Multi-scale Mechanical Behavior of Rock Mass Under Thermal-Solid Coupling: From Mesoscopic Damage to Macroscopic Fracture

The study investigates the multiscale mechanical behavior of rocks in thermal-solid coupling environments, focusing on the transition from micro-damage to macro-fracture, which is critical for applications such as geothermal energy development, nuclear waste disposal, and oil and gas reservoir heating. A multiscale analysis framework was developed, combining the poly-crystalline discrete element method (PDEM) for microscale modeling and the finite discrete element method (FDEM) for macroscale simulation. The PDEM model characterized the effects of temperature, pressure, and displacement constraints on rock strength and elastic modulus at the microscale, revealing that the dominant factors controlling rock mechanical properties vary with temperature. Specifically, thermal expansion of clay minerals predominates in the range of 25–400 ℃, while crack propagation becomes the primary mechanism in the 400–800 ℃ range, leading to an initial increase followed by a decrease in both strength and elastic modulus with rising temperature. A normalized constitutive model was developed, showing that rock strength and elastic modulus follow a linear relationship with displacement constraints, with temperature-dependent coefficients following single exponential and Boltzmann distributions, respectively. The model was validated against experimental data under borehole heating conditions, demonstrating that thermal gradients induce non-uniform rock expansion, leading to stress concentration at boundaries and sequential crack propagation from the borehole wall to surrounding rock. This study presents a novel multiscale coupling approach that bridges microscale damage mechanisms with macroscale fracture behavior, providing a robust toolset for predicting thermally induced rock failure and offering significant implications for geothermal energy extraction, underground waste storage, and enhanced oil recovery applications.