Abstract: The strength of rock material is composed of cohesive strength and friction strength, with residual friction strength being dependent on the confining pressure level. Based on experimental data from rock materials subjected to varying confining pressures and freeze-thaw cycles, we determined the statistical damage model parameters associated with confining pressures. The microelement strength of rock, measured directly by axial strain during the post-peak strain softening stage was assessed, revealing that the final value of the evolution of statistical damage variables in the residual damage stage is less than 1. Utilizing the Lemaitre strain equivalence assumption, we proposed a strain softening constitutive model to characterize the stress drop in rock materials. This model converts the peak stress of rock into nominal stress using residual damage variables. By comparing the model with experimental data, we verified its validity under various confining pressures and low freeze-thaw cycles. Using the model parameters and peak strain test data from rock materials under varying confining pressures and freeze-thaw cycles, we obtained the strain lag factor that characterizes the residual strain lagging behind the peak strain. We also determined the stress drop rate of rock material, expressed by the strain lag factor during the strain softening stage. Additionally, we identified the range of increased brittleness and plasticity of rock materials during this stage and verified the relationship between Weibull shape parameters and fractal dimensions concerning the failure probability of rock microelements under triaxial stress states.

Key words: freeze-thaw rock, peak strain, strain softening, stress drop, stress drop rate, residual damage