## 4 Discussions  
### Analysis of Dynamic Parameters  
In order to better evaluate the dynamic characteristics of granite, the results of three parameters, peak stress, peak strain, and peak modulus, are selected in this paper. To specifically analyze the extent to which confining pressure and initial damage affect the dynamic mechanical properties of granite samples, the peak stress of 270.10 MPa for no-damaged rock samples under a confining pressure of 5 MPa is used as a reference. The damage-weakening factor \(f_{s}\) is introduced for calculation and analysis, with its calculation given by Equation (5) [30]:  
\[f_{s}=\left(\frac{\sigma_{n}-\sigma_{1}}{\sigma_{1}}\right)\times 100\%, \tag{5}\]  
In the equation, \(\sigma_{n}\) is the peak stress of granite samples under different states, and \(\sigma_{1}\) is the peak stress of no-damaged rock samples under a confining pressure of 5 MPa. If \(f_{s}\) is positive, this means that the peak stress of the rock sample has increased compared to \(\sigma_{1}\); otherwise, it means a decrease. The \(f_{s}\) calculation results of the weakening coefficient of granite rock samples in different states are shown in Table 2.  
Figure 7: CT scan slices of rock samples in different states. It can be seen from Table 2 that the peak stress of granite samples under different confining pressures decreases continuously with the increase in initial damage, and the peak stress of rock samples with different initial damage increases continuously with the increase in confining pressure. The increase in confining pressure will reduce the impact of initial damage on the peak stress, which is particularly obvious at a confining pressure of 20 MPa. The analysis of the internal reasons shows that the initial damage leads to the increase in porosity inside the rock sample, especially the increase in the connectivity between the pores and fissures, which leads to the acceleration of the penetration of the pores and fissures and the decrease in the bearing capacity of the rock sample under the impact load. When confining pressure is applied, the micropores inside the rock sample are closed, and the void between the matrix of the rock sample is reduced. With increasing confining pressure, the pore cracks caused by the initial damage will be further compacted until they are completely closed, which is reflected in the increase in peak stress in the dynamic characteristics of the rock. To analyze the effects of confining pressure and initial damage on the deformation of granite samples under impact load, this paper selected peak strain and peak modulus as the analysis objects. Peak strain is the strain value corresponding to the peak stress in the stress-strain curve of a rock, and peak modulus is the ratio of peak stress to peak strain. Compared with the modulus of elasticity, the peak modulus expresses the rock's ability to resist deformation at various stages under impact loading [31]. The relationships between the peak strain and peak modulus of rock samples and the confining pressure under different states are shown in Figures 8 and 9. As illustrated in Figure 8, confining pressure and initial damage have varying impacts on the peak strain of granite samples under impact load. Specifically, at the same confining pressure, an increase in initial damage leads to a higher peak strain in the samples. Additionally, for samples with the same initial damage state, peak strain increases with higher confining pressure. However, this promoting effect of confining pressure on peak strain diminishes as confining pressure continues to rise. This behavior can be explained by the fact that initial damage increases the porosity within the samples, introducing irreversible deformation. Under impact load, this deformation is further exacerbated. While confining pressure enhances the sample's ability to resist elastic deformation, once the deformation exceeds the elastic limit of the sample, confining pressure becomes a force that exacerbates deformation.