## 3 Results and Discussion
### FEA Simulation for Evaluating the Dynamics of Hanji and Cornstarch in Response to Bullet Impact  
The effects of Hanji and cornstarch suspension layers on the bullet velocity were estimated via FEA simulations. The mechanical properties of these materials are provided in Table S1, as obtained from previous studies [25,26]. We specifically assumed the cornstarch suspension to exhibit solid-like behavior under high shear rates during bullet impact [18]. [PERSON] et al. researched the rheological behavior of cornstarch suspensions and estimated the complex shear modulus and yield stress in the solidified region [25,26]. These values were converted into properties of elasticity using the appropriate Poisson's ratio of 0.3. To establish the failure criteria, predefined shear failure values obtained from mechanical properties, namely the ultimate strength points of materials, were used by comparing the equivalent plastic strain (PEEQ) calculated through the built-in function in ABAQUS/CAE (see supporting information) [30]. For the composite plate, the sealing of the cornstarch suspension, which was used in the bulletproof test, was neglected to estimate only the performance of composite materials. The boundary conditions were set with the sides of the composite plate being fixed in all directions. During the initial simulations performed with plates comprising 15-mm-thick Hanji and 30-mm-thick cornstarch suspension layers, the bullet penetrated the composite plates (Figure 3). During the penetration bullet, Hanji absorbed most of the impact stress rather than the cornstarch suspension. This difference in stress absorption between these materials was attributed to the superior elastic properties of Hanji. However, in the case of the perforated hole size, the cornstarch suspension layer had much larger holes than Hanji. The impact stress of the bullet after penetrating the Hanji layer was still high enough to cause failure of the cornstarch suspension layer. The suspension showed more plastic deformation at failure, despite having a low elastic modulus and strength. Thus, FEA simulations indicated the different failure modes of the materials caused by the bullet impact. From this difference in the failure modes between these materials, we anticipated that each layer of these materials would resist the bullet impact through different mechanisms. To analyze these mechanisms, the momentum of bullets was investigated in terms of changing the linear and angular velocities during penetration (Figure 4). It was found that the linear velocity of the bullet was decreased by each layer of the materials with different slopes showing the gradients between time and linear velocity. Therefore, as the degree of the negative slope in the specific material region became stronger,  
Figure 2: Schematics depicting the bulletproof experiments for (**a**) composites with only Hanji layers and (**b**) hybrid composites composed of Hanji and cornstarch suspension (STF) layers. The respective normalized penetrated areas are also presented in the graphs. the material was effective at decreasing the linear velocity rapidly. According to the regression of the first linear equation in each penetrating region, the cornstarch suspension showed a slightly larger degree of the negative slope than Hanji as given below:  
\[\text{Linear Velocity}\;(\text{Hanji})=-0.274t+408.15 \tag{5}\]  
\[\text{Linear Velocity}\;(\text{cornstarch suspension})=\;-0.363t+407.99 \tag{6}\]  
where \(t\) is the time in \(\upmu\)s during penetration. Therefore, compared to Hanji, the STF was effective in decreasing the linear velocity of the bullet. The linear velocity of the bullet in the initial FEA simulations was decreased by approximately 11% by the composite plate. On the other hand, the angular velocity was not affected by the composite plate. This was because the total penetration time was extremely short for the bullet to interact with the materials.