Loading/Unloading Response of Crystalline Rocks with Varied Thermal-Damaged Degrees to Cyclic Compression Using a Grain-Based Model

Characterization of mechanical behaviors of thermal-damaged rocks holds significant implications for deep-ground engineering such as nuclear waste disposal. A thermal-damaged rock can present an abnormal negative Poisson's ratio (NPR) and compression-hardening memory (CHM). However, thus far, how the NPR and CHM influenced the loading/unloading behaviors of thermal-damaged rocks remains unclear from the macro- to micro-scale. This study first experimentally explored the macro-behaviors of Crystalline Rocks with varied Thermal-Damaged Degrees under cyclic unconfined compression. Subsequently, Grain-Based Models considering heterogeneity were established to reproduce experimental results and further quantitatively analyze macro- to micromechanical behaviors of thermal-damaged Crystalline Rocks. Results showed that the CHM and NPR had prominent influences on the Loading/Unloading Response of thermal-damaged Crystalline Rocks, such as non-linear stress-strain relations and tensile stress concentration characteristics. With enhancing the thermal-damaged degree, the grain contact had a more significant reduction of compressive contact stiffness than the shear one and an increment of maximum allowable closure, thereby enhancing promoted NPR and CHM effects. Moreover, the CHM could weaken the NPR owing to enhancement of the compressive contact stiffness under Cyclic Compression. The CHM and NPR affected the magnitude and proportion of the tensile stress and formation of intergranular and intragranular failure which was influenced by the maximum historic compressive stress. Due to the NPR effect, the failure was initially concentrated at both ends of the synthetic sample. Under Cyclic Compression and increasing compressive stress, the NPR effect was attenuated, resulting in a more uniform failure pattern.