## 3 Experimental Results and Discussion
### Out-of-Plane Test
#### 3.1.2 Dynamic Compression  
(a) Mechanical Behavior  
For the non-damaged tests of the \(\Phi 9\times 4\) specimen under 0.4 MPa, shown in Figure 7, the stress-strain curve could be divided into four zones [17]. Figure 4: The quasi-static compressive behavior under out-of-plane loading: (**a**) stress–strain curve; (**b**) first derivative of stress vs. strain curve. Figure 5: The failure pattern of the bigger specimen: (**a**) top view of the two pieces; (**b**) shear failure feature. Figure 6: The failure pattern of the smaller specimen: (**a**) top view of the specimen; (**b**) inner feature of the specimen. Zone 1: This period is called the self-installation zone. The strain rate increased rapidly, but the stress fluctuated around 0. This is because the parallelism of the specimen faces in contact with the bars was not 100% guaranteed. Zone 2: This period is called the under-loading zone. Once the perfect contact was ensured, the strain rate began to drop. As the strain rate increased, the stress increased as well. However, the stress-strain relationship showed two different periods in this zone. During the 2.1 period, the stress-strain curve was not linear, possibly due to the pores among the specimen being compressed. During the 2.2 period, the stress-strain curve was linear and stable, due to the material being more uniform with the specimen yet to be damaged. Zone 3: This period is called the soften zone. The strain rate tended to zero and the stress tended to an average value. The stress reached the maximum value when the strain rate reached zero. As shown in Figure 7b, the material behavior showed a softening phenomenon. Zone 4: This period is called the spring-back zone. The strain rate changed sign, and the stress started to decrease. The specimen began to relax and then recovered. This shows that, even though several fibers were squeezed out, the main body of the specimen suffered no serious damage. Therefore, the specimen still had the ability to spring back. For the damaged tests of the \(\Phi 9\times 4\) specimen under 0.8 MPa, shown in Figure 8, the stress-strain curve could be divided into five zones [17]. Zone 1 and Zone 2 were the same as those for the non-damaged tests. Figure 8: Evolution of the stress and strain rate curves of damaging test under out-of-plane loading: (**a**) stress and strain rate vs. time curves; (**b**) stress vs. strain curve. Figure 7: Evolution of the stress and strain rate curves of non-damaging test under out-of-plane loading: (**a**) stress and strain rate vs. time curves; (**b**) stress vs. strain curve. Zone 3: This period is called the soften zone. As shown in Figure 8b, there was serious fluctuation in the stress curve. This was possibly because the microscopic damage under higher strain rate was more serious than that under lower strain rate. Zone 4: This period is called the fracture zone. The accumulation of microscopic damage resulted in the appearance of a macroscopic defect. The fracture of the material generated a rapid fall in the stress with a rapid increase in the strain rate. The presence of a second peak characterized the onset of macroscopic damage [48]  
Zone 5: This period is called the compact zone. The damaged specimen became more compact; the rest of the specimen could hold the loading temporarily, which caused a sharp drop in the strain rate. It is obvious that the spring-back behavior disappeared. (b) Damage Kinetics  
The high-speed camera records show that some fibers were squeezed out from the sides during the loading before the specimen was damaged completely, which can be seen in Figure 9. This is because the specimens were made of 0\({}^{\circ}\)/90\({}^{\circ}\) layers, and there were no woven fibers to limit the outward movement. In addition, the friction coefficient of UHMWPE is very low [10], while the strength of UHMWPE is higher than that of the resin matrix. Thus, the fibers near the side tended to be squeezed out.