Numerical investigation on the cooling-related mechanical properties of heated Australian Strathbogie granite using Discrete Element Method

A series of laboratory tests was carried out to study the cooling-related mechanical properties of Australian Strathbogie granite, which were heated from room temperature to various high temperatures (200, 400, 600 and 800 °C) followed by two different cooling methods (exposed in air for slow cooling and put into water for rapid cooling). Using a two-dimensional discrete element code, UDEC, a grain-breakable model was firstly established, in which the grains were made breakable by connecting the centroid and midpoints of edges for each grain. Therefore, both the inter-granular and intra-granular fractures can be handled in the model for capturing the fracturing mechanism of rock under heating or cooling treatment. In the experiments and simulation, three types of granite with different grain sizes were considered, which are fine grain (FG), medium grain (MG) and coarse grain (CG). Besides, the effects of grain size homogeneity and mineral composition were also taken into consideration in the simulation. It was found that the strength and deformation characteristics from the numerical simulation had a good consistency with that from the experiments. For all types of granite, no significant variations in compressive strength and failure strain were found at lower heating temperature (below 400 °C) under both slow and rapid cooling conditions. Compressive strength reduced sharply as heating temperature rising from 400 to 800 °C, while failure strain exhibited the opposite trend and was more pronounced for rapid cooling. Fracturing analysis indicated that granite exhibited more fractures after heating and cooling treatments compared with that without cooling treatment. Compared with slow cooling, rapid cooling introduced more fractures, especially intra-granular fractures, and damaged rock more severely. CG was found to have more inter-granular fractures but fewer intra-granular fractures than FG and MG granite at all testing temperatures except for 200 °C. Granite with heterogeneous grain size distribution exhibited lower strength and more intra-granular fractures than that with uniform grain size distribution after cooled from the same temperature. The increasing quartz content introduced higher mineral heterogeneity in granite during heating and cooling due to thermal expansion differences. Granite with higher quartz content displayed lower strength and more fractures than that with lower quartz content after heating and cooling treatments.