# Rock Indentation Behavior: Effects of Penetration Rates and Indenter Types
## 3 Results and Discussion
### Effect of Loading Condition on Rock Indentation  
Figure 6 presents the effects of loading rates on the fractured area, peak force, and maximum indentation depth under different loading conditions. Accordingly, it is evident that the rock fractured area subjected to the spherical indenter is significantly larger than that of the rock under the conical indenter. The time required for the rock to fracture under the spherical indenter is also shorter, indicating a higher fracturing rate. However, the indentation depth of the spherical indenter is less than that under the conical indenter. Furthermore, the peak force required to fracture the rock with the spherical indenter is significantly higher than that with the conical indenter under the same loading rate. According to rock fracture mechanics, during the rock indentation process, the rock accumulates strain energy due to the compressive forces. This strain energy is instantaneously released during the rock fragmentation stage, primarily used to create new fractured surface areas. The larger the loading pressure, the greater the amount of strain energy that can be released, leading to a larger fractured surface area and a higher degree of rock fragmentation. The conical indenter can repeatedly compress and penetrate the rock, which results in a reduction of the peak force. Meanwhile, cracks tend to propagate along fracture planes that consume less energy, resulting in a larger compression depth but a relatively smaller degree of rock fragmentation. Conversely, the spherical indenter undergoes only one compression and fragmentation stage and is unable to continuously penetrate the rock, resulting in a higher peak force and a smaller indentation depth. Additionally, the increased peak force contributes to a larger rock surface area.  
Figure 6: _Cont._The loading rate can be considered an important factor affecting rock-breaking efficiency, showing a linear relationship with fracture area and peak force. A higher loading rate leads to more energy input into the rock mass per unit time, causing a faster rate of internal crack formation and resulting in a larger newly fractured area during rock fragmentation. Additionally, the effect of rock brittleness on rock indentation is demonstrated in Figure 7. Rock brittleness can affect the correlation between the penetration rate and maximum indentation depth. Furthermore, higher rock brittleness results in a greater  
Figure 6: Influence of loading rate on rock fragmentation: (**a**) loading rate—fracture area; (**b**) loading rate—indentation depth; (**c**) loading rate—peaking pick force.  
degree of rock fragmentation under the same loading rate. Research by [PERSON] [36] and [PERSON] [37] has found that during the process of rock indentation, the propagation of rock cracks is controlled by the tensile strength of the rock. When the released strain energy exceeds the rock's tensile capacity, the higher the rock brittleness, the easier it is for the cracks to propagate.  
Furthermore, the relationship between the maximum pressure obtained from the rock indentation tests and the fractured area (Figure 7) demonstrates a strong correlation between the maximum loading pressure and the rock fractured area during the penetration process (R\({}^{2}\) = 0.719). Moreover, the greater the brittleness of the rock, the greater the dispersion of this fitting relationship.  
Therefore, under the same loading conditions, rock fracturing efficiency and the degree of fragmentation under the spherical indenter are both greater than those under the conical indenter. Correspondingly, the peak loading force required by the spherical indenter is also higher. Additionally, both higher loading rates and greater rock brittleness contribute to a higher degree of rock fragmentation. Hence, loading rates, pick type, and rock brittleness collectively affect rock fragmentation efficiency.