# Study on Damage Behavior and Its Energy Distribution of Deep Granite at High-Temperature Conditions
## Acoustic Emission Test of Fractured Rock under Triaxial Loading
### High-Temperature Damage Test and Analysis of Rock  
The cracks induced by stress propagate mainly between a single (intragranular crack) or even several (transgranular crack) minerals. Therefore, the resulting cracks tend to be parallel to the maximum principal stress, that is, there is significant anisotropy. Thermal cracks mostly occur between adjacent grains (intergranular cracks). Since the distribution of minerals in granite is statistically isotropic, thermal stress-induced cracks are basically isotropic. In order to analyze the influence of fracture density on the energy distribution of rock mass during deformation and failure. Therefore, the spatial distribution of cracks is mainly considered in this experiment, and the crack type is the second. Therefore, granite samples with different cracks were prepared by thermal damage. The rock samples in this paper are made of uniformly textured granite without obvious cracks and are made of complete continuous rock blocks. The size of the specimen was \(\varphi\)30 mm \(\times\) 60 mm cylinder, meeting the requirements of the specification height to diameter ratio 2.0-2.5. The average density of the specimen at room temperature was 2.68 g/cm\({}^{3}\). The granite specimens with the size of \(\varphi\)30 mm \(\times\) 60 mm were heated in a programmable tube furnace to prepare the granite specimens with cracks. In order to obtain granite samples with different degrees of fracture, four target temperatures were set during the test, namely 300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, and 600 \({}^{\circ}\)C, and three samples were heated at each target temperature. The heating path is as follows, the sample is heated from room temperature to the target temperature at a heating rate of 55 \({}^{\circ}\)C/min, then keep the temperature for 1 h, and, finally, the sample is cooled to room temperature at a cooling rate of 5 \({}^{\circ}\)C/min. In order to quantitatively describe the damage degree of rock after heating, the parameters that can characterize the physical and mechanical damage of material were selected, namely, porosity and change rate of p-wave velocity before and after heating. In order to measure the change of sample porosity after high-temperature treatment, the sample was cooled to room temperature and weighed (\(M_{s}\)) after high-temperature heating treatment. According to the preparation method of saturated sample suggested by ISRM, the sample was saturated and weighed (\(M_{sat}\)). The effective porosity of the sample (hereinafter referred to as porosity) \(n\) can be calculated,  
\[n=\frac{M_{sat}-M_{s}}{\rho_{w}V} \tag{1}\]  
where, \(V\) and \(\rho_{w}\) are the volume and density of water, respectively. The non-metallic ultrasonic instrument RS-ST01C was used to measure the p-wave velocity of rock samples before and after high-temperature treatment. The variation tendency of porosity and p-wave velocity of samples treated at different temperatures are shown in Figure 2.  
From Figure 2a, it can be seen that the higher the target temperature, the greater the porosity of the sample, which were 0.377, 0.916, 1.122, 1.669, and 3.226, respectively. In the range of 500-600 \({}^{\circ}\)C, the growth rate of porosity is 8.5 times that of normal temperature. In another picture, the p-wave velocity of saturated sample is higher than that of dry sample under the same test conditions. The higher the target temperature, the wave velocity of the dried sample decreases monotonically with the increase in the target temperature. The velocity deceleration is the slowest in the range from 0 \({}^{\circ}\)C to 300 \({}^{\circ}\)C, and the velocity at 300 \({}^{\circ}\)C is 0.836 times that of the initial sample. At 300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, and 500 \({}^{\circ}\)C, the rock wave velocity decreases step by step, and the crack development scale is relatively stable.