## 4 Analysis of Test Results
### Analysis of Uniaxial Compression Test Results
#### 4.2.1 Deformation Characteristics
At the same time, the decrease of the rigidity will also affect the resistance of the sample to deformation, so that the strength and deformation resistance of the sample are weakened. However, [PERSON] et al. (2018) considered that 600 \({}^{\circ}\)C is the threshold temperature for strengthening and weakening fine sandstone samples. Moreover, as the temperature increases, the intragranular stress further increases, causing the sample to produce micro-cracks or to expand, widen, and connect the primary cracks. Macroscopically, the mechanical properties of the coarse sandstone are degraded, and the peak strength and average modulus of the sample decrease with increasing temperature. Both groups of samples are medium-hard brittle rocks, and the effect of temperature on the post-ductility change is not obvious. The strain showed a softening phase after the peak stress was reached, the stress decreases rapidly, and the strain does not change much, showing brittle failure characteristics. The relationship between uniaxial compression deformation parameters and temperature of group A samples is shown in Figure 9. Here, it is necessary to specify that the initial modulus \(E_{0}\), the deformation modulus (secant modulus) \(E_{50}\) and the definition are according to the procedure shown in ([PERSON] and [PERSON], 2017). Elastic modulus (Young's modulus) is \(E_{\mathrm{T}}\). The initial modulus \(E_{0}\) is the slope of the stress-strain curve passing through the origin in the initial compaction stage. The deformation modulus \(E_{50}\) is the slope of the 50% peak intensity connected to the origin of the uniaxial compression axial stress-strain curve. The modulus \(E_{0}\) and the deformation modulus \(E_{50}\) are related to the nonlinear deformation effect in the initial compaction stage of the loading process. The deformation properties of the rock cannot be well characterized. Only the deformation characteristics with stress below 50% of the peak strength can be described. The elastic modulus refers to the slope of the straight line segment on the uniaxial compression axial stress-strain curve; the elastic modulus can characterize the proportional relationship of the stress-strain variation of the material, its value has little effect on the test loading conditions and has a clear mechanical meaning ([PERSON] and [PERSON], 2017). The peak strain \(\varepsilon_{0}\) represents the strain value corresponding to the peak stress, and the axial compressive strain of the sample consists of the closure of the crack, the slip between the particles, and the elastic strain of the material. The initial stage of the sample loading contains the above three parts. The strain in the elastic and yield stages is mainly composed of the slip between the particles and the elastic compression of the material. The strain in the failure stage is mainly the slip of the macroscopic failure surface. It can be seen from Figure 9 that the effect of high temperature on the initial modulus \(E_{0}\), the deformation modulus \(E_{50}\), and the elastic modulus \(E_{\text{T}}\) of the sample is not the same. The initial modulus \(E_{0}\) is particularly affected by high temperature, which is mainly manifested by the fact that the initial stage of loading of the sample is affected by the compaction of the micro-cracks generated by the high temperature. After the high temperature of 100-900 \({}^{\circ}\)C, as shown in Figure 9a. Compared with the sample at room temperature of 25 \({}^{\circ}\)C, the average modulus \(E_{0}\) of the sample after 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, 600 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, 800 \({}^{\circ}\)C and 900 \({}^{\circ}\)C decreased by 20.3%, 35.1%, 32.4%, 64.0%, 52.7%, 78.4%, 81.9%, and 85.3% respectively.