Dynamic Properties of Thermally Treated Granite Subjected to Cyclic Impact Loading

The Earth’s deep rock mass is subjected to various complex temperature and stress perturbations. To study the effect of temperature and dynamic disturbance on the damage mechanical properties of these rocks, a series of laboratory tests was carried out by means of a modified split Hopkinson pressure bar system. Specimens were heat treated from room temperature up to a maximum of 800 °C and then cooled to room temperature. During repeated loading tests, the dynamic incident energy was kept constant in each cycle. For samples under the same treated temperature, the dynamic strength and deformation capacity degraded gradually with increasing impact number. Furthermore, rock strength decreased with increasing treated temperature, with the number of impacts before failure reduced accordingly. Since damage can be initiated by thermal treatment and then aggravated by dynamic disturbance, according to the observed decrease in rock strength and increase in strain rate under the different temperature conditions, a temperature of 400 °C was found be a significant failure threshold. The maximum strain was employed to describe damage evolution during cyclic impact loading, indicating that fatigue damage is gradually accumulated and maintains a three-segment growth with an increase in repeated impacts. Under the coupled effect of temperature and cyclic impact loading, specimens exhibited two different failure modes: split tensile failure and unloading failure. The micro-properties of fracture morphology in granite after different temperature and repeated impact were also discussed in detail.