Fatigue damage behaviors of pre-flawed granite containing hole and fissures exposed to variable-frequency multi-level constant amplitude (VFMLCA) cyclic loads

Rock mass is often encountered with complicated stress disturbance caused by human-induced engineering loads and the environmental loads. Cyclic or fatigue loading path with fixed loading frequency are widely performed. To date, however, damage and fracture evolution of the pre-flawed rock under variable-frequency multi-level constant amplitude (VFMLCA) cyclic loads was not well understood. In the present study, granite samples containing a circular hole and two fissures were prepared to conduct VFMLCA cyclic loading tests. The fatigue strength, deformation, energy conversion, damage propagation, and crack coalescence were comprehensively investigated. Testing results show that the fatigue strength increases with increasing fissure angle. Rock volumetric strain and the dissipated energy are the maximum for rock with a 50° fissure angle. The dissipated energy increases with decreasing of loading frequency and an exponential relation is revealed between them. In addition, a damage evolution model that can well describe the two-stage and three-stage damage evolution for each cyclic loading level and the entire loading process is proposed and proved. Moreover, three failure modes of single tensile coalescence, double tensile coalescence, and double shear coalescence were revealed from the reconstructed CT images. It is suggested that the double shear coalescence is prone to forming large crack network.