Thermal cracking mechanism of granite during heating and cooling processes

Due to the limitations in high-temperature test equipment, the studies on the real thermal cracking of rocks in laboratory typically involve inverse analysis based on microstructure observations of cooled specimens. The real-time cracking evolution at high temperatures cannot be obtained through this method. Therefore, in this study, a thermo-mechanical coupled UDEC grain-based model for granite is established based on the modified joint constitutive law considering temperature and crack slip effects so as to investigate the real-time thermal cracking behavior of granite during heating and cooling processes. It is found that the thermally induced microcracking in granite begins to occur at around 75℃ under heating conditions. The number of microcracks rapidly increase near the α→β quartz phase transition temperature, but the microcrack density does not change significantly during the cooling process. Although the change in the crack number caused by the cooling effects is negligible, it can lead to an increase or decrease in the crack opening. During the heating process, the initiation of microcracks is mainly formed by the local stress accumulation due to different thermal expansions of the adjacent grains. The microstructure changes caused by quartz transition can enhance the interaction between different grains, leading to the increasing compression and shear motion on the grain level. This results in thermal-induced cracks continuing to deform and develop. During the cooling process, the local microscopic stress release due to the thermal cracking during heating and the shrinkage of different mineral crystals due to the cooling effects make the number of microcracks hardly change, but their morphological characteristics can change more significantly. This greatly affects the macroscopic stress-strain behaviors of granite after cooling. The findings of thermo-mechanical coupling tests on granite based on the discrete element numerical simulations are interpreted in a micro-meso-scale manner, revealing the real-time thermal cracking mechanism of granite during heating and cooling, further promoting the understanding of the thermo-mechanical coupling of high-temperature rocks.