A testing method for the tensile strength of rock based on gradient loading and end face effect

Abstract

A method for testing the tensile strength of rock based on gradient loading and end face effect belongs to the technical field of rock tensile strength testing. This method measures the mechanical parameters of the rock sample, then grinds the inclined end face, and uses the displacement control method to uniaxially load until the rock sample S<subgt;1< / subgt> cracks, so as to determine the location where the tensile crack occurs, and then measures the strain during the development process of the tensile crack, thereby calculating the tensile strength of the rock sample. This method is a simple and effective method for testing the tensile strength of rock, which can avoid the technical difficulties in direct tensile test and Brazilian splitting test of rock, directly measure the tensile strength of rock by using the rock slab cracking simulation system, and reduce the labor intensity.

Description

Technical Field

[0001] The present invention belongs to the technical field of rock tensile strength testing, and particularly relates to a rock tensile strength testing method based on gradient loading and end face effect. Background Art

[0002] With the gradual depletion of shallow mineral resources, resource development has been continuously moving towards the deep part of the earth. Deep well and tunnel excavation has become the new normal in water conservancy, transportation, mining, and geothermal development. During the deep excavation process, as a typical form of tensile failure, the phenomenon of surrounding rock slab cracking has become increasingly prominent, posing a severe challenge to the safe construction of deep buried well and tunnel projects. How to accurately measure the rock tensile strength is the key to revealing the occurrence mechanism of deep surrounding rock slab cracking, and is of great significance for formulating reasonable and effective slab cracking control schemes and ensuring the safety of deep buried tunnel construction.

[0003] At present, the testing of rock tensile strength is generally divided into two types: direct tensile test and indirect tensile test. Although the direct tensile test of rock is more in line with the actual tensile situation of the rock, due to difficulties in specimen preparation, stress concentration easily occurring at the contact between the fixture and the specimen, and possible eccentricity during the tensile process, etc., it is difficult to implement the direct tensile test of rock. The most common indirect tensile test method - Brazilian splitting test - also has defects such as stress concentration at the loading point and eccentric loading, and there are errors in its test results. Moreover, both of these two conventional methods require additional processing of specimens and test equipment with specific requirements, which not only increases the workload of tensile strength testing but also increases its difficulty. How to measure the rock tensile strength with a simpler method during the rock slab cracking simulation test is a technical problem that urgently needs to be solved. Summary of the Invention

[0004] The purpose of the present invention is to bypass the existing rock tensile strength testing path and provide a rock tensile strength testing method based on gradient loading and end face effect. This method is a simple and effective rock tensile strength testing method, which can avoid the technical difficulties in the direct tensile test of rock and the Brazilian splitting test, directly measure the rock tensile strength using a rock slab cracking simulation system, and reduce the labor intensity.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A rock tensile strength testing method based on gradient loading and end face effect of the present invention specifically includes the following steps:

[0007] Step 1: Determine the mechanical parameters of the rock sample, including elastic modulus E, Poisson's ratio μ, and maximum displacement l max ;

[0008] Step 2: Determine the position where cracking occurs

[0009] The determination method is as follows: First, a cuboid rock sample with a length, width, and height of a mm × a mm × b mm is made, and it is required that b ≥ 2×a; then the end face of the rock sample is carefully polished so that one of the a×a end faces forms an inclined end face; the shape of the inclined end face is a rectangle, and its four sides can be divided into two groups, and the two sides in each group are parallel to each other; among them, in the horizontal plane, the two sides of the first group are obliquely intersecting with the horizontal plane, and the height difference formed by the inclination is d0 (d0 < l max ); the two sides of the second group are parallel to the horizontal plane, and there is a height difference between the two sides parallel to the horizontal plane, one side is on the relatively low side, and the other side is on the relatively high side. Among them, the a×b side corresponding to the relatively low side in the inclined end face is denoted as the left side face, and the a×b side corresponding to the relatively high side in the inclined end face is denoted as the right side face, and the two a×b side faces perpendicular to the left side face and the right side face are denoted as the front side face or the back side face; this rock sample is called a quasi-cuboid rock sample and is denoted as S1;

[0010] Place the rock sample S1 in the loading chamber of the RMT pressure testing machine, separate the inclined end face of the rock sample S1 from the bearing plate with a gasket, and use the displacement control method to uniaxially load until the rock sample S1 starts to crack, then stop loading and quickly unload, observe the inclined end face and determine the position where the tensile crack of the rock sample S1 occurs, and mark the fracture plane formed at the position where the tensile crack occurs as the tensile fracture surface;

[0011] Among them, the tensile fracture surface, the left side face, and the right side face are parallel to each other.

[0012] Furthermore, for the marking method of the tensile fracture surface, the horizontal distance from the tensile fracture surface to the left side face of the rock sample is selected to mark the position where the tensile crack occurs, and it is denoted as L lie ; or the horizontal distance from the tensile fracture surface to the right side face of the rock sample is used to mark the position where the tensile crack occurs, and it is denoted as L lie ′;

[0013] Furthermore, in the rock sample S1, the inclined end face is perpendicular to the front side face or the back side face.[[ID= / / ID=19]]

[0014] The RMT pressure testing machine mentioned above is an RMT pressure testing machine in which the bearing plate of the press is not connected with a spherical end.

[0015] The stiffness of the bearing plate > the stiffness of the rock S1 > the stiffness of the gasket, and the gasket is preferably a polytetrafluoroethylene plate.

[0016] The determination method for the start of cracking is as follows: The start of cracking moment is determined according to the acoustic emission signal, or for rocks with greater brittleness, it is determined by the first clear splitting sound generated by the start of cracking.

[0017] The key of the present invention lies in first creating a tensile crack and determining the position where the tensile crack occurs. Only by creating a tensile crack and determining the position where the tensile crack occurs can strain gauges be pasted in advance at the positions where tensile cracks may occur to accurately measure the tensile strength of the rock.

[0018]

[0018] One of the reasons for the rock sample cracking is the end gradient stress. Because, for the two opposite sides of the inclined end face of the rock sample S1, one side is lower and the other side is higher. After being compressed, the longitudinal stress on the relatively higher side is greater, and the longitudinal stress on the relatively lower side is smaller. In the width direction of the rock sample S1 (the horizontal projection direction of the side with a length greater than a in the inclined end face), a stress gradient of longitudinal stress is formed. In such a gradient stress environment, transverse tensile stress will be generated in the width direction of the rock sample S1, and it is easier to generate tensile fracture surfaces that are nearly parallel to the left and right side surfaces of the rock sample S1. Under the condition of gradient stress, the rock sample S1 can not only generate tensile fracture surfaces, but also the tensile fracture surfaces are nearly parallel to the left side surface or the right side surface.

[0019] Another reason for the rock sample cracking is the end effect. The end effect refers to that after the rock S1 is compressed, since the stiffness of the bearing plate is greater than that of the rock S1, the transverse deformation after compression is smaller, and an extrusion and frictional effect will be generated on the end face of the rock S1. However, after the rock S1 is compressed, if there is a gasket between the bearing plate and the rock sample S1, and the stiffness of the gasket is smaller than that of the rock, such as a polytetrafluoroethylene plate, etc., the transverse deformation of the gasket after compression is larger, and it is difficult to release due to the frictional effect, and a tensile frictional force will be generated on the end face of the rock S1. Specifically, after the rock sample is compressed, the polytetrafluoroethylene plate deforms first, but this deformation is uneven. Near the edge, due to the transverse free space, the transverse deformation is larger after longitudinal compression; while for other parts of the polytetrafluoroethylene plate, such as the middle, due to the smaller transverse free space, the transverse deformation is difficult to release after longitudinal compression, and it will be subjected to the frictional resistance from the end face of the rock sample. The action and reaction effects are opposite, and the polytetrafluoroethylene plate here will also generate transverse tension on the end face of the rock sample S1, and this tension will increase with the axial compression until cracking occurs.

[0020]

[0019] Therefore, in this case, although the rock sample S1 is compressed, tensile cracking will also occur, and when cracking occurs, the end load is less than the uniaxial compressive strength of the rock sample S1. The position where cracking occurs can be determined from Step 2. If a rock sample made in the same way as in Step 2 is made and Step 2 is repeated, the position where new rock sample cracking develops will be the same as or very close to that of S1.

[0021]

[0021] Step 3: Strain measurement during the process of crack development

[0022] The specific method is: First, make a rock sample the same as the rock sample S1, denoted as rock sample S2. Paste a longitudinal strain gauge at the position corresponding to the tensile fracture surface on the front or rear side of the rock sample. The long side of this longitudinal strain gauge is parallel to the tensile fracture surface to measure the longitudinal strain ε y x and set another transverse strain gauge at a position 3 - 5 mm away from the inclined end face at the position corresponding to the tensile fracture surface on the front or rear side of the rock sample to measure the transverse strain ε x, wherein, the longitudinal strain gauge and the transverse strain gauge are in a T shape, and the transverse strain gauge is closer to the inclined end face. Then, place a brand-new gasket between the bearing plate and the end face of the rock sample S2, and repeat Step 2 to conduct the test. At the same time, during the loading and crack initiation process, synchronously measure the strain of the rock sample at the moment before the tensile crack initiation.

[0023] In Step 3 described above, the gasket is preferably a polytetrafluoroethylene plate.

[0024] In Step 3 described above, the corresponding widths of the longitudinal strain gauge and the transverse strain gauge are 3 mm, and the lengths are 10 mm.

[0025] Step 4: Calculation of the tensile strength of the rock sample

[0026] According to the strain measured at the moment before the tensile crack initiation, using the elastic modulus E and Poisson's ratio μ measured in Step 1, and combining with Hooke's law, calculate the transverse tensile stress when the rock cracks, which is the tensile strength of the rock. The deformation on the surface of the rock sample can be simplified as a plane strain problem. For a plane strain problem, according to Hooke's law, σ x is:

[0027]

[0028] In the formula, σ x is the transverse tensile stress, E is the elastic modulus, μ is Poisson's ratio, ε x is the measured transverse strain, and ε y is the measured longitudinal strain.

[0029] The beneficial effects of the present invention are as follows:

[0030] The present invention provides a method for monitoring the tensile strength of a rock. By accurately determining the crack position, the test position can be determined in advance, so as to accurately obtain the tensile strength. This method is simple, the error of multiple monitoring data is small, it can avoid the technical difficulties in the direct tensile test and Brazilian splitting test of the rock, directly measure the tensile strength of the rock using the rock slab cracking simulation system, and reduce the labor intensity. Description of the drawings

[0031] Figure 1 is the step flow chart of the method for testing the tensile strength of a rock based on gradient loading and end face effect of the present invention.

[0032] Figure 2 is a schematic diagram of the specially made quasi-rectangular parallelepiped rock sample S1.

[0033] Figure 3 is a photo of the cracked rock sample S1 under compression.

[0034] Figure 4It is a schematic diagram of uniaxial loading and strain monitoring of rock sample S2. In the figure, 1 is the upper bearing plate, 2 is the polytetrafluoroethylene plate, 3 is the rock sample, 4 is the transverse strain gauge, 5 is the longitudinal strain gauge, and 6 is the lower bearing plate.

[0035] Figure 5 It is the strain-time curve of the loading and cracking process of rock sample S2. Specific implementation manners

[0036] The present invention will be further described below in conjunction with the accompanying drawings and specific examples:

[0037] As Figure 1 shown, a rock tensile strength testing method based on gradient loading and end face effect includes the following steps:

[0038] Step 1: Determine the mechanical parameters of the rock sample, including elastic modulus E, Poisson's ratio μ, and maximum displacement l max . The measured elastic modulus E of the rock sample is 8.5×10 4 MPa, μ = 0.25, l max = 0.98 mm.

[0039] Step 2: Determine the position where tension cracking occurs. The determination method is as follows: Fabricate a cuboid rock sample with dimensions of a mm×a mm×b mm, where a = 40 mm, b = 100 mm, b = 2.5×a, meeting the requirement of b≥2×a, and the parallelism is controlled within 0.01 mm. Then carefully polish the end faces of the rock sample to make the end face formed by a×a of the rock sample symmetric front and back, tilted left and right, with the left side lower and the right side higher, and the difference is d0, d0 = 0.3 mm, meeting the requirement of d0 < l max = 0.98 mm. This rock sample is called a quasi-cuboid rock sample (as Figure 2 shown), denoted as S1; where z0 = y0 = a, x1 = x2 = b, x3 = x4, x3 + d0 = x1.

[0040] Next, place the rock sample in the loading chamber of an RMT press (requirement: the bearing plate of the press is not connected with a spherical end), separate the upper end face of the rock sample from the bearing plate with a polytetrafluoroethylene plate, and uniaxially load it at a rate of 0.1 mm / min by displacement control until the rock sample S1 starts to crack. When hearing the first clear splitting sound of the rock sample (indicating that the rock sample has started to crack), stop loading and quickly unload. Finally, observe and determine the position where the rock sample tension cracks (as Figure 3 shown), and measure the distance L lie from the cracking position to the left side of the rock sample = 24 mm.

[0041] Step 3: Strain measurement during the process of tensile fracture development. The specific method is as follows: First, prepare a rock sample identical to S1, denoted as rock sample S2. At a position 5 mm from the upper end face and L lie (L lie = 24 mm) from the left side face of the rock sample S2 on its front side, paste 2 strain gauges, corresponding to the transverse strain gauge 4 and the longitudinal strain gauge 5. Among them, paste a longitudinal strain gauge 5 at the position corresponding to the tensile fracture plane on the front side of the rock sample. The long side of this longitudinal strain gauge 5 is parallel to the tensile fracture plane to measure the longitudinal strain ε y . Set another transverse strain gauge 4 at a position 5 mm from the inclined end face at the position corresponding to the tensile fracture plane on the front side of the rock sample to measure the transverse strain ε x . Among them, the longitudinal strain gauge 5 and the transverse strain gauge 4 are in a T shape, and the transverse strain gauge 4 is closer to the inclined end face. The size of the strain gauge is 3 mm × 10 mm. Then, longitudinally clamp the rock sample 3 between the upper loading plate 1 and the lower loading plate 6. Place a brand-new polytetrafluoroethylene plate 2 between the upper loading plate 1 and the inclined end face of the rock sample 3, and then repeat Step 2 to conduct the test. At the same time, during the loading and crack initiation process, synchronously measure the strain of the rock sample S2. The loading schematic diagram of the rock sample S2 is as shown in Figure 4 . The strain-time curve during the loading of the specimen to crack initiation is as shown in Figure 5 . It can be seen from Figure 5 that in the later stage of loading, sudden changes occur in both the transverse and longitudinal strains, indicating the occurrence of tensile fractures. Take the strain one second before the sudden change as the critical fracture strain. At the time of impending fracture, the measured transverse strain ε x = 739.9×10 -6 , and the measured longitudinal strain ε y = -1777×10 -6 .

[0042] Step 4: Calculation of the tensile strength of the rock sample. Substitute the elastic modulus E = 8.5×10 4 MPa, Poisson's ratio μ = 0.25 obtained in Step 1, and the critical fracture transverse strain ε x = 739.9×10 -6 , longitudinal strain ε y = -1777×10 -6 obtained in Step 3 into formula (1) to calculate the transverse tensile stress σ b as follows:

[0043]

[0044] Therefore, the transverse tensile stress of the rock when approaching tensile fracture calculated hereby is the tensile strength of the measured rock sample: σ b = 26.8 MPa.

[0045] The parts not described in the present invention can be realized by adopting or referring to the existing technologies.

[0046] The greatest advantage of the present invention is that the tensile strength of the rock sample can be measured only by the method of uniaxial loading. Without adding direct tensile tests and indirect tensile tests, while conducting the rock slab cracking simulation test, the tensile strength of the rock can be measured, which can greatly reduce the difficulty and working intensity of measuring the tensile strength and improve the test efficiency of the tensile strength.

[0047] Comparative Example 1

[0048] Same as Example 1, except that the stiffness of the gasket used > the stiffness of the rock, then it is difficult for the rock sample to undergo tensile fracture after being compressed. Specifically, after the rock sample is compressed, there is also an end effect at its end. Only at this time, the end effect will generate frictional force with compressive properties perpendicular to the pressurizing direction at the end face. The rock sample will finally show shear failure, and it is difficult to obtain the tensile failure form, and thus it is difficult to measure the tensile strength of the rock sample. On the contrary, if the stiffness of the gasket < the stiffness of the rock, the end effect can generate tensile frictional force at the end of the rock sample, ultimately leading to tensile failure of the rock sample. The use of a gasket with a small stiffness is one of the keys to the success of this method.

[0049] Comparative Example 2

[0050] Same as Example 1, except that no gasket is provided, then it is also difficult for the rock sample to undergo tensile fracture after being compressed. Specifically, without a gasket, the end face of the rock sample will be in direct contact with the pressure machine bearing plate (steel plate). At this time, the same as in Comparative Example 1, the stiffness of the steel plate > the stiffness of the rock, and the end face will generate frictional force with compressive properties, and it is difficult for the rock sample to undergo tensile failure.

[0051] Comparative Example 3

[0052] Same as Example 1, except that two strain gauges are arranged to form an inverted T shape, that is, the longitudinal strain gauge is located above the transverse strain gauge, and the longitudinal strain gauge is closer to the inclined end face, which will lead to a smaller final tensile strength test result. Specifically, after the rock sample is compressed, tensile cracks first occur at the inclined end face (on the upper part) of the rock sample and expand downward. Moreover, the tensile frictional force generated by the end effect is the largest at the inclined end face and gradually decreases downward. Only when the transverse strain gauge is at a position closer to the inclined end face above can the tensile strength be measured more accurately. If the two strain gauges are arranged to form an inverted T shape, making the transverse strain gauge farther away from the inclined end face, the final tensile strength test result will be smaller.

Claims

1. A method for testing the tensile strength of rocks based on gradient loading and end face effect, characterized in that, It includes the following steps: Step 1: Measure the mechanical parameters of the rock sample, including elastic modulus E, Poisson's ratio μ, and maximum displacement l max ; Step 2: Determine the location where tensile cracks occur The determination method is as follows: First, a cuboid rock sample with a length, width, and height of amm × amm × bmm is made, and it is required that b ≥ 2 × a; then the end faces of the rock sample are carefully polished so that one end face with a × a forms an inclined end face; the height difference between the two sides formed by the inclination is d0, where d0 < l max ; this rock sample is called a quasi-cuboid rock sample and is denoted as S1; Place the rock sample S1 in the loading chamber of the RMT pressure testing machine, separate the inclined end face of the rock sample S1 from the bearing plate with a gasket, uniaxially load it to the onset of cracking of the rock sample S1 by means of displacement control, then stop loading and quickly unload, observe the inclined end face and determine the location where tensile cracks occur in the rock sample S1, and mark the fracture plane formed at the location where tensile cracks occur as the tensile fracture surface; wherein, the stiffness of the bearing plate > the stiffness of the rock S1 > the stiffness of the gasket; Step 3: Measure the strain during the development process of tensile cracks The specific method is as follows: First, make a rock sample identical to rock sample S1, denoted as rock sample S2. Paste a longitudinal strain gauge at the position corresponding to the tensile fracture plane on the front or rear side of the rock sample. The long side of this longitudinal strain gauge is parallel to the tensile fracture plane to measure the longitudinal strain ε y , and set another transverse strain gauge at a position 3 - 5 mm away from the inclined end face at the position corresponding to the tensile fracture plane on the front or rear side of the rock sample to measure the transverse strain ε x , where the longitudinal strain gauge and the transverse strain gauge are in a T shape, and the transverse strain gauge is closer to the inclined end face; Then, place a brand-new gasket between the bearing plate and the end face of the rock sample S2, and repeat Step 2 to conduct the test. At the same time, during the loading and cracking process, synchronously measure the strain of the rock sample at the moment before the onset of tensile cracking; Step 4: Calculate the tensile strength of the rock sample According to the strain measured at the moment before the onset of tensile cracking, using the elastic modulus E and Poisson's ratio μ measured in Step 1, and combining with Hooke's law in the generalized sense, calculate the transverse tensile stress when the rock cracks, which is the tensile strength of the rock.

2. The rock tensile strength testing method based on gradient loading and end face effect according to claim 1, characterized in that, The shape of the inclined end face is rectangular, and its four sides are divided into two groups, with the two sides in each group being parallel to each other; wherein, in the horizontal plane, the two sides of the first group are obliquely intersecting with the horizontal plane, and the two sides of the second group are parallel to the horizontal plane. One of the two sides parallel to the horizontal plane is on the relatively lower side, and the other side is on the relatively higher side. Among them, the a×b side face corresponding to the relatively lower side of the inclined end face is denoted as the left side face, the a×b side face corresponding to the relatively higher side of the inclined end face is denoted as the right side face, and the two a×b side faces perpendicular to the left side face and the right side face are denoted as the front side face or the rear side face.

3. The method for testing the tensile strength of rock based on gradient loading and end-face effect according to claim 2, wherein The tensile fracture surface, the left side face, and the right side face are parallel to each other.

4. The rock tensile strength testing method based on gradient loading and end face effect according to claim 2, characterized in that, The marking method of the tensile fracture surface selects the horizontal distance from the tensile fracture surface to the left side of the rock sample to mark the occurrence position of the tensile fracture, denoted as L lie ; or select the horizontal distance from the tensile fracture surface to the right side of the rock sample to mark the occurrence position of the tensile fracture, denoted as L lie '.

5. The rock tensile strength testing method based on gradient loading and end face effect according to claim 1, wherein, In the rock sample S1, the inclined end face is perpendicular to the front and rear side faces.

6. The rock tensile strength testing method based on gradient loading and end face effect according to claim 1, characterized in that, The selected RMT pressure testing machine is the RMT pressure testing machine in which the bearing plate of the press is not connected with a spherical end.

7. The rock tensile strength testing method based on gradient loading and end face effect according to claim 1, characterized in that The gasket is a polytetrafluoroethylene plate.

8. The rock tensile strength testing method based on gradient loading and end face effect according to claim 1, characterized in that The determination method of the onset of cracking is: the onset moment is determined according to the acoustic emission signal, or for rocks with greater brittleness, it is determined by the first clear splitting sound generated by the onset of cracking.

9. The rock tensile strength testing method based on gradient loading and end face effect according to claim 1, characterized in that, In Step 3, the widths of the longitudinal strain gauge and the transverse strain gauge are 3 mm, and the lengths are 10 mm.

10. The rock tensile strength testing method based on gradient loading and end face effect according to claim 1, characterized in that, In the fourth step described above, the deformation on the surface of the rock sample can be simplified to a plane strain problem. For a plane strain problem, according to the generalized Hooke's law, σ x is as follows: where σ x is the transverse tensile stress, E is the elastic modulus, μ is the Poisson's ratio, and ε x is the measured transverse strain, and ε y is the measured longitudinal strain.

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