## 4 Discussion
This behavior is caused by  
Figure 14: (**a**) Different failure patterns observed inside the rock samples and the failure mechanisms of rock under water jet impact: (**b**) shock stress wave effect, (**c**) pressured water wedge effect, and (**d**) water flow scouring effect [52; 53; 54; 55]. the fact that the shear stress in the contact region of water jet-coal exceeds the coal fracture strength, owing to the extremely high \"water-hammer\" pressure. This will lead to tremendous damage in the coal, as revealed in the SEM image by [PERSON] et al. [52], which is beneficial to generating the powder particles. The coal particles are then washed away by the water flow, forming a broken pit. In addition, the impact stress waves will also transmit inside the coal, thus generating internal cracks including circumferential fractures, radial fractures and conical fractures. As illustrated in Figure 14a, the former two kinds of fractures usually emerge surrounding the broken pit with a high density of small fractures. Owing to the combination of the pressureed water wedge effect and water flow scouring effect, fine coal particles may be formed in this zone. Please note that coal has abundant joints and fissures (Figure 15). The coal mass will be broken severely because of the interference and reflection of waves. For the conical fractures, they initiate below the broken pit and expand outward. When these fractures interconnect with other cracks or inherent fissures in coal, large macroracks will be generated. Owing to the presence of the pressureed water wedge, volume split fragmentation occurs, forming fragments with large sizes, as marked with 1, 2 and 3 in the gray-shaded areas. Because the jet impact velocity has a significant influence on the coal failure patterns, there are differences in the size distributions of coal fragments under the various jet velocities. When the jet ejects from the bit, the kinetic energy can be expressed by Equation (10). The impact energy is proportional to the cube of the jet velocity. When the jet velocity increases from 219 m/s to 237 m/s, the impact energy will increase by 27%. In addition, the \"water-hammer\" pressure and the stagnation pressure substantially increase. As a result, the internal particles of coal will endure higher shear stress and tensile stress, causing more damage and higher fragmentation of coal. [PERSON] et al. [52] found that the number of cracks increased, while [PERSON] [56] found that the secant angle of conical fractures decreased with the increase in jet velocity. Therefore, the percentage of small coal fragments will increase as the jet velocity increases, leading to a decrease in the dominant fragment size. \[\omega=\frac{\rho_{w}\pi R^{2}v^{3}}{2} \tag{10}\]  
Figure 15: Schematic diagram of cleats and fracture structure in coal and rock mass. where \(\omega\) is the kinetic energy of the water jet and \(R\) is the diameter of the water jet. The coal strength also has an important influence on the coal fragmentation under water jet impact in WJD. On the one hand, coal with higher strength, such as a high compressive strength or shear strength, has a stronger ability to resist destruction under the same impact energy loading. The damage area is also smaller compared with low-strength coal. On the other hand, fewer inherent fissures usually occur inside coal with a higher strength, weakening the interference and reflection of stress waves. As a result, the fragmentation degree of coal subjected to water jets in the M7 coal seam is lower than that in the M8 coal seam, which results in a large dominant fragment size and small fractal dimension when the impact energy is the same.