Mechanical behavior, energy evolution and damage mechanism of anisotropic shale under triaxial multilevel cyclic loading: an experimental investigation

This paper aims to investigate the fatigue response of layered shale under triaxial cyclic loading. In our experiments, multilevel constant-amplitude cyclic loading was conducted under 5 MPa confining pressure. The anisotropic mechanical behaviors and energy evolution of shale specimens were analyzed for different bedding plane angles, in addition, the damage mechanism was discussion by observing microstructure of the fracture surface. Results indicated that the fatigue parameters, energy dissipation and damping characteristics were influenced by interbed structure. Increased bedding angle from 0° to 90°, strength, lifetime, axial strain and deformation modulus first decreased and subsequently increased, and the 60° interbedding orientation was the turning point for above parameters to change. As the stress level increased, the strain, elastic modulus, energy and damping ratio of the specimen showed step-like rise. The sudden jump in axial stress caused much more damage than those from fatigue loading. Macroscopic failure mode analysis revealed, with increasing bedding angle, the specimen showed a changeover from local shear failure to overall shear failure to severe splitting failure. Microscopic cracks at the fracture surface of specimen after cyclic loading was dominated by intergranular fracture, and the severity of macroscopic failure was positively dependent on the proportion of intragranular fracture and transgranular fracture.