## 2 Material and Methods
### Numerical Method  
The high-speed fragment model is designed with a cylindrical structure, with a diameter of 12 mm and a height of 30 mm. The speed is set to 200 m/s. The material is 45\(\#\) steel. In the simulation process, in order to eliminate the influence of fragment deformation on structural damage characteristics, the finite element model of the fragment is set as a rigid body. The finite element model of the impact structure is shown in Figure 2.  
LS-DYNA is used in the numerical simulation. In order to approximate the actual situation, the contact between the skin and the core is a fixed connection failure method. The  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Material** & \(\rho\) **(kg/m\({}^{3}\))** & **G (GPa)** & **A (GPa)** & **B (GPa)** & **n** & **c** & **m** & **T\({}_{\text{m}}\) (K)** & **T\({}_{\text{r}}\) (K)** \\ \hline
7075 Aluminum & 2810 & 26.9 & 0.520 & 0.477 & 0.52 & 0.0025 & 1.61 & 893 & 293 \\
2024 Aluminum & 2780 & 28.6 & 0.265 & 0.426 & 0.34 & 0.015 & 1 & 775 & 300 \\ Stainless steel & 7890 & 77 & 0.278 & 1.3 & 0.8 & 0.072 & 0.81 & 1800 & 298 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Metal material parameters.  
Figure 2: Finite element model of impact structure.  
contact surface between the skin and the core is firmly connected, allowing the two surfaces to slide and separate from each other after failure. The deformation of the honeycomb core is a complex process. In addition to the contact between the skin and the core, the honeycomb cells themselves also interact. Therefore, the honeycomb core is automatically set to face-to-face contact. The composite honeycomb structure adopts an eight-node hexahedral solid element. Since the composite model is symmetrical, in order to reduce the number of calculation units, a quarter model is established for calculation. In order to prevent the displacement of the edge of the composite honeycomb structure, the degree of freedom of the solid elements at the edge in the X, Y, and Z directions is limited. The rest of the solid elements have six degrees of freedom for the displacement and rotation of X, Y and Z. Therefore, in order to approximate the actual situation, the contact between the skin and the core is a fixed connection failure method. The contact surface between the skin and the core is firmly connected, allowing the two surfaces to slide and separate from each other after failure.  
In order to analyze the influence of cell size, material and composite method on the anti-damage performance of the composite honeycomb sandwich structure, a total of 18 groups of structures were designed for numerical simulation of high-speed fragment impact. According to the simulation results, several groups of composite honeycomb sandwich structures with the best protective performance were selected for experimental research. See Table 4 for details.  
Among them, Al/Al indicates that the material of the upper and lower honeycomb cores is 2024 aluminum; Al/St indicates that the upper core material is 2024 aluminum, and the lower core material is stainless steel; St/St indicates that the material of the upper and lower honeycomb cores is stainless steel; and St/Al indicates that the upper core material is stainless steel, and the lower core material is 2024 aluminum. NOMEX indicates that the core material is NOMEX; \(d3\) indicates that the core cell diameter is 3 mm; and \(d6.4\) indicates that the core cell diameter is 6.4 mm.