# Experimental Study on Dynamic Mechanical Properties of Sandstone Under Different Damage Degrees
## 4 Specimen Preparation and Experimental Procedure  
In this study, red sandstone with high integrity and good homogeneity was used for both dynamic uniaxial and triaxial impact experiments. First, a vertical coring machine \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Part Name** & **Length (m)** & **Diameter (mm)** & **Material** & \begin{tabular}{c} **Density** \\ **(kg/m\({}^{3}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Poisson’s** \\ **Ratio** \\ \end{tabular} &
\begin{tabular}{c} **Elastic Longitudinal** \\ **Wave Velocity (m/s)** \\ \end{tabular} \\ \hline Incident bar & 2 & 50 & 40 Cr & 7697 & 0.28 & 5410 \\ Transmission bar & 1.5 & 50 & 40 Cr & 7697 & 0.28 & 5410 \\ Energy absorbing bar & 0.5 & 50 & 40 Cr & 7697 & 0.28 & 5410 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical parameters of each bar in SHPB apparatus. Figure 6: Large-diameter SHPB apparatus. was used to extract cores from a homogeneous sandstone block that exhibited no obvious defects or color variations. The diameter of the coring drill bit was 50 mm, and the core diameter was slightly less than 50 mm. The extracted cores were then cut into segments approximately 35 mm in length. The microstructure and mineral composition of the red sandstone are shown in Figure 7.  
In order to obtain rock samples with different degrees of damage, various drilling methods were applied to the cut rock specimens, as shown in Figure 8.  
The impact drilling method is shown in Figure 8a. Two different power levels of electric hammers, 650 W and 1300 W, were used to drill holes in the red sandstone, with a hole diameter of 8 mm. Each power level was used to drill 16 sandstone specimens. The non-impact drilling method is shown in Figure 8b, where a diamond bit was used for core drilling with a center hole diameter of 8 mm after coring. Similarly, 16 rock specimens were  
Figure 8: Schematic diagram of drilling. (**a**) Impact drilling. (**b**) Non-impact drilling. Figure 7: Sandstone: (**a**) naked-eye observation, (**b**) observation under plane-polarized light, (**c**) observation under cross-polarized light (the letters Qtz, PI, and Cal represent quartz, plagioclase, and calcite, respectively), and (**d**) percentage of minerals [32,33]. drilled using the non-impact drilling method. According to the requirements for testing the mechanical properties of rocks, the 48 drilled sandstone specimens were processed into 30 mm thick rock specimens, with the flatness and perpendicularity tolerances of both end faces controlled within 0.02 mm. The drilled sandstone was cut into 48 rock specimens using a cutting machine, each with a length of 31 mm. Then, an SHM-200 double-end face rock grinder was used to grind the top and bottom surfaces of these 48 cylindrical rock specimens until they were flat. After grinding, the length of each specimen was 30 mm, with the flatness and perpendicularity tolerances of the end faces controlled within 0.02 mm. The completed rock specimens are shown in Figure 9. From left to right, the images display three types of rock specimens: those drilled using high-impact power (labeled HS), low-impact power (labeled LS), and the non-impact drilling method (labeled NS). Before conducting the experiment, randomly select four rock specimens from each group to measure their physical geometric parameters and longitudinal wave velocity. The test data are presented in Table 2. Using the wave velocity data, a trend chart showing the variations in wave velocity for different rock specimen types was plotted, as shown in Figure 10. The chart shows that the wave velocity of NS rock specimens is higher than that of LS specimens, which in turn is higher than that of HS specimens. In addition, lower wave velocity indicates more internal fractures and greater rock damage. Therefore, HS rock specimens suffer the most severe surrounding rock damage, LS specimens undergo moderate damage, and NS specimens have the least damage.