Stress relaxation behavior of sandstone considering thermal-damage after loading–unloading histories

The time-dependent behavior of rocks is crucial for the long-term stability of geotechnical structures, particularly concerning stress relaxation under coupled thermo-mechanical conditions. This study investigates the stress relaxation behavior of red sandstone subjected to high-temperature treatments (25, 250, 500, 750, and 1000 °C). Triaxial relaxation tests under cyclic loading reveal that elevated temperatures significantly enhance stress relaxation, characterized by a two-stage relaxation process with an initial rapid decrease followed by a gradual decrease in stress. The results indicate that higher temperatures and initial stresses accelerate the relaxation process, while plastic strain accumulates with deviatoric stress and loading cycles, particularly at elevated temperatures due to increased microstructural damage. Mineral decomposition and microcrack propagation at high temperatures account for the observed stress relaxation and plastic deformation. This research provides valuable insights into the stability assessment of geotechnical structures under thermal and mechanical loading conditions, highlighting the implications for geothermal energy extraction and wellbore stability. These findings contribute to a better understanding of rock behavior in complex thermal environments, ultimately informing more effective engineering practices.