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Recently, auxetic metamaterials have turned into an area of growing interest, because of their deformation uniqueness, design flexibility, and functional diversity. To achieve their programmable design of densification and energy absorption, the variable stiffness factor method (VSF) emerged. However, the energy absorption capacity of the metamaterials designed by this method has a significant reduction. In this work, to remedy this defect, several different lightweight specimens are proposed and fabricated to extend the stage of energy absorption and retain the tunability. Then quasi-static compression tests and finite element methods are carried out to analyze the mechanical properties of the designed boundary-constrained structure (BCS) and X-shape constrained structure (XCS). The accuracy of the densification points and energy absorption capacity of the two structures at different VSF values are also studied. The results show that the densification points of the designed structures can be adjusted quantitatively, and their deformation modes are more stable than conventional structures during compression. Furthermore, their specific energy absorption is four and five times higher than that of non-lightweight structures with VSF = 40%, respectively. These findings contribute to advancing the implementation of auxetics in applications of multistage protective structures. • A lightweight metamaterial based on the variable stiffness factor (VSF) method is proposed. • The proposed structures realize multi-level energy absorption and stable deformation. • The SEA of proposed structures was 4 and 5 times higher than that of VSF = 40% structure. [ABSTRACT FROM AUTHOR]

• A novel EARE is designed by introducing an elliptical annular into RE cell. • EARE produces extra vertical support without hindering lateral deformation. • EARE concurrently enhances auxetic performance, stiffness, and SEA. • Adjusting geometric parameters rationalizes designing mechanical properties. Auxetic structures have sparked a lot of interest due to their excellent mechanical characteristics and unusual negative Poisson's ratio influence. However, the large porosity property results in low stiffness, which limits their application scope. Current approaches to augment stiffness often result in diminished auxetic performance. Therefore, it is challenging to simultaneously enhance the auxetic, stiffness and energy absorption capacity of auxetic structures. To address this issue, this paper proposes a novel elliptical annular re-entrant honeycomb (EARE) by introducing an elliptical annular structure into a conventional re-entrant honeycomb (RE) cell, which can provide extra longitudinal support without hindering lateral deformation. The plateau stress, Poisson's ratio and energy absorption properties of the EARE are investigated by quasi-static compressive tests and finite element modeling. The results show that the EARE honeycomb has two plateau stages due to the supporting effect of the elliptical annular structure. Compared with the conventional RE honeycomb with the same wall thickness, the EARE honeycomb not only exhibits a stronger auxetic effect embodied in the Poisson's ratio reduced by 5.19%, but also has a higher average plateau stress and a higher specific energy absorption of 171.63% and 28.03%, respectively, which significantly improves the stiffness and energy absorption performance. The parameterized analysis reveals that by appropriately adjusting the geometric parameters, the stiffness, energy absorption, and auxetic effect of the EARE honeycomb can be enhanced. The research findings address the conflict between auxetic performance and honeycomb stiffness, providing new insights into the optimization of honeycomb structure design. [Display omitted] [ABSTRACT FROM AUTHOR]

Negative Poisson's ratio honeycombs have been widely studied in recent years due to their unique mechanical properties. A tetrachiral honeycomb (TCH) exhibits a bulging effect when subjected to in-plane impact, reducing its Poisson's ratio effect. To improve the performance of TCH, this study proposes two types of axisymmetric tetrachiral honeycombs (ATCH-1 and ATCH-2). First, the in-plane impact performances of TCH, ATCH-1, and ATCH-2 are compared via experiments, and the accuracy of the finite element model is validated. Then, the effects of the different parameters on the deformation modes, energy absorption characteristics, and negative Poisson's ratio effects of the three types of honeycombs are analyzed via finite element models. ATCH-1 and ATCH-2 have higher specific energy absorption (SEA) and a more significant negative Poisson's ratio effect than TCH at an impact velocity of 10 m/s. However, the axisymmetric honeycombs do not have advantages at impact velocities of 50 m/s and 100 m/s. The cylindrical radius r significantly affects the deformation mode of the three types of honeycombs. The axisymmetric design provides a new concept for the novel negative Poisson's ratio honeycomb design. [ABSTRACT FROM AUTHOR]

Abstract: Systems with a negative Poisson’s ratio (auxetic) exhibit the unusual yet very useful property of getting wider rather than thinner when uniaxially stretched. A novel mechanism to generate auxetic behaviour at tailor-made values which may be implemented in composites manufacture using readily available materials is proposed. FEA simulations are used to provide a proof of principle for this concept and an analytical model is proposed so as to elucidate the requirements for auxetic behaviour. It is shown that the simulations and analytical model give comparable results and confirm that this system may reach giant negative values of the Poisson’s ratio. [Copyright &y& Elsevier]

Abstract: Honeycombs, as cores in sandwich panels, are in wider use in transport applications where density-specific performance is critical. The ability of honeycombs to withstand through-thickness compression, and in particular resist buckling, is a key performance factor in many applications. However reports of the systematic study of buckling limits in such materials are scant. Honeycomb unit cells with a range of geometries, including conventional hexagonal, re-entrant hexagonal geometries, and modifications of both thereof, were tested mechanically and simulated via finite element modelling. The primary figure of merit was the density-specific peak compressive stress immediately prior to onset of buckling collapse. The conventional hexagonal honeycomb unit cell, as examined here, is not well optimised for density-specific peak stress; in comparison the re-entrant unit cell examined here had approximately 13% higher density-specific peak stress. Modifications to these geometries, such as face stiffeners and fillets, increased absolute values of peak stress but in most cases were deleterious or at best neutral to density-specific peak stress due to the subsequent increase mass of the honeycombs. [ABSTRACT FROM AUTHOR]

Aiming at achieving excellent energy absorption capacity, a novel periodic auxetic reentrant honeycomb structure is proposed by defining an octagon honeycomb unit cell with various shape parameters and reentrant angles. Additive manufacturing is applied to fabricate three groups of resin samples with different design parameters. Quasi-static compression experimental test is conducted obtaining the effects of negative Poisson's ratio (NPR) on the compressive stress–strain relationship of the proposed structure. The results show the excellent lateral material shrinkage under axial compression and distinct hardening process with sharp stiffness increase when the structure experiences densification. The experiment results are then used to validate the corresponding 3D finite element models. Good agreements are found between the experiment and finite element simulation results. Furthermore, a geometry parameter study is used to analyze the effect of design parameters on energy absorption capacity of the structure and the results can be a good reference for future optimal design of similar meta -materials. To summarize, in addition to the good energy absorption capacity, thanks to the obvious NPR effects and lateral shrinkage behavior, the newly proposed honeycomb meta-material with specific auxetic reentrant design becomes a solid material with low porosity and high stiffness after full compression to provide further protection or support for its neighbor objectives. [ABSTRACT FROM AUTHOR]

In recent years, auxetic cellular materials such as auxetic reentrant honeycombs were widely studied due to its attractively negative Poisson's ratio. In this manuscript, the structure response of end-clamped sandwich beam with auxetic reentrant hexagonal aluminum honeycomb core locally subjected to foam projectile with high-velocity impact was experimentally and numerically studied. The reentrant honeycomb cores with different cell-wall thicknesses were fabricated by selective laser melting (SLM). According to experiment results, the failure modes of the sandwich beam were investigated. It was found that sandwich beam with different honeycomb cell's thicknesses could fail in different modes. Meanwhile, the local negative Poisson's ratio deformation was observed in the thin honeycomb core. Based on this, a finite element (FE) model was constructed to analysis the dynamic deformation evolution of two face sheets and auxetic reentrant honeycomb core. The local auxetic and expand deformations of honeycomb core were both found. Finally, the residual deflections of sandwich beam under impulsive loadings were discussed. Unlabelled Image • The structure response of sandwich beam with reentrant honeycomb core under locally impact was firstly studied. • The local negative Poisson's ratio deformation was observed in the reentrant honeycomb core. • The relationship between residual deflection of sandwich beam with reentrant honeycomb core and local impulse was linear. [ABSTRACT FROM AUTHOR]