figure 4As shown, it is a schematic diagram of the bonding centering device. It consists of an upper pull head fixture 1, a coal rock sample fixture 2, and a lower pull head fixture 3. The upper pull head fixture 1 consists of a level A4, an upper pull head connection screw hole 5, and a height fixing screw sleeve A6. The level A4 and three height fixing screw sleeves A5 are used to adjust the three-point height of the upper pull head fixture 1 to ensure that the upper pull head fixture 1 is horizontal. The upper pull head connection screw hole 5 fixes the upper pull head 21 through the upper connector 20; the coal rock sample fixture 2 consists of a level B7, a coal rock sample sleeve hole 8, a sleeve plate fixing screw hole 9, a coal rock sample sleeve plate 10, and a height fixing screw sleeve B11. The level B7 and three height fixing screw sleeves B11 are used to adjust the three-point height of the coal rock sample fixture 2 to ensure that the coal rock sample fixture 2 is horizontal. The coal rock sample sleeve plate 10 limits the size of the coal rock sample and is connected to the coal rock sample fixture through the sleeve plate fixing screw hole 9. The coal-rock sample first passes through the hole on the coal-rock sample sleeve plate 10, and then passes through the coal-rock sample sleeve hole 8 to ensure alignment; the lower pull head fixture 3 is composed of a column 12, a level C13, a lower pull head connection screw hole 14, and a leveling knob 15. The level C13 and the leveling knob 15 are used to adjust the three-point height of the lower pull head fixture 3 to ensure that the lower pull head fixture 3 is horizontal. The lower pull head connection screw hole 14 fixes the lower pull head 24 through the lower connector 26. The column 12 connects the upper pull head, the coal-rock sample and the lower pull head fixture to ensure that the centers of the three fixtures are consistent and the force lines are aligned.

[0037] like Figure 5 As shown, it is a schematic diagram of the tensile centering device; the tensile centering device consists of an upper tensile test fixture 16, an upper tensile adapter 17, an upper ball joint box 18, a universal ball joint 19, an upper connector 20, an upper pull head 21, an upper grouting hole 22, a coal rock sample 23, a lower pull head 24, a lower grouting hole 25, a lower connector 26, a lower ball joint box 27, a lower tensile adapter 28, and a lower tensile test fixture 29. The tensile test instrument stretches the upper tensile adapter 17 and the lower tensile adapter 28 through the upper tensile test fixture 16 and the lower tensile test fixture 29, and finally breaks the coal rock sample 23. During the stretching process, the universal ball joint 19 ensures that the test process device and the coal rock sample 23 are aligned at any time to prevent bending moment and torque from being generated at the end, thereby causing stress concentration, affecting the test results, and causing test failure. The upper tensile adapter 17 is used to connect the upper ball joint box 18 and the upper tensile test fixture 16, and the lower tensile adapter 27 is used to connect the lower ball joint box 27 and the lower tensile test fixture 29. The upper connector 20 is used to connect the upper pull head 21 and the upper ball joint box 18, and the lower connector 26 is used to connect the lower pull head 24 and the lower ball joint box 27. The bonding slurry is injected into the upper and lower pull heads 2124 through the upper and lower grouting holes 2225. The plastic sealing ring 32 is used to prevent slurry leakage, and the thickness of the plastic sealing ring 32 can be adjusted automatically to meet the requirements of connecting coal and rock samples 23 of different sizes with the upper and lower pull heads 2124.

[0038] like Figure 6 The figure shows a schematic diagram of a pull head and a connector; the connector 2026 plays a key role in both the bonding and centering stages. In the bonding and centering stage, the inner thread 30 of the connector 2026 is used to assemble with the upper and lower pull heads 2124, and the outer thread 31 is used to assemble with the upper pull head connection screw hole 5 and the lower pull head connection screw hole 14; in the stretching and centering stage, the inner thread 30 of the connector 2026 is used to assemble with the upper and lower pull heads 2124, and the outer thread 31 is used to assemble with the upper ball joint box 18 and the lower ball joint box 27. The connector not only realizes the connection between the coal rock sample 23 and the bonding and centering device, but also realizes the assembly of the coal rock sample 23 and the stretching and centering device, which plays an important role in this patent.

[0039] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A centering device for direct tensile testing of coal and rock samples with different diameters, characterized by: It includes a bonding centering device, in which a tensile centering device is arranged, and the bonding centering device is used to ensure that eccentric bonding occurs when installing the coal rock sample; The tensile centering device is used to ensure that the force lines of the coal and rock samples are consistent during the tensile process; The bonding centering device comprises an upper pull head fixture (1), a coal rock sample fixture (2) and a lower pull head fixture (3) which are arranged from top to bottom, wherein an upper pull head connection screw hole (5) is arranged at the center of the upper pull head fixture (1), a coal rock sample sleeve hole (8) is opened at the center of the coal rock sample fixture (2) for allowing a coal rock sample (23) to pass through, the coal rock sample (23) is cylindrical in shape, and the coal rock sample sleeve hole (8) is larger than the diameter of the coal rock sample (23), and coal rock samples (23) of different diameters are fixed by arranging coal rock sample sleeve plates (10) of different sizes on the coal rock sample sleeve hole (8); and a lower pull head connection screw hole (14) is arranged at the center of the lower pull head fixture (3). A coal rock sample sleeve plate (10) is provided at the coal rock sample sleeve hole (8), and the coal rock sample sleeve plate (10) is adapted to coal samples of different diameters, and any coal sample smaller than the coal rock sample sleeve hole (8) can be adapted thereto. The upper pull head fixture (1), the coal rock sample fixture (2) and the lower pull head fixture (3) are horizontally connected by three vertically arranged height-adjustable columns (12), and the height of the columns (12) is adjusted to ensure that the upper pull head fixture (1), the coal rock sample fixture (2) and the lower pull head fixture (3) are always parallel, and at the same time ensure that the upper pull head connecting screw hole (5), the coal rock sample sleeve hole (8) and the lower pull head connecting screw hole (14) are always on a perpendicular midline from the center of the circle; The tensile centering device comprises an upper tensile mechanism and a lower tensile mechanism for generating reverse tensile forces upward and downward on the coal rock sample (23) to be tested, wherein the upper tensile mechanism comprises an upper pull head (21) arranged at the top of the coal rock sample (23), the top of the upper pull head (21) is connected to an upper ball joint box (18) through an upper connector (20), a universal shaft is arranged upward in the upper ball joint box (18), and the universal shaft is connected to an upper tensile test fixture (16) generating the upper tensile force through an upper tensile adapter (17) at the top; the lower tensile mechanism comprises a lower pull head (24) arranged at the bottom of the coal rock sample (23), a lower connector (26) is arranged at the bottom of the lower pull head (24), a lower ball joint box (27) is connected to the bottom of the lower connector (26), a universal shaft is arranged downward in the lower ball joint box (27), and the universal shaft is connected to a lower tensile test fixture (29) generating the upper tensile force through a lower tensile adapter (28) at the bottom; The sizes of the upper pull head connecting screw hole (5) and the lower pull head connecting screw hole (14) are precisely matched with the upper pull head (21) and the lower pull head (24), respectively, so that when the upper pull head (21) is placed in the upper pull head connecting screw hole (5) and the lower pull head (24) is placed in the lower pull head connecting screw hole (14), the upper pull head (21) and the lower pull head (24) are also ensured to have the same center of the circle and aligned force lines, on the basis of ensuring that the centers of the circles of the upper pull head connecting screw hole (5) and the lower pull head (24) are maintained on a perpendicular midline.

2. The centering device for direct tensile testing of coal and rock samples with different diameters according to claim 1 is characterized by: The portion of the universal joint shaft disposed in the upper ball joint box (18) and the lower ball joint box (27) is divided into a universal ball joint (19). When a tensile force is applied to the coal rock mass sample (23), the universal ball joint (19) ensures that the force lines are aligned in a tensile test.

3. The centering device for direct tensile test of coal and rock samples with different diameters according to claim 1 is characterized by: The upper pull head (21) and the lower pull head (24) are both provided with a cavity for accommodating a portion of the coal rock sample (23). The side walls of the upper pull head (21) and the lower pull head (24) are respectively provided with an upper grouting hole (22) and a lower grouting hole (25) which are connected to the internal cavity. Adhesive is injected into the cavity of the upper pull head (21) and the lower pull head (24) through the upper grouting hole (22) and the lower grouting hole (25), so that the upper pull head (21) and the lower pull head (24) are connected and fastened to the upper and lower ends of the coal rock sample (23).

4. The centering device for direct tensile test of coal and rock samples with different diameters according to claim 3 is characterized by: A plastic sealing ring (32) for preventing leakage of adhesive is provided between the cavity openings of the upper pull head (21) and the lower pull head (24) and the coal rock sample (23), and the thickness of the plastic sealing ring (32) is adjusted as required.

5. The centering device for direct tensile test of coal and rock samples with different diameters according to claim 3 is characterized by: The upper pull head holder (1), the coal rock sample holder (2) and the lower pull head holder (3) are respectively provided with a level gauge A (4), a level gauge B (7) and a level gauge C (13).

6. The centering device for direct tensile test of coal and rock samples with different diameters according to claim 3 is characterized by: A height-adjusting screw sleeve A (6) is provided on the column (12) between the upper pull head fixture (1) and the coal rock sample fixture (2), and a height-adjusting screw sleeve B (11) is provided on the column (12) between the coal rock sample fixture (2) and the lower pull head fixture (3). A level C (13) and a leveling knob (15) are used to adjust the three-point height of the lower pull head fixture (3) to ensure that the lower pull head fixture (3) is level; the column (12) connects the upper pull head, the coal rock sample and the lower pull head fixture to ensure that the centers of the three fixtures are consistent and the force lines are aligned.

7. The centering device for direct tensile test of coal and rock samples with different diameters according to claim 3 is characterized by: The bottom of the pull-down head holder (3) is provided with an adjustable leg, and the adjustable leg is provided with a leveling knob (15).

8. A method for using the centering device for direct tensile testing of coal and rock samples of different diameters according to any one of claims 1 to 6, characterized in that Here are the steps:

1. Bonding and centering stage: First, insert the leveling knob (15) into the hole of the lower pull head holder (3), assemble the inner thread (31) of the lower connector (24) with the lower pull head (24), and then assemble the outer thread of the lower connector (24) with the lower pull head connecting screw hole (14) on the lower pull head holder (3). At this time, use the leveling instrument C (13) to adjust the three-point height of the lower pull head holder (3) through the leveling knob (15) to make the bubble in the leveling instrument C (13) level. At this time, the plane of the lower pull head holder (3) is overall horizontal, and the three columns (12) are inserted into the lower pull head holder (3) in sequence; Then, the coal rock sample (23) is first placed in the lower pull head (24) that has been assembled with the lower pull head fixture (3), and a coal rock sample sleeve (10) whose circle center size matches the coal rock sample (23) to be tested is selected to restrict the coal rock sample (23), and the coal rock sample sleeve (10) is assembled with the coal rock sample fixture (2) through the sleeve fixing screw hole (9); first, three height fixing screw sleeves B (11) are installed in the column (12), and then the coal rock sample is fixed The device (2) is installed into the column (12), the coal-rock sample (23) passes through the holes in the coal-rock sample holder (2) and the coal-rock sample sleeve (10), and the height of the three height fixing screw sleeves B (11) is adjusted by the level B (7) so that the bubble in the level B (7) on the coal-rock sample (23) holder (2) is level, and the coal-rock sample (23) and the lower pull head (24) are basically aligned in center through the fixing action of the coal-rock sample holder (2) and the lower pull head holder (3); Third, assemble the inner thread (30) of the upper connector (20) with the upper pull head (21), assemble the outer thread (31) of the upper connector (20) with the upper pull head connecting screw hole (5) in the upper pull head fixture (1), install three height fixing screw sleeves A (5) into the column (12), and then insert the upper pull head (21) assembled with the upper pull head fixture (1) into the coal rock sample (23), and insert the upper pull head fixture (1) into the column (12), and adjust the height of the three height fixing screw sleeves A (5) by the level gauge A (4) so ​​that the bubble in the level gauge A (4) on the upper pull head fixture (1) is horizontal, and at this time, the centers of the upper pull head (21), the coal rock sample (23) and the lower pull head (24) are consistent; Finally, the adhesive is injected into the upper pull head (21) through the upper grouting hole (22), and the lower pull head (24) is injected through the lower grouting hole (25); the upper and lower pull heads (21) (24) are kept stationary and the bonding and centering stage is completed while the adhesive completes bonding of the upper and lower pull heads (21) (24) with the coal and rock sample (23); 2. Stretching and centering stage: Unlock the nut on the upper pull head fixture (1), loosen the height fixing screw sleeve A (5) and the height fixing screw sleeve B (11), first pull out the column (12), rotate the upper pull head fixture (1) to release the fixation on the upper pull head (21), take out the coal rock sample sleeve (10) and the coal rock sample fixture (2) from the coal rock sample (23), and then rotate the lower connector (24) to take the sample out of the lower pull head fixture (3), so that the coal rock sample (23) bonded to the pull head can be obtained; Place the universal ball joint (19) at the tail end of the upper tensile adapter (17) into the upper ball joint box (18), and connect the outer circle thread (31) on the upper connector (20) to the upper ball joint box (18); place the universal ball joint (19) at the tail end of the lower tensile adapter (28) into the lower ball joint box (27), and connect the outer circle thread (31) of the lower connector (26) to the lower ball joint box (27); at this time, the assembly of the tensile device has been completed; clamp the upper and lower tensile adapters (17) (28) with the upper and lower clamps of the tensile machine, and start tensile loading. At this time, the universal ball joint (19) will flexibly adjust the upper and lower centers of gravity of the coal rock sample (23) as the test progresses to meet the centering requirements of the tensile stage.

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