## 2 Fragment Oblique Impact Honeycomb Sandwich Structure Test
### Numerical Simulation Model  
In order to realize honeycomb sandwich structures with different structural parameters, this paper uses a parametric modeling method to obtain a numerical simulation model with adjustable structural parameters. The composite honeycomb structure core and the overall modeling process use Truegrid 3.1.3 software. The model is re-established by changing the size parameters of the honeycomb cell. The solver part uses ANAYS LS-DYNA to set and calculate the control parameters. The honeycomb structure in the numerical simulation of this paper adopts the Lagrangian grid. The Lagrangian grid is a hexahedral solid element. Each hexahedron has eight nodes. The oblique impact process of the fragment is a pure solid grid contact. In order to approximate the actual situation, the contact between the skin and the core layer 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 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. In order to achieve consistency with the experiment, the surrounding of the honeycomb sandwich structure is fixed, and the constraints include the movement and rotation of the X, Y, and \(Z\) directions. After the numerical simulation is completed, the numerical simulation results processing software, Ls-prepost, is used to quantify the simulation results. The LS-DYNA 18.2 display dynamic damage analysis software was used to simulate and analyze the 16 groups of models and 2 groups of comparative models. The six types of composite honeycomb sandwich structures are shown in Figure 3.  
The MAT_JOHNSON_COOK model is selected as the material model of the four composite honeycomb sandwich structures designed in this paper. This model can better describe the basic properties of the model of metal solid materials, including density \(\rho\) and shear modulus \(G\). The equation of state can reflect the influence of the strain rate effect based on the inherent properties of the material (initial yield stress \(A\), strain hardening modulus \(B\), strain rate sensitivity coefficient \(C\), hardening index \(n\)) and integrate the temperature dependence of the material into the material equation (temperature softening coefficient \(m\), material melting temperature \(T_{\text{m}}\), room temperature \(T_{\text{r}}\)), in which the metal material is accompanied by the EOS_GRUNIEISEN equation of state. The material model parameters of 2024 aluminum, 304 stainless steel, and 7075 aluminum alloy are shown in Table 1. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Material** & \(\rho\) **(kg/m3)** & \(G\) **(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 1: Material parameter. In the two groups of structural models compared, nylon material and fiber material are non-metallic materials, and the density is low, at 144 kg/m\({}^{3}\) and 1740 kg/m\({}^{3}\), respectively. Therefore, * MAT_PLASTIC_KINEMATIC is selected for the two materials. The material model can better describe the strain effect and failure mode of the two non-metallic materials. The Young's modulus of NOMEX and FIBER are 1.13 GPa and 2.30 GPa, respectively. Poisson's ratios are 0.38 and 0.307, respectively. The yield stress of the material is 3.34 \(\times\) 10\({}^{-5}\) GPa and 9 \(\times\) 10\({}^{-5}\) GPa, respectively, and the failure strain of the material is 0.8 and 0.9, respectively.