Damage evolution and constitutive model of limestone considering fissure angle and wet-dry cycles under axial stress

Fissured limestone in the hydro-fluctuation belt of the Three Gorges Reservoir is prone to degradation under wet-dry cycles, posing a significant risk in geologically hazardous regions. However, the damage evolution mechanisms and mechanical response behavior of fissured rock masses under combined wet-dry cycles and axial stress remain poorly understood, and effective non-contact early warning technologies are still lacking. To address this, we conducted microscopic analysis, uniaxial compression testing, and digital image correlation monitoring to investigate the deterioration characteristics of limestone with varying fissure angles and wet-dry cycles. Moreover, we proposed a statistical early-warning method grounded in the divergence rate and statistical characteristics of strain and displacement fields. A piecewise constitutive model incorporating macroscopic, wet-dry cycle and microscopic damage was established incorporating the compaction stage. Results show that wet-dry cycles significantly alter the pore structure and calcite content, and that limestone degradation arises from the combined effects of physical reaction, chemical reaction, and axial stress. Peak stress and elastic modulus increase with fissure angle but decrease with cycle count. The failure mode is jointly influenced by fissure angle and number of wet-dry cycles, with fissure angle playing a dominant role. The statistical indicators and divergence rates of the strain and displacement fields enable identification of three distinct stages of failure: stable, crack propagation, and failure. The model predictions align well with experimental stress–strain curves. This study provides new insight into the damage mechanisms of fissured limestone and offers theoretical support for precursor identification and geohazard mitigation in reservoir environments.