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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]

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]

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]