## 3 Results and Discussion
### Hole Diameters in the Witness Clay Pieces after Penetration  
During bulletproof tests in the field, the cornstarch suspension showed outstanding resistance to bullet impact as the normalized perforated area decreased. Comparisons between the normalized perforated areas calculated in the experiments and simulations showed that as the thickness of the STF layers increased, the area of the witness clay penetrated by the bullet decreased owing to the decreasing momentum of the bullet (Figure 2b). However, the normalized perforated area increased with increases in the Hanji layer thickness, even though the linear velocity of the bullet was decreased in the Hanji region (Figure 2a). Thus, it was assumed that in addition to the momentum of the bullet, the impact stress also propagated through the composite plate, and it was eventually transferred to the clay piece. During penetration, Hanji had a much higher order of stress distribution over a broad region compared to that of the cornstarch suspension (Figures 5 and 6). It could be interpreted that the witness clay received a higher level of stress from the end of the Hanji layers. Based on the bullet momentum and impact stress propagation, the results of bulletproof performance tests conducted in the field and those of the additional FEA simulations were examined to evaluate the normalized perforated area. [PERSON] et al. and several researchers calculated von Mises stress distribution in layered materials through FEM and utilized their calculations to analyze their own issues [35; 36; 37]. Under the same conditions used during FEA to analyze the momentum of bullet, nodes on the 40 mm \(\times\) 40 mm region on the back side were selected as the medium for stress propagation to the clay (Figure S5). Until the time that the bullet penetrated the plate completely, the von Mises stresses on these nodes were added up. Therefore, the stress momentum generated by stress propagation and bullet momentum generated by the linear velocity at the specific time were calculated using the sum of the von Mises stresses at the nodes as follows:  
\[M_{stress}=t_{p}\times A\times\sigma_{Tot} \tag{10}\]  
\[M_{bullet}=m_{bullet}\times v_{p} \tag{11}\]  
where \(t_{p}\) and \(v_{p}\) respectively represent the time and velocity of the bullet at complete penetration, \(\sigma_{Tot}\) is the total von Mises stress at the selected nodes until \(t_{p}\), \(A\) is the area of selected nodes, and \(m_{bullet}\) is the mass of the bullet. According to FEA results, the stress momentum in the Hanji layers was much higher, of the order of approximately \(\sim 10^{7}\), than that of the cornstarch suspension having the same thickness (Figure 7a,b). Specifically, the stress momentum values of the Hanji layers (with thicknesses of 30, 45, and 60 mm) largely increased to 1155.0, 1947.7, and 3012.2 kg\(\cdot\)m/s, respectively, while those of the cornstarch layers with the same thicknesses increased slightly to \(0.9\times 10^{-3}\), \(1.3\times 10^{-3}\), and \(1.7\times 10^{-3}\) kg\(\cdot\)m/s compared to the stress momentum values of the Hanji layers. Furthermore, the linear velocity of the bullet in the cornstarch suspension decreased considerably, resulting in a lower bullet momentum compared to that in Hanji (Figure 7c,d). The bullet momentum values in the Hanji layers (30, 45, and 60 mm) were 3.1917, 3.0684, and 2.9420 kg\(\cdot\)m/s, respectively, while those in the cornstarch layers (with the same thicknesses) were respectively 3.1704, 3.0355, and 2.9068 kg\(\cdot\)m/s.