Particle flow simulation of the crack propagation characteristics of granite under cyclic load

The microcracks in natural rock masses considerably impact the stability of the underground engineering structures. The mechanical properties of the cracked rock masses contribute considerably to the strength of the rock masses and their compression failure mechanism. The instability and failure of the surrounding rocks are often induced by the propagation and penetration of these internal cracks. In practical engineering, rock mass excavation is a process involving dynamic disturbance. The mechanical properties of the rocks under cyclic load are considerably different from those of the rocks under static load. The characteristics and development of microcracks are the main factors influencing rock fatigue failure. From the microscopic viewpoint, the particle-based discrete element method is used to conduct the cyclic loading and unloading tests of the preexisting cracked granite. First, the microcompositions of granite are determined using image processing techniques, and the micromechanical parameters are calibrated based on the indoor uniaxial compression test results. The stage of crack development during rock failure is analyzed by compiling particle flow code to track the type and propagation process of cracks. Results indicate that the orientations of new cracks in fractured rocks with different dip angles are similar to those of the preexisting cracks. Further, the relation between the crack initiation angle and the inclination angle of the preexisting cracks is obtained according to the tendency of new cracks. The crack initiation angle of shear and tension cracks decreases and increases monotonically, respectively, when the inclination angle β ≤ 45° and β ≥ 60°. The cyclic disturbance load increases the axial deformation of the fractured rock mass, and the axial cumulative residual strain curve exhibits an inverse S-shape when entering the acceleration stage faster with the increasing upper stress limit. The peak strength of the model specimen shows a decreasing trend followed by an increasing trend with the increasing fracture inclination. The peak strengths of the laboratory-intact rock are 63% to 89%, indicating an obvious deterioration phenomenon in the rock materials. The growth of shear and tension cracks show different characteristics under cyclic load; the growth rate of tension cracks is considerably higher than that of shear cracks during the unstable crack development stage. The results presented in this study may be used as reference to investigate the deformation and failure mechanisms of rock materials.