Effect of Cell-Wall Angle on the Mechanical Properties of 3D-Printed Hierarchical Re-Entrant Honeycomb

Abstract

Both hierarchical and auxetic structures have shown unusual mechanical properties and draw great attention for multiple engineering applications. Recently, a triangular 2nd order of hierarchy has been successfully integrated into re-entrant honeycomb, one specific type of auxetic structures, by the emerging additive manufacturing method. The resulted hierarchical re-entrant honeycomb (H-ReH) outperforms the conventional re-entrant honeycomb (C-ReH) in stiffness, initial buckling strength, densification strain and specific energy absorption capacity (SEA). However, the optimized designs of the cell structures in H-ReH are still elusive and yet to be explored, which is critical for advanced safety applications. The mechanical performance and deformation mode of H-ReH are mainly determined by the geometric parameters of the structure, among which the cell-wall angle is one of the most critical design parameters. To this end, we designed H-ReHs with three different cell-wall angles, i.e. 60°, 75° and 90°. C-ReHs with the same three angle designs were processed through the same 3D-printing method as reference samples. The mechanical performance of the fabricated specimens was characterized by the uniaxial quasi-static compression tests. The evolution of the strain field in all the samples was measured and analyzed by the Digital Image Correlation (DIC). The results show that the angle designs have significant influences on the elastic modulus, strength, structural stability, and SEA of H-ReH. By increasing the angle from 60° to 75°, the densification strain and the SEA are increased by 60% and 75%, respectively. This is due to the altered deformation modes of the H-ReHs with different cell-wall angles. By contrast, the C-ReHs are found to be nearly inert to the angle change, due to its bending-dominated behavior regardless of the cell-wall angle change. When further increase the cell-wall angle to 90°, both H-ReH and C-ReH exhibit notable enhancement on the elastic modulus and the strength, but at a much-compromised structural stability. The vertical member of both structures buckles and fractures at a small strain. In conclusion, this study has demonstrated that the mechanical properties of H-ReH is sensitive to the cell-wall angle. Furthermore, the H-ReH has much better mechanical tunability over C-ReH through the angle designs due to its unique deformation mechanisms. These findings will guide the future design of H-ReH and other types of lightweight robust materials and structures.