Damage characteristics of sandstone subjected to freeze–thaw cycles under different stress paths

The damage characteristics and failure modes of rock specimens subjected to different freeze–thaw (FT) cycles are vital to rock engineering in cold regions. Hence, in this study, the physical and mechanical parameters of sandstone specimens under different FT cycles were measured, and their deterioration laws were analyzed. The damage features of the rocks in different FT states under loading along various stress paths, including the main-frequency variation and crack propagation path evolution, were statistically analyzed. Two damage variables, one related to the acoustic emission (AE) and the other related to the rock strength, were combined to reveal the AE characteristics and damage evolution laws in the different damage states. The results showed that FT cycling led to a decrease in the peak stress, an increase in the number of microcracks, an increase in the AE activity, a shift in the main-frequency distribution toward the low-frequency range, and a shift in the damage mode from shear damage to a mixed damage dominated by tensile damage. The degree of strength deterioration in the rock specimens was affected by FT cycling under the different stress paths in the following influence order: conventional uniaxial loading > equal-amplitude cyclic loading> multistage cyclic loading; cyclic loading was more likely to induce tensile rupture than conventional uniaxial loading. The AE characteristic parameters were consistent with the stress–strain curve variations, which could reflect the damage evolution process of the rock specimens. The main frequencies showed a band-like evolution trend with the loading process and could be divided into three bands: a low-frequency interval (0–50 kHz), a medium-frequency interval (50–150 kHz), and a high-frequency interval (150–300 kHz); the main frequencies were concentrated in the low-frequency interval. With the increase in the number of FT cycles, the main frequency in the high-frequency interval and the number of corresponding microcracks gradually decreased.