## 4 Discussions
### Quantitative Fracture Analysis  
To conduct a more detailed quantitative analysis of the changes in internal fissures of the group of no-damaged rock samples subjected to impact loads under different confining pressures, three-dimensional reconstruction of the two-dimensional CT images was performed using Avizo 2020.1 software. The three-dimensional reconstruction models of the group of no-damaged granite rock samples subjected to impact under different confining pressures are shown in Figure 10.  
From the three-dimensional reconstruction models of rock samples under different conditions shown in Figure 10, the impact of confining pressure on the development of fissures in the rock samples is evident. For the untreated rock samples, there are only a few isolated, small-volume pores with limited connectivity. After impact, numerous through fissures develop, which are the primary cause of the rock's fracturing. As confining pressure increases, the number of through fissures decreases. Particularly, when the confining pressure reaches 15 MPa, the number of through fissures within the rock samples is notably reduced, and at 20 MPa, no large-volume fissures are observed. Figure 9: Peak modulus of rock samples after impact in different states. Under impact loading, the influence of confining pressure on the internal development of fissures is more clearly depicted in the three-dimensional reconstruction models. From the longitudinal section of the rock sample in Figure 10, the evolution law of the pore fracture can be observed. As discussed in the analysis of granite dynamic deformation, impact loading causes axial deformation in the rock sample, and due to the Poisson effect, transverse deformation also occurs. However, the presence of confining pressure restricts transverse deformation and alters the development direction of the fissures, causing them to gradually shift towards the center of the rock sample. The specific impact of confining pressure on fissures' development is illustrated in Figure 11. Increased confining pressure makes the internal through fissures of the rock sample more linear, with this effect being particularly pronounced at 10 MPa. The primary cause of rock failure is the propagation and development of internal fissures. Quantitative analysis of these fissures using the three-dimensional reconstruction  
Figure 11: Schematic diagram of the influence of confining pressure on impact fissure development. Note: Fs is impact load; Fw is confining pressure force. Figure 10: Three-dimensional reconstruction models of rock samples under different states. models of the rock samples provides a fundamental explanation of how confining pressure affects the macroscopic mechanical properties of granite samples after impact. In this analysis, the internal fissures of the rock samples are categorized into fissures and independent fissures. The main cause of rock failure is the formation of through fissures that connect and penetrate the rock sample under external forces, while the remaining fissures that do not penetrate the rock are considered independent fissures. The volume of fissures and the maximum length of through fissures are selected as parameters for quantitative analysis. The detailed analysis results are presented in Table 3.  
From Table 3, it is observed that as confining pressure increases, the total volume of internal fissures in the rock samples decreases after impact. Specifically, the proportion of the volume occupied by through fissures decreases, while the proportion occupied by independent fissures increases. Additionally, the maximum length of through fissures also diminishes. These quantitative analysis results demonstrate the suppressive effect of confining pressure on the development of internal fissures in the rock samples.