Numerical Investigation of the Triaxial Direct Shear Failure Mechanism of a Granite Polycrystalline Model under Thermomechanical Coupling: Insights from Nanoindentation and FDEM

The long-term mechanical behavior of rocks under thermally coupled environment is an important issue affecting the stability of underground engineering. In this paper, the triaxial direct shear mechanical properties and failure modes of Beishan granite under thermal coupling were revealed by nanoindentation experiments combined with finite-discrete element method (FDEM). The experimental results show that the long-term strength parameters of Beishan granite obtained according to the Mohr-Coulomb criterion show a tendency of cohesion increasing and then decreasing with increasing temperature, and the change of friction angle is not significant. For the same temperature condition, the confining pressure level is elevated, and the percentage of creep strain under the same load ratio increases. For the same temperature condition, the creep strain percentage under the same load ratio increases and then decreases when the enclosure pressure level increases. Increasing confining pressure from 9 to 22 MPa resulted in a 238% rise in transgranular cracks and 36% more shear cracks. Conversely, raising temperature from 30°C to 110°C increased total intergranular and transgranular cracks by 56% but reduced shear cracks by 20%. These findings offer theoretical insights and methods for predicting the long-term stability of nuclear waste repositories.