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The literature lacks comparisons of analytical and numerical calculations that have been verified experimentally for elastic constants of auxetic cells in cores manufactured from wood materials. The aim of this study was to determine the effect of auxetic cell geometry and the type of material used in their manufacture on elastic properties of the honeycomb panel core. This paper describes properties of the materials, from which core cells were modeled and presents mathematical models of cell properties. The method of numerical optimization of cell shape was specified, and the numerical calculations concerning modeled cells are given along with the course and results of experimental tests. Additionally, the results of analytical, numerical, and experimental tests were compared. Cell geometry had a considerable effect on elastic properties of honeycomb panel cores, particularly the angle of the cell wall. Moreover, geometric imperfections had a significant effect on the results of analytical calculations. Based on numerical calculations, satisfactory consistency between these results and experimental tests was obtained. [ABSTRACT FROM AUTHOR]

Development of advanced materials is often time consuming and expensive because of the large number of variables involved and experiments needed. An effective experimentation strategy would accelerate development by reducing the required amount of experiments without sacrificing the obtainable information. In this paper, the development of auxetic polyurethane (PU) foams was discussed as a case study. Auxetic materials are materials with a negative Poisson's ratio and have potential in many structural and functional applications. Auxetic PU foams are the most studied auxetic materials, and their manufacturing and properties are affected by many processing and environmental factors. This paper introduces a sophisticated design of experimental methodology to help reduce the experimental effort while effectively screening these factors. This methodology is then applied in an experiment to illustrate its utility and distinct advantages that greatly facilitate material development. [ABSTRACT FROM AUTHOR]

This paper dealt with the vibration characteristics of an auxetic rectangular plate under in-plane compression. Firstly, the equivalent bending stiffness matrix of a star-shaped auxetic plate was obtained using Castigliano's theorem and the homogenization technique. Then, employing the classical plate theory (CPT) in conjunction with the Rayleigh–Ritz method, the natural frequencies of auxetic plate were extracted. The chebyshev polynomial series has been selected to define the assumed displacement fields of the plate. Convergence study for the Rayleigh–Ritz method was conducted. The accuracy of the proposed mathematical model for the star-shaped auxetic plate was validated using the results from a finite element analysis (FEA). Effects of unit cell geometric parameters on the natural frequencies of the plate were examined. The auxeticity angle of star-shaped pattern had a significant effect on the natural frequencies. The present approach can be extended into other auxetic patterns, such as re-entrant bowtie auxetics. [ABSTRACT FROM AUTHOR]

Auxetic structures possess negative Poisson's ratio (NPR). They exhibit enhanced indentation resistance, synclastic deformation, high fracture toughness and high energy absorption. Several sorts of auxetic structures exist, such as re-entrant, arrowhead, chiral, etc. The re-entrant structure is the most common sort of auxetic structures. This paper investigates the mechanical behavior of the re-entrant auxetic structure using experimental and numerical methods. Two modified re-entrant topologies are proposed based on the original (conventional) re-entrant topology. Using these modified topologies, two re-entrant auxetic sandwich structures are designed and 3D printed using fused deposition modeling (FDM) out of polylactic acid (PLA). Conducting compression, three and four-point bending tests, the compressive and flexural performance of the two new re-entrant auxetic sandwich structures is studied and compared with the original (conventional) re-entrant structure. More specifically, Poisson's ratio, compressive modulus, flexural stiffness and maximum loads of the structures are focused and studied. The mechanical properties of auxetic sandwich structures are improved using modified topologies. The new re-entrant auxetic sandwich structures show 11% higher normalized compressive modulus and 9% higher normalized flexural stiffness than the original re-entrant structure. [ABSTRACT FROM AUTHOR]

In the current paper, analysis of torsional vibration behavior of non-circular nanorods made of auxetic honeycomb material is investigated for the first time. The triangular and elliptical cross-sections are selected to be analyzed. Moreover, one of the distinct properties of auxetic honeycomb materials is that they possess a negative Poisson's ratio, which can be effective on the design and analysis of structures. In order to capture a small size or scale effect, the constitutive relation of nanorods is modified based on Eringen's nonlocal elasticity theory (ENET). Hamilton's principle is employed to obtain the kinematic relation of nanoscale rods. The nonlocal derived governing equations of non-circular auxetic nanorods are solved by utilizing an analytical method with respect to different boundary conditions. In order to verify the correctness of obtained outcomes, the results are compared to previous investigations, and good agreement can be observable. The influences of various parameters such as both Clamped–Clamped (C–C) and Clamped-Free (C-F) boundary conditions, nonlocal parameters, inclined angles, and geometrical ratios are explored and illustrated in the framework of several figures, which can be seen in detail. [ABSTRACT FROM AUTHOR]

The objective of this research is to understand more fully the out-of-plane auxetic behavior of paper. In the reported research, initial characterization of the auxetic properties of common, commercially available papers is described. The results of these experiments suggest that the auxetic response seen in paper structures is related to the cellulose fiber network structure, as previously suggested, and is related to inter-fiber bonding and fiber organization in the sheet. These results also indicate that further study is needed to understand how materials and processing variables influence auxetic behavior. A mathematical model, attempting to explain auxetic response in an idealized arrangement of fibers is included. Ultimately, the fundamental understanding resulting from this research should lead to new product development opportunities for forest product-based paper materials as well as to the establishment of predictive structure-property relations for auxetic materials using paper as a model system. [ABSTRACT FROM AUTHOR]

Kirigami, the ancient paper art of cutting, has recently emerged as a new approach to construct metamaterials with novel properties imparted by cuts. However, most studies are limited to thin sheets‐based 2D kirigami metamaterials with specific forms and limited reconfigurability due to planar connection constraints of cut units. Here, 3D modular kirigami is introduced by cutting bulk materials into spatially closed‐loop connected cut cubes to construct a new class of 3D kirigami metamaterials. The module is transformable with multiple degrees of freedom that can transform into versatile distinct daughter building blocks. Their conformable assembly creates a wealth of reconfigurable and disassemblable metamaterials with diverse structures and unique properties, including reconfigurable 1D column‐like materials, 2D lattice‐like metamaterials with phase transition of chirality, as well as 3D frustration‐free multilayered metamaterials with 3D auxetic behaviors and programmable deformation modes. This study largely expands the design space of kirigami metamaterials from 2D to 3D. [ABSTRACT FROM AUTHOR]

Based on the variational differential quadrature (VDQ) method, the bending and buckling characteristics of circular plates made of functionally graded graphene origami-enabled auxetic metamaterials (FG-GOEAMs) are numerically studied in this paper. It is considered that the plate is composed of multiple GOEAM layers with graphene origami (GOri) content that changes in layer-wise patterns. The results from genetic programming-assisted micromechanical models are also employed in order to estimate the material properties. The plate is modeled according to the first-order shear deformation plate theory whose governing equations are obtained using an energy approach in the context of VDQ technique. The governing equations are given in a new vector-matrix form which can be easily utilized in coding process of numerical methods. By means of VDQ matrix differential and integral operators, the governing equations are discretized and solved to calculate the lateral deflection and critical buckling load of plates under various boundary conditions. Selected numerical results are presented to investigate the influences of boundary conditions, GOri content, folding degree and distribution pattern on the buckling and bending behaviors of FG-GOEAM plates. [ABSTRACT FROM AUTHOR]

The problem of obtaining anisotropic auxetic composite laminates, i.e. having a negative Poisson's ratio for at least some directions, is examined in this paper. In particular, the possibility of obtaining auxeticity stacking uni-directional identical plies is considered. It is shown that if the ply is composed by isotropic matrix and fibers, then it is impossible to obtain totally auxetic orthotropic laminates, i.e. auxeticity for each direction, unless at least one among matrix and fibers is auxetic itself. Moreover, it is shown what are the conditions, in terms of the mechanical properties of the constituents and of the volume fraction of the fibers, to fabricate uni-directional plies with which to realize laminates having a negative Poisson's ratio for some directions. Several existing materials are also examined. All the analysis is done using the polar formalism, very effective for the study of plane anisotropic problems. [ABSTRACT FROM AUTHOR]

AbstractAuxetics have emerged recently as an exciting class of mechanical metamaterials that are identified by negative Poisson’s ratio. The mechanical properties of rationally designed auxetic metamaterials are mostly influenced by their topological characteristics rather than the material properties of their constituent material. The goal of this paper is to use optimization algorithms to achieve the desired target mechanical properties for reentrant auxetic metamaterials. The GEKKO optimization package in Python is used for optimizing the geometry of auxetic lattice structures. Several mechanical properties including Poisson’s ratio, relative Young’s modulus, relative yield stress, and relative energy absorption capability, as well as the noted properties normalized with respect to relative density and relative Young’s modulus are chosen for being minimized or maximized. ANSYS finite element package is also implemented for validation of the results obtained from the optimization algorithm. According to the results, to achieve the maximum magnitude of Poisson’s ratio (positive and negative), the size of the unit cell in the lateral direction must be selected to be maximum and the thickness must become minimum. Moreover, to achieve the maximum value of the relative Young’s modulus or energy absorption in any direction, the size of the unit cell in that direction must be maximized. Also, to achieve the maximum amount of relative yield stress in both directions, the unit cell must have a maximum thickness and an internal angle close to zero. [ABSTRACT FROM AUTHOR]

This paper introduces a novel metasurface based on Auxetic metamaterials for attenuating and isolating low-frequency Rayleigh waves. Distributed scatterers are embedded in the Auxetic substrate to form the unit cell, and the meticulously designed layout is intended to induce more multipolar resonance modes. Firstly, the propagation characteristics of Rayleigh waves in the elastic half-space containing metamaterial domain are investigated under the dynamic effective framework of elastic media, and the effective parameter combinations for shielding Rayleigh waves are summarized, which provide a guide for customized design. Then, a new ultra-low frequency band gap is revealed via eigenstate analysis and effective parameter calculation. More noteworthy is the discovery of bridging effects in hybrid bands. Finally, multiple bandgaps, including ultra-low frequency bandgap, are integrated through hybrid bands, achieving an ultra-wide attenuation from 3.2 Hz to 20 Hz, almost perfectly covering the main frequency range of real seismic Rayleigh waves. At the same time, a reduction of up to 96.3% in the response peak is also validated in time-domain testing. More promisingly, there is still room to improve the low-frequency performance of the metasurface by optimizing the effective modulus of the Auxetic metamaterial. Undoubtedly, it further explores the physical boundary of extremely compact metamaterials in modulating even larger wavelength, which will inspire more potential Auxetic metamaterials for applications in emerging fields. [ABSTRACT FROM AUTHOR]

In this paper, low‐velocity impact properties and impact damage responses of a novel type of 3D hybrid auxetic structures made from carbon fiber‐reinforced polymer (CFRP) laminates were investigated experimentally and numerically. The structures were first fabricated with three different rib's width ratios t1/t0, and then impacted with three different impact energies of 30, 40, and 50 J using a drop tower impact system equipped with a cylindrical impactor. The finite element (FE) models were built in ABAQUS/Explicit, and the Hashin criteria were applied to predict the failure of the structures. The contact force histories, energy histories, displacements, and failure modes were compared between the experiment and FE simulation at all three impact energy levels. The failure morphologies and mechanisms were obtained and studied. The impact testing results suggest that the 3D auxetic structure with a higher t1/t0 ratio can lead to higher peak contact force and sustain higher impact energy. The failure of the structure starts from the middle area and propagates to the upper and lower areas. This research provides a better understanding of the impact responses and failure modes of the 3D auxetic lattice structure, which can promote the potential applications of the auxetic materials. Highlights: Drop weight low‐velocity impact responses of 3D novel hybrid auxetic lattice structures are investigated through both experiment and finite element simulation.The effects of structural parameters on the impact performance of the 3D auxetic structures at different impact energy levels are analyzed.The failure morphologies and mechanisms of the 3D auxetic structures are studied. [ABSTRACT FROM AUTHOR]

In this paper, the free vibration characteristics of stiffened sandwich plates with auxetic core and functionally graded piezoelectric face sheet are investigated. The sandwich plate configuration comprises three distinct layers: a top layer consisting of functionally graded piezoelectric material (FGPie), a core layer composed of auxetic material exhibiting a negative Poisson's ratio, and a bottom layer made of functionally graded material (FGM). The sandwich plate is reinforced by isotropic stiffeners along the x and y directions. A novel model is established based on the four-variable shear deformation refined plate theory and the pb2-Ritz method. This model offers the flexibility to analyze plates with diverse mechanical boundary conditions and supports two types of electrical boundary conditions: closed circuit and open circuit. The accuracy and convergence of the proposed model are validated through comparative analyses against published results, thereby confirming the reliability when analyzing the vibration characteristics of this complex sandwich structure. Furthermore, some new investigations are carried out to explore the influence of various parameters on the vibrational characteristics of the stiffened piezoelectric auxetic sandwich (SA-FGPie) plates. The results suggest that the properties of the reinforcing stiffeners, such as quantity or dimensions, alter the free vibration characteristics of the SA-FGPie plates significantly. In addition, parameters such as the volume fraction indexes of the FGPie and FGM layers, electrical boundary conditions or FGPie layer thickness, as well as geometric parameters of the auxetic unit cell also have a significant influence on the vibration response of SA-FGPie plates. However, the influence of these parameters depends on the specific boundary conditions. [ABSTRACT FROM AUTHOR]

This study aimed to improve the energy absorption capabilities of auxetic structures, which are used in sandwich structures in engineering. To achieve this, the study used a combination of CFRP and aluminum materials in a hybrid double arrow-head auxetic (DAHA) structure. This paper conducted experiments and simulations to analyze the mechanical properties of the hybrid DAHA structures and found that the mixture ratio of hybrid material, ply angles of CFRP laminates, and inclination angles of beams significantly influenced the energy absorption performance of the structures. Additionally, the proposed hybrid DAHA structures outperformed other control groups when the thickness ratio of CFRP was 75%. The inclination angles of beams also affected the energy absorption capacity by changing the structural void of DAHA structures. These findings offer new insights into enhancing the energy absorption capacity of hybrid auxetic structures. The proposed hybrid structures can absorb more energy by 56.4% with a lower peak crushing force comparing with the traditional metal auxetic structure. Therefore, the CFRP/Aluminum hybrid auxetic structure has a bright future in the energy absorption fields. [ABSTRACT FROM AUTHOR]

This paper describes the nonlinear vibration of a novel smart plate with an auxetic metamaterial core and piezoelectrically actuated multiscale hybrid nanocomposite porous layers (GPL/CF/PVDF) using Reddy's higher-order shear deformation theory (HSDT). Using the rule of mixture (ROM) and Halpin–Tsai model (HT), the current properties of electroelastic layers were determined. Smart plate equations are detected using Hamilton's principle, Maxwell's law, and von Karman nonlinearity terms. The generalized differential quadrature method (GDQM) is then employed to discretize the governing equations of a sandwich plate for various boundary conditions. In addition, the smart plate's nonlinear frequency ratio and nonlinear natural frequency are detected using the angle of auxetic cell, the electric voltage, and the graphene nanoplatelet weight fraction. In addition, the influence of the porosity constant, the thickness of the auxetic core, and the thickness of the smart graphene-reinforced hybrid nanocomposite layer on (ω nl / ω l) and ω nl were computed and presented in each figure. [ABSTRACT FROM AUTHOR]

Helical Auxetic Yarn (HAY) possesses negative Poisson's ratio. These yarns can be successfully used as reinforcing elements in composite materials. This paper proposes a novel tri-component helical auxetic structure capable of enhancing the initial modulus of HAY. The novel structure comprises a high modulus thin secondary core II positioned along the first low modulus thick primary core I. An additional stiff strand wraps the cores I and II. Using statistical techniques, evaluation of the secant modulus and not Young's modulus was carried out and validated. The resultant structure was found to possess a relatively high elastic modulus in the range of 11–15 GPa. The structure exhibited an auxetic phenomenon of −62. It was revealed that such high stiffness auxetic yarn could successfully be utilized as reinforcing elements in high stiffness cost-effective auxetic composite structures. [ABSTRACT FROM AUTHOR]

This paper presents a novel rhombic star-shaped honeycomb (RSSH) configuration with tunable Poisson's ratio and elastic modulus, and an RSSH with the U-shaped ligaments (URSSH) configuration is generated to analyze the effect of the curved ligaments on the elastic properties of RSSH. Cellular structure of RSSH combines with a rhombic and an improved star-shaped geometry, which can also be subdivided into flexible segments and rigid segments. In-plane elastic properties of the RSSH cell, including Poisson's ratio and elastic modulus, are both derived by analytical model based on flexibility modeling method and Castigliano's second theorem. Based on the guidance of theoretical model, quasi-static tensile experiments and finite element simulation of RSSH and URSSH specimens are carried out with different Poisson's ratio. The experimental results are in good agreement with the theoretical predictions and finite element analysis results. Length of cellular ligaments Lt1, dt have linear relationship with mechanical properties of the RSSH cell under constant angle θ1 and make the cellular configurations with different Poisson's ratio more obtainable. The effect of the symmetry of the flexible segments on the cellular mechanical properties is also analyzed through theoretical prediction. The introduction of asymmetry can bring wider design space of RSSH mechanical properties, especially Poisson's ratio. This work demonstrates the promising potential for applying RSSH configurations in 2D morphing structures with tunable auxetic behaviors. [ABSTRACT FROM AUTHOR]

Compared with dense thermoelectric materials (TEMs), the auxetic honeycomb porous TEMs can enhance mechanical performance and energy conversion efficiency. The Fourier heat conduction law becomes unavailable due to the thermal wave speed in the porous TEMs is limited. Considering the non-Fourier law, this paper analyzes the thermal shock fracture characteristics of the porous TEMs bonded to an elastic substrate. A theoretical model for the effects of non-Fourier heat conduction, thickness of TME, shape and dimensions of honeycomb cell on strength failure and fracture failure is proposed. The analysis demonstrates that the maximum thermal stresses in the porous TEMs are underestimated without considering the non-Fourier heat conduction. The stress level in the auxetic honeycomb-based TEMs is much smaller than that in the traditional honeycomb TEMs with the positive Poisson's ratio architecture. It is found that a thicker TEM, a bigger relative density and a larger length ratio of the horizontal to the inclined strut can result in a smaller thermal stress. Based on the strength and fracture criterions, the envelope of safety angle between the horizontal and the inclined strut is identified. The simplified expressions of the safety angle envelope versus the applied thermal load are given. [ABSTRACT FROM AUTHOR]

Compared with dense thermoelectric materials (TEMs), the auxetic honeycomb porous TEMs can enhance mechanical performance and energy conversion efficiency. The Fourier heat conduction law becomes unavailable due to the thermal wave speed in the porous TEMs is limited. Considering the non-Fourier law, this paper analyzes the thermal shock fracture characteristics of the porous TEMs bonded to an elastic substrate. A theoretical model for the effects of non-Fourier heat conduction, thickness of TME, shape and dimensions of honeycomb cell on strength failure and fracture failure is proposed. The analysis demonstrates that the maximum thermal stresses in the porous TEMs are underestimated without considering the non-Fourier heat conduction. The stress level in the auxetic honeycomb-based TEMs is much smaller than that in the traditional honeycomb TEMs with the positive Poisson's ratio architecture. It is found that a thicker TEM, a bigger relative density and a larger length ratio of the horizontal to the inclined strut can result in a smaller thermal stress. Based on the strength and fracture criterions, the envelope of safety angle between the horizontal and the inclined strut is identified. The simplified expressions of the safety angle envelope versus the applied thermal load are given. [ABSTRACT FROM AUTHOR]

This research paper introduces an innovative technique for measuring displacement using auxetic tubular structure (ATS). The proposed displacement measurement method is based on tubular structures with a negative Poisson's ratio. It capitalizes on the underlying principle that the elastic deformation-induced change in transmittance of the ATS can be translated into a corresponding modification in the output current of the solar cell. This method allows for the conversion of the variation in light transmission into a corresponding variation in output voltage. The construction of the ATS can be achieved through 3D-printing technology, enhancing the accessibility of displacement measurement and design flexibility. The experimental results demonstrate that the proposed measurement method exhibits a linear error of less than 8% without any subsequent signal processing and achieves a sensitivity of 0.011 V/mm without signal amplification. Furthermore, experimental results also show that the proposed method has good repeatability and can maintain a high level of reliability and sensitivity when using different measurement devices. This confirms the effectiveness and feasibility of the proposed method, showing a favorable linear relationship between the input and output of the measurement system with an acceptable sensitivity, repeatability, and reliability. [ABSTRACT FROM AUTHOR]

Auxetic honeycomb has unique mechanical properties such as good energy absorption capacity, tensile strength and fracture toughness, etc. Therefore, honeycomb with a negative Poisson's ratio is used widely in medical, biological, aerospace and other fields. This honeycomb has large deformations in energy absorption and vibration reduction. It is very important to study the nonlinear constitutive of the honeycomb structure. Therefore, this paper establishes the nonlinear constitutive relationship of the auxetic honeycomb structure under large deformations. This constitutive relation includes the in-plane stress, in-plane strain, Young's modulus and Poisson's ratio of the negative Poisson's ratio honeycomb. The finite element model of the negative Poisson's ratio honeycomb cells is established, and the calculated results of finite element model are compared with that of the theoretical calculation results. On this basis, the influence of the geometric parameters on the mechanical properties of the structure is studied. The results of this paper will provide a theoretical basis for the further study of the auxetic honeycomb sandwich structure and provide a basis for the engineering application of honeycomb structures. [ABSTRACT FROM AUTHOR]

Auxetic structures (AXSs) are a novel class of materials with unique mechanical deformation behavior associated with negative Poisson ratio. The combination of AXS configurations with various types of materials has unveiled a wide field of applications, including military high-velocity protection against explosions and ballistic projectiles. However, the characteristic geometric re-entrant model of AXSs imposes limitations and difficulties when using conventional manufacturing methods to assemble the structure lattice. Additive manufacturing (AM) has recently been explored as a more efficient and cost-effective method to fabricate AXSs, regardless of the type of material. This review paper focuses on the development and applications of AM processed AXSs. The review highlights the significance and great potential for this class of materials that can be produced relatively fast and at a low cost. The advantages of AXS/AM are expected to extend to important industrial sectors, particularly for military ballistic armor, where the feasibility for products with improved properties is critical. The use of AM offers a viable solution to overcome the difficulties associated with the conventional manufacturing methods, and thus offers greater design flexibility, cost efficiency, and reduced material waste. This review paper aims to contribute to the understanding of the current state-of-the-art and future research prospects for the production and applications of AXS/AM. [ABSTRACT FROM AUTHOR]

Purpose: This paper aims to evaluate the resin infiltration influence on the mechanical properties of components 3D printed by the material extrusion-based additive manufacturing (AM), also known as fused deposition modeling and commonly uses the acrylonitrile butadiene styrene (ABS) as depositing material. Improvements in their mechanical properties are desirable due failure resulting from the extrusion process. In this way, resin infiltration is considered a candidate solution to enhance 3D printed components' strength. Design/methodology/approach: The mechanical properties of AM samples produced with and without the resin infiltration were assessed under torsion, tensile and flexural stresses. Torsional tests are rarely found applied for this case, an alternative torsion test developed by one of the authors was used. The torsion modulus (G) is obtained without the Poisson's ratio, which is usually unknown for recently made composites. Scanning electron microscopy was also done to verify the resin infiltration on the samples. Findings: Results demonstrated that the resin infiltration on ABS can improve the mechanical properties of samples compared to non-infiltrated. The tensile and bending strength increased more than 6%. Both Young's and torsion modulus also presented a significant increase. The samples did not present any considerable change in their weight property. Originality/value: This paper discusses on resin infiltration on print ABS, as to produce a composite material, enhancing ABS properties without gaining weight. This paper also used the torsion modulus instead of the common approach of bringing only tensile and flexure strength. [ABSTRACT FROM AUTHOR]

This paper presents for the first time two types of metamaterials based on the fragmentation–reconstitution of rotating units in order to produce Poisson's ratio discontinuity at the original state. For both metamaterials, each rotating unit takes the form of a rhombus that comprises eight sub-units. During on-axis stretching, each rhombus fragments into eight rotating sub-units. When the prescribed strain is reversed, these eight sub-units reconstitute back into a single rotating rhombus such that they rotate as a rigid body. Using geometrical construction, the incremental Poisson's ratio was established at the original state. In the case of large deformation, the finite Poisson's ratio was formulated in conjunction with the maximum allowable rotations for full stretching along both axes and for full compression. The family of on-axes Poisson's ratio versus rotational angles for various shape descriptors displays a fork-shaped distribution with discontinuity at the original state. Two major distinguishing factors of these metamaterials—property discontinuity at the original state with constant and variable Poisson's ratio under compression and tension, respectively—allow them to function in ways that cannot be fully performed by conventional materials or even by auxetic materials and metamaterials. [ABSTRACT FROM AUTHOR]

• Effect of negative poisson's ratio on nonlinear free vibration and postbuckling behaviour of FG-GOEAM beam is investigated in detail. • Metamaterial properties of GOEAM are determined by physics-embedded machine learning assisted micromechanics model. • FG-GOEAM beam greatly outperforms its pristine metallic counterpart due to GOri reinforcement. • Added mass model is used to include the effect of fluid surrounding the FG-GOEAM beam. This paper presents a numerical study on nonlinear free vibration and postbuckling behaviours of functionally graded (FG) graphene origami (GOri)-enabled auxetic metamaterial (GOEAM) tapered beams immersed in fluid, with a particular focus on the effect of negative Poisson's ratio (NPR) on the nonlinear frequencies and postbuckling equilibrium paths. The metamaterial properties of the novel GOEAM are determined by a physics-embedded machine learning based micromechanics model. The beam deformation is governed by Timoshenko beam theory and von Kármán nonlinearity, and the governing equations are solved using the differential quadrature method (DQM). The fluid pressure exerted on the surface of the beam is calculated using the velocity potential function and Bernoulli's equation. Comprehensive parametric studies demonstrate that due to the use of GOri reinforcement, an FG-GOEAM beam with NPR outperforms its metallic counterpart with considerably increased nonlinear fundamental frequency and enhanced postbuckling resistance. The nonlinear free vibration and postbuckling behaviours of the beams can be effectively tuned through GOri parameters. [ABSTRACT FROM AUTHOR]

This study aims to improve the impact protection performance of composite structures by combining a honeycomb core with negative Poisson's ratio and graphene platelets reinforced (GPR) face sheets. The paper investigates the nonlinear repeated low-velocity impact responses of auxetic honeycomb composite plates, taking into account loading-unloading-reloading processes. Effective material properties of the auxetic honeycomb core and GPR face sheets are obtained by using the proposed modified Gibson function and Halpin-Tsai model. Then, taking into account geometric nonlinearity, the nonlinear equations of motion for the system were derived by Hamilton's principle. Afterward, the time-varying contact force between the composite plate and a spherical impactor is defined by the modified nonlinear Hertz contact theory. The Galerkin method and variable-step Runge-Kutta algorithm are selected to obtain nonlinear impact responses. The proposed methods are verified by finite element simulation and experiment. Finally, the study evaluates the effects of key parameters on the nonlinear repeated low-velocity impact responses. [ABSTRACT FROM AUTHOR]

Purpose: Auxetics are the class of cellular materials with a negative Poisson's ratio. This paper aims to study the low-cost 3D printing capabilities and printing variations and improve the indentation performance of the re-entrant diamond auxetic metamaterial by tuning the structural parameters that have not been reported. Design/methodology/approach: The design of experiment strategy was adopted to study the influence of re-entrant angle, diamond angle and thickness-to-length ratio on relative density, load, stiffness and specific energy absorption (SEA) during indentation experimentally. Grey relational analysis was chosen as a multi-objective optimisation technique to optimise structural performance. Surrogate models were proposed to uphold the metamaterial's tailorability for desired application needs. The fit and efficacy of the proposed models were tested using specific statistical techniques. The predominant deformation mechanisms observed with the alteration in structural parameters were discussed. Findings: The improvements noticed are 48 times hike in load, 112 times improvement in stiffness and 10 times increase in SEA for optimised structures. The surrogate models are proven to predict the outputs accurately for new input parameters. In-situ displacement fields are visualised with an image processing technique. Originality/value: To the best of the authors' knowledge, the indentation performance of the re-entrant diamond auxetic metamaterials has not been reported and reported for the first time. The influence of geometrical parameters on the newly developed structure under concentrated loading was evaluated. The geometry-dependent printing variations associated with 3D printing have been discussed to help the user to fabricate re-entrant diamond auxetic metamaterial. [ABSTRACT FROM AUTHOR]

Re-entrant auxetics offer the potential to address lightweight challenges while exhibiting superior impact resistance, energy absorption capacity, and a synclastic curvature deformation mechanism for a wide range of engineering applications. This paper presents a systematic numerical study on the compressive and flexural behaviour of re-entrant honeycomb and 3D re-entrant lattice using the finite element method implemented with ABAQUS/Explicit, in comparison with that of regular hexagonal honeycomb. The finite element model was validated with experimental data obtained from the literature, followed by a mesh size sensitivity analysis performed to determine the optimal element size. A series of simulations was then conducted to investigate the failure mechanisms and effects of different factors including strain rate, relative density, unit cell number, and material property on the dynamic response of re-entrant auxetics subjected to axial and flexural loading. The simulation results indicate that 3D re-entrant lattice is superior to hexagonal honeycomb and re-entrant honeycomb in energy dissipation, which is insensitive to unit cell number. Replacing re-entrant honeycomb with 3D re-entrant lattice leads to an 884% increase in plastic energy dissipation and a 694% rise in initial peak stress. Under flexural loading, the re-entrant honeycomb shows a small flexural modulus, but maintains the elastic deformation regime over a large range of strain. In all cases, the compressive and flexural dynamic response of re-entrant auxetics exhibits a strong dependence on strain rate, relative density, and material property. This study provides intuitive insight into the compressive and flexural performance of re-entrant auxetics, which can facilitate the optimal design of auxetic composites. [ABSTRACT FROM AUTHOR]

• This paper firstly reviews recent development of auxetic composite with fillings. • A key focus lies in the enhancement mechanism and parametric design of auxetic composite with fillings. • Auxetic composites and traditional composites were compared in terms of mechanisms and energy absorption. • It summarises current limitations and future research regarding the design, fabrication, and applications of auxetic composites. Auxetic materials have garnered significant attention due to their lightweight and excellent energy absorption capabilities. Nonetheless, they often display relatively lower stiffness when compared with conventional materials. To address this limitation and enhance their mechanical properties, researchers have explored various avenues, including designing hybrid auxetic structures by combining two or more auxetic unit cells and developing auxetic composites using multiple materials. While previous reviews extensively covered hybrid auxetic structures, discussing their classification, design methodologies, fabrication techniques, applications and mechanical behaviours, there has been a noticeable gap in the literature concerning auxetic composites with fillings. Therefore, this paper concentrates on auxetic composites with fillings, delving into their classifications, mechanical responses, and underlying mechanisms. This review article also critically examines different design factors that influence the performance of auxetic composites and compares them with conventional counterparts in terms of mechanisms and mechanical properties. Overall, auxetic composites exhibit superior mechanical characteristics compared to equivalent conventional materials. However, several challenges and limitations persist regarding the design, fabrication, and applications of auxetic composites. [ABSTRACT FROM AUTHOR]

Auxetic materials are attracting increasing interest due to their extraordinary or even abnormal mechanical properties. Different from the traditional truss-lattice structures, this paper proposes a series of novel auxetic plate-lattice structures with an excellent auxetic effect. Utilizing a dual-materials glue-free assembly design involving hyperelastic and plastic materials, these plate-lattices exhibit numerous advantages such as disassembly, assembly, replacement, recyclability, reusability, excellent energy absorption, high specific stiffness, and impact resistance performances. Quasi-static/dynamic tests and simulations are conducted to assess mechanical properties, including elastic constants, deformation modes, and mechanical responses. Simultaneously, the effect of the principal geometric parameter, concave angle, on the structural response was analyzed with various impact velocities. Findings suggest that the concave angle influences the structural elastic constants under compressive loading. The stress–strain response exhibits a distinct dual-plateau phenomenon. Taking the A65 (concave angle is 65°) configuration as an example, the stress on the second plateau is approximately 2.7 times that of the first plateau, significantly enhancing the structure's energy absorption capability. Under dynamic impact loadings, different configurations and varying impact velocities affect the structure's energy absorption performance. This paper introduces a series of assemblable plate-lattice structures with an auxetic effect, offering design insights and guidance tailored to the convenient transportation, unconventional structures, and rapid assembly needs of lightweight, impact-resistant mechanical metamaterials. [ABSTRACT FROM AUTHOR]

Auxetic structures exhibit unusual changes in size, expanding laterally upon stretching instead of contracting. This paper presents this effect in a failsafe mode in structures made of rigid squares. We applied the concept of auxetic structures made of rigid rotating squares (from Grima and Evans) and offer a novel solution for connecting them. By introducing axes of rotation on the surface of the squares, a reliable working system is obtained, free from stress, in which the squares can come into contact with each other and completely cover the surface of the structure, or, in the open position, form regularly arranged pores. Herein, we present a new 2D auxetic metamaterial that is mathematically generated based on a theoretical relationship of the angle between the edges of a square and the position of the axis of rotation. Physical models were generated in the form of a planar structure and in the form of a circular closed structure. Such physical models confirmed our initial considerations and the geometrical relationships, offering new application possibilities. The novel structure that was designed and manufactured for the purpose of the paper can be considered as a new proposal in the market of auxetic materials. [ABSTRACT FROM AUTHOR]

Research on auxetic metamaterials is important due to their high performance against impact loadings and their usefulness in actuators, among other applications. These metamaterials offer a negative Poisson's ratio at the macro level. However, usual auxetic metamaterials face challenges in (1) grading the effect, (2) coupling and combining auxetic metamaterials with non-auxetic materials due to boundary compatibility, (3) obtaining the same auxetic behavior in all directions in the transverse plane, and (4) adapting the regular geometry to the component design boundary and shape. The goal of this paper is to present a novel, recently patented tunable 3D metamaterial created to reproduce a wide spectrum of 3D auxetic and non-auxetic Poisson's ratios and Young's moduli. This wide range is obtained using the same basic unit cell geometry and boundary connections with neighboring cells, facilitating designs using functionally graded metamaterials as only the connectivity and position of the cell's internal nodes are modified. Based on simple spatial triangularization, the metamaterial is easily scalable and better accommodates spatial curvatures or boundaries by changing the locations of nodes and lengths of bars. [ABSTRACT FROM AUTHOR]

As a measuring device, an auxetic piezo-resistant sensor must have the characteristics of the measuring equipment. One of these features is to have the correct and same answer in different strains and directions (same answer in different conditions). This issue should be considered in the case of auxetic piezo-resistant sensors because of having different Poisson's ratios at different strains and also being generally anisotropic. In this paper, the problem of instability of Poisson's ratio during the strain for auxetic structures was investigated and showed that the sensing performance of re-entrant auxetic piezo-resistant sensors is nonlinear due to the nonlinearity of Poisson's ratio over a broad range of stain values. To investigate this issue, highly sensitive, stretchable, piezoresistive sensors based on graphite powders sandwiched between silicon rubber layers were prepared using a re-entrant auxetic structure. The sensing performance of the auxetic sensors was identified and compared with previous sensors. The presented sensor showed piezoresistive enhancement under tension loading, with improvement up to 176 and 380% (in terms of gauge factor) in comparison with previous mix auxetic and conventional composite sandwich sensors, respectively. Furthermore, we proposed an auxetic structure that has a constant Poisson's ratio over a broad range of strains. Using this auxetic structure made it possible to design an auxetic sensor with linear sensitivity over a broad spectrum of strain values. The auxetic sensor with a constant value of Poisson's ratio has stable sensitivity changes at different strains. It means that the sensor sensitivity has a fixed slope. In the next works, we intend to use isotropic auxetic structures to make piezo-resistant auxetic sensors. [ABSTRACT FROM AUTHOR]

As interesting analog of black phosphorus, group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se) are greatly expected to have potential applications in photodetectors, photovoltaics, thermoelectric, and piezoelectric devices. In this paper, we conduct a thorough investigation on the mechanical properties of four MX monochalcogenides, i.e., single- and multi-layered GeS, GeSe, SnS, and SnSe, including their ideal tensile strength, evolution of monolayer bulking height, and phase transition under uniaxial strain within the framework of density functional theory. The ultimate tensile strengths of the four monochalcogenides are found rather low, while the failure strains are remarkably high, showing ultrahigh mechanical flexibility. Meanwhile, the ideal strength demonstrates strong anisotropy, which may induce the exfoliated MX thin flakes showing an obvious in-plane cleavage tendency of much easier to crack along the zigzag orientation. In addition, it is found that all of them are potential auxetic materials with large negative Poisson's ratios, just like single-layer black phosphorus. Furthermore, we find all the monolayer monochalcogenides will demonstrate geometrical phase transition when uniaxial compressive strains are applied. It is expected that these newly found unique features of monolayer group-IV monochalcogenides and their derivatives make them attractive candidates for their incorporation in future mechanical, nanoelectronics, and thermal applications. [ABSTRACT FROM AUTHOR]

For the practical use of innovative, high-energy-density batteries, loading of high-capacity active materials at high gravimetric and volumetric fractions is required. In this work, a flexible self-supporting paper of few-wall carbon nanotubes (FWCNTs) was used as three-dimensional conductive matrices to capture Si and S at ∼70 mass% without metal foils nor polymeric binders. The full cell comprised of the Li x Si-CNT and S-CNT electrodes with a negative to positive (N/P) capacity ratio of ∼1 realized high initial energy densities of 810 W h kg−1 and 680 W h L−1 based on the total mass and volume of the electrodes, respectively. However, the cell showed a poor cycle performance due to the depletion of active Li. The increased N/P capacity ratio of 2.7 drastically improved the cycle performance, realizing an energy density of 610 W h kg−1 at first discharge and 360 W h kg−1 after 100 cycles based on the total mass of the electrodes. The full cell architecture based on 3D current collector of FWCNTs is a promising solution for loading active materials at high fractions and achieving high energy densities at the total electrode level. Image 1 [ABSTRACT FROM AUTHOR]

In this paper, the energy absorption capacity of continuous fiber-reinforced thermoplastic auxetic structures is examined experimentally and the results compared with both numerical and analytical methods. To this extent, a 3 D printing technology of Fused Deposition Modeling (FDM) is implemented for fabricating the auxetic honeycombs and quasi-static compression test is conducted to extract the load-displacement behavior of the structure. Both hollow and foam filled lattice structures are tested, and the results revealed that the absorbed energy increased by 20% for PLA and 70% for PA specimens, when using foam. Finite element analysis is also performed using the explicit solver. The onset of failure is determined using the maximum stress criterion and VUSDFLD subroutine and the damage growth is modeled by decreasing the mechanical properties of the elements. The obtained results are in relatively good agreement with the experimental analysis. In addition, a theoretical formulation is developed and the probabilistic analysis is performed to draw the honeycomb failure design envelope, which is a practical tool for designing of various honeycomb configurations. The failure envelope shows that decreasing the strut angle increases the honeycomb failure load, however, the auxetic property of the structure decreases. [ABSTRACT FROM AUTHOR]

In order to improve the specific stiffness of negative Poisson's ratio metamaterial structures. In this paper, a novel circular NPR (negative Poisson's ratio) metamaterial structure was designed. The equivalent Poisson's ratio and the equivalent elastic modulus of the circular NPR were deduced, and the mechanical properties of the circular NPR structure and the cube NPR structure were compared. Meanwhile, the influence of geometrical size on the Poisson's ratio of the whole structure was studied. The results show that the circular structure has the properties of negative Poisson's ratio. Compared with the cube NPR structure, under the same external dimensions, the weight of the circular NPR structure was reduced by 16.7%. When the two structures were applied the same load, the Von Mises stress of the circular NPR structure decreased by 71.6% compared with that of the cube NPR structure in the elastic range. Due to its light weight and greater resistance to deformation, the specific stiffness of the circular NPR structure was larger. Meanwhile, by changing the geometric size, the negative Poisson's ratio of the circular NPR structure can be regulated. [ABSTRACT FROM AUTHOR]

Auxetic materials exhibit a negative Poisson's ratio, i.e., they become thicker rather than thinner in at least one dimension when strained. Recently, a nematic liquid crystal elastomer (LCE) was shown to be the first synthetic auxetic material at a molecular level. Understanding the mechanism of the auxetic response in LCEs is clearly important, and it has been suggested through detailed Raman scattering studies that it is related to the reduction of uniaxial order and emergence of biaxial order on strain. In this paper, we demonstrate direct observation of the biaxial order in an auxetic LCE under strain. We fabricated ~100 μm thick LCE strips with complementary geometries, exhibiting either planar or homeotropic alignment, in which the auxetic response is seen in the thickness or width of the sample, respectively. Polarized Raman scattering measurements on the planar sample show directly the reduction in the uniaxial order parameters on strain and suggest the emergence of biaxial order to mediate the auxetic response in the sample thickness. The homeotropic sample is studied via conoscopy, allowing direct observation of both the auxetic response in the width of the sample and increasing biaxiality in the LCE as it is strained. We verified that the mechanism of the auxetic response in auxetic LCEs is due to the emergence of the biaxial order and conclude such materials can be added to the small number of biaxial nematic systems that have been observed. Importantly, we also show that the mechanical Frèedericksz transition seen in some LCEs is consistent with a strain-induced transition from an optically positive to an optically negative biaxial system under strain, rather than a director rotation in a uniaxial system. [ABSTRACT FROM AUTHOR]

In this paper, the stiffness of carbon nanotube bundle (CNTB) reinforced composite is predicted by means of the Mori-Tanaka method and Eshelby method. The curvature of one CNTB on uniaxial aligned distributed condition is investigated, and the influence of porosity and agglomeration on the CNTB/polymer composite is discussed in the case of randomly distribution of the CNTB in matrix. Numerical examples and analysis show influence of the contents of carbon nanotubes (CNTs) in the CNTB, the content of CNTB, agglomeration, waviness, porosity and curvature have great effect on the elastic modulus and Poisson's ratio of the whole composite. [ABSTRACT FROM AUTHOR]

Single-walled carbon nanotubes (SWCNTs) have superior mechanical properties which originate from a strong C-C covalent bond and unique nanostructure. Chirality, one of the helical structural parameters of SWCNTs, leads to differences in mechanical performance. In this work, molecular dynamics (MD) simulation was performed to analyze engineering Poisson's ratio (EPR) and incremental Poisson's ratio (IPR) of SWCNTs with different chiral angles, respectively, under tensile and compressive load, as well as the chiral effect on rigidity. We reported the minimum EPR for (4, 1) SWCNT and obtained the distribution and trend of EPR which is dependent on chiral index m. In addition, a new observation showed two exactly opposite trends of EPR existing not only in tension and compression but also in the longitudinal and radial directions. Furthermore, we found that the critical strain, over which SWCNT would be auxetic, ranged from 6% to 18% and was also chirality-dependent. Three representative SWCNTs with chiral angle of 0° (zigzag), 10.89° (chiral), and 30° (armchair) were selected for the mechanism study of auxeticity. Finally, a method of the contribution to radial strain for two main deformation modes proposed in this paper could well explain the negative IPR phenomenon. [ABSTRACT FROM AUTHOR]

Material design plays a pivotal role in industries dedicated to lightweight manufacturing. In response to the needs of such industries, topology optimization (TO) has evolved beyond structural optimization to include designing material microstructures for improved structural performance. While conventional approaches often prioritize stiffness-oriented designs as a foundational aspect of structural integrity, it is equally imperative to emphasize strength-based design principles to ensure robustness against localized stress concentrations and potential failure modes. This paper explores the application of stress-constrained topology optimization (SCTO) for designing auxetic microstructures. The study commences with a comprehensive parametric analysis to examine the influence of the P-norm parameter, variations in volume fraction, and initial design adjustments on the resulting microstructures. Several optimized structures, accompanied by stress contours, are generated to showcase how the optimization process responds to these parameters. The approach employs the density-based solid isotropic material with penalization material interpolation scheme, and it utilizes the stress penalization technique to mitigate the singularity phenomenon. The multi-constraint optimization employs the method of moving asymptotes algorithm to update design variables. The effectiveness of the proposed approach to utilize actual strain fields, derived from macroscopic finite element analysis under service loads, for TO of an auxetic microstructure according to strength requirements is demonstrated. The application of the designed microstructure is explored through its integration into an auxetic cushion pad. This analysis confirms the microstructure's effectiveness in practical applications, demonstrating its potential for broader usage in similar applications. • Modified SIMP TO with MMA designs auxetic microstructures under stress constraints. • Strain–energy homogenization method evaluates microstructure's effective properties. • Optimization parameters impact homogenized properties and auxetic design. • Adjoint method computes stress constraint derivative. • Case study designs auxetic microstructure using macroscopic strain field. [ABSTRACT FROM AUTHOR]

• An atomic lattice-mimicry method is proposed to design macroscopic metamaterials with scaled-up NPR mechanism. • The effective Poisson's ratio and normalized Young's modulus of the metamaterial are strain-independent. • An optimized auxetic metamaterials is achieved, with 3 times enhancement of NPR and 2 orders of magnitude enhancement of normalized Young's modulus. Metamaterials with strain-independent negative Poisson's ratio (NPR) and Young's modulus are essential for the applications with extreme loading conditions. However, strain-independent auxetic metamaterials are rare, and there is a lack of design inspiration for them. In this paper, we present a novel auxetic metamaterial inspired from the atomic lattice with strain-independent NPR. The novel NPR properties due to the unique atomic lattice keep valid when scaling up by 6 orders of magnitude to macroscopic metamaterials, with the similar deformation mechanism. The NPR and Young's modulus keep constant with increasing loading strain. These strain-independent auxetic effects are robust with various designable geometric parameters in wide ranges. Furthermore, the geometric parameters optimization for the auxetic metamaterial is conducted through developing the matrix displacement method. An optimized metamaterial is achieved with 3 times enhancement of auxeticity and 2 orders of magnitude enhancement of Young's modulus, compared to the initio architectures. This work provides a wealth of inspirations from atomic lattice in nature to design strain-independent macroscale auxetic metamaterials. [Display omitted] [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]

[Display omitted] • A novel multiscale modeling method was developed for the auxetic behavior of negative Poisson's ratio woven fabric (NPRWF). • The finite element model of NPRWF was innovatively constructed based on the simulated constitutive curve of negative Poisson's ratio yarn (NPRY). • The effectiveness of the theoretical model and the finite element model of NPRWF were verified with good agreement. Auxetic behavior is critical to determining the deformation and mechanical performance of materials. Auxetic mechanisms of the woven fabric have previously not been well understood, especially for those containing negative Poisson's ratio yarn (NPRY). Most theoretical models were developed from the macro geometric structure which lacks analysis of yarn levels. This paper aims to use numeral analysis combined with finite element modeling to investigate the auxetic behavior of the NPRY-based fabric. Firstly, the theoretical model that reveals the relationship between the auxetic behaviors of yarn and NPRY-based fabric was established based on component yarn interweaving rules, deformation, and stress. Following, a strain-dominated deformation simulation was further performed to clarify the micro stress and auxetic mechanism of fabric by calculating the stress–strain response and NPR value. Moreover, the experiments of NPRY-based fabric were also developed to validate the theoretical and simulation results, Results indicate that the model can well predict the auxetic behavior of fabric with an error under 7%. The micro–macro analysis of the auxetic behavior from yarn to fabric provides a fundamental understanding of the auxetic mechanism of NPRY-based fabric. Additionally, the yarn-to-fabric model can used to study the auxetic behavior of NPR fabric with variable parameters thus supplying theoretical guidance for engineering. [ABSTRACT FROM AUTHOR]

• The proposed design exhibits better energy absorption characteristics. • Reinforcing ligaments ensure uniform deformation and improved stability. • The proposed design maintains a constant Poisson's ratio under relatively large deformation. • The theoretical derivation is simple and applicable to other auxetic structures. The traditional anti-tetra-missing rib structure often suffers from uneven stress deformation and poor energy absorption, which limits its engineering applications. To address these limitations, this paper proposed a novel windmill-shaped auxetic structure by incorporating reinforcing ligaments into the traditional anti-tetra-missing rib structure. The incorporation of these reinforcing ligaments ensures that both the outer and inner ligaments of the windmill-shaped auxetic structure are adequately supported, leading to more uniform structural deformation and enhanced energy absorption capabilities. Further modifications, such as reducing the length of the outer and inner ligaments and increasing their thickness within a reasonable range, significantly enhance the energy absorption capacity. These findings provide a theoretical basis for the design and practical application of auxetic metamaterials. [Display omitted] [ABSTRACT FROM AUTHOR]

Bi-level tailoring of cellular metamaterials involving a dual design space of unit cell and elementary beam level architectures has recently gained traction for the ability to achieve extreme elastic constitutive properties along with modulating multi-functional mechanical behavior in an unprecedented way. This article proposes an efficient analytical approach for the accurate evaluation of all constitutive elastic constants of asymmetric multi-material variably-thickened hexagonal lattices by considering the combined effect of bending, stretching, and shearing deformations of cell walls along with their rigid rotation. A tri-member unit cell is conceptualized, wherein all nine constitutive constants are obtained through the mechanics under one cell wall direction and subsequent repetitive coordinate transformations. We enhance the design space of lattice metamaterials substantially here by introducing multiple exploitable dimensions such as asymmetric geometry, multi-material unit cells and variably-thickened cell walls, wherein the conventional monomaterial auxetic and non-auxetic hexagonal configurations can be analyzed as special cases along with other symmetric and asymmetric lattices such as a range of rectangular and rhombic geometries. The generic analytical approach along with extensive numerical results presented in this paper opens up new avenues for efficient optimized design of the next-generation multi-functional lattices and cellular metamaterials with highly tailored effective elastic properties. • Bi-level tailoring of cellular metamaterials involving a dual design space of unit cell and elementary beam-level architectures. • Accurate evaluation of all constitutive elastic constants of asymmetric multi-material variably-thickened hexagonal honeycomb lattices. • Expanded design space by introducing multiple exploitable dimensions such as: asymmetric geometry, composite multi-material unit cells, variably-thickened cell walls (derived through physics-based rationale) and their coupled effect. • New avenues for efficient optimized design of the next-generation multi-functional lattices and cellular metamaterials with highly tailored effective elastic properties. [ABSTRACT FROM AUTHOR]

The reasonable design of man-made multifunctional metamaterials with advanced mechanical and physical properties that are not found in nature is both challenging and important. In this paper, novel chiral structures are designed by adding V- and Tri- chiral elements to the typical hexagonal honeycomb substrate, so as to realize the modulation of in plane mechanical properties. Finite element simulations show that the Poisson's ratio of the hexagonal V-chiral structure is negative when the strain is greater than 0.4, and that of the hexagonal Tri-chiral structure is always negative. Quasi-static compression tests show that the introduction of chiral properties results in a stronger load-bearing capacity, and the hexagonal Tri-chiral structure has a stronger energy absorption (EA) and specific energy absorption (SEA) capability. A parametric study is carried out by means of FEM, by adjusting the geometrical parameters of the proposed structures, the performance in terms of energy absorption and compressive stiffness can be improved. Subsequently, Bloch-Floquet theorem is employed for the dispersion relations of the chiral metamaterials and the results show that compared to conventional hexagonal honeycomb, chiral metamaterials are able to generate band gaps in a lower frequency range. Numerical simulation results show that the bandwidth of the proposed novel chiral metamaterials can be tuned through a reasonable selection of hexagonal edge length, wall thickness and radius. Experimental investigations are conducted to test the vibration transmission rate of Structure Ⅲ, and the results prove that the structure is capable of vibration suppression in the target frequency band range. Finally, by filling Structure Ⅲ with steel columns, the structure generates band gaps in the lower frequency range, hence the application of this chiral metamaterial is broadened. • A novel design method for multifunctional auxetic metamaterials based on chiral units is proposed. • The Poisson's ratio, energy absorption, load bearing capacity and dispersion relation of the structures are analyzed in detail. • The static and dynamic characteristics of the proposed structures are verified by experiments. [ABSTRACT FROM AUTHOR]

The intrinsic auxeticity of materials has long been attributed to their unique geometric configurations. Triggering and enhancing auxeticity presents significant challenges, as many nanomaterials display subtle auxetic effects with a negative Poisson's ratio (NPR) typically above -0.4. This paper introduces three materials with identical symmetry—two-dimensional pentagonal structures Penta-B 2 × 2 Y 2 (i.e., Penta-B 2 C 4 , Penta-B 2 N 4 , and JP-B 2 C 2 N 2)—to investigate a novel mechanism influencing material auxeticity: electron transfer. Specifically, variations in electron transfer cause Penta-B 2 N 4 and JP-B 2 C 2 N 2 to show axial auxetic effects, whereas Penta-B 2 C 4 demonstrates opposite behavior. The axial NPR of Penta-B 2 N 4 is ∼ -0.05, while for JP-B 2 C 2 N 2 , it reaches ∼-0.46, significantly surpassing that of typical intrinsic auxetic materials. This represents an increase of approximately 920 % compared to Penta-B 2 N 4. Additionally, JP-B 2 C 2 N 2 features semi-metallic properties, where the conduction band minimum (CBM) and the valence band maximum (VBM) are tangent to the Fermi level. Consequently, minor alterations in external conditions can induce a transition between semiconductor and metallic states in JP-B 2 C 2 N 2. Thus, the 2D material JP-B 2 C 2 N 2 emerges as a promising candidate for nanoelectronic and electromechanical applications. Furthermore, this study also enhances the academic understanding of auxetic properties in nanomaterials by linking their mechanical and electronic characteristics and laying a theoretical foundation for further experimental exploration of auxeticity. [Display omitted] [ABSTRACT FROM AUTHOR]

• In order to improve the bonding strength between the structures, a new fiber-reinforced CorTube structure is designed and fabricated. • The auxetic characteristic and the effective young's modulus of the structure are analyzed theoretically, and the relationship between the configuration parameters and the theoretical solutions is discussed. • The deformation characteristics and failure forms of the structure under low-speed impact load are studied. • The influence of impact position and impact energy on the mechanical properties of the structure is revealed. Auxetic structures are gaining great attention due to their unique contraction deformation characteristics under compression and impact. In this paper, high performance carbon fiber-reinforced composites are used to fabricate the auxetic structure consist of corrugated sheets and tubes (CorTube). The quasi-static uniaxial compression and low-velocity impact properties of composite CorTube structure are explored. The response of the composite CorTube structures under quasi-static compression loads are analyze through a combination of theoretical analysis, simulations, and experimental tests. Additionally, drop-weight impact tests are conducted using a rigid impactor with a hemispherical head to examine the effects of impact energy levels, impact locations, and corrugated sheet thickness on the impact response of CorTube structure. Enhancing corrugated sheet-tube bonding via modified cross members and reducing tube crushing during quasi-static compression are notable findings. The results also highlight the remarkable auxetic properties of the composite CorTube under low-speed impact, and the impact resistance could be enhanced by increasing the corrugated sheet thickness and stiffness. Various failure modes were observed, including cracks, pits, tube crushing, and delamination. Significantly, peak-impacted specimens exhibited greater maximum displacement with lower peak impact forces. This study offers insights into the deformation and failure modes of auxetic CorTube structures under low-velocity impact. [ABSTRACT FROM AUTHOR]

As destructive power of firearms raises over the years, ballistic armors are in continuous need of enhancement. For soft armors, this improvement is invariably related to the increase of stacked layers of high-strength fiber fabrics, which potentially restrains wearer mobility. A different solution was created in the early 2000s, when a research work proposed a new treatment of the ballistic panels with non-Newtonian colloidal shear thickening fluid (STF), in view of weight decreasing with strength reinforcement and cost-effective production. Since then, databases reveal a surge in publications generally pointing to acceptable features under ballistic impact by exploring different conditions of the materials adopted. As a result, several works have not been covered in recent reviews for a wider discussion of their methodologies and results, which could be a barrier to a deeper understanding of the behavior of STF-impregnated fabrics. Therefore, the present work aims to overview the unexplored state-of-art on the effectiveness of STF addition to high-strength fabrics for ballistic applications to compile achievements regarding the ballistic strength of this novel material through different parameters. From the screened papers, SiO 2 , Polyethylene glycol (PEG) 200 and 400, and Aramid are extensively being incorporated into the STF/Fabric composites. Besides, parameters such as initial and residual velocity, energy absorbed, ballistic limit, and back face signature are common metrics for a comprehensive analysis of the ballistic performance of the material. The overview also points to a promising application of natural fiber fabrics and auxetic fabrics with STF fluids, as well as the demand for the adoption of new materials and more homogeneous ballistic test parameters. Finally, the work emphasizes that the ballistic application for STF-impregnated fabric based on NIJ standards is feasible for several conditions. [ABSTRACT FROM AUTHOR]