## 5 Conclusions  
This paper presents a numerical study on the impact behavior of a novel chiral auxetic possessing three-dimensional (3D) deformation capability. The new 3D chiral auxetic is designed by orthogonally assembling two 2D chiral auxetics but replacing the circular nodes-based conventional chiral auxetics (ACs) with hexagonal nodes-based chiral auxetics (AHs). The capability of the ABAQUS/Explicit model is validated against experimental results. The Poisson's ratio of 2D AH is numerically obtained close to the theoretical value of \(-1\), but parameter \(L\) determines the different value of Poisson's ratio between 2D AC and 2D AH when considering the axial and shear deformations of ligaments, although this deflection is very limited. Thereafter, the study on compressive behavior of 3D AH is performed numerically using a model composed of \(5\times 5\times 8\) 3D unit cells to eliminate the boundary effect. Based on the simulation results, the main conclusions can be drawn as follows:  
1. The deformation modes of 3D AH highly depend on impact velocities. The \(\overline{\sigma}\) and \(\overline{U}\) increase as the impact velocity increases. When the impact velocity remains constant, larger initial impact mass results in an increase in maximum crushing deformation. 2. Increasing the beam radius leads to a decrease in specific energy absorption under both low-speed and high-speed compressive loadings. However, the energy absorption capability of 3D AH is not sensitive to the yield strength of nodes. 3. The impact behavior that occurred in the X direction results in higher impact stress and better energy absorption performance than the impact in the Y direction in the initial stage of compression (approximately 25% for low-speed impact and 19% for high-speed impact). Afterwards, the crushing deformation in the Y direction gives rise to a stronger capability of plastic energy dissipation than the impact in the X direction. In practice, the 3D AH can be employed in the crashworthiness design and optimization of auxetic structures as energy absorbers used for structural protection and safety, since it ensures bidirectional shrinkage at the loaded zone and then results in higher resistance  
Figure 13: Normalized plastic energy dissipation against crushing strain of 3D AH with different impact directions, (**a**) impact at a velocity of 5 m/s; (**b**) impact at a velocity of 50 m/s. against impact and blast loads. The behavior and failure modes under various loadings, e.g., local impact loading and blast loading, are of larger practical relevance and will be the focus of future work. Additionally, the experimental study on impact behavior of 3D AH considering the effect of material properties, e.g., aluminum alloy and plastic used in 3D printing, will be carried out in our future work. The direct comparison of the impact mechanical properties of different 3D auxetic structures accounting for the same impact scenarios is of importance for optimum choice in practical application, which is an ongoing work and will be presented in a future publication. Methodology, D.G. ; supervision, C.Z. ; writing--original draft, D.G. ; writing--review and editing, J.Z. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (grant No. : 52201331), the National Key R&D Program of China (grant No. : 2021 YFB2600800), the Natural Science Foundation of Liaoning Province (grant No. : 2023-MS-217), the Key Laboratory of Liaoning Province (2022 JH13/10200060), the QNPY Foundation of Shenyang University of Technology (grant No. : 070/200005808) and the Talent Introduction Plan of Shenyang University of Technology (grant No. : 060/100030095). And the APC was funded by the National Key R&D Program of China (grant No. : 2021 YFB2600800). Not applicable. The data presented in this study are available on request from the corresponding author.