## 6 Conclusions  
In this paper, the self-developed C-CAST system is introduced, and the stress-strain curve characteristics, mechanical properties and macro failure mode of coal samples under different loading rates and inclination angles are studied. Based on acoustic emission technology, the distribution of tension and shear cracks in coal samples after loading is determined. The main conclusions are summarized as follows:  
* The combined compression and shear loading experimental system is developed, the specific assembly steps and use methods of the system are described, and the strength calculation principle of coal samples under the coupled compression-shear loading is introduced. Compared with the traditional uniaxial compression experimental system, the system can realize uniaxial loading of coal samples under the inclined condition. As a result, the sample is in the coupled compression-shear loading mechanical state, and the mechanical status of inclined pillars affected by coupled compression-shear loading can be effectively reflected.
* With the increase of inclination angle, the peak shear stress of coal samples increases gradually, while the peak axial stress and elastic modulus slightly increase and then decrease, and reach the maximum value at the inclination angle of 5\({}^{\circ}\). Within the inclination angle range of 0\({}^{\circ}\) to 15\({}^{\circ}\), with the increase of loading rate, the peak shear stress, peak axial stress and elastic modulus of coal sample first increase and then decrease; the loading rate corresponding to the maximum point decreases with the increase of inclination angle. Within the range of 20\({}^{\circ}\) to 25\({}^{\circ}\), the peak axial stress and elastic modulus gradually decrease with the increase of the loading rate. It indicates that the shear stress component can weaken the strengthening effect of the loading rate on the strength of coal samples.
* The influence of inclination angle and loading rate on the failure characteristics of coal samples is significant. With the increase of the inclination angle, the failure mode is transformed from tensile splitting failure (0\({}^{\circ}\)-5\({}^{\circ}\)), tension-shear composite failure (10\({}^{\circ}\)) to single shear failure mode (15\({}^{\circ}\)-25\({}^{\circ}\)); the loading rate has little effect on the failure mode, but has a significant effect on the failure degree.
* When the loading rate is 1.0 and 10 mm/min, within the inclination angle of 0\({}^{\circ}\)-5\({}^{\circ}\), the proportion of tensile cracks after coal samples failure is significantly greater than that of shear cracks; the tensile failure is the main failure mode of the samples. Within the 10\({}^{\circ}\)-25\({}^{\circ}\), the proportion of shear cracks increases gradually with the increase of inclination angle, and both of them are more than 50%. The shear failure is the main failure mode. This law is consistent with the macro failure mode of the samples.  
Conceptualization, Y.C. and P.W.; methodology, H.C. and H.P.; data curation, L.C.; writing--original draft preparation, P.W. and K.Z.; writing--review and editing, Y.C. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by National Natural Science Foundation of China (Grant Nos. 51974295, U1803118, 51974296).  
The authors declare no conflict of interest.