# Rock Indentation Behavior: Effects of Penetration Rates and Indenter Types
## 3 Results and Discussion  
_3.1. Rock Indentation Process_  
It is essential to understand the key characteristics of the rock fragmentation process (rock fragmentation stage and crack propagation stage) to gain a deeper understanding of the rock fragmentation mechanism during the indentation process. Figure 2 shows the force-penetration curves under different loading conditions. Accordingly, a blunt indenter requires a significantly greater rock-breaking force compared to a sharp indenter, with a larger chip-fractured area. In addition, the required rock-fracturing force decreases as the rock brittleness decreases. During the rock indentation process with a sharp indenter, rock indentation involves multiple cycles of repeated compression and fracturing processes, with the number of cycles increasing as the rock brittleness rises. In contrast, a blunt indenter undergoes only a single compression and fracturing process. The force-displacement curves of the sharp indenter show a comparatively gentle transition, indicating that rocks can be fractured with a relatively low force but result in significant penetration displacement. Conversely, the force-displacement curves of the spherical indenter demonstrate a sharper inclination, implying that a higher force is required for rock fracturing, accompanied by a comparatively reduced indentation displacement. Figures 3 and 4 show the rock indentation process under different indenters, as recorded by the high-speed camera. Representative moments of each stage are selected and marked in the lower right-hand corner of Figures 3 and 4. Accordingly, it is evident that the rock indentation process using a sharp indenter can be divided into the rock compression stage, crack propagation stage, and fragmentation stage. In contrast, in the indentation process of a blunt indenter, there is the absence of a pronounced crack propagation stage; instead, rock failure occurs instantaneously after the compression stage. Figure 3: Rock breaking stage—sharp indenter. (**a**) Limestone sharp indenter; (**b**) granite sharp indenter. Figure 2: Force–penetration curves with different loading conditions in indentation tests. _Stage 1: Rock compression stage_  
The uneven compression stress distribution induced by the indenter escalates with the increment of the applied force. When the stress surpasses the rock's shear strength, it precipitates the formation of a multitude of rock chips and the appearance of internal cracks. As the indenter continues to penetrate the rock, the rock undergoes multiple cycles of compression and shear fragmentation. In addition, the number of these cycles is entirely determined by the rock's brittleness. The upper boundary of the force-penetration curves marks the end of the compression process, while the lower boundary signifies the end of the rock's shear fragmentation and the beginning of the next compression stage. These rock chips are mostly fine powder-sized particles, with only a few larger rock debris (approximately 2-3 mm in diameter). The compression stage of the blunt indenter endures a longer duration compared to that of the sharp indenter. In addition, there are no rock chips resulting from shearing; rather, the process generates smaller rock debris due to compressive failure. _Stage 2: Crack propagation stage_  
Cracks initiate in the core fragmentation zone and propagate with each release of compressive strain energy during the multiple cycles of compression and fragmentation stages. In the final compression-fragmentation stage, the cracks reach their maximum extent, and two distinct cracks can clearly be seen converging at a single point on the rock surface. Therefore, the crack propagation stage extends from the end of the first compression phase to the end of the last compression phase.