# Impact Testing of 3D Re-Entrant Honeycomb Polyamide Structure Using Split Hopkinson Pressure Bar
## 4 Results and Discussion
### Energy Absorption  
Figure 8 shows the energy absorption curves of different specimens at different impact velocities. Energy absorption is defined as the area enclosed by the stress-strain curves at a certain strain, namely, deformation energy per unit volume. In general, when the strain value is the same, C2 has the best energy absorption capacity, C5 comes second, and C3 has the worst. Therefore, when the structure has no obvious defect, the smaller the re-entrant angle (\(\theta\)) or the more stable the structure, the stronger the energy absorption capacity; when the structure has obvious defects, the energy absorption capacity is poor.  
Figure 6: Stress–strain curves of the specimens under different working conditions. (a) v0 = 14 m/s, (b) v0 = 18 m/s, (c) v0 = 22 m/s.  
Figure 9 compares the energy absorption of specimens C2, C3 and C5 under different impact velocities and quasi-static tests. On the whole, the energy absorption capacity of specimens under impact loading is significantly enhanced compared with that of quasi-static specimens; the larger the impact velocity is, the more energy is absorbed by the specimen. Table 3 compares the energy absorption capacity of the specimens under SHPB tests and quasi-static compression tests at the same strain of 0.2. It can be found that the energy absorption capacity of the SHPB specimen is several times higher than that of the quasi-static specimen. Among them, the most stable structural impact specimen, C5, is particularly outstanding, with the highest energy absorption ratio of 15.7 times that of the corresponding quasi-static specimen. Specim C2 has the highest density, and its energy absorption capacity is the highest in both quasi-static and impact tests, which is 1.701 J/cm\({}^{3}\) at the impact velocity of 22 m/s and is 5.1 times that in the quasi-static test. Specim C5 has the lowest density, and its energy absorption capacity is the lowest in the quasi-static test; however, it has the second highest energy absorption capacity in the impact test, of 1.222 J/cm\({}^{3}\) at the impact velocity of 22 m/s, indicating the advantages of a stable structure in energy absorption under impact loading.  
Figure 7: Comparison of stress–strain curves of specimens under different working conditions. (**a**) C2, (**b**) C3, (**c**) C5.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**V** & **C1** & **C2** & **C3** & **C4** & **C5** \\ \hline quasi-static & 0.109 & 0.333 & 0.092 & 0.145 & 0.078 \\ \hline
14 m/s & 0.333/3.1 & 0.934/2.8 & 0.278/3.0 & 0.585/4.0 & 0.834/10.7 \\ \hline
18 m/s & 0.509/4.7 & 1.389/4.2 & 0.438/4.8 & 0.378/2.6 & 0.773/9.9 \\ \hline
22 m/s & 0.664/6.1 & 1.701/5.1 & 0.537/5.8 & 0.733/5.1 & 1.222/15.7 \\ \hline \end{tabular} Note: Under dynamic loading, the first value is the absorbed energy value, and the second value is the ratio of the absorbed energy of the impact specimen to that of the corresponding quasi-static specimen.  
\end{table}
Table 3: Comparison of the energy absorption (J/cm\({}^{3}\)) of the specimens under SHPB tests and quasi-static compression tests.  
Figure 8: Energy absorption curves of different specimens at different impact velocities. (**a**) v0 = 14 m/s, (**b**) v0 = 18 m/s, (c) v0 = 22 m/s.