Thermo-mechanical response and fracture mechanisms of granite investigated through experiments and 3D FDEM simulations

The exploitation of geothermal energy and unconventional oil and gas resources critically depends on understanding how high-temperature rock masses cool and fracture. However, the effects of thermal treatment on crack propagation and fracture mechanisms in rock masses remain inadequately understood. This study investigates the thermo-mechanical behavior and fracture patterns of granite using a combined experimental and computational approach. The methodology integrates semi-circular bend (SCB) tests with three-dimensional finite-discrete element method simulations incorporating thermo-mechanical coupling (3D FDEM-TM). SCB test results revealed a quadratic decrease in fracture toughness with increasing temperature, with maximum reductions of 61.73% under air cooling and 73.83% under water cooling at 400 °C. Water-cooled specimens consistently exhibited lower fracture toughness than air-cooled ones, with reductions ranging from 3.33% to 36.34%. Scanning electron microscopy (SEM) analyses identified a transition from transgranular to intergranular cracking as temperature increased, a phenomenon more pronounced in water-cooled specimens. 3D FDEM-TM simulations showed that water cooling induces significantly larger temperature gradients compared to air cooling. Tensile cracks predominated during thermal treatment, but their proportion decreased with rising temperature. Under mechanical loading, air-cooled granite exhibited an initial increase in crack number up to 200 °C, followed by a decline, whereas water-cooled granite showed a continuous reduction in crack number with increasing temperature. The failure mechanism transitioned from load-induced crack initiation at lower temperatures (100–200 °C) to thermal crack coalescence at higher temperatures (300–400 °C). The proposed 3D FDEM-TM framework simultaneously accounts for mineralogical heterogeneity, crack-dependent thermal resistance, and the complete heating–cooling–mechanical loading sequence. These findings provide a basis for selecting thermal stimulation strategies for geothermal energy development and deep rock engineering.