# Rock Indentation Behavior: Effects of Penetration Rates and Indenter Types
## 1 Introduction  
Conical picks are widely used in coal mining machines and roadheaders [1, 2]. A proper understanding of the rock fragmentation mechanism of conical picks could effectively improve rock-breaking efficiency and guide the arrangement, sizing, and other aspects of the clutter head design [3, 4]. Rock indentation tests are commonly employed to investigate rock failure mechanisms due to their simplicity and effectiveness in simulating rock fracture processes compared to complex conical picks. Therefore, a significant amount of research has focused on the rock-breaking process, theoretical models of rock breakage, and the prediction of rock-breaking parameters with indenters [5, 6, 7]. Rock indentation tests are typically simplified to wedge-shaped or other generalized indenters that penetrate the rock surface. This enables a quantitative description of the rock indentation behavior of indenters, such as nanoindentation testing at a nanometer grain scale, point load testing at a centimetric grain scale, and indenters at an engineering scale [8, 9, 10, 11]. The indenter initially establishes an effective stress field on the rock surface. When the applied stress exceeds the rock's compressive strength, it results inthe formation of a fractured core zone and rock chips. Under applied load, the indenter penetrates the rock surface, forming pits [12]. Simultaneously, internal cracks develop at the edges of the crushing core zone due to tensile stresses. Rock fragmentation occurs when the input energy exceeds the rock's total strain energy. The compression and fracturing phases occur cyclically, with 70-85% of the penetration energy consumed during the compression phase [13]. Most researchers have focused on the rock fragmentation process and phenomena to investigate the fragmentation mechanisms. For example, [PERSON] [14] compared the indentation test results of glass, ceramics, and siliceous shale, identifying crack propagation patterns during the rock indentation of siliceous shale. In addition, others have studied how tool parameters, rock mechanical properties, and confining pressure affect rock indentation, analyzing correlations between tool parameters (e.g., bit type, bit angle, and indentation depth) and rock mechanical characteristics [15; 16; 17; 18; 19]. [PERSON] conducted a study on the percussive drilling of Red Bohus granite using conical tools [19]. The research reveals the effects of bit weight, confining pressure, and impact energy on the drilling process. Meanwhile, it is also of utmost importance to determine the value of the thrust increment required to initiate chip formation (only as an order of magnitude) [20]. However, laboratory studies primarily capture the surface features of rock indentation, making it challenging to observe internal crack propagation and fracture evolution patterns. Consequently, some researchers have employed numerical simulation methods to visualize crack propagation and fracture evolution during rock indentation [21]. These studies also explored the effects of factors such as the loading rate, confining pressure, and temperature on rock indentation and validated their findings against physical experiments [22]. In terms of rock indentation theoretical models, the rock cavity expansion model is considered the most classical model. [PERSON] [8] proposed a modified rock penetration and fracture cavity expansion model using linear fracture mechanics and provided formulas for calculating the radius of the plastic deformation zone and the crack length. Based on this model, [PERSON] [23] and [PERSON] [24] identified key factors (rock strength parameters, indenter geometry, and confining pressure) influencing rock fracturing during penetration by wedge-shaped indenter experiments. However, this cavity expansion model is a continuous analytical model based on elastoplastic deformation theory, which is not entirely applicable to brittle materials like rock.