## 3 Experimental Results and Discussion
### Out-of-Plane Test
#### 3.1.2 Dynamic Compression
Furthermore, as the strain rate increased, the soften zone of the stress-strain curve tended toward the linear period. Therefore, the slope of the stress-strain curve in the soften zone was higher and the fracture strain became smaller to some extent as the strain rate increased to 2989, 3213, and 3316 s\({}^{-1}\), as shown in Figures 11 and 12. This phenomenon also occurred in the non-damaged tests of specimens 8 x 8 x 4 when the strain rate became higher. To sum up, with the increasing strain rate, the maximum stress, fracture stress, and energy absorption also increased. In contrast, it seems that the maximum strain increased at first and then decreased when it became fracture strain. As shown in Figure 13a-d for the \(\Phi 9\times 4\) specimens under 0.2-1.2 MPa impact, as the strain rate increased, more fibers were squeezed out. Therefore, the little square at the center of the end face became more and more clear. As to the specimens under 3045.0018 \(\pm\) 78.6690 s\({}^{-1}\) strain rate, some were non-damaged and some were completely damaged, which means that this strain rate represents the critical fracture value. As for the damaged specimens shown in Figure 13e,f, the main body was crushed into several relatively big blocks. The traces of the crushed parts indicate that they were damaged by inner shear stress. Figure 12: The changes in material properties of the \(\Phi 9\times 4\) specimens with increasing strain rate: (**a**) max stress and energy absorption vs. strain rate curve; (**b**) max strain vs. strain rate curve. Figure 5:0 The morphological differences of \(\Phi 9\times 4\) specimens under increasing strain rate (s\({}^{-1}\)): (**a**) 1666.6250 \(\pm\) 2.5290; (**b**) 2232.7599 \(\pm\) 146.0478; (**c**) 2588.1668 \(\pm\) 164.9590; (**d**) 3045.0018 \(\pm\) 78.6690; (**e**) 3210.3948 \(\pm\) 121.6243; (**f**) 3284.0803 \(\pm\) 45.5605. As mentioned in Reference [21], compaction is the major failure mode in the out-of-plane direction, which represents the energy absorption mechanism. Similarly, as more compaction occurs, more fibers are squeezed out and more blocks are crushed; furthermore, with increasing strain rate, the energy absorption also increases. [PERSON] also [21] found that the laminated angle has an effect on the dynamic energy absorption of composites at higher strain rates. Placing laminae in a well-proportioned manner on the projective plane effectively enhances the energy absorption load under transversal impact. The friction between fibers perhaps has a relationship with the placing angle and energy propagation, which needs deeper study in further work. (d) Effect of Size  
The non-linear and linear periods were mainly considered for size effects. For the influence of thickness, as shown in Figure 14a, if the thickness was different, the stress-strain relationships were obviously different. As shown in Figure 15a, with an increase in the thickness, the slope of the non-linear period became steeper and shorter. As shown in Figure 16b and Table 2, the thickness seems to not significantly affect the modulus values of the linear period. Therefore, the thickness seems to affect the self-installation zone and the non-linear period, but it seems to not affect the linear period. Since the self-installation zone is inevitable for the SHPB apparatus, the thickness seems to mainly affect the non-linear period. Figure 5:1 The stress–strain curves of different specimens: (**a**) effect of thickness; (**b**) effect of diameter/area; (**c**) effect of cross-section shape. Figure 5:2 The first derivative of stress–strain curves of different specimens: (**a**) effect of thickness; (**b**) effect of diameter/area; (**c**) effect of cross-section shape. With regard to the influence of diameter/area, as shown in Figure 14b, the diameter or area seems to not significantly affect the stress-strain relationship. Considering Figure 15b and that the modulus of specimens with different diameter in Table 2 was similar, the diameter or area also seems to mainly influence the non-linear period.