## 3 Experimental Results and Discussion
### Analysis of Crack Propagation Characteristics  
To analyze the crack propagation characteristics induced by joints, detailed investigations were conducted at locations where large rock chips formed during cutting. Large rock chips typically form when the _FC_ peaks [36]. In this study, plots of cutting force versus cutting distance were synchronized with the 3D-scanned cutting grooves to identify the positions where _FC_ peaked. At these peak points, transverse profiles were extracted to analyze the patterns of crack propagation. The analysis focused on cutting cases with joint spacings of 60 mm and 90 mm and cutting depths of 6 mm and 9 mm, considering \(d\) ranging from 0.1\(J_{s}\) to 0.2\(J_{s}\). This range was selected based on the findings in Section 3.2 to investigate variations in crack propagation before the fragmentation zone becomes confined by the joint plane.  
To interpret the observed crack behavior, it is useful to refer to [PERSON] [39]. He found that, in rock cutting using point-attack picks such as conical picks, cracks typically initiate at the tip of a pick and propagate toward the free surface, forming a V-shaped groove (shown as a black dotted line in Figure 13). He also observed that the fracture surfaces form an included angle (\(\Phi\)) relative to the central vertical axis of the groove.  
Figure 13 illustrates three types of crack propagation patterns observed in jointed rock cutting. The first pattern involves a crack that initiates at the pick tip and terminates at the joint plane without reaching the free surface (1 in Figure 13). This behavior is clearly observed at \(Y=118\) mm in Figure 14c and at \(Y=69\) mm in Figure 15d. This observation is consistent with the findings of [PERSON] et al. [7] and [PERSON] et al. [17], who noted that crack propagation tends to terminate upon encountering a joint plane. The second pattern features a crack that propagates through the joint plane and continues to the free surface on the opposite side (2 in Figure 13), as seen in Figure 15c. [PERSON] et al. [40] noted that cracks may continue to propagate beyond thin joints, though their length is typically reduced by the presence of the joint. This observation is consistent with this study, where the crack extends past the joint plane but terminates earlier than in intact rock. The third pattern shows a crack terminating at a free surface just ahead of or within the joint plane, without crossing it (3 in Figure 13). This pattern is exemplified in Figure 14b and at \(Y=140\) mm in Figure 15b.  
These crack propagation patterns near the joint plane suggest that the reduction in \(FC_{m}\) may be attributed to shorter crack propagation lengths caused by joint interference. In the first pattern, the crack follows the shortest path to the joint plane. In the second and third patterns, although the cracks reach the free surface, the \(\Phi\) becomes smaller compared to that in intact rock, further reducing overall propagation length.  
Figure 14: Crack profiles at \(p=6\) mm: (a) \(J_{s}=60\) mm, \(d=6\) mm; (b) \(J_{s}=60\) mm, \(d=12\) mm; (c) \(J_{s}=90\) mm, \(d=9\) mm; (d) \(J_{s}=90\) mm, \(d=18\) mm.  
Figure 13: Crack propagation patterns in jointed rock mass cutting compared with the theory of [PERSON] [39] in intact rock.