Elastoplastic coupling mechanism and evolution model of bedded sandstone under freeze–thaw cycles

This study examines the coupled influence of freeze–thaw (F–T) cycles and bedding anisotropy on the elastoplastic response of bedded sandstone. Cyclic triaxial tests and theoretical analysis show that the apparent elastic modulus evolves in two distinct stages with increasing plastic strain: it rises during compaction due to pore closure and declines after yielding as a result of crack propagation. In contrast, the apparent Poisson's ratio increases monotonically. F–T cycling not only degrades stiffness but also acts as an “anisotropy amplifier,” with the most significantly reduction occurring at intermediate bedding orientations ( β ≈ 45°). Based on the experimental data, quantitative relationships between the apparent elastic parameters and plastic strain, bedding dip angle, and number of F–T cycles are established. These are further used to derive evolution equations for the five independent elastic constants of a transversely isotropic model, forming a constitutive framework that captures damage-induced anisotropic coupling. The model provides key equations for high-fidelity thermomechanical numerical analysis and supports damage-state-based early warning and differentiated design for infrastructure in cold regions.