## 3 Results and Discussion
### Effect of Confining Pressure  
Models of uniform centripetal pressure (UCP) and some parallel pressures were established to investigate the influence of confining pressure on damage development (Figure 19). In UCP, the coal was subjected to uniform centripetal pressure \(\mathrm{P_{C}}\). Models of parallel pressures included models of uniform pressures (uniform \(X\)-axis pressure (UXP), uniform \(Y\)-axis pressure (UYP)) and non-uniform pressures (two non-uniform \(X\)-axis pressures (NXPs) and two non-uniform \(Y\)-axis pressures (NYPs)). In NXP (a) and NYP (a), the maximum pressure (\(\mathrm{p_{max}}\)) pointed to the center of the coal and decreased linearly along both sides to 0 MPa at the end. In NXP (b) and NYP (b), the pressure pointing to the center of the coal was 0 MPa and increased linearly along both sides to \(\mathrm{p_{max}}\) at the end (Table 7). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Model** & **Pressure Direction** & **Maximum Pressure Point** & **Minimum Pressure Point** \\ \hline UCP & Center & Uniform & Uniform \\ UXP & \(X\)-axis & Uniform & Uniform \\ UYP & \(Y\)-axis & Uniform & Uniform \\ NXP (a) & \(X\)-axis & Center & Sides \\ NXP (b) & \(X\)-axis & Sides & Center \\ NYP (a) & \(Y\)-axis & Center & Sides \\ NYP (b) & \(Y\)-axis & Sides & Center \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Confining pressure table. Figure 18: \(D\) and the changes in damage distribution in (**A**) S\({}_{0}\)M, (**B**) S\({}_{180}\)M, and (**C**) RM. (**a–c**) Damage distribution before and after PDAC. The damage paths of S\({}_{0}\)M and RM decreased with increasing \(\rm{p_{C}}\) (0-1.8 MPa) by 15 s microwave irradiation under UCP (Figure 20). Tensile and shear stresses in the coal matrix were opposed by \(\rm{p_{C}}\), which limited the formation and development of damage. Under 1 MPa, all damage in the central region of the coal sample disappeared, leaving only the boundary still experiencing damage caused by close pyrites (A\({}_{0}\), A\({}_{\rm{R}}\), C\({}_{\rm{R}}\), and H\({}_{\rm{R}}\)). Under 1.4 MPa, the damaged area near A\({}_{0}\) was reduced, and the damaged area near A\({}_{\rm{R}}\) disappeared. This was due to the average temperature of A\({}_{0}\) (122.45 \({}^{\circ}\)C) being higher than A\({}_{\rm{R}}\) (104.64 \({}^{\circ}\)C), resulting in higher thermal stress, making the damage caused by A\({}_{0}\) more difficult to inhibit. Furthermore, although the average temperature of C\({}_{\rm{R}}\) (96.70 \({}^{\circ}\)C) was lower than that of A\({}_{\rm{R}}\) (104.64 \({}^{\circ}\)C), C\({}_{\rm{R}}\) was closer to the boundary, resulting in the coal matrix between it and the boundary still being damaged. Consequently, the damage caused by pyrites in some hot spots in static models was more difficult to inhibit by \(\rm{p_{C}}\) compared to RM, and inhibition by \(\rm{p_{C}}\) was more effective in relation to internal damage in the coal sample. The damage percentage in S\({}_{0}\)M and RM decreased with increasing \(\rm{p_{C}}\) in UCP (Figure 21). The percentage of areas damaged in S\({}_{0}\)M decreased from 6.54% without pressure to 0.31% under 1.8 MPa, representing a 95.25% percentage reduction. Similarly, the percentage of damaged areas in RM decreased from 13.97% to 0.38, representing a 97.30% percentage reduction. Therefore, \(\rm{p_{C}}\) significantly inhibited the formation and development of damage caused by A\({}_{0}\). The damage percentage reduction in S\({}_{0}\)M decreased from 13.97  
of damage, which was consistent with a previous study [15], in which increments in cuttings and crack volumes before and after microwave radiation of coal cores under isotropic stress were significantly lower than those under no stress. The intermediate pressure (p\({}_{\text{mid}}\)) was p\({}_{\text{X}}\) and p\({}_{\text{Y}}\) in the uniform parallel pressure model, while the p\({}_{\text{mid}}\) was p\({}_{\text{max}}\)/2 in the non-uniform parallel pressure model.