Investigations of the Progressive Damage and Failure Behaviors of Thermally Treated Granite Under the Coupling Action of Biaxial Stress Constraint and High Strain Rate

The elevated temperature, high geostress environment, and strong dynamic loading are important factors that induce disasters in deep rock engineering work. The progressive damage and failure behaviors of granite under the coupling of elevated temperature, biaxial stress constraint, and high strain rate were studied by true triaxial split Hopkinson pressure bar (SHPB) tests. The results show that the P-wave velocity attenuation rate kp increases exponentially with temperature, and kp is closely related to the thermal damage of granite grain structures. Under the biaxial stress constraint, the dynamic strength and elastic modulus of granite both increase at first, then decrease with the increase of temperature and a thermal enhancement effect is observed at 150 °C. The threshold temperature for the transition of dynamic failure from brittleness to plasticity is 500 °C. The dynamic strength generated by axial compression and lateral expansion increases exponentially with the strain rate. The lateral stress σ2 enhances the dynamic strength of granite. Under true triaxial stress condition (σ1 > σ2 > σ3 ≠ 0), the enhancement of the dynamic strength is further improved, and the lateral expansion of samples develops along the direction of the minimum principal stress. Under the same stress constraint, the number of impacts that can be borne by granite samples is linearly negatively correlated with heat treatment temperature. The samples are compacted apparently in the early stage of cyclic loading, and the maximum strain experiences three stages: a decrease, slow increase, and sharp increase. When the heat treatment temperature is 700 °C and above, the slow increase stage no longer occurs.