Numerical study of the influence of grain size and heterogeneity on thermal cracking and damage of granite

In the process of geothermal or deep energy resource exploitation, rock damage and failure due to thermal cracking pose a great challenge to construction safety. To investigate the effects of grain size on thermal damage of the typical hot dry rock, granite, a grain-based numerical model study was conducted. The result shows that, with temperature increasing, the thermal crack pattern changes from mainly tensile crack along grain boundaries to mainly shear crack within intra-grain. When intra-grain cracking occurs, more strength damage can be induced in coarse-grained or homogeneous granite. The quartz particle grains contained in those samples were larger, contributing to a higher thermal expansion coefficient and thereby causing a more significant strength reduction. Thermal damage was calculated and plotted against volumetric thermal strain. The same curve pattern was observed in samples with different sizes and heterogeneity, indicating that the damage variation depends on the volumetric thermal strain. The relational model for the curves was further fitted. In fine-grain or heterogeneous granite, the rate of damage increasing along with volumetric thermal strain is higher. This is mainly due to larger amounts of grain boundaries in the samples, which act as weak bonding and are easier to break at a certain thermal strain. Confining pressure around the specimen can significantly suppress thermal strain and inhibit thermal damage, which corresponds to less value of model parameter related to peak damage and a higher value of that related to curve shape. Our work clarified the major factor determining thermal damage of granite with different sizes and proposed a relation model, which can offer as an instruction for the development of the high temperature geological repository.