## 4 Discussion
### Comparison with Other Studies  
Based on a review of the available research on the changes in the physical and mechanical properties of sandstone after high temperature exposure, the test results of sandstone with different bedding angles in this paper were compared with those in other studies. Due to the limitations of physical properties such as the mass, volume, and density of sandstone after high-temperature treatment and the data of mechanical properties under triaxial conditions in the literature, we selected only the longitudinal wave velocity, uniaxial compressive strength, and elastic modulus for comparison, which was facilitated by normalizing the data.  
The two change trends of the sandstone's longitudinal wave velocity and elastic modulus with increasing temperature can be seen in Figure 14a,c: 1 The temperature of the sandstone's longitudinal wave velocity and elastic modulus below \(400\)\({}^{\circ}\)C shows a slow declining trend, and when the heating temperature exceeds \(400\)\({}^{\circ}\)C, the longitudinal wave velocity and elastic modulus rapidly decrease (as shown in this article for \(\beta=0\)\({}^{\circ}\) and \(30\)\({}^{\circ}\) sandstone); 2 The longitudinal wave velocity and elastic modulus of the sandstone increase below 400 \({}^{\circ}\)C. When the heating temperature is higher than 400 \({}^{\circ}\)C, the longitudinal wave velocity and elastic modulus decrease rapidly (as in this paper for \(\beta=45^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) sandstone).  
Figure 14: **(a) Relationship between normalized \(v_{p}\) and temperature [13; 31; 32; 33]; (b) normalization \(\sigma\) relationship between \(\sigma_{c}\) and temperature [33; 34; 35; 36; 37; 38]; (c) relationship between normalized \(E\) and temperature [8; 32; 37; 38].**  
The change trend with temperature of the uniaxial compressive strength of sandstone can be divided into three types, as shown in Figure 14b: 1 The uniaxial compressive strength of sandstone at temperatures below 400 \({}^{\circ}\)C increases with temperature, and when the temperature rises to 600 \({}^{\circ}\)C, the strength of sandstone decreases (as shown in this paper for \(\beta\) = 0\({}^{\circ}\) sandstone). 2 Under heating conditions of 200 \({}^{\circ}\)C, the uniaxial compressive strength of sandstone is obviously improved, and the strength gradually decreases with increasing temperature (as in this paper for \(\beta\) = 45\({}^{\circ}\) sandstone). 3 The uniaxial compressive strength of sandstone increases at 200 \({}^{\circ}\)C, decreases slightly at 400 \({}^{\circ}\)C, and increases again at 600 \({}^{\circ}\)C (\(\beta\) = 60\({}^{\circ}\) and 90\({}^{\circ}\) sandstone).  
Through the extraction and collation of the test data available in previous studies and the comparison with the experimental results in this paper, we found that the change laws of the sandstone's longitudinal wave velocity, uniaxial compressive strength, and elastic modulus with temperature reported by different scholars can find a better corresponding relationship with the change trend summarized in this paper. The specific results are shown in Table 2. Therefore, we believe that in the various studies on sandstone longitudinal wave velocity, the difference in uniaxial compressive strength and elastic modulus with temperature is caused by the different angles between the sampling direction and bedding plane, and the bedding angle affects the change trend of these physical and mechanical indices.