Study on three-dimensional mesoscopic evolution characteristics and disturbance factors of coal open fractures

The evolution of fractures is affected by internal and external environments such as the distribution characteristics of mineral particles and confining pressure conditions. In order to explore the macro-meso evolution characteristics and influencing factors of fractures in fractured coal under different confining pressures, based on the industrial CT scanning system and its equipped triaxial loading system, the triaxial static load test of fractured coal was carried out. The interaction mechanism of internal and external conditions of primary fractures, mineral particles and confining pressure is reasonably explained by multi-angle joint characterization. the results showed：① The confining pressure will shift the significant area of the initial damage of the coal body, so that it transits from the fracture tip to the upper and lower ends of the coal body with the increase of confining pressure, and it is easier to connect between micropores and large-sized fractures than between micropores and micropores, and new macroscopic cracks are generated. ② The increase of confining pressure will lead to the change of three-dimensional dynamic fractal dimension from slow increase, rapid increase and steady increase to the development stage of steady increase, rapid increase and slow increase, which can be used to characterize the time evolution law of fractures. ③ The fractured coal body exhibits longitudinal tensile failure under uniaxial or low confining pressure. The high confining pressure will change its failure mode and tend to shear failure, and enhance the strength of the coal body through two ways. ④ The deviation between the theoretical value and the experimental value of the cracking angle increases with the increase of confining pressure, which is consistent with the discrete numerical relationship caused by the distribution of mineral particles in coal. ⑤ According to the stress composition of the fracture and the distribution characteristics of mineral particles, the fracture propagation behavior is divided into three types: direct drive, bypassing mineral particles and staggering mineral particles. The above propagation behavior depends on the confining pressure to change the force component of the crack. The cracks under the action of relative pure tensile stress, tensile-shear composite stress and relative pure shear stress correspond to the above three propagation behaviors, that is, the influence form of crack propagation is mainly confining pressure, supplemented by the occurrence form of mineral particles.