Lightweight but high energy absorption performance structures are desired to satisfy demands in applications of aerospace, automotive, manufacturing, as well as packaging. Cellular structures consisting of spatially distributed mass and voids have been widely used for impact mitigation scenarios. Recent advancements in additive manufacturing have resulted in processes to precisely control the complex microstructure of these structures, leading to an improved mechanical performance. Recently, a new class of bio-inspired auxetic structure, namely the hierarchical re entrant honeycomb (H-ReH) has been developed. The mechanical properties of the H-ReH are controlled by multiple structural parameters, in which the slenderness ratio (SR) of structural members is among the most critical ones. Based on classical beam theory, the deformation modes of each structural member are highly sensitive to the SR, which determines the overall stability and crashworthiness of the structure. It is necessary to systematically investigate and quantify the effect of SR on the mechanical properties of H-ReHs. In this study, H-ReHs with SR ratios in a wide range have been manufactured by polyjet 3D printing technique. The mechanical properties of the printed H-ReHs have been fully characterized by quasi-static compression tests and full-field Digital Image Correlation (DIC) technique. The evolution of strain field in the structures under compression has been analyzed. The experimental results indicated that the SR is related to the transition between bending- to stretching- dominated deformation modes. The effect of SR becomes more significant when the relative density of the H-ReHs is lower. In addition, the specific energy absorption (SEA) as well as the energy absorption plateau stress have been improved by the SR at the same relative density. These findings will guide future design and material selection of lightweight but high energy absorption structures.
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