Experimental and model investigation on fatigue properties of sandstone after fully coupled thermo-hydro-mechanical cycles

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Cold regions often feature complex geological environments where various physical phenomena interact, with a particularly notable thermo-hydro-mechanical (THM) coupling. In this study, fully coupled THM cyclic tests were conducted, followed by fatigue tests, to explore how THM treatment influences the fatigue properties of damaged sandstone. Experimental results indicate that rock fatigue deformation, damage evolution, and failure characteristics are highly sensitive to initial damage caused by coupled THM treatment. Rocks subjected to multiple THM cycles exhibit lower initial irreversible strain, shorter fatigue life, lower critical total dissipated energy, and higher irreversible strain increments, indicating accelerated deterioration. After THM coupling, rock fatigue failure shows complex crack networks, with macroscopic failure modes shifting from shear to tensile failure. Polarized microscopy and acoustic emission analyses reveal that this transition stems from micro-scale transgranular to intergranular fractures. We introduced a fractional order-based damage fatigue model to quantitatively describe the rock viscoelastic parameters after different initial damage treatments. Rock viscoelastic-plastic parameters decrease with increasing coupled THM cycles. Finally, we discussed the feasibility of applying these results to the long-term stability analysis of rock slopes. This study provides unique insights and modeling tools from fully coupled THM experiments to understand the rock fatigue characteristics, offering potential applications for slope stability assessment.