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Metamaterials that exhibit negative Poisson's ratio (NPR) are known as auxetics. They laterally expand or contract when they are axially tensioned or compressed. A re–entrant chiral auxetic structure (RCA) is a recently developed structure, which combines the topological features of re–entrant and chiral honeycombs. Comparative study between the RCA structure and popular benchmarks subjected to uniaxial compression has been conducted experimentally and numerically. Specimens have been fabricated from polyamide12 (PA12) using Multi Jet Fusion (MJF). Numerical models have been developed using ABAQUS/Explicit and verified by the experiments. The experimental measurements manifest the high accuracy of the MJF process to produce robust components with precise details. It has been found that the RCA structure outperforms the other types of honeycombs in terms of strength and specific energy absorption when loaded in the Y direction, while only the tetrachiral honeycomb surpasses the RCA structure in terms of specific energy absorption when loaded in the X direction. The auxeticity (NPR) of the RCA structure compressed in the Y direction was larger than that of the other honeycombs. Numerical models have been employed to study the effects of friction and the number of cells on the mechanical response of the RCA structure. [ABSTRACT FROM AUTHOR]

• An innovative hybrid tube with auxetic outer layer was studied experimentally. • The crashworthiness indicators of auxetic tubes and hybrid tubes were measured and compared. • The effects of the material of auxetic tubes and the shape of holes were examined. Hybrid tubes of different forms have been designed and used in various sectors such as automotive, medical, pipeline, and aerospace industries for protective equipment, oesophageal stents, oil and gas pipes, and superior vibration dampers. Auxetic materials and structures have received increasing attention due to their unique properties such as negative Poisson's ratio. They laterally expand under uniaxial tension or laterally contract under uniaxial compression. In this work, an innovative hybrid tube with an auxetic tube as the outer tube and a conventional tube as the inner tube has been investigated. All the inner tubes were made of extruded aluminum alloy 6063 with T5 heat treatment, and the outer tubes were made of stainless steel 304 and 316L with oval holes, respectively. Quasi-static compressive tests have been conducted to study the crashworthiness of such hybrid tubes. The effects of outer tube material and geometry on the deformation mode, force–displacement curve and energy absorption of hybrid tubes have been discussed. The results show that the outer auxetic tube changes the deformation of the inner tube. The load carry capacity and energy absorption of a hybrid tube increase with the strength of the outer auxetic tube. [ABSTRACT FROM AUTHOR]

Auxetic materials exhibit interesting mechanical properties. In this work, a novel auxetic unit cell was proposed with an aim to improve the strength and energy absorption of the auxetic structure constituting these unit cells. In the developed unit cell, the vertical member of a conventional re-entrant unit cell was replaced by a diamond. The corresponding structure was named re-entrant diamond structure. Specimens were fabricated using Multi-Jet Fusion (MJF) technique and experimentally tested to investigate their mechanical performance under uniaxial quasi-static and dynamic compression. Finite element model was also developed in ABAQUS/Explicit and validated using experimental results. Moreover, in order to improve stiffness of the diamonds, a cross-link member was introduced in each diamond. Subsequently, a parametric study was conducted to study the influence of diamond angle (θ 2) and length ratio (L 2 /L 1) on the deformation mode, stress–strain curve, Poisson's ratio, and energy absorption of the re-entrant diamond structures with and without cross-links. This study revealed that larger diamond angle (θ 2) and smaller length ratio (L 2 /L 1) are desirable for high strength. Furthermore, the re-entrant diamond structures with and without cross-links were compared with the conventional re-entrant and hexagonal honeycombs in terms of strength and energy absorption. [ABSTRACT FROM AUTHOR]