## 3 Experimental Results and Analysis
### Macro- and Microscopic Failure Features  
The fracture behavior of rock is closely related to the impact velocity and microstructural changes during impact testing [36]. Hence, studying the fracture state during impact can show how saturation affects the mechanical properties of rock at low temperatures. After the tests, frozen sandstone fragments with different saturations were collected and dried. The results are presented in Table 3. It is obvious that the sandstone samples are completely broken at different impact velocities due to their relatively low strength. At low impact velocity (4 m/s), there were large broken blocks in the samples with different saturations, especially blocks approximately 5.9 cm in length in the sample with 50% saturation. Figure 15: Evolution of rock energy distribution with saturation of (**a**) \(S_{\text{r}}=\) 0%, (**b**) \(S_{\text{r}}=\) 50%, and (**c**) \(S_{\text{r}}=\) 100%. At this time, the pore ice within the sample played a role in supporting the pore wall and cementation, so that some of the crack development at the pore tip was suppressed under the action of the stress waves, reflecting the increase in the volume of macroscopically fractured blocks. Meanwhile, the plastic deformation before the peak during the impact test implies the development and extension of the internal microcracks, so the increase of plastic deformation indicates the increase of fragmentation degree. As can be seen in Figure 8, the plastic deformation increases with saturation and then decreases, which corresponds to the same trend in the fragmentation degree. Moreover, plastic deformation increases with the impact velocity, while the powder particles of the broken samples gradually increase in size. At an impact velocity of 7 m/s, the samples were broken into powder, indicating that the frozen sample mainly changed from tensile failure to complex fracture. Therefore, the final failure morphology of the samples can be used to infer the role of pore ice. In addition, as shown in Figure 17, SEM images are used to infer changes in the microscopic morphology of the red sandstone after freezing. The images demonstrate that saturation has a significant effect on the surface morphology of the sample, and that many microcracks have formed after the freezing treatment. In the original state, the internal structure of the sandstone is relatively loose (Figure 17a) and has many small primary cracks, pores, and other defects, some of which can be up to 4 m in width. After freezing, the surface structure becomes dense due to thermal contraction of the mineral particles, and some of the original cracks close, with a maximum crack width as small as 2 m (Figure 17b). In Figure 17c, some of the previously closed cracks reactivate and propagate at low saturation as water freezes to ice at low temperatures. When the saturation increases to 50% (Figure 17d), some cracks develop further and connect, but most cracks are less than 1 m in width, and the damaging effect of pore ice on the matrix is still less than its strengthening effect. However, when the sample saturation reaches 80%, the increasing pore water facilitates the continued expansion of the pore ice, leading to the development of larger cracks, some of which are up to 6 m in width (Figure 17e). When the sample is fully saturated, a block appears on the fracture surface, indicating that the volume of ice expands even after the cracks are penetrated, causing the block to be extruded; meanwhile, the cracks on the fracture surface reach approximately 9 m in width (Figure 17f). In summary, as the saturation increases, microcracks begin to develop inside the sample, while the strengthening of the pore ice prevents the manifestation of the internal damage to a certain extent. However, when the saturation exceeds 50%, the pores inside the sample cannot provide sufficient expansion space for the ice, resulting in a fracture network inside the sample. As a result, the strength and deformation resistance tend to decrease as the sample approaches full saturation.