A micromechanical fatigue damage model for quasi-brittle rocks subjected to triaxial compressive cyclic loads

This paper is devoted to a novel micromechanics-based damage constitutive model for describing the mechanical behavior of quasi-brittle rocks under cyclic loading considering the effects of confining pressure. The proposed model is successfully employed to simulate cyclic loading tests under various confining pressures and upper limit stresses. Inspired by previous studies that focus on time-dependent creep deformation of quasi-brittle materials, the number of cyclic loading is regarded as equivalent to time scale. In the same spirit, the scalar-valued internal damage variable used to describe the degradation of rock materials is decomposed into two parts, namely an instantaneous component due to monotonic loading, and a fatigue-related component attributed to cyclic loading. Within the framework of irreversible thermodynamics, we present a Coulomb-type friction criterion and a strain energy release rate-based damage criterion. To describe the evolution of fatigue damage, we also propose a damage criterion based on the microstructure evolution. By the use of returning mapping procedure, the constructed model is applied to simulate two representative quasi-brittle rocks, namely sandstone and argillaceous quartz siltstone, and successfully captures all significant behavior features.