Dynamic compression and Brazilian Disc (BD) tests were performed on heated sandstone with split Hopkinson pressure bar (SHPB) at different strain rates. The sandstones were treated under the temperatures of 20 °C, 200 °C, 400 °C, 800 °C and 1200 °C. The full-field and real-time fracturing processes were captured by the high-speed 3D-DIC technique with resolution of 256 × 256 pixels and 200,000 frames per second (fps). The effect of heat on crack initiation stress thresholds, and the stability of crack development were investigated together with the dynamic Poisson's ratio and elastic modulus. The density and wave velocity were found to decrease with temperature increase of the heat treatment. The results also showed that strain rate effect still exists in the high temperature treated sandstones; however, within a critical range (temperatures between 400 °C and 800 °C), the reduction in compressive and tensile strength is followed by a rise. The thermal effect on the distribution and evolution of the strain localization under compressive loading were discussed.
With an increasing demand in resource and space, the utilization of underground spaces, where high temperature may occur, is more and more important. The thermal effect on the rock mechanics will influence the efficiency of the rock excavation and the safety of the rock engineering. The underground spaces, also, experience dynamic loadings from various sources such as impact, explosion and earthquake. However, temperature and strain rate have opposite effects on the stress and strain. Increasing the strain rate or decreasing the temperature will lead to higher stress levels, but lower values of strain (Zhang and Zhao, 2014). Therefore, the understanding of the coupled effect of high temperature and strain rate on the dynamic behaviour of rocks is essential.
The thermal effect on rock dynamics has attracted extensive attentions from researchers. The dynamic fracture toughness of Fangshan gabbro and Fangshan marble subjected to high temperature was measured by (Zhang et al., 2001) with the short rod (SR) method on SHPB. It was found that temperature variation affects the dynamic fracture toughness of the two rocks to a limited extent within the tested temperature ranges. This result was different from the results obtained under the static loading condition. Yin et.al. investigated effect of thermal treatment on the dynamic fracture toughness of Laurentian granite (LG) conducted based on notched semi-circular bend (NSCB) test (Yin et al., 2012). The thermally induced micro-cracks within the rock samples were then examined by scanning electron microscope (SEM). They found that at temperatures below 250 °C, the thermal expansion of grains led to an increase in the toughness of the rock. At treatment temperatures above 450 °C, the sources of weaknesses such as grain boundaries and phase transition of silicon were depleted resulting in decrease of fracture toughness. Similar pattern was also found in tensile strength in Brazilian disc tests done by (Yin et al., 2015) on Laurentian granite after being treated with high temperature. These results showed that dynamic tensile strength first increases and then decreases with a linear increase of loading rate. Liu and Xu employed the SHPB method to conduct uniaxial compression and split tensile tests on Qinling biotite/granite samples, which were treated under high temperatures and then cooled naturally to room temperature (Liu and Xu, 2014). These researchers also concluded the effect of high temperature on the dynamic tensile and compressive strength. Huang and Xia used computed tomography (CT) to quantify the damage induced by the heat-treatment and correlated it with the dynamic compressive strength of Longyou sandstone which was obtained by SHPB (Huang and Xia, 2015). Further investigations by (Liu and Xu, 2015) were carried out on the influences of coupled temperature/strain rate effect on dynamic compressive mechanical behaviours of sandstone. No obvious strain rate effect was observed in tests conducted under high temperature compared to ones at room temperature when ratios of dynamic compressive strength, peak strain, and energy absorption ratio of rock were studied. Similar research was implemented on granite by (Fan et al., 2017), and results showed that the dynamic energy absorption capacity increases below 400 °C but then decreases as the temperature increases to 800 °C. The effect of thermal treatment on energy absorption capacity was more obvious under a smaller impact pressure in granite samples. The dynamic mechanical behaviours of coal samples exposed to elevated temperatures were also examined with SHPB unit (Yu et al., 2017). In this study, the anthracite specimens were preheated up to 500°C in an oxygen-free environment. The results showed that coal gradually loses its dynamic bearing and anti-deformation capacities with increase in temperature, especially after 300°C.
