Macro and micro mechanical behaviors and micro damage theory of rock at low temperature freeze-thaw cycles

Aiming at challenges posed by rock freeze-thaw (FT) in cold regions rock mass engineering, it is of great significance to analyze its macro- and micro- mechanical properties and damage laws for the smooth progress of construction. In this study, indoor freeze-thaw cycle (FTC) tests on sandstone were conducted to analyze the mass change rate, density change rate, longitudinal wave velocity change rate, microstructure change and mechanical properties of sandstone after FTC. A microscopic FT damage variable reflecting the FT damage was defined based on the changes of rock porosity before and after the FTC, enabling the derivation of the total damage variable under the coupled action of FTC and mechanical loading. A damage evolution equation and a microscopic damage constitutive model for rock under coupled FTC and confining pressure were established by using Lemaitre's strain equivalence principle, the theory of continuous damage mechanics, and the assumption that the failure of rock micro-units follows the SMP criterion. The rationality and accuracy of the model were verified using triaxial compression test data for FT-damaged rock. The results show that both macro- and micro- mechanical properties of sandstone are degraded under the action of FTC, resulting in significant damage. The developed microscopic damage constitutive model can reflect the stress-strain characteristics of the whole process of FT rock triaxial compression, with excellent agreement observed between experimental and theoretical curves. This validates the reliability of the model and the methodology for determining its parameters. Additionally, defining the microscopic FT damage variable based on rock porosity changes is demonstrated to be a feasible and highly accurate approach to reflect rock FT damage degree. This model expands the damage model for rock under the coupling effect of FTC and confining pressure, further illuminating the damage mechanism and failure law in such environments. The findings provide references for the construction of rock mass engineering in cold regions.