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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]

Due to the negative Poisson's ratio (NPR) effect, auxetic structures such as re-entrant honeycombs show excellent energy absorption and force mitigation capabilities under crushing load. Compared with two-dimensional (2D) auxetic structure, three-dimensional (3D) ones are expected to have better performance, but they are more difficult to fabricate. In this study, a hierarchical fabrication method for metallic 3D re-entrant honeycomb (3D-RH) using traditional manufacturing techniques is proposed. A 3D-RH specimen was crafted and tested under quasi-static crushing condition. The finite element (FE) model was established and verified by the experimental results. The numerical simulation results showed that the friction condition between 3D-RH specimen and loading surfaces has a great influence on the deformation mode of specimen. Moreover, the energy absorption capability of 3D-RH was numerically compared with a similar structure, i.e., the warp and woof periodic auxetic cellular (WWPAC) structure. The results showed that the specific energy absorption (SEA) of 3D-RH is 1.59 times that of WWPAC and the 3D-RH outperformed the latter in lightweight performance. Finally, the blast resistance of 3D-RH sandwich panel was numerically studied. Compared with 2D re-entrant expanded honeycomb (2D-REH) core, 3D-RH core can reduce the maximum deflection of lower face-sheet by 14.6%, and increase SEA by 20.5%, showing better blast resistance. • A hierarchical fabrication method of metallic 3D-RH has been proposed. • The method uses traditional manufacturing techniques to reduce fabrication cost. • The friction coefficient can alter the deformation mode of 3D-RH under crushing. • The energy absorption per unit mass of 3D-RH is greater than that of the WWPAC. • The 3D-RH shows better anti-blast performance than the 2D-REH. [ABSTRACT FROM AUTHOR]

An analytical framework has been developed for predicting the equivalent in-plane elastic moduli (longitudinal and transverse Young’s modulus, shear modulus, Poisson’s ratios) of irregular auxetic honeycombs with spatially random variations in cell angles. Employing a bottom up multi-scale based approach, computationally efficient closed-form expressions have been derived in this article. This study also includes development of a highly generalized finite element code capable of accepting number of cells in two perpendicular directions, random structural geometry and material properties of irregular auxetic honeycomb and thereby obtaining five in-plane elastic moduli of the structure. The elastic moduli obtained for different degree of randomness following the analytical formulae have been compared with the results of direct finite element simulations and they are found to be in good agreement corroborating the validity and accuracy of the proposed approach. The transverse Young’s modulus, shear modulus and Poisson’s ratio for loading in transverse direction (effecting the auxetic property) have been found to be highly influenced by the structural irregularity in auxetic honeycombs. [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]

Auxetic metamaterials have generated a great deal of interest in recent years. In this letter, a novel approach which can be used to produce such auxetic metamaterials is presented. This method involves the introduction of patterned slit perforations within a sheet/block of material in order to create systems which resemble a large variety of auxetic systems ranging from rotating units to chiral honeycombs. These perforated systems have also been shown to have the potential to exhibit giant negative Poisson's ratios. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

This paper assesses the thermal shock behaviors of the auxetic honeycomb layer (HL) through consideration of crack propagation and new crack initiation. All material properties are temperature-dependent. To calculate the thermal stress field with no crack, the finite difference approach is used. On this basis, a theoretical model for thermal stress intensity factor and thermal stress between cracks is developed. The model combines the equivalent fracture toughness and equivalent tensile fracture strength as the failure criteria. The critical temperature for crack propagation and new crack initiation in the HL is determined. The results reveal that the thermal shock resistance (TSR) of the HL is overestimated if the temperature-dependent material properties (TDMPs) are not taken into account. Multiple cracking can dramatically improve the TSR of the HL. Compared to the non-auxetic HL, the TSR of the auxetic HL is significantly enhanced. Furthermore, the HL with lower relative density generally possesses higher TSR, and the corresponding physical mechanisms are revealed. • Thermal shock resistance of auxetic honeycomb layer is assessed considering multi-cracking. • Thermal shock resistance is overestimated if temperature dependence of material properties is ignored. • Multiple cracking increases the thermal shock resistance of the structure. • Auxetic property improves the thermal shock resistance well. [ABSTRACT FROM AUTHOR]

This work proposed an improved modification on the common re-entrant hexagonal honeycomb (RHH) by replacing the solid junction of the cell walls with a small hollow circle, and thus achieving a novel kind of honeycomb. A simple energy-based approach was adopted to develop theoretical models for predicting the in-plane equivalent elastic moduli of the new honeycomb. Finite element analysis was employed to verify the theoretical models and then the effects of the micro geometrical parameters on the in-plane elastic behaviors were further discussed. The results showed that the new honeycomb possesses an obvious negative Poisson's ratio effect, meanwhile, can achieve higher specific Young's modulus and shear modulus than the RHH. Moreover, the new honeycomb was found to exhibit a more tailored anisotropy than the RHH from the off-axial properties analysis. As a result, the new honeycomb may be an effective substitute for the RHH in application and the new layout can be a good guide for the improved design of auxetic honeycombs. [ABSTRACT FROM AUTHOR]

• 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]

• Nonlinear thermo-buckling and postbuckling analysis. • The imperfect eccentrically stiffened FGM double curved shallow auxetic shells. • The shell subjected to uniform external pressure, axial compressive, thermal loads. • Using analytical method, the FSDT, the Galerkin and stress function methods. • Taking account the effect of pores and temperature-dependent material properties. It is the first time the nonlinear thermo-mechanical analysis (buckling and postbuckling) of imperfect eccentrically stiffened (ES) double curved shallow shells made of auxetic material under uniform external pressure, axial compressive and thermal loads has been studied by adopting analytical method. The double curved shallow sandwich shell is constructed by an auxetic honeycomb core layer with negative Poisson's ratio and two isotropic homogeneous skin layers. In the present study, the outer surface of the structure is reinforced by a system of FGM stiffeners and taking the influence of pores in the stiffeners into account to increase the load capacity of the structure. Besides, the material properties of the shells and stiffeners are both temperature-dependent. The first-order shear deformation theory is applied to derive equilibrium and compatibility equations of double curved shallow auxetic shell, considering both the geometrical nonlinearity in the Von Karman sense and initial geometrical imperfections and shells placed on the Winkler – Pasternak foundation. The Galerkin and stress function methods are proposed to determine the critical load value and the deflection– load curve by taking the root form of the two-term rotation angles to give the Galerkin integral an equilibrium equation. Several comparisons with results in other literature are made to examine the reliability and accuracy of the method used in the present study. Finally, the effects of thermal environment, reinforced stiffeners, elastic foundation, pores and parameters of auxetic core layer on the buckling and postbuckling loads are evaluated in this paper. The numerical and graphical results show that the reinforced stiffeners, auxetic core, temperature and porous distribution have significantly influenced on the buckling and postbuckling loads of ES double curved shallow auxetic honeycomb sandwich shells. [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]

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]

Cellular auxetic materials possess a negative Poisson's ratio (NPR) and attractive mechanical properties; however, these materials are limited by their low stiffness and strength and strong anisotropy. To address these issues, we developed a novel auxetic honeycomb with a fully triangular architecture inspired by the anti-tetra chiral configuration. The new stretch-dominant structure was fabricated using a three-dimensional printing process. The resulting deformation behavior of the triangular auxetic honeycomb structure was compared with that predicted from finite element (FE) simulations; these revealed an NPR and high stiffness, in which the ratio of specific stiffness to specific density was 0.12. The simulations allowed for an NPR in both the longitudinal and transverse directions concurrently. Based on the mechanism revealed by the FE simulations, the novel triangular honeycomb can be tuned to have the same behavior in two orthogonal directions, while retaining high stiffness and an NPR. [ABSTRACT FROM AUTHOR]

This work investigates the effects of cracking on the overall properties of brittle auxetic/non-auxetic honeycombs. The micro numerical model and macro theoretical model are established simultaneously. The analytical expressions of crack opening displacement (COD) are derived and the results agree well with numerical simulations. The fracture toughness of honeycombs is calculated by combining the stress in the numerical model and the stress intensity factor (SIF) in the theoretical model. The reductions in the equivalent stiffness and equivalent thermal stress coefficient (TSC) of the cracked honeycombs are predicted. The negative Poisson's ratio (NPR) effects on those fracture mechanics parameters and equivalent thermomechanical properties are studied. And considering the orthogonal anisotropic properties of honeycombs, the cracking along two orthogonal directions is examined. It reveals that the significant NPR effects can only be detected in a single direction and the effects in another direction are inconspicuous. In addition, the effects of crack length, medium dimensions, and cell-wall angle of honeycomb cells on the overall performance of the cracked medium are also evaluated by parametric studies and the results are displayed graphically. [ABSTRACT FROM AUTHOR]

In this study, a new vertical strut combined re-entrant auxetic structure was designed and its mechanical behaviors have been studied. An analytical model was established based on energy theorems considering bending, shearing and stretching. Numerical simulations and experiments were both carried out to confirm the validity of the analytical model based on beam theory. Results exhibited a good agreement between the refined analytical model, the numerical simulation and experimental study. Four deformation mechanisms were analyzed and the effect of deformation on the effective Poisson's ratio (EPR) and elastic module were investigated. A parametric study was further performed to quantitatively illustrate the influences of the geometrical parameters on the effective elasticity properties of the designed structure. The results showed that the lightweight structures can significantly enhance the auxetic behavior and achieve a high degree of anisotropy with wide vertical strut lengths and internal angles compared with conventional re-entrant honeycomb configurations. • An analytical model of a vertical strut combined auxetic structure was established. • Vertical strut and re-entrant angle mainly determined effective elasticity properties. • Four deformation mechanisms were considered and influenced auxetic behaviors. • Numerical simulations and experiments both validate the analytical model. [ABSTRACT FROM AUTHOR]

In this article, a novel re-entrant octagonal honeycomb (ROH) with negative Poisson's ratio (NPR) configuration is proposed owing to its superior energy absorption and lightweight design performances. Regarding the effects of adjacent unit cells, a mechanics-based homogenization approach is put forward and employed to derive static mechanical properties including elastic modulus and Poisson's ratio of two-dimensional (2D) and three-dimensional (3D) ROHs under axial loading. Based on above, the predominant compression failure mode is explored. Then, the static mechanical properties and finite element model of ROH are validated by experiment and demonstrate high accuracy. Subsequently, to obtain dynamic responses of the ROH under dynamic impact loading, the relations among dynamic plateau stress and quasi-static plateau stress, initial density, dense strain and impact velocities are established under the conditions of mass conservation and momentum theorem based on one-dimensional shock wave theory and ideal rigid-perfectly plastic material model. Besides, dynamic energy absorption models of the ROH under dynamic loading are acquired in consideration of the effects of impact velocities on dense strain, the results show that theoretical models agree quite well with simulation results. On this basis, the application of ROH on automotive energy absorber is studied with the purpose of acquiring superior energy absorption and satisfying lightweight requirements simultaneously. Based on the decoupling thought of stiffness and pedestrian protection, a new design method of carbon fiber reinforced plastics (CFRP) hood with ROH is put forward in consideration of energy absorption and lightweight requirements. Of this, ROH is regarded as sandwich structure to meet the demands of energy absorption, CFRP plates are treated as outer and inner skins to satisfy the needs of stiffness. Firstly, multi-objective optimization models are set up under the constraints of reasonable structural parameters and negative Poisson's ratio. Then, the NSGA-Ⅱalgorithm is adopted to achieve the optimal Pareto fronts with the targets for maximum unit mass energy absorption and minimum density. Afterwards, the optimization on thicknesses of CFRP layers is carried out to achieve minimum mass under the constraint conditions that the static stiffness of CFRP hood is larger than that of steel hood. In addition, the layering sequence of inner and lower skins are also optimized. Finally, improved overall stiffness, optimized HIC values and similar lightweight space of CFRP hood with ROH are achieved in comparisons with CFRP hood without ROH. Compared with steel hood, the pedestrian protection performance of CFRP hood with ROH is greatly enhanced, 1st free mode increases by 33.9%, and torsional stiffness and bending stiffness increase by 23.1% and 30.8%, respectively. While the weight decreases by 40.1%. The results indicate that the CFRP hood with ROH presents outstanding advantages of pedestrian protection and lightweight design in terms of HIC and stiffness performance. [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]

• The mechanical properties and failure stress of re-entrant honeycombs is formulated analytically. • The stretching, flexing and hinging mechanisms of the auxetic honeycomb are considered. • Using the Euler buckling and Tsai-Wu criterion, the critical stress of the honeycomb is obtained. • Effects of geometric parameters and loading conditions on the mechanical properties are studied. In the present study, a novel theoretical model is developed, based on classical laminate theory, to predict the equivalent mechanical properties of the re-entrant lattice structures, which composed of continuous fiber reinforced composite struts. Three main mechanism of stretching, flexing and hinging are considered and a general closed-form formulation is derived to estimate the auxetic honeycomb's elastic and shear modulus as well as Poisson's ratios. In spite of previous studies in which the response of honeycomb structures is modeled using beam theory, here, each strut of unit cell is expressed as a composite laminate with orthotropic mechanical properties and classical laminate theory is implemented to calculate the mechanical constants. In addition, using the available relations, the elastic buckling stress and failure load of the auxetic unit cell is evaluated. The proposed model is validated using experimental results, which are available from the previous studies as well as FE simulations. A parametric study is also conducted on the model to study the effects of cell angle and honeycomb's density on the mechanical properties and failure stresses of the re-entrant structure. The results show that, the proposed analytical model can finely predict the equivalent and failure properties of the auxetic honeycomb structures. Furthermore, it is observed that, for the considered re-entrant composite structures with the cell angles between −25 to 25, the vertical strut fails due to the damage evolution and for the unit cells with other cell angles, the buckling failure is determinative. [ABSTRACT FROM AUTHOR]

In this study, two dimensional double-U auxetic honeycombs (2D DUHs) composed of curved shell members have been proposed based on traditional double-V (concave quadrilateral or re-entrant double-arrowhead) auxetic honeycombs (2D DVHs). Theoretical analysis, numerical simulation and compression experiments are adopted to characterize the mechanical properties including equivalent Young's modulus, equivalent Poisson's ratio, and quasi-static collapse process of 2D DVHs and DUHs. Specimens of the 2D DUHs and DVHs made of Aluminum alloy (A6061) are fabricated through wire cut electrical discharge machining. Both numerical and experimental results illustrate that concave DUHs exhibit tunable negative Poisson's ratio and equivalent stiffness level compared to DVHs. Furthermore, DUHs have smooth geometry configurations that can reduce stress concentration at elastic region and exhibit stronger auxetic behavior during large deformation. Those mechanical properties of 2D DUHs satisfy the requirements for practical engineering applications such as energy absorption. The dynamic crushing responses of 2D DVHs and DUHs are also investigated, and it is found that the crushing collapse modes and plateau stress improvement of 2D DUHs are related to the crushing velocity. Finally, the local impact responses of 2D DVHs and DUHs are studied numerically, the influences of impact velocity and indenter size are considered. This study provides useful instructions for the design, fabrication and analysis of auxetic structures, which has a broad prospect in some advanced technical applications. • 2D double-U auxetic honeycombs are proposed based on traditional double-V auxetic honeycombs. • DUHs exhibit tunable Poisson's ratio and have equivalent stiffness and strength level compared to DVHs. • DUHs have smooth structural connections that can reduce stress concentration and manufacturing damage. • DUHs present stronger auxetic behavior which leading to better load resistance and energy absorbing capacity. [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]

Continuous fiber reinforced thermoplastic composite (CFRTPC) auxetic honeycomb structures were fabricated using the 3D printing technology with a specific printing path planning. For comparison, auxetic honeycombs were also fabricated with pure polylactic acid (PLA). In-plane compression tests were conducted, with corresponding damage types explored using Scanning Electron Microscopy (SEM) images. A printing path-based finite element (FE) method was developed to mimic both small and large deformations of CFRTPC auxetic honeycombs, while analytical model was proposed to predict their effective stiffness and Poisson ratio. Good agreement was achieved among analytical predictions, FE simulation results and experimental measurements. A systematic parametric study was subsequently carried out to quantify the dependence of in-plane mechanical properties on geometrical parameters. Compared with pure PLA structures, the presence of continuous fibers efficiently prohibited crack propagation in the matrix for each ligament of CFRTPC auxetic honeycombs. Adding continuous fibers increased the mass only by 6%, but led to dramatic increase in compressive stiffness and energy absorption by 86.3% and 100% respectively and smaller Poisson ratios. The proposed 3D printing technology has great potential in integrated fabrication of continuous fiber reinforced composite lightweight structures having complex shapes, attractive mechanical properties, and multifunctional attributes. • The continuous fiber reinforced composite auxetic honeycomb was fabricated by 3D printing technology. • A combined experimental, analytical and numerical approach was employed to investigate the in-plane behaviors. • Adding continuous fibers lead to dramatic increase in mechanical properties and smaller Poisson ratios. [ABSTRACT FROM AUTHOR]

• The experimental and numerical methods were used to investigate the dynamic response of end-clamped shallow sandwich arch with HCNPR under local impulsive. • The local negative Poisson's ratio deformation was observed in the reentrant honeycomb core. • The dynamic deformation evolution of the face sheets and honeycomb core was studied. In recent years, auxetic metallic cellular material such as auxetic reentrant honeycomb is one of the research hotspot in metallic cellular materials due to its attractively negative Poisson's ratio. In this manuscript, the dynamic response of the end-clamped shallow sandwich arch with aluminum face sheets and auxetic reentrant hexagonal aluminum honeycomb core subjected to foam projectile with high-velocity impact was experimentally and numerically investigated. The reentrant honeycomb cores with different cell-wall thicknesses were fabricated by selective laser melting (SLM). The deformation processes of honeycomb core were captured by a high-speed camera. And deflection history of back face sheet was obtained using three-dimensional digital imaging correlation method (DIC) analysis system. Failure modes and deformation mechanisms of the sandwich arch were investigated. According to experiment results, the local negative Poisson's ratio deformation was found in honeycomb core. Then, a finite element (FE) model was established to study the dynamic deformation evolution of two face sheets and auxetic reentrant honeycomb core. The results showed that the honeycomb cells were not always undergoing shrinkage deformation and flowing into the loading center, while the local expand deformations of honeycomb core were also found. [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]

In this paper, not only the effect of cell-wall thickness on the deformation mode of metallic auxetic reentrant honeycomb was firstly studied, but also the negative Poisson's ratio (NPR) effect on the crushing stress was also clarified by the quasi-static compressive experiments. Firstly, two types of typical reentrant hexagonal honeycombs with thin-walled and thick-walled cells were manufactured and compressive tests were carried out to characterize the deformation modes, negative Poisson's ratio distribution and crushing stress. Experiment results showed that the deformation mode of thin-walled honeycomb was quite different from that of thick-walled honeycomb, which has not been found in the existing reported papers. Secondly, a finite element (FE) model of the reentrant honeycomb was established and the FE results were systematically compared with the experimental results. The effects of honeycomb cell's number on the compressive deformation and the crushing stress of the honeycomb were studied. The results showed that once the honeycomb contained more than four cells in both transverse and horizontal directions, both the compressive deformation and the crushing stress were close to the experimental results. Finally, the energy absorption characteristics of reentrant honeycomb during compression were discussed. Unlabelled Image • The experimental and numerical methods were used to accurately investigate the compressive behavior of the reentrant honeycomb. • The negative Poisson's ratio (NPR) effect on the crushing stress was clarified by the quasi-static compressive experiments. • Experiment results found that the NPR effect only had an amplified effect in the strain hardening stage. • In order to accurately investigate the compressive behavior of the reentrant honeycomb, the minimum cell's number should be greater than four. [ABSTRACT FROM AUTHOR]

Microstructured honeycomb materials may exhibit exotic, extreme and tailorable mechanical properties, suited for innovative technological applications in a variety of modern engineering fields. The paper is focused on analysing the directional auxeticity of tetrachiral materials, through analytical, numerical and experimental methods. Theoretical predictions about the global elastic properties have been successfully validated by performing tensile laboratory tests on tetrachiral samples, realized with high precision 3D printing technologies. Inspired by the kinematic behaviour of the tetrachiral material, a newly-design bi-layered topology, referred to as bi-tetrachiral material, has been theoretically conceived and mechanically modelled. The novel topology virtuously exploits the mutual collaboration between two tetrachiral layers with opposite chiralities. The bi-tetrachiral material has been verified to outperform the tetrachiral material in terms of global Young modulus and, as major achievement, to exhibit a remarkable auxetic behaviour. Specifically, experimental results, confirmed by parametric analytical and computational analyses, have highlighted the effective possibility to attain strongly negative Poisson ratios, identified as a peculiar global elastic property of the novel bi-layered topology. Unlabelled Image • A novel layered topology of auxetic cellular materials based on the tetrachiral honeycomb microstructure, named bi-tetrachiral material, is proposed. • The global elastic properties of 3D printed samples of the bi-tetrachiral material are experimentally identified from quasi-static tensile tests. • Computational finite element models and beam lattice models efficiently reproduce the experimental response of the bi-tetrachiral material. • The mutual collaboration of two tetrachiral layers with opposite chirality prevents the development of global angular strains under tensile loads. • Parametric analyses show that Poisson ratios close to −1 are achievable by regulating the distance between the interlayer constraints. [ABSTRACT FROM AUTHOR]

• A new NPR 2D honeycomb was developed based on the horizontal series-connected parallelograms. • A combination of analytical, numerical and experimental method was adopted to investigated the in-plane effective modulus of the new 2D honeycomb. • Dependence of the mechanical properties of the new 2D homeycomb on the geometric parameters was exploited. • New mechanism for generating NPR was discovered. • The new 2D honeycomb was extended to three dimensions. This paper details a new 2D auxetic honeycomb that was structured by mirroring the horizontal series-connected parallelograms along the vertical direction. A combination of analytical, numerical, and experimental methods was used to gain a deep understanding of the elastic behavior of the new honeycomb and examined its dependence on the geometric parameters. The results showed that the new honeycomb can achieve a negative Poisson's ratio via a novel mechanism. Furthermore, the newly developed 2D honeycomb was extended to three dimensions, and three kinds of 3D structures were conceived via different splicing methods. A preliminary numerical study was conducted to explore the mechanical properties of the 3D structures. An auxetic effect was found in two principal orthogonal coordinate directions of those 3D structures when they were compressed along the other principal axis. This work may provide a new insight into the design of both the 2D and 3D auxetics. [ABSTRACT FROM AUTHOR]

This paper presented a detailed study of the negative Poisson's ratio (NPR) effect on shrinkage deformation and crushing stress of metallic auxetic reentrant honeycomb under low, medium and high-velocity compressions. Instead of the traditional calculating method for the crushing stress of auxetic honeycomb, the new theoretical formula considering the NPR effect was proposed. The theoretical and finite element (FE) results showed that the crushing stress of reentrant honeycomb had been underestimated according to the traditional nominal stress-strain curve. Not only had that, but also the theoretical results showed that the NPR effect could enhance the inertial effect during dynamic compression. Subsequently, under the medium and high-velocity compressions, the dynamic deformations of reentrant honeycomb were obviously different from that under low-velocity compression and those of metallic cellular materials with positive Poisson's ratio (such as Al foam and traditional hexagonal honeycomb) under medium or high-velocity compressions. In view of these differences, a new shrinkage deformation mechanism of reentrant honeycomb was proposed and a characterization parameter in contrast to the Poisson's ratio was presented to describe its dynamic shrinkage deformation capacity. [ABSTRACT FROM AUTHOR]

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

We report that carbon honeycomb, a new three-dimension carbon allotrope, exhibits large negative Poisson's ratio, as large as −0.32, in tensile revealed via molecular dynamics simulations. The Poisson's ratio of carbon honeycomb is anisotropic, and sensitive to temperature. The carbon honeycomb has phase transformation from normal to auxetic by tensile, along both zigzag and armchair directions. The critical strain for the normal-auxetic transition along the cell-axis direction reduces with respect to an increase in temperature. Combined with high strength of 50 GPa, such a unique and adjustable negative Poisson ratio suggests broad engineering applications of carbon honeycomb. [ABSTRACT FROM AUTHOR]