## 3 Experimental Results Analysis
### Effect of Loading Rate and Inclination Angles on Peak Axial Stress  
Figure 9 shows the peak axial stress values of coal samples under different loading rates and inclination angles. For simplicity, only the data corresponding to the maximum of the variation coefficient under each inclination angle are given in the table. As shown in Table 2, the standard deviation and coefficient of variation of the experimental data obtained in this experiment are, respectively, within the range of 0.43-1.00 MPa and 10.12-11.13%, indicating the reliability of the experimental data [48]. Figure 9 shows the curve of average peak axial stress of coal sample changing with loading rate and inclination angle. As shown in Figure 9a, when the loading rate is constant, with the increase of the inclination angle of coal samples, the peak axial stress slowly increases and then gradually decreases, and reaches the maximum at the inclination angle of 5\({}^{\circ}\). For example, when the inclination angle increases from 0\({}^{\circ}\)-5\({}^{\circ}\) at the loading rate of 3.5 mm/min, the peak axial stress increases from 11.74 to 12.25 MPa with an increase of 4.34%; when the inclination angle increases from 5\({}^{\circ}\)-10\({}^{\circ}\), the peak axial stress decreases from 12.25 to 7.92 MPa, with a decrease of 35.35%. In Figure 9b, when the inclination angle is within 0\({}^{\circ}\)-15\({}^{\circ}\), the peak axial stress increases first and then decreases with the increase of loading rate. The larger the inclination angle, the lower the loading rate required for the sample to reach the maximum value. For example, when the inclination angles of coal samples are 0\({}^{\circ}\), 5\({}^{\circ}\), 10\({}^{\circ}\) and 15\({}^{\circ}\), the loading rates corresponding to the peak axial stress are 3.5, 3.5, 2.0 and 0.5 mm/min, respectively. When the inclination angle is within the range of 20\({}^{\circ}\)-25\({}^{\circ}\), the peak axial stress shows a nonlinear decrease with the increase of the loading rate. According to the variation characteristics of peak axial stress at the inclination angle of 0\({}^{\circ}\)-15\({}^{\circ}\), the following explanation is given: in the range of low loading rate, with the gradual increase of loading rate, the internal cracks of coal samples cannot be fully developed, which effectively reduces the weakening effect of failure propagation on coal samples strength. Therefore, the characteristics of the high loading rate and high strength are observed. However, only when the solid structure around the fracture in the coal rock has high bearing capacity, will the coal rock show high strength performance under the high loading rate, and the rock material meets this requirement. For coal samples with the low strength, the increase of loading rate restrains the initiation and propagation of cracks, but it greatly increases the load-bearing capacity of the solid structure around the cracks at the same time. As a result, solid structure around the cracks break prematurely due to the overload. Therefore, within the range of high loading rate, the peak axial stress of the sample decreases gradually. The difference of peak axial stress changing with the loading rate under different inclination angles is explained as follows: with the increase of inclination angle, the shear stress component also  
Figure 9: Variation law of peak axial stress of coal sample with loading rate and inclination angle: (a) Relationship between peak axial stress and inclination angle; (b) relationship between peak axial stress and loading rate. increases gradually, and the shear load is conducive to the initiation and propagation of micro cracks in the sample [40; 41; 49]. Therefore, the strengthening effect of loading rate on coal strength is weakened, and this effect is more obvious with the increase of inclination angle, until the strengthening effect of loading rate on the strength of the sample disappears completely.