Lightweight materials and structures with extraordinary mechanical properties are desired for automotive, aerospace, and biomedical applications. Conventional lightweight materials such as foams have low density, large deformation and adjustable yield strength, and thus they are widely used in engineering applications. However, the mechanical performance is still far from satisfactory, due to the bending-dominated deformation mechanism determined by the stochastic microstructures. Inspired by nature, a novel type of lightweight materials, namely the hierarchical re-entrant honeycombs (H-ReHs) has been developed by replacing the traditional cell walls of re-entrant honeycombs by a second-order triangular hierarchy. The H-ReHs comprise fully controllable cellular structures ranging from microscale to macroscale. In addition, the geometry and pattern of the structures are precisely controlled via the emerging additive manufacturing techniques. We hypothesize that the H-ReHs outperform conventional lightweight materials and structures due to the microstructure of the material system, the unique interaction between the structural hierarchies, and the negative Poisson's ratio effect. To test this hypothesis, H-ReHs with different geometric designs and printing materials have been manufactured. The mechanical performance of the H-ReHs has been fully characterized by quasi-static compression tests, dynamic impact tests, and quasi-static oblique tests. To reveal the deformation mechanisms of the H-ReHs, high-speed camera and 3D-digital image correlation have been utilized to analyze the internal strain field of the specimens under various loading conditions. It has been demonstrated that the 3D-printed H-ReHs exhibit enhanced specific stiffness, specific initial-buckling strength, structural stability, and specific energy absorption capacity in comparison to regular re-entrant honeycombs (R-ReHs). The H-ReHs exhibit stretching-dominated elastic behavior due to the second-order triangular hierarchy. The post-buckling behavior of the H-ReHs is more stable than the R-ReHs under various strain rates due to the interaction between the first and the second structural hierarchies. These findings provide insights into the design of next-generation lightweight yet mechanically robust materials and structures for future applications and mitigate energy crisis.
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