## 3 Experimental Results and Discussion
### Out-of-Plane Test
#### 3.1.2 Dynamic Compression
As shown in Figure 9 for the damaged test of the \(\Phi 9\times 4\) specimen under 1.2 MPa, some features were apparent. A comparison with Figure 9a-c shows that the specimen became brighter. This was potentially because the space in the specimen was squeezed out and, thus, the specimen became compact. During this period, the input bar moved quickly toward the output bar until around 56.16 \(\upmu\)s. Then, the relative speed of the input bar and the output bar became lower; this state was maintained for a while until around 126.88 \(\upmu\)s. During this period, small amounts of fibers were squeezed out of the specimen, as indicated by the arrow in Figure 9d. At 126.88 \(\upmu\)s, a large amount of fiber was squeezed out, as indicated in the circle of Figure 9e. From this moment on, the input bar again movedquickly toward the output bar. This was potentially because, along with large amount of fiber being squeezed out, the specimen began to fail and could not maintain the loading between bars anymore. These large chunks of fibers being squeezed out could also be seen in other records of the damaged tests. During this period, large chunks of fibers were squeezed out constantly along with the moving input bar. Finally, when the input bar stopped moving, only a small portion of the specimen remained between the two bars, as shown in Figure 9f. Correspondingly, the specimen was damaged completely and there was certainly no spring-back. As shown in previous SEM micrographs of a specimen near a transmission bar [22], severe damage was observed in the form of matrix fracture, resulting in delamination of a few layers near the edge and complete exposure of a large number of fiber bundles with negligible matrix bonding. The extent of damage was found to increase with the increase in specimen strain rates. At higher strain rate impact, the specimen exhibited breakage of the fiber-matrix interfacial bond via the occurrence of various failure mechanisms such as fiber breakage, fiber-matrix debonding, matrix cracking, delamination, and exposure of broken fibers due to separation from the reinforcing matrix material. The squeezed fiber bundles, breakage, and delamination can also be seen in Figure 9. These detailed fracture features could be helpful for establishing a relevant damage model. As shown in Figure 10, the maximum stress of the \(\Phi 9\times 4\) specimens under different impact pressures increased with the increasing strain rate. If the specimen was not damaged completely, the maximum stress points usually happened around a certain time. If the specimen was damaged completely, the maximum stress points occurred earlier and earlier. Correspondingly, each zone in the damaged tests happened earlier and earlier under higher and higher strain rate. (c) Effect of Strain Rate  
As Figure 11 shows, as the strain increased, it seems that the stress values under different strain rates increased along the same curve, where only the peak points were different. Figure 11: Dynamic behavior of \(\Phi 9\times 4\) specimens. Figure 10: The evolution of stress with increasing strain rate. If the specimen was not damaged completely, the peak stress refers to the maximum stress that the specimen could sustain under a certain strain rate. After the peak point, the specimen still had the ability to spring back, which could also be seen in the high-speed camera records. However, the spring-back curve was not the same as the loading curve. This was potentially because some fibers were squeezed out of the main body of the specimen and, thus, a certain degree of permanent damage was caused. Furthermore, the slopes of previous spring-back curves under different strain rates were similar [49], which means that the specimens retained similar properties. If the specimen was damaged completely, the peak stress refers to the fracture stress of the specimen. As shown in Figures 11 and 12 after 2989 s\({}^{-1}\), the fracture stress also increased as the strain rate increased.