Formation and fractal characteristics of main fracture surface of red sandstone under restrictive shear creep

Taking red sandstone specimen as the research object, this paper makes a rather systematic research on the formation and morphology of the main control fracture surface of the red sandstone under restrictive shear creep based on the basis of fractal theory by using three major self-designed research devices, that are, shear creep experimental device for soft coal or rock material, three-dimensional automatic measure and control experimental device for microstructure evolution based on micro-analysis, and three-dimensional mesoscopic monitoring software for dynamic evolution of micro-structure on the surface of coal-rock. The results show that restrictive shear creep of red sandstone experiences three stages, from the initial, steady to accelerated creep stage. The mechanism of creep action is the crystal slip under shear creep load and the slip due to dislocation. The formation of the main crack on the surface of the red sandstone under restrictive shear creep consists of four kinds of crack propagation modes: merging propagation, tensile propagation, intergranular propagation, bypassing crystal propagation. The changes of the profile along the shear direction are great in the distribution of the fracture surface, which shows a double crest trend in the second half, but the profile along the vertical shear direction declines relatively gently, slightly lower in the middle; in addition, the complexity of the roughness in the main fracture surface of the red sandstone specimen along the shear direction is greater than that in the vertical shear direction. A new method is proposed to calculate the fractal dimension of the fracture surface, and based on this method, it is found that the fractal characteristics of the main fracture surface of the red sandstone specimen show that the fractal dimension along the shear direction increases first and then decrease, and appears maximum value in the middle. And the changes of fractal dimension of the vertical shear profile are smaller than that in the shear direction, so does the fractal dimension.