To investigate the acoustic emission (AE) characteristics and damage mechanical behaviors of Huashan granite, two groups of uniaxial compression tests were carried out: the first group of tests was performed on samples after thermal treatment without a cooling-off period (i.e., tested at real-time temperature; RT samples); and a second group of tests was performed on samples after the samples had cooled to room temperature (i.e., tested at post-temperature, PT samples). The experimental results showed that AE energy increased with the increase of treatment temperature, and it became quite large for temperatures over 500°C. The AE accumulative energy counts of the RT sample were higher than the PT sample. It was found in uniaxial compression tests of heat-treated granite that the mutation point in the ∑N/∑E curve could be taken as the indicator of the crack damage threshold (${\sigma }_{cd}$ ). The peak strain (the strain at the peak of the stress–strain curve) of the RT sample was larger than the PT sample, implying that the ductility of the former was more obvious, while peak stress and elastic modulus were smaller. For treatment temperatures of 500°C and below, the damage of the sample increased exponentially with external load, whereas as the temperature increased to 700°C, the damage-time curve was hyperbolic without an obvious inflection point. The sample under uniaxial compression exhibited axial splitting failure at low temperatures, and the failure mode became more complicated as the temperature rose.

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°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 $18\mathrm{m}/\mathrm{s}$ . 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.

The material testing machine and the split Hopkinson pressure bar (SHPB) were adopted, respectively, to conduct the static and dynamic compression tests on granite specimens heat treated by different temperatures. The effects of strain rate and heat-treatment temperature on the mechanism of energy evolution of the specimen during deformation and failure process were studied. The results show a significant strain rate effect on the granite, with the energy dissipation density increasing with increasing impact velocity (or strain rate), regardless of the treatment temperature. The specimens heat treated at 300 °C and 700 °C have the minimum and maximum energy dissipation densities, respectively. The specimen in the SHPB tests easily broke into pieces or even powder; while under static compression, only macroscopic fracture surfaces and spalling phenomenon on the specimen were detected. The energy dissipation density is inversely proportional to the compressive strength of the specimen. The rate of energy dissipation change is defined, which can be used to identify the stages in the deformation process of rock and to determine the position of the failure point in the stress-strain curve. For both the dynamic and static compression tests, the value of energy utilization ratio is relatively low, with a maximum value of about 35%.