Research on damage evolution mechanism of sandstones with different lithologies under freeze-thaw cyclic loads

Rock is the most common engineering material in nature, among which sandstone has the widest distribution. It is characterized by its loose structure, strong water absorption, and decreased strength when wet. Especially in cold regions, significant seasonal and diurnal temperature fluctuations can lead to the formation of unique frozen rock bodies. As the seasons and day-night cycles change, these can transition into thawed states, generating immense local tensile and compressive stresses which result in irreversible damage to rocks with low cementing strength, making them highly susceptible to stability disasters. Therefore, studying the damage degradation mechanisms of rocks under freeze-thaw cyclic loads is crucial for the safety evaluation and disaster prediction of engineering projects in cold regions. However, rocks of different lithologies in various engineering contexts exhibit significant differences, leading to distinct damage evolution mechanisms under freeze-thaw cycles and loads, resulting in varying failure effects. In light of this fact, to explore the damage evolution mechanisms of sandstones with different lithologies, this study comprehensively employs theoretical analysis and experimental methods to conduct freeze-thaw and load tests on Green Sandstone (QSY) and Yellow Sandstone (HSY) within a temperature range of -30 to 30°C for 0, 10, 20, 30, and 40 cycles. The following results were obtained: The variation patterns of peak stress and elastic modulus in QSY and HSY under increasing pore numbers and pore sizes during freeze-thaw cycles have been identified. Differences in rock damage resulting from variations in mineral composition between the two rock types have been clarified. It has been confirmed that QSY exhibits more pronounced degradation under identical conditions, with a macroscopic fracture surface forming after 40 freeze-thaw cycles. Five evolutionary stages of QSY and HSY under cyclic freeze-thaw and loading conditions have been established. The analysis revealed that the trends in event rate and cumulative event count align with the characteristics observed in the stress-strain curve. Notably, the emergence of a high event rate without a corresponding low event rate was identified as a precursor to specimen failure, with 30 freeze-thaw cycles marking the transition point where QSY shifts from brittle to plastic failure behavior. In contrast, HSY consistently exhibits brittle failure throughout the process. A damage model under combined freeze-thaw and loading conditions has been developed, elucidating the evolution characteristics across three stages-quiescent, accelerated, and stable. The correlation between the number of freeze-thaw cycles and the initial damage value has been determined, confirming that QSY is more susceptible to freeze-thaw effects under comparable conditions. Furthermore, the accelerated damage stage has been identified as the critical phase leading to rock failure and instability.