Effect of Thermal Treatment on the Dynamic Fracture Toughness of Laurentian Granite

The effect of thermal treatment on the dynamic fracture toughness of Laurentian granite (LG) was investigated in this work. Notched semi-circular bend (NSCB) LG specimens are heat treated at temperatures up to 850 °C. The micro-cracks in the rock samples induced by thermal treatment are examined by scanning electron microscope (SEM). The microscopic observations are consistent with the subsequent P-wave velocity measurements, which shows that the P-wave velocity decreases with the treatment temperature monotonically when the temperature is higher than 250 °C. Dynamic fracture toughness measurements are then carried out on these samples with the dynamic load exerted by a modified split Hopkinson pressure bar (SHPB) system. The relationship between fracture toughness and treatment temperature is investigated. Experimental results show that fracture toughness increases with the loading rate but decreases with the treatment temperature. However, when the heating temperature is below 250 °C and above 450 °C, the dependence of dynamic fracture toughness on the temperature is different from other temperatures, which can be explained by the physical processes at the microscopic level of the rock due to heating. At treatment temperatures below 250 °C, the thermal expansion of grains leads to an increase in the toughness of the rock. At treatment temperatures above 450 °C, the sources of weakness such as grain boundaries and phase transition of silicon are depleted, and as a result the decrease in fracture toughness is not as significant as other treatment temperature ranges.