## 3 Experimental Results and Discussion
### In-Plane Test
#### 3.2.2 Dynamic Compression  
(a) Mechanical Behavior  
The whole first-derivative course of the specimen under 0.2 MPa loading is shown in Figure 22. After a fast increase, the first derivative fluctuated around a certain constant. This certain period of the stress-strain curve can be seen as linear. The first derivative then dropped slowly and finally fluctuated around zero. According to the analysis, for the loading evolution of the \(8\times 8\times 8\) specimen under 0.2 MPa loading, shown in Figure 23, the stress-strain curve could be separated into six zones. Figure 21: The fracture pattern of the bigger specimen: (**a**) side view of one kind of kink band; (**b**) side view of another kind of kink band. Figure 22: The first derivative of stress vs. strain curves of \(8\times 8\times 8\) specimen. Zone 1: This period is called the self-installation zone. The strain rate increased slowly, and the stress fluctuated near a very small value. This is because the parallelism of the specimen faces in contact with the bars was not 100% guaranteed. This period was very short. Zone 2: This period is called the under-loading zone. The strain rate increased rapidly, and the stress also increased quickly to a maximum value. As shown in Figure 23b, this zone could be divided into a linear period and non-linear period. This zone represents the main dynamic behavior of the specimen under in-plane loading. Zone 3: This period is called the fracture zone. The strain rate kept increasing, while the stress decreased after the fracture point. This shows that the specimen's holding ability did not vanish suddenly but gradually. Zone 4: This period is called the distorted zone. The strain rate was almost constant in this zone. The fluctuation of stress was big. After a drop, the stress increased to a second peak followed by another drop. This was possibly because the damaged specimen was squeezed, thereby becoming more compact, but it was simultaneously folded and distorted. As a result, the compact specimen could hold the loading to some extent. The distorted structure resulted in the stress fluctuating very unsteadily. Zone 5: This period is called the unloading zone. The strain rate decreased in this zone. The stress also decreased relatively rapidly. Zone 6: This period is called the ending zone. The strain rate became zero and the loading ended. Finally, the stress fluctuated around a small value. (b) Damage Kinetics  
As showed in the circles of Figure 24b in comparison with Figure 24a,c for the \(8\times 8\times 8\) specimen under 0.2 MPa loading, when the corner of the specimen in contact with the input bar was squeezed vertically, the specimen failed at this moment. This moment corresponds to the fracture point on the stress-strain curve shown in Figure 23. At first, the fracture happened at the side of the specimen near the input bar. Then, as more layers were squeezed apart vertically, the stress fell gradually from 23.69 to 52.12 us. In this period, as the input bar moved forward, the fracture persisted and the layers on the contact surface were gradually squeezed vertically. The remainder of the specimen could still hold loading to some extent, as shown in Figure 24c. Finally, the remainder of the specimen was shortened, preventing it from maintaining contact. After this, as the input bar kept moving forward, the whole body of the specimen was squeezed, distorting it as shown in Figure 24d. In addition, because the morphology of the layers squeezed vertically was similar, it could be concluded that the parallel degree of the two ends near the input and output bars was good. Moreover, the fracture pattern under high-strain-rate loading was a kind of delamination similar to that presented in References [18; 50; 51], as opposed to the kink bands seen in quasi-static loading [18]. Figure 24: _Cont._  
(c) Effect of Strain Rate  
As shown in Figure 25, with the strain rate increasing, the energy absorption increased as well, albeit slowly.