Effects of Confining Pressure and Temperature on the Energy Evolution of Rocks Under Triaxial Cyclic Loading and Unloading Conditions

Abstract

The deformation and failure of rocks result from the dissipation and release of their internal energy. The energy evolution throughout the processes of deformation and failure in rock is a critical research topic. The triaxial cyclic loading and unloading tests under five confining pressures were carried out on high-temperature rock samples to investigate the influences of the confining pressure ($\sigma_{3}$) and temperature (T) on their energy evolution and distribution characteristics. The energy densities of rock samples under various confining pressures were calculated by determining the area between the loading and unloading curves, including axial energy densities ($u_{1}^{0}$, $u_{1}^{e}$, $u_{1}^{d}$) and circumferential strain energy density ($u_{3}^{0}$). The energy accumulation and dissipation and the effect of $\sigma_{3}$ and T on the energy distribution laws of loaded rock samples were analysed. The characteristic energy density ($u_{1}^{t}$) was used to analyse the accumulation, dissipation and release of energy of the loaded rock sample. $u_{1}^{t}$ increased with the increase in $\sigma_{3}$ and decreased with the increase in T. Furthermore, $u_{3}^{0}$ increased with the increase in $\sigma_{3}$, which effectively limited the energy dissipation and release due to fracture or failure of the rock sample. With the increase in T, the circumferential strain of the rock sample increased, which led to an increase in $u_{3}^{0}$. At the pre-peak stage, energy accumulation characterised the energy behaviour of the loaded rock sample, and the proportion of the elastic energy density ($k_{1}^{e}$) was large. At the post-peak stage, energy release and dissipation characterised the energy behaviour of the loaded rock sample, the dissipated energy density proportion ($k_{1}^{d}$) increased gradually, and the change law for $k_{1}^{e}$ and $k_{1}^{d}$ was considerably affected by the confining pressure and temperature effect. The dissipated energy density of the loaded rock sample was used to establish the energy damage variable and analyse the evolution law of the dissipated energy damage variable of the high-temperature rock sample with $\sigma_{3}$ and T. The results of this study can provide guidance for the research on high-temperature rock damage mechanisms and prevention of dynamic disasters of rock underground engineering.