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Due to the high energy absorption capability and excellent bending strength of the sandwich panels, they are mostly preferred as protective structures against explosive blast attacks. The evaluation of their blast-mitigation characteristics through experiments is highly dangerous, costly, time-consuming, and polluting for the environment. Therefore, in this presented work, a series of numerical analyses were performed to evaluate the blast mitigation of the outstanding sandwich panels with honeycomb, corrugated, auxetic, and foam cores. The masses of sandwich panels were kept constant, and their areal densities were maintained constant throughout the study to effectively compare their performance under identical air blast loading conditions. To apply the air blast loads to the designed sandwiches, 1–3kg of spherical-shaped trinitrotoluene charges were used for a 100mm stand-off distance. The sandwiches are made of high-strength AL-6XN steel and crushable aluminum foam. The rate-dependent Johnson–Cook constitutive model of plasticity and the crushable foam model with volumetric hardening were used for the evaluation of sandwich panels’ plastic deformations. The findings of the work depicted that the honeycomb core is more efficient than the other core structures of the same masses because the sandwich panels with honeycomb core provide smaller back skin deflections, globalized core crushing, and higher core energy absorption than the other sandwich panels for extreme conditions of air blast loading. [ABSTRACT FROM AUTHOR]

The application of bionic structures for the design of energy-absorbing structures has been proposed recently. The rapid advancement of additive manufacturing technology provides technical support for the fabrication of non-traditional structures and further improves the energy-absorbing properties of bionic structures. This work proposes a novel bionic hybrid structure that consists of honeycomb-inspired thin-walled tubes filled with weevil-inspired diamond TPMS (triple periodic minimal surface) structures. The energy-absorbing properties and the deformation behaviors of these topologies under axial crushing loads were investigated using combined numerical simulations and experimental tests. First, the effect of filling quantity and filling distribution on energy absorption of the hybrid structures was investigated. Results show that honeycomb tubes and diamond TPMS structures produce a synergistic effect during compression, and the hybrid structures exhibit excellent stability and energy absorption capacity. The bionic hybrid structure improves specific energy absorption (SEA) by 299% compared to honeycomb tubes. Peak crush force (PCF) and SEA are more influenced by filling quantity than by filling distribution. The effects of diamond TPMS structure volume fraction and honeycomb tube wall thickness on the energetic absorptive capacity of the hybrid structure were furthermore investigated numerically. Finally, a multi-objective optimization method was used to optimize the design of the bionic hybrid structure and balance the relationship between crashworthiness and cost to obtain a bionic hybrid energy-absorbing structure with superior performance. This study provides valuable guidelines for designing and fabricating lightweight and efficient energy-absorbing structures with significant potential for engineering applications. [ABSTRACT FROM AUTHOR]

A modified 3D re-entrant honeycomb is designed and fabricated utilizing Laser Cladding Deposition (LCD) technology, the mechanical properties of which are systematically investigated by experimental and finite element (FE) methods. Firstly, the influences of honeycomb angle on localized deformation and the response of force are studied by an experiment. Experimental results reveal that the honeycomb angles have a significant effect on deformation and force. Secondly, a series of numerical studies are conducted to analyze stress characteristics and energy absorption under different angles (α) and velocities (v). It is evident that two variables play an important role in stress and energy. Thirdly, response surface methodology (RSM) and the Non-Dominated Sorting Genetic Algorithm II (NSGA-II) are implemented with high precision to solve multi-objective optimization. Finally, the final compromise solution is determined based on the fitness function, with an angle of 49.23° and an impact velocity of 16.40 m/s. Through simulation verification, the errors of energy absorption (EA) and peak crush stress (PCS) are 9.26% and 0.4%, respectively. The findings of this study offer valuable design guidance for selecting the optimal design parameters under the same mass conditions to effectively enhance the performance of the honeycomb. [ABSTRACT FROM AUTHOR]

A honeycomb structure is a sandwich structure widely used in fuselage, among which the hexagonal honeycomb core is the most widely used. The energy absorption characteristics and impedance ability of the structure are the main reasons that directly affect the energy absorption characteristics of the honeycomb sandwich structure. Therefore, it is necessary to study the out-of-plane mechanical properties of the composite honeycomb sandwich structure. Based on the numerical simulation results, the energy absorption characteristics of several composite honeycomb sandwich structures are verified by drop hammer impact experiments. The research shows that the transient energy absorption characteristics of the composite honeycomb sandwich structure are mainly related to the cell size of the honeycomb structure. The smaller the size of the front cell, the stronger the overall impact resistance; the strength of the composite honeycomb sandwich structure exceeds that of 7075 aluminum alloy-NOMEX and carbon fiber-NOMEX honeycomb sandwich structures. In this paper, the energy absorption characteristics of composite honeycomb sandwich structures under different impact energy are compared and studied. The displacement, force and energy curves of energy absorption characteristics related to time variables are analyzed. The difference in protective performance between the composite honeycomb sandwich structure and existing airframe structure is compared and studied. The optimal structural design parameters of composite honeycomb sandwich under low-speed impact of drop hammer are obtained. The maximum energy absorption per unit volume of the designed honeycomb sandwich structure is 171.7% and 229.8% higher than that of the NOMEX-AL and NOMEX-C structures. The 6.4 mm and 3 mm cell sizes show good characteristics in high-speed buffering and crashworthiness. The composite honeycomb sandwich airframe structure can improve the anti-damage performance of the UAV airframe structure, ensure the same thickness and lightweight conditions as the existing honeycomb sandwich airframe structure, and improve the single-core bearing mode of the existing airframe structure. [ABSTRACT FROM AUTHOR]

In order to improve the impact resistance of sandwich panels under low-velocity impact, the lotus leaf vein is selected as a biological prototype to design a bio-inspired honeycomb (BIH) sandwich panel. ABAQUS is used to establish and effectively verify the finite element (FE) model of the BIH sandwich panel. To systematically compare and study the mechanical properties of BIH and conventional hexagonal honeycomb sandwich panels under low-velocity impact, the maximum displacement of face-sheets, the deformation mode, the plastic energy consumption and the dynamic response curve of the impact end are presented. At the same time, the performance differences between them are revealed from the perspective of an energy absorption mechanism. Furthermore, the influence of the circumscribed circle diameter ratio of the BIH trunk to branch (γ), the thickness ratio of the trunk to branch (K) and the impact angle (θ) on impact resistance is studied. Finally, the BIH sandwich panel is further optimized by using the response surface method. It can be concluded that, compared to conventional hexagonal honeycomb sandwich panels, the addition of walls in the BIH sandwich panel reduces the maximum deformation of the rear face-sheet by 10.29% and increases plastic energy consumption by 8.02%. Properly adjusting the structural parameters can effectively enhance the impact resistance of the BIH sandwich panel. [ABSTRACT FROM AUTHOR]

Honeycomb structures are commonly adopted due to their superior energy absorption capacity. In this study, a new bionic glass sponge–type honeycomb structure (BSH) with a quadrilateral octagonal mesh microstructure inspired by the sea sponge structure was proposed. The in-plane crushing performance of the BSH with different geometrical parameters under different crushing speeds was investigated by ABAQUS/Explicit. The numerical findings suggested that the BSH displayed stronger energy absorption in contrast to the square, hexagonal and hierarchical honeycombs at both quasi-static and dynamic crushing conditions. More plastic hinges and more unit walls involved in deformation resulted in a high energy absorption capacity. In addition, three typical deformation modes of the BSH under different loading speeds were discussed, and the empirical model to predict the plateau stress of the BSH was established based on the shock wave theory. Finally, the effect of boundary segmentation parameter m on crushing performance was also illustrated. The energy absorption capacity reaches a maximum at m = 3 for quasi–static loading, whereas at m = 5 for dynamic loading. These findings provide valuable insights into the optimization of bionic honeycombs. [ABSTRACT FROM AUTHOR]

• A central self-similar strategy is proposed to develop bio-inspired honeycombs. • Central self-similar degradable polymer honeycombs are additively manufactured. • The interaction effect of different connections between the two walls can promote EA. • Theoretical models of the proposed central self-similar honeycombs are built. At present, improving the energy absorption capacity of lightweight degradable polymer honeycombs is crucial for one of the future engineering applications and bio-inspired strategy based on hierarchy is considered an efficient method for designing honeycomb structures. To improve the energy absorption performance and stability of polymer honeycombs, this study develops three central self-similar bio-inspired honeycombs combining the microstructure of horsetails and designs different connections between the two walls and connecting structures for the central self-similar honeycombs. Bio-inspired honeycombs are fabricated using toughened polylactic acid (PolyMax™ PLA) through fused deposition modelling and subjected to axial compression tests. The finite element (FE) models of the bio-inspired honeycombs are developed to simulate the axial compression process and verified with experimental results. Results show that the connecting structures distributed on the outer wall of the central self-similar honeycombs considerably improve the energy absorption performance of the bio-inspired honeycombs. In the double-cell honeycombs, connecting structures interact with the cell walls, leading to the formation of more folding lobes on the cell walls and increased cell wall utilisation. Furthermore, the effects of geometrical parameters on the energy absorption performance of the bio-inspired honeycombs are investigated. Suitable connecting structure size and cell wall thickness can improve the energy absorption performance and stability of the honeycombs. In addition, the theoretical calculation models of the three bio-inspired honeycombs are established, and the errors with the FE calculation results are within 6 %, which verifies the accuracy of the theoretical models. [Display omitted] [ABSTRACT FROM AUTHOR]

• A cost-effective fabrication method is proposed. • More pronounced NPR effects can be attained by implemented misplaced design. • The comparative method of specific plateau stress is proposed. • The optimized MRHC structure possesses an excellent energy absorption performance. Two-stage plateau stress structures have emerged as a novel research focus in the field of impact protection and energy absorption. The proposal of structures exhibiting a two-stage plateau stress region is increasingly prevalent. However, the majority of currently available two-stage plateau stress structures are predominantly fabricated using 3D printing technology, which can incur substantial costs. Additionally, the majority of currently proposed two-stage plateau stress structures exhibit diminished plateau stress values and possess limited capabilities for energy absorption. The objective of this study is to develop an economically viable metamaterial that exhibits dual high-level plateau stresses and demonstrates exceptional energy absorption capabilities in two in-plane directions. Based on the concept of misplaced design and traditional reinforced honeycomb structures (RHC), a novel metamaterial named misplaced reinforced-honeycomb (MRHC) has been developed. Aluminum alloy component and stainless stee component are fabricated. After the implementation of the misplaced design, the MRHC structure exhibits a pronounced enhancement in negative Poisson's ratio effect. The influence of misplaced design on the mechanical performance of the MRHC structure, as well as the impact of horizontal cell wall length (l 3) and rotation angles (α and β), has been investigated. The multi-objective optimization has been employed to determine the optimal rotation angles α and β. The optimized MRHC structure was compared to the previously proposed two-stage plateau stress structures, revealing its superior energy absorption capabilities in both the in-plane X and Y directions. The proposed MRHC structure exhibits promising potential for future applications in impact buffering and collision protection fields. [Display omitted] [ABSTRACT FROM AUTHOR]

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]

This study proposed a design method of honeycomb structure based on triply periodic minimal surfaces (TPMS), namely gyroid-honeycomb. The honeycomb samples were fabricated by selective laser melting (SLM) with Ti6Al4V (TC4) material. Through the uniaxial quasi-static compression tests and numerical simulations, the mechanical responses and energy absorption capability of honeycomb structures under different compressive velocities were investigated. Compared with conventional truss-honeycomb structure, the gyroid-honeycomb exhibited reduced stress fluctuation and enhanced energy absorption capability under in-plane compressive direction. Particularly, gyroid-honeycomb also demonstrated a superior energy absorption capability under both low and high compressive velocities. According to the scanning electron micrograph (SEM) image of the fracture surface, the gyroid-honeycomb exhibited a unique buckling failure mode along with a large plastic deformation and negative Poisson's ratio. The different compressive responses can be attributed to the distinct stress distribution in gyroid-honeycomb according to the simulation results. Notably, the gyroid-honeycomb structure displayed a remarkable capacity for energy absorption across various compressive velocities. These findings provide valuable insights into the application of the novel TPMS-based honeycomb structures in enhancing the crashworthiness protection for vehicles. [Display omitted] • The design method of Gyroid-honeycomb is proposed based on implicit functions of minimum surface. • Gyroid-honeycomb structure exhibits less stress fluctuation during uniaxial compression. • Deformation mechanism of negative Poisson's ratio is observed in gyroid-honeycomb. • The energy absorption capability of gyroid-honeycomb is enhanced under low and high compressive velocities. [ABSTRACT FROM AUTHOR]

• Novel design strategy with cost-efficiency of multi-cell structure for energy absorption. • Shape of node-locked cell can be expanded for NMH. • Various bending shape selected for on-demand design of the node-locked effect. • Irregular localized deformation is beneficial to energy absorption of NMH. Material reconfiguration at node positions is currently a prominent method for enhancing the energy absorption capability of honeycomb structures. However, the complexity of cellar configuration poses challenges in the manufacture with cost-efficiency due to intricate preparation processes, which limits its widespread application. In this work, an innovative approach was proposed to fabricate the multi-cell honeycomb structure by nesting and assembling node-locked components with a specific shape. The stable energy absorption of multi-cell structure with node-locked design strategy was preliminarily validated by the numerical and experimental results. Additionally, the effect of the bent panels number and the cell size ratio were investigated numerically and theoretically to reveal the energy absorption mechanism of node-locked multi-cell honeycomb (NMH). It is demonstrated that the strategy by assembling node-locked components can significantly simplify preparation processes of multi-cell structure while maintaining excellent energy absorption capabilities, this offers a better balance between the cost-efficiency and superb energy absorption for multi-cell structures. [Display omitted] [ABSTRACT FROM AUTHOR]

• Inspired by self-similar evolution, a novel concave-convex hierarchical honeycomb (NCHH) is proposed. • The NCHH show the characteristics of concave and convex, and the energy absorption characteristics of concave honeycomb is better than that of convex honeycomb. • Reasonable configuration of self-similarity coefficient, angle and wall thickness can effectively improve the crashworthiness of the structure under axial impact. A novel bi-directional hierarchical honeycomb is proposed. A finite element numerical model was constructed using Abaqus/Explicit, the model's accuracy was verified, and a series of in-plane impact studies were carried out. Firstly, the mechanical properties of axial impact were analyzed for different bi-directional hierarchical honeycombs. The results show that the mechanical properties of NBHH- h 1 h 2 2 h 3 -III and NBHH- h 1 h 2 h 3 h 4 -IV are the best and almost identical. Subsequently, the mechanical properties of NBHH-X h x Y h y -IIs with different layers were analyzed. Research results show that NBHH- h 3 3 h 4 -II has a 77.96 % increase in specific energy absorption (SEA) compared to NBHH- h 1 3 h 2 -II, and NBHH- h 1 3 h 2 -II has a 116.02 % increase in SEA compared to NBHH-3 h 1 h 2 -II. A parametric study of NBHHs was conducted. The mechanical properties of NBHHs with different layer ratio coefficients were analyzed, and it was found that the mechanical properties of NBHHs with l 2 = 12 and k = 0.2 are the best. Finally, the effects of wall thickness and velocity on NBHHs were analyzed. Increasing the wall thickness appropriately can effectively improve the impact resistance of the structure. Bi-directional hierarchical design provides a practical reference for enhancing the in-plane mechanical properties of honeycombs. [ABSTRACT FROM AUTHOR]

The battery-pack system of electric vehicles is prone to collide with low obstacles on the road, causing battery short circuits and even explosions. It poses a great safety threat to passengers and drivers. The honeycomb structure's high energy absorption and lightweight properties have made it a popular choice in the automotive industry. This paper designs different bio-inspired honeycomb structures to a battery-pack system of electric vehicles to improve the crashworthiness performance. The effects of different bio-inspired honeycomb structures on the crashworthiness of a battery-pack system during frontal impact are analyzed based on a nonlinear finite element model. First, the geometric parameters of seven different bio-inspired honeycomb individual units are described. The overall structure of the honeycomb is applied to a battery-pack system. Second, the nonlinear finite element model of a battery-pack system and honeycomb structures are established and verified. Then, collision simulations are conducted. The deformation and the maximum stress of a battery-pack's bottom shell are computed. The energy absorbed by the honeycomb structures during frontal impact are investigated. The results indicate that the proposed bio-inspired honeycomb structure mimicking grass stems improves the safety performance of battery-pack systems most. Finally, a parametric design is carried out on the bio-inspired honeycomb structure. The effects of wall thicknesses and the number of replacement hexagons on the crashworthiness performance are analyzed. The honeycomb structure preforms best when thickness is 1 mm and the number of replacement hexagons is 2 and 4. The optimized bio-inspired honeycomb structure reduces the deformation of the battery-pack' bottom shell by up to 30%, and maximum stress by 10%. • The deformation, maximum stress, and energy absorption are investigated. • The effects of seven bio-inspired honeycomb structures are analyzed. • Parametric design is carried out to further improve the crashworthiness performance. • The deformation of the bottom shell can be reduced by up to 30%. [ABSTRACT FROM AUTHOR]

Honeycomb structure as a thin-walled structure has excellent mechanical properties. In this work, three types of novel embedded chiral honeycomb (ECH) structures are proposed based on a hybrid strategy and manufactured by using the additive manufacturing (AM) technique. Quasi-static uniaxial compression tests in lateral and diagonal loading are carried out. The loading-displacement curves and deformation modes of ECHs are recorded and analyzed. The numerical models are established and its simulation results are in good agreement with the experiments. The results show chiral units have a significant enhancement on macro mechanical properties. It is found that ECH4 has the best energy absorption characteristics (EA of 49.52 J, SEA of 1.55 J/g) in lateral loading and ECH3 has the best (EA of 62.25 J, SEA of 2.14 J/g) in diagonal loading. The deformation patterns of structures are also discussed. Moreover, the effects of geometric parameters on the mechanical responses of ECHs are analyzed, including the effects of the circular node radii and the wall thickness. The optimal interval of parameter design is finally determined. The findings from this study are helpful for a new topology optimization design for hexagonal honeycomb structures with enhanced mechanical properties. [ABSTRACT FROM AUTHOR]

An all carbon fiber reinforced polyetheretherketone (CF/PEEK) composite honeycomb is prepared by the multi-spot ultrasonic welding in this work. The interfacial responses and flat compression failure behaviors of the honeycomb are firstly analyzed. It is found that ultrasonic welding has both positive and negative effects on honeycomb performance. High-frequency vibration results in fiber waviness, which is beneficial to elevating both the bending resistance of the joints and the compression strength of the honeycomb. However, melted EDs also lead to resin-rich areas at the interface. Based on the findings, the multiwall carbon nanotubes/PEEK (MWCNTs/PEEK) energy directors are developed, providing new insights on optimizing the properties of the interface. The experimental results indicate that the interfacial node tensile strength is enhanced by 400 % (1267 kPa) with the MWCNTs content increasing to 7.5 wt%, and the optimized CF/PEEK honeycomb shows more superior specific energy absorption and specific compressive strength than those of the commercial light-weight honeycombs. Moreover, the joint failure behaviors are investigated thoroughly. The addition of MWCNTs is found to induce fiber bridging between the cell walls and facilitate stress transfer through the joint interface. The optimized ultrasonic welding method provides a new opportunity for efficient and volume manufacturing of carbon fiber reinforced composite honeycombs without dimension limitation and large mold costs. [Display omitted] [ABSTRACT FROM AUTHOR]

• Face-centered hierarchical strategy is proposed to develop a novel honeycomb. • Face-centered hierarchical network provides strong lateral support to resist the multi-crushing loads. • Effect of hierarchy order and thickness gradient is significant on the multi-crushing properties. • Face-centered hierarchical honeycomb possesses the more stable and efficient multi-crushing properties. The undesirable resistance to multi-crushing loads limits the development of regular honeycomb structures in advanced engineering fields. Therefore, a novel hierarchical strategy, namely face-centered hierarchical strategy, is proposed to improve the multi-crushing properties of honeycombs. The honeycombs with the face-centered hierarchical network are manufactured by selective laser melting to test their energy absorption mechanism against the oblique crushing load. The multi-crushing properties of face-centered hierarchical honeycomb (FCHH) are explored by a numerical investigation. The carrying capacity and energy absorption of the FCHH decrease with the increasing crushing angle. The parameter configuration design indicates that the hierarchy order has a positive effect on the multi-crushing resistance of the FCHH. The face-centered hierarchical network not only provides the strong lateral support, improving the collapse of the honeycomb, but also reduces the folding wavelength, improving its material utilization. Specifically, the specific energy absorption and crushing force efficiency of the FCHH improve by 27.65 % and 30.83 % respectively as the hierarchy order increases from 1 to 3 at the crushing angle of 30°. Furthermore, when the thickness gradient factor is close to 1, the FCHH obtains a more reasonable material distribution to resist the oblique crushing load. Compared with other hierarchical honeycombs for the oblique crushing resistance, the FCHH demonstrates a more stable carrying process and more efficient energy absorption. This research expects to propose a promising reinforcement strategy to guide the design of multi-crushing properties for honeycombs. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Effect of failure mode on bending behavior of a multi-cellular auxetic and non-auxetic structures. • Among multicellular beams, ReH offers superior bending resistance over a wide deformation range. • A new dimensionless assessment criterion, failure sensitivity, is presented. • The bending performance index of multicellular beam structures with respect to solid section is defined. In recent years, metallic honeycomb structures have been popularly researched due to their significant influence on structural strength and rigidity. In this study, the bending response of multi-cellular beam structures with four (hollow tube (HT), square (Squ), hexagonal (Hex) and octagonal (Octa) and Re-entrant honeycomb (ReH)) cross-sections are numerically investigated. Furthermore, using the nonlinear finite element codes LS-DYNA, a comparative study of the energy absorption characteristics between structures with auxetic and non-auxetic beam cross-sections was carried out. The ReH specimen used in the finite element (FE) validation study was manufactured using the Direct metal laser sintering (DMLS) additive manufacturing method to accommodate the complex geometries. FE method analysis are carried out to systematically investigate the influence of the geometrical configuration and identify the failure mechanism on the bending performance. The results show that in the HT structure, an upper corner fracture occurred because of folding in the contact area of the indenter. For Squ, Hex and Octa beam structures, failure occurs due to localized stress caused by buckling in the cell walls. On the other hand, the cross-sectional area of ReH structure tends to shrink under the bending load. Thus, the influence of the local buckling effect could be minimized despite the high displacements. This situation has ensured that the reduction in the moment of inertia of the cross-section remains limited. Furthermore, the specific energy absorption (SEA) capacity of the ReH beam significantly performed 11.3, 3.76 and 1.77 times better than the multi-cellular beam with Hex, Squ and Octa honeycomb beams, respectively. Accordingly, it was understood that the failure of the re-entrant cross-section under severe deformation was more limited than the others. This study is expected to contribute to evaluating the load-bearing capacity of metallic honeycomb structures, including understanding the failure process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel viscoelastic cell arrays are incorporated in a bicycle helmet liner. • Shear and compressive response of the cells are controlled via changing their curvature. • Significant improvements with the novel cell arrays over conventional EPS foam. • Shear-compliant cells can significantly reduce rotational kinematics and brain strain. • Placement of cells with varying amounts of shear stiffness should be optimised. Rotational head motion is one of the major contributors to brain tissue strain during head impacts, which damages axons and vessels and leads to traumatic brain injury. Helmet technologies have come to market promising enhanced protection against such rotational head motion. We recently introduced novel air-filled viscoelastic cell arrays and showed that their shear response under oblique impacts can be tailored through altering the cell wall curvature. We found that concave cells provide shear stiffness that is a few folds larger than that of convex cells. Here we test whether altering the cell curvature can reduce head rotational kinematics and brain strain and whether the viscoelastic cell arrays outperform the reference EPS foam-based liner. To test these hypotheses, we incorporate the viscoelastic cell arrays in a bicycle helmet liner. We use validated finite element models of the helmet and replace the liner with validated finite element models of the cellular cell arrays. We simulate oblique impacts at different locations to represent a wide range of real-world bicycle head impacts. In all cases, the head kinematics and brain deformation metrics indicate significant improvements with the novel cell arrays over the conventional EPS liner. We show that the shear-compliant cell arrays can reduce head rotational acceleration by as much as 64 % and brain strain by 69 %, but not in all impact locations. Cell arrays with similar axial stiffness yet lower shear stiffness often bottomed out, indicating that a considerable amount of energy is dissipated via cell shearing around the impact zone. Our results show that placement of cells with varying amounts of shear stiffness should be optimised, with the most shear-compliant cells near the crown and the least near the temples. This study shows the promising performance of viscoelastic cell arrays in protecting the head and brain under oblique impacts and provides avenues for optimising the distribution of their compressive-shear properties. [ABSTRACT FROM AUTHOR]

• A novel meta -honeycomb was obtained and fabricated, which reversed the energy absorption method of traditional materials. • The enhanced bandgap property was established by multi-resonator. • The dynamic response of the meta -honeycombs was measured under the impact and blast loads. • The optimization of the meta -honeycomb was verified by coupling different resonators. Conventional honeycomb materials are difficult to resist multiple blast because the enormous energy generated by the explosion would cause plastic deformation of the material, which weakens the strength of the structure. In this paper, a novel meta -honeycomb is designed by adding local resonance system to hierarchical cells, which is based on the multi-resonator-enhanced bandgap property, to withstand multiple blast. This meta -honeycomb reverses the energy absorption mode of traditional honeycomb materials; that is, the energy absorption by plastic deformation of the material becomes the reflection of the stress wave through the equivalent negative mass property generated by the local resonance system, which significantly reduces the deformation of the material and the reaction force at the support under impact loads. In particular, for the secondary explosion loading, the maximum deformation of the meta -honeycomb is reduced by 74% compared with the traditional hierarchical honeycomb. Furthermore, it is demonstrated that the design of meta -honeycomb can be optimized by coupling the bandgap generated by the negative mass property, thereby improving the energy absorption efficiency. This research result provides a new concept for the design of blast-proof materials, which has a high potential for a wide range of engineering applications. [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]

Existing auxetic honeycomb meta-structures are generally integrated structures with complicated microstructural geometry which hence should be fabricated by costly additive manufacturing (AM) technology. The high cost and low production efficiency of AM technology is still the main bottleneck restricting the engineering applications of such kind of structures. In this work, a prefabricated auxetic honeycomb meta-structure with modularized components was proposed based on mortise and tenon (M&R) principle originated in ancient China. The modularized components of the prefabricated auxetic honeycomb are all tubular structures which can be easily fabricated by traditional manufacturing with low cost and high technical maturity. The mechanical properties of the proposed meta-structure were then detailed investigated numerically under quasi-static and dynamic loading conditions. The referenced integrated auxetic honeycomb meta-structure was also considered as a benchmark for comparison. Results show that the proposed prefabricated design exhibits lower strength and stiffness compared with its integrated counterpart, owing to the introduction of more flexibility. Consequently, the proposed design processes a much more stable stress plateau stage with a larger deformation range and effectively reduces the reactive force of the protected structure. The present work paves a novel avenue to design protective meta-structures with low-cost and high performance. [ABSTRACT FROM AUTHOR]

• Three novel lattices by inserting honeycomb struts into the traditional 3D auxetic structure were proposed. • The tensile and compressive mechanical behaviours of hybrid lattices were studied and compared. • 3D hybrid lattices are more effective in tension-dominated applications. Lattice structures have the potential for application in tensile conditions. However, limited studies have addressed tensile behaviours of such structures and compared them with their compressive behaviours. In this paper, re-entrant structure as the foundation was selected to develop new 3D hybrid auxetic honeycomb lattice structures, each characterized by a unique unit cell configuration that features interconnected centre auxetic honeycomb, differentiating it from 2D hybrid auxetic honeycomb structures. A series of uniaxial quasi-static tensile and compressive tests, followed by simulations were conducted on these structures to investigate and compare their energy absorption characteristics. The results show that there were significant differences between these structures in terms of their mechanical behaviours under tension and compression. Especially, the smallest value of energy absorption under tensile load was 86% higher than the largest value under compressive load. The stiffnesses of these lattice structures under tensile loading were about 84–122 times of those of the identical corresponding structures when they were compressed. Meanwhile, the progressive stretching, buckling and collapse mechanisms observed in these structures exhibited excellent stiffnesses and energy absorptions under tensile and compressive load compared to traditional 3D auxetic as the basic structure. The outer frame structure of lattice structures had a direct influence on Poisson's ratio. The work indicates that the stiffness and energy absorption of the traditional 3D auxetic structure can be enhanced by embedding auxetic and honeycomb umbrella shaped elements. [Display omitted] [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]