## 3 Experimental Results and Discussion  
### Drop Hammer Test  
Based on the results of numerical simulation, four groups of 304 stainless steel honeycomb cores with the best protective performance were selected from 16 groups of structures with 2024 aluminum and 304 stainless steel as honeycomb cores for drop hammer impact experiments and compared with 7075 aluminum and NOMEX honeycomb core composites and unidirectional carbon fiber and NOMEX honeycomb core composites. Figure 7 is the honeycomb core layer pattern.  
Figure 6: Total energy absorption curves of 8 structures. (**a**) The total energy absorption per unit time of 1#-4# structure; (**b**) the total amount of energy absorption per unit time of 9#-10# structure.  
The core layer was cut into the expected experimental size of 150 mm \(\times\) 150 mm, and the core layer was bonded to the skin by K-7007 room temperature curing special strong adhesive. The bonded honeycomb sandwich structure was placed at a ventilated place, and a 50 kg load was applied to the upper surface of the specimen for 20 days to achieve the purpose of firm bonding.  
There are six types of composite honeycomb sandwich structures in the drop hammer impact experiment, which are the diameter of the double-layer honeycomb cell is 3 mm, the diameter of the upper honeycomb cell is 3 mm, the diameter of the lower honeycomb cell is 6.4 mm, the diameter of the upper honeycomb cell is 6.4 mm, the diameter of the lower honeycomb cell is 3 mm, the diameter of the double-layer honeycomb cell is 6.4 mm, and Al/NOMEX and C/NOMEX. The size of the specimens is 150 mm \(\times\) 150 mm. The diameter of the NOMEX honeycomb cell is 3 mm, and the thickness of the core layer is 15 mm. The thickness of single layer core of 304 stainless steel honeycomb is 8 mm; 7075 aluminum skin thickness of 1 mm; and the thickness of the unidirectional carbon fiber skin is 2 mm. Figure 8 is the six types of composite honeycomb sandwich structures.  
Figure 7: Honeycomb core layer pattern. (**a**) \(D\) = 3 mm 304 Stainless steel honeycomb core. (**b**) \(D\) = 6.4 mm 304 Stainless steel honeycomb core. (**c**) \(D\) = 3 mm NOMEXhoneycomb. (**d**) 7075 aluminum skin.  
Figure 8: Six types of composite honeycomb sandwich structures. (**a**) C/NOMEX. (**b**) Al/NOMEX. (**c**) Top \(D\) = 3 mm/Bottom \(D\) = 3 mm. (**d**) Top \(D\) = 3 mm/Bottom \(D\) = 6.4 mm. (**e**) Top \(D\) = 6.4 mm/Bottom \(D\) = 3 mm. (**f**) Top \(D\) = 6.4 mm/Bottom \(D\) = 6.4 mm.  
Where \(D\) = 3 mm represents the cell diameter of the honeycomb core is 3 mm, \(D\) = 6.4 mm represents the cell diameter of the honeycomb core is 6.4 mm, TOP represents the upper layer, and the bottom represents the lower layer. AL/NOMEX represents the core NOMEX material, skin 7075 aluminum, C/NOMEX represents that the core material is NOMEX, and the skin material is unidirectional carbon fiber.  
The STLH-150 drop weight test machine, produced by Shangtai Experimental Instrument Co., Ltd. (Jinan, China), was used to carry out impact experiments on the samples. The maximum impact energy of the experimental machine is 150 J, the diameter of the impact head is 20 mm, the reserved size of the punch is 75 mm \(\times\) 125 mm, and the remaining height space of the impact is 300 mm. The specimen is fixed by the base of the impact support fixture and the fixed clamp with a rubber head. The feedback data of the drop test machine are collected by the instrumented drop control system, and the process of the drop penetrating the composite honeycomb sandwich structure is collected by using the high-speed photography equipment. The experimental device is shown in Figure 9.