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Metastructures have drawn extensive attention from researchers due to their excellent mechanical properties. In this paper, to further enhance the mechanical properties of the re-entrant origami honeycomb metastructure, the bio-inspired hierarchical design is introduced on the re-entrant origami honeycomb metastructure, a novel hierarchical re-entrant origami honeycomb (HROH) metastructure is proposed. The crashworthiness of the novel HROH is researched under low-velocity and high-velocity impact. The results indicate a significant improvement in the energy absorption capacity of the novel HROH compared to both the conventional hierarchical re-entrant honeycomb and the re-entrant origami honeycomb. The plateau stress of the novel HROH under different compression velocities are derived by the two-scale method. And the theoretical results are in good agreement with the numerical results. Moreover, the influences of the dihedral angles and wall thickness on impact resistance also are studied. To increase the wall thickness will improve the plateau stress and SEA–strain curve of the HROH, while increasing the dihedral angles will diminish the plateau stress and SEA–strain curve. In addition, to obtain the optimal structure of the HROH, a multi-objective optimization method based on surrogate models and the non-dominated sorting genetic algorithm II (NSGA-II) is employed. The results exhibit high accuracy in the crashworthiness optimization. This study provides a crucial reference for the design and application of buffer protection structure in fields such as automotive, aerospace and defense. • A novel re-entrant origami honeycomb is proposed by introducing hierarchical design. • The two-scale method is used to derive theoretical expression of the novel honeycomb. • Energy absorption and deformation mode of the novel honeycomb are studied. • A multi-objective optimization method is employed to obtain the optimal structure. [ABSTRACT FROM AUTHOR]

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]

[Display omitted] • A new auxetic honeycomb was designed by hybriding rhombic and star configurations. • A theoretical model was developed to predict the elastic modulus and Poisson's ratio. • The proposed honeycomb possesses higher auxetic effect and larger specific modulus than the star honeycomb. • The proposed honeycomb exhibits enhanced plateau stress and specific energy absorption compared with conventional auxetic honeycombs. Auxetic structures with negative Poisson's ratio exhibit excellent performance in cushioning, shear resistance and energy absorption, but their load-bearing capacity is usually poor. To address this drawback of the conventional Auxetic structure, a star-rhombic honeycomb (SRH) design is proposed in this paper to improve its load-bearing capacity. Analytical models of the elastic modulus and Poisson's ratio of this structure for different loading directions are developed. Quasi-static compression experiments were conducted on SRH specimens fabricated by selective laser melting (SLM) technique. Good agreement was achieved between the simulated and experimental stress response curves and deformation patterns. It was found that the SRH structure possessed better elastic modulus and energy absorption capacity than the SH structure without sacrificing the auxetic properties. The elastic modulus and Poisson's ratio of SRH under the x loading direction are more sensitive to the changes of structural parameters. An optimum energy absorption performance is achieved for an appropriate ratio of the inner rhombic strut thickness to outer reentrant strut thickness (k = 0.5). The specific energy absorption of SRH is improved by 136% and 75% compared to that of the conventional reentrant honeycomb (RH) and star honeycomb (SH), respectively. It has advantages for application scenarios with requirements for both auxetic and load bearing properties. [ABSTRACT FROM AUTHOR]

• Energy absorption of re-entrant honeycombs in tension and compression is compared. • Analytical models are established and validated by FE and experimental results. • Re-entrant honeycombs are more effective in tension-dominated applications. Energy absorption is essential for materials/structures in engineering application. It is related to their deformation mechanisms and force–displacement (stress–strain) relationships. Re-entrant honeycombs are expected to show better energy absorption performance in tension than in compression due to a longer stroke of plastic deformation. To examine this, the work aims to systematically examine the energy absorption performance of re-entrant honeycombs under both tensile and compressive loads in the two loading directions. Analytical models for re-entrant units are established based on an idealised deformation mechanism, which are validated using Finite element (FE) analysis and the experimental result in the reference. The results demonstrate that when loaded in the X 2 direction, re-entrant honeycombs are more effective in tension rather than compression in terms of energy absorption, which means that one can take the full advantage of re-entrant honeycombs in tension-dominated energy dissipation applications. However, when loaded in the other direction, the performance depends on initial cell angles of the structures and θ 0 = 45° is the divide. [ABSTRACT FROM AUTHOR]

• An improved Diabolo shaped honeycomb is proposed. • The energy absorption characteristics and stability of DSH are investigated. • The plateau stress and specific energy absorption of DSHs are better than those of RHHs. Due to the excellent energy absorption performance of re-entrant hexagonal honeycomb (RHH) with negative Poisson's ratio (NPR) property, an improved Diabolo shaped honeycomb (DSH) based on RHH is proposed in this paper. With the application of bionic principle, three DSHs with adding ribs are further extended in design. A comparative analysis between RHH and DSHs under axial compression is performed by the finite element calculation software ABAQUS/ EXPLICIT, and subsequently the energy absorption performance and energy absorption stability of DSHs under oblique compression are investigated. The relationship between the cell wall thickness of the honeycomb with the best energy absorption effect in DSHs and the compression crashworthiness criteria is also predicted. The results show that the plateau stress and specific energy absorption of DSHs under axial compression are better than those of RHHs, and the energy absorption capacity of RHH is improved by a novel design. In addition, the energy absorption capacity and stability of the honeycomb under oblique compression are influenced by the number position of the ribs and the cell wall thickness. Therefore, the improved design of NPR metamaterials with oblique compression can provide a reference for the practical application of honeycombs with good energy absorption capacity and stability. [ABSTRACT FROM AUTHOR]

• Auxetic energy absorption structures with re-entrant configurations manufactured by 3D-printed superior elastic-plastic stainless steel. • High speed drop hammer impact tests conducted under varying input impact energies. • Comparison results with a convex honeycomb structure indicate excellent energy absorption performance of auxetic specimens. • Different crushing modes verified for the auxetic specimens with different impact energies and side length-to-thickness ratios. • The side length-to-thickness ratio was found to have a significant effect on the anti-impact performance of auxetic specimens. In order to fully comprehend dynamic responses of metallic auxetic honeycombs, drop hammer impact tests and corresponding finite element (FE) analyses were conducted. The three-dimensional (3D) printing technology was adopted to manufacture stainless steel specimens with varying cell side length-to-thickness ratios and cell configurations. The specimens were crushed under three distinct energy inputs, and the deforming process and relevant mechanical parameters, e.g., the Poisson's ratio, failure mode, plateau stress and energy absorption capacity were studied. The FE results can generally compare well with the experimental ones and facilitate clarifying the impact mechanisms of the experimental results. Improvements of the auxetic specimens in energy absorption were verified through comparison with that of the corresponding convex honeycomb. The results reveal that the specimen with an auxetic honeycomb has higher plateau stress and specific energy absorption with less deformation when the geometric size is the same. A small side length-to-thickness ratio and input impact energy can lead to greatly improved energy absorption efficiency. [ABSTRACT FROM AUTHOR]

• A novel self-similar hierarchical triangular honeycomb (SHTH) is developed. • The 3D metal printing technology is used to carry out the experimental research in this study. • The crashworthiness performances of SHTHs are investigated. • The theoretical models of SHTHs are established based on equivalent simplified collapse mode. Hierarchical structures and materials have attracted extensive attention due to their advantage in improving mechanical properties. Therefore, to design lightweight structures with superior mechanical properties, a novel self-similar hierarchical triangular honeycomb (SHTH) is developed by sequentially arranging the similar sub-triangles on the side of an ordinary triangular honeycomb (OTH) in this study. The crushing mechanism of SHTHs is investigated using experimental, numerical, and theoretical methods. The numerical simulation is verified by the experimental data of metal 3D printed specimens. Firstly, comparative studies on the crushing performances of the SHTH and the OTH are performed. The results demonstrate that the hierarchical configuration has great improvement in deformation mode and crashworthiness indicator for triangular honeycombs. Then, the crushing performances of SHTHs with different hierarchy factors and relative densities are discussed. The results show that the relative density and the hierarchy factor have significant effects on the in-plane mechanical behaviors of SHTHs. Moreover, the SHTHs exhibit better crashworthiness in the X-direction (i.e., along the side direction of the primary triangular cell) than in the Y-direction (i.e., normal to the side direction of the primary triangular cell), and the crushing performances of SHTHs are tunable by relative density and hierarchy factors. Finally, the theoretical models on the crushing responses of SHTHs are first derived based on equivalent simplified collapse mode and then verified by experimental and numerical data. The results further reveal the influence mechanism of hierarchical arrangement in improving the crushing performance of honeycomb structures, and a novel design concept for engineering collision protection is developed in this study. [ABSTRACT FROM AUTHOR]

• 1. A sandwich beam with a novel auxetic core was constructed. • 2. The relationships between geometric parameters and the bending response of the sandwich beam s were discussed. • 3. The sandwich beam with star-triangular auxetic core has better bending performance than the classical auxetic core. In recent years, the enhanced auxetic honeycomb structure has attracted attention owing to its high strength and negative Poisson's effect. The purpose of this study is to explore the bending response and energy absorption of the star-triangular honeycomb core (STH-C) and sandwich beam with an STH-C (STH-SW). Three-point bending tests and a series of numerical simulations are conducted to investigate the bending performance of the sandwich beams. The results indicate that STH-C exhibits only local bending deformation in the loading region, whereas the cells of the STH-SW undergo shrinkage deformation in the loading region, and the neighboring cells undergo tensile deformation. In addition, the deformation mode of the STH-SW is influenced by the compression position and structural parameters. Increasing the face sheet thickness and changing the thickness arrangements is beneficial to improving the load-bearing capacity of the sandwich beams, especially when t f / t b > 1. Furthermore, the load-bearing capacity of the sandwich beams increase with the increase of the cell-wall angle and t / l , and increasing the t / l can change the cells from a global deformation to a local deformation mode. Subsequently, the complex proportional assessment method (COPRAS) is implemented to determine the influence weights of the structural parameters on the load-bearing capacity of the sandwich beams. The results demonstrate that the cell structure parameters have a more significant influence on the bending performance, especially the t / l. Finally, the bending performance of the STH-SW is compared with that of sandwich beams with different cores (star-shaped, re-entrant hexagonal and triangular corrugated structures). The results show that the energy absorption capacity of the STH-SW is 24.34% and 37.97% higher than that of sandwich beams with star-shaped and re-entrant hexagonal cores, respectively. Although the energy absorption capacity of the STH-SW is 10.25% less than that of multi-layer triangular corrugated core sandwich beam, there is no initial peak load for the STH-SW. This study is expected to provide some insights for further investigations on the bending performance optimization of sandwich beams with auxetic cores. [ABSTRACT FROM AUTHOR]