Method for testing tensile strength of surrounding rock by considering non-linear elastic deformation

Abstract

The invention provides a surrounding rock tensile strength testing method considering non-linear elastic deformation, and relates to the technical field of rock tensile strength testing. The method specifically comprises the following steps: making a cuboid rock sample containing a rectangular groove and a standard cylindrical rock sample by using a rock block to be tested; pasting a strain gauge at the center of the rectangular groove in the cuboid rock sample; putting the rectangular-groove-containing cuboid rock sample attached with the strain gauges into a press machine for testing the tensile strength of the rock sample for a compression splitting test, and determining the crack initiation strain of the rectangular-groove-containing cuboid rock sample by using monitoring data in the compression splitting test process; and performing a uniaxial compression experiment on the standard cylindrical rock sample, constructing a rock sample stress-strain nonlinear elastic relation model of the standard cylindrical rock sample according to experimental data, and further calculating the tensile strength of the rock block to be tested. The method can avoid the problem of stress concentration easily occurring in the traditional test method, so that the tensile strength of the rock can be measured more accurately and conveniently.

Description

Technical Field

[0001] The present invention relates to the technical field of rock tensile strength testing, and particularly to a method for testing the tensile strength of surrounding rock considering non-linear elastic deformation. Background Art

[0002] During the excavation of deep-buried tunnels, due to the complex geological conditions of deep-buried tunnels, tensile failure often occurs in the surrounding rock. Therefore, it is very necessary to determine the rock tensile strength to study the mechanical behavior and failure characteristics of rocks under tensile stress.

[0003] At present, the main rock tensile strength testing techniques are direct tensile tests and Brazilian splitting tests. Among them, the direct tensile test of rocks measures the rock tensile strength and tensile deformation modulus through uniaxial tension. However, due to eccentricity and difficulty in centering during the test process of rock samples, it will affect the test results. The Brazilian splitting test measures the rock tensile strength through an indirect tensile method, and this method is relatively convenient to measure. However, the Brazilian splitting test is based on the assumption of elastic mechanics, assuming that the rock is a linear elastic body, while the rock contains pores and has obvious non-linear characteristics after being stressed, especially the non-linear deformation characteristics of sedimentary rocks with higher porosity are more obvious. When using the Brazilian splitting test to measure the rock tensile strength, due to the porosity of the rock not being considered, the test results may deviate greatly. Moreover, for rocks with relatively low hardness, during the Brazilian splitting test, the tensile failure of the rock sample often does not start from the center, but first cracks near the contact point between the rock sample and the pressure plate, and then the rock sample undergoes tensile failure due to the wedge effect, resulting in the measured tensile strength being smaller than the true value. Therefore, it is very necessary to develop a method for testing the rock tensile strength that can consider both the non-linear characteristics of rock deformation and avoid the problem of test failure caused by stress concentration near the Brazilian fracture line load. Summary of the Invention

[0004] Aiming at the deficiencies of the above-mentioned prior art, considering the non-linear characteristics of the deformation during the stress process of the rock sample, the present invention designs a cuboid rock sample with a rectangular groove and proposes a method for testing the tensile strength of surrounding rock considering non-linear elastic deformation, aiming to avoid the problem of easy stress concentration under the action of the disc line load in the Brazilian splitting test, so as to be able to measure the rock tensile strength more accurately and conveniently.

[0005] A method for testing the tensile strength of surrounding rock considering non-linear elastic deformation proposed by the present invention includes the following steps:

[0006] Step 1: Obtain the rock block to be tested and divide it into two parts, one part is used to make a cuboid rock sample with a rectangular groove, and the other part is used to make a standard cylindrical rock sample;

[0007] Step 2: Paste a strain gauge with a length of M and a width of N at the exact center of the rectangular groove in the cuboid rock sample with a rectangular groove, and the length of the strain gauge is parallel to the length of the rectangular groove;

[0008] Step 3: Place the cuboid rock sample with a rectangular groove and a strain gauge attached into a press for testing the tensile strength of the rock sample to conduct a compression splitting test, and record the monitoring data during the compression splitting test;

[0009] Step 4: Extract the crack initiation strain of the cuboid rock sample with a rectangular groove based on the monitoring data during the compression splitting test, and determine the crack initiation strain of the cuboid rock sample with a rectangular groove;

[0010] Step 5: Conduct a uniaxial compression test on the standard cylindrical rock sample, construct the stress-strain curve of the standard cylindrical rock sample based on the experimental data, and then fit the stress-strain curve of the rock sample by using the method of nonlinear fitting to construct the nonlinear elastic relationship model of the stress-strain of the standard cylindrical rock sample;

[0011] Step 6: Calculate the tensile strength of the rock sample to be tested according to the crack initiation strain of the cuboid rock sample with a rectangular groove and the nonlinear elastic relationship model of the stress-strain of the standard cylindrical rock sample;

[0012] The manufacturing method of the cuboid rock sample with a rectangular groove described in Step 1 is as follows: Determine that the length of the cuboid rock sample with a rectangular groove is e, the width is f, and the height is g, where e≥g≥f, and the units of e, g, and f are all mm; the width f of the cuboid rock sample is ≥20 mm, and ensure that the length and height of the cuboid rock sample meet the rock sample size specifications of the press for testing the tensile strength of the rock sample; carefully polish the cuboid rock sample so that the non-parallelism of the end face of the cuboid rock sample is less than 0.05 mm, the side face is perpendicular to the end face, and the error is less than 0.25°; select any one end face of the polished cuboid rock sample to cut a rectangular groove, the length of the rectangular groove is r, the width is q, and the height is t, and the units of r, q, and t are all mm; the length of the rectangular groove is parallel to the length of the cuboid rock sample, and the width of the rectangular groove is equal to the width of the cuboid rock sample, q = f; the value range of the height of the rectangular groove is:

[0013] The height-diameter ratio of the standard cylindrical rock sample described in Step 1 is 2:1;

[0014] The strain gauge described in Step 2 is used to measure the loading strain ε of the cuboid rock sample; the length M of the strain gauge is greater than 10 times the maximum particle size of the rock sample minerals and less than half of the length r of the rectangular groove;

[0015] The monitoring data described in Step 3 includes: strain, load, and digital images of the rock sample;

[0016] The experimental process of the compression splitting test is as follows: The compression splitting test uses displacement control. Set the loading rate k of the press, where k is greater than the minimum loading rate of the press and the value of k is less than 1 mm / min. Place the rectangular-grooved cuboid rock sample with strain gauges on the loading bin of the press, and ensure that the direction of the rectangular groove with the strain gauges is downward. The loading bin includes an upper bearing plate and a lower bearing plate. After the rock sample is placed, first perform a preloading of 1 kN to make the press and the rock sample in full contact. At this time, the rock sample and the press are in a stable state. The press applies pressure at the loading rate k until cracks are observed in the rock sample, then stop loading and unload to complete the compression splitting test. During the compression splitting test, the strain and load of the rectangular-grooved cuboid rock sample are monitored in real time. At the same time, a digital image of the rectangular-grooved cuboid rock sample is taken in real time by a camera.

[0017] The specific content of step 4 is as follows: According to the strain and load recorded during the compression splitting experiment, draw the strain-time curve of the rectangular-grooved cuboid rock sample during the loading process. Observe the strain-time curve of the rock sample during the loading process. When the strain-time curve shows a decrease in strain, tension is occurring. When the strain-time curve reaches the peak value, the rock sample undergoes tensile failure, and the peak strain in this strain-time curve is defined as the initial cracking strain ε of the rectangular-grooved cuboid rock sample. 0 and ε 0 is negative.

[0018] The construction method of the stress-strain non-linear elastic relationship model of the standard cylindrical rock sample is as follows: It is stipulated that tensile stress and tensile strain are negative, and compressive stress and compressive strain are positive. The stress-strain non-linear elastic relationship model of the standard cylindrical rock sample is expressed as:

[0019]

[0020] Rewrite the expression of the above stress-strain non-linear elastic relationship model of the standard cylindrical rock sample into a relationship of σ with respect to ε, and obtain:

[0021]

[0022] where ε is the strain; σ is the stress; a, b, and c are all undetermined coefficients.

[0023] The beneficial effects of adopting the above technical solutions are as follows:

[0024] (1) The design of the rectangular groove is beneficial to more accurately measure the cracking strain: Compared with the circular arc groove, when the cuboid rock sample with a rectangular groove designed by the method of the present invention is subjected to a compression splitting test, for the position where cracking is likely to occur in the middle of the rectangular groove, the tensile stress distribution near this position is more uniform. Furthermore, by choosing to attach a strain gauge at the exact center of the rectangular groove, the strain at the moment of rock sample cracking can be measured more accurately. However, this is not the case for the circular arc groove because the tensile stress distribution at the position where cracking is likely to occur in the circular arc groove is non-uniform, and the strain gauge has a certain length. The strain measured by the strain gauge is the average strain within a certain range, which will be much smaller than the actual value, and the greater the non-uniformity of the stress distribution, the greater this measurement deviation will be.

[0025] (2) The method of strain measurement makes the determination of the crack initiation moment more accurate: Using a high-definition camera to determine the crack initiation is a commonly used traditional method. However, only when the crack initiates and propagates to a certain extent can the crack be visually detected in the image. In other words, the moment when the crack is seen in the digital image is not the crack initiation moment but the moment when the crack has propagated to a visible extent after initiation, lagging behind the initiation moment. When a crack is generated, the areas on both sides of the crack will inevitably unload pressure, causing the strain to decrease. In the method of the present invention, the strain gauge is pasted on one side of the rock sample where a crack will be generated. Before the crack is generated, the measured strain continuously increases, and once the crack is generated, the measured strain will gradually decrease. Therefore, in the method of the present invention, by plotting the strain-time curve of the rock sample during the loading process, the moment corresponding to the peak strain is determined as the crack initiation moment, accurately determining the crack initiation moment.

[0026] (3) Considering the non-linearity of the rock stress-strain relationship makes the measurement result of the rock tensile strength more accurate: Since rocks generally have porosity, this leads to the stress-strain relationship of rocks generally showing non-linearity. The indirect measurement Brazilian splitting method also assumes that the rock is a homogeneous isotropic elastic body and does not consider the non-linear elastic characteristics of rock deformation. The method of the present invention first determines the crack initiation moment and the strain at the crack initiation moment through strain measurement, and then calculates the stress corresponding to the strain at the crack initiation moment based on the measured non-linear relationship model of rock stress-strain, and takes this stress as the tensile strength of the rock. The method of the present invention fully considers the non-linear characteristics of rock deformation during the measurement process, and the measurement result is more reasonable.

[0027] (4) Applicable to the measurement of the tensile strength of soft rock: Since the ability of soft rock to resist the intrusion of external forces is weak, during the conventional Brazilian splitting test, it often occurs that near the line load, the rock sample first generates a V-shaped notch due to stress concentration, and then the rock sample splits due to the wedge effect, making it difficult to observe the expected phenomenon of first splitting near the midpoint of the circular rock sample and then expanding towards the two ends of the line load direction, resulting in a smaller measured tensile strength. In the method of the present invention, the upper and lower stress-bearing surfaces of the produced rock sample are both flat surfaces. When compressed, the force received is a surface load, rather than a line load or a point load, thus eliminating the problem of local stress concentration damage of the rock sample caused by the line load or point load, and is particularly applicable to the tensile strength test of soft rock with relatively small hardness, such as siltstone, coal rock, etc.

[0028] In summary, the method of the present invention not only takes into account the non-linear characteristics of the deformation during the stress-bearing process of the rock sample, but also avoids the stress concentration near the line load during the Brazilian splitting process by compressing the special designed cuboid rock sample with a rectangular groove, and can more accurately measure the tensile strength of rocks with relatively small hardness. Description of the Drawings

[0029] Figure 1 is a flow chart of a method for testing the tensile strength of surrounding rock considering non-linear elastic deformation in this embodiment;

[0030] Figure 2 is a schematic diagram of a cuboid rock sample with a rectangular groove in this embodiment;

[0031] Figure 3 is a schematic diagram of the compression splitting test of a cuboid rock sample with a rectangular groove in this embodiment;

[0032] Figure 4 is a schematic diagram of a cuboid rock sample with a rectangular groove splitting in this embodiment;

[0033] Figure 5 is a strain-time curve diagram of a cuboid rock sample with a rectangular groove during the compression splitting process in this embodiment;

[0034] Figure 6 is a fitting curve diagram of the stress-strain non-linear elastic relationship of a cylindrical standard rock sample in this embodiment;

[0035] In the figure: 1 - upper bearing plate; 2 - lower bearing plate; 3 - rock sample; 4 - strain gauge. Specific Embodiments

[0036] For the convenience of understanding the present application, the following combines the drawings and embodiments to further describe in detail the specific embodiments of the present invention. The following embodiments are used to illustrate the present invention, but are not used to limit the scope of the present invention. On the contrary, the purpose of providing these embodiments is to make the disclosure content of the present application understood more thoroughly and comprehensively.

[0037] A method for testing the tensile strength of surrounding rock considering non - linear elastic deformation in this embodiment is as follows Figure 1 As shown, this method includes the following steps:

[0038] Step 1: Obtain the rock block to be tested and divide it into two parts. One part is used to make a cuboid rock sample with a rectangular groove, and the other part is used to make a standard cylindrical rock sample.

[0039] The method for making the cuboid rock sample with a rectangular groove is as follows: Determine that the length of the cuboid rock sample with a rectangular groove is e, the width is f, and the height is g, where e ≥ g ≥ f, and the units of e, g, and f are all mm; the width f of the cuboid rock sample is ≥ 20 mm, and ensure that the length and height of the cuboid rock sample meet the rock sample size specifications of the press for testing the tensile strength of the rock sample; carefully polish the cuboid rock sample so that the non - parallelism of the end face of the cuboid rock sample is less than 0.05 mm, the side face is perpendicular to the end face, and the error is less than 0.25°; select an end face of the polished cuboid rock sample to cut a rectangular groove, the length of the rectangular groove is r, the width is q, and the height is t, and the units of r, q, and t are all mm; the length of the rectangular groove is parallel to the length of the cuboid rock sample, and the width of the rectangular groove is equal to the width of the cuboid rock sample, q = f; the value range of the height of the rectangular groove is:

[0040] In this embodiment, by designing a cuboid rock sample with a rectangular groove, the problem of linear load stress concentration existing in the traditional rock tensile strength testing methods such as the Brazilian splitting method and the three - point complete method is avoided.

[0041] The height - to - diameter ratio of the standard cylindrical rock sample is 2:1.

[0042] In this embodiment, first, obtain a large and uniform blue sandstone block as the rock block to be tested, and cut the rock block to be tested into two parts. One part is used to make a cuboid rock sample with a rectangular groove, and the size of the length and height of the cuboid rock sample is determined according to the rock sample size that the press can load, and the recommended value is 50 - 150 mm; in order to ensure that tensile failure can occur when the rock sample is compressed, the width of the cuboid rock sample needs to be greater than or equal to 20 mm, and the recommended value is 30 - 60 mm; generally, the maximum height that the press can place is 200 mm, so in this embodiment, the made cuboid rock sample with a rectangular groove has a length, width, and height of 100 mm × 50 mm × 100 mm; the other part is used to make a standard cylindrical rock sample with a diameter of 50 mm and a height of 100 mm. Carefully polish it so that the non - parallelism of the end face of the rock sample is less than 0.05 mm, the side face is perpendicular to the end face, and the error is less than 0.25°; as Figure 2As shown, a rectangular groove is cut on the EFXY plane of the cuboid rock sample. The rectangular groove is cut from the midpoint of EF, and the length of the rectangular groove is parallel to the length of the cuboid rock sample. It is recommended that the length r of the rectangular groove be e / 2, the width q = f, and the height be In this embodiment, the length, width, and height of the rectangular groove are 50 mm, 50 mm, and 20 mm respectively.

[0043] Step 2: Paste a strain gauge with a length of M and a width of N at the center of the rectangular groove in the cuboid rock sample with the rectangular groove, and the length of the strain gauge is parallel to the length of the rectangular groove.

[0044] The strain gauge is used to measure the loading strain ε of the cuboid rock sample; the length M of the strain gauge is greater than 10 times the maximum particle size of the rock sample minerals and less than half of the length r of the rectangular groove.

[0045] In this embodiment, there is no specific limit for the width of the strain gauge. Generally, it is recommended that N be 3 mm. The maximum particle size of the rock sample minerals of the rock sample to be tested is 0.3 mm. Therefore, a strain gauge with a length of 10 mm and a width of 3 mm is pasted at the center of the rectangular groove, and it is ensured that the length of the strain gauge is parallel to the length of the rectangular groove for measuring the loading strain of the rock sample.

[0046] Step 3: Place the cuboid rock sample with the rectangular groove and the strain gauge pasted on it into a press for testing the tensile strength of the rock sample to conduct a compression splitting test, and record the monitoring data during the compression splitting test.

[0047] The monitoring data includes: strain, load, and digital images of the rock sample.

[0048] The experimental process of the compression splitting test is as follows: The compression splitting test uses displacement control, and the loading rate k of the press is set, where k is greater than the minimum loading rate of the press and the value of k is less than 1 mm / min; place the cuboid rock sample 3 with the rectangular groove and the strain gauge 4 pasted on it in the loading bin of the press, and ensure that the direction of the rectangular groove with the strain gauge pasted on it is downward; the loading bin includes an upper bearing plate 1 and a lower bearing plate 2; after the rock sample 3 is placed, a preloading of 1 kN is first carried out to make the press and the rock sample fully contact. At this time, the rock sample 3 and the press are in a stable state; the press applies pressure at the loading rate k until it is observed that the rock sample cracks, then stops loading and unloads to complete the compression splitting test; during the process of the compression splitting test, the strain and load of the cuboid rock sample with the rectangular groove are monitored in real time, and at the same time, the digital image of the cuboid rock sample with the rectangular groove is taken in real time by a camera.

[0049] In this embodiment, in order to facilitate the observation of the gestation process of slab cracking, the press is a rigid press, and the loading rate should be small enough. The test uses displacement control, and the loading rate is set to 0.05 mm / min. As Figure 3As shown in the figure, a rectangular groove-containing cuboid rock sample is placed with the groove facing down in the loading bin of a press. After a preloading of 1 kN, the loading is stopped, and this is the stable state at this time. The purpose of preloading is to ensure full contact between the press and the specimen, thereby reducing experimental errors. Then, the pressure is applied at the above-mentioned predetermined loading rate until the rock sample undergoes tensile failure, that is, when cracks appear in the rock sample, the loading is stopped and unloaded. During the experiment, strain and the camera are monitored synchronously. After the experiment ends, various experimental monitoring data including strain, load, and digital images are saved; as Figure 4 shown, the rectangular groove-containing cuboid rock sample undergoes splitting.

[0050] Step 4: Extract the initial cracking strain of the rectangular groove-containing cuboid rock sample based on the monitoring data during the compression splitting experiment, and determine the initial cracking strain of the rectangular groove-containing cuboid rock sample.

[0051] The specific content of the above-mentioned Step 4 is: Based on the strain and load recorded during the compression splitting experiment, plot the strain-time curve of the rectangular groove-containing cuboid rock sample during the loading process. Observe the strain-time curve of the rock sample during the loading process. When the strain-time curve shows a decrease in strain, tension cracking is occurring; when the strain-time curve reaches the peak, the rock sample undergoes tensile failure, and the peak strain in this strain-time curve is defined as the initial cracking strain ε 0 of the rectangular groove-containing cuboid rock sample, and ε 0 is negative.

[0052] In this embodiment, as Figure 5 shown, based on the strain recorded during the compression splitting experiment, plot the strain-time curve of the rectangular groove-containing cuboid rock sample during the loading process. Observe the strain-time curve of the rock sample during the loading process. When the strain-time curve shows a decrease in strain, tension cracking is occurring. According to the stress-time curve monitored by the press, the moment when the curve reaches the peak is the moment of tensile failure because the curve will have an obvious drop after this moment, so it can be clearly known that failure has occurred. Record the peak strain of the measured strain, that is, the initial cracking strain ε 0 =-0.1345%.

[0053] Step 5: Conduct a uniaxial compression experiment on a standard cylindrical rock sample, construct the stress-strain curve of the standard cylindrical rock sample based on the experimental data, and then fit the stress-strain curve of the rock sample by using the method of nonlinear fitting to construct the nonlinear elastic relationship model of the stress-strain of the standard cylindrical rock sample;

[0054] The construction method of the nonlinear elastic relationship model of the stress-strain of the standard cylindrical rock sample is: It is stipulated that tensile stress and tensile strain are negative, and compressive stress and compressive strain are positive. The nonlinear elastic relationship model of the stress-strain of the standard cylindrical rock sample is expressed as:

[0055]

[0056] where ε is the strain; σ is the stress; a, b, and c are all undetermined coefficients of the model.

[0057] Rewrite formula (1) as a relational expression of σ with respect to ε, and obtain:

[0058]

[0059] Adopt the method of non - linear fitting to fit the stress - strain curve of the standard cylindrical rock sample, and determine the values of the undetermined coefficients a, b, and c.

[0060] In this embodiment, according to the standard of engineering rock mass test methods (GBT50266 - 2013), carry out the uniaxial compression test on the standard cylindrical rock sample to obtain the uniaxial compression stress - strain curve of the rock sample. Through the method of non - linear fitting, perform non - linear fitting on the compaction - linear elastic stage of the stress - strain curve to obtain the fitting curve. As Figure 6 shown, the measured points are evenly distributed on both sides of the fitting curve, indicating that the fitting effect is relatively good. The undetermined coefficients in the non - linear elastic relationship model of the stress - strain of the standard cylindrical rock sample are determined by non - linear fitting as shown in Table 1.

[0061] Table 1 Table of undetermined coefficients of the non - linear elastic relationship model of the stress - strain of the standard cylindrical rock sample

[0062] a b c 11231 0.0031 0.0549

[0063] Step 6: Calculate the tensile strength of the rock sample to be tested according to the crack - initiation strain of the rectangular - groove - containing cuboid rock sample and the non - linear elastic relationship model of the stress - strain of the standard cylindrical rock sample.

[0064] In this embodiment, substitute the crack - initiation strain of the rectangular - groove - containing cuboid rock sample into formula (2), and the tensile strength σ of the rock can be calculated t , that is:

[0065]

[0066] σ t =-4.3398 MPa (4)

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope defined by the claims of the present invention.

Claims

1. A method for testing the tensile strength of surrounding rock taking into account nonlinear elastic deformation, characterized in that: The method comprises the following steps: Step 1: Obtain the rock block to be tested and divide it into two parts, one of which is used to make a rectangular parallelepiped rock sample with a rectangular groove, and the other is used to make a standard cylindrical rock sample; Step 2: Paste a strain gauge with a length of M and a width of N at the center of the rectangular groove in the rectangular rock sample containing the rectangular groove, and the length of the strain gauge is parallel to the length of the rectangular groove; Step 3: Place the rectangular rock sample with a rectangular groove and a strain gauge in a press for testing the tensile strength of the rock sample to perform a compression splitting test, and record the monitoring data during the compression splitting test; Step 4: extract the cracking strain of the rectangular parallelepiped rock sample containing the rectangular groove according to the monitoring data during the compression splitting experiment, and determine the cracking strain of the rectangular parallelepiped rock sample containing the rectangular groove; Step 5: Perform a uniaxial compression test on the standard cylindrical rock sample, and construct a rock sample stress-strain curve of the standard cylindrical rock sample based on the experimental data, and then fit the rock sample stress-strain curve by using a nonlinear fitting method to construct a rock sample stress-strain nonlinear elastic relationship model of the standard cylindrical rock sample; Step 6: Calculate the tensile strength of the rock block to be tested based on the crack initiation strain of the rectangular parallelepiped rock sample with rectangular grooves and the rock sample stress-strain nonlinear elastic relationship model of the standard cylindrical rock sample.

2. A method for testing the tensile strength of surrounding rock taking into account nonlinear elastic deformation according to claim 1, characterized in that: The method for making the rectangular rock sample containing the rectangular groove in step 1 is as follows: determine that the length of the rectangular rock sample containing the rectangular groove is e, the width is f, and the height is g, wherein e≥g≥f, and the units of e, g, and f are all mm; the width f of the rectangular rock sample is ≥20 mm, and ensure that the length and height of the rectangular rock sample meet the rock sample size specifications of the press used to test the tensile strength of the rock sample; carefully polish the rectangular rock sample so that the non-parallelism of the end face of the rectangular rock sample is less than 0.05 mm, the side face is perpendicular to the end face, and the error is less than 0.25°; select any end face of the polished rectangular rock sample to cut a rectangular groove, wherein the length of the rectangular groove is r, the width is q, and the height is t, and the units of r, q, and t are all mm; the length of the rectangular groove is parallel to the length of the rectangular rock sample, and The width of the rectangular groove is equal to the width of the rectangular rock sample, q=f; the value range of the height of the rectangular groove is:

3. A surrounding rock tensile strength testing method considering nonlinear elastic deformation according to claim 2, characterized in that: The height-to-diameter ratio of the standard cylindrical rock sample in step 1 is 2:

1.

4. A surrounding rock tensile strength testing method considering nonlinear elastic deformation according to claim 3, characterized in that: The strain gauge in step 2 is used to measure the rock sample loading strain ε of the rectangular rock sample; the length M of the strain gauge is greater than 10 times the maximum particle size of the rock sample mineral particles and less than half the length r of the rectangular groove.

5. A method for testing the tensile strength of surrounding rock taking into account nonlinear elastic deformation according to claim 4, characterized in that: The monitoring data in step 3 include: strain, load and digital image of rock sample; The experimental process of the compression splitting test is as follows: the compression splitting test adopts displacement control, and the loading rate k of the press is set, wherein k is greater than the minimum loading rate of the press, and the value of k is less than 1 mm / min; the rectangular rock sample 3 with a rectangular groove and affixed with a strain gauge 4 is placed in the loading chamber of the press, and it is ensured that the rectangular groove with the strain gauge is directed downward; the loading chamber comprises an upper pressure plate 1 and a lower pressure plate 2; after the rock sample 3 is placed, a preload of 1 kN is first performed to make the press and the rock sample fully contact, and at this time the rock sample 3 and the press are in a stable state; the press applies pressure at the loading rate k until cracks are observed in the rock sample, then stops loading and unloading, and completes the compression splitting test; during the compression splitting test, the strain and load of the rectangular rock sample with the rectangular groove are monitored in real time, and at the same time, a digital image of the rectangular rock sample with the rectangular groove is captured in real time by a camera.

6. A surrounding rock tensile strength testing method considering nonlinear elastic deformation according to claim 5, characterized in that: The specific content of step 4 is: according to the strain and load recorded during the compression splitting experiment, the strain-time curve of the rectangular rock sample containing rectangular grooves during the loading process is drawn, and the strain-time curve of the rock sample during the loading process is observed. When the strain decreases in the strain-time curve, tensile cracking is occurring; when the strain-time curve reaches a peak value, the rock sample undergoes tensile failure, and the peak strain in the strain-time curve is taken as the crack initiation strain ε0 of the rectangular rock sample containing rectangular grooves, and ε0 is a negative number.

7. A surrounding rock tensile strength testing method considering nonlinear elastic deformation according to claim 6, characterized in that: The method for constructing the rock sample stress-strain nonlinear elastic relationship model of the standard cylindrical rock sample is as follows: tensile stress and tensile strain are set to be negative, and compressive stress and compressive strain are set to be positive, and the rock sample stress-strain nonlinear elastic relationship model of the standard cylindrical rock sample is expressed as: Rewrite the expression of the rock sample stress-strain nonlinear elastic relationship model of the above standard cylindrical rock sample into the relationship between σ and ε, and we get: Where ε is strain; σ is stress; a, b, and c are all unknown coefficients.

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