Fatigue behavior and damage evolution of granite under tensile and compressive loads

Fatigue damage induced by cyclic loading critically governs the long-term stability of underground rock engineering structures. While the mechanical behavior of rock under cyclic compression has been extensively investigated, the fundamental disparity between tensile and compressive fatigue mechanisms remains elusive, largely due to the lack of a unified experimental framework. In this study, a series of monotonic and constant-amplitude cyclic loading tests were conducted on granite under both uniaxial compression and direct tension. The results highlight fundamentally divergent damage trajectories, where cyclic compression is characterized by elastic shakedown and compaction hardening, with energy dissipation governed primarily by internal friction. Notably, postfatigue monotonic tests reveal a stress-dependent dual mechanism in compression: low-amplitude cycling enhances the residual strength of granite by up to 46.9% (from 97.9 to 143.8 MPa) via mechanically induced closure of preexisting microdefects, whereas high-amplitude cycling precipitates damage accumulation. Conversely, cyclic tension exhibits progressive stiffness degradation driven by direct bond breakage, with negligible hardening effects. To quantify these disparities, a novel damage variable was derived by integrating deformation kinematics and energy principles. Comparative validation confirms that, unlike stiffness, energy, or conventional strain-based indices, this newly derived framework robustly captures the three-stage damage evolution and provides a unified description for both loading modes. Ultimately, this study elucidates the mechanistic transition from friction-dominated compressive fatigue to cleavage-dominated tensile fracture, offering a rigorous quantitative basis for the stability assessment of rock masses under complex cyclic stress states.