Study on the evolutionary characteristics of strain energy on crack propagation in coal and rock under bending load

The failure of coal mine overburden is mainly caused by fractures under bending loads. The energy evolution characteristics of coal and rock fractures are closely related to coal mine disasters such as rock burst. To obtain the characteristics of energy release and accumulation of coal and rock under bending load, three‐point bending tests of coal, mudstone, and sandstone were carried out respectively. The strength characteristics and fracture propagation process of coal and rock under bending load were studied. The strain energy evolution rules of coal and rock were calculated and obtained. The fracture mechanism of coal and rock was discussed by analyzing the critical strain energy release rate. The results show that the fracture complexity of sandstone and mudstone is greater than that of coal. The microstructure and its directivity in coal and rock indirectly affect their fracture characteristics through the elastic modulus characteristics. The distribution of parameters such as peak load of fracture, fracture energy, and crack length of coal and mudstone samples is discrete, while that of sandstone samples is concentrated. The deformation energy density of coal and rock basically shows a linear increase trend at the prepeak stage. The deformation energy density evolution characteristics at the postpeak stage are mainly affected by the load drop. It is important to establish the internal relationship between the meso structural characteristics and macro mechanical properties for solving engineering problems.

load is also of great significance for early warning of coal mine dynamic disasters. 9,10n recent years, scholars have paid more attention to the fracture of coal and rock mass under bending load.The characteristic parameters of crack propagation were obtained through laboratory tests and calculation in combination with relevant theories of fracture mechanics, and then the instability mechanism of rock in the process of fracture was explored.Wei et al. 11 studied the bending fracture mechanical behavior and acoustic emission characteristics of sandstones with different grain sizes, and found that the peak load and fracture toughness gradually decreased and the number of acoustic emission events gradually increased with the increase of rock particle size.Zuo et al. 12,13 studied the influence of loading rate on acoustic emission (AE) characteristics of sandstone failure and micromorphology of the fracture surface, and found that the number of AE ringing decreased with the increase of loading rate.The proportion of meso transgranular fracture increases with the increase of loading rate.Yang et al. 14,15 obtained the full field displacement data of sandstone through the digital speckle correlation method, and determined the length of the rock fracture process area according to its deformation characteristics.Gong et al. 16 used the test method of Yang to determine the fracture process area, obtained high-precision fullfield deformation characteristics of the whole process of rock fracture, and determined the length of the fracture process area and the crack opening displacement when mudstone was damaged.
The difference of diagenetic lithology is an important factor affecting the fracture characteristics of coal and rock.At present, based on the analysis of the fracture characteristics of coal and rock, the influence of their physical properties on the mechanical properties of the fracture process is gradually being considered.Wang et al. 17,18 conducted three-point bending tests on different kinds of sandstones, and found that different mineral compositions and grain morphology of rocks would lead to certain differences in the fracture toughness of rocks.Kang et al. 19 explored the anisotropic characteristics of shale and mudstone fracture failure process under threepoint bending conditions, and obtained the change characteristics of shale and mudstone fracture toughness and tensile strength respectively.Through three-point bending test, Huang et al. 20 found that when the I-type fracture extends to the rock property alternation interface, the crack tip would be passivated, resulting in shear strain, and would change to the composite fracture shape.Tan et al. 21characterized the anisotropy in deformability and strength of rocks of different lithology through the concept of equivalent volume, and explored the mechanical properties of rocks under different lithology and stress states.
Type I fracture of coal and rock is accompanied by energy storage and dissipation.It is of great significance to analyze the mode I crack propagation and fracture mechanism of coal and rock from the perspective of energy.Vladyslav et al. 22 carried out bending load failure test on prefabricated fissured granite, and found that the changing characteristics of rock fracture energy and peak load increase with the increase of eccentricity.Li et al. 23 carried out the loading and unloading fracture test of red sandstone and found the linear energy storage and consumption characteristics in the process of rock tensile failure.Pinazzi et al. 24 conducted bending load fracture test on mudstone and sandstone, and found that the rock fracture energy increased with the increase of loading rate.The rate of energy rise of sandstone fracture is higher than that of mudstone.Mohamed et al. 25 carried out three-point bending tests on marble, limestone and sandstone, and studied the nonlinear evolutionary characteristics of fracture energy during rock deformation.
The instability and failure of coal mine roof are usually caused by the expansion and coalescence of the internal fissures of coal and rock, and then the macroscopic fracture.Therefore, it is more practical to study the deformation and fracture evolution process of coal and rock under bending load from the perspective of energy.Considering that the roof strata are mainly sandstone, mudstone, and coal, the three-point bending test of sandstone, mudstone, and coal is carried out in combination with the digital speckle correlation method.The paper analyzes the strain and displacement information in the fracture process area, studies the development characteristics of rock deformation localization in different media and the corresponding crack initiation and expansion state, discusses the displacement and energy evolution characteristics of coal and rock localization zone, and reveals the internal mechanism that causes the fracture difference in different coal and rock media.

| THREE POINT BENDING TEST OF DIFFERENT COAL AND ROCK MEDIA
As shown in Figure 1, the coal and rock samples used in the three-point bending test were taken from T3292 working face and its roof of 9# coal seam in Kailuan mining area, China.The lithology is coal, mudstone, and sandstone.The coal and rock stratum where the sample is located is the middle Carboniferous Tangshan Formation stratum, which is a marine continental sedimentary coal stratum.
The sample is a cuboid with a length of 240 mm × high of 30 mm × thick of 30 mm. 26 The prefabricated crack is 10 mm long and 2 mm wide, and the span of the lower fulcrum is 200 mm (Figure 2).
The samples are divided into three groups according to lithology, and there are three samples in each group.The number of coal samples is C1-C3, the number of mudstone samples is M1-M3, and the number of sandstone samples is S1-S3.The basic mechanical parameters of the samples are summarized in Table 1.
Digital speckle displacement monitoring system is used for three-point bending test of different coal rock media, as shown in Figure 3. Digital speckle displacement monitoring mainly adopts German DMK-GX2000 CCD industrial camera.The image is in bmp format with a resolution of 1840 × 1600 pixel, and the acquisition rate is designed as 10 frames/s.The white LED cold light source is used to supplement the light of the sample.Before the test, the observation surface of the sample shall be pretreated to ensure that it is flat and does not reflect light, and the speckle is randomly distributed.The observation range is 20 × 20 mm square area from the front of the sample.After calibration, the surface resolution of the sample is 0.16 mm/pixel.After the test, the stored speckle image is processed by MATLAB to obtain the crack propagation and deformation information during the whole fracture process of the sample.
The loading system used in the test is RLJW-2000 rock servo pressure testing machine. 27Before formal loading, preload the sample to ensure full contact between the indenter and the sample.The contact line of the upper indenter is symmetrical in the center of the sample.During the test, the displacement control is used for loading, the loading rate is 0.01 mm/min, and the loading speed is uniform until the sample is damaged.The variation of bending load on the sample with displacement is shown in Figure 4.With the increase of the vertical pressure, the bending load of the sample increases gradually, and the change rate of the loaddisplacement curve increases first and then decreases.
When the peak load is reached, the tensile stress in the middle of the sample under the bending moment reaches the limit value and breaks, resulting in a sudden drop in bending load.The fracture characteristics of different coal and rock media are quite different.The brittleness of coal and sandstone is relatively high, and the rock will lose stability rapidly after reaching the peak load.The brittleness of mudstone is relatively low, and the rock loses stability slowly after reaching the peak load.The ratio of rock strain before and after the peak load is approximately 1.

| Strength characteristics of coal and rock under bending load
According to the genetic classification of coal and rock, sandstone and mudstone in this paper belong to sedimentary rock, and coal belongs to metamorphic rock.Sandstone, mudstone, and coal have significant differences in mineral composition and cementation degree, resulting in their different strength characteristics under the same bending load conditions.The peak load of the sample at the moment of fracture is positively related to its limit stress.The fracture energy can directly reflect the ability of coal and rock to resist failure, and can represent the fracture strength of coal and rock.The fracture energy can be calculated from the load displacement curve according to the below formula. 28 where W is the fracture energy of coal and rock mass, P is the load acting on coal and rock, δ is the deformation of coal and rock.
The peak load, fracture energy, and other parameter data of coal and rock samples are summarized in Table 2, and the variation rule of each parameter is shown in Figure 5.According to Table 2 and Figure 5, the average peak load of sandstone is 2.64 kN, the average peak load of mudstone is 2.41 kN, and the average peak load of coal sample is 1.39 kN.The difference in peak load of sandstone, mudstone, and coal may be related to many factors such as the degree of internal cementation or metamorphism, the development of primary fractures, and the internal pore structure.After analyzing the test results, it was found that the peak load dispersion of sandstone, mudstone, and coal is quite different, and the dispersion of mudstone is greater than that of sandstone and coal.It is speculated that the sandstone and coal have fewer internal cracks and holes, and their homogeneity is high.The internal structures between samples have little difference, and the peak load is close.The mudstone has low homogeneity and is rich in microcracks, micropores, bedding, and clay particles, resulting in a large difference in peak load between mudstone samples.
The development of internal cracks in coal and rock can be characterized by compressional wave velocity.In general, the higher the integrity of coal and rock, the fewer the internal longitudinal cracks, and the greater the elastic modulus, the faster the compressional wave propagates in them.According to the statistics in Table 2, the average compressional wave velocity of sandstone is 2940 m/s that of mudstone (2590 m/s).It shows that the internal longitudinal cracks of sandstone are less than that of mudstone, and the compactness of sandstone is better than that of mudstone.The peak load of sample S2 with the fastest compressional wave velocity is not the maximum.It shows that the compressional wave velocity can reflect the development of internal cracks in coal and rock to a certain extent, but it is not positively related to the fracture strength.The fracture mechanism of coal and rock is complex, which is related to material properties, internal composition, and other factors.
Fracture energy refers to the deformation energy generated during the whole fracture process of coal and rock from initial fracture to complete failure.Analyzing the data in Table 2 and Figure 5, the fracture energy of sandstone is 290.44-329.14N mm, with an average of 307.04 N mm.The fracture energy of mudstone is 254.79-300.23 N mm, with an average value of 274.78 N mm.The fracture energy of coal is 191.22-222.68N mm, with an average value of 206.15 N mm.The fracture energy of coal and rock is related to its brittleness, cementation degree, primary crack development, and internal mesostructure.Sandstone is relatively brittle, with large elastic modulus and high internal uniformity.Therefore, the fracture energy required to reach the failure threshold is the largest, and the energy release rate is the fastest when the fracture is complete (S1-S3 curves in Figure 3).Due to the influence of internal cementation, the brittleness of mudstone and coal is reduced, the plasticity is enhanced, and the internal cracks are developed.Therefore, the fracture energy required for failure is reduced, and the instantaneous energy release rate of fracture is slowed down (M1-M3 and C1-C3 curves in Figure 3).In addition, the fracture energy of mudstone has a large dispersion, which is similar to the peak load distribution characteristics.The analysis results show that the internal homogeneity of mudstone is poor, leading to a large difference in the mesostructure between samples, resulting in a large dispersion of fracture energy of different mudstone samples.

| Crack propagation characteristics of fracture process under bending load
To study the crack propagation characteristics of coal and rock under bending load, the digital speckle correlation method was used to analyze the longitudinal crack propagation process of coal and rock surface.According to the images collected during the fracture process, the crack morphology, crack length, and lasting fracture time of coal and rock can be analyzed.Figure 6 shows the crack morphology of coal and rock under bending load.
The crack propagation process of sandstone is the simplest, with relatively straight fracture lines and few bending points.The complexity of the crack propagation process of mudstone is in the middle.The crack propagation process of coal is the most complex, with horizontal deviation in the crack initiation position, which is not at the midpoint of the sample.Sandstone has the latest crack initiation time and the fastest expansion speed after crack initiation.It rapidly extends to the top of the samples.The average duration of the fracture process is 1.1 s.The fracture initiation time of mudstone is earlier than that of sandstone, the crack propagation speed is lower than that of sandstone, and the crack extends gradually from the bottom to the top.The average duration of the fracture process is 2.5 s.The coal has the earliest crack initiation time.When the main crack extends to the vicinity of the primary crack, they quickly connect.The fracture process is not gradually extending from the bottom to the top, but the process of the main crack expanding and connecting with the primary crack.The average duration of the fracture process is 4.2 s.
The fracture morphology of coal and rock after failure contains a lot of useful information, which is the direct result of crack connection, and also the indirect reflection of the fracture process of rock particles.The fracture surface of coal is relatively rough, and the fracture surface is in step shape, with several primary fractures developing in the middle.The fracture surface of mudstone is in the form of undulating steps, the roughness of fracture surface is smaller than that of coal, and microcracks and micropores are distributed on the section.The roughness of sandstone section is less than that of coal and mudstone, and there are a few impurities distributed on the fracture surface, which have an important impact on the formation of fracture.According to the observation results of scanning electron microscope, the fracture surface of coal under bending load is rough, with a large number of sharp cluster particles left after fracture, scale like shallow pits caused by tensile stress and hill steps caused by tensile shear stress.The mudstone section is uneven, with large micropores distributed on the surface, and the intergranular fracture is much larger than the transgranular fracture and composite fracture.The roughness of sandstone meso fracture morphology is smaller than that of mudstone.After amplification, a large number of intergranular fractures can be observed, and their grain size is smaller than that of mudstone.Transgranular fractures and composite fractures are rarely found (Figure 7).
To study the crack propagation characteristics of coal and rock under bending load, the principal strain difference method 29 is used to determine the localization region.The difference of principal strain in the divided area is more localized than that in the surrounding area.The strain evolution of sandstone sample S1 is shown in Figure 8A-D.Under the 0.2P max load condition, the sample is in the compaction stage, and the color of the strain nephogram is basically consistent, indicating that the surface deformation of the sample is uniform (Figure 8A).Under the 0.6P max load condition, the sample is in the elastic stage, and the strain value has a slight increase compared with the 0.2P max load condition.The deformation localization phenomenon starts to appear in the center of the prefabricated crack, and there is no crack initiation on the sample surface before the stress level (Figure 8B).Continue to load to the | 2263 0.79P max load condition, the range of deformation concentration area and deformation of the sample are increased, and the localization band is basically stable forming (Figure 8C).When the load condition reaches 0.95P max , the maximum deformation of the sample reaches 0.12 mm, and the deformation is further concentrated.As the loading continues, the localization band extends to the upper end, and the sample finally fails as a whole (Figure 8D).
After analyzing the crack propagation in Figure 8, the rock deformation concentration area is consistent with the final failure area.The deformation concentration area gradually develops from the center of the prefabricated crack to the top and evolves into a localization zone, finally forming a macro crack connecting the prefabricated crack and connecting with the microcrack near the main crack.In addition, the time of sandstone crack initiation is late, once the crack initiation and propagation speed will be fast, and it will rapidly extend to the top to complete the whole fracture process.The crack initiation time of mudstone is earlier than that of sandstone, and the crack propagation speed is also fast, but slower than that of sandstone.The crack propagation process is relatively complete, extending and penetrating from bottom to top.Coal has the earliest crack initiation time.Due to the existence of primary cracks, cracks parallel to the fracture direction generally deform and expand in advance before crack initiation.When the main cracks are close to the primary cracks, they are quickly connected.The fracture of coal is not like the gradual extension and connection of sandstone from bottom to top, but the process of the main crack expanding and connecting with the primary crack.
The fracture opening deformation of sandstone is concentrated, with an average of 6.55 mm.The fracture opening deformation of coal and mudstone is relatively discrete, and the average is 1.29 and 4.16 mm respectively.The fracture opening deformation of sandstone is greater than that of coal and mudstone.The reason is that the sandstone has high brittleness.Once the crack is transmitted to the top of the sample, the sample is basically broken into two parts, resulting in the rapid increase of the crack width.The plasticity of coal and mudstone is relatively high.After the main crack is transmitted to the top of the sample, the fracture part of the sample may be locally bonded.The two sides of the main crack of coal and mudstone are still in the occluding state, and the crack width will not further increase, so the fracture opening deformation is smaller than that of sandstone.Because the mud content of each sample is different, the dispersion of mudstone fracture opening deformation is large.In summary, the fracture surfaces of coal, sandstone, and mudstone are mainly tensile fracture surfaces and intercrystalline fracture surfaces.This is closely related to the failure of the rock section caused by tensile stress under bending load.Primary microcracks in coal, clay minerals in mudstone, and crystals in sandstone have an important influence on the formation of fracture.The composition and structure of the rock itself are the key factors affecting fracture (Figure 9).

| ANALYSIS OF STRAIN ENERGY EVOLUTION CHARACTERISTICS
The study of energy evolution in the process of rock deformation and failure plays an important role in further exploring the mechanism of rock engineering disasters.Analyzing the energy evolution characteristics of the rock deformation localization zone during the whole loading process can deepen the understanding of the energy accumulation and release in the process of rock deformation and failure.

| Analysis method of deformation energy evolution
In this paper, a quantitative analysis method for deformation energy evolution is proposed.Based on the final failure mode of rock and the strain field before failure, the boundary line of deformation localization zone is determined according to the intersection line of uniform deformation field and nonuniform deformation field, and the calculated deformation field is divided into deformation localization zone and the area outside the deformation localization zone.The energy analysis area is the deformation localization zone I and Ⅱ shown in Figure 10.Average the strain components of all data points in the analysis area to represent the strain components of the rock in the calculation area.
Each strain component measured in the loading process is processed with the above method to calculate the value of any strain component in the whole loading process.According to the research conclusion in literature, 30,31 during the whole process of rock loading, the area outside the deformation localization zone basically maintains an elastic state.Therefore, the amount of deformation energy density outside the deformation localization band of rock can be obtained by the below formula,  ( ) where, E is the elastic modulus, μ is the Poisson's ratio, ε 1 and ε 2 are the principal strains of rock surface, and U is the deformation energy density.

| Analysis of deformation energy evolution
The coal and rock are subjected to bending load acting on the middle of the top.According to the symmetry of the rock and load, the deformation energy evolution characteristics of area I and area II are basically similar.This paper unifies the analysis of area I of coal and rock.Figures 11 and 12 show the deformation energy evolution curve of coal and rock in area I.The characteristics of deformation energy evolution can be analyzed from the prepeak stage, peak stage, and postpeak stage of bending load.At the prepeak stage, the energy change of coal, mudstone, and sandstone at the initial stage of loading is mainly embodied in energy accumulation, and the energy evolution curve is basically consistent.When the loading displacement is 0.31, 0.56, and 0.62 mm respectively, the deformation energy density evolution curve of coal, mudstone, and sandstone will bifurcate.The energy accumulation rate of coal is the fastest, and that of sandstone is the slowest.However, the energy accumulation rate of sandstone will exceed that of mudstone when the loading displacement is 0.16-0.29 mm.It is speculated that it is caused by the connection between the primary crack and the main crack in the mudstone.
At the peak stage, the energy of coal, sandstone, and mudstone accumulates to the maximum.In addition, coal, sandstone, and mudstone begin to release energy gradually at the peak load.The energy evolution curve of sandstone keeps a nonlinear evolution trend of increasing and decreasing fluctuations, indicating that the energy accumulated and released by sandstone is basically the same, and the effect of load reduction is not obvious.The energy evolution curves of mudstone and coal show a decreasing trend at the peak load, indicating that the energy released by mudstone and coal is greater than the energy accumulated, and the load reduction has an obvious impact on mudstone and coal.
In the postpeak stage, the deformation energy density is similar to the evolution characteristics of the deformation localization, mainly reflecting two characteristics.One is that the sudden decrease of rock bearing capacity corresponds to the change of energy release characteristics outside the deformation localization zone.The reduction of rock bearing capacity is related to the failure of the partial weakening area, and at the same time, it causes the energy adjustment outside the localization zone, not only the energy release, but also the energy accumulation caused by the energy transfer.Second, the change of rock energy after peak load is different, and different rock properties have different changing characteristics, which are mainly affected by the shape of deformation localization zone of rock.According to the scanning results of coal and rock under electron microscope, mudstone has large grains, weak cementation between grains, and a large number of clay minerals are mixed between grains, leading to the reduction of mudstone cohesion.In addition, there are many microcracks and micropores in mudstone, which reduces the fracture strength and fracture energy of mudstone.The difference of micro cracks and micropores in different mudstones is very large, which is an important reason for the strong dispersion of mudstones.
The internal mesostructure of sandstone is single, the distribution of micropores is less, and they are not connected with each other.The structural characteristics of sandstone are relatively uniform, and the dispersion of fracture strength and fracture energy between different sandstone samples is small.The fracture of coal and rock under bending load is mainly mode I fracture, and its mechanical model of crack propagation is shown in Figure 13.Based on the theory of elastic fracture mechanics, the critical strain energy release rate γ c of crack tip propagation can be calculated by the below formula.(3) where, T cr is the critical tension of rock crack propagation, a cr is the critical crack length of rock, E is the elastic modulus of rock, b is the crack width, and h is half of the calculated area width at the crack tip.At the tip of the mode I crack, when the partial derivative ∂γ/∂a of strain energy release rate γ with respect to crack length a is <0, γ decreases after crack propagation, and the crack will expand stably.When ∂γ/∂a > 0, γ increases after crack propagation, the crack will expand unstably.According to Equation (3), the fracture of coal and rock is mainly controlled by its elastic modulus E under the condition of the crack with same size and the same tension T. The smaller the elastic modulus E of rock, the greater the critical strain energy release rate G required for fracture.Therefore, the energy release rate of coal (1.9 GPa) with smaller elastic modulus is higher than that of sandstone (3.3 GPa) and mudstone (7.8 GPa) with larger elastic modulus during fracture (Figures 14-16).The elastic modulus is anisotropic.There are many microcracks and micropores in coal and mudstone.These weak structures have strong directionality.Assuming that the weak structure is consistent with the direction of bending load, it will significantly affect its fracture characteristics, making the dispersion of coal and mudstone samples significantly increased.However, there are only a few micropores in sandstone, and these micropores do not have obvious directivity, so the dispersion of sandstone samples is relatively weak.
Studying the bending load performance of different rocks is of great significance for guiding the support and stability evaluation of geotechnical engineering in the field.However, only laboratory test research has been conducted in this paper, and only three samples including coal, mudstone, and sandstone have been considered.In the future, three-point bending mechanical tests will be further conducted on other lithologic samples.The relationship between its mechanical properties and Crack propagation characteristics, fracture energy, and energy release rate is analyzed to provide theoretical reference for the research of fracture mechanism of rock mass in engineering site.

| CONCLUSION
1.The fracture complexity of sandstone and mudstone is greater than that of coal.The fracture duration of sandstone and mudstone is longer than that of coal, and the peak load of fracture of sandstone and mudstone is higher than that of coal.The microstructure and its directivity in coal and rock indirectly affect their fracture characteristics through the elastic modulus characteristics.2. The peak load of fracture, fracture energy, and crack length of coal and mudstone samples is discrete, while that of sandstone samples is concentrated.This is closely related to the homogeneity of coal and rock samples, there are many microcracks and micropores in coal, mudstone is rich in clay particles, and there are only a few micropores in sandstone.The homogeneity of sandstone is higher than that of coal and mudstone.3. The characteristics of energy release and accumulation during coal and rock fracture are related to the evolution of deformation localization.The deformation energy density of coal and rock basically shows a linear increase trend at the prepeak stage.The deformation energy density of coal and rock at the peak load stage is related to the displacement evolution characteristics of the deformation localization region.The deformation energy density evolution characteristics at the postpeak stage are mainly affected by the load drop.4. The theoretical calculation results show that the smaller the elastic modulus E of rock, the greater the critical strain energy release rate G required for fracture.In this experiment, the energy release rate of coal (1.9 GPa) with smaller elastic modulus is higher than that of sandstone (3.3 GPa) and mudstone (7.8 GPa) with larger elastic modulus during fracture.

F I G U R E 1
Coal and rock samples.(A) Coal.(B) Mudstone.(C) Sandstone.F I G U R E 2 Dimension diagram of three-point bending samples.

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Microscopic observation of fracture profiles in different coal and rock masses.(A) Sandstone.(B) Mudstone.(C) Coal.

F G U R E 8
Crack process of sandstone.(A) Crack tip formation.(B) Stable crack propagation.(C) Peak strength.(D) Unstable crack propagation.

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Evolution characteristics of shear strain in coal and rock masses.(A) Mudstone.(B) Sandstone.(C) Coal.I G U R E 10 Analytical method of displacement evolution of deformation localization band.(A) Coal.(B) Mudstone.(C) Sandstone.I G U R E 11 Characteristics of deformation evolution during the rock bending failure process.(A) Coal.(B) Mudstone.(C) Sandstone.(D) Deformation characteristic curve.

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I G U R E 12 Evolution curves of deformation energy density.F I G U R E 13 Mechanical model of crack I.

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I G U R E 14 AE waveform monitored when coal are under bending load.(A) C-1.(B) C-2.(C) C-3.

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I G U R E 15 AE waveform monitored when sandstone is under bending load.(A) S-1.(B) S-2.(C) S-3.I G U R E 16 waveform monitored when mudstone are under bending load.(A) M-1.(B) M-2.(C) M-3.ZHANG ET AL. | 2269 Mechanical parameters of sampled coal and rock.
T A B L E 1 F I G U R E 3 Test loading and monitoring systems.F I G U R E 4 Load-displacement curves of coal and rock samples.
Cracking load, compressional wave velocity, and fracture energy of coal and rock samples.
T A B L E 2 F I G U R E 5 Cracking load and fracture energy of coal and rock samples.