# Investigation of Rock-Breaking Mechanisms Based on the Adaptive Matching Method for Drilling Loads
## 3 Results and Discussion
### Effect of the Initial Axial Force  
In this study, the initial axial force (equivalent to the weight on bit, or WOB, which acts on the PDC drill bit while drilling in bottomhole) is used to characterize the applied initial axial force, indicating the applied axial force at the beginning of the simulation. This parameter directly dictates the initial cutting depth and corresponding reaction force profile, thereby influencing both the cutting velocity and displacement progression. Figure 8 shows the results of the kinematic behavior and reaction force characteristics of the PDC cutter under varying initial axial forces, ranging from 350 N to 550 N, at a fixed load factor of 0.6. Additionally, Figure 9 presents the corresponding rock fragmentation characteristic parameters throughout the simulation period.  
Under the condition of the same driven force acting on the PDC cutter, the variation frequency of the axial force and cutting displacement exhibited minor differences throughout the simulation period. Compared to an applied axial load of 400 N, the PDC cutter achieved greater displacement under an axial load of 600 N. This is because higher axial force increases the cutting depth during rock breaking, leading to elevated reaction force on the cutter. These amplified forces induce temporary acceleration followed by cutter seizure. Once seized, displacement progression halts while the applied axial load is dynamically adjusted.  
Figure 8: Motion and reaction force of the PDC cutter under different initial WOBs.  
As shown in Figure 8a,b, when the applied axial force (denoted WOB in the figure) reaches 550 N, the PDC cutter experiences minimal displacement and seizes earliest in both simulation time and displacement progression. Similarly, reaction forces in the X (cutting direction) and Z (axial direction) directions mirror the displacement trends. Specifically, Figure 8c,d reveal that upon cutter seizure, the X direction's reaction force exhibits regular oscillatory behavior, whereas the axial reaction force fluctuates irregularly.  
The PDC cutter could become stuck during simulation under the action of different initial axial forces, leading to a distinct rock fragmentation state and cutting displacement characteristics. Figure 9 demonstrates that the total number of cracks, the maximum displacement, and the work power monitored in the simulation process firstly increase and then decrease while increasing the initial axial force, whereas MSE exhibits the opposite tendency.  
Specifically, when the initial axial force is below 450 N, the maximum displacement in the X direction reaches the programmed limit of 140 mm. At 450 N, both work power and the number of total cracks attain peak values, indicating optimal rock failure efficiency. A higher MSE corresponds to greater energy consumption, while more cracks signify more complete rock fragmentation. Figure 9 further reveals the minimum MSE occurs at an axial force of 500 N, though the maximum displacement remains below 140 mm at this load due to cutter seizure upon encountering large particles or clusters.  
Based on the above parametric analysis, an initial axial force of 450 N acting on the PDC cutter was selected in this study for the subsequent simulation. This kind of setting ensures complete simulated rock destruction while minimizing energy consumption and maximizing crack generation during the rock-breaking simulation.