Negative Poisson's Ratio Behavior Research Articles

Compressive properties and energy absorption of 4D printed auxetic mechanical metamaterials

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.

Epoxy matrix nano composites: modulus, strength and ductility enhancement through auxeticity of α-Cristobalite filler

The negative Poisson's ratio (NPR) nano-size α-Cristobalite particle/epoxy composites were prepared and tensile tested. The elastic modulus and strength of the composites were improved as the particle volume fraction increased from 0 to 0.02. Unlike the conventional particle reinforced composites, the fracture strain increased with the nano α-Cristobalite addition, an effect which was ascribed to the intrinsic NPR behavior of the filler.

Design, microstructure, and mechanical property of negative Poisson's ratio porous structure fabricated by LPBF of AlCoCrFeNi2.1 eutectic high-entropy-alloy

Negative Poisson's ratio (NPR) materials are a type of structural materials that show an atypical deformation response. In general, metal-based porous materials with NPR do not have good mechanical properties due to the substrate's limited yield strength and plasticity, as the NPR behavior is unlikely to play a continuous role in the deformation. AlCoCrFeNi2.1, a eutectic high-entropy alloy (EHEA) with high yield strength and high ductility was used to fabricate the NPR structures by laser powder bed fusion (LPBF) to enhance the structures mechanical properties and NPR behavior. Structures with four different sets of structural parameters were designed for this study. The microstructure and mechanical properties of AlCoCrFeNi2.1 EHEA, synthesized by LPBF were studied, and the strength and energy absorption effect of NPR porous structure materials were investigated by static and dynamic compression experiments. Research showed the large number of inter-phase interfaces provided by the nano-scale cellular eutectic microstructure effectively restrict dislocation movement, thereby enhancing the strength and toughness of this EHEA alloy, endowing it with superior impact resistance, fracture resistance, and mechanical energy absorption. The compressive performance and the mechanical energy absorption capacity of the 3D EHEA-based NPR porous structure strongly depend on the inclination angle Ф between the thick rod and the horizontal direction in the 2D single-body model design. The strength and energy absorption of the structure with a high inclination angle of 75° material are both higher than those of the structure with a low inclination angle of 45° by almost a factor of two.

Magnetic anisotropy of high-entropy oxides with negative Poisson's ratio

High-entropy oxide (HEO) materials have recently been extensively studied and attracted unprecedented attention. The HEO can be regulated by introducing different atomic species, resulting in excellent physical properties. Herein, we fabricated the spinel HEO (Cr,Mn,Fe,Co,Ni)3O4 (CMFCN) film with various of thicknesses on perovskite STO substrate by PLD technology. Interestingly, all the CMFCN films exhibit negative Poisson's ratio behavior and demonstrate ferrimagnetic behavior at room temperature. The strain-induced Poisson's ratio significantly affects the magnetism of the HEO film. The magnetic anisotropy decreases as the Poisson's ratio decreases, while coercivity increases from ∼270 to ∼1700 Oe. The axis of easy magnetization changes from in-plane to out-of-plane. These results indicate that the thickness-modulated magnetic of HEO epitaxial films has potential applications in integrating magnetic insulating oxides with spintronics.

Pattern Transformation Inspired Multifunctional Cylindrical Vessels with Programmable Stoma‐Shaped Biomimetic Openings

AbstractPattern transformation in a periodic porous structure has inspired multifarious mechanical metamaterials/metastructures due to the induced unusual negative Poisson's ratio behavior of macroscopic materials. Recently, it has been leveraged to architect a variety of designable and multifunctional structural members. Inspired by this design methodology, a novel porous cylindrical shell, which is perforated by a large number of staggered openings, is constructed and investigated meticulously. A stable, anti‐disturbed, and controllable waisted deformation of the architected cylindrical shell will be triggered under an axial compression. A stoma‐shaped biomimetic hole and graded distribution of initial openings are proposed to ensure that the holes distributed throughout the shell can be closed up concurrently while the closed states of holes can be flexibly programmed. To explore the applications of such shells, a handy cylindrical vessel is elaborately designed and its multiple functions including reagent release, underwater sampling, and flow control are exhibited by experiments. The results reflect that the designed vessel can be facilitated with many advantages such as uniform release, quick action, easy actuation, and repeated usage. Moreover, it also may open a new avenue for metamaterials in the fields of biomedical engineering, underwater detection, fluid machinery, etc.

Compressive mechanical properties of layer hybrid lattice structures fabricated by laser powder bed fusion technique

A layered hybrid lattice structure consisting of a body-centered cubic (BCC) structure and an N-type structure with a negative Poisson's ratio effect is designed. In order to assess the effect of the position and number of N-type structure layers in the overall lattice structure on the mechanical properties, three kinds of hybrid lattice structures with different positions and numbers of N-structure layers are manufactured by using laser powder bed fusion (LPBF) technique. Compression experiments are carried out on the three kinds of layered hybrid lattice structures and the BCC lattice structure for comparisons. The entire loading process is recorded and the deformation pattern of the lattice structure is captured. The experimental results show that the presence of the N-type structure has a significant effect on the deformation pattern of the lattice structure. Comparing the mechanical response and energy absorption efficiency, it is found that the hybrid lattice structure performs better than the BCC structure when the structural strain is small. In addition, finite element models of the N-type structure unit-cell and hybrid lattice structures are established. Numerical simulations are conducted to verify the negative Poisson's ratio behavior of the N-structure and predict the deformation modes of the layered hybrid lattice structure.

Fabrication and crushing response of graded re-entrant circular auxetic honeycomb

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

Emerging negative Poisson's ratio driven by strong intralayer interaction response in rectangular transition metal chalcogenides

Auxetic behavior quantified by the negative Poisson's ratio (NPR) is commonly attributed to geometry evolution with re-entrant mechanism or other mechanical factors, which is thought to be independent of electronic structures. Materials with NPR behavior dominated by electronic effects, combined with other remarkable properties, may yield unexpected multi-functionalities. Herein, from first-principles calculations, by studying a class of two-dimensional (2D) transition metal chalcogenides (TMCs), namely X2Y2-type (X = Cu, Ag, Au; Y = O, S, Se) rectangular TMCs (R-TMCs), we identify that the monolayer R-Cu2Se2 unconventionally demonstrates a structure-independent NPR. In contrast, the NPR is absent in other R-TMCs. The emerging NPR is attributed to the strong strain response of intralayer interaction in R-Cu2Se2, which can be traced to the lone pair electrons and weak electronegativity of Se atoms under multi-orbital hybridization. The emerging NPR would make R-Cu2Se2 a promising candidate in electronics protection, and our study would provide valuable clues and useful guidance for designing advanced auxetic materials.

Effects of geometry and boundary constraint on the stiffness and negative Poisson's ratio behaviour of auxetic metamaterials under quasi-static and impact loading

Auxetic metamaterials exhibit a negative Poisson's ratio and superior energy dissipation characteristics under dynamic (impact) loading. This makes them attractive candidates for components that provide crash or impact protection (such as crumple zones) in automotive and aerospace applications. However, little prior work on the effect of boundary constraint on the behaviour of these structures exists. Therefore, the primary goal of this investigation is to assess the effect of boundary constraint on the energy-absorbing properties of auxetic metamaterials. Specifically, auxetic metamaterials (lattice structures) were fabricated from stainless steel powders via 3-D laser printing (selective laser melting). Additional structures were printed with constraining walls attached in order to evaluate the effects of boundary constraint on the auxetic behaviour. A comprehensive investigation of the mechanical performance of the structures was then carried out under both quasi-static and dynamic (impact) loading conditions. Poisson's ratio and elastic modulus values were determined from the experimental results and compared with theoretical calculations. Dissipation of vertical forces into the lateral boundaries was confirmed by about 20% less negative Poisson's ratios, for otherwise identical auxetic structures. In addition, dynamic loading simulations have shown that with optimized geometric parameters, auxetic metamaterials are capable absorbing over 90% of the energy of an impact. However, critical overestimation of the energy absorbing abilities of auxetic crash protectors could occur if the effects of lateral constraint are not carefully considered.

Three-dimensional composites with nearly isotropic negative Poisson's ratio by random inclusions: Experiments and finite element simulation

Composites with negative Poisson's Ratio (NPR), also known as auxetic composites, are of great interests due to excellent properties. Auxetic composites of various types have been developed, however most of them have anisotropic NPR behaviour. This paper reports a new type of three-dimensional (3D) composites with nearly isotropic NPR by randomly embedding rigid re-entrant equilateral triangle inclusions into soft matrix. Four types of inclusions with different structural parameters including re-entrant angle, prism length and rib thickness were first made from two types of carbon fibre prepregs, and then embedded into silicone rubber matrix with different inclusion numbers to fabricate 3D composites with various volume fractions of inclusions. Both experimental and finite element analyses were conducted to investigate the deformation behaviours under compression and the effects of different structural parameters and materials. The results show that the NPR of the composites can be realised with suitable structural parameters and properties. The lowest NPR of −0.19 was obtained via embedding 6 mm length inclusions made of one-layer 3K twill weave carbon fibre prepreg with an inclusion number of 800. The study provides a new approach for the development of 3D composites with isotropic NPR for the practical applications of auxetic materials.

Crashworthiness analysis of cylindrical tubes with coupling effects under quasi-static axial loading: An experimental and numerical study

In most cylindrical tubes, the occurrence of negative stiffness under compression is unavoidable. This downward trend in the force–displacement relationship means a decrease in the energy-absorption capacity. To this end, this paper introduces a new assembly method comprising two concentric cylindrical tubes. The inner cylinder possesses positive Poisson's ratio behavior, whereas the outer cylinder exhibits negative Poisson's ratio behavior. When compressed, the outer and inner cylinders shrink and expand, respectively, creating surface contacts between the two cylinders, called coupling effects. This property not only avoids the occurrence of negative stiffness in outer cylindrical tube, but also increases the energy-absorption capacity in an upward trend in the force–displacement relationship. To confirm this claim, three different types of cylindrical tubes, possessing positive Poisson’s ratio, zero Poisson's ratio, and negative Poisson’s ratio behaviors, are considered. A finite-element analysis is implemented to simulate deformation patterns of cylindrical tubes. Then, to verify the results of finite-element analysis, a laser-cutting method is applied to fabricate cylindrical tubes from stainless steel tubes. The results show that the proposed assembly method increases the energy-absorption capacity by up to 95% compared to the well-known auxetic tube. Next, a parametric study is performed, in which the gap space between the two cylinders is considered as a design variable. The results reveal the smaller the gap space, the higher the energy-absorption capacity. The absorbed energy in the assembled cylinders without gap space is 17.6 J, which is 36% greater than that of cylinders with 13 mm gap space. The effects of relative density and crushing speed are also evaluated. When compared to the crushing speed, the energy-absorption capacity is highly dependent on relative density.

An Auxetic Metamaterial with Tunable Positive to Negative Hygrothermal Expansion by means of Counter‐Rotating Crosses

A type of mechanical metamaterial is proposed, whereby the effective coefficients of thermal and moisture expansions (CTE and CME, respectively) can be tuned from positive to negative values while maintaining a consistently auxetic (negative Poisson's ratio) behavior for the overall structure. Each unit consists of a pair of equal‐armed crosses pin jointed at their centers, whereas their ends are connected via bridging beams using pin joints to facilitate free rotation. The bridging beams are of lower modulus and higher hygrothermal expansivity to permit negativity in metamaterial's overall Poisson's ratio and hygrothermal expansion. Two sets of effective Young's modulus, CME and CTE models are developed—one set for large deformation and the other for infinitesimal deformation. Results indicate that the effective Young's modulus is reduced while the expansion coefficients become more negative as the inclination angle of the equal‐armed crosses increases, and that the application of tensile and compressive loads increases and decreases the effective Young's modulus, respectively. The establishment of effective zero thermal and moisture expansion conditions facilitates the application of the metamaterial structure that requires auxetic property within highly fluctuating environmental condition.

A Novel Hyperbolic Two-Dimensional Carbon Material with an In-Plane Negative Poisson's Ratio Behavior and Low-Gap Semiconductor Characteristics.

Poisson’s ratio is one of the fundamental features of materials, reflecting the transverse strain response to the applied certain axial strain. The materials with a negative Poisson’s ratio, also known as auxetic materials, are rare in nature and attract lots of research interest because of their unusual mechanical behavior and extensive applications in mechanical nanodevices. Here, we proposed and studied a novel hyperbolic two-dimensional graphene-like structure (GLS) showing a negative Poisson’s ratio behavior by first-principles calculation. The thermodynamic, dynamic, lattice dynamic, and mechanical stabilities of the GLS were carefully studied. In addition, we also explored the electronic structure, mechanical characteristics, and optical-electronic characteristics. The GLS not only displays a negative Poisson’s ratio in certain directions but also shows low-gap semiconductor characteristics and superior electronic conductivity. It is a potential sunscreen material because of the outstanding reflection and absorption for ultraviolet and infrared light.

In-plane elastic constants of a new curved cell walls honeycomb concept

Abstract This paper is focused on the identification of the in-plane elastic constants of a new design of auxetic (negative Poisson's ratio) honeycomb configuration with curved cell walls by using analytical and numerical homogenization techniques. The sensitivity of the elastic constants is determined against the various cell geometry parameters. Good agreement between the analytical and numerical simulations is observed. We show that the specific curved wall honeycomb configuration proposed in this paper possesses a high in-plane shear compliance, tailored anisotropy and the possibility of inducing a negative Poisson's ratio behaviour in baseline honeycomb configurations that would have otherwise positive in-plane Poisson's ratios.

Dome-Shape Auxetic Cellular Metamaterials: Manufacturing, Modeling, and Testing

We present in this work the manufacturing, modelling and testing of dome-shaped cellular structures with auxetic (negative Poisson's ratio) behaviour. The auxetic configurations allow the creation of structures with synclastic (i.e. dome-shaped) curvatures, and this feature is used to evaluate the performance of cellular metamaterials under quasi-static indentation conditions. We consider here different cellular geometries (re-entrant, arrow-head, tri-chiral, hexagonal) and the implications of their manufacturing using 3D printing techniques with PLA material. The dome-shaped configurations are modelled using full-scale non-linear explicit FE models that represent both the geometry and approximate constitutive models of the PLA filament material derived from tensile tests on dogbone specimens. The cellular metamaterials samples are subjected to indentation tests, with maps of strains obtained through DIC measurements. The correlation between experimental and numerical simulations is good, and shows the peculiar indentation behaviour of these cellular structures. We also perform a comparative analysis by simulation of the force/displacement, strain and fracture history during quasi-static loading, and discuss the performance of the different cellular topologies for these dome-shape metamaterial designs.

Elastomer-Free, Stretchable, and Conformable Silver Nanowire Conductors Enabled by Three-Dimensional Buckled Microstructures.

Many three-dimensional (3D) nanomaterial-based assemblies need incorporation with elastomers to attain stretchability-that also compromises their pristine advantages for functional applications. Here, we show the design of elastomer-free, highly deformable silver nanowire (AgNW) conductors through dip-coating AgNWs on a 3D polymeric scaffold and following a simple triaxial compression approach. The resulting 3D AgNW conductors exhibit good stability of resistance under multimodal deformation, such as stretching, compressing, and bending as well as comparable conductivity with those elastomer-based ones. Moreover, the buckled structures endow our 3D conductors with novel negative Poisson's ratio behavior, which can offer good comfortability to curvilinear surfaces. The combination of mechanical properties, conductive performance, and unique deformation characteristics can satisfy multiscale conformal mechanics with a soft, curvilinear human body.

Finite element simulation of an auxetic plied yarn structure

Auxetic plied yarn is a novel type of yarn structure formed with two types of component yarns with different moduli. Although the effect of structural parameters on its negative Poisson's ratio behavior has been investigated by both experimental and geometrical analyses, the influence of material properties, including yarn tensile modulus and friction, has not been studied yet. In this work, these two factors are further investigated via finite element simulation. The finite element model is first created by using the commercial software ANSYS 15.0. Then, the simulation results are compared with the experimental and analytical data. Finally, the effects of the tilt angle of stiff yarn, yarn friction and tensile modulus are discussed. It is expected that the outcomes of this work would be useful to guide the design and fabrication of this type of auxetic yarn structure.

Macroscopic mechanical response of chiral-type cylindrical metastructures under axial compression loading

Chiral cellular structures inspired by the microstructure of biomaterials can display auxetic performances, such as negative Poisson's ratio (NPR), negative stiffness, energy dissipation, and acoustic absorption. In this study, novel chiral-type cylindrical shells were designed and fabricated via 3D printing method. Theoretical analysis, finite element analysis, and experiments were conducted to investigate the mechanical properties and deformation characteristics of cylindrical shell with various categories of chiral-type cells. Results revealed that the anti-chiral shell and chiral–axial shell can achieve auxetic behavior, namely, NPR behavior and compressive-twist response, which are beneficial for energy absorption and vibration isolation performance. Given the distinction in the geometrical configuration of unit cell, the cylindrical shells exhibited extremely diverse mechanical properties. The analytical formulae of axial compressive modulus were deduced based on Euler–Bernoulli beam theory, and the applicability and accuracy were verified. The new mechanical metastructures offer potential applications as smart actuators, biomechanical devices, and sensors in various industries.

Compressive property of Al-based auxetic lattice structures fabricated by 3-D printing combined with investment casting

Auxetic lattice structures are a new kind of structures that show unique mechanical performance, particularly negative Poisson's ratio behavior. In the present study, an Al-based auxetic lattice structure has been fabricated by 3-D printing and investment casting, and the compressive mechanical behavior was studied. It is shown that increasing the length of re-entrant struts or re-entrant angle leads to decreased compression strength but increased Poisson's ratio. If the relative density of the structures keeps constant, the elastic modulus and compression strength will be enhanced as the Poisson's ratio increases owing to the auxetic effect.

Auxetics and Other Systems of Anomalous Characteristics

Auxetics and Other Systems of Anomalous Characteristics