Investigation of degradation characteristics of sandstone under cyclic loading based on crack initiation and damage stress thresholds

Abstract

While conventional cyclic loading schemes typically rely on arbitrary fractions of peak strength, this study establishes loading bounds based on the crack-initiation (σci) and damage (σcd) stress thresholds. This physically based approach restricts the applied stress to the critical interval for stable micro-crack propagation, ensuring that each cycle effectively induces substantial internal deterioration. Consequently, it establishes a rigorous mechanistic link between the cyclic loading history and macroscopic mechanical degradation. Therefore, the crack initiation and damage stress thresholds were identified by analyzing the volumetric strain curves. Guided by these thresholds, the cyclic loading range was established from 30 MPa to 60 MPa. Both cumulative strain and cyclic strain increments exhibited a typical three-stage evolution: initial compaction, steady-state damage, and accelerated instability. Cyclic loading induced a nonlinear reduction in rock strength, with the uniaxial compressive strength decreasing by 27.8% after 21 cycles. The evolution of the secant modulus reflects a transition from initial compaction hardening to softening, driven by cumulative plastic damage. The damage variable D exhibited characteristic stages of rapid initial accumulation, steady growth, and a terminal surge. This evolution characterizes the degradation of surrounding rock under mining disturbances, providing a theoretical foundation for stability assessments under complex stress states.