# Nonlinear Impact Force Reduction of Layered Polymers with the Damage-Trap Interface
## 3 Experimental Results  
Figure 5 shows very different maximum impact forces measured directly from the test machine for two kinds of PMMA material systems with and without the 5083 adhesive layers. At the impact energy of 20 J, the maximum impact force of the layered PMMA specimen was reduced by 60% compared to the identical specimen without bonding as shown in Table 1. In the case of layered PMMA, 8 J, 11 J, and 18 J impact experiments were not performed as a significant difference was not observed from the impact energy reported in Table 1. For the layered Polycarbonate specimen with the 5083 adhesive, the maximum impact force was reduced by at least 20% even though the maximum impact energy was 120 J as shown in Figure 6. Table 2 shows the maximum impact force measured from the test machine as a function of impact energy. If we define an allowable impact force of 12 kN for the Polycarbonate plate according to our experimental records, the allowable impact energy of the baseline Polycarbonate plate should not exceed 30 J. But the allowable impact energy of the layered Polycarbonate plate with the damage-trap material interface could reach 70 J without any final failure, or 133% increase in terms of the allowable impact energy for the same allowable maximum impact force. Therefore, this simple material design could yield significant energy absorption.  
Figure 4: (**a**) A drop-weight impact test machine; (**b**) a loaded specimen at the test section.  
Figure 5: Variations of the maximum impact force for both baseline and layered PMMA specimens as a function of impact energy.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Impact Energy (J)**} & \multicolumn{2}{c}{**Max. Impact Force (kN)**} & \multirow{2}{*}{**Percentage Reduction**} \\ \cline{2-2}  & **Baseline** & & **Layered PMMA** \\ \hline
1.00 & 3.95 \(\pm\) 0.64 & 2.30 \(\pm\) 0.38 & 41.77 \\
5.00 & 7.94 \(\pm\) 0.81 & 3.87 \(\pm\) 0.43 & 51.26 \\
8.00 & 9.83 \(\pm\) 1.37 & - & - \\
11.00 & 11.39 \(\pm\) 1.53 & - & - \\
12.00 & 11.95 \(\pm\) 1.24 & 4.54 \(\pm\) 0.58 & 62.01 \\
15.00 & 12.75 \(\pm\) 1.49 & 4.50 \(\pm\) 0.73 & 64.71 \\
18.00 & 12.35 \(\pm\) 1.23 & - & - \\
20.00 & 12.51 \(\pm\) 1.87 & 4.66 \(\pm\) 0.96 & 62.75 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Maximum impact force for baseline and layered PMMA specimens.  
Figure 6: Variations of the maximum impact force for both baseline and layered Polycarbonate specimens as a function of impact energy.  
Therefore, we find at least one mechanism of the damage-trap material interface, i.e., it reduces the maximum impact force acting on the layered polymers. As a result, impact damage is reduced. However, the above result was based on experimental observation. In order to find the relationship between the failure mechanisms and material properties, we present a simplified model. For a projectile impact test, the first stage is elastic indentation/contact, and the second stage is a complicated failure process. It should be noted that the impact failure of layered materials with highly mismatched properties requires three-dimensional numerical simulations to obtain a complete understanding [20; 21]. In this experimental paper, we only employ a simplified elastic model in order to understand some mechanics insight for our experimental studies, which will be helpful for new material designs.