## 3 Test Results
### Change in Compressive Strength and Elastic Modulus
The elastic modulus was calculated according to the stress-strain curve of the mechanical compression test for each sample. The average elastic modulus was used as the elastic modulus, that is, the slope of the nearly straight line (elastic stage) on the stress-strain curve of the sample. As shown in Figure 9a, the variation trend of the elastic modulus of sandstone with different bedding angles can also be divided into three cases: (1) When the temperature is below 400 \({}^{\circ}\)C, the elastic modulus of sandstone decreases slightly at \(\beta\) = 0\({}^{\circ}\) and 30 \({}^{\circ}\)C. When \(T\) > 400 \({}^{\circ}\)C, the elastic modulus of sandstone starts to decrease, and the higher the heating temperature is, the faster the elastic modulus decreases. (2) When \(\beta\) = 45\({}^{\circ}\), the elastic modulus of sandstone is basically unchanged at \(T\) = 200 \({}^{\circ}\)C, while at \(T\) = 400 \({}^{\circ}\)C, the elastic modulus increases and is higher than that at room temperature. Similarly, when \(T\) > 400 \({}^{\circ}\)C, the elastic modulus of sandstone decreases, which can be observed in Figure 9a. In the temperature range of 400-600 \({}^{\circ}\)C, the decrease rate of the elastic modulus of \(\beta\) = 45\({}^{\circ}\) sandstone is higher than that at 600-800 \({}^{\circ}\)C, which indicates that more fractures developed and connected in the sandstone during the temperature rise of 400-600 \({}^{\circ}\)C [21]. (3) When \(\beta\) = 60\({}^{\circ}\) and 90\({}^{\circ}\), a heating temperature below 400 \({}^{\circ}\)C increases the elastic modulus of sandstone, which is more obvious at \(\beta\) = 60\({}^{\circ}\), and the elastic modulus increases by approximately 9% compared with that at room temperature. A heating temperature above 400 \({}^{\circ}\)C makes the elastic modulus of sandstone decrease rapidly, and the decreasing rate is higher than \(\beta\) = 0\({}^{\circ}\) and 30\({}^{\circ}\) because the matrix of sandstone is relatively thicker in the axial direction when \(\beta\) = 60\({}^{\circ}\) and 90\({}^{\circ}\). It can be inferred that in this experiment, the crack development and penetration phenomenon caused by high temperature exposure were more likely to occur in the matrix. Figure 8: (a) Change trend of \(\sigma_{ct}/\sigma_{c0}\) with temperature for rock samples with different bedding angles; (b) \(\sigma_{c}\) variation trend with dip angle for rock samples treated at different temperatures. Figure 9: (a) Variation trend of \(E_{t}/E_{0}\) with temperature for rock samples with different bedding dip angles; (b) variation trend of \(E\) with dip angle for rock samples with different temperature treatments. The change trend of the elastic modulus of rock samples with different bedding dip angles is relatively consistent with temperature, and it is generally shown that when \(T\) < 400 \({}^{\circ}\)C, the elastic modulus changes little and may increase, decrease, or remain unchanged. When \(T\) > 400 \({}^{\circ}\)C, the elastic modulus of rock samples with all bedding dip angles decreases rapidly. When \(T\) = 1000 \({}^{\circ}\)C, the elastic modulus only reached 30% at room temperature. Figure (b)b shows the change trend of the sandstone's elastic modulus with the dip angle at the same heating temperature level, and the minimum value of the elastic modulus usually appears at \(\beta\) = 45\({}^{\circ}\). When the heating temperature is 600 \({}^{\circ}\)C and below, the elastic modulus of the \(\beta\) = 30\({}^{\circ}\) and 60\({}^{\circ}\) rock samples is obviously higher than that of the rock samples with other bedding dip angles. When \(T\) > 600 \({}^{\circ}\)C, except for the small elastic modulus of sandstone at \(\beta\) = 45\({}^{\circ}\), the elastic modulus of rock samples with other bedding dip angles shows little difference.