Search

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

We propose in this paper analytical and numerical models that describe the in-plane uniaxial elastic properties (Young's moduli and Poisson's ratios) of a honeycomb structure with curved walls. We perform a parametric analysis of the mechanical performance of this honeycomb also by taking into account the different types of deformations acting inside the cell walls. The curved wall honeycomb possesses higher magnitudes of the Poisson's ratio ν12 in the auxetic configuration compared to classical center symmetric configuration with straight cell wall. The presence of the curvature also allows creating configurations with positive Poisson's ratio even for negative internal cell angles, and makes this honeycomb design attractive for mechanical tailoring. [ABSTRACT FROM AUTHOR]

In this paper, we introduce methods of constructing structures with rigid sliding units that exhibit auxetic behavior, namely, have negative Poisson's ratio. First, we illustrate how we can construct 2-dimensional (2D) auxetic structures with bow-tie shaped rigid sliding hexagonal units. Next, we show that we can construct 3-dimensional (3D) auxetic structures by arranging hourglass shaped rigid sliding units in regular triangular configuration or in regular square configuration. Also, we show, through theoretical and numerical analysis, that these structures indeed exhibit auxetic behaviors. [ABSTRACT FROM AUTHOR]

Iron-gallium alloy (galfenol) is a body centered cubic (BCC) magnetostrictive alloy that, like many other BCC metals, is auxetic in the h110i{100} directions. For compositions of 12- 33 at.% gallium, Poisson ratio (n) values as low as 0.7 are observed along these directions in response to a uniaxial elastic force (tensile tests, resonant ultrasound spectroscopy, etc.). In 2014, Raghunath and Flatau first reported that galfenol also exhibits positive strain in both these directions when the uniaxial force applied along the h110i{100} directions is provided solely by a magnetic field, i.e., with no external mechanical/elastic force or stress being applied to the alloy [Raghunath and Flatau, IEEE Trans. Magn. 50(11), 1-4 (2014)]. The observed response is a result of the intrinsic, atomic-scale, bi-directionally coupled magnetoelastic properties of these alloys. In this paper, we advance the understanding of the intrinsic atomic-scale magnetoelastic coupling of the alloy that leads to the observed magnetoelastic auxetic-like behavior by presenting simulations developed based on density functional theory, which we show match the experimental findings and energy-based simulations. The elastic anisotropy and the electronic mechanisms that lead to magnetoelastic auxetic-like behavior in galfenol are discussed. [ABSTRACT FROM AUTHOR]

This paper reports a development of bi-stretch auxetic woven fabrics by using non-auxetic yarns and weaving technology. The fabric structure is first designed based on a re-entrant hexagonal geometry by combination of loose weave and tight weave in a woven fabric structure, and then fabricated on a rapier weaving machine by using both non-elastic and elastic yarns. Two variations of the fabric are produced by using different elastic yarn arrangement in weft direction. The designed auxetic geometry is finally achieved after a washing process due to creation of non-uniform contraction or shrinkage profiles within the fabric structural unit cell. The testing results showed that the developed fabrics exhibit negative Poisson ratio effect in both weft and warp directions in a large range of tensile strain. [ABSTRACT FROM AUTHOR]

This paper reports a new type of auxetic yarn made with circular braiding technology to overcome the yarn slippage problem in the conventional helical auxetic yarn structure. Various auxetic yarn structures are firstly produced on a vertical circular braiding machine with 16 yarn carries by changing the number and arrangement of stiff yarn as well as diameter of elastic yarn and core yarn, and then tested under single and repeating tensile conditions. The results show that not only yarn slippage problem could be overcome in the newly developed auxetic yarn structure, but also an early activation and a higher magnitude of negative Poisson's ratio could be achieved. The results also show that all structural parameters including stiff yarn number and arrangement, core yarn, and elastic yarn diameter have an influence on the auxetic effect of the novel yarn structure. Since the new auxetic yarn is produced by using standard braiding manufacturing technique and conventional non-auxetic yarns, there are few technological barriers for its largescale manufacture for practical application. [ABSTRACT FROM AUTHOR]

This paper reports quasi-static and low-kinetic energy impact testing of auxetic and conventional open-cell polyurethane foams. The auxetic foams were fabricated using the established thermo-mechanical process originally developed by Lakes. Converted foams were subject to compression along each dimension to 85% and 70% of the unconverted dimension during the conversion process, corresponding to linear compression ratios of 0.85 and 0.7, respectively. The 0.7 linear compression ratio foams were confirmed to have a re-entrant foam cell structure and to be auxetic. Impact tests were performed for kinetic energies up to 4 J using an instrumented drop rig and high speed video. A flat dropper was employed on isolated foams, and a hemispherical-shaped dropper on foams covered with a rigid polypropylene outer shell layer. The flat dropper tests provide data on the rate dependency of the Poisson's ratio in these foam test specimens. The foam Poisson's ratios were found to be unaffected by the strain rate for the impact energies considered here. Acceleration-time data are reported along with deformation images from the video footage. The auxetic samples displayed a six times reduction in peak acceleration, showing potential in impact protector devices such as shin or thigh protectors in sports equipment applications. [ABSTRACT FROM AUTHOR]

Auxetic fabrics have gained an increasing attention in the auxetic material area in recent years. Especially, auxetic fabrics fabricated based on both weft-knitted and warp-knitted structures have made some important progress. However, all the knitted auxetic fabrics reported so far are fabricated based on 2D knitted structures, although some of the weft-knitted auxetic fabrics are finally formed into 3D forms. This paper reports a novel kind of 3D auxetic fabric fabricated based on a warp-knitted spacer structure with adoption of a special geometrical configuration which is formed with parallelograms. The auxetic behavior of these fabrics is discussed in terms of tensile direction, geometrical parameter, and repeated extension. The shape fitting ability on a spherical surface is also demonstrated. The results shows that the 3D auxetic fabrics have auxetic behavior in all the fabric plane directions and the highest auxetic effect is obtained when stretched in the weft direction of the fabric. The 3D auxetic fabrics also display an excellent shape fitting ability, which makes them very attractive for various applications where fitting to the human body's shape is highly required, such as bra cups, protectors for knees and elbows, shoes covers, etc. [ABSTRACT FROM AUTHOR]

Chiral systems are a class of structures, which may exhibit the anomalous property of a negative Poisson's ratio. Proposed by Wojciechowski and implemented later by Lakes, these structures have aroused interest due to their remarkable mechanical properties and numerous potential applications. In view of this, this paper investigates the on-axis mechanical properties of the general forms of the flexing anti-tetrachiral system through analytical and finite element models. The results suggest that these are highly dependent on the geometry (the ratio of ligament lengths, thicknesses, and radius of nodes) and material properties of the constituent materials. We also show that the rigidity of an anti-tetrachiral system can be changed without altering the Poisson's ratio. The anti-tetrachiral system, with the unit cell shown in red. [ABSTRACT FROM AUTHOR]

Two-parameter analysis of auxetics among the cubic crystals is proposed. A brief analysis of the equivalence of this two-parameter consideration and other approaches is given. The main result of this paper is the classification of partial auxetics with a single dimensionless complex, which is composed of the crystals elastic compliances. The auxetic surface separates the regions with negative and positive Poisson's ratio. The character of its changes with change of the dimensionless complex is determined. The critical value of the complex where a topological rearrangement of the auxetic surface occurs is obtained. The distribution of partial auxetics in zones with different values of the dimensionless complex was found. The sign of another dimensionless parameter influences the location of a region with negative Poisson's ratio relative to the auxetic surface. View of the auxetic boundary for a cubic crystal when the dimensionless elastic parameter is close to the critical value [ABSTRACT FROM AUTHOR]

The design, manufacturing and characterization of warp knit textile structures with enhanced drapeability and energy absorption is reported in this paper. Four textile structures were produced, all based on a triangular or double arrowhead structure, which is known to lead to a negative Poisson's ratio ν. Mechanical testing has confirmed that textile structures can be produced which are auxetic at ± 45° to the warp direction, with ν of up to −0.22 ± 0.03. [ABSTRACT FROM AUTHOR]

A new method for the production of thin flat and curved auxetic foam sheets by uniaxial compression is reportted in this paper. Detailed optical microscopy and Poisson's ratio measurement by videoextensometry has revealed that the auxetic behaviour is due to a crumpling of the pores through the foam thickness, caused by a uniaxial compression of 40-60% in combination with shear for the curved foams. Poisson's ratio values of up to −3 have been recorded for the curved foams and −0.3 for the flat sheets. [ABSTRACT FROM AUTHOR]

The utility of auxetic materials in applications such as foams and composites is contingent upon the performance of these materials under cyclic loading. Recently, several textile materials (knitted, woven, and nonwoven fabrics) have been found to exhibit auxetic properties, either through design and/or the use of a postprocessing treatment. In the wake of these advancements, this research examines the reversibility of the out‐of‐plane auxetic response in previously studied polyester fiber needle‐punched nonwovens. Herein, a heat‐compressed nonwoven is strained cyclically to different levels, and the thickness is measured before and after straining. While axial extension is found to be elastic for up to 3% strain, thickness increases (auxetic responses) are almost completely reversible for 1% axial strain and highly reversible for 2% and 3% axial strain. Irreversible changes can be clearly observed starting at strains of 2% and larger, especially for a higher number of cycles. Combining these results with the previous understanding from microscopic and tomographic imaging, possible reversible (straightening of bent fibers and fiber bundles) and irreversible (slipping or breaking of fibers) structural deformations leading to thickness changes are identified. These processes influence the magnitude of the auxetic response within and beyond the elastic strain regime, respectively. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations in the isobaric-isothermal ensemble with variable shape of the periodic box are used to study elastic properties of two-dimensional (2D) model systems of hard discs with parallel layers of hard cyclic hexamers. Both the particles in their pure phases form elastically isotropic crystals. However, the crystal of discs shows positive and that of hexamers - negative Poisson's ratio (PR). The studied systems with layers are of low symmetry. Hence, a general analytic formula for the orientational dependence of PR in 2D systems is applied to such systems. It is shown that, by changing parameters of the particles and their relative concentration, as well as orientation of the layers, one can obtain systems which are non-auxetic (their PR is positive), auxetic (their PR is negative), or partially auxetic ones (their PR is negative for some directions and positive for other ones). If the disks and hexamers are interpreted as small molecules, the obtained results indicate that by introducing auxetic nano-layers one can strongly modify PR of crystals or even thin layers. The periodic box with N = 224 particles can be thought of as a model of a thin layer or as a model crystalline structure with parallel horizontal layers built of four hard cyclic hexamer rows in the hard disc system. [ABSTRACT FROM AUTHOR]

We have examined the effect of temperature, pressure, and time on the magnitude of induced auxetic response in postmanufacture- processed needle-punched nonwovens. Different combinations of these processing parameters - temperatures ranging from room temperature to above the glass transition temperature (Tg) of constituent fibers, pressure of 0.49 or 2.45 MPa, and duration of 4 or 24 h - were used to process these nonwovens. This resulted in some permanent compression (defined as "compression set") and in out-of-plane auxetic behavior of the fabric, both of which were found to be proportional to the severity of the processing conditions employed. Milder processing conditions were found to be insufficient in producing significant compression set and hence produced little or no auxetic response. Using micro-CT and optical video microscopy techniques, it was found that the processing conditions caused the fiber bundles present in needle-punched nonwovens to assume a buckled/tilted state, which opened up during mechanical extension and hence were instrumental in the observed thickness increase. Out-of-plane Poisson's ratio with strain for as-received and treated nonwovens. [ABSTRACT FROM AUTHOR]

This paper describes the fabrication of polymeric auxetic foam using a quasi tri‐axial compression method. In this method, the conversion process is executed by applying a quasi tri‐axial compression on conventional closed‐cell polyurethane foams. In order to achieve optimum auxetic characteristics, the densification points in various directions are evaluated. In addition, the heating time of the compressed foam is also determined and applied to minimize the residual stress due to the volumetric compression. It leads to the fabrication of auxetic foams with achievable maximum re‐entrancy and minimum collapsed cells without surface creasing. The results also show that the optimized auxetic foam exhibits isotropic behavior compared to its pre‐cursor foam as verified by the morphological analysis. The produced foam has all the desired properties of auxetic foams. The auxetic foam with a negative Poisson's ratio may be have great potential as the supplementary materials in energy absorption and structural applications. [ABSTRACT FROM AUTHOR]

We describe here a heat-compression protocol used on needle-punched commercial nonwoven fabrics which induces an out-of-plane auxetic response in these treated fabrics. The as-received samples, however, are not auxetic and show a decrease in thickness when stretched uniaxially. Using micro-CT imaging it was found that the vertical fiber bundles/columns, produced in the needle punching step of fabric manufacture, are tilted and buckled as a result of the heat-compression treatment. It is suggested that it is likely the reorientation of these columns during subsequent uniaxial strain that drives thickness increase (auxetic response). Variation of instantaneous Poisson's ratio with axial strain for as-received and treated nonwovens. [ABSTRACT FROM AUTHOR]

Two-dimensional dilational materials, for which the only easy mode of deformation is dilation are reviewed and connections are drawn between models previously proposed in the literature. Some models, which appear to be dilational materials, but which in fact are not, are also discussed. Finally, four new examples of three-dimensional dilational materials are given. [ABSTRACT FROM AUTHOR]

Elastic properties of the face centered cubic (fcc) phase of particles, interacting through hard-core repulsive Yukawa potential are determined by Monte Carlo (MC) simulations in the NpT ensemble with variable shape of the periodic box. The effects of the Debye screening length ( [ABSTRACT FROM AUTHOR]

Elastic properties of fcc phases of soft polydisperse spheres, interacting through inverse-power pair potentials [ABSTRACT FROM AUTHOR]

Buckling is one type of unstable structural response which has been known for centuries. Although buckling is generally regarded as a type of undesirable phenomenon, many investigations reported that it can be utilized to generate buckling‐induced metamaterials and structures with negative Poisson's ratio. Recent studies demonstrate that buckling‐induced auxetic behavior will disappear when the base material is changed from elastomer to metal. A pattern scale factor (PSF) method to recover auxeticity of metallic metamaterials and structures was mentioned in previous studies, but the method has not been specifically introduced. Herein, a more detailed introduction of the PSF method is made and some successful case studies are presented. [ABSTRACT FROM AUTHOR]

Five transverse rod (TR)‐containing main‐chain liquid crystalline polymers (LCPs) are examined in both the powder and fiber forms by wide angle X‐ray diffraction. Data from the diffraction experiments are consistent with the site‐connectivity‐driven TR‐reorientation mechanism for intrinsic auxetic character in these macromolecules. Shifts in peak maxima, intensity distributions in the interchain interaction region, and calculated volume increase accompanying fiber formation are all in line with expectations from considerations of this mechanism. The narrow window for experimental conditions necessary for fiber preparation and subsequent X‐ray observation of this phenomenon is detailed. A close examination of space‐filling Corey–Pauling–Koltun (CPK) molecular models reveals the details of local packing of rods and connected polymer chains in the quiescent nematic polymer melt prior to fiber drawing. It is found that two rod‐reorientation possibilities exist each of which leads to increased interchain separation as required for auxetic response. Suggestions for future experiments and materials design for new LCPs are described. [ABSTRACT FROM AUTHOR]

Elastic properties and Poisson's ratio of the face centered cubic (fcc) phases of three systems of particles which interact, respectively, through: 1) hard sphere potential (HSP), 2) repulsive Yukawa potential (RYP), and 3) hard‐core repulsive Yukawa potential (HCRYP), studied by Monte Carlo (MC) simulations in the NpT ensemble, are compared in a broad range of pressures between melting and close packing. Partial auxeticity is observed in each of the considered systems and the role of the RYP and HSP, incorporated in HCRYP, is revealed. This study confirms intuitive expectations that the elastic properties of the system with particles interacting through HCRYP at high pressures are well approximated by hard spheres, whereas in the low pressures regime they are close to those obtained for the system with pure Yukawa interactions. It is also demonstrated that in the "transition region", when the HCRYP system is neither close to RYP nor to HSP system, its Poisson's ratio in the directions [110][11¯0] exceeds the ranges given by RYP and HSP for those directions. [ABSTRACT FROM AUTHOR]

Honeycombs made from more than one material are proposed and analyzed with the help of finite element simulations. It is shown that such honeycombs are designed to change their shape when subjected to a change in temperature or pressure and can be tailor‐made to exhibit particular thermomechanical properties. It is also shown that the anomalous properties of negative thermal expansion/compressibility can be manifested from honeycombs built from common everyday conventional materials. Applications of such constructs are discussed including their role in the manufacture of smart grips which tighten upon heating. [ABSTRACT FROM AUTHOR]

Honeycombs or foams with reentrant microstructures exhibit effective negative Poisson's ratio. Although they are light weight due to inherently empty space, their overall stiffness and damping are somewhat limited. With judiciously chosen filler material to fill the voids in star‐shaped honeycomb, it is numerically demonstrated its auxeticity may be enhanced. By combining the filler and skeleton, the hierarchical composite materials are constructed. The magnitude of the enhancement depends on inner and outer filler's modulus mismatch, as well as the types of filling. Filler's auxeticity also largely enhances overall auxeticity of the outer‐ and all‐filled honeycomb. In addition, for outer‐filled honeycomb, its effective viscoelastic modulus and damping are significantly increased, while maintaining relatively light weight, due to local stress concentration. [ABSTRACT FROM AUTHOR]

A chiral 3D lattice is designed, made by 3D printing, and studied experimentally. The lattice exhibits squeeze–twist coupling and a Poisson's ratio near zero. Squeeze–twist coupling does not occur in classical elasticity which makes no provision for chirality. By contrast, chiral effects are allowed in Cosserat elasticity. An experimental squeeze–twist coupling strain ratio on the order of unity and a Poisson's ratio near zero are in reasonable agreement with prior finite element analysis of a lattice with similar structure, for which negative Poisson's ratio is anticipated for a sufficient number of cells. [ABSTRACT FROM AUTHOR]

A 3D lattice structure consisting of pivoting cubes with flexible links at the corners is presented. It exhibits anisotropic negative Poisson's ratio -0.54 and -0.75. Size effects occur in torsion and bending; these effects are consistent with Cosserat elasticity but not with classical elasticity. Cosserat elastic solids exhibit sensitivity to strain gradients; size effects occur in torsion and bending; also reduction in stress concentrations. The observed Cosserat effects also reveal anisotropy. [ABSTRACT FROM AUTHOR]

Perforated systems constitute one of the most important classes of mechanical metamaterials. In this work, two types of "I"-shaped slit perforation patterns are proposed which may be used to design perforated systems that mimic the deformation mechanisms of a variety of re-entrant and anti-tetrachiral honeycomb systems. Using finite element analysis, it is shown how these systems have the potential to exhibit a large spectrum of negative Poisson's ratios, ranging from extremely negative to zero, which are retained over a wide tensile strain range. A detailed analysis of the deformation behavior of these systems is also presented along with a comparison of the changes in overall expansion and Poisson's ratios of both systems observed upon loading with those predicted by previously formulated theoretical models of re-entrant and anti-tetrachiral systems. It is hoped that this work will be of considerable aid in the efforts of scientists to understand the underlying principles governing the production of auxetic mechanical metamaterials through the use of perforations and also stimulate further research on how these and similar mechanisms may be implemented in perforated systems to design other metamaterials with anomalous mechanical properties. [ABSTRACT FROM AUTHOR]

Poly(phenylacetylene) sheets that mimic the geometry of a wine-rack-like structure have been predicted to exhibit negative Poisson's ratios off-axis. However, their potential to exhibit negative linear compressibility (NLC) has remained largely unexplored. In this work, the compressibility and other mechanical properties of wine-rack-like poly(phenylacetylene) networks with 1,2,4,5 tetra-substituted phenyls as well as their equivalent with allene or cyclobutadiene centres are simulated to assess their ability to exhibit negative linear compressibility on-axis and off-axis. It is shown that some of these systems can indeed exhibit negative linear compressibility whilst others exhibit a near-zero compressibility. The results are compared to the compressibility properties of other poly(phenylacetylene) networks reported in literature as well as with those predicted from the analytical model for an idealised wine-rack structure deforming through hinging. Results suggest that these mechanical properties are arising from a wine-rack-like mechanism, and there is a good agreement with the theoretical model, especially for systems with longer acetylene chains whose geometry is closer to that of the idealised wine-rack. [ABSTRACT FROM AUTHOR]

We utilized molecular statics (MS) simulations to investigate the auxeticity of single layer black phosphorus (SLBP). Our simulation results show that the SLBP has a negative in-plane Poisson's ratio in the zigzag direction when the applied strain along the armchair direction exceeds 0.018. We show that the interplay between bond stretching and bond rotating modes determines the in-plane Poisson's ratio behavior. While the bond stretching mode always tends to increase the in-plane auxeticity, the bond rotating mode might increase or decrease the in-plane auxeticity. Furthermore, we show that graphite also exhibits an in-plane negative Poisson's ratio at finite strains due to a similar mechanism. [ABSTRACT FROM AUTHOR]

Microstructured plates, consisting of various conventional and re-entrant cells, are numerically constructed and analyzed for their effective elastic properties under in-plane deformation. The finite element numerical method is adopted. The calculated effective Poisson's ratios of the plates are found to be in the range between −1 and 1, in consistency with the theory of two-dimensional elasticity. Auxetic angles need to be greater than about 20° in order to obtain negative Poisson's ratio. Increasing the auxetic angles reduces the effective pure shear modulus. Elastically anisotropic characteristics of the homogenized plate are analyzed with the calculated effective Young's modulus, Poisson's ratio, and pure shear modulus. [ABSTRACT FROM AUTHOR]

Although high heels are known to affect the biomechanics of human movement, most notably by increasing forefoot plantar pressure, it is commonly worn due to the influence of fashion. To deal with the resulting clinical issues, some prescriptions include the use of cushioning pads to redistribute the plantar loads and increase comfort. This preliminary study has sought to investigate whether auxetic foam would be a good candidate for the redistribution of plantar loafing in the forefoot when compared to a more traditional material. From the results obtained, it was shown that auxetic foam demonstrated a decrease in forefoot peak plantar pressure while a commercial product, a material traditionally used for plantar cover paddings, increased pressure in the region of the second-fourth Metatarsophalangeal Joint. This demonstrates the potential for auxetic materials to become the material of choice for such applications, especially since it can be further remodeled until its properties can produce a significant forefoot pressure reduction. [ABSTRACT FROM AUTHOR]

The microstructure of the manmade auxetic materials always involve inherited porosity and sharp corners, where the stress concentration occurred. We have proposed a novel kind of non-porous smooth curve sheet which exhibits a negative Poisson's ratio and has a low stress concentration factor. The auxetic response was validated by good qualitative and quantitative agreements between the tensile experiments and numerical analysis of array model. Using the experimentally validated finite element model, the parametric study on the evolution of the macroscopic Poisson's ratio, the stress concentration factor on the geometry parameters and the parental material properties were investigated. This one phase auxetic smooth curve sheet, which is simple to manufacture and assemble into 3D structures, may be used in a variety of industrial applications. [ABSTRACT FROM AUTHOR]

This article describes the rules and procedures for designing a three-dimensional (3D) antitetrachiral structure that achieves a negative Poisson's ratio. Numerical analysis and experimental validation showed good agreement between the deformations and Poisson's ratios. Moreover, the proposed auxetic structure displayed anisotropic behavior in x- and y-directional compression as well as isotropic behavior in z-directional compression. The parametric study and design examples presented in this article demonstrate the relationships among the Poisson's ratio, effective Young's modulus, porosity, and geometry of the proposed structure, and provide insights into the potential applications for future auxetic designs. [ABSTRACT FROM AUTHOR]

Auxetic composites have attracted considerable attention in recent years and have demonstrated a high potential of applications in different areas due to their wonderful properties as compared to non-auxetic composites. In this study, a threedimensional (3D) auxetic textile structure previously developed was used as reinforcement to fabricate auxetic composites with conventional polyurethane (PU) foam. Both the deformation behavior and mechanical properties of the auxetic composites under compression were analyzed and compared with those of the pure PU foam and non-auxetic composites made with the same materials and structural parameters but with different yarn arrangement. The results show that the negative Poisson's ratio of composites can be obtained when suitable yarn arrangement in a 3D textile structure is adopted. The results also show that the auxetic composites and non-auxetic composites have different mechanical behaviors due to different yarn arrangements in 3D textile structure. While the auxetic composites behave more like damping material with lower compression stress, the nonauxetic composite behaves more like stiffer material with higher compression stress. It is expected that this study could pave a way to the development of innovative 3D auxetic textile composites for different potential applications such as impact protection. [ABSTRACT FROM AUTHOR]

This report employed molecular statics simulation and densityfunctional- theory calculation to study the Poisson's ratios of face-centered-cubic materials. We provide numerical and theoretical evidences to show that cubic materials can exhibit auxetic behavior in a principal direction under proper loading conditions. When a stress perpendicular to the loading direction is applied, cubic materials can exhibit a negative Poisson's ratio at finite strain. The negative Poisson's ratio behavior, including its direction and value, is highly dependent on the direction and magnitude of the transversely applied stresses. As a result, we show that it is possible to tune the direction and magnitude of the negative Poisson's ratio behavior of cubic materials by controlling the transverse loadings. [ABSTRACT FROM AUTHOR]

Hard-body models reproduce many fundamental features of real physical systems and are convenient tools to study the role of molecular shape and its influence on macroscopic, for example, elastic, properties of condensed matter. In this article, a degenerate (aperiodic) crystal of hard dimers with arrays of nanochannels introduced in the direction [001] is studied by Monte Carlo simulations. The nanochannels are filled by hard spheres of diameter s' which can be different from the diameter s of spheres forming dimers. It is shown that, by modifying the ratio s'/s, one can qualitatively modify the Poisson's ratio of the system. In particular, one can obtain partial auxetics (i.e., systems with negative Poisson's ratio in some directions) with auxetic directions [110] or [111]. The applied method is convenient to determine Poisson's ratio in all directions for crystals of arbitrary symmetry. Explicit formulae for Poisson's ratio in main directions of the studied structures are given. Structure of hard dimers (green atoms) with an array of nanochannels (red atoms). Diameters of dimer atoms have been intentionally decreased to reveal underlying molecular structure (black segments) and channel orientation. [ABSTRACT FROM AUTHOR]

We conducted molecular statics simulations to investigate the negative Poisson's ratio (auxetic behavior) of periodic porous graphene structures based on the rotating rigid unit mechanism. To obtain a negative Poisson's ratio, simple voids were periodically introduced into graphene. We showed that the Poisson's ratio of the designed graphene structure is strongly dependent on the aspect ratio of the voids, and it can approach the theoretical limit of -1.0. More importantly, the graphene periodic structure maintains its auxetic behavior even under large strains (ε~0.20). Hence, it can be employed in a wide range of applications requiring structures that can endure large deformation. In addition, we found that the key factor in the auxeticity of the investigated structures is the deformation occurring at the void tips. [ABSTRACT FROM AUTHOR]

Auxetic polyproplyene fibres have been produced on a largescale industrial extruder for the first time. A first batch of tests identified a coarse processing window which was then more closely defined by subsequent tests. The effects of barrel temperature, screw speed, take-up speed and quenching (through the parameters of air gap and bath temperature) on the Poisson's ratio and Young's modulus of the fibres were examined. It was found that a temperature of 200 °C, screw speed of 12.5 rpm and take-up speed of either 1.5 or 3.5 rpm produced fibres with a high degree of auxeticity up to 5% strain - significantly increasing the auxetic strain range previously reported for lab-scale extrusions at lower extruder temperatures. Quenching with a water bath reduced the auxeticity, so was not employed as a processing parameter. [ABSTRACT FROM AUTHOR]

Chiral three-dimensional isotropic cubic lattices with rigid cubical nodules and multiple deformable ribs are developed and analyzed via finite element analysis. The lattices exhibit geometrydependent Poisson's ratio that can be tuned to negative values. Poisson's ratio decreases from positive to negative values as the number of cells increases. Isotropy is obtained by adjustment of aspect ratio. The lattices exhibit significant size effects. Such a phenomenon cannot occur in a classical elastic continuum but it can occur in a Cosserat solid. [ABSTRACT FROM AUTHOR]

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

Analytical model derivations for predictions of the on-axis Poisson's ratios of the 'α-cristobalite' framework of corner-sharing tetrahedra are presented in full, for the first time, for a previously reported second tetrahedral rotation mechanism. Parametric analyses are performed for each of the three concurrent models (two 2-mode models where dilation acts with each rotation mechanism (the CTM1 and CTM2 models), and the model where all three mechanisms operate (CTM3)) to explore the Poisson's ratio dependency on the key model geometrical and deformation strength parameters. Predictions in good agreement with all experimental on-axis Poisson's ratios specifically for silica α-cristobalite are possible for the new CTM3 model comprising two tetrahedral rotation mechanisms acting concurrently with tetrahedral dilation when direction-dependent strength parameters are employed. This is a significant improvement over the previous analytical model incorporating the tetrahedral dilation and first tetrahedral rotation mechanisms and which only achieved good agreement for loading along the longitudinal x3 axis. [ABSTRACT FROM AUTHOR]

Special soliton analytical solutions are seeked for the so-called double dispersion equation (or Porubov's equation), which describes the propagation of longitudinal waves in an elastic rod. Using the F-expansion method, a new interesting class of traveling solitary waves is obtained. As a byproduct, the results obtained by other authors for some special cases are regained. Some numerical simulations are also performed. Splitting of various initial pulses during propagation into a sequence of solitary waves is considered. The dependence of the amplitude and the velocity of the solitary waves on Poisson's ratio is discussed in detail. Collisions between some obtained solitary waves are also presented. The simulation results are compared with the obtained exact analytical solutions-the latter reflect the perfect balance between the nonlinearity and dispersion. The comparison indicates that the obtained exact solutions are useful for describing more complicated wave fields studied by numerical simulations. It also indicates that the numerical results for auxetic materials are in better agreement with theoretical solutions than in the case of ordinary materials. Furthermore, one can conclude that the Secant pulse generates solitary waves closer to the analytical predictions than a Gaussian pulse. This is similar to the case of variational method, where the Secant trial function is also more proper than the Gaussian one. [ABSTRACT FROM AUTHOR]

Auxetics, i.e. systems with a negative Poisson's ratio, exhibit the unexpected property of becoming wider when stretched and narrower when compressed. This property arises from the manner in which the internal geometric units within the system deform when the system is submitted to a stress and may be explained in terms of 'geometry-deformation mechanism' based models. This work considers realistic finite implementations of the well known rotating squares system in the form of (i) a finite planar structure and (ii) a tubular conformation, as one typically finds in stents. It shows that although the existing models of the Poisson's ratios and moduli based on periodic systems may be appropriate to model systems where the geometry/deformation mechanism operate at the micro- or nano- (molecular) level where a system may be considered as a quasi infinite system, corrections to the model may need to be made when one considers finite structures with a small number of repeat units and suggests that for finite systems, especially for the 2D systems, the moduli as predicted by the periodic model may be significantly overestimating the moduli of the real system, even sometimes by as much as 200%. [ABSTRACT FROM AUTHOR]

Using Finite Element computer simulations, Poisson's ratio (PR) is determined for anti-chiral structures built on rectangular lattices with disorder introduced by stochastic distributions of circular node sizes. The investigated models are parameterized by the lattice anisotropy, the rib thickness, and the radii distribution of circular nodes. Three approaches are developed. The first approach, exact in the limit of infinitely large system and infinitely dense mesh, uses only planar elements (CPS3). Two other approaches are approximate and exploit one-dimensional elements utilizing the Timoshenko beam theory. It is shown that in the case of sufficiently large anisotropy of the studied structures PR can be highly negative, reaching any negative value, including those lower than [ABSTRACT FROM AUTHOR]

Auxetics exhibit the unusual property of a negative Poisson's ratio, meaning they get wider rather than thinner when stretched. This work employs force-field based simulations to study the effect of solvent molecules on the structure and mechanical properties of porous polyphenylacetylene auxetic organic networked polymers. In particular, nano-level re-entrant honeycombs 1,4- and 2,8-reflexyne were exposed to three types of solvent molecules (ethanol, propanol and benzene) at different concentrations by performing Monte-Carlo based sorption simulations in two scenarios, one where the solvent was permitted into the pores of the system upon exposure, and one were the solvent could also enter in between the layers of the polymer. Results indicate that the presence of solvent molecules within the molecular framework of the networked polymers has a significant effect on their density and stiffness but does not influence the Poisson's ratio of the material. The effect on the stiffness was found to be highly dependent on various factors, including the properties of the network and the type and amount of solvent adsorbed. These results have important implications in future case scenarios which may involve the synthesis of such systems, and it throws light on how the mechanical properties could be affected due to contamination, while also identifying new methods on how to tailor such properties by exposing the material to specific solvents. [ABSTRACT FROM AUTHOR]

Materials exhibiting auxetic behaviour get fatter when stretched (i.e. possess a negative Poisson's ratio). This property has been closely related to particular geometrical features of a system and how it deforms. One of the mechanisms which is known to have a potential to generate such behaviour is that of rotating rigid units. Several models based on this concept have been developed, including two-dimensional as well as three-dimensional (3D) models. In this work, we propose a new 3D structure constructed from rigid cuboids which also deform through relative rotation of the units. In particular, analytical models for the mechanical properties, namely the Poisson's ratio and the Young's moduli, are derived and it is shown that for loading on-axis, these systems have the potential to exhibit negative values for all the six on-axis Poisson's ratios. [ABSTRACT FROM AUTHOR]

A number of basic features of the mechanical properties of a cubic symmetry solid material are identified and discussed within the framework of the XY-plane approach, where X = G/ K, Y = G/ W, K is the bulk modulus and G, W are shear moduli. An anisotropic part of the Debye temperature is analysed, which was found to be weakly dependent on X and is mainly determined by Y which quantifies the material elastic anisotropy. The anisotropic part has a considerable effect on the Debye temperature of partially auxetic materials. An empirical ductility-brittleness criterion is considered, which indicates that many auxetics may deform in a brittle manner. A possible correlation between directional tensile deformation and the surface of the most extreme Poisson's ratio (PR) values is suggested. Finally, the reasons for the paucity of cubic materials in the part of the XY-plane with low Y values is investigated using static microscopic models in which the particles interact with nearest and next nearest neighbours only. An example of a type of microscopic model system located in this 'empty' region has been found, which consists of a system under tension, where the interparticle interaction decays with distance in an oscillatory way. The results obtained suggest that a rather unconventional form of the interparticle interaction set and thermodynamic conditions may therefore be required to develop a solid material with these mechanical properties. [ABSTRACT FROM AUTHOR]

An investigation of Poisson's ratios and band gap behaviors of 2D hierarchical re‐entrant lattice structures is conducted using finite element method (FEM). The structure with a hierarchy order n (n≥1) is constructed by replacing each outmost vertex of the re‐entrant octagons of a hierarchical structure of hierarchy order n−1 with a smaller self‐similar re‐entrant octagon. The dispersion relation and transmission spectrum of the proposed hierarchical structures are analyzed based on the Bloch's theorem. The effects of geometrical parameters, order of rotational symmetry and orientation angle of some types of re‐entrant polygon cores on the lattice Poisson's ratio and band gap structures are also investigated. Results show that the re‐entrant structures with first order hierarchy exhibit a wider band gap and a stronger attenuation effect compared with structures without structural hierarchy. The first order 2D hierarchical re‐entrant structure with a negative Poisson's ratio of −0.032 exhibits the widest band gap, 17.8% wider than that of the zeroth order 2D re‐entrant structure. The order of rotational symmetry and the orientation angle of the re‐entrant polygon cores of the lattice have a strong impact on the lattice Poisson's ratio and band structures. [ABSTRACT FROM AUTHOR]