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

The 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 degrees C, 200 degrees C, 400 degrees C, and 600 degrees C. Then, the samples were repeatedly impacted in a modified split Hopkinson pressure bar (SHPB) with the striking velocities 12, 15, and 18m/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 degrees 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 degrees C to 400 degrees 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 degrees 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 degrees C. This showed that the hardening effect induced by temperature overtakes the damage-softening effect. The 600 degrees 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 degrees C and 600 degrees C.