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Auxetic honeycomb sandwich structures (AHS) composed of a single material generally exhibit comparatively lower energy absorption (EA) and platform stress, as compared to traditional non-auxetic sandwich structures (TNS). To address this limitation, the present study examines the use of aluminum foam (AF) as a filling material in the re-entrant honeycomb sandwich structure (RS). Filling the AHS with AF greatly enhances both the EA and platform stress in comparison to filling the TNS with AF, while the auxetic composite honeycomb sandwich structure effectively addresses interface delamination observed in traditional non-auxetic composite sandwich structures. Subsequently, the positive–negative Poisson's ratio coupling designs are proposed to strengthen the mechanical features of a single honeycomb sandwich structure. The analysis results show that the coupling structure optimizes the mechanical properties by leveraging the high bearing capacity of the hexagonal honeycomb and the great interaction between the re-entrant honeycomb and the filling material. In contrast with traditional non-auxetic sandwich structures, the proposed auxetic composite honeycomb sandwich structures demonstrate superior EA and platform stress performance, suggesting their immense potential for utilization in protective engineering. [ABSTRACT FROM AUTHOR]

By utilizing 3D printing technology, experimental three-point bending (TPB) tests, and finite element analysis, six honeycomb structures with a variety of overall Poisson's ratios (PR) are studied and compared in terms of bending properties and failure mechanisms. Four novel honeycombs that are designed by hybridizing hexagonal and re-entrant units outperform benchmark conventional honeycombs in terms of load-carrying capacity. Architected hybrid geometry honeycombs with zero PR show excellent specific energy absorption capability in comparison to benchmark honeycombs, absorbing approximately 136.9% and 475.1% more energy under TPB. 3D-printed honeycombs consisting of hexagonal units face layer separation damage mode under bending, while honeycombs with re-entrant cells in their lattice fail with joint shear due to the angle of their struts towards loadings. Designing honeycombs with a hybrid geometry lattice can enhance the load-carrying capacity, specific energy absorption, flexibility, and flexural modulus of the structure under bending. Due to their superior performance, the proposed architected hybrid geometry honeycombs with various Poisson's ratios own promising applications in automotive, protective, and construction industries. [ABSTRACT FROM AUTHOR]

Auxetic metamaterials possess superior properties and have drawn extensive attention in the past decades. Herein, two curved ribs are added to the conventional re‐entrant (CRE) honeycomb structure to form a modified auxetic re‐entrant (MRE) honeycomb structure for improving the energy absorption capacity of the structure. In‐plane quasi‐static compression response of the honeycomb in large deformation is numerically explored first. Then, quasi‐static compression test is conducted to verify the validity of the numerical simulation. Both experimental and numerical simulation results reveal that during the quasi‐static compression process, two plateau stresses occur in the load–displacement curves of the structure, and the second plateau stress is much higher than the first one. The occurrence time of the second plateau stress can be controlled by the distance between the concave curved ribs in the structure. The experimental results are in good agreement with the finite element analysis results. Due to their desirable mechanical properties, the MRE honeycomb structures have great potential for applications in civil engineering, vehicle crashworthiness, and protective infrastructure. [ABSTRACT FROM AUTHOR]

• A novel bi-material multistable auxetic honeycomb (BMAH) with reusable and enhanced energy-absorbing phases was designed. • The crushing stress–strain curve shows three stress plateaus, and BMAH maintains a stable Poisson's ratio even at high strain levels, transitioning from near-zero to negative and then to positive. • Under the selected material combination, the second stress plateau is 8 times greater than the first, and the third stress plateau is 17 times greater than the first, significantly improving energy absorption efficiency. • A theoretical model was established to reveal the deformation mechanism of BMAH and predict the plateau stresses and strain range for each deformation stage. • Effects of geometrical parameters and crushing velocity on BMAH stress plateaus and Poisson's ratio were investigated, and a programmable guide for designing stress plateaus and specific energy absorption is presented. The ideal energy absorption structures, in addition to possessing reusability and high energy absorption efficiency, should also possess multiple desired properties to fulfill various functional applications. In this study, a bi-material multistable auxetic honeycomb (BMAH) is fabricated by using bi-material 3D printing technology. Under in-plane crushing, multiple stress plateaus can be achieved and the Poisson's ratio can be stably tuned, allowing for a transition from near-zero to negative and then to positive. The above characteristics are attributed to the specimen's multi-path deformation. The first stress plateau is only associated with recoverable elastic deformation. The second and third stress plateaus, which are 8 times and 17 times higher respectively than the first, are associated with plastic deformation, significantly enhancing energy absorption efficiency. The deformation mechanism is theoretically analyzed, and the effects of geometric parameters on the performance of BMAH are investigated. In addition, the effect of crushing velocity on the crushing behavior of the BMAH is also discussed. As the crushing velocity increases, the total number of stress plateaus transitions from three initially to two and then to one. The developed BMAH exhibits significant potential applications in multi-stage energy absorbers and smart sensors. [ABSTRACT FROM AUTHOR]

• 3D origami-inspired auxetic honeycomb with omni-directional high stiffness is proposed. • Theoretical models for predicting the elastic mechanical properties of the CFRP composite 3D honeycombs are developed. • The 3D honeycomb can be applied where good load-bearing capacity and obvious auxetic effect are simultaneously desired. • The 3D honeycomb suggests a potential for 3D isotropic auxetic design. Traditional 3D auxetic materials face significant limitations, marked by low stiffness and pronounced anisotropy. To address these challenges, here a new kind of 3D origami-inspired auxetic honeycomb with omni-directional high stiffness is proposed by us. Two simplified theoretical models based on different assumptions are developed to predict structural homogenized elastic properties along the principal axes. Combined theoretical and numerical studies are carried out to reveal the auxetic mechanism and dominated deformation characteristics of the 3D honeycomb. In order to gain a understanding of its elastic anisotropy, directional dependence of structural homogenized Young's moduli, Poisson's ratios and shear moduli are also investigated in detail. A detailed examination of the dependence of structural homogenized Young's moduli, Poisson's ratios, and shear moduli on loading direction is conducted to comprehend its elastic anisotropy. This paper introduces a feasible method for the rapid manufacturing of the 3D honeycomb using high-performance continuous carbon fiber reinforced composite (CFRP) based on hot-pressing molding. Subsequently, the compressive elastic behaviors of CFRP 3D honeycombs are characterized and evaluated through experimental tests. A comparison with traditional 3D auxetic materials reveals the 3D honeycomb's superiority in both multi-axial loading and rapid manufacturing. Additionally, the demonstrative example suggests that the 3D honeycomb can offer nearly isotropic Poisson's ratio of about −0.9. These results not only demonstrate that the 3D honeycomb is an excellent candidate for applications where both good load-bearing capacity and notable auxetic effect are simultaneously desired but also highlight its potential for 3D isotropic auxetic design. [ABSTRACT FROM AUTHOR]

• Auxetic origami honeycomb structure 3D-printed using stainless steel. • Impact tests on cushioning performance of auxetic origami structures conducted. • Auxetic origami structures exhibit higher plateau stress and specific energy absorption. • A smaller width-to-thickness ratio leads to higher energy absorption. • The folding angle significantly affects the stress and energy absorption. The honeycomb structure with a negative Poisson's ratio, inspired by the Miura origami unit, exhibits three-dimensional negative Poisson's ratio characteristics and effective energy dissipation performance. This unique property provides significant potential in the field of protection applications. This study aims to comprehensively understand the impact of auxetic and origami structures on the cushioning performance of honeycomb structures. For this purpose, 316 L stainless steel specimens were fabricated using 3D printing technology and subjected to drop hammer impact tests, and the results were verified by finite element simulation. This paper focuses on assessing the deformation mode and energy dissipation performance of origami auxetic honeycomb structures with varying width-to-thickness ratios and folding angles under different impact energy levels. The results indicate that the negative Poisson's ratio origami specimens exhibit higher plateau stress and specific energy absorption compared to their positive and zero Poisson's ratio origami counterparts with the same geometry. In addition, a smaller width-to-thickness ratio and higher input impact energy enhance the cushioning performance of honeycomb structures. [ABSTRACT FROM AUTHOR]

• Innovative metamaterial design approach proposed to improve energy absorption capacity. • Varying the structural parameter (D/d) enhanced the metamaterial auxetic effect. • HPH metamaterials absorb energy effectively horizontally with their honeycomb structure. • HPH metamaterials show excellent reusability at lower compression ratios. Hexagonal perforated honeycomb (HPH) metamaterials were designed, fabricated and investigated in this work. Initially, the mechanical properties and deformation modes of various proposed HPH metamaterials (HPH-U, HPH-1 L and HPH-3 L) under quasi-static vertical compression were analyzed using experimental and finite element methods. The findings reveal significant improvements in mechanical properties and energy absorption due to the honeycomb and hierarchical expandable design. Notably, the auxetic effect is diminished. Subsequently, the impact of key dimensional parameters on the mechanical properties and Poisson's ratio behavior was explored. The results highlighted that an increase of elliptical perforation major axis to minor axis ratio enhanced the auxetic effect but compromised load-bearing and absorptive capacity. Additionally, HPH metamaterials displayed exceptional performance under quasi-static horizontal compression, with their honeycomb structure enabling them to withstand both vertical and horizontal impacts while exhibiting excellent energy absorption capabilities. The study also identified relative density as a crucial factor influencing horizontal impact performance. Finally, the study subjected the HPH metamaterials to multiple quasi-static vertical repetitive compressions, meticulously analyzing variations in mechanical properties under diverse compression ratios. Due to distinctive structural design and impressive mechanical properties, HPH metamaterials demonstrate immense potential for applications in automotive engineering and sports protection fields. [Display omitted] [ABSTRACT FROM AUTHOR]

Auxetic structures, which have a negative Poisson's ratio, have good mechanical energy/impact absorption properties. These structures have found application in sandwich composites in the aerospace and defence industries, in the production of armour or protective sports equipment. In this study, the mechanical behaviour of the auxetic honeycomb structure in different directions (in-plane and out-of-plane) under impact loading was investigated. For the in-plane (x and y) and out-of-plane (z) directions, boxes with an auxetic honeycomb structure were produced by material extrusion method using Power ABS filament. In the experimental study, dynamic tests were carried out with a drop test machine. Besides, explicit analyses were performed by creating finite element models for these 3 directions. The experimental and numerical results have shown that the energy absorption property of auxetic honeycomb geometry is superior in the case of out-of-plane loading, in agreement with each other. In in-plane loadings, crush force efficiency (CFE) and crush forces were lower. [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]

• 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]

Mechanical metamaterials represent a class of deformable systems, which exhibit macroscopic deformations, mechanical, and/or thermal properties. These emerge due to the structure of their subunits rather than their materials composition and typically exhibit anomalous (normally negative) macroscopic structural, mechanical, or thermal property/properties caused by a change in shape/size of the system. Herein, a class of honeycombs is discussed, which push to the extreme the classical definition of "mechanical metamaterials," exhibiting temperature‐tunable Poisson's ratio properties. More specifically, centrosymmetric honeycombs with T‐shaped joints constructed from different materials are shown to exhibit temperature‐dependent Poisson's ratio values, which can be either positive or negative (auxetic) depending on the external stimulus the system is subjected to. The sign and magnitudes of the Poisson's ratio values are explained in terms of particular geometries that these composite honeycomb systems attain at different temperature conditions. In particular, auxeticity is attributed to the transformation of the T‐shaped units to re‐entrant units. Practical aspects on how these properties may be achieved are discussed, including the possibility that the changes in shape, and hence Poisson's ratios, are induced via different extents of dryness on opposite surfaces of ligaments made from absorbent materials. [ABSTRACT FROM AUTHOR]

Frequencies and energies of vibrations for a honeycomb sandwich plate with negative Possion's ratio are investigated. Based on Reddy's third-order shear deformation plate theory, von Karman type nonlinear theory and Hamilton theory, the equations of motion for the plate are obtained, free vibrations and vibrations with the damping and in-plane force of the plate are considered, respectively. The changes of real parts of the eigenvalues with the damping and in-plane forces of the plate are obtained. In order to obtain the analytic solutions of the frequencies and energies of the plate, lateral free vibrations are considered by using the Hamilton energy method, and we compare the results with that of the previous free vibrations. The potential energies and the total energies variation with parameters of the plate are given. The research results from the two methods show that the change trends of the frequencies and energy of the plate are exactly consistent, but the values of natural frequencies and potential energies are lager in lateral vibrations. [ABSTRACT FROM AUTHOR]

• In-plane crushing responses of the tetra-chiral (TC) honeycombs were investigated by using experimental and numerical methods. • Theoretical formulas were derived to predict plateau stresses of the TC honeycomb under both quasi-static and dynamic crushes. • Effects of unit cell radius ratio on crushing strength, dynamic sensitivity index and effective Poisson's ratio (EPR) of the TC honeycomb were investigated. • Stress and EPR evolutions of the TC honeycomb with different orthogonal array ratios were also investigated. Tetra-chiral (TC) honeycombs are a type of auxetic materials showing negative Poisson's ratio (NPR), which are promising for impact energy absorption. This study aims to analyze the in-plane crushing response of the TC honeycombs under both quasi-static and dynamic loading conditions through numerical simulations and theoretical analyses. The numerical modeling techniques in LS-DYNA were validated using the quasi-static crush test data of TC honeycomb specimens obtained with a 3D-printer. Numerical simulations revealed the "Z" mode deformation and the "bulge effect" under quasi-static crushing, and the row-by-row "I" mode deformation under dynamic crushing of the TC honeycombs. By referring to the simulation results, theoretical models were derived based on the representative unit for predicting the TC honeycomb's crushing strength, i.e. plateau stress, under both quasi-static and dynamic loads. Good agreement was found between the theoretical and the numerical predictions with a maximum relative error of 7%. Parametric analyses showed that the unit cell configuration has a great effect on the crushing strength of TC honeycomb under in-plane crushing. The dynamic sensitivity index was defined to quantitatively evaluate the sensitivity of the TC honeycomb's crushing strength to the loading speed, and was found to depend on the unit cell geometry. The TC honeycomb was found to present a varied effective Poisson's ratio (EPR) with strain under both quasi-static and dynamic in-plane crushing; smaller radius ratio and lower crushing speed were found to yield more obvious NPR effect. Moreover, relative dimension of the TC honeycomb represented by the orthogonal array ratio do not affect its plateau stress under either quasi-static or dynamic crushing, while large orthogonal array ratio results in more obvious "bulge" and NPR effects under quasi-static crushing. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The yield surfaces of several honeycombs with tunable Poisson's ratio, i. e. hexagonal honeycomb, augmented hexagonal honeycomb, re-entrant honeycomb, and augmented re-entrant honeycomb, are investigated by both theoretical analysis and numerical simulations. A theoretical model is developed to predict the mechanical response of these honeycombs considering the coupling effect of the shear stress and axial stress. Explicit expressions of the yield strength are derived for the four types of honeycombs. Good consistency is obtained between the analytical solutions and the numerical simulations. The results of the study provide a new idea for designing new honeycomb materials with tunable Poisson's ratio for engineering applications in the future. [ABSTRACT FROM AUTHOR]

This paper describes the mechanical properties of the three-dimensional(3D) double-V honeycomb intersected by a two-dimensional double-V structure with negative Poisson's ratio. Considering the effect of the adjacent unit cells, the Poisson's ratio and Young's modulus of the honeycomb under axial loading are derived theoretically. The finite element (FE)model of the 3D double-V honeycomb (DVH) is conducted to verify the theoretical solutions. It is found that the constraint of the adjacent cells does not affect the negative Poisson's ratio but enhance Young's modulus a lot, and the two properties are determined by the stuffer/tensor angles. Then, the prototypes of the 3D DVH is manufactured to carry out the quasi-static experiments. The stiffness and the collapse stress of the theoretical, numerical and experimental solutions are compared, and it is evident that these results agree very well which indicates the analyses of these solutions are convinced. And the quasi-static collapse stress increases with the increment of tensor angle θ 1 and the reduction of stuffer angle θ 2 . [ABSTRACT FROM AUTHOR]

This paper presents a finite-element analysis of honeycomb and re-entrant honeycomb structures made of a two-phase composite material which is optimized with respect to selected parameters. It is shown that some distributions of each phase in the composite material result in the counter-intuitive mechanical behaviour of the structures. In particular, negative values of effective Poisson's ratio, i.e., effective auxeticity, can be obtained for a hexagonal honeycomb, whereas re-entrant geometry can be characterized by positive values. Topology optimization by means of the method of moving asymptotes (MMA) and solid isotropic material with penalization (SIMP) was used to determine the material's distributions. [ABSTRACT FROM AUTHOR]

Honeycomb structures are essentially constituted of a repetition of regularly-arranged and loaded sub-structures. The present study carries out a parametrically investigation of the behavior of a multi re-entrant honeycomb structure with variable stiffness and Poisson’s ratio effects. A refined analytical model is specifically developed and compared to full-scale numerical simulations. The analytical model developed is based on energy theorems and takes into full consideration bending, shearing and membrane effects. The influence of the cell walls thickness on the elastic homogenized constants is investigated. The results obtained show a good agreement between the refined analytical approach developed and the numerical computations carried out. [ABSTRACT FROM AUTHOR]

• Two different assembled honeycomb structures were designed and studied. • The structures can be transformed by changing the length of connection plates. • The energy absorption capacity of re-entrant honeycomb was investigated. • Re-entrant honeycomb has higher energy absorption than hexagonal honeycomb. Fabricating auxetic metamaterials via an assembling process is a novel and economic method. In this work, an assembled structure that could realize the conversion of positive and negative Poisson's ratio by changing the length of the connection plates was proposed. Two different honeycomb structures of the assembled re-entrant honeycomb (ARH) and assembled hexagonal honeycomb (AHH) were designed, manufactured and studied numerically and experimentally. Dimensions of the two assembled honeycomb structures are consistent. The nominal stress–strain curves of the two assembled honeycombs have similar initial peak stresses, but the assembled re-entrant honeycomb has a higher bearing capacity and shorter stress plateau. Subsequently, the mechanical properties of ARH were studied by changing the three parameters of corrugated plates. Besides, the energy absorption and specific energy absorption of the assembled re-entrant honeycomb were investigated through the numerical method. Finally, the assembled honeycomb structure with the suitable parameter was determined. Tunable mechanical properties of the assembled auxetic structures can be obtained by varying the length of the connection plates. This strategy of assembly can reduce the costs of manufacturing and improve transportation efficiency. The proposed assembled structures are promising in civil and protective engineering. [ABSTRACT FROM AUTHOR]

• An aluminum foam filled re-entrant aluminum honeycomb structure (FRAH) is designed and investigated. • The proposed FRAH possesses higher specific energy absorption compared to the structure without filling aluminum foam. • The concept of the functional gradient (FG) is introduced into the composite re-entrant honeycombs. • 4. The FG design of the composite structure could reduce the initial peak stress and increase the platform stress value. Auxetic materials have been widely studied in recent years due to their excellent mechanical properties and special deformation modes. However, the application of auxetic materials in practical engineering is limited by the low stiffness. Therefore, in order to broaden the range of applications, it is significant to improve the stiffness of auxetic materials. In this study, the re-entrant aluminum honeycomb is combined with an aluminum foam. The mechanical properties and deformation modes of the aluminum foam filled re-entrant aluminum honeycomb are studied experimentally and numerically. In addition, the effects of the geometrical parameters of such a honeycomb are investigated. The results indicate that the stiffness and the energy absorption capacity can be effectively improved by filling the re-entrant honeycomb with aluminum foam. Due to the high compressibility of aluminum foam, the structure can still retain the auxetic effect at a large strain. In addition, the concept of the functional gradient is introduced into the field of composite re-entrant honeycomb, and the influence of angle gradient and thickness gradient on the structure is analyzed. The results show that the composite structure with the designed functional gradient outperforms the uniform composite structure in terms of initial peak stress and the stress value at the platform stage. Due to its superior performance, the proposed auxetic composite structures hold promising applications in the fields of vehicle engineering, protective engineering and aerospace engineering. [Display omitted] [ABSTRACT FROM AUTHOR]

A contrast in Poisson ratio is a possible strategy to enhance the stiffness of composite structures. In solid materials Poisson ratio is hardly tailorable unless cellular architectures are considered. Here, we first investigated the effect of a single re-entrant inclusion acting as a defect into a regular (non-re-entrant) honeycomb lattice. Building on this, we generated regular patterns of re-entrant inclusions into a regular hexagonal cellular matrix and we characterized the apparent stiffness and Poisson ratio of the obtained structures. We also explored the role of the intrinsic material properties of the inclusion as well as of its closest environment on the interplay between the deformations of different phases in the lattice. Our main finding is that a small fraction of re-entrant inclusions (around 12%) is sufficient to generate a substantial augmentation in stiffness (300%) at constant overall relative density and without inducing strong anisotropy. Eventually, we fabricated by 3D polyjet printing bi-material composite architectures to demonstrate the superior mechanical behavior obtained exploiting the Poisson effect. [ABSTRACT FROM AUTHOR]

In the present work, a new design of honeycomb is proposed by embedding the rhombic configuration into the normal re-entrant hexagonal honeycomb (NRHH), in order to enhance the honeycomb’s in-plane mechanical properties. Both theoretical analysis and numerical simulations are employed to calculate the in-plane mechanical properties of the new honeycomb under uniaxial compression, including Young’s modulus, Poisson’s ratio and critical buckling strength. The results show that the new honeycomb can maintain auxetic performance, while both the in-plane Young’s modulus and the critical buckling strength are significantly improved compared to the NRHH. Comparisons between the present design and other exiting enhanced periodic topologies are also carried out. With respect to them, the present design features superior performances. For these outstanding properties, this layout may provide a new concept for the optimization and design of auxetic materials. [ABSTRACT FROM AUTHOR]

• Metallic graded RECH prototypes fabricated through a low-cost step-by-step method. • Graded RECH takes advantage of structural hierarchy and functional gradient. • Gradient design causes multi-step deformation mode and multi-plateau stress. • Improved SEA and auxeticity of graded RECH under different crushing velocities. As lightweight energy-absorbing materials, auxetic honeycombs have demonstrated promising applications in engineering fields. In this study, the graded re-entrant circular auxetic honeycombs (RECH) were proposed, and their metallic prototypes were fabricated through a low-cost step-by-step method. The novel design of graded RECH takes advantage of structural hierarchy on the meso-scale and functional gradient on the macro-scale. The in-plane crushing behaviors of uniform (U) RECH, positive gradient (PG) RECH and negative gradient (NG) RECH with the same relative density were investigated under different crushing velocities. The finite element (FE) models were validated by comparing against the experimental and theoretical results. The results showed that the gradient design can control the multi-step deformation modes of RECH, which leads to the multi-plateau characteristics and reduced initial stress peaks in stress-strain curves. Under quasi-static crushing, the specific energy absorption (SEA) of PG-RECH is higher by 43.9% than that of U-RECH owing to more plastic deformations of hierarchical circular walls at a large strain. Under dynamic impacts, the SEA of NG-RECH is 2.7 times and 1.5 times those of PG-RECH and U-RECH at a small strain, respectively. Finally, the NG-RECH shows more negative Poisson's ratio (NPR) behavior than the U-RECH under dynamic crushing. This study offers new routes to the fabrication and design of auxetic metamaterials. [Display omitted] [ABSTRACT FROM AUTHOR]

Auxetic structures with negative Poisson's ratio (NPR) have been widely studied because of their great potentials in energy absorption. In order to further understand the 3D auxetic structures, especially the 3D re-entrant honeycombs, the crushing response of metallic 3D re-entrant honeycomb (3D-RH) is theoretically studied in this paper. First, the finite element (FE) model of the 3D-RH was built and validated. The numerical simulation results show that the 3D-RH exhibits different deformation modes under low- and high-speed crushing. Secondly, a set of numerical simulations was used to determine the critical velocity that distinguishes the two deformation modes. Based on the predicted deformation profile, the crushing strength of 3D-RH under low- and high-speed crushing was derived in the form of theoretical equations. The relative errors between the theoretical and numerical results are less than 10%. Thirdly, the structural parameters were studied to reveal their effects on the crushing strength of 3D-RH. The results showed that the oblique wall length and cell wall thickness have significant effects on the crushing strength, while the cross length and cell wall width have little effect on the crushing strength. In addition, the re-entrant angle has no obvious effect on the low-speed crushing strength, but has a significant effect on the high-speed crushing strength. Finally, considering the rotational symmetry of the 3D-RH unit, the 3D-RH-nRP unit was found, which is a new type of 3D re-entrant unit with multiple re-entrant parts. The corresponding equations of crushing strength were derived. The FE results showed that the equations can well estimate the crushing strength of 3D-RH-nRP structures. • The crushing strength of 3D re-entrant honeycomb was theoretically studied. • Effects of geometrical parameters on the crushing strengths were analyzed. • A type of 3D re-entrant honeycomb with multiple re-entrant parts was proposed. [ABSTRACT FROM AUTHOR]

Auxetic honeycombs usually feature a single plateau stress, which hinders their multifunctional applications. In this work, we aim at investigating the two plateau characteristics of re-entrant honeycomb (REH) with negative Poisson's ratio (NPR) in the concave direction through experimental, numerical and theoretical methods. A series of REH prototypes with different geometric parameters were fabricated by utilizing the additive manufacture (AM) technique, and were tested under quasi-static crushing. It was found that due to the transitional rectangle structures formed during crushing, the REH specimens exhibit a two-step deformation mode and thus have two plateau stresses in the stress–strain curves. The second plateau stress of the baseline specimen is more than twice the first plateau stress. Both the numerical and the theoretical predictions agree well with the experimental results. Detailed parameter analysis showed that larger cell wall thickness, smaller straight wall length and smaller inclined angle lead to larger first and second plateau stresses. In addition, the effect of crushing velocity on the crushing behavior of the REH was discussed. Under dynamic crushing, no critical strain is found to divide the first and second plateau stages, and the two plateau characteristics disappear. This study may provide guidelines for the design of multifunctional honeycombs with two plateau stages. [Display omitted] • Two plateau characteristics of re-entrant honeycomb (REH) in concave direction through experimental, numerical and theoretical methods. • REH prototypes with different geometric parameters were fabricated by utilizing additive manufacture (AM) technique. • Formation of transitional rectangle structure caused two-step deformation mode of REH and two plateau stages. • Second plateau stress of REH was more than twice the first plateau stress. • Effects of geometric parameters and crushing velocity on two plateau characteristics of REH were investigated. [ABSTRACT FROM AUTHOR]

In the present work, two new designs of honeycomb structures were presented based on the basic re-entrant structure namely, splined-reentrant and stiffened-reentrant honeycombs. The new structures were designed in order to enhance the in-plane stiffness and maintaining the auxetic behavior of the structure. In this study, finite elements modeling and experimental work were carried out to evaluate the in-plane properties of the new designs. The effect of the geometrical parameters such as rib length and rib thickness of the unit cell of the new designs on the in-plane properties was investigated. Finite elements results showed that the in-plane stiffness of the new designs was improved significantly compared to the basic re-entrant structure. Also, the stiffened-reentrant structure showed better enhancement of the stiffness than the splined-reentrant structure. For example the modulus of elasticity of the stiffened-reentrant structure exceeds 16× that of the basic re-entrant structure in x-direction and was over two times in the y-direction with lower values of the negative Poisson's ratio. Compression tests were carried on honeycomb samples made of steel using laser cutting technique with different geometrical parameters. Test results for the three designs were compared with the finite element results and they were in a good agreement. [ABSTRACT FROM AUTHOR]

[Display omitted] • An assembled auxetic chiral honeycomb (AACH) is designed and manufactured by 3D printing. • The AACH could exhibit lower peak force and high stress plateau. • The external energy is mainly absorbed by the vertical wave plate. • The AACH provides a new idea for the industrial manufacturing of auxetics. Auxetic metamaterials tend to be fabricated using additive manufacturing and laser cutting due to their special porous microstructures. Although the auxetic metamaterials have many applications currently, the high cost and the low efficiency of available manufacturing are adverse to expend their application range. In this work, the auxetic chiral honeycomb is assembled crosswise using the slotted wave plate. Effects of wave radius, plate thickness, slot percentage, and base material on the Poisson's ratio and mechanical performance are explored experimentally and numerically. The results show that assembled auxetic chiral honeycomb (AACH) exhibits lower peak force and high plateau stress than the conventional assembled one. With the increase of wave radius and plate thickness, the energy absorption (EA) and specific energy absorption (SEA) would increase. As for the different material combinations, when the base materials in vertical and horizontal wave plates adopt stainless steel and aluminum, respectively, the AACH would exhibit desirable EA, SEA, and auxetic behavior. These findings provide a new approach to the manufacture of auxetics at a low cost, which is beneficial for potential applications. [ABSTRACT FROM AUTHOR]

• A simple re-entrant auxetic metamaterial is designed by rotating and connecting 2D re-entrant honeycomb cells. • The energy absorption of the proposed metamaterial is discussed under different design parameters. • The proposed 3D re-entrant auxetic lattice has desirable stiffness and specific energy absorption. • The initial peak stress and energy absorption capacity of the metamaterial can be adjusted via changing design parameters. As a branch of auxetics, the re-entrant hexagonal honeycomb has many superior mechanical performances. However, most research focused on two-dimensional (2D) re-entrant structures or three-dimensional (3D) structures without high compressibility. The energy absorption capacity of such structures is deficient for practical applications. Therefore, it is important to investigate the 3D re-entrant honeycomb structures which can sustain large deformation in order to make the most use of the materials. In this work, a simple 3D re-entrant unit cell is designed, manufactured and examined. The influence of geometric parameters on the deformation mode and energy absorption capacity is investigated numerically. The experimental results are in good agreement with the finite element prediction. The proposed 3D re-entrant auxetic metamaterial not only possesses a greater bearing capacity but also presents a stable compression deformation. The structure exhibits a desirable energy absorption behavior, including obvious auxetic behavior and a long stress plateau. By adjusting the geometrical parameters of the structure, the performances of energy absorption and compression stiffness can be improved. These findings provide a new idea to design 3D auxetic metamaterials and promote their utilization in protective structures. [Display omitted] [ABSTRACT FROM AUTHOR]

Double arrowed honeycomb (DAH) is a typical auxetic material with negative Poisson's ratio (NPR) and excellent mechanical properties, showing a wide application prospect in aerospace, marine, automotive and other fields. In this work, we introduce the double circular arc walls into the regular DAH unit cell to improve the strength and energy absorption capability of the material. According to the position of the arc wall, three circular double arrowed honeycomb (CDAH) unit configurations were proposed, including upper-circular (U-type), lower-circular (L-type), and full-circular (F-type). The prototype specimens of DAH and CDAHs were fabricated by 3D printing and tested under quasi-static crushing. The finite element (FE) models of the specimens were established and validated by the experimental results. The experimental and numerical results showed that the three types of CDAHs have higher crushing stress, crush force efficiency (CFE) and specific energy absorption (SEA) than the DAH with the same geometric parameters, which is due to the formation of more plastic hinges at the joints and the middle of the arc walls. Specifically, the CFE of L-type CDAH is 25.8% higher than that of DAH, and the SEA of F-type CDAH is 85.9% higher than that of DAH. The detailed parameter analyses of CDAH revealed that small short inclined wall angle and lower circular arc radius, and large friction coefficient value, long inclined wall angle and cell wall thickness all lead to large SEA. Among the three types of CDAH, the U-type has the greatest SEA and the most NPR effect under the same relative density. Finally, a wide range of SEA and Poisson's ratio of the CDAH can be obtained by rationally designing the arc wall position and the geometric parameters of the unit cell. [Display omitted] • Quasi-static crushing behaviors of three novel types of circular double arrowed honeycombs (CDAHs) through experimental tests and numerical simulations. • More Energy absorption than regular double arrowed honeycomb (DAH) due to formation of more plastic hinges of arc walls. • Good agreement between experimental and numerical results in terms of deformation mode, crushing stress and specific energy absorption (SEA). • Effects of cell number, geometric parameter and relative density on crushing responses of CDAHs. [ABSTRACT FROM AUTHOR]

Auxetic honeycombs have a variety of potential applications in aerospace engineering due to their excellent mechanical and multidisciplinary properties. A novel class of auxetic honeycombs have been invented recently named as Re-entrant circular (REC) honeycombs. The REC honeycombs can be obtained by replacing the sloped cell walls of the well-known re-entrant (RE) unit honeycombs with double circular arc walls to dissipate more energy. This study aims to investigate the in-plane dynamic crushing responses of the novel REC honeycombs through numerical simulations and theoretical analyses. LS-DYNA based numerical simulations revealed the "X" and "I" combined deformation mode of the REC honeycomb under low velocity crushing, and the "I" mode under high velocity loading. The geometric parameters of unit cell and the crushing velocity were both found to have great effects on the proximal end crushing stress of the REC honeycomb. It was also shown that the REC honeycomb presents obvious negative Poisson's ratio (NPR) effect, attributed to its re-entrant characteristic. Based on the meso-scale deformation mechanisms of the representative unit cell, theoretical models have been established to predict the plateau stress of the REC honeycomb under different crushing velocities. The theoretical predictions were in good agreement with the numerical simulation results. Moreover, the dynamic sensitivity index and the deformation mode map of the REC honeycomb were obtained to evaluate its dynamic response sensitivity to the loading velocity. Finally, the in-plane crushing characteristics of the REC and the RE honeycombs were compared. The REC honeycomb was found to show higher specific energy absorption (SEA) and penetration resistance than its RE counterpart, due to the more plastic hinges formed during the dynamic crushing process. [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]

• A novel composite is proposed by filling layered re-entrant honeycombs with concrete. • The mechanical properties and failure modes of two types of specimens under uniaxial compression are investigated. • The proposed auxetic composite exhibits an enhanced stiffness and shear resistance. • The effects of cell angle and cell wall thickness on the mechanical properties are investigated. Auxetic materials exhibit desirable mechanical properties, e.g., fracture resistance, shear resistance and energy dissipation due to their unique deformation characteristics. Honeycomb sandwich structures also have superior properties, e.g., energy absorption performance and low density. These desirable characteristics enable them as promising alternatives for building construction to meet the modern construction requirement which a higher safety standard is needed. In this study, a novel concrete composite with auxetic layered honeycomb was designed, manufactured and investigated for exploring the load-bearing performance of auxetic metamaterials under quasi-static compression. The layered honeycomb sandwich structure was analyzed by quasi-static compression test and its energy absorption performance was evaluated. Numerical models, validated by experimental results, were employed in parametric studies to further examine their performance. The results show that the layered structure has higher initial peak stress and stable platform stress. Moreover, it could improve the shear resistance and overall stability of the structure. These findings are beneficial to the applications of auxetic metamaterials in building construction. [ABSTRACT FROM AUTHOR]

A novel class of two-dimensional auxetic structures based on the pentagon motif is proposed. Their mechanical properties are investigated by combining molecular mechanics simulations with a simple three-parameter mechanical model which assumes perfectly elastic behavior. It is predicted that the proposed structures – termed as double re-entrant honeycomb – may possess unique mechanical characteristics, which include complete and perfect auxeticity, as well as the negative Poisson's ratio observed in both the tensile and compressive regimes. The behavior of the considered structures is explained in relation to well-known auxetic models. It is shown that the considered structures simultaneously implement three different mechanisms leading to a negative Poisson's ratio: the opening of the re-entrant units, the rotation of the squares, and the flattening effect. [Display omitted] • A novel class of 2D auxetic structures composed of pentagons is proposed. • The proposed structures combine two overlapping re-entrant honeycomb structures. • Simultaneous implementation of three different auxetic mechanisms is possible. • The proposed structures offer unique properties: complete and perfect auxeticity. [ABSTRACT FROM AUTHOR]

In this work, the static and transient conductivity properties of a novel centersymmetric honeycomb structure are evaluated using analytical and finite element (FE) models. The honeycomb structure features a unit cell geometry allowing in-plane auxetic (negative Poisson's ratio) deformations, and geometry parameters to be used to design the honeycomb configurations for multifunctional applications. The equivalent thermal conductivity along the two principal directions of the multi-re-entrant unit cell is calculated using a theoretical approach based on Fourier's law and electric-thermal analogy. To validate the theoretical models, a FE analysis has been carried out on periodic unit cells under thermal steady-state conditions, showing an excellent agreement with the theoretical results. A transient thermal analysis, considering convection and radiation, has been performed numerically to evaluate the possible use of this honeycomb configuration for sandwich panels in thermal protection systems. Parametric analysis on the thermal conductivity and maximum temperatures acquired during convection—radiation is performed vs. the nondimensional geometry parameters of the honeycomb cell. The auxetic honeycomb configurations show higher out-of-plane conductivity, strong in-plane thermal anisotropy, and the lowest peak temperatures during heat transfer between the bottom and top faces of honeycomb panels. [ABSTRACT FROM AUTHOR]

This work illustrates the manufacturing and tensile testing of a novel concept of honeycomb structures with hexagonal and auxetic (negative Poisson's ratio) topology, made of shape memory alloy core material. The honeycombs are manufactured using Nitinol ribbons having 6.40mm of width and 0.2mm of thickness. The ribbons were inserted in a special dye using cyanoacrilate to bond the longitudinal strips of the unit cells. Tensile tests at room temperature (full martensite) were performed on the honeycombs. Finite element models of the honeycomb assemblies under tensile loading have been developed using nonlinear beam elements from a commercial code. Good agreement is observed between numerical nonlinear simulations and the experimental results. [ABSTRACT FROM AUTHOR]

Auxetic honeycombs showing negative Poisson's ratio (NPR) effects have attracted extensive attentions due to their excellent mechanical properties, among which those with the innovative re-entrant circular (REC) unit cells have proven to present higher energy absorption capabilities than their equivalents with conventional re-entrant (RE) units, under quasi-static loading. In the current study, the quasi-static crushing strengths and energy absorption capabilities of the REC honeycombs with different geometric parameters are thoroughly investigated, through finite element (FE) based numerical simulations in combination with theoretical analyses. It was found that the crushing stress, the energy absorption characteristic and the auxeticity, i.e. the NPR value of the REC honeycomb can be tailored by adjusting the geometrical parameters of its unit cell to trigger specific deformation mechanisms in both meso (unit cell) and macro (total honeycomb) scales. Specifically, based on the cell wall interaction pattern, the REC unit cell configuration map can be divided into three distinctive regions, which are the penetration region, the interference region and the regular region, respectively. LS-DYNA based numerical simulations revealed three typical macro-scale deformation modes, i.e. the "X", the "I" and the "V" modes of the REC honeycomb in the three regions, corresponding to three meso-scale interaction patterns of the unit cell, including the "overlap", the "rectangle" and the "re-entrant" patterns. The quasi-static crushing stresses of the REC honeycombs with different geometric parameters were further compared and divided into four distinctive stages: the elastic stage, the plateau stage, the enhancement stage and the densification stage, respectively. Larger radius to height ratio and length to height ratio were found to result in larger plateau strain and smaller plateau stress. Good agreements were achieved between the numerical and theoretical predictions of the plateau strain and plateau stress of the REC honeycombs. Finally, the quasi-static energy absorption capabilities and the NPR values of the REC honeycombs with different geometric parameters were revealed and discussed, to assist engineering applications of such materials. [Display omitted] • Three regions in re-entrant circular (REC) unit cell configuration map. • "X", I" and "V" deformation modes in macro-scale. • "Overlap", "rectangle" and "re-entrant" deformation patterns in meso-scale. • Quasi-static crushing stresses of REC honeycombs with four distinctive stages. • Energy absorption and auxeticity of REC honeycombs with different parameters. [ABSTRACT FROM AUTHOR]

• Dynamic responses of auxetic DAHSPs under paper tube-guided air blasts is experimentally conducted. • Large-scale metallic DAHSPs with diverse core configurations are designed and fabricated via folding and vacuum brazing. • Fair comparisons on the blast resistance among different design strategies to vary the core relative density is performed. • Optimal thickness allocation within each layer/among various layers and foam filling for performance promotion are pursued. • Numerical simulations reveal the shock focusing effect and indicate reasonable agreement with the experimental data. Auxetic double arrowhead honeycombs (DAHs) featured by negative Poisson's ratio is attracting increasing attention owing to their tailorable mechanical properties and outstanding energy absorption capability. In this paper, the two-dimensional metallic DAHs were produced and used as the sandwich cores to construct the large-size double arrowhead honeycomb sandwich panels (DAHSPs). A comprehensive experimental investigation on their dynamic responses under the paper tube-guided air blasts was conducted. It majorly focused on the influence of core configurations induced by the variation of core wall thickness, horizontal distance, inclined angles, thickness allocations and foam filling on the deformation/failure modes, associated mechanisms and blast resistance of the target panels. Fair comparisons among different design strategies were performed to evaluate their relative performance. Experimental results show that relative to increasing the core wall thickness, the decrease of horizontal distance is more remarkable to reduce the back face deflection upon the similar weight expense, whereas the effect of inclined angles is limited due to the competition between enhanced bending stiffness and deteriorated transmitted load. A more efficient approach to decrease the back face deformation is to enlarge the inclined angles when the core relative density is relatively low but to decrease the horizontal distance at a high level of relative density. A further improvement of blast resistance can be achieved by allocating more mass to the stuffer wall rather than the tendon one, or by introducing a gradient DAH core instead of the homogeneous one. Compared to adjusting the metallic DAH core itself, the lightweight polyurethane (PU) foam filling is a more competitive manner to promote the blast performance. The preliminary numerical simulations confirmed the shock focusing effect of the paper tube support and indicated reasonable agreement with the experimental results. The present study is supposed to shed light on the design, manufacturing and optimization of auxetic sandwich structures for protection applications. [Display omitted] [ABSTRACT FROM AUTHOR]

Nowadays, sandwich plates with cellular core structures are gaining the attention of researchers owing to their outstanding features. For a better insight into auxetic structures, in this study, we propose an excellent computational approach based on polygonal meshes to comprehensively examine the free vibration, buckling and dynamic instability behaviors of the auxetic honeycomb sandwich plate structures. A generalized C 0 -type higher-order shear deformation theory ( C 0 -HSDT) in conjunction with both Laplace and quadratic serendipity shape functions is employed to approximate the strain fields for polygonal plate elements. The sandwich plate structures are constituted by an auxetic honeycomb core layer with negative Poisson's ratio and two skin layers reinforced by graphene nanoplatelets (GNPs). Ultra-light features of the plate structures can be obtained by using the auxetic honeycomb cells with negative Poisson's ratio while the GNPs are embedded into skin layers to enhance the structural stiffness. In order to determine the dynamic instability region of the sandwich plate, Bolotin's approach is utilized in the current research. Several numerical examples are carried out to investigate the influences of geometrical parameters of auxetic cell and GNPs reinforcement on the structural behaviors. The results obtained in the current research can be considered as benchmark ones to investigate auxetic sandwich plate structures. [ABSTRACT FROM AUTHOR]

Honeycombs with re-entrant (RE) unit cells were found to have negative Poisson's ratios (NPRs) and enhanced energy absorption capabilities than regular hexagon honeycombs. For further improved energy absorption capacity, a novel re-entrant circular (REC) honeycomb configuration is proposed in this work by replacing the sloped cell wall of the regular re-entrant honeycomb with double circular arc cell walls, which can dissipate extra energy due to more formed plastic angles during the crushing process. The in-plane quasi-static crushing response of the REC honeycomb with large deformation was investigated theoretically and numerically. The numerical modeling methods in LS-DYNA software were validated by using the quasi-static crush test data of 3D-printed REC honeycomb specimens. Experiment and numerical simulations both revealed that quasi-static load can result in an "X" mode deformation of the REC honeycomb. Based on the simulated deformation profiles of the representative unit, theoretical models were derived to predict the REC honeycomb's crushing strength, i.e. plateau stress. Good agreements were found between the theoretical and the numerical results within a relative error about 9%. Parametric analyses further showed that the unit cell configuration has a great effect on the crushing strength of the REC honeycomb. The REC honeycomb presents a varied Poisson's ratio with the global strain of the honeycomb; smaller radius to height ratio and length to height ratio were found to yield more pronounced NPR effect. Moreover, compared with the regular RE honeycomb, the specific energy absorption of the REC honeycomb is much higher due to the introduction of the double circular arc cell walls. Image 1 • "X" deformation mode of re-entrant circular (REC) auxetic honeycombs can be observed in experiment and simulation. • More Energy absorption than regular re-entrant (RE) honeycombs due to more formed plastic hinges under the quasi-static load. • Good agreement between numerical and theoretical prediction of the plateau stress. • Unit cell configuration show a great effect on crushing strength and Poisson's ratio. [ABSTRACT FROM AUTHOR]

This paper presents a stiffening method by which new inclined walls are added to the classical re-entrant honeycomb cell. This modification boosts the in-plane rigidity of the re-entrant cell without significant loss from auxetic behaviour. This study focuses on the in-plane elasticity moduli and negative Poisson's ratios along the principal axes. Analytical expressions that calculate the elasticity moduli and negative Poisson's ratios are derived; both finite element modelling and experiments reported in the literature validate the expressions of this work. Further, the variation in the in-plane elastic properties of the core cell is examined by altering the new wall's geometric, sectional, and material parameters. The results from the analytical expressions and the finite element models match very closely and demonstrate the boosted rigidity of the cell proposed in this work. This work also provides a benchmark of the new cell against the classical cell that can be used to tailor the in-plane elasticity moduli and negative Poisson's ratios to suit the needs of different applications. • Enhancement stiffness of re-entrant honeycomb core cell. • Advanced tunable mechanical properties by new derived analytical expressions. • A benchmark of analytical, numerical and experimental results. [ABSTRACT FROM AUTHOR]

This paper is focused on the identification of the in-plane elastic constants of a new design of auxetic (negative Poisson's ratio) honeycomb configuration with curved cell walls by using analytical and numerical homogenization techniques. The sensitivity of the elastic constants is determined against the various cell geometry parameters. Good agreement between the analytical and numerical simulations is observed. We show that the specific curved wall honeycomb configuration proposed in this paper possesses a high in-plane shear compliance, tailored anisotropy and the possibility of inducing a negative Poisson's ratio behaviour in baseline honeycomb configurations that would have otherwise positive in-plane Poisson's ratios. • New refined analytical model for the in-plane elastic constants of the curved wall centresymmetric honeycomb. • High shear compliance of the honeycomb proposed compared to other centresymmetric configurations. • High degree of tailoring the auxetic effect and the anisotropy. [ABSTRACT FROM AUTHOR]

To improve the anti-explosion performance of blast wall in offshore platforms, an auxetic re-entrant blast wall (ARBW) was proposed and designed based on the indentation resistance effect of an auxetic structure. Based on the numerical nonlinear dynamic analysis method verified by the explosion experiment of a conventional steel corrugated blast wall (CBW), the failure mechanisms of ARBW, steel honeycomb sandwich blast wall (HSBW) and CBW were investigated under distributed impulse loads. Computational results demonstrated the excellent anti-explosion performance of the proposed ARBW design. Concerning the minimal deformation at the mid-point of the proposed protective structures, the ARBW performed best. As regards the minimal deformation at the connection, both ARBW and HSBW worked well. The stress distribution of the connection illustrated the different energy absorption and transmission modes of the three blast walls. [ABSTRACT FROM AUTHOR]

• The analytical dynamic model of an auxetic honeycomb sandwich plate is constructed. • Compare the auxetic honeycombs and honeycombs with positive Poisson's ratio on impact resistance and energy absorption. • Explore the change of geometric parameters of honeycomb core cell result in change of Poisson's ratio and then result in change of the responses of the plate. • Five kinds of different impact loads are considered and the effect of different loads on the plate are compared. In this paper, we study the nonlinear transient responses of an auxetic (negative Poisson's ratio) honeycomb sandwich plate under impact dynamic loads. The partial differential equations of the honeycomb sandwich plate based on Reddy's higher shear deformation theory and Hamilton's principle are obtained. The nonlinear ordinary differential equations are then derived by Galerkin truncation method. The dynamic responses of the honeycomb sandwich plate subjected to different loads, such as step loading, air-blast loading, sinusoidal loading, triangular loading and incremental loading, respectively, have been investigated. The contributions of total thickness, core thickness ratio, Poisson's ratio, cell inclination angle and blast types to transient responses of the plate are discussed in detail by numerical simulations. The effect of geometrical parameters on the dynamics of the plate and the effect of different blast loads on the plate are obtained. It is found that the honeycomb sandwich plate with negative Poisson's ratio would be a better choice compared with the positive one for some structures under dynamic loads. [ABSTRACT FROM AUTHOR]