## 4 Analysis of the Rock Breaking Process  
### Force-Penetration Depth Curve and AE Events  
The force-penetration depth curves and acoustic emission (AE) events caused by five types of carbide buttons are depicted in Figure 8. In the spatial distribution graph of the AE events, the size of points represents their energy magnitude, while the color indicates the penetration depth at which they formed. The force-penetration depth curves for all five buttons exhibited a consistent trend of multiple loading-unloading cycles. During each loading-unloading cycle, the number of AE events gradually increased and reached a peak when the peak force suddenly decreased. Take the conical button as an example. The indentation force gradually increased as the penetration depth increased during stage I. It corresponded with the closure of pre-existing microflaws, and only a few AE events were detected. In stage II, the indentation force increased approximately linearly with the penetration depth. The number of AE events also increased gradually, mainly resulting in microcracks. The rock deformed in an elastic-plastic manner. In stage III, the indentation force increased nonlinearly with the increase of penetration depth, indicating that the rock exhibited anisotropic characteristics. The indentation force fluctuated until it reached the peak value, and the rock was severely damaged. During this stage, a large number of AE events were generated. Subsequently, the indentation force dropped sharply and entered unloading stage IV, accompanied by a large number of AE events. The particles at the contact between the button and the rock were completely destroyed, forming a crushed zone. When reaching stage V, the load transfer capacity of the crushed zone was almost zero, and the indentation force increased very slowly until the rock was recompacted and entered a new loading and unloading cycle. Due to the loss of symmetry in the stress distribution under the carbide button after the rock was damaged, the fluctuation of indentation force was more obvious in the new cycle. The peak forces of the saddle, spherical, wedge, parabolic, and conical buttons were 135, 118, 116, 53, and 49 kN, respectively. This indicated that the saddle button required the maximum thrust provided by the equipment during rock breaking.  
Figure 8: \(Cont\).  
Figure 7: Crack propagation mode after the spherical button penetrated rock for 3 mm.  
The contact between conical, spherical, and parabolic buttons and rocks is point contact. They have the same AE event distribution pattern, i.e., a hemispherical high-density AE event aggregation zone was formed beneath the button during the first loading and unloading cycle. New AE events subsequently occurred uniformly outside the zone as the carbide buttons continued to penetrate into the rock. Among these three buttons, the spherical button produced the largest AE zone range and most AE events during the first loading and unloading cycle. This is because its profile has the largest curvature, which allows it to penetrate the largest volume at the same depth. The contact between the  
Figure 8: The force-indentation depth curve and acoustic emission (AE) events.  
wedge button and the rock can be described in a linear manner. AE events first occurred at the chamfers on both ends of the button, and a slit-shaped high-density AE events aggregation zone was formed beneath the wedge button during the first loading and unloading cycle. New AE events in the following loading cycles mainly occurred beneath the zone. For the saddle button, its contact with the rock started at point contact and gradually transformed into an arc-shaped line contact as the penetration depth increased. AE events first occurred at the contact point between the saddle button and the rock, and a slit-shaped high-density AE events aggregation zone was formed beneath the saddle button in the first cycle. Different from other buttons, new AE events mainly occurred at both ends of the slit-shaped AE zone.