Experimental Study on Damage Mechanical Characteristics of Heat-Treated Granite under Repeated Impact

AbstractThe mechanical behaviors of rock under the combined action of high temperature and cyclic loading are a current concern in rock mechanics. In this study, biotite granite samples were thermally treated at the temperatures 20°C, 200°C, 400°C, and 600°C. Then, the samples were repeatedly impacted in a modified split Hopkinson pressure bar (SHPB) with the striking velocities 12, 15, and 18 m/s. The dynamic stress-strain curves were measured, and the failure modes were observed. The experimental results showed that the samples heat-treated at each temperature had a damage threshold and that the threshold was the lowest after the 600°C treatment. The damage induced by the repeated impact was not significant if the loading was less than some critical value. The stress-strain responses including the strain rate and peak stress under repeated impact were similar at treatment temperatures ranging from 20°C to 400°C. The stress-strain curve first went up and then shifted right. Within the range of this study, the average strain rate was the highest for the sample after the 600°C treatment whereas the peak stress was the lowest. An apparent deterioration of the mechanical properties was detected. The resistance to the repeated impact was the strongest for the sample heat-treated at 200°C. This showed that the hardening effect induced by temperature overtakes the damage-softening effect. The 600°C treatment sample had the most severe failure degree and the most complex failure mode, possessing the characteristics of axial splitting and dynamic crushing. Thus, a threshold temperature for biotite granite may exist between 400°C and 600°C.