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Auxetic structures have theoretically superior bending resistance that was previously inaccessible. Compared with the planar two-dimensional (2D) auxetic structures, the three-dimensional (3D) counterparts may exhibit auxetic behavior in multiple directions. Recently, increasing studies have been devoted to studying the mechanical properties of 3D auxetic structures. In this paper, two 3D honeycomb structures were proposed, namely 3D re-entrant honeycomb and 3D hexagonal honeycomb. Subsequently, the mechanical properties, deformation modes, and deformation mechanisms of these two honeycomb structures under three-point bending were investigated experimentally and numerically. Compared with the 3D non-auxetic hexagonal honeycomb, the 3D auxetic re-entrant honeycomb exhibits higher ductility and fracture resistance. In addition, unlike the conventional honeycomb structure, the mid-span section of the 3D auxetic honeycomb exhibits a trapezoidal deformation mode under bending. Finally, the influence of the number of unit cells and geometric parameters on the mechanical properties of the 3D honeycomb structure were systematically studied by parametric analysis. The excellent bending performance provides a basis for the application of auxetic honeycomb in the fields of biomedicine, soft robots, and buffer devices. [Display omitted] • Two 3D honeycomb structures, namely 3D auxetic honeycomb and 3D non-auxetic honeycomb, were designed and fabricated. • The mechanical properties, failure modes, and deformation mechanism of two specimens are investigated under three-point bending. • Compared with the 3D non-auxetic honeycomb, the 3D auxetic honeycomb exhibits better ductility and a trapezoidal deformation pattern at midspan. [ABSTRACT FROM AUTHOR]

Double arrowed honeycomb (DAH) is a typical auxetic material with negative Poisson's ratio (NPR) and excellent mechanical properties, showing a wide application prospect in aerospace, marine, automotive and other fields. In this work, we introduce the double circular arc walls into the regular DAH unit cell to improve the strength and energy absorption capability of the material. According to the position of the arc wall, three circular double arrowed honeycomb (CDAH) unit configurations were proposed, including upper-circular (U-type), lower-circular (L-type), and full-circular (F-type). The prototype specimens of DAH and CDAHs were fabricated by 3D printing and tested under quasi-static crushing. The finite element (FE) models of the specimens were established and validated by the experimental results. The experimental and numerical results showed that the three types of CDAHs have higher crushing stress, crush force efficiency (CFE) and specific energy absorption (SEA) than the DAH with the same geometric parameters, which is due to the formation of more plastic hinges at the joints and the middle of the arc walls. Specifically, the CFE of L-type CDAH is 25.8% higher than that of DAH, and the SEA of F-type CDAH is 85.9% higher than that of DAH. The detailed parameter analyses of CDAH revealed that small short inclined wall angle and lower circular arc radius, and large friction coefficient value, long inclined wall angle and cell wall thickness all lead to large SEA. Among the three types of CDAH, the U-type has the greatest SEA and the most NPR effect under the same relative density. Finally, a wide range of SEA and Poisson's ratio of the CDAH can be obtained by rationally designing the arc wall position and the geometric parameters of the unit cell. [Display omitted] • Quasi-static crushing behaviors of three novel types of circular double arrowed honeycombs (CDAHs) through experimental tests and numerical simulations. • More Energy absorption than regular double arrowed honeycomb (DAH) due to formation of more plastic hinges of arc walls. • Good agreement between experimental and numerical results in terms of deformation mode, crushing stress and specific energy absorption (SEA). • Effects of cell number, geometric parameter and relative density on crushing responses of CDAHs. [ABSTRACT FROM AUTHOR]