## 4 Results and Discussion
### Numerical Modeling Analysis  
It has been experimentally proved that Configuration B with the B4C/UHMW-PE composite plate as the rear wall was the best shield configuration among the three. Numerical simulations were used to analyze the above experimental results in more detail. The experimental conditions of Configuration A were also numerically calculated for comparison. The variation in the projectile residual velocity for the five conditions under the two shield configurations is shown in Figure 9. It is worth noting that the termination time (0.4 ms) was long enough for the projectile to reach a complete stop and short enough to prevent a redundant computation time. The numerical results were consistent with the experimental results. The debris cloud formed after the projectile penetrated the first two bumpers, which was almost identical to the debris cloud image captured in the experiment, as shown in Figure 10. The image scale was calibrated before the test, so we got the velocity of the debris head with 1.01 km/s. The velocity of the debris head obtained from the simulation was 0.95 km/s, which showed good agreement with the experimental results. Figures 11 and 12 show the damage contours of the bumpers and the rear plates, both in the experiments and the simulations. The comparison in damage size and the pattern between the simulation and the experimental results exhibited a good agreement. Through the above results, the reliability of the numerical calculation model was verified. Figure 8: The areal density of the rear walls and the experimental results. Figure 9: The projectile residual velocity versus time. To further compare and analyze the impact process of Configuration A and Configuration B, we carried out a numerical simulation of the Al 7075-T651 plate as the rear wall at an impact velocity of 1.86 km/s, in which the areal density of the rear wall was the same as that of Configuration B-2. Figure 11: The experiment and simulation damage contours of **(a)** Bumper I and **(b)** Bumper II. Figure 10: The debris cloud shape of Bumper II of Configuration B-1 by the experiment and the simulation. The numerical calculation was noted as A-4. The variation in the projectile residual velocity with time for Configurations A-4 and B-2 is shown in Figure 13. For the first \(\sim\)0.13 ms, the projectile perforated two bumpers and slowed down to \(\sim\)1050 m/s. In the first \(\sim\)0.01 ms of the remaining projectile impacting the rear wall, the projectile slowed down to \(\sim\)440 m/s and the deceleration behavior was almost the same for the two configurations but, following this, time deviations occurred, as seen in Figure 13. The projectile velocity in Configuration B-2 decreased more slowly and eventually dropped to zero, while the rear wall of Configuration A-4 was perforated and the projectile residual velocity was \(\sim\)219 m/s. In accordance with this, the B\({}_{4}\)C/UHMW-PE composite plate intercepted the projectile more effectively than the single Al 7075-T651 plate. The energy absorption of the rear plate during the process of the remaining projectile and debris impact, as well as the energy transfer information during the impact process, were obtained from the simulation. Figure 14a,b shows the kinetic energy and total energy histories of the Al 7075-T651 rear wall for Configuration A-4 and the B\({}_{4}\)C/UHMW-PE composite plate for Configuration B-2. As the projectile penetration progressed, the kinetic energy of the rear plate increased initially and then gradually decreased, as seen in Figure 14a. At \(\sim\)0.14 ms, the kinetic energy of the rear plate of both configurations reached the maximum value. At this moment, the Al 7075-T651 plate in Configuration A-4 was perforated and the B\({}_{4}\)C layer in Configuration B-2 was broken. It can be seen from Figure 14a that the B\({}_{4}\)C layer with Configuration B-2 had the lowest kinetic energy due to the constraint of the UHMW-PE layer. The total energy of the rear plate characterized the energy absorption of the projectile.