In hot dry rock reservoirs, natural fissures and elevated temperatures significantly influence crack initiation and propagation behaviors. However, the mode I–II fracture mechanism of crystalline rocks under high-temperature conditions remains unclear. This study examined cracked straight-through Brazilian disc (CSTBD) granite by combining digital image correlation (DIC) and acoustic emission (AE) techniques under varying temperatures and fissure inclination angles. This paper then proposed an improved thermo-mechanical coupled peridynamic model that integrates a multi-layer thermo-mechanical coupling algorithm, a refined rock model based on crystal and microporosity structures, and crack mode recognition methods to understand the fracture evolution of high temperature treated granite from macro- and micro-perspectives. The results showed that the fissure inclination angle β governed crack initiation location and failure mode. As β increased, failure modes transitioned from tension to tension-shear, shear, compression-shear, and tension, with initiation sites shifting from the fissure tip to its edge and then to its surface. Temperature modulated rock toughness, crack trajectories, and brittle-ductile behavior; both K IC and K IIC decreased linearly with increasing temperature, and post-peak mechanical behavior evolved from brittle to ductile, with 500 °C as the threshold. Thermal expansion mismatches among mineral phases activated an internal microcrack network, inducing secondary cracks during primary crack growth. Fissure inclination and temperature exerted synergistic control. Inclination and temperature were the dominant factors at ambient-low temperatures and high temperature, respectively. These findings offer valuable guidance for fissure optimization design and thermal energy enhancement in enhanced geothermal system (EGS) engineering.
Li et al. (Mon,) studied this question.