Harnessing structural hierarchy and multi-material approaches to improve crushing performance of re-entrant honeycombs

This study proposes to improve the crushing performance of re-entrant honeycombs by considering both mechanistic and material ingredients. First, a set of novel hierarchical configurations possessing hexachiral auxetic subordinate cells was designed by replacing the solid cell walls of the conventional re-entrant honeycomb. Then the voids of the modified re-entrant honeycombs and the proposed hierarchical metamaterials were filled with a flexible material by using a multi-material 3D printer. Parametric numerical analyses were performed using validated Finite Element models to identify optimal metamaterial architectures. A set of samples, including traditional and developed configurations, were 3D printed and subjected to uniaxial compression loading. The deformation mechanism, compressive strength, and energy absorption of the samples were compared at different strain levels. The results show that introducing hexachiral hierarchies along the ribs prolongs the duration of plastic deformation and keeps the auxeticity of the samples at larger strains. This provides a stable and long post-yield plateau in the compressive stress-strain curve that brings considerable improvement in energy absorption and compressive strength. In addition, multi-material approaches with the combination of rigid photopolymer and flexible rubber-like material also contribute to stabilizing the deformation mechanism of both conventional and developed hierarchical re-entrant auxetic configurations. The proposed hierarchical auxetic structures present more specific compressive strength and specific energy absorption (up to ~150 %) compared to conventional re-entrant auxetic honeycombs. Overall, the proposed hybrid designed strategies can be leveraged to manipulate the collapse mechanism and improve crushing performance.