## 5 Discussion  
The initiation and propagation of microcracks in coal rock are the main reasons for the final failure. Besides, the increase of shear stress component accelerates the initiation and propagation of microcracks, reduce their peak strength, and promote the deformation and failure of samples [51]. From the experimental results, when the inclination angle changes from 0\({}^{\circ}\) to 5\({}^{\circ}\), the peak strength of the sample increases slightly. This is mainly because the strength performance of coal and rock is closely related to the external loading conditions and properties of the material itself. When the specimen is loaded at a low inclination angle, the bonding and friction effects between the particles inside the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Inclination Angle (\({}^{\circ}\))** & **Tensile Crack/Total Crack (\%)** & **Shear Crack/Total Crack (\%)** \\ \hline
0 & 69.56 & 30.44 \\
5 & 67.46 & 32.54 \\
10 & 62.75 & 37.25 \\
15 & 48.35 & 51.65 \\
20 & 45.05 & 54.95 \\
25 & 39.81 & 60.19 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Proportion of tensile and shear cracks under different inclination angles at loading rate of 10 mm/min. Figure 15: The proportion of tensile and shear cracks under different inclination angles at loading rate of 10 mm/min. material are slightly improved compared with that under the vertical loading [52]. However, the shear stress component of the sample at the inclination angle of 5\({}^{\circ}\) is small, ranging from 0.7-1.05 MPa. Although it promotes the initiation and propagation of microcracks in the specimen, its effect cannot overcome the bonding and friction effects between solid coal and rock particles around the cracks. As a result, the peak axial stress increases slightly. With the increase of the inclination angle of the specimen, the shear stress component increases gradually. At this time, the promoting effect of shear stress on the crack fracture is greater than the bonding and friction between the particles inside the specimen, leading to the deformation and failure under the low vertical load. [PERSON] [41] and [PERSON] [49; 53] carried out similar tests based on granite and basalt samples by this experimental system, and reported that the strength and elastic modulus of rock gradually decreases with the increase of the inclination angle. This conclusion is slightly different from the results obtained in this study. The main reason is as follows: the peak strength of granite and basalt is greater than 100 MPa (hard rock), while the strength of coal is less than 20 MPa (soft rock) under the conventional uniaxial compression. For hard rock, when the inclination angle is increased to 5\({}^{\circ}\), the generated shear stress component is 9.1-9.5 MPa [41]. As a result, the effect of shear stress on crack propagation is greater than the bond and friction between particles in the sample, and the peak strength of the sample decreases with the increase of inclination angle. Many studies have shown that the compressive strength and elastic modulus of rock are not inherent properties of rock, but depend on the external environment and loading modes [54; 55], [PERSON] [51] pointed out that the failure direction of rock sample is parallel to the direction of maximum principal stress. When the axial direction of the specimen is parallel to the loading direction of the testing machine, the split failure in the axial direction is the main failure mode of the specimen. When the axial direction of the specimen has a certain angle with the loading direction of the testing machine, the shear failure of the specimen is the main failure mode. The larger the inclination angle, the more significant the trend of shear failure. This conclusion is consistent with that in the study conducted by [PERSON] [41] and [PERSON] [49; 53]. The failure process of coal and rock under load is essentially a process in which the tensile and shear cracks cross each other and extend to the surface of the sample.