## 3 Experimental Results and Discussion
### Analysis of Energy Absorption Characteristics of Composite Honeycomb Sandwich Structure  
The energy absorption index is one of the main factors considered in structural design. In the process of drop impact, the gravitational potential energy of the drop is converted into kinetic energy to penetrate and compress the structure, but the kinetic energy is quadratic, and the force analysis process is more complicated. Therefore, the energy absorption characteristics of the structure are analyzed by returning to the gravitational potential energy itself. Since the potential energy U has a linear relationship with the experimental force F, and the potential energy U also has a linear relationship with the displacement S, the bilinear function is used to represent the energy absorption characteristics of the experimental specimen, that is, U = FS [24]. The force-time and displacement-time curves obtained by the impact of the specimen were integrated by using the acquisition system of the drop test machine, and the force-displacement curves under different impact energies were obtained to judge the energy absorption characteristics of the structure. The force-displacement curves of the six structures under different impact energies are shown in Figure 11.  
Figure 11: Load–displacement curves of six different honeycomb sandwich structures. (**a**) Top 3/Bottom 3. (**b**) Top 3/Bottom 6.4. (**c**) Top 6.4/Bottom 3. (**d**) Top 6.4/Bottom 6.4. (**e**) Al-NOMEX. (**f**) C-NOMEX. The total energy (\(TEA\)) absorbed by the composite honeycomb sandwich structure during impact is caused by impact. The total energy value is the area enclosed by force-displacement, and the expression is [25]  
\[TEA=\int Fds \tag{2}\]  
The total energy absorption of the honeycomb sandwich structure is obtained by summing the integral area of the curve. The total energy absorption of the 12 groups of structures is shown in Table 9.  
Through Tables 7-9, it can be found that with the increase in the damage degree of the composite honeycomb sandwich structure, the total absorption energy of the structure gradually increases. Because Al-NOMEX and C-NOMEX are completely penetrated under different impact energies, the energy absorption limit of the structure is about 80 J and 64 J. However, through the instantaneous damage compression and energy absorption characteristics, it can be seen that for the buffer structure, the composite honeycomb sandwich structure with the upper and lower core cell diameters of 6.4 mm has a good energy absorption effect in high-speed buffering, and the structure collapses quickly, which has a good reference for the design of one-time buffer structure. The structure with the cell diameter of the upper and lower core layers of 3 mm has the smallest impact displacement of the drop hammer under the same energy impact, the deformation of the underlying skin is not obvious, and the crashworthiness advantage is obvious, which has a good reference for the design of the anti-collision structure. Due to the different structural configurations, the equal displacement method cannot be used to judge the overall energy absorption characteristics of the structure. In order to more intuitively observe the energy change when the drop hammer penetrates the structure, the energy-time curve is drawn according to the feedback information, as shown in Figure 12.  
Through the energy-time curve, it can be seen that the limit energy of the drop is transferred to the specimen, and the time to reach the maximum value is different. The shorter the time to reach the maximum value, the stronger the resistance characteristics of the structure, and the stronger the cushioning performance of the structure. In the process of impact, the transient energy of the two groups of curves Figure 12e,f did not reach the maximum value. It can be seen that the advantages of Al-NOMEX and C-NOMEX structures in both anti-impact performance and energy absorption characteristics are not obvious.