Experimental study on the failure process of sandstone subjected to cyclic loading and unloading after high temperature treatment

To understand the influence of temperature on sandstone failure process, the cyclic loading and unloading tests under the action of seepage and stress were carried out on sandstone after different temperature (25–800 °C) treatments. Based on the experimental results, the stress-strain curve, Young's modulus and crack volume strain of rock were analysed during cyclic loading and unloading, and the energy calculation equation was established considering the confining pressure and seepage pressure. The main research results are as follows: (1) with the increase in temperature, the stress-strain curve of the rock sample after heat treatment shows obvious signs of hysteresis loop, showing the characteristic of a three-stage evolution, that is, “sparse→dense→sparse”. (2) the Young's modulus in the unloading process is clearly larger than that in the loading stage, but both increase first and then decrease with the increase in temperature, with 400 °C as the critical temperature. (3) temperature can affect the deformation characteristics of rock, which results in an evident correlation between the crack volume strain and the temperature during each cycle of loading and unloading. (4) based on the established energy calculation equation, the energy evolution characteristics in the process of rock deformation and failure are obtained. Compared with the sample treated at lower temperatures than 400 °C, the cumulative dissipation energy of the sample increases after higher temperature (above 400 °C) treatment. The results are expected to enhance the understanding of mechanical properties and failure process of sandstone after temperature treatment, and provide guidance in engineering stability analysis under multi-physical field coupling.