Study on Fatigue Damage Characteristics of Sandstone with Different Inclination Angles Under Freeze–Thaw Cycle Conditions

Fractured rock masses in cold regions are subject to long-term seasonal freeze–thaw cycles. To investigate the fatigue damage characteristics of sandstone with different fracture inclinations under freeze–thaw cycling conditions, samples containing fractures of varying inclinations were prepared using sandstone from Altay, Xinjiang. After vacuum saturation and freeze–thaw cycling treatment (−30 °C to 30 °C), uniaxial cyclic loading tests were conducted to analyze strain, elastic modulus, Poisson’s ratio, and damage variables. The results showed that under cyclic loading, the strain of the sandstone exhibited a “stepwise accumulation” characteristic, with peak and residual strain increasing with the progression of the cycle. Among them, the specimen with a fracture angle of 45° exhibited the fastest strain increase before failure. The peak elastic modulus showed a “continuous decrease within each stage and an initial increase followed by a decrease between stages,” while the residual elastic modulus continued to decrease, with both experiencing a sudden, sharp drop at the end of the cycle. The peak Poisson’s ratio decreases with the number of cycles in the early stage, then transitions to logarithmic growth in the later stage, rapidly increases near failure, and finally, the residual Poisson’s ratio in the final cycle exceeds the peak Poisson’s ratio; the evolution of damage variables exhibits an S-shaped three-stage characteristic, with the initial stage showing an irreversible deformation growth rate exceeding 10% due to compaction. In the middle stage, it grows steadily due to microcrack propagation, and in the final stage, it approaches 1. Samples with steep inclination angles exhibit earlier damage initiation and faster growth rates. The study reveals that crack inclination angle influences the evolution rhythm by regulating the proportion of compaction and shear damage, providing a theoretical basis for assessing the engineering stability of fractured rocks in cold regions.