## 5 Results and Discussion  
### Comparison of Rock-Breaking Mechanism of 3-RDE Cutter and Conventional Cutter  
In this section, the rock breaking process with the conventional cutter and 3-RDE cutter is analyzed, respectively. The cutting depth is set as 1.5 mm. It can be obviously seen from Figure 7 that the cutting forces and their fluctuations (representing as the error bars in Figure 7a,b), as well as MSE, of 3-RDE cutter are significantly smaller than that of conventional cutter. This indicates that 3-RDE cutter can achieve a better performance than the conventional cutters, representing as a stronger ability to penetrate into the formation, less torque required, more stable cutting behavior, and higher rock breaking efficiency. The service life is one of the important factors that determine the performance of the cutter. To some extent, the contact stress of the cutter can be employed to assess the wear of cutters [20; 21]. Figure 8 shows the contact stress nephogram of conventional cutter and 3-RDE cutter. It can be seen that the contact stress of the cutters only exist in the region where the cutters interacts with the rock, while the stress in other regions is zero. However, there are significant differences in the distribution of the contact stress in the cutter-rock interaction region. For the conventional cutter, the contact stress concentrates on the bottom of the outer edge, while the contact stress, for the 3-RDE cutter, mainly distributes in the region nearby the convex ridge, forming a \"convex ridge effect\". Moreover, the amplitude of the contact stress of 3-RDE cutter is just about 0.5 times of that of conventional cutter, indicating that the 3-RDE cutter can significantly reduce the contact stress. This is one of the reasons why the 3-RDE cutter has higher wear resistance than the conventional cutter. Figure 6: Comparison between the numerical and experimental results: **(a)** The axial force; **(b)** The tangential force; **(c)** The Mechanical Specific Energy (MSE). Figure 7: Comparison of cutting forces and MSE between conventional cutter and 3-RDE cutter: **(a)** F\({}_{n}\); **(b)** F\({}_{h}\); **(c)** MSE. Figure 9 shows the plastic strain nephogram during rock breaking with the conventional cutter and 3-RDE cutter. By plotting the plastic strain on the un-deformed rock shape, one can figure out that for the 3-RDE cutter the characteristics of the plastic deformation zone of the rock are significantly different from those of the conventional cutter. The former consists of an elongated \"red\" zone and a wide \"green\" zone, where the plastic strain in the \"red\" zone is greater than that in the \"green\" zone (Figure 9b). The reason for this phenomenon is that the 3-RDE cutter interacts the rock with a similar \"V\" shape, and the convex ridge is the first spot that contacts with the rock. Then, a large local concentrated force is generated around the tip area (the red point in Figure 10b), which is higher enough to cause plastic deformations of the rock. Finally, a local fracture is formed, releasing the rock's internal stress. As the 3-RDE cutter continues to move forward, the working faces on both sides gradually come into contacting with the rock, and bevel the rock. Consequently, under the combined action of crushing and shearing, the rocks in front of the working faces are rapidly broken, eventually forming the combined plastic deformation zone in Figure 9b. In contrast, the fractured area caused by conventional cutter almost entirely turns on the \"red\" region, and this region is significantly wider than that formed by 3-RDE cutter. Obviously, the peak of the plastic strain in Figure 9a is greater than that in Figure 9b. This can be attributed to the fact that the entire cutting area of the conventional cutter squeezes the rock synchronously (Figure 10a), and the maximum stress is located at the outer edge of the cutter, where the rock gets damaged due to shear stress.