## 4 Conclusions  
The composite honeycomb sandwich structure is more and more applied to the field of UAV, and the impact resistance of the honeycomb structure is becoming more and more important. The performance of the composite honeycomb structure under impact is compared by numerical simulation, and its damage characteristics and the residual velocity of the fragment are analyzed. Four structures with better energy absorption characteristics are obtained by using the energy absorption criterion. In order to verify the accuracy of the theoretical model and the calculation method of fragment residual, the damage performance of the composite honeycomb sandwich structure impacted by fragments at different speeds was experimentally studied. The actual damage characteristics of the honeycomb sandwich structure were analyzed. Based on the energy absorption characteristics of the structure, the response differences of different structures to impact energy were obtained. The main conclusions are as follows:  
(1) The upper and lower core layers are all stainless steel. The structure with a cell diameter of 3 mm has the strongest impact resistance, and the energy absorption is about 30.6% higher than the other three design structures, which is consistent with the experimental test and numerical simulation results. In the test speed range, the unit volume energy absorption of the optimized composite honeycomb sandwich structure reaches \(521.6\times 10^{3}\,\mathrm{J/m^{3}}\)-\(659.6\times 10^{3}\,\mathrm{J/m^{3}}\). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**NO.**} & \multicolumn{2}{c}{**Total Energy Absorption per**} & \multirow{2}{*}{**NO.**} & \multicolumn{2}{c}{**Total Energy**} & \multirow{2}{*}{**Energy Absorption per**} \\  & **Absorption (N/m)** & & & & **Absorption (N/m)** & **Unit Volume (\(\times 10^{3}\) J/m\({}^{3}\))** \\ \hline
1\# & 72 & 168.4 & 13\# & 229 & 535 \\
2\# & 89.9 & 210.3 & 14\# & 89.3 & 208.9 \\
3\# & 65.3 & 152.7 & 15\# & 138 & 322.8 \\
4\# & 193 & 451.5 & 16\# & 232 & 542.7 \\
5\# & 37.4 & 87.5 & 17\# & 220 & 514.6 \\
6\# & 44.8 & 104.8 & 18\# & 200 & 467.8 \\
7\# & 66 & 154.4 & 19\# & 138 & 322.8 \\
8\# & 50.5 & 118.1 & 20\# & 340 & 795.3 \\
9\# & 163 & 381.3 & 21\# & 244 & 570.8 \\
10\# & 167 & 390.6 & 22\# & 282 & 659.6 \\
11\# & 202 & 472.5 & 23\# & 274 & 641 \\
12\# & 232 & 542.7 & 24\# & 223 & 521.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Energy absorption of composite honeycomb sandwich structure (Calculated data). (2) The damage diameter of the front skin of the composite honeycomb sandwich structure is similar to the diameter of the fragment, and the damage diameter of the rear skin is different. The carbon fiber skin is blocked by the damaged material. The aperture is small, and the mechanical response of the whole structure is more obvious when the skin material is metal. The different degrees of damage to the rear skin of the composite honeycomb sandwich structure are mainly affected by the skin material and the honeycomb diameter. When the honeycomb diameter is 3 mm, the connection effect on the rear skin is more obvious, resulting in increased collateral damage at the rear. (3) The four groups of composite honeycomb sandwich structures designed have obvious deformation in the process of impact, except that the deformation of the bottom skin of the ST-6-6 structure is not obvious, and the deformation is more obvious with the increase in structural resistance. The velocity load characteristics of the composite honeycomb sandwich structure are obvious, and the residual velocity curve of the fragment after the low-speed impact structure can show a linear trend. Through the numerical simulation and experimental test of the four composite honeycomb sandwich structures, it is shown that the material model and simulation method used in the numerical simulation have certain engineering significance and can be used to test the energy absorption effect of the structure in the high-speed range.