Uniaxial Compression Damage Model of Rock Considering the Mechanical Behavior of Microcracks

Abstract

The present invention discloses a uniaxial compression damage model of rock considering the mechanical behavior of microcracks, which reflects that rock failure is caused by the slip and propagation of microcracks. According to the microcrack slip model and the principle of energy balance, the uniaxial compression stress-strain relationship of rock is established, and it is considered that microcracks obey the Weibull distribution model. Furthermore, taking the strain energy density criterion as the microcrack propagation criterion, the iterative method is used to solve the propagation length of the wing crack in the mixed-mode fracture, and the evolution equation of the rock damage variable expressed by the propagation length of the wing crack is obtained. Thus, a uniaxial compression damage model of rock is proposed and its rationality is verified. The influence of microcrack length, friction coefficient and rock fracture toughness on the mechanical properties of rock is studied by parameter sensitivity analysis. The research results show that the macroscopic mechanical properties of rock are the macroscopic manifestation of its mesoscopic mechanical parameters. Therefore, the rock damage constitutive model established from the mesoscopic perspective can better reveal its deformation and failure mechanism and has good application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of rock mechanics analysis, and particularly relates to a uniaxial compression damage model of rock considering the mechanical behavior of microcracks. Background Art

[0002] The initiation and propagation of microcracks under external loads are the fundamental mechanical mechanisms that lead to the progressive damage and ultimate failure of rocks. Although significant progress has been made in the research on rock damage models based on macroscopic damage mechanics of continuum mechanics, it mainly establishes corresponding macroscopic phenomenological damage constitutive equations within the framework of thermodynamics by introducing associated and non-associated plastic flow rules. Therefore, it fails to well reflect the mesoscopic phenomena of the influence of the mechanical behaviors such as the closure, frictional slip, propagation, and interaction of microcracks on the deformation and failure characteristics of rocks, and thus it is difficult to reveal the internal mechanical mechanism of rock failure.

[0003] The emergence, development, and application of mesoscopic mechanics with mesoscopic elements (such as microcracks, microvoids, etc.) as the research objects provide a powerful tool for the research on rock damage theory and thus develop various rock mesoscopic damage models. Zhou et al. assumed that microcracks follow a random distribution and proposed a corresponding uniaxial compression damage model of rock in combination with the initiation and propagation criteria of microcracks under compressive loads; however, the contribution of the displacements generated due to the initiation, propagation, and frictional sliding of microcracks under compressive loads to the total deformation of the rock was not considered.

[0004] In summary, from the current research status at home and abroad, it can be seen that predecessors have conducted in-depth research on rock damage constitutive models from the perspective of mesoscopic mechanics, but they have not well considered the interaction between microcracks, especially the inhibitory effect between them, on their mechanical properties during the loading process of rocks, resulting in incorrect calculation of the number of activated microcracks in the loaded rock, and there is a certain error between the calculation results of the theoretical model and the measured results. Summary of the Invention

[0005] In order to solve the above problems, the present invention proposes a uniaxial compression damage model of rock considering the mechanical behavior of microcracks.

[0006] The uniaxial compression damage model of rock considering the mechanical behavior of microcracks in the present invention is as follows:

[0007] σ = E(1 - D)ε (27)

[0008] In the formula, D is the rock damage variable, σ is the stress, ε is the strain, and E is the elastic modulus of the rock.

[0009] The calculation formula of the rock damage variable is

[0010]

[0011] where N is the density of microcracks in the rock, l is the tensile wing crack, l * = 0.27a, l ** = 0.083a, 4bh represents the unit area, θ is the inclination angle of the microcrack, f is the friction coefficient, and a represents the half-length of the crack.

[0012] The stress intensity factor at the tip of the microcrack is

[0013]

[0014] where τ * is the effective shear stress, θ is the inclination angle of the microcrack, l * = 0.27a, l ** = 0.083a, a represents the half-length of the crack, K I represents the stress intensity factor of type I, K II represents the stress intensity factor of type II.

[0015] The propagation length of the microcrack is calculated using the strain energy density criterion, that is, when the strain energy density S of the wing crack is greater than the minimum strain energy density S c , the microcrack begins to initiate and propagate. The strain energy density S of the wing crack can be solved by the following formula:

[0016]

[0017] where

[0018] K I represents the stress intensity factor of type I, K II represents the stress intensity factor of type II, θ is the inclination angle of the microcrack, E is the elastic modulus of the rock, and v is the Poisson's ratio of the rock.

[0019] The formula for calculating the minimum strain energy density S c is

[0020]

[0021] where K Ιc is the static fracture toughness of the rock; S c is also called the fracture threshold, E is the elastic modulus of the rock, and v is the Poisson's ratio of the rock.

[0022] The length of the microcrack at a certain moment is calculated by the following formula

[0023]

[0024] where l tis the wing crack length at a certain moment t, S is the strain energy density of the wing crack, S c is the fracture threshold.

[0025] The number of microcracks activated during the damage of the rock is

[0026] N = (1 - D n )N0 = (1 - D n )kε m (26)

[0027] In the formula, N is the number of microcracks, D n is the rock damage at a certain time step n, N0 is the number of microcracks activated per unit volume under a given strain ε, N0 = kε m , k and m are parameters describing the material failure characteristics.

[0028] The beneficial effects of the present invention are as follows.

[0029] The present invention discloses a uniaxial compression damage model of rock considering the mechanical behavior of microcracks, which reflects that rock failure is caused by the slip and propagation of microcracks. According to the microcrack slip model and the energy balance principle, the uniaxial compression stress-strain relationship of rock is established, and it is considered that microcracks obey the Weibull distribution model. Furthermore, taking the strain energy density criterion as the criterion for the propagation of microcracks, the iterative method is used to solve the propagation length of the wing crack in the mixed-mode fracture, and the evolution equation of the rock damage variable expressed by the propagation length of the wing crack is obtained. Thus, a uniaxial compression damage model of rock is proposed and its rationality is verified. The influence of microcrack length, friction coefficient and rock fracture toughness on the mechanical properties of rock is studied by parameter sensitivity analysis. The research results show that the macroscopic mechanical properties of rock are the macroscopic manifestation of its mesoscopic mechanical parameters. Therefore, the rock damage constitutive model established from the mesoscopic perspective can better reveal its deformation and failure mechanism and has good application prospects. Description of the Drawings

[0030] Figure 1 is the rock specimen containing microcracks under uniaxial compression and the microcrack slip model.

[0031] Figure 2 is the comparison diagram of sandstone test and calculation curves under uniaxial compression load.

[0032] Figure 3 is the curve of axial stress, damage and axial strain under uniaxial compression.

[0033] Figure 4 is the mechanical properties of the specimen at different microcrack lengths.

[0034] Figure 5 is the mechanical properties of the specimen at different microcrack friction coefficients.

[0035] Figure 6 are the mechanical properties of specimens with different rock fracture toughnesses. Specific implementation manners

[0036] The embodiments of the present invention will be described in detail below. Examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present invention, and should not be construed as limiting the present invention.

[0037] First, based on the microcrack slip model and the principle of energy balance, the uniaxial compression stress-strain relationship of rocks was established.

[0038] Figure 1 (a) is a rock element containing a single microcrack. The mechanical behavior of the initiation and propagation of the microcrack under the compression load is as Figure 1 (b) shown. Under the compression load, the microcrack surface first closes, and then shear stresses are generated on its upper and lower surfaces, and a sliding trend appears on the microcrack surface. Due to the closure of the microcrack surface, the frictional force acting on it will prevent the rock block from slipping. When the shear stress component on the microcrack surface exceeds its frictional force, the specimen will undergo frictional slip along the microcrack surface, and then stress concentration will be generated at the tip of the microcrack. As the compression load continues to increase, when the stress intensity factors at the tips P and P' of the microcrack satisfy the microcrack initiation and propagation criterion, tensile wing cracks Q and Q' will be generated at the tip of the microcrack, as Figure 1 (b) shown. As the microcrack continues to expand, the tensile wing cracks will gradually tend to expand in the direction of the compression load and finally lead to axial splitting failure of the rock specimen.

[0039] Under uniaxial compression, the normal stress σ θ and shear stress τ θ on the microcrack surface are respectively:

[0040] σ θ = σcos 2 θ (1)

[0041] τ θ = σsinθcosθ (2)

[0042] In the formula: θ is the inclination angle of the microcrack.

[0043] Let the friction angle of the microcrack surface be and the corresponding friction coefficient Under uniaxial compression, the shear stress on the microcrack surface will cause a sliding trend of the microcrack, and correspondingly, the normal stress acting on the microcrack surface will generate a frictional force that prevents the microcrack from slipping. It can be seen that when the specimen slides along the microcrack surface, correspondingly, the effective shear stress τ *It should be greater than 0. Therefore, from equations (1) and (2), the slip driving force on the microcrack surface can be obtained as follows:

[0044]

[0045] Under uniaxial compression load, as Figure 1 shown, the stress intensity factor at the tip of the microcrack is:

[0046]

[0047] In the formula, τ * is the effective shear stress, θ is the inclination angle of the microcrack, l * = 0.27a, l ** = 0.083a, a represents the half-length of the crack, K I represents the stress intensity factor of type I, and K II represents the stress intensity factor of type II.

[0048] According to the principle of energy balance, the work W1 done by the load on the elastic body is equal to the elastic strain energy U e released due to crack propagation and the energy W f dissipated due to frictional sliding between the crack surfaces, that is:

[0049] W1 = U e + W f (5)

[0050] As Figure 1 (a) shown, assume that the strain generated by the microcrack under the compressive stress is Δε1. Then, due to the Poisson's ratio effect, a small strain value will also appear in the direction perpendicular to the compressive stress for the microcrack, denoted as Δε2. The work done by σ is:

[0051] W1 = 4bhσΔε1 (6)

[0052] Among them: 4bh represents the unit area.

[0053] In the elastic range, the linear relationship between Δε1, Δε2 and the applied stress can be written as:

[0054]

[0055] In the formula: [S ij (i,j = 1,2) is the compliance tensor caused by a single microcrack. According to symmetry, S ij = S ji . From equations (6) and (7), we can obtain:

[0056] W1 = 4bhS 11 σ 2 (8)

[0057] According to fracture mechanics, the elastic strain energy released due to the propagation of microcracks is (for plane stress problems):

[0058]

[0059] Substituting Equation (4) into Equation (9) gives:

[0060]

[0061] Under the action of shear stress, if the sliding distance of the specimen along the microcrack surface is δ, the energy consumed due to frictional slip is:

[0062] W f = 2aδ·fσ θ (11)

[0063] Assuming that the microcrack spacing is not considered, the sliding distance δ is:

[0064]

[0065] Where: l ** = 0.083a.

[0066] Substituting Equation (12) into Equation (11) gives:

[0067]

[0068] From (5), (10), (13), by comparing the coefficients, the components of the compliance tensor caused by a single crack are: Where:

[0069] B i is a parameter related to the tensile wing crack l,

[0070] If the microcrack density in the rock is assumed to be N, and all microcracks have the same size, are parallel and uniformly distributed, and their interaction is ignored, the strain caused by the propagation of microcracks is:

[0071]

[0072] Assuming that the strain of the rock mass is small strain, then the total strain ε should be the elastic strain ε e (before the crack propagates) and the strain ε d caused by the propagation of microcracks, that is:

[0073] ε = ε e + ε d (15)

[0074] According to Hooke's law, the stress σ and the elastic strain ε e should satisfy:

[0075] ε e = D:σ (16)

[0076] where: D is the compliance tensor. For plane stress problems, Using equations (14) - (16), the total strain is:

[0077]

[0078] where: E and v are the elastic modulus and Poisson's ratio of the rock, respectively.

[0079] Equation (17) is obtained based on plane stress problems. For plane strain problems, only replace E with E / (1 - v 2 ), and v with v / (1 - v 2 ) is sufficient.

[0080] Then, under uniaxial compression loading, and combined with Figure 1 (b), assuming the stress and strain in the direction of the compression loading are σ and ε respectively, then from equation (17), the corresponding stress - strain relationship can be obtained as:

[0081] ε = σ / E + NS 11 σ (18)

[0082] Substitute S 11 into equation (18), and through transformation, we can get:

[0083]

[0084] Introduce the rock damage variable D to describe the weakening of the elastic modulus of the rock due to the increase in micro - crack density and the propagation of micro - cracks, that is:

[0085]

[0086] where:

[0087]

[0088] Secondly, taking the strain energy density criterion as the micro - crack propagation criterion, using the iterative method to solve the propagation length of the wing crack in mixed - mode fracture, and obtaining the evolution equation of the rock damage variable expressed by the propagation length of the wing crack. Thus, a new uniaxial compression damage model of rock is proposed and its rationality is verified.

[0089] Calculate the propagation length of the wing crack using the strain energy density criterion, that is, when the strain energy density S of the wing crack is greater than the minimum strain energy density S c , the micro - cracks begin to initiate and propagate.

[0090] The strain energy density S of the wing crack can be solved by the following formula:

[0091]

[0092] Where: When θ3 = 0, S is the strain energy density in the direction of the wing crack.

[0093] The minimum strain energy density S c The calculation formula is:

[0094]

[0095] Where: K Ιc is the static fracture toughness of the rock; S c is also called the fracture threshold.

[0096] The length l of the wing crack at a certain moment t t is calculated by the following formula:

[0097]

[0098] Grady et al. proposed a microcrack evolution model when studying the problem of oil shale blasting. It uses the Weibull distribution model to describe the number of activated microcracks, that is:

[0099] N0 = kε m (25)

[0100] In the formula: N0 is the number of activated microcracks per unit volume under a given strain ε; k and m are parameters describing the material failure characteristics.

[0101] At a certain time step n, assuming the rock damage is D n , then the number of activated microcracks N considering damage can be expressed as:

[0102] N = (1 - D n )N0 = (1 - D n )kε m (26)

[0103] According to the Lemaitre equivalent principle, the uniaxial compression damage model of the rock can be expressed as:

[0104] σ = E(1 - D)ε (27)

[0105] Where: D is the rock damage variable, calculated by formula (21).

[0106] The model calculation process is:

[0107] ① First, the initial stress is taken as zero, then a stress increment Δσ is given, and the stress intensity factor at the tip of the microcrack is calculated by Equation (4). Then, it is judged whether the microcrack propagates according to Equations (21) - (24). If the microcrack propagation condition is not satisfied, then N = 0 and l = 0, and then the corresponding strain increment Δε under this stress increment Δσ is calculated by Equation (27). Then the stress is gradually increased until the microcrack begins to propagate, and let this moment be t n , at this time, the stress and strain are σ n and ε n . This stage corresponds to the linear elastic stage before the microcracks in the rock do not propagate.

[0108] ② Let the strain increment at time t n+1 be Δε n+1 , and the number of activated microcracks N n is calculated by Equation (26) (at this time, take D n = 0) and the microcrack propagation length l n = 0, and then substitute it into Equation (21) to calculate the corresponding damage increment ΔD n+1 , and calculate the corresponding stress increment Δσ n+1 by Equation (27).

[0109] ③ Substitute the damage increment ΔD n+1 at time t n+1 into Equation (26) to calculate the increment of the number of activated microcracks ΔN n+1 , and substitute it into Equation (21) to obtain the increment of the microcrack propagation length Δl n+1 corresponding to the damage increment ΔD n+1 .

[0110] By performing cyclic iterative solutions for ② and ③, the stress - strain curve of the rock in the damage stage can be obtained, and then combined with the stress - strain curve in the elastic stage obtained from ①, the complete compressive stress - strain curve of the rock can be obtained.

[0111] Next, the experimental results of Zhou et al. are used to verify this model. The rock used in the experiment is sandstone taken from the construction site of Xiangjiaba Hydropower Station in Sichuan Province. The rock parameters are: elastic modulus E = 34 GPa, Poisson's ratio υ = 0.3, fracture toughness K IC = 1.1 MPa·m 1 / 2 , density 2600 kg / m 3 . The specimen is a standard cylindrical shape with a height of 100 mm and a diameter of 50 mm. The uniaxial compression test curve is as Figure 2 . Using this experimental result to verify the uniaxial compression damage model of rock proposed in this paper, the relevant parameters are taken as: k = 5e22 m -3, m = 7.1, microcrack size 2a = 2e-4 m, friction coefficient f = 0.26, microcrack inclination angle θ = 45°. The theoretical calculation curve is as shown in Figure 2 , and it can be seen that:

[0112] (1) The theoretical curve is in good agreement with the test results, especially in the linear elastic stage. There is a certain error near the peak stage. The theoretical and test peak strengths are 131.03 MPa and 137.68 MPa respectively, and the latter is about 95.17% of the former, that is, the error is very small. After the peak strength, the shapes of the two curves are basically the same.

[0113] (2) According to the constitutive model proposed in this paper, the elastic limit strength of the rock is 79.3 MPa, that is, when the compressive strength is greater than this value, damage begins to occur in the specimen; its peak compressive strength is 131.03 MPa, that is, the former is about 60.5% of the latter. Many scholars believe through research that the elastic limit strength of the specimen is about 30% - 70% of its peak strength, which is consistent with the previous research results.

[0114] Finally, the influence of microcrack length, friction coefficient and rock fracture toughness on the mechanical properties of rocks was studied by parametric sensitivity analysis.

[0115] The calculated parameters of the rock are taken as: density 2500 kg / m 3 , elastic modulus E = 40 GPa, Poisson's ratio v = 0.2, and the calculation model is as shown in Figure 1 (b). The shape and size of the specimen are the same as before, and the calculation is carried out according to the plane stress problem. The microcrack parameters are taken as: microcrack inclination angle θ = 45°, microcrack size 2a = 60 μm, friction coefficient f between microcracks = 0.7, Weibull function characteristic parameter k of microcracks = 4e23 / m 3 , m = 6.5, fracture toughness K IC = 0.4 MPa·m 1 / 2 . Then the axial stress, damage and axial strain curves of the rock under uniaxial compression are as shown in Figure 3 . It can be seen that when the external force is small, the microcracks will not expand. At this time, the stress-strain curve is linear and the damage of the specimen is 0, that is, no new damage occurs. When the external force exceeds the elastic limit of the rock, the microcracks begin to expand and new damage begins to occur in the specimen. However, at this time, the microcracks belong to stable expansion, that is, if the microcracks are to continue to expand, an external force still needs to be applied and the stress-strain curve still shows an upward trend. When the specimen reaches the peak strength, the corresponding damage is about 0.2. When the stress peak is reached, the stress of the specimen begins to decrease, the damage increases rapidly with the strain, and the microcracks will converge to form macroscopic cracks, eventually leading to the failure of the rock, which is consistent with the corresponding test results.

[0116] Taking this model as an example, the effects of microcrack length, friction coefficient, rock fracture toughness, etc. on the uniaxial compressive mechanical properties of rocks are studied.

[0117] (1) Effect of microcrack length

[0118] Take the microcrack lengths 2a as 60μm, 80μm, 100μm, and 120μm respectively. The elastic modulus of the rock E = 34GPa, Poisson's ratio υ = 0.3, fracture toughness K IC = 1.1MPa·m 1 / 2 , and density 2600kg / m 3 . The specimen is a standard cylindrical shape with a height of 100mm and a diameter of 50mm. The material failure characteristic parameters are: k = 5e22m -3 , m = 7.1, friction coefficient f = 0.26, and microcrack inclination angle θ = 45°. The curve of the uniaxial compressive stress-strain of the rock varying with the microcrack length is as shown in Figure 4 . Generally speaking, as the microcrack length increases, the uniaxial compressive peak strength of the specimen approximately linearly decreases, and at the same time, the peak strain of the specimen also gradually decreases. This is because under the same external load, as the microcrack length increases, the stress intensity factor at the crack tip gradually increases, and under the condition of constant fracture toughness, the external force required for microcrack propagation should decrease. Therefore, the peak strength and damage variable of the specimen show a decreasing and increasing trend respectively as the microcrack length increases.

[0119] (2) Effect of microcrack friction coefficient

[0120] Take the microcrack friction coefficients f as 0.5, 0.6, 0.7, and 0.8 respectively. The elastic modulus of the rock E = 34GPa, Poisson's ratio υ = 0.3, fracture toughness K IC = 1.1MPa·m 1 / 2 , and density 2600kg / m 3 . The specimen is a standard cylindrical shape with a height of 100mm and a diameter of 50mm. The material failure characteristic parameters are: k = 5e22m -3 , m = 7.1, microcrack size 2a = 2e-4m, and microcrack inclination angle θ = 45°. Then the curve of the uniaxial compressive stress-strain of the rock varying with the microcrack friction coefficient is as shown in Figure 5 . Generally speaking, as the microcrack friction coefficient increases, the uniaxial compressive peak strength of the specimen increases, but the increase amplitude is different. When the microcrack friction coefficient is 0.5 - 0.7, the peak strength of the specimen increases less, approximately linearly; while when the microcrack friction coefficient is 0.7 - 0.8, the peak strength of the specimen increases rapidly. This is because as the microcrack friction coefficient increases, the ability of the microcrack to resist slip increases, and then the ability of the specimen to resist failure also increases accordingly, and finally leads to the improvement of the peak compressive strength of the specimen. At the same time, from Figure 5(c) It can also be seen that as the friction coefficient of microcracks increases, the strain corresponding to the onset of damage also increases significantly, indicating that as the friction coefficient of microcracks increases, the ability of the specimen to resist failure increases non-linearly.

[0121] (3) Influence of rock fracture toughness

[0122] Take the micro-rock fracture toughness K IC to be 0.3 MPa·m 1 / 2 , 0.4 MPa·m 1 / 2 , 0.5 MPa·m 1 / 2 and 0.6 MPa·m 1 / 2 , the elastic modulus of the rock E = 34 GPa, Poisson's ratio υ = 0.3, and density 2600 kg / m 3 . The specimen is a standard cylindrical shape with a height of 100 mm and a diameter of 50 mm. The material failure characteristic parameters are: k = 5e22 m -3 , m = 7.1, friction coefficient f = 0.26, microcrack size 2a = 2e-4 m, and microcrack inclination angle θ = 45°. Then the curve of the uniaxial compressive stress-strain of the rock varying with the rock fracture toughness is as Figure 6 . Generally speaking, as the rock fracture toughness increases, the uniaxial compressive peak strength of the specimen increases, but the increase amplitude is different. When the rock fracture toughness is 0.3 - 0.5, the peak strength of the specimen increases less and approximately linearly; while when the microcrack friction coefficient is 0.6, the peak strength of the specimen increases rapidly. This is because as the rock fracture toughness increases, the ability of microcracks to resist crack initiation increases, and then the compressive strength of the specimen also increases accordingly, and finally leads to the increase of the peak compressive strength of the specimen. At the same time, from Figure 6 (c) It can also be seen that as the rock fracture toughness increases, the strain corresponding to the onset of damage also increases significantly, indicating that as the rock fracture toughness increases, the ability of the specimen to resist failure increases non-linearly.

[0123] The results of the parameter sensitivity analysis show that as the microcrack length increases and the friction coefficient decreases, both the uniaxial compressive strength and the peak strain of the rock decrease. When the microcrack length is 60 - 120 μm, the uniaxial compressive strength decreases approximately linearly; while when the microcrack friction coefficient gradually increases from 0.5 to 0.8 and the rock fracture toughness increases from 0.3 MPa·m 1 / 2 to 0.6 MPa·m 1 / 2 , the uniaxial compressive peak strength of the rock first increases slowly and then increases rapidly, indicating that both the microcrack friction coefficient and the rock fracture toughness have a greater impact on the peak strength of the specimen, and the degree of their influence shows a non-linear characteristic.

[0124] Although the above embodiments have been shown and described, it is to be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions, and variations made by those of ordinary skill in the art to the above embodiments are within the scope of protection of the present invention.

Claims

1. A uniaxial compression damage model of rock considering the mechanical behavior of microcracks, characterized in that, The uniaxial compression damage model of the rock is as follows: σ = E(1 - D)ε (27) where D is the rock damage variable, σ is the stress, ε is the strain, and E is the elastic modulus of the rock; The calculation formula for the rock damage variable is Wherein, N is the density of microcracks in the rock, l is the tensile wing crack, l * = 0.27a, l ** = 0.083a, 4bh represents the unit area, θ is the inclination angle of the microcrack, f is the friction coefficient, and a represents the half-length of the crack.

2. The uniaxial compression damage model of rock considering the mechanical behavior of microcracks according to claim 1, characterized in that, The stress intensity factor at the tip of the microcrack is where τ * is the effective shear stress, θ is the microcrack dip angle, l * = 0.27a, l ** = 0.083a, a represents the crack half-length, K I represents the mode I stress intensity factor, K II represents the mode II stress intensity factor.

3. The uniaxial compression damage model of rock considering the mechanical behavior of microcracks according to claim 1, characterized in that, The propagation length of the microcracks is calculated using the strain energy density criterion, that is, when the strain energy density S of the wing crack is greater than the minimum strain energy density S c , the microcracks begin to initiate and propagate, and the strain energy density S of the wing crack can be solved by the following formula: In the formula, K I represents the stress intensity factor of type I, K II represents the stress intensity factor of type II, θ is the microcrack dip angle, E is the elastic modulus of the rock, and v is the Poisson's ratio of the rock.

4. The uniaxial compression damage model of rock considering the mechanical behavior of microcracks according to claim 3, characterized in that, The minimum strain energy density S c is calculated by the formula where K Ιc is the static fracture toughness of the rock; S c is also called the fracture threshold, E is the elastic modulus of the rock, and v is the Poisson's ratio of the rock.

5. The uniaxial compression damage model of rock considering the mechanical behavior of microcracks according to claim 3, characterized in that The length of the microcrack at a certain moment is calculated by the following formula where l t is the wing crack length at a certain moment t, S is the strain energy density of the wing crack, and S c is the fracture threshold.

6. The uniaxial compression damage model of rock considering the mechanical behavior of microcracks according to claim 1, characterized in that The number of microcracks activated during rock damage is N = (1 - D n )N0 = (1 - D n )kε m (26) where N is the number of microcracks, D n is the rock damage at a certain time step n, N0 is the number of microcracks activated per unit volume under a given strain ε, and N0 = kε m , where k and m are parameters describing the failure characteristics of the material.

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