## 4 Result and Discussion
### Multiple Dynamic Impact Test  
The stress-strain curve of U-SF2 in the impact compression test is shown in Figure 11, and the failure modes are analyzed in Figure 12. From Figure 11, the stress peak decreases with the increase of the number of shocks. In the course of the multiple-impact compression tests, each impact load damaged the specimen to some extent. In Figure 12 1 st, although the specimen did not show any damage and visible deformation in its appearance, a micro-crack had started to develop from the inside of the specimen. In addition, in the first impact, a small number of cracks were generated. With the continuous application of impact load, the cracks further developed and the damage gradually intensified. From Figure 12 2 nd and 3 rd, it can be seen that the cracks inside the specimen had developed to a certain extent. The cracks were connected and extended until they reach the outer surface, and visible deformation was observed in the specimen. After the fourth impact, obvious cracks appeared in the specimen. Six shocks later, the specimen was seriously damaged, the peak of the impact stress decreased significantly, and the specimen soon lost its bearing capacity and became fragmented. In the above process, the fibers showed a good toughening effect. Especially when the fiber doping was increased to 4% and 6%, the development of cracks inside the specimen usually required more than five impact loads before they developed into mutual penetration and were visible on the outside.  
Figure 10: Failure pattern of fiber-oriented UHPC impact compression test.  
The stress-strain curve of D-SF2 in the impact compression test was shown in Figure 13, and the comparison of the failure morphologies of directional fibers was shown in Figure 14. The peak stress, resistance to multiple-impact compression, and dynamic energy absorption properties were more obviously improved at 2% fiber doping. However, the enhancement of the stress-strain curve of D-SF2 is not a significant effect.  
Figure 11: Stress–strain curves of non-oriented multiple-impact compression of fibers: (**a**) U-SF2; (**b**) U-SF4; and (**c**) U-SF6.  
Figure 12: Failure pattern of fiber non-oriented multiple-impact compression specimens.  
of dynamic impact mechanical properties by high fiber doping was not obvious. The reason was that the ultimate anisotropy was sought in fiber-oriented distributed UHPC. It is for the sake of the more prominent load in one direction, at the expense of the mechanical properties in other directions, in order to achieve the best use and more efficient effect. For example, the stress state of the flexural beam can be basically determined, and its damage area was generally the span of the beam, the bottom tensile part; therefore, by improving the fiber-bridging capacity of this part, it is possible to achieve an overall performance improvement effect. The contribution of oriented fibers to the improvement of the impact resistance compression performance was smaller.  
Figure 14: Failure mode of fiber-oriented multiple-impact compression specimens.  
Figure 13: Fiber-oriented UHPC multiple-impact compression stress–strain curves: (**a**) D-SF2; (**b**) D-SF4; and (**c**) D-SF6.