Search

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]

[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]

This paper investigated the quasi-static and dynamic behavior of 3D auxetic metamaterial structures made from carbon fiber reinforced polymer (CFRP) laminated composite. The aim of the study was to enhance design methodologies for load-bearing and energy absorption applications of these 3D novel structures, filling the research gap in understanding their response to quasi-static and especially dynamic loadings. The two novel 3D structures were designed and fabricated by using an interlocking assembly method based on the 2D auxetic CFRP sheets, which were formed with hybrid double-arrow-head with re-entrant and star unit-cells and made with plain weave carbon epoxy prepregs. The finite element (FE) method was adopted to analyze the mechanical characteristics of the structures under the quasi-static and dynamic loading, and Hashin failure criteria were used to define damage in the structures. The study showed that the designed 3D auxetic CFRP structures simultaneously exhibit superior auxeticity, load-bearing, and energy absorption capacity. [ABSTRACT FROM AUTHOR]

Auxetic mechanical metamaterials contract or expand laterally when subjected to compressive or tensile load in the axial direction and offer potential benefits in areas such as flexible electronics, aerospace, and soft actuators. Nevertheless, when metamaterials are fabricated using conventional methods, their configurations and properties are fixed, which prohibits adaptation to the specific geometric requirements of their application environment. This limits their further development. This paper proposes an auxetic mechanical metamaterial with a negative Poisson's ratio and energy absorption capability. Compression specimens are fabricated with LCD photo-curable printing technology. The deformation mechanism and in-plane compression characteristics are examined through experimental and numerical simulation methods. The influence of unit cell geometrical control parameters on Poisson's ratio and energy absorption are also explored. The results indicate the dominant deformation mechanism of metamaterial during in-plane compression is bending deformation. The negative Poisson's ratio behavior is more pronounced when compressing along the X direction than along the Y direction, and the specimens also exhibit energy absorption capacity. In addition, the metamaterials prepared by 4D printing technology can be transformed from one configuration to another under external stimuli stimulation and force, and exhibit different mechanical properties, realizing programmable and reconfigurable, further expanding their application range. [ABSTRACT FROM AUTHOR]

The main objective of this paper is to evaluate the dynamic behaviour of sandwich toroidal shells composed of a core with negative Poisson ratio (auxetic honeycomb material) and nanocomposite enriched coating layers under impulsive/periodic pressures considering the geometrical nonlinearity. The set of nonlinear equilibrium equations based on the first-order shear deformation theory are and the resultant nonlinear differential motion equations are approximately solved by the Galerkin method and 4th-order Runge–Kutta solution, respectively. The achieved responses show that the amplitude of outward deformations is intensively increased when the proposed sandwich shells are subjected to rectangular periodic and impulsive loadings which shows the destructive effects of this type dynamic loading. The performed parametric studies shows that these destructive effects can be controlled by changing the geometrical properties of the honeycomb cells and the volume fractions of the nanocomposites in the coating layers. It is notable that in the previous studies the nonlinear dynamic response of toroidal shells subjected to various periodic and impulsive pressures was not investigated. However, in the present work, the mentioned dynamic responses were achieved and investigated without spending time-consuming computational. In fact, the novelty of this work is to develop such a robust and efficient tool. [ABSTRACT FROM AUTHOR]

• Two novel auxetic structures with different arrangements of a new enhanced trigram frame unit-cell were proposed. • The mechanical performance of the designed structures was evaluated through theoretical analysis, finite element simulation, and experimental tests. • Single and multi-objective optimization methods were implemented to optimize the geometry parameters of the selected auxetic structure. • The exceptional mechanical characteristics of the present proposed structures, surpassing previous designs in terms of stiffness and specific stiffness, highlight their tremendous potential for load-bearing applications. Superior stiffness is deemed essential for structures intended for several applications. The stiffness enhancement can be achieved by adopting structures that deform based on the stretching-dominated mechanism instead of the bending-dominated mechanism. This paper proposes a novel auxetic unit cell design and two distinct lattice structures dominated by stretching when subjected to quasi-static compressive and tensile loadings. The structures are manufactured via the fused deposition modeling (FDM) 3D printing method with the use of polylactic acid (PLA) filament as the patent material. The investigation also includes theoretical analyses and finite element (FE) simulations, all yielding consistent results. Response Surface Methodology (RSM) is also employed to understand the influence of three geometric parameters. Besides, single-objective optimization is conducted to maximize the stiffness through geometry design, and multi-objective approaches are implemented to improve the stiffness while simultaneously reducing relative density. Finally, different unit cells, which deform based on the stretching-dominated mechanism, are selected to represent their respective classifications, enabling a comparison of their potential stiffness capacity with the designed structures in this study. Remarkably, it is highlighted that the proposed unit cells and structures outperform others in terms of specific stiffness, making them a promising choice for use in structural engineering. [ABSTRACT FROM AUTHOR]

This paper examines the impact of polyurethane foam (PUF) filling on the dynamic response of honeycomb sandwich beams (HSBs) through the blast testing. The HSBs are composed of steel face sheets and an aluminium alloy honeycomb core filled with PUF. A comparative analysis of damage modes is conducted between conventional hexagonal and auxetic HSBs, considering both foam-filled and unfilled configurations. The results reveal that the HSBs with foam filling demonstrate significantly enhanced blast performance compared to their counterparts without foam filling. This superiority is evident in both global deformation and local damage characteristics. Furthermore, a well-validated numerical model is developed to examine the damage mechanisms of PUF-filled HSBs under blast loading. The investigation reveals that appropriate PUF filling methods can effectively enhance bending resistance while minimizing the decline in the energy absorption efficiency. Finally, a multiobjective optimization was conducted using the back-propagation (BP) neural network and the SPEA2 algorithm to identify the optimal position for PUF. By utilizing a reduced foam filling approach as opposed to completely filling the structure, the two optimized configurations show a decrease in deformation of 5.38 % with an equivalent specific energy absorption (SEA) or an increase in the SEA by 5.26 % with an equivalent deformation. [ABSTRACT FROM AUTHOR]

This paper presents the design, 3D printing, and testing of this novel type of auxetic structure formed with arc-shaped unit-cells (UCs). The mechanical behavior of two different re-entrant Dumbell (RED) and Multiple-Arc structures are evaluated. The metamaterials are fabricated using 3D printing via Fused Deposition Modeling (FDM) techniques. Quasi-static compressive loading tests are performed on these metamaterials, and the results are verified using Finite Element (FE) simulation and theoretical analysis. The results of the study reveal that the RED configuration, which includes Dumbell-shaped struts that exhibit negative Poisson's ratio (NPR) themselves, outperforms the Multiple-Arc, the traditional re-entrant circular (REC), and conventional re-entrant auxetic structures in terms of specific stiffness (E s) specific energy absorption (SEA) and NPR values. In addition, the effects of the geometry parameters, including strut thickness to height ratio, length to height ratio, and angle of Dumbell strut on the SEA and NPR values of the RED structure, are investigated. The findings indicated that the SEA values improved as the thickness-to-height ratio and Dumbell strut angle increased, and the length-to-height ratio decreased. Additionally, the NPR value showed enhancement with increased thickness-to-height ratio and Dumbell strut angle. However, the thickness-to-height ratio did not significantly influence the NPR value. [ABSTRACT FROM AUTHOR]

3D printable multi-material honeycombs have attracted increasing interest recently due to the improved elastic moduli, buckling and energy absorption properties. This paper proposes an analytical homogenization for the equivalent in-plane elastic moduli (EIEM) of multi-material honeycombs. First, the axial and bending stiffness of a cantilever beam consist of three sections made of different materials is formulated. Then based on unit cell method and cantilever beam model, the closed-form expressions of EIEM are proposed by fully considering the deformations of both joints and cell walls with arbitrary stiffness, which are sufficiently general to be applied to hexagonal, auxetic and rectangular multi-material honeycombs and validated very well by numerical simulations and experiments. Furthermore, the effects of geometric and material distribution ratios on EIEM are discussed. The results show that the geometric and material distribution ratios of inclined cell wall have a significant effect on all five EIEM while that of vertical cell wall shows a significant effect on shear modulus but only a slight effect on y- direction elastic moduli. Compared with single-material honeycombs, changing joint stiffness has a significant effect on the equivalent Young's moduli and shear modulus but a slight effect on Poisson's ratios. • Closed-form expressions of equivalent elastic moduli for multi-material honeycombs. • Axial and bending stiffness formulated for a dual-material cantilever beam. • Analytical expressions of moduli match well with numerical and experimental results. • Dependency of material and geometric distributions on moduli investigated. • Proposed method applied to multi-material hexagonal and auxetic honeycombs. [ABSTRACT FROM AUTHOR]

Auxetic materials exhibit many superior properties but drawback in low stiffness owe to the substantial porosity. Some narrow-ribs enhanced auxetic honeycombs have been designed for improving the in-plane stiffness. For these narrow-ribs enhanced auxetic structures, local buckling likely appears as the reinforcing walls are designed in low thickness for superior auxeticy. In this paper, a novel narrow-ribs enhanced auxetic honeycomb is designed by embedding rhombus into each cell of the normal re-entrant hexagonal honeycomb (NRHH). Analytical model is built to investigate the elastic behaviors of the new proposed honeycomb under uniaxial compression, in which the incremental method is firstly introduced for post-local-buckling analysis. Numerical simulations are carried out to verify the analytical results. It is interesting that bilinear elastic characteristic is found for the stress-strain and transverse strain-longitudinal strain curves of the new proposed honeycomb on account of the local buckling. Parameters studies are performed and the new proposed honeycomb is proved to exhibit superior mechanical properties compared to other narrow-ribs enhanced honeycombs. Significantly, the incremental method brought forward in the present paper for the local buckling analysis can be popularized to other honeycombs enhanced with narrow-ribs. Thus, the bilinear elastic characteristic is a general feature for these honeycombs. [ABSTRACT FROM AUTHOR]

Abstract Recently, auxetic honeycombs have attracted considerable attention for their excellent mechanical properties, especially the potential applications for energy absorption. In this paper, a novel re-entrant auxetic honeycomb is proposed and named as re-entrant star-shaped honeycomb (RSH), with the in-plane impact responses explored theoretically and numerically. Three types of microstructural deformation modes are observed under different impact velocities, including low-velocity mode, medium-velocity mode and high-velocity mode. Moreover, a deformation map is summarized to illustrate the effects of the impact velocity and the relative density on the deformation modes. Two typical plateau stress regions are detected under low-velocity impact loading, and the second plateau stress is almost twice higher than the first one. The transverse contraction of the RSH mainly occurs at the first plateau region, which is different for the classical re-entrant honeycomb (RH). In addition, the absorbed energy of the RSH decreases slightly and then increases with the impact velocity. The results of the finite element simulations suggest that the RSH shows more excellent impact resistance, compared with the RH and SSH with same cell wall thickness. Furthermore, the effects of the cell wall thickness and the impact velocity on the crushing strength of the RSH are discussed, and the theoretical results are in good agreement with the finite element simulations. [ABSTRACT FROM AUTHOR]

Abstract This paper mainly concerns effects of the auxetic structure on the crash performances in terms of the axial crash force, specific energy absorption, and deceleration, which are evaluated with the conventional and honeycomb structures. Based on the systematic design procedure, the re-entrant units for the auxetic structure are regularly arranged in the tube wall, which are produced by the additive manufacturing with SUS316L metal powder. Under the low impact condition, the auxetic does not only exhibit higher specific energy absorption, but also demonstrate substantially low deceleration due to effect of the densification during the axial crash compared with the conventional tube. Furthermore, while the honeycomb tube demonstrates oscillating behavior in the deceleration, the auxetic tube tends to maintain steady deceleration after the first peak during the axial crash, which is able to guarantee enhanced damping performances under the low impact condition. [ABSTRACT FROM AUTHOR]

• An innovative 3D auxetic structure (3D CATS) was proposed based on the coupling connection of the tube and auxetic units. • The force responses exhibit multiple stages, and the tube and auxetic units played different roles during different stages. • The interaction between the auxetic units and the tube completely changes the deformation modes of the tube. • The SEA of the structure was significantly superior to most existing 2D and 3D auxetic structures. • Two sets of parameters were introduced to vary the compression displacement and force in the plateau stage. In this paper, a novel 3D composite auxetic tubular structure (3D CATS) connected by a thin-walled (TW) circular tube was proposed, and two sets of parameters h 1 , h 2 , and β , n were introduced to construct structural families with various geometries and mechanical properties. The mechanical responses of the 3D CATS were preliminarily explored through the quasi-static experiment. The good agreement between the experimental results and the finite element (FE) simulation verified the accuracy of the FE model. Subsequently, the coupling mechanism between the tube and auxetic units was revealed by analyzing the deformation modes and force responses. The results indicated that the deformation of auxetic units caused more plastic hinges in the tube to enhance the contact force seriously. Then, the 3D CATS was proved to exhibit better specific energy absorption (SEA) compared with other auxetic structures. Furthermore, the influence of h 1 , h 2 , and β , n on the responses was also discussed. The results showed that h 1 , h 2 can change the compression displacement points and forces, while β , n can adjust the stiffness and forces. The results can provide an important reference for the design and study of variable mechanical properties for innovative 3D auxetic structures with enhanced performance. [ABSTRACT FROM AUTHOR]

• 3D novel auxetic structures with improved stiffness are developed based on the stretch-dominated mechanism and made from carbon fibre reinforced polymer composite via interlocking assembly method. • The compressive properties of the developed structures are investigated by theoretical analysis, finite element simulation, and experimental test, and the results from these analyses are compared. • The unit-cell optimized geometry is proposed by multi-objective optimization method. This paper presents a study of 3D novel hybrid auxetic structures made with carbon fiber reinforced polymer (CFRP) laminate composite based on the stretch-dominated mechanism. The homogenized mechanical characteristics of the designed structures, including Young's modulus, shear modulus, and Poisson's ratio, are evaluated via theoretical analysis and finite element simulation. The results exhibit that the structures have negative Poisson's ratio values in all the selected ranges of the design geometry parameters. The multi-objective optimization method is employed to achieve the optimized geometry parameters based on maximizing the stiffness and minimizing the relative density responses. Meanwhile, 2D fabricated auxetic sheets are assembled to build 3D structures through the interlocking method, and quasi-static compressive tests are performed to study their mechanical behaviors. The axial and lateral strains are evaluated by the strain gauge and video extensometer to measure the negative Poisson's ratio values through compressive tests. The results indicate that the proposed structures exhibit superior stiffness and high elongation percentage, suggesting them as a strong candidate for the next generation of load-bearing auxetic structures. [ABSTRACT FROM AUTHOR]

The paper is concerned with mechanical behaviour characterisation of auxetic cellular structures built from inverted tetrapods. The auxetic cellular structures were fabricated from the Ti-6Al-4 V alloy powder using the selective electron-beam melting method (SEBM). The fabricated specimens were experimentally subjected to different loading conditions by varying the loading direction and velocity (strain rate). In addition, the influence of the porosity on the mechanical response and deformation behaviour was analysed based on experimental compression tests supported by infrared thermography. The experimental results show that the porosity and loading direction have a major influence on the mechanical response, while the strain rate influence is negligible at analysed strain rates. The experimental results were also used to validate the computational lattice model of analysed structures computed by the LS-DYNA finite element code. The experimental and computational results were in good agreement for most of analysed geometries and loading cases. [ABSTRACT FROM AUTHOR]

This paper presents a new bi-material microstructural design method for chiral auxetic metamaterials. Based on the independent point-wise density interpolation (iPDI) and a bi-material model, optimal design problem of periodic unit cells is formulated using nodal density variables. The design objective is to minimize the Poisson’s ratio while satisfying the specified volume constraints of the hard and soft materials, and the effective elastic properties of the bi-material microstructure are computed by the asymptotic homogenization method under periodic boundary conditions. This topology optimization problem is solved with a gradient-based mathematical programming algorithm on the basis of the sensitivity analysis. Several numerical examples, regarding design of anisotropic, orthogonal anisotropic and isotropic bi-material microstructures of chiral auxetic metamaterials, are given to demonstrate the effectiveness of the method. It is shown that the proposed bi-material design optimization method can be used to improve the performance of chiral auxetic metamaterials through enlarging the design space. [ABSTRACT FROM AUTHOR]

In this paper, an analytical multi-parameter model based on the classical lamination theory (CLT) is developed and the study of the stiffness response for unidirectional composite laminates with out-of-plane ply waviness is followed. A range of parameters associated with the waviness defect, including the waviness geometry, the fiber off-axis angle, the extent of wavy region and the waviness pattern, are incorporated into the analytical model. The fiber volume fraction and the type of reinforcement are also taken into account in the parametric analysis. The three-dimensional stiffness parameters of thick wavy laminates are investigated systematically for different cases. An observation of the negative in-plane Poisson’s ratios for wavy laminated composites reinforced with carbon fibers is presented under the given assumptions of CLT. This phenomenon provides a possible, new technique to design the auxetic materials. The modeling approach provides an effective procedure to quantify the effects of ply waviness on the stiffness of composite laminates. [ABSTRACT FROM AUTHOR]

The mechanical properties of artery stent are of key importance to the mechanical integrity and biomechanical performance reliability of stent-plaque-artery system. In this paper, making use of auxetic deformation features of chiral structures and mechanical benefits of structural hierarchy, two types of innovative chiral stents with auxetic properties are proposed: (a) anti-tetrachiral stent with circular and elliptical nodes; (b) hierarchical anti-tetrachiral stents with circular and elliptical nodes. Firstly, the in-plane mechanical properties of anti-tetrachiral structures are investigated theoretically, and uniaxial tensile experiments are performed for verification; Secondly, design procedures of anti-tetrachiral stent and hierarchical anti-tetrachiral stent with circular and elliptical nodes are elaborated. Effects of stent geometrical parameters on the tensile mechanical behaviors of these stents are studied with finite element analysis (FEA). It is found that the mechanical behaviors of anti-tetrachiral stent can be tailored through adjusting the levels of hierarchical structures and unit cell design parameters. Finally, the deformation of anti-tetrachiral and hierarchical anti-tetrachiral stents during stenting process are investigated with FEA. It is found that the proposed anti-tetrachiral and hierarchical anti-tetrachiral stents exhibit remarkable radial expanding abilities while maintaining axial stability, thus show promising performances for practical clinical applications. [ABSTRACT FROM AUTHOR]

This paper investigates the energy absorption and deformation characteristics of sandwich structures with auxetic tetrachiral core under high velocity projectile impact. The finite element model of the structure is developed using the Johnson-Cook material model and is used to study the effects of projectile impacts for velocities between 150 and 450 m/s. The FE model is validated using in-house experiments on 3D printed PLA sandwich with tetrachiral core under hemispherical projectile impact and also using experimental data reported in the literature. Further, a semi-analytical model is developed to estimate the residual velocities and energy absorption of sandwich structure under blunt projectile impact, which shows good agreement with the FE results with an error of less than 8%. Finally, the performance of the sandwich structure with tetrachiral cores is studied under projectile impact for velocities ranging from 150 to 350 m/s and compared with sandwich structures with re-entrant and double arrowhead cores. The results show that for velocities above the ballistic limit, the energy absorbed by the facesheets, and core are nearly constant and is greatest for blunt projectile impact, followed by hemispherical, ogive, and conical geometries. Furthermore, sandwich structures with auxetic cores exhibit better energy absorption characteristics compared to those with hexagonal honeycomb cores. [ABSTRACT FROM AUTHOR]

Composites with auxetic effect have been drawing more attention in recent years due to their unique features and wide applications. In this study, a kind of novel multilayer orthogonal structural composite with auxetic effect was fabricated for energy absorption and impact resistance. Non-auxetic composite with the same raw materials but with different reinforcement structure was also fabricated for comparison in order to better understand the effect of reinforced structure on the deformation mechanism and mechanical responses of the composites. Since the quasi-static compressive properties and Poisson’s ratio effect of composites had been investigated previously, this paper only focused on the study of the low-velocity drop-weight impact tests of composites under impact energies from 7.25 J to 65.25 J. It was shown that both auxetic and non-auxetic composites were strain rate sensitive from 34.23 s −1 to 111.48 s −1 , but they exhibited totally different mechanical responses due to different deformation and damage mechanism. Pull out tests demonstrated that strong interfacial bonding between the reinforcements and matrix could ensure the desired deformation of structural reinforcements and auxetic effect of the composite. It was also concluded that the auxetic composite had better energy absorption performance in medium strain range. [ABSTRACT FROM AUTHOR]

[Display omitted] • Spinal interbody cages adapted for an auxetic, and honeycomb using standard CAD software operations and manufactured with SLM. • Auxetic-based cages exhibit higher strength properties than honeycomb-based cages. • The stiffness of the auxetic cages is comparable to stiffness of human cortical bone. • The honeycomb cages exhibit lower value of Young's modulus closer to that for vertebrae. This paper presents a method for modeling a biomedical device by adaptation a commercially available interbody cage for an auxetic metamaterial and honeycomb structure using standard operations in Computer-Aided design software. The mechanical properties of experimental prototypes of Ti-6Al-4 V cages made by selective laser melting using computer modeling (finite element analysis), static and low-cycle fatigue compression tests (up to 3500 cycles) are characterized. 3D printed cells with structures with an angle of inclination between cell edges of less than 90˚ (auxetic metamaterial) are shown to exhibit higher static compressive strength and fatigue resistance than cells based on structures with an angle of inclination greater than 90˚ (honeycomb structure). The changes in the inclination angle differently affect to the porosity and consequently to mechanical behavior of auxetic metamaterial and honeycomb structure. The Young modulus of the auxetic-based interbody cage is 6.68 ± 0.28 GPa and comparable to the elastic modulus of human cortical bone. The honeycomb-based cage exhibits lower values of Young modulus (1.19 ± 0.03 GPa) close to that of trabecular bone and vertebrae. Importantly, the auxetic-based cage is not destroyed after 3500 cycles under 14 kN load with residual deformations ≤ 1 % (0.21 ± 0.10 mm displacements). [ABSTRACT FROM AUTHOR]

This paper aims to understand the role of directional material properties on the mechanical responses of origami structures. We consider the Miura-Ori structures our target model due to their collapsibility and negative Poisson's ratio (NPR) effects, which are widely used in shock absorbers, disaster shelters, aerospace applications, etc. Traditional Miura-Ori structures are made of isotropic materials (Aluminum, Acrylic), whose mechanical properties like stiffness and NPR are well understood. However, how these responses are affected by directional materials, like Carbon Fiber Reinforced Polymer (CFRP) composites, needs more in-depth understanding. To that end, we study how fiber directions and arrangements in CFRP composites and Miura-Ori's geometric parameters control the stiffness and NPR of such structures. Through finite element analysis, we show that Miura-Ori structures made of CFRP composites can achieve higher stiffness and Poisson's ratio values than those made of an isotropic material like Aluminum. Then through regression analysis, we establish the relationship between different geometric parameters and the corresponding mechanical responses, which is further utilized to discover the Miura-Ori structure's optimal shape. We also show that the shear modulus is a dominant parameter that controls the mechanical responses mentioned above among the individual composite material properties within the Miura-Ori structure. We demonstrate that we can optimize the Miura-Ori structure by finding geometric and material parameters that result in combined stiffest and most compressible structures. We anticipate our research to be a starting point for designing and optimizing more sophisticated origami structures with composite materials incorporated. [ABSTRACT FROM AUTHOR]

• An anti-tetrachiral cellular structure composed of flexible beams is proposed. • Closed-form solutions of elastic constants of the structure are obtained. • The light-weight structure shows highly tunable elastic moduli and Poisson's ratio. To provide better solutions for compliant structures, in this paper, an anti-tetrachiral cellular structure with low in-plane stiffness and high auxeticity is proposed. Closed-form solutions of the structure's elastic indicators are theoretically obtained via the energy method, and then numerically verified by using finite element method. The in-plane elastic performance is also discussed through parameter analysis. Results show that the structure has very much low effective moduli and negative Poisson's ratio, exhibiting significantly enhanced deformability and auxeticity. Compared with the conventional anti-tetrachiral structure with rigid central rings, the proposed structure with fully flexible beams shows superior in-plane compliance. As a kind of auxetic structure with highly tunable effective moduli and Poisson's ratio, the proposed structure has great engineering potential in developing new types of morphing, protecting or buffering structures in aerospace, marine, medical and other fields. [ABSTRACT FROM AUTHOR]

• We propose two types of 2D auxetic metamaterials for repeated energy dissipation. • Different directions have different force-displacement (F - d) characteristics and hence the energy dissipation. • 2D auxetic metamaterials perform better than the multistable structure with zero Poisson's ratio. • Rotation of the unit cell in structure affects the F - d responses. • Auxetic behaviors vary with loading directions. Multistable mechanical metamaterials are periodic structures whose unit cells show bistable configurations. The snap-through behavior of the unit cell enhances the energy dissipation of the metamaterials. The materials are reusable and can dissipate energy multiple times since the deformation is within the elastic region. This paper presents two types of two-dimensional auxetic metamaterials: the shuriken-shaped motif with four axes of symmetry and the triangle motif with six axes of symmetry. The cylindrical and three-dimensional (3D) configurations are designed with the geometric topology method. We carry out experiments with 3D printed samples, finite element analysis (FEA), and theoretical investigation to reveal the various mechanisms of energy dissipation. The results demonstrate that the planar metamaterials exhibit different energy dissipation along various loading directions. Each type of metamaterial exhibits discrepant force-displacement characteristics. The triangle motif performs better than the shuriken-shaped motif in terms of energy dissipation along some loading directions. The proposed auxetic metamaterials perform better than the conventional multistable structures with zero Poisson's ratio. Energy dissipation and auxetic behavior of the typically cylindrical and 3D metamaterials are verified numerically and experimentally. [ABSTRACT FROM AUTHOR]

• We propose two types of 2D auxetic metamaterials for repeated energy dissipation. • Different directions have different force-displacement (F - d) characteristics and hence the energy dissipation. • 2D auxetic metamaterials perform better than the multistable structure with zero Poisson's ratio. • Rotation of the unit cell in structure affects the F - d responses. • Auxetic behaviors vary with loading directions. Multistable mechanical metamaterials are periodic structures whose unit cells show bistable configurations. The snap-through behavior of the unit cell enhances the energy dissipation of the metamaterials. The materials are reusable and can dissipate energy multiple times since the deformation is within the elastic region. This paper presents two types of two-dimensional auxetic metamaterials: the shuriken-shaped motif with four axes of symmetry and the triangle motif with six axes of symmetry. The cylindrical and three-dimensional (3D) configurations are designed with the geometric topology method. We carry out experiments with 3D printed samples, finite element analysis (FEA), and theoretical investigation to reveal the various mechanisms of energy dissipation. The results demonstrate that the planar metamaterials exhibit different energy dissipation along various loading directions. Each type of metamaterial exhibits discrepant force-displacement characteristics. The triangle motif performs better than the shuriken-shaped motif in terms of energy dissipation along some loading directions. The proposed auxetic metamaterials perform better than the conventional multistable structures with zero Poisson's ratio. Energy dissipation and auxetic behavior of the typically cylindrical and 3D metamaterials are verified numerically and experimentally. [ABSTRACT FROM AUTHOR]

This paper investigates tunable nonlinear bending behaviors of functionally graded composite beams made of graphene origami (GOri)-enabled auxetic metal metamaterials (GOEAMs) within the theoretical framework of the first-order shear deformation theory and von Kármán type nonlinearity. The beam is comprised of multiple GOEAM layers with GOri content and folding degree being variables to effectively control its auxetic property that is graded from layer to layer across its thickness direction. Our developed genetic programming (GP)-assisted micromechanical models are used to estimate the position- and temperature-dependent Poisson's ratio and other material properties of each GOEAM layer in the beam. The nonlinear governing equations of the FG-GOEAM beam are derived by the principle of virtual work and numerically solved by the differential quadrature (DQ) method. A detailed parametric investigation is conducted to examine the effects of GOri content, folding degree, and temperature on the tunability of the nonlinear bending deflection and normal stress of the FG metamaterial beam. Numerical results offer significant insights into the design of FG-GOEAM beam structures with enhanced bending performances. [ABSTRACT FROM AUTHOR]

The paper describes a vibroacoustics analysis of auxetic gradient honeycomb composite structures with hexagonal configurations. We examine two classes of gradient cellular layout – one with continuously varying internal cell angle, the other with gradient cell wall aspect ratio across the surface of the honeycomb panel. The structural dynamics behaviour of the two gradient honeycomb configurations is simulated using full-scale Finite Elements and Component Mode Synthesis (CMS) substructuring. Samples of the gradient honeycombs have been manufactured by means of 3D printing techniques, and subjected to modal analysis using scanning laser vibrometry. We observe a general good comparison between the numerical and the experimental results. A numerical parametric analysis shows the effect of the gradient topology upon the average mobility and general vibroacoustics response of these particular cellular structures. [ABSTRACT FROM AUTHOR]

This paper presents a systematic study of the helical auxetic yarn (HAY) via careful in-house fabrication and characterisation of a wide range of polymeric fibres and yarns. It provides a better understanding of the auxetic behaviour of the HAY in order to tailor their properties for specific applications. The study focused on three parameters: component moduli, the core/wrap diameter ratio and the initial wrap angle. The results show that a larger difference in component moduli, a higher core/wrap diameter ratio and a lower initial wrap angle can produce a larger maximum negative Poisson’s ratio value and thereby a better auxetic performance for HAYs. All three parameters could be carefully utilised when in combination to achieve the required auxetic behaviour of HAYs. Moreover, the instantaneous true Poisson’s ratio analysis accurately presents the instantaneous behaviour of highly strain dependent HAYs. [ABSTRACT FROM AUTHOR]

Lattice structures with Plateau borders (LSPB) have attracted increasing interests recently due to the improved stiffness, strength, energy absorption properties. Undoubtedly, vertexes with Plateau borders (VPB) play a significant role on the dynamic elastic moduli. This paper proposes an analytical framework of the frequency-dependent equivalent in-plane dynamic elastic moduli (Young's moduli E 1 (ω) , E 2 (ω) , Poisson's ratio ν 12 (ω) , ν 21 (ω) and shear modulus G 12 (ω)) of LSPB. First, dynamic stiffness (DS) matrix of a lattice cell edge (based on rod and Timoshenko theories) connected to VPBs (modelled as rigid bodies) at both ends is formulated. Then, based on the above DS matrix and the unit cell method, closed-form expressions of equivalent in-plane dynamic elastic moduli are proposed, which are sufficient general to be applied to four types of lattices. The effects of mass, inertia moment and size of VPB on the equivalent dynamic elastic moduli are studied, with both physical and mathematical interpretations. Furthermore, the proposed expressions are applied to honeycomb, rectangular, auxetic and rhombus LSPB and some interesting and important observations are made. This research provides analytical expressions for broadband dynamic elastic moduli of LSPB, which can be directly used in the design and optimization of composite structures with lattice cores. [ABSTRACT FROM AUTHOR]

Traditional auxetic structures only exhibit lateral contraction when axially compressed. In this paper, a strategy of bi-directional auxetic metamaterials with tunable effective Poisson's ratio is proposed. The bi-directional auxetic metamaterials based on two-dimensional double arrow structures are designed and then fabricated through additive manufacturing. The deformation mechanisms of the proposed structures are investigated through numerical simulations combined with experiments in the present study. The results show that the relative specific stiffness and the effective Poisson's ratios of the proposed structures depend on geometrical parameters and geometrical configuration. The relative specific stiffness of the three-dimensional cubic structure (TCU) is between the internal and external stiffness of the structure, as is the case of the effective Poisson's ratio. The axisymmetric deformation mechanism of the three-dimensional cylindrical structure (TCY) generates transversely isotropic Poisson's ratio and enhances stiffness compared to the TCU with the same geometric parameters. This work systematically characterizes the deformation mechanism of innovatively designed structures and widens the potential applications of metamaterials in the fields of machinery, encompassing biomedicine, and electronic devices. [ABSTRACT FROM AUTHOR]

In the present paper, distributions of lateral deflections and in-plane normal and transverse shear stresses of circular/annular sandwich plates with orthotropic composite face sheets and auxetic (with negative Poisson’s ratio) cores are determined, for the first time, employing a zigzag theory whose results are corrected based on the three-dimensional theory of elasticity. The governing equations are developed based on the principle of minimum total potential energy and solved using a differential transform whose center is located at the outer radius of the plate. A comprehensive parametric study including evaluation of effects of various geometric and material parameters, lamination schemes, and boundary conditions is accomplished. Accuracy of the results is verified by results of the 3D theory of elasticity extracted from the ABAQUS code. Results reveal that the core auxeticity increases the rigidity of the soft core drastically and consequently, decreases the lateral deflection of the plate and moderates the interfacial jumps in the through-thickness distribution of the transverse shear stress, so that a semi-continuous parabolic distribution may be established. The core auxeticity may considerably reduce the maximum radial stresses of the face sheets and influence of the auxeticity is more remarkable for plates with more rigid constructions or boundary conditions. [ABSTRACT FROM AUTHOR]

A comprehensive characterization of the novel class of anti-tetrachiral cellular solids, both considering the static and the dynamic response, is provided in the paper. The heterogeneous material is characterized by a periodic microstructure made of equi-spaced rings each interconnected by four ligaments. In the most general case, rings and ligaments are surrounded by a softer matrix and the rings can be filled by a different material. First, the first order linear elastic homogenized constitutive response is estimated resorting to two different microstructural models: a discrete model, in which the ligaments are modeled as beams and the presence of the matrix is neglected and the equivalent elastic properties are evaluated through a simplified analytical approach, and a more detailed continuous model, where the actual properties of matrix, ligaments and rings, varying in the 2D domain, are considered and the first order computational homogenization is adopted. Special attention is given to the dependence of the 2D overall Cauchy-type elastic constants on the mechanical and geometrical parameters characterizing the microstructure. The results, indeed, show the existence of large variations in the linear elastic constants and degree of anisotropy. A comparison with available experimental results confirms the validity of the analytical and numerical approaches adopted. Finally, the rigorous Floquet–Bloch approach is applied to the periodic cell of the cellular solid to evaluate the dispersion of propagation waves along the orthotropic axes in the framework of elasticity and to detect band gaps characterizing the material. A numerical approach, based on the first order computational homogenization, is also adopted and the rigorous and approximate solutions are compared. [ABSTRACT FROM AUTHOR]

• This paper proposes a three-dimensional structure based on a triangle unit. • The analytical formulas for PR and CTE are given. • This structure can achieve both negative PR and negative CTE. • The mechanism can be regarded as a new concave angle mechanism. Auxetic materials and negative thermal expansion materials show great application potential in many important fields. In this paper, a novel three-dimensional truss structure with a tunable Poisson's ratio and tailorable coefficient of thermal expansion was developed. Since the deformation of each rod was dominated by axial tension or compression, the structure showed high stiffness. Equations regarding equivalent elasticity parameters and coefficients of thermal expansion of the structure were obtained by displacement and unit force methods. A numerical simulation verification was performed by ANSYS 16.0 software. The verification results indicated that the formulas have high precision and wide application scope. Through parametric analysis, we obtained the conditions for achieving a negative Poisson's ratio and negative thermal expansion coefficient. We also concluded that this material could realize a three-dimensional negative Poisson's ratio and bidirectional negative thermal expansion simultaneously by choosing reasonable geometric parameters and material combinations. [ABSTRACT FROM AUTHOR]

• Several optimal designs of lightweight auxetic metamaterials are proposed. • The proposed auxetic metamaterials possess better energy absorption capacity. • The effect of the amount and location of the removed material is studied. • The potential applications of lightweight auxetic metamaterial are discussed. In the past decades, auxetic metamaterials have attracted extensive attention due to their desirable properties. In order to further improve the energy absorption properties of auxetic metamaterials, this work investigates the design of lightweight auxetic metamaterials based on the elliptic perforated plate. The material in the large deformation region is retained and the material in the small deformation region is removed as much as possible. The experimental and numerical results indicate that the mechanical properties of the novel metamaterials are almost unchanged, but the specific energy absorption is substantially enhanced owing to the reduced material overall and the distribution of the optimized materials to the most needed area. In this paper, the influence of the amount and mode of the material removal on the mechanical properties of the auxetic metamaterial is explored through the numerical method. The proposed auxetic metamaterials with lightweight characteristics have potential applications in many fields, e.g., civil engineering, aerospace and vehicle engineering. [ABSTRACT FROM AUTHOR]

Abstract: Scientific investigations of structures characterised by auxetic properties conducted so far have led to analyses on a molecular and macroscopic scale and few studies have been concerned with descriptions of auxetic models which could find application in objects of everyday use. The aim of this paper was to carry out numerical modelling and physical verification of furniture seat quality made up of auxetic pressure springs. The authors decided to determine system stiffness and distribution of contact pressure on their surface. The performed detailed numerical analysis made it possible to evaluate the comfort of seat utilisation as well as to assess the impact of seat modification with NHE35 foam on changes of this comfort. In addition, it was demonstrated that numerical modelling may provide a useful tool for rapid prototyping of complex, multilayer systems manufactured from materials with non-linear characteristics. Results of numerical calculations were verified in the laboratory determining the scale of errors with which these results should be interpreted. [Copyright &y& Elsevier]

In this paper, we report a new multi-scale NPR composite based on auxetic fabric/elastomer matrix (AF/EM) composite structure. The multi-scale AF was obtained via using helical auxetic yarn (HAY) as weft yarns and constructing re-entrant hexagonal (REH) geometry structure on the macro scale, then the AF was combined with EM to fabricate the auxetic composite (AC). The AF and AC exhibit the NPR of −1.492 and −0.379. Simultaneously, high tensile strain over 108% (AF) and 82% (AC) can be subjected, indicating good mechanical performance. The superior performance of the AC is its ability to maintain NPR and resilience, with elastic recovery ratio over 94% and work recovery coefficient with a stable value over 80% after 10 cycles. Also, the damping properties results further evidence the good elasticity of our AC. The paper shows that our AF and AC are promising candidates in protective composites. [ABSTRACT FROM AUTHOR]

• A novel auxetic chiral lattice composite is proposed by designing joints of the reinforcement phase. • The mechanical properties and deformation characteristics of four specimens are investigated under uniaxial compression. • The novel auxetic chiral lattice composite exhibits an enhanced stiffness and SEA capacity. • The effects of geometrical parameters and volume fraction of the reinforcement phase on the novel auxetic composites are investigated. Auxetic materials and structures have many potential applications due to their counter-intuitive deformation behavior and desirable mechanical properties. However, auxetic structures have some drawbacks, such as relatively low stiffness and stability. To improve their mechanical performance, one common method is to fill soft materials in auxetic frames, i.e. , auxetic two-phase composites. In this paper, a novel method to further enhance the mechanical properties of auxetic composites has been proposed, which is implemented by designing joints of frames. Mechanical properties and deformation characteristics of these novel composites and their conventional counterparts are investigated experimentally and numerically. The results of finite element analysis and experiments exhibit a good agreement. The energy absorption capacity and auxeticity of the proposed composites could be enhanced by optimizing the design of joints. Subsequently, parametrical studies are conducted to quantify the effects of the geometrical parameters and the volume fraction of the frame. [ABSTRACT FROM AUTHOR]

• Effectiveness of re-entrant auxetic structure under close-in and far-field blasts. • Evaluation of protective efficacy of auxetic honeycomb-core sandwich panel. • Performance comparison with equivalent conventional honeycomb. This paper aims to scrutinise the effectiveness of a re-entrant auxetic honeycomb-core sandwich panel (AHSP) to protect reinforced concrete (RC) slab under close-in and far-field detonations of high explosive. A series of quarter symmetric 3D comprehensive numerical models were developed by using Arbitrary Lagrangian Eulerian (ALE) formulation in LS-DYNA hydrocode. Strain rate effects of the rate sensitive materials were employed in the model to consider dynamic behaviour of the materials during blast loading. The developed numerical model was validated with the documented experimental results. Deformation patterns, energy absorption, overpressure damping, blast pressure deflection, and stress-transmission to the protected structure were investigated with the computational models. Furthermore, the performance of AHSP sacrificial protective structure was compared with an equivalent areal density conventional honeycomb-core sandwich panel (CHSP) structure. The results showed that AHSP not only absorbed higher blast energy but also acted as a pressure deflector by forming a densified concave depression under close-in blast load. AHSP damped higher magnitude of blast overpressure in both close-in and far-field blasts. Moreover, a uniform redistribution of the stress on the protected structure was observed with AHSP protection. The overall response of AHSP was found to be better than CHSP to protect the RC slab. [ABSTRACT FROM AUTHOR]

• Two new types of auxetic honeycombs with curved inserts were proposed. • The slope-deflection equation for an arc bar was established. • Curved inserts are essential in tailoring the mechanical properties. • A special dominated deformation mode intermediate between pure bending and stretching was achieved. Imperfect situation, like the non-uniform cross-section or non-straight profile of cell walls, can in certain case be a positive contributor for properties of cellular solids. However, auxetic cellular structures with imperfect profiles are rarely reported in the literature. Based on the re-entrant hexagonal honeycomb (RHH), this paper developed two new types of honeycombs with curved inserts and focused on the issue that how microscopic curved inserts influence the auxetic structure's macroscopic mechanical properties? Theoretical models for the in-plane equivalent elastic moduli of the two types of new honeycombs, including the Young's modulus, Poisson's ratio and shear modulus, were first established, and then verified by numerical simulations. The results showed that the two types of new honeycombs possess the same ability to achieve auxeticity, in the meantime, exhibit improved Young's modulus and shear modulus compared to the RHH. Furthermore, the new honeycombs demonstrate more flexibility in design as the curved inserts can offer more geometric parameters for tailoring their macroscopic mechanical properties. The study also showed that the curved inserts played a key role in determining the dominated deformation mode of the new honeycombs, and a special dominated deformation mode intermediate between pure bending and stretching was found in these honeycombs. This work would provide a useful guide for the development and optimization of auxetic materials. [ABSTRACT FROM AUTHOR]

This paper investigates the thermo-mechanical postbuckling behaviors of sandwich plates with auxetic graphene-reinforced metal matrix composite (GRMMC) core resting on elastic foundations. Two cases of the compressive postbuckling of sandwich plates subjected to uni-axial compression in thermal environments and the thermal postbuckling of sandwich plates subjected to a uniform temperature rise are considered. The auxetic GRMMC core having negative Poisson's ratio (NPR) is assumed to be piece-wise functionally graded (FG) by changing the volume fraction of graphene through the thickness domain of the core. The material properties of both metal face sheets and the GRMMC core are assumed to be temperature dependent. The Reddy's third order shear deformation plate theory and the von Kármán-type kinematic nonlinearity are used to formulated the governing equations of the sandwich plate which account for the plate-foundation interaction and the thermal effects. The postbuckling solutions are obtained by employing a two-step perturbation approach. The results reveal that the compressive postbuckling and thermal postbuckling behaviors of sandwich plates with auxetic GRMMC core are significantly influenced by the FG pattern, the face sheet-to-core-to-face sheet thickness ratio, and foundation stiffness. [ABSTRACT FROM AUTHOR]

Auxetic metamaterials, which expand transversely when longitudinally stretched, are widely used in engineering fields, such as flexible electronics and biomedicine. However, they have fixed configurations and properties without the ability to tune and adapt after fabrication. In this paper, we propose three kinds of 4D printed programmable metamaterials (triangular, square and honeycomb lattice metamaterials). The mechanical properties of the designed metamaterials for different topological angle θ and radius R are studied through experiments and finite element analysis (FEA). Results show that the triangular and square lattice metamaterials achieve large deformation and auxetic behaviors, and due to shape memory effects, the Poisson's ratios and the elastic moduli of the metamaterials can be programmed by tuning the topological parameters and temperature. Furthermore, the proposed cylindrical shells with desired mechanical properties and configurations demonstrate potential applications in biomedical scaffolds. [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]

Highly compressible and stretchable lattice materials are perfectly suitable to be exploited in a range of cutting edge engineering applications such as low band-gap acoustic metamaterials, vibration absorbers, soft robotics, stretchable electronics and stent devices. Physics-based understanding and efficient computational methods are of paramount importance for the analysis and design of such cellular metamaterials. This paper develops the analytical framework to understand the nonlinear mechanics of hexagonal lattices subject to in-plane compressive and tensile stresses. Nonlinear equivalent elastic moduli and Poisson's ratios of the stressed lattice are expressed through the coefficients of the stiffness matrices of the constitutive beam elements. The stiffness coefficients, in turn, are derived from the transcendental displacement function which is the exact solution of the corresponding governing ordinary differential equation with appropriate boundary conditions. The closed-form analytical expressions of the equivalent elastic properties of the lattice are expressed in terms of trigonometric functions for the case of compressive stress and hyperbolic functions for the case of tensile stress. The general expressions are then used to investigate three special cases of wide interest, namely, auxetic hexagonal lattices, rhombus-shaped lattices and rectangular lattices. Analytical expressions are validated using independent nonlinear finite element simulation results. Numerical results are displayed for applied compressions and tensions in both directions separately and together. The equivalent elastic moduli show a softening effect under compression and a stiffening effect under tension. The Poisson's ratios are not significantly affected by the applied stresses. The proposed analytical approach and the new closed-form expressions provide a computationally efficient and physically intuitive framework for the analysis and parametric design of lattice materials under external stresses. [ABSTRACT FROM AUTHOR]

This paper developed a thermal–mechanical analysis model to describe the thermal shock behavior of ceramic sandwich structures (CSSs) with an auxetic honeycomb core. The transient temperature distribution and the associated thermal stress field of CSSs subjected to a thermal shock were determined. The crack growth behavior was discussed based on the thermal stress intensity factor (TSIF), crack location, face-sheet thickness, and internal cell orientation of the honewycomb core in detail. The results revealed that the auxetic and non-auxetic honeycomb cores have the similar temperature distribution profile however the auxetic honeycomb core can substantially reduce the overall thermal stress level. Furthermore, by equating the maximum TSIF and equivalent fracture toughness, the critical temperature for which the honeycomb core can sustain without failure was obtained. The results showed that CSSs with auxetic honeycomb core possessed a higher thermal shock resistance than that of non-auxetic. This research work was the first attempt to identify the the thermal shock resistance of CSSs with an auxetic honeycomb core. The results would provide fresh insight into the design and selection of CSSs applied in an extreme temperature environments. [ABSTRACT FROM AUTHOR]

In this paper we will investigate the impact of in-plane negative Poisson's ratio (NPR) on the post-buckling responses of graphene-reinforced metal matrix composite (GRMMC) laminated plates under uni- and bi-axial in-plane compressive loads. The effects of temperature variation and foundation support on the post-buckling responses of GRMMC laminated plates are also taken into consideration. The graphene volume fractions in each layer of a GRMMC plate may vary along the plate thickness direction to achieve a functionally graded (FG) arrangement. The GRMMC layers have temperature-dependent material properties that can be modeled by the extended Halpin–Tsai model. Employing the Reddy's third order shear deformation plate theory, the governing equations containing both thermal and foundation effects for the post-buckling problem of FG-GRMMC plates are formulated. The von Kármán geometrical nonlinearity is also considered. The post-buckling responses of the FG-GRMMC laminated plates are obtained by solving the governing equations using a two-step perturbation approach. The results have revealed that in-plane NPR has a substantial impact on the post-buckling responses of GRMMC laminated plates. [ABSTRACT FROM AUTHOR]

• Proposed cell-based auxetic structure that exhibits axial twist in response to extension with pre-deformed ligaments. • Three methods of characterization including: analytical, FEM, and experimental. • Analysis of structure parameters and their effect on the twist-deformation response. • Discussion on the structures potential for future in mechanical actuator development and composites. Recent advances in artificial metamaterials have allowed unique structures to deform in distinct modes not previously found in traditional material form. Auxetic structures that induce twist through deformation have been a focus of recent research. In this paper a cell-based tubular structure with pre-deformed ligaments is proposed which exhibits efficient extension induced twist (EIT) without the limitations of buckling. An analytical model of the structure is proposed using cell parameters to characterize the non-linear twist-deformation relationship. Samples of the structure were fabricated using the Ultimaker 3 platform with elastomer based TPU 95a build material and PVA support material. The analytical model and FEM were validated by tensile experiments, where fabricated samples reached 60 degrees of non-linear axial twist over 40 mm of deformation. In order to further characterize the structure's twist response, key design parameters of the lattice structure were varied and modelled. Actuation through tensile elastic deformation has key advantages in reliability and reduced complexity. Due to these advantages, this technology has potential applications in aerospace, biomedical and robotic fields. [ABSTRACT FROM AUTHOR]

• A novel 3D NPR material design method based on the tension-twist coupling effects have been proposed. • Eleven types of novel 3D lattice materials with NPR and PPR properties are proposed. • The numerical and experimental results show that the proposed novel 3D NPR and PPR materials exhibit desirable PR properties. In recent years, as the development of additive manufacturing technology, 3D negative Poisson's ratio (NPR) materials attract many researchers' interest. In this paper, a novel 3D NPR material design method based on the tension-twist coupling effect has been proposed. A novel category of 3D NPR and positive Poisson's ratio (PPR) materials by linking multilayer 2D NPR materials through 3D chiral chains are proposed. The sign of the Poisson's ratio depends on the linkage pattern of the 3D chiral chain. Based on this method, 11 types of novel 3D NPR and PPR materials are presented. Experimental and numerical methods are adopted to study the mechanical properties of the proposed novel 3D NPR and PPR materials. Results show that the novel 3D materials present desirable NPR and PPR as we expected, which proved the effectiveness of the proposed design method. [ABSTRACT FROM AUTHOR]