Crack propagation mechanisms in single-fractured red sandstone under freeze–thaw cycles considering water content and fracture inclination

To elucidate the coupled effects of water content and fracture inclination on crack propagation and failure evolution in single-fractured red sandstone under freeze–thaw conditions in cold regions, freeze–thaw cycling–uniaxial compression tests were conducted on specimens with varying water contents and fracture inclinations. The analysis integrates full-field strain monitoring using particle image velocimetry (PIV), microstructural characterization via scanning electron microscopy (SEM), and the Sneddon elastic solution. The results demonstrate that increasing water content generally reduces the peak strength, elastic modulus, and residual load-bearing capacity of the rock, although the magnitude of degradation varies with fracture inclination, while promoting a transition in failure mode from brittle to ductile. Among all cases, specimens with a fracture inclination of 45° exhibit the highest water sensitivity, with an average reduction in peak strength of 38.2%. PIV results indicate that the deformation of fractured rock evolves from localized stress concentration to distributed damage propagation. SEM observations reveal that freeze–thaw cycling coupled with high water content promotes pore expansion, cementation degradation, and microcrack connectivity. Theoretical analysis further indicates that fracture inclination governs the direction of crack propagation, whereas water content modifies the internal stress field within fractures through ice-induced expansion pressure. When the water content exceeds approximately 50%, the crack propagation angle tends to stabilize. This study establishes a cross-scale correlation of damage evolution in single-fractured rock under freeze–thaw action, providing a theoretical basis for rock mass stability evaluation and engineering disaster prevention design in cold regions.