## 5 Numerical Simulation of Core Discing Mechanism Considering Drilling Pressure  
Based on the analysis of the drilling parameters and the shape of the core discs' surfaces, it is concluded that the core discing phenomenon under low confining pressure is tensile failure that responds well to the drilling pressure. The drilling process results in stress concentration on the core [33]. The peak stress produced by the stress concentration can often exceed the yield limit of rock and cause the redistribution of stress. The previous numerical simulation studies of core discing mainly considered the influence of confining pressure on the internal stress field of the rock mass, and the model of gradually releasing the \"ring\" [13] was often used to simulate the stress release process, ignoring the interaction  
Figure 13: Fractal dimension of upper and lower surfaces. (**a**) Test 3; (**b**) Test 4; (**c**) Test 5.  
between the drill bit and the rock under different drilling parameters. To reveal the mechanism of core discing under low confining pressure, the drilling parameters were obtained according to the core drilling tests, and the interaction between the drill bit and the rock was considered in the numerical simulation. A 3D core drilling model was established in ANSYS. The size of the model was 100 mm \(\times\) 100 mm \(\times\) 100 mm. The borehole diameter was 32 mm, the core diameter was 24 mm, and the drilling depth was 75 mm. Half of the model is shown in Figure 14. The mechanical parameters of the numerical model were selected according to the tests, as shown in Table 3.  
The top, bottom, left, and right surfaces of the model were fixed. The annular area at the borehole bottom imposed the drilling pressure and torque. Numerical simulations were carried out on the three drilling tests (test 3, 4, and 5) with the core discing phenomenon. The drilling pressure was divided into two parts in test 3, test 4, and test 5: the complete core section and the discing core section. The average drilling pressure of the complete core section and the maximum drilling pressure of the discing core section were adopted in the numerical model of test 3, test 4, and test 5, respectively. Six simulation tests were carried out. The simulation parameters are shown in Table 4.  
The normal stresses on the core surface show a ring-shaped tensile stress zone near the core root, which is consistent with the shape of the rock disc. The closer it is to the core root, the more significant tensile stresses are. The core root area is enlarged, and the position of \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Simulation Test** &
\begin{tabular}{c} **Drilling** \\ **Pressure/MPa** \\ \end{tabular} & **Bit Torque/N \(\cdot\)m** & **Corresponding Condition** \\ \hline
1 & 31.34 & 3.41 & Test 3 Complete Core Section \\
2 & 42.43 & 5.53 & Test 4 Complete Core Section \\
3 & 50.70 & 8.24 & Test 5 Complete Core Section \\
4 & 50.51 & 18.15 & Test 3 Discing Core Section \\
5 & 58.52 & 19.62 & Test 4 Discing Core Section \\
6 & 68.62 & 21.70 & Test 5 Discing Core Section \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Boundary parameters of the numerical model. Figure 14: \(1/2\) numerical model. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Poisson Ratio** & \begin{tabular}{c} **Young’s** \\ **Modulus** \\ **/GPa** \\ \end{tabular} & \begin{tabular}{c} **Bulk** \\ **Modulus** \\ **/GPa** \\ \end{tabular} & \begin{tabular}{c} **Shear** \\ **Modulus** \\ **/GPa** \\ \end{tabular} & \
\begin{tabular}{c} **Tensile** \\ **Modulus** \\ **/GPa** \\ \end{tabular} &
\begin{tabular}{c} **Compressive** \\ **Strength** \\ **/MPa** \\ \end{tabular} \\ \hline
0.23 & 51.63 & 34.00 & 25.00 & 21.42 & 246.21 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Material parameters of the numerical model. the core root is marked with a black dashed line, as shown in Figure 15. By convention, tensile stresses are positive and compressive stresses are negative. Figure 15 shows that tensile stresses are generated on the core surface.