## 3 Results and Analysis
### Crack Propagation  
The typical fracture conditions in the internal rock are shown in Figure 9a, which consists mainly of three parts. The first part is the plastic zone below the cutter, whose typical shape, which is essentially an inverted triangle, is shown in Figure 9b. The second part is the crushing zone (the yellow outlines in Figure 9a formed between the two plastic zones, as shown in Figure 9c. The third part is the relatively intact rock mass formed by the internal cracks, as shown in Figure 9d.  
Figure 10 shows the typical development of surface cracks. As can be seen, surface cracks can be divided into two types, depending on the locations of the surface cracks. The first type is the surface crack between the two cutting grooves, named surface crack A. It is generated after two indentations, usually starting from the edge of one groove and developing to the other, and its development is  
Figure 8: The distribution of AE amplitude in a range of 40–100 dB: **(a)** the first indentation process, **(b)** the second indentation process.  
Figure 9: Typical fracture conditions. **(a)** fracture condition; **(b)** typical plastic zones; **(c)** crushed zone; **(d)** intact rock mass.  
directly related to the fragmentation of the rock between the cutters. Therefore, the research on its development degree and path is very important. As shown in Figure 10, apart from surface crack A, there is another type of surface crack on the surface of the specimen, which is named surface crack B. Normally, surface crack B starts from the edge of one groove and develops toward the edges of the specimen.  
As shown in Figure 11, when the water content was 0, there were two cracks between the grooves, but they were not completely coalesced. The rock mass between the two cutting grooves was not highly fragmented. When the water content was 0.11%, two cracks appeared between the grooves, one of which was connected to the two grooves by mutual coalescence. When the water content increased to 0.22%, there were three cracks between the grooves, all of which were coalesced with the two grooves but relatively dispersed. When the water content was 0.32%, there were five cracks between the grooves, all of which were coalesced with the two grooves, and the rock mass between the grooves was highly fragmented. When the rock specimen was saturated, the rock mass was severely broken between the grooves, forming eight cracks that coalesced with the grooves. Due to the sufficient development of the cracks between the grooves, the rock mass between the grooves was fully broken. Therefore, with the increase of the water content, the fragmentation of the rock mass between the grooves gradually increased after the indentation.  
Based on the development of the internal cracks mentioned above, it can be seen from the typical failure and crack development profile that the internal cracks of the inter-groove rock mass start from the adjacent plastic zones and coalesce with each other, and then, the surface cracks segment the rock mass, forming a complete rock chip. Thus, the contour of the crushing zone between the cutters is determined jointly by the surface cracks and the internal cracks of the rock mass.  
Figure 10: Typical surface cracks.