Effects of High Temperature on the Mechanical Behaviors of Sandstone under True-Triaxial Unloading Conditions

To investigate the influence of high-temperature treatment on the mechanical behaviors of sandstone under unloading conditions, a series of true-triaxial unloading tests with a stress path of loading σ 1 and unloading σ 3 were performed on thermally treated sandstone specimens. Before testing, scanning electron microscopy (SEM) observations and physical property analysis demonstrated that high-temperature treatment effectively weakens the density and P-wave velocity of the specimen, and the development of microcracks is increasingly evident as the temperature exceeds 350 °C. The results of unloading tests reveal that as the temperature increases, the peak strength first increases and then dramatically decreases, with a threshold temperature of 800 °C, while the peak strain exhibits a continuously increasing trend. The changes in stress σ 3 and strain ε 3 at the unloading stage substantially increase negatively with temperature. In addition, as the temperature increases, the conversion rates of U 1 , U e and U p first increase and then decrease. The damage variable defined by strain energies at the peak stress point increases with the temperature. The dominant failure mode changes from shear fracture to tensile fracture as the temperature exceeds 650 °C. These results can provide some guidance for the design and construction of rock structures subjected to high temperature.