## 5 Conclusions  
The dynamic stress-strain curve, dynamic compressive strength, dynamic strain, dynamic modulus of elasticity, specimen crushing pattern, energy dissipation density, and specimen crushing average size and fractal dimension were obtained from SHPB impact compression tests on prefabricated fractured 45\({}^{\circ}\) dip angle water-saturated sandstone specimens at different impact air pressures. The main conclusions are as follows:  
1. The dynamic stress-strain curve clusters of water-saturated fissured sandstone and natural fissured sandstone specimens are basically similar, which can be approximately divided into four stages, including the elastic stage, the crack extension stage, the plastic yielding stage, and the damage stage.
2. The dynamic compressive strength of the water-saturated fractured sandstone specimens is higher than that of the natural sandstone specimens. This is mainly due to the surface tension generated by the free water at the fissure tip of the water-saturated specimen, which hinders the instantaneous expansion of cracks and generates the Stefan effect. The dynamic strain of the water-saturated fissured sandstone specimen is slightly lower than that of the natural sandstone specimen. Both of them have a strain rate effect.
3. In the case of increasing impact air pressure, the specimen undergoes tensile and shear composite damage, the number of cracks is dense, and the size of the broken block is obviously reduced.
4. The energy-consuming density of crushing a water-saturated fissured sandstone specimen tends to increase quadratically with strain rate, and more energy needs to  
Figure 15: Variation in fractal dimension of fragments with crushing energy dissipation density.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen** & **Slope** & **Correlation** & **Fractal** \\
**Number** & & **Coefficient** & **Dimension** \\ \hline PJ42-01 & 1.1448 & 0.8627 & 1.8552 \\ PJ42-04 & 0.9357 & 0.9286 & 2.0643 \\ PJ42-08 & 0.9147 & 0.9437 & 2.0853 \\ PJ42-10 & 0.8694 & 0.9281 & 2.1306 \\ PJ42-13 & 0.7526 & 0.9266 & 2.2474 \\ PJ42-17 & 0.6986 & 0.9199 & 2.3014 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Calculation results of fractal dimension of water-saturated fissured sandstone specimens with different strain rates.  
be absorbed in the process for the germination, extension, and penetration of cracks within the specimen.
5. The average particle size of specimen crushing gradually decreases with the increase in crushing energy dissipation density; the higher the energy absorption of the rock specimen, the higher the energy dissipation in the deformation and destruction process of the specimen, which leads to the formation of more cracks, and the rock specimen will be crushed into small fragments or even powder form. As the crushing energy dissipation density increases, the fractal dimension of sandstone fragments also increases, which satisfies the multiplicative power function relationship. The fractal dimension is suitable as a quantitative index to characterize sandstone crushing.  
Conceptualization, Q.P.; software, Q.P.; validation, S.S.; formal analysis, S.S. and X.L.; data curation, S.S. and S.W.; writing--original draft, S.S. and Y.X.; writing--review and editing, J.H. and W.H. All authors have read and agreed to the published version of the manuscript.  
National Natural Science Foundation of China: no. 52074005 and no. 52074006; The Anhui Natural Science Foundation: no. 1808085 ME134; The Anhui Postdoctoral Science Foundation: no. 2015B058.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.  
We express our great appreciation to the editors and reviewers who gave valuable suggestions on this research.  
The authors declare no conflicts of interest.