# Impact Testing of 3D Re-Entrant Honeycomb Polyamide Structure Using Split Hopkinson Pressure Bar
## 5 Conclusions  
In this study, five 3D re-entrant (RH) structures made of polyamide with varying geometries were manufactured and SHPB impact tests were conducted with different impact velocities. The impact loading process can be explained and understood deeply by analysing the waveform of incident wave, reflection wave and transmitted wave. The test results of stress-strain curves, energy absorption ability and failure mode under impact loading were analysed and compared with the quasi-static test results. Among the tested specimens, both the strength and energy absorption capacity of all impact specimens were significantly improved compared with quasi-static specimens. C2 displayed the strongest energy absorption capacity, which was 1.701 J/cm\({}^{3}\) at the impact velocity of 22 m/s and was 5.1 times that in the quasi-static test. The energy absorption of C5 was 1.222 J/cm\({}^{3}\) at the impact velocity of 22 m/s and was 15.7 times that in the quasi-static test. C3 displayed the lowest flow stress and the weakest energy absorption capacity due to the half-reduced thickness of the oblique strut cross section (\(t_{2}\)), which was 0.537 J/cm\({}^{3}\) at the impact velocity of 22 m/s and was 5.8 times that in the quasi-static test. It was found that the structural geometric parameters had a great effect on the peak yield stress, flow stress and energy absorption capacity of the impact specimen. In general, a smaller re-entrant angle (\(\theta\)) can improve the yield stress, peak stress and energy absorption ability, such as C2. A longer horizontal length of the oblique strut (\(L_{0}\)) is helpful to improve the peak stress and energy absorption capacity, such as C5. However, the obvious weakening of structural elements is detrimental to the strength and energy absorption capacity, such as C3. The test  
Figure 10: Failure patterns of specimens. (**a**) Impacted surface of impact specimen. (**b**) Side surface of impact specimen. (**c**) Compressed surface of quasi-static specimen. (**d**) Side of quasi-static specimen.  
results can provide a reference for the structural optimization design of similar 3D auxetic structures under medium impact velocity. The trial specimens are limited and more works are required in the future to provide more accurate design predictions.  
Conceptualization, J.C.; methodology, W.T.; software, W.T.; validation, J.C. and S.P.; formal analysis, W.T.; investigation, S.P.; resources, J.C. and W.T.; data curation, W.T.; writing--original draft preparation, J.C. and W.T.; writing--review and editing, J.C.; visualization, W.T.; supervision, J.C.; project administration, J.C.; funding acquisition, J.C. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Guangdong Province higher vocational college high-level professional group construction project, grant number GSPZYQ2020015.  
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also form part of an ongoing study.  
The authors declare no conflict of interest.