3D-Printed Hierarchical Re-Entrant Honeycomb With Improved Structural Stability Under Quasi-Static Compressive Loading

Re-entrant honeycombs with negative Poisson’s ratio have shown great potential as lightweight energy absorbers for various applications. However, due to its bending-dominated behavior, the structural stability and energy absorption capacity of reentrant honeycombs are yet to be further improved. It has been demonstrated that hierarchical structures exhibit a combination of lightweight and superior mechanical properties. We hypothesize that by introducing the triangular hierarchical substructures into the conventional cell walls, the bending-dominated behavior of re-entrant honeycombs can be converted into the stretching-dominated one. Consequently, the overall structural stability of the hierarchical re-entrant honeycombs can be promoted through local deformation of hierarchy, which can potentially benefit the energy absorption capacity of the resulted structure. To test our hypothesis, we first fabricate the hierarchical reentrant honeycombs with length scale ranging from micrometer to centimeter using Polyjet 3D-printing technique. Regular reentrant honeycombs with solid struts have been fabricated as baseline structures. The mechanical performance of the honeycombs has been characterized through uniaxial quasi-static compression tests. Besides, the local deformation mechanisms of the hierarchical structure have been revealed by the Digital Image Correlation (DIC). In comparison to the regular re-entrant honeycomb, the global failure strain of hierarchical re-entrant honeycomb is enhanced by 36%. This is due to the improved structural stability from local fracture and densification of the triangular hierarchy. Both the regular and hierarchical honeycombs exhibit the same specific energy absorption capacity. As predicted by the existing scaling laws, the hierarchical re-entrant honeycomb has great potential to outperform regular one by optimizing the relative density of the structure. A finite element model of the hierarchical re-entrant honeycomb has been developed by using commercial software Abaqus/CAE 2020. The model has been calibrated by the experimental data. Within the elastic region, the simulated deformation modes show good agreement with experimental observations. When the relative density of the regular re-entrant honeycombs equals to the hierarchical ones, the model predicts that the hierarchical re-entrant honeycombs have superior energy absorption performance with enhanced stiffness and yield strength in comparison to the regular ones. In conclusion, this study has demonstrated that by introducing hierarchical structure into re-entrant honeycomb, the structural stability has been improved. Furthermore, the hierarchical structure endows re-entrant honeycomb with lightweight yet competitive energy absorption capacity.