Keywords: 
• mesostructure /
• cyclic disturbance /
• crack orientation /
• crack propagation /
• particle flow

Abstract: Micro cracks in natural rock mass have a significant impact on the stability of underground engineering structures. The mechanical properties of cracked rock mass determine the strength of rock mass and the mechanism of its compression failure. The instability and failure of surrounding rock are often induced by the expansion and penetration of these internal cracks. In practical engineering, rock mass excavation is a process of dynamic disturbance. The mechanical properties of rock under cyclic load are significantly different from those under static load. Its own characteristics and the development of microcracks are the main factors of rock fatigue failure. From the microscopic point of view, the particle discrete element method is used to carry out cyclic loading and unloading tests of pre-existing cracked granite. Firstly, the micro-component of granite is determined by MATLAB image processing technology, and the micro-mechanical parameters are calibrated based on the indoor uniaxial compression test results. By compiling particle flow code to track the type and expansion process of cracks, the stage characteristics of crack development in rock failure process are analyzed. The research results show that the orientation of new cracks in fractured rocks with different dip angles is basically the same as that of prefabricated cracks. According to the dominant tendency of new cracks, the relationship between the crack initiation angle and the inclination angle of prefabricated cracks is obtained: when the inclination angle β ≤ 45°, the crack initiation angle of shear and tension cracks decreases monotonically, while when the inclination angle β ≥ 60°, the crack initiation angle of shear and tension cracks increases monotonically; Cyclic disturbance load increases the axial deformation of fractured rock mass, and the axial cumulative residual strain curve presents an inverse S-shape, which enters the acceleration stage faster with the increase of the upper stress limit. The peak strength of the model specimen shows a trend of decreasing first and then increasing with the increase of fracture inclination. The peak strength is 63 %~89 % of the uniaxial compressive strength of intact rock in the laboratory, reflecting obvious rock deterioration phenomenon. Under cyclic load, the growth curves of shear crack and tension crack show obvious variation characteristics, which has certain reference significance for reasonably defining the deformation and failure stage of rock.