Experimental and numerical investigation on the failure mechanical behavior and crack evolution mechanism of hollow sandstone under triaxial cyclic loading

The damage evolution mechanism of hollow sandstone under triaxial cyclic loading is crucial for evaluating the safety and stability of deep wells and deep tunnels. The strength, deformation, and damage evolution characteristics of hollow sandstone specimens (inner diameter of 15 mm and internal pressure of 0 MPa) were investigated by triaxial cyclic loading experiments and X-ray micro-CT scanning. Based on the experimental observations, a nonlinear parallel-bonded stress corrosion (N-PSC) model was developed in the particle flow code (PFC) to capture the damage behavior of hollow sandstone under cyclic loading. The fundamental principle of the N-PSC model is to reproduce cyclic damage by nonlinearly modifying the bond diameter. The results show that the peak strength of hollow sandstone under triaxial cyclic loading is lower than that obtained under conventional triaxial loading. The specimens exhibit three distinct stages of nonlinear damage development: initial damage, stable damage, and accelerated damage stages. Numerical specimens based on the N-PSC model can better reflect the macroscopic mechanical properties of hollow sandstone, including stress–strain curves, strength and deformation characteristics. The experimental energy analysis and numerical crack evolution jointly reveal the damage accumulation mechanism of hollow sandstone under cyclic loading. Microcracks initiate once the cyclic stress exceeds the crack damage threshold and rapidly propagate and coalesce in the post-peak stage to form a macroscopic fracture zone. Under triaxial cyclic loading, microcracks in hollow sandstone initiate from the inner hole wall and gradually extend towards the external wall. The specimens exhibit overall shear failure accompanied by hole spalling. At high confining pressure, stress concentrations at the specimen ends lead to severe end damage and localized failure. This research provides a theoretical explanation for the damage process and crack evolution mechanism of hollow rocks under cyclic loading.