Effects of microstructural alteration by thermal expansion and contraction on compressive behaviors of crystalline rocks: A nonlinear grain-based modeling comparative study

The multiscale mechanical behavior of crystalline rocks is governed by their microstructure, which is critical for engineering applications such as cold-region construction, geothermal energy exploitation, and underground nuclear waste disposal. Temperature-induced Cool-Damage (C-D) cycles and Thermal-Damage (T-D) both degrade microstructures, weaken mechanical properties, and alter deformation and failure processes, yet comparative studies on their distinct effects remain limited. This study combines uniaxial compression tests, Universal Discrete Element Code (UDEC) simulations, and thermal-mechanical coupling analyses to evaluate microstructural deterioration and mechanical responses of Suizhou granite under C-D and T-D conditions. Results show that both treatments reduce peak strength and elastic modulus while enhancing stress-strain nonlinearity through compression hardening. At the microscale, temperature disturbances restructure stress transfer pathways and create heterogeneous stress fields that accelerate crack initiation and complex propagation. Notably, T-D samples exhibit a Negative Poisson's Ratio (NPR) effect, manifested as lateral contraction during axial loading. NPR suppresses tensile contact stresses typical of conventional deformation but induces localized stress concentrations, leading to premature intergranular cracking and extensive intragranular failure. This behavior arises from disproportionate stiffness attenuation, with normal stiffness degrading more severely than shear stiffness. Comparative analysis further indicates that heating promotes tensile microcrack formation and coalescence into connected networks, whereas cooling produces fragmented, weakly connected cracks and additional shear cracking at grain contacts, leading to a balanced degradation of both shear and normal stiffness and limiting NPR development. These findings elucidate the differences between expansion-driven and contraction-driven damage, and provide new insights into predicting rock behavior under complex environmental conditions.