## 3 Test Results and Analysis
### Changes in Mechanical Properties
#### 3.2.2 Strength Characteristics  
Figure 8 shows, under different cooling methods, the variation curves of uniaxial compressive strength of granite with temperature change. It can be seen from the figure that, through natural cooling and water cooling, the peak strength of the samples can form into two stages. The temperature threshold is 400 \({}^{\circ}\)C. The first stage is from 200 \({}^{\circ}\)C to 400 \({}^{\circ}\)C and the strength of the samples increases with the temperature at a high level under different treatment methods. At this stage, the water in different forms existing in granite samples slowly escapes after the high temperature. The expansion of particles inside the sample makes these cracks and pores close again. The deformation and crack propagation caused by thermal stress are suppressed to a certain extent. The second stage is from 400 \({}^{\circ}\)C to 600 \({}^{\circ}\)C. In this stage, the peak strength of the sample decreases step by step with the increase in temperature. As the temperature rises, the mineral particles in the sample keep expanding uncoordinatedly. This leads to a greater thermal stress and more microcracks inside the sample. The temperature is higher, and the phenomenon is more obvious, which accelerates the penetration of the cracks in the sample, but the strength is opposite. By comparing the compressive strength changes of the samples under different cooling methods, it can be seen that the compressive strength of natural cooling samples at different temperatures is higher than that of water cooling. We can conclude that the degree of deterioration of rock strength by the water cooling method is greater than that of natural cooling. The influence of cold shock damage is greater. High temperatures will change the mineral particles inside the rock to some extent. The internal particles will recrystallize after the cold shock, which will cause more internal pores in the rock [19]. Further, with the samples cooled by water, the periphery of the samples is cooled rapidly, but the interior remains at a high temperature, which causes the temperature difference between the inside and outside of the samples to produce tensile stress. The more serious the damage is, the lower the strength of the sample will be. The elastic modulus and deformation modulus of the sample can nicely reflect the deformation characteristics. In this paper, the elastic modulus parameters of rock are taken from the partial slope of the approximate straight line in the axial direction--the stress-strain curve of the sample, whose slope of the straight line is 40-60% peak stress. Figure 9 shows the rule of elastic modulus changing with the temperature under different  
Figure 8: Change of peak strength of sample after different cooling methods. cooling methods. It can be found from Figure 9 that the elastic modulus of the sample after different cooling methods changes slightly at first, and then decreases gradually as the temperature goes up. This suggests that, as the temperature rises, the change of particles inside the sample attracts the reducing resistance to deformation. The difference in the elastic modulus between different cooling methods is small when it is at a low temperature. However, when the temperature become higher, the elastic modulus of the samples under natural cooling is higher than that under the water cooling. Note that, when the temperature is lower, natural cooling and water cooling have a smaller effect on the deformation resistance of the sample. The deformation characteristics of rock can be expressed by deformation modulus. The secant slope of any point on the stress-strain curve connected with the origin coordinate represents the deformation modulus corresponding to that point. Here, the deformation modulus is taken from the stress-strain ratio of the corresponding point of half of the peak stress intensity. Figure 10 shows the curves of the deformation modulus following with the temperature change in different cooling methods.