## 4 Results and Discussion  
### Experiment Results  
The role of the first two bumpers was to break the projectile and reduce its kinetic energy. The typical damage of the first two layers is shown in Figure 5. It can be obtained that Bumper I exhibited only one perforated hole with a diameter of about 18.6 mm, which proved that the projectile's posture was good. On the other hand, Bumper II exhibited multiple small perforated holes and craters around a big, irregularly-shaped perforated hole. The projectile carried a large initial kinetic energy due to its large mass and high velocity. After the projectile perforated Bumper I, a debris cloud was formed which then impacted Bumper II. Partially broken projectile and debris clouds still penetrated Bumper II and caused perforation or other forms of damage to the rear wall. \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Symbol** & **Values** \\ \hline
**Strength: Orthotropic yield** & & \\ Plasticity constant 11 & \(A_{11}\) & 0.016 \\ Plasticity constant 22 & \(A_{22}\) & \(6\times 10^{-4}\) \\ Plasticity constant 33 & \(A_{33}\) & \(6\times 10^{-4}\) \\ Plasticity constant 12 & \(A_{12}\) & 0 \\ Plasticity constant 13 & \(A_{13}\) & 0 \\ Plasticity constant 23 & \(A_{23}\) & 0 \\ Plasticity constant 44 & \(A_{44}\) & 1 \\ Plasticity constant 55 & \(A_{55}\) & 1.7 \\ Plasticity constant 66 & \(A_{66}\) & 1.7 \\ Eff. stress \#1 (GPa) & \(\sigma_{\#\#1}\) & \(1.48\times 10^{-3}\) \\ Eff. stress \#2 (GPa) & \(\sigma_{\#\#2}\) & \(7.0\times 10^{-3}\) \\ Eff. stress \#3 (GPa) & \(\sigma_{\#\#3}\) & 0.027 \\ Eff. stress \#4 (GPa) & \(\sigma_{\#\#4}\) & 0.04 \\ Eff. stress \#5 (GPa) & \(\sigma_{\#\#5}\) & 0.05 \\ Eff. stress \#6 (GPa) & \(\sigma_{\#\#6}\) & 0.06 \\ Eff. stress \#7 (GPa) & \(\sigma_{\#\#7}\) & 0.08 \\ Eff. stress \#8 (GPa) & \(\sigma_{\#\#8}\) & 0.098 \\ Eff. stress \#9 (GPa) & \(\sigma_{\#\#9}\) & 0.2 \\ Eff. stress \#10 (GPa) & \(\sigma_{\#\#10}\) & 1 \\ Eff. plastic strain \#1 & \(\varepsilon_{\#\#11}\) & 0 \\ Eff. plastic strain \#2 & \(\varepsilon_{\#\#2}\) & 0.01 \\ Eff. plastic strain \#3 & \(\varepsilon_{\#\#3}\) & 0.1 \\ Eff. plastic strain \#4 & \(\varepsilon_{\#\#4}\) & 0.15 \\ Eff. plastic strain \#5 & \(\varepsilon_{\#\#5}\) & 0.175 \\ Eff. plastic strain \#6 & \(\varepsilon_{\#\#6}\) & 0.19 \\ Eff. plastic strain \#7 & \(\varepsilon_{\#\#7}\) & 0.2 \\ Eff. plastic strain \#8 & \(\varepsilon_{\#\#8}\) & 0.205 \\ Eff. plastic strain \#9 & \(\varepsilon_{\#\#9}\) & 0.21 \\ Eff. plastic strain \#10 & \(\varepsilon_{\#\#10}\) & 0.215 \\ \hline \end{tabular}
\end{table} 
Table 4: _Cont._The results of the impact tests are listed in Table 5. The typical experimental results of damage of the rear wall in each configuration are presented in Figures 6 and 7. According to the damage of the rear wall, it is evident that the rear walls of Configurations A-1, A-2, B-1 and C were penetrated (i.e., CP) while the rear walls of Configurations A-3 and B-2 were not penetrated (i.e., PP). The rear wall of Configuration A-1 (i.e., 5 mm-thick Al 7075) was perforated with a diameter of 20 mm, and there were two obvious impact craters near the perforated area. The rear wall of Configuration A-2 (i.e., 8 mm-thick Al 7075) was penetrated and the back was scabbing. The Configuration A-3 rear wall (i.e., 12 mm-thick Al 7075) was not perforated, and there were some craters on the front side, with no bumps on the back. From Figure 7, it can be seen that the B\({}_{4}\)C board was fractured in all of the configurations. In Configuration B-1, the projectile perforated the UHMW-PE layer and caused the fiberboard to bulge by approximately 83 mm. However, in Configuration B-2, the UHMW-PE layer was not perforated due to the increased thickness of B\({}_{4}\)C and UHMW-PE. The UHMW-PE board posed several long cracks and bulged by 9 mm. In Configuration C, the rear wall was completely perforated. The outermost Al 7075-T651 layer posed six approximately symmetric long cracks, starting from the perforated hole.