## 3 Results and Analysis
### SHPB Dynamic Results and Analysis  
The SHPB test was carried out following the above-mentioned test method, and the test results are shown in Table 7 and Figures 6 and 7. Figures 6 and 7 show the splitting tensile stress-strain curves of each group of specimens after the polypropylene fiber was mixed with steel fiber at different volume contents and draw ratios, respectively. The peak stress of the single addition steel fiber group increased by 15.7% compared to the reference group. Compared to the single addition steel fiber group, the peak stress of the 0.05%, 0.1%, and 0.15% SPA groups increased by 6.4%, 15.7%, and 4.0%; the peak stress in the 0.05%, 0.1%, and 0.15% SPB groups increased by 14.7%, 24.2%, and 12.6%; the peak stress in the 0.05%, 0.1%, and 0.15% SPC groups increased by 1.2%, 5.1%, and 2.9%, respectively. This shows that with the increase in polypropylene fiber volume content and the length-diameter ratio, the peak stress of each hybrid fiber group first increased and then decreased in comparison to the single addition steel fiber group. This was due to the fact that when the fiber volume and length-diameter ratio were moderate, the mixing of the two fibers had a positive effect on the peak stress of the concrete. When the fiber volume was too high or the length-diameter ratio was too large, the peak stress was negatively affected. The change rule of the peak strain of the concrete specimens was similar to the peak stress rule. The peak strain of the single addition steel fiber group increased by 8.5% compared to the reference group. Compared to the single addition steel fiber group, the peak strain of the 0.05%, 0.1%, and 0.15% SPA groups increased by 49.7%, 52.8%, and 34.7%; the peak strain of the 0.05%, 0.1%, and 0.15% SPB groups increased by 61.13%, 65.28%, and 60.1%; and the peak strain of the 0.05%, 0.1%, and 0.15% SPC groups increased by 2.6%, 32.64%, and 18.7%, respectively. This shows that with the increase in polypropylene fiber volume content and length-diameter ratio, the peak stress of each hybrid fiber group has increased, but the lifting effect is obviously decreasing, indicating that the incorporation of hybrid fibers can significantly enhance the dynamic splitting deformation behavior. The area enclosed by the stress-strain curve of concrete in Figures 6 and 7 and Table 7 represents the dynamic toughness index under dynamic loading. The toughness index of the single addition steel fiber group increased by 8.5% compared to the reference group. Compared to the single addition steel fiber group, the 0.05%, 0.1%, and 0.15% SPA groups' toughness index increased by 41.7%, 72.9%, and 33.3%; the 0.05%, 0.1%, and 0.15% SPB groups' toughness index increased by 52.1%, 81.9%, and 46.5%; and the 0.05%, 0.1%, and 0.15% SPC groups' toughness index increased by 27.8%, 38.3%, and 31.3%, respectively. These results show that the toughness index of the hybrid fiber concrete was significantly higher than that of the single addition steel fiber group, which indicates that the hybrid fiber was able to significantly enhance the dynamic toughness of the concrete. When the length-diameter ratio of the polypropylene fiber was constant, the dynamic toughness of concrete first increased and then decreased with the increase in the volume of the polypropylene fiber, and it reached its maximum value when the volume of polypropylene was 0.1%. This shows that the incorporation of hybrid fibers significantly improved the dynamic toughness of concrete since when micro-cracks began to appear inside the specimen, the incorporation of an appropriate amount of fibers effectively inhibited the development of macroscopic cracks, increased the residual stress of the concrete, and improved the toughness. Since the time of impact loading was short, the DIC method was used here to observe the change in strain distribution before crack generation. Figure 8 shows strain nephograms for each specimen, illustrating two different change processes.