# Rock Indentation Behavior: Effects of Penetration Rates and Indenter Types
## 2 Methodology
### Experimental Procedure  
Rock indentation tests under various loading conditions were conducted under a conical indenter with a loading rate of 1 mm/min to study the phases of the rock indentation process. Subsequently, 18 samples of each rock type were tested using both conical and rounded blunt indenters at loading rates of 1 mm/min, 2 mm/min, 3 mm/min, and 4 mm/min. For each loading condition, three specimens of each rock type were tested as follows. _Step 1: Setting up the testing apparatus:_  
To ensure the accuracy of the test results, all equipment was inspected and calibrated, with particular attention paid to the sensors and the high-speed camera. The displacement sensor was adjusted to remain perpendicular to the horizontal rod beneath the rock sample's support layer. The indentation testing machine was configured to allow free adjustment of the indenter position, while the high-speed camera was set up to clearly record the entire rock fracturing process. \begin{table}
\begin{tabular}{c c c c} \hline
**Rock Types** & **Artificial Rock** & **Limestone** & **Granite** \\ \hline \(\sigma\)c (MPa) & 19.57 & 69.52 & 142.55 \\ \(\sigma\)t (MPa) & 2.21 & 7.52 & 10.07 \\ BI (\(\sigma\)c/\(\sigma\)t) & 8.85 & 9.24 & 14.2 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical properties of rock specimens. _Step 2: Indentation test preparation:_  
Rock specimens were first categorized and labeled according to the test conditions. The indentation testing apparatus was then powered on, and all components were fine-tuned. The axial loading device and the frame of the indentation testing machine were adjusted according to the height of the specimens. Using the instrument control system, the position of the indenter was precisely aligned with the center of the rock specimens, and all sensor readings were zeroed. The parameters of the high-speed camera, including lighting and angle, were optimized to ensure optimal recording conditions. A preliminary indentation test was conducted on a test specimen to verify the apparatus setup, during which the equipment parameters were adjusted according to the observations and results. _Step 3: Rock indentation process identification:_  
For each type of rock specimen, indentation tests were conducted using a conical indenter with a loading rate of 1 mm/min. Monitoring was stopped once visible fracturing occurred in the rock. The fractured rock chips were then collected, measured, and analyzed. _Step 4: Rock indentation tests with different loading conditions:_  
For each type of rock specimen, indentation tests were conducted using conical and rounded blunt indenters at loading rates of 1 mm/min, 2 mm/min, 3 mm/min, and 4 mm/min. Three tests were performed for each combination of rock type and loading condition. Monitoring was stopped upon visible fracturing of the rock. The fractured pieces and particles were then collected, measured, and analyzed. After each test, the platform was cleaned in preparation for the next experiment. _2.4. Indices for Indentation Evaluation_  
During the rock indentation process, the accumulated strain energy is released in various forms, including thermal energy, kinetic energy, surface energy, and radiation energy. Among these, approximately 90% of the energy is released as surface energy, which contributes to the generation of new rock surface area [32; 33; 34]. The newly generated surface area of the rock is widely recognized as an effective index for evaluating rock fracturing. In addition, the maximum indentation pressure and the maximum indentation depth are also frequently used as criteria for assessing rock fracturing [16; 35]. Therefore, this study uses the newly generated surface area of the rock, the maximum loading pressure, and the maximum indentation depth as quantitative indices for indentation evaluation.