## 2 Material and Methods
### Numerical Simulation Results  
Taking the fragment penetrating 1# structure as an example, the simulation process is shown in Figure 3. At 50 us, the first layer of the skin begins to sag and deform, and the upper core layer is deleted because the material deformation exceeds the set failure maximum value, and some core layer materials are exposed around the sag. At 100 us, the fragment penetrates the second layer of skin, and the lower core layer begins to fail and delete, and the middle and bottom skins deform seriously in the direction of fragment impact, the connection between the plugging block and the structure is reduced, and the gap is clearly visible. At 150 us, the fragment penetrated the underlying skin. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Outer Panel Skin** & **Number** & **Material** & **Upper Layer** & **Bottom Layer** & **Number** & **Material** & **Upper Layer** & **Bottom Layer** \\ \hline \multirow{6}{*}{The skin material in 1#} & Al/Al & \(d3\) & \(d3\) & \(10\)\% & St/St & \(d3\) & \(d6.4\) \\  & 2\# & Al/Al & \(d3\) & d6.4 & \(11\)\% & St/St & \(d6.4\) & \(d3\) \\  & 3\# & Al/Al & \(d6.4\) & \(d3\) & \(12\)\% & St/St & \(d6.4\) & \(d6.4\) \\ \cline{1-1}  & 4\# & Al/Al & \(d6.4\) & \(d6.4\) & \(13\)\% & St/Al & \(d3\) & \(d3\) \\ \cline{1-1}  & 7075 aluminum. & 5\# & Al/St & \(d3\) & \(d3\) & \(14\)\% & St/Al & \(d3\) & \(d6.4\) \\ \cline{1-1}
18\# is unidirectional carbon fiber. & 6\# & Al/St & \(d3\) & \(d6.4\) & \(15\)\% & St/Al & \(d6.4\) & \(d3\) \\ \cline{1-1}  & 7\# & Al/St & \(d6.4\) & \(d3\) & \(16\)\% & St/Al & \(d6.4\) & \(d6.4\) \\ \cline{1-1}  & 8\# & Al/St & \(d6.4\) & \(d6.4\) & \(17\)\% & NOMEX-AL & & \(d3\) \\ \cline{1-1}  & 9\# & St/St & \(d3\) & \(d3\) & \(18\)\% & NOMEX-C & & \(d3\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Material parameter. Considering the influence of materials on the energy absorption characteristics of the structure under the same structural size, this paper divides the 1# and 9#, 2# and 10#, 3# and 11#, and 4# and 12# structures into 4 groups and compares and analyzes the energy absorption effect. Figure 4 is the comparison of skin energy absorption effect. Figure 4 shows the comparison of skin energy absorption per unit time of 8 groups of structures with the same composition. The amount of skin bearing capacity depends on the support degree and the maximum deformation degree of the core layer during the deformation process. Due to the inertia effect of the core layer of the honeycomb sandwich structure, the energy absorption effect of the first layer skin is not much different. At the same time, due to the different support degrees of the core layer, the energy absorption of the middle skin changes, and the two curves are obviously separated. The plastic deformation space of the structural skin with a low support degree increases, and the energy absorption is relatively improved. The structure with high support is obviously subjected to shear action, and the crushing time and density will increase accordingly, which will apply a large amount of load to the bottom skin in advance and deform, resulting in a relatively low level of the energy absorption curve in the later stage. Figure 3: Progress of fragmentation impacting composite honeycomb sandwich structure of 1#. (a) 0 \(\upmu\)s. (b) 50 \(\upmu\)s. (c) 100 \(\upmu\)s. (d) 150 \(\upmu\)s. (e) Structural failure section. From Table 5, it can be found that the energy absorption per unit volume of the structure with the core material of 304 stainless steel is higher than that of the structure with the core material of 2024 aluminum, and the same type of structure is 24.7%, 13.2%, 17.6% and 11.3% higher, respectively. It can be seen that the energy absorption characteristics of the composite honeycomb sandwich structure have a great relationship with the material composition of the structure.