## 2 Dynamic Compression Test at the High Strain Rate
### Comparison at Different Strain Rates  
The mean values of the dynamic compression test results (the strain rate of 1.3 \(\times\) 10\({}^{3}\) s\({}^{-1}\), 2.0 \(\times\) 10\({}^{3}\) s\({}^{-1}\), 4.6 \(\times\) 10\({}^{3}\) s\({}^{-1}\), 6.67 \(\times\) 10\({}^{-4}\)) are fitted into the strain rate curves and the stress-strain curves at different strain rates, as shown in Figures 16 and 17, respectively. The compression of the quasi-static and the strain rate of 4.6 \(\times\) 10\({}^{3}\) s\({}^{-1}\) both experience the elastic stage, the plateau stage and the densification stage. However, the strain rates of 1.3 \(\times\) 10\({}^{3}\) s\({}^{-1}\) and 2.0 \(\times\) 10\({}^{3}\) s\({}^{-1}\) only experience the elastic stage and the plateau stage, as shown in Figure 17. It can be seen that there is no fluctuation at Quasi-static state, only one fluctuation at strain rate of 1.3 \(\times\) 10\({}^{3}\), two fluctuations at a strain rate of 2.0 \(\times\) 10\({}^{3}\), and three fluctuations at a strain rate of 4.6 \(\times\) 10\({}^{3}\). The fluctuation appears in the plateau stage, which is because the cellular wall instability and buckling during the compression. It can be seen that the fluctuations increase with the strain rate increases. Figure 16: Mean stress rate curve at various strain rate. Figure 17: Mean stress–strain curve at various strain rate. The yield strength, the average plateau stress and the initial densification strain of the aluminum honeycomb structure at different strain rates are shown in Table 6. The yield strength ratio at the strain rate of \(2.0\times 10^{3}\) s\({}^{-1}\) to the strain rate of \(1.3\times 10^{3}\) s\({}^{-1}\) is 1.03:1, while the average plateau stress ratio of them is 1.02:1. Additionally, the yield strength and the average plateau stress at the strain rate of \(2.0\times 10^{3}\) s\({}^{-1}\) and \(1.3\times 10^{3}\) s\({}^{-1}\) are both higher than that at the strain rate \(6.67\times 10^{-4}\). It is indicated that the aluminum honeycomb structure processes the stress hardening effect. The yield strength ratios at the strain rate \(4.6\times 10^{3}\) s\({}^{-1}\) to the strain rate \(1.3\times 10^{3}\) s\({}^{-1}\) and \(2.0\times 10^{3}\) s\({}^{-1}\) are 0.91:1 and 0.88:1, respectively, while the average plateau stress ratios of them are 0.95:1 and 0.93:1. It is indicated that the dynamic compression mechanical properties at the high strain rate of \(4.6\times 10^{3}\) s\({}^{-1}\) process the stress softening effect. The stress hardening effect and softening effect are both the combination of the characteristic of the honeycomb structure itself and the sealed gas effect. The characteristic of the honeycomb structure makes it exhibit higher stress in the dynamic compression tests at the high strain rate, however, the sealed gas effect plays the dominant role. As the loading time is short in the dynamic compression tests at the high strain rate, the cell walls are not damaged, and the gas in the honeycomb cells can be approximately sealed, leading to the higher gas pressure. As the impact bar velocity increases, the yield strength and the plateau stress at the strain rate of \(2.0\times 10^{3}\) s\({}^{-1}\) are higher than those at the strain rate of \(1.3\times 10^{3}\) s\({}^{-1}\). However, as the impact bar velocity increases further, the cell walls seems partly damaged in the dynamic compression tests at the strain rate of \(4.6\times 10^{3}\) s\({}^{-1}\) and the gas in the honeycomb cells can be partly sealed, leading to the lower gas pressure, so the yield strength and the plateau stress at the strain rate of \(4.6\times 10^{3}\) s\({}^{-1}\) are lower than those at the strain rate of \(1.3\times 10^{3}\) s\({}^{-1}\) and \(2.0\times 10^{3}\) s\({}^{-1}\). Additionally, the gas flow out fully and the sealed gas effect is negligible in the process of the quasi-static compression, leading to the ambient gas pressure in the honeycomb cells, therefore the yield strength and the plateau stress at quasi-static are the lowest.