## 2 Numerical Simulation Analysis of Fragment Impact Composite Honeycomb Sandwich Structure
### Numerical Simulation Results Analysis  
The overall damage and core damage effects of the 1#-16# structure in the numerical simulation are shown in Figure 2.  
Figure 2: _Cont._For these two composite methods, the minimum opening diameter of the skin is concentrated in the middle skin position, where the strength of the lower material is high, which indicates that the strength of the lower core material has a great influence on the opening diameter of the middle skin. The higher the strength, the greater the bearing capacity of the middle skin; the deformation to the impact direction of the fragment is completely limited; and the shear effect around the fragment increases, resulting in the minimum opening of the structure. The position of the middle skin is at the middle skin position, and the opening is relatively neat, and the tensile strength of the material is not obvious. Based on the overall damage to all structures and the damage to the core layer, the influence of the 16 composite methods designed on the degree of structural deformation is different. Among them, the overall deformation of the small-sized high-strength material composite structure is small, and the impact resistance is relatively high. The second is the honeycomb sandwich structure composed of two different strength materials, and the structure composed of low-strength and high-plastic materials has a relatively poor effect. For a structure with the same average cell density and different combinations of upper and lower core materials, the damage situation is not much different, but it has a certain influence on the deformation degree of the overall structure. The lower the strength of the lower layer material, the greater the overall deformation of the structure. On the contrary, with the increase in strength of the lower core layer, the deformation of the overall structure is relatively small. It can be seen that the anti-damage performance of the composite honeycomb sandwich structure is determined by the composite method, material selection, and core size ratio.  
According to the law of conservation of energy, the total energy absorption of the structure in each case is obtained, as shown in Table 3.  
Due to the different composition of the structure, a simple analysis of the overall energy absorption of the composite honeycomb sandwich structure cannot reflect the real energy absorption characteristics of the structure. Therefore, this paper measures the energy absorption effect of the structure by the energy absorption index per unit volume, that is:  
\[EPV=\frac{EAT}{V} \tag{2}\]  
_EAT_ is the total energy absorption of the structure, and \(EPV\) is the energy absorbed by the unit volume of the structure. Table 4 shows the numerical simulation results of energy absorption per unit volume for 18 types.  
Figure 2: Damage to 16 groups of honeycomb structures. (**a**–**p**) are separately for 1#-16#.  
From the energy absorption per unit volume, it can be seen that the energy absorption effect of the designed 16 groups of composite honeycomb sandwich structures is much larger than that of the two groups of structures. The minimum energy absorption per unit volume is 53% and 307% higher than that of NOMEX-AL and NOMEX-C structures, and the maximum value is 102.3% and 438.8% higher, respectively.  
Combined with the overall failure mode of 16 groups of structures, the core failure mode, and the energy absorption per unit volume, it can be found that among all the composite methods, the structure with the best energy absorption effect and the best impact resistance and deformation ability is the 9# structure with the upper and lower core cell diameters of 3 mm, and the energy absorption per unit volume is 596.5 J/m\({}^{3}\).