Study on the mechanical response characteristics of sandstone under elasticity plasticity and full path unloading after peak stress

When underground tunnels in coal mines traverse geological structurally abnormal zones (faults, collapse columns, fractured zones, etc.), excavation-induced unloading leads to instability and failure of the engineering rock mass. Rock masses in fractured zones are in elastic, plastic, and post-peak stress states, and the process of excavation through these zones essentially involves unloading under full stress paths. To explore the mechanical response of sandstone under different stress levels, based on the investigation of possible stress paths, a systematic study is conducted on various unloading paths, including elastic-axial compression with unloading of confining pressure, elastic-constant principal stress with unloading of confining pressure, plastic-axial compression with unloading of confining pressure, plastic-constant principal stress with unloading of confining pressure, plastic-constant axial D₁ displacement with unloading of confining pressure, plastic-equal proportional unloading of axial and confining pressures, and post-peak-synchronous unloading of axial and confining pressures. Characteristics of full stress-strain curves under seven unloading paths are obtained. The deformation patterns caused by unloading are analyzed, and the relationship between unloading paths and strain increments is investigated. Results show that, as the degree of unloading increases, the unloading deformation modulus ( E ) in the elastic stress state exhibits a trend of initial increase, then stabilization, followed by decrease, while in the plastic stress state, E gradually decreases. Both elastic and plastic states show an increasing trend in Poisson's ratio (µ). The normalized plastic shear strain γ^p/γ^p_max and dilation angle (ψ) conform to a single exponential function, and there is a negative correlation between initial confining pressure and dilation angle. These findings support the enrichment and development of unloading rock mechanics.