Search

This is the fifteenth volume of I Physica Status Solidi B i focussed on materials and models showing negative Poisson's ratio (PR), which are often called auxetics, and other systems of unusual, e.g., "negative", characteristics. SP [ sp [1] SP ] sp This time it contains 19 papers which are grouped into three sections. This non-classical effect cannot occur in classical thermo-elasticity but is allowable in Cosserat solids. SP [ sp [17] SP ] sp Next, I Elena Pasternak and Arcady Dyskin i propose a mechanism to control the constriction of elastic rod modulus by lateral compression. This, amongst other things, opens a way to design materials ensuring vibration isolation. SP [ sp [18] SP ] sp I Haifeng Ruan and Dong Li i propose a hierarchical acoustic metamaterial with a spider-web-like structure. [Extracted from the article]

The paper discusses the problem of recovery of the microstructure of the least compliant bodies of non-homogeneous optimal isotropic properties predicted by the Isotropic Material Design method. The three-dimensional microstructure is assumed as constructed by a subsequent lamination process in which two isotropic materials of ordered properties are used, the process being repeated three or six times, subsequently. The Hooke tensors closest to the optimal ones are found by making use of the analytical Francfort--Murat formulae for effective moduli of third and sixth rank sequential laminates. The ability to model an auxetic behavior within the subdomains where the optimal Poisson's ratio assumes negative values is also shown. [ABSTRACT FROM AUTHOR]

Helical arrangements are very common both in nature and in artificial structures. In this paper, an analysis of a prismatic composite bar is presented. The cross-sectional shape of the bar is described by a regular polygon. The composite is built of two homogeneous isotropic materials characterised by different Poisson's ratios one of which can be negative. Spatial distribution of materials in the composite is based on helical shape. Both materials are connected with the use of a transition zone of assumed width. The bar is subjected to torsional loading and its behavior is investigated by means of FEM method. Both material properties and the parameters of helical shape are investigated and the result of numerical analysis is presented. [ABSTRACT FROM AUTHOR]

The present paper concerns two methods: IMD (isotropic material design) and YMD (Young modulus material design) concerning Pareto optimal distributions of the material and its isotropic properties. The merit function is the weighted sum of compliances corresponding to n independent loading conditions. The unit cost of the design is assumed as equal to the trace of the elastic moduli tensor C. In the IMD method, the design variables are the bulk and shear moduli. In the YMD method, the Poisson ratio is assumed as given, possibly non-uniformly distributed in the design domain, the Young modulus being the only design variable. Both the methods reduce to the auxiliary minimization problem involving statically admissible stress fields corresponding to the subsequent loading conditions. The integrand is expressed by a norm of the collection of these stresses, hence has linear growth. The numerical method proposed refers to the primal formulation: it is based on piece-wise polynomial approximation of the set of statically admissible stresses. The exemplary Pareto optimal solutions show that admitting negative values of Poisson's ratio may contribute to a decrease of the total compliances. [ABSTRACT FROM AUTHOR]

The contact problem between two bodies: plate and semi-cylindrical bar is studied in this paper. The analysis concerns the contact pressure distribution along the contact region, as well as the deflection of the plate. Furthermore, the case of a composite plate covered with the auxetic layer is also considered. The influence of the Poisson's ratio of the layer on the behavior of the plate is studied. The contact pressure distribution along the contact area is analyzed using finite element analysis. [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]

An introduction is presented in which the editor discusses various reports within the issue on topics including the Poisson's ratio, auxetics and textile fabrics.

An introduction is presented in which the editor discusses various reports within the issue including the auxetics, conventional foams, and negative Poisson's ratio.

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 presents an experimental study of tensile and deformation behavior of an auxetic plied yarn structure. The basic configuration of the plied structure formed with two soft yarns and two stiff yarns was firstly introduced. Based on this 4-ply helix structure, six kinds of yarn samples were made by different combinations of soft yarns, stiff yarns and twist levels. These yarn samples were first tested and the effects of different design parameters were analyzed. Apart from the tensile test, changes in the internal structure of two samples upon stretching were also studied through microscopic examination. Finally, double helix yarn and 6-ply auxetic yarn were made for a comparative purpose to investigate the effect of helical structure. The study shows that all of the 4-ply auxetic yarn samples exhibit negative Poisson's ratio (NPR) throughout the entire tensile process, and the magnitude of NPR gets larger with a smaller soft yarn diameter, a higher tensile modulus of stiff yarn and a lower twist level. The study also shows that auxetic behavior inside the 4-ply structure is generated by the interplay between the soft yarns and stiff yarns and 4-ply auxetic yarns have the advantages to produce immediate auxetic effect upon stretching. [ABSTRACT FROM AUTHOR]

In this paper, a novel three-dimensional (3D) cellular structure with negative Poisson's ratio was designed by alternating cuboid surface indents on the vertical ribs of the unit cells. The Poisson's ratio and Young's modulus of structures with different geometric parameters were determined using the finite element method (FEM) as a function of these parameters. Samples with identical geometric variables were fabricated via 3D printing, and their through-thickness direction Poisson's ratios were measured and compared with simulation results. Results showed that the Poisson's ratio of the 3D cellular structures can be tuned from positive to negative and can reach a minimal value of −0.958. Good agreement was found between the experimental results and the simulation. This lattice structure is considerably stiffer than re-entrant negative Poisson's ratio foam with the same solid phase. The design concept developed here can be optimized for specific applications via geometric parameters manipulation. [ABSTRACT FROM AUTHOR]

This paper proposes an in-plane negative area compressibility (NAC) material not due to Lifshitz mode, but due to all around in-plane pressure. The NAC system consists of an array of two-dimensional material in the form of n-sided polygon with n/2 number of connecting rods, where n is an even number. Two types of NAC microstructural geometries are introduced herein: 8-sided polygon in square array and 6-sided polygon in hexagonal array. The polygons are permitted to change in shape - and hence change in area - at constant side lengths with accompanying translational motion of the connecting rods, thereby changing the overall area of the cell. Results reveal that negative compressibility is observed in the square array NAC when the material shape is in between a regular octagon and a square, and in the hexagonal array NAC when the material shape is in between a regular hexagon and an equilateral triangle. [ABSTRACT FROM AUTHOR]

A table of contents for the periodical is presented.

In the present paper, density functional theory (DFT) based first-principles methods are applied to investigate the mechanical and bonding properties of the newly synthesized T2 phase superconductor Ta5GeB2 for the first time. The calculated lattice constants are in reasonable agreement with the experiment. The elastic constants ( Cij), bulk modulus ( B), shear modulus ( G), Young's modulus ( Y), Poisson's ratio ( v), Pugh ratio ( G/ B), and elastic anisotropy factor A of Ta5GeB2 are calculated and used to explore the mechanical behavior of the compound. To give an explanation of the bonding nature of this new ternary tetragonal system, the band structure, density of states, and Mulliken atomic population are investigated. The estimated Debye temperature and Vickers hardness are also used to justify both the mechanical and bonding properties of Ta5GeB2. [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]

Auxetics are a modern class of material fabricated by altering the material microstructure. Unlike conventional materials, auxetics exhibit a negative Poisson's ratio when subjected to tensile loading. These materials have gained popularity within the research community because of their enhanced properties, such as density, stiffness, fracture toughness and dampening. This paper provides a critical oversight of the auxetic field with particular emphasis to the auxetic foams, due to their low price, easy availability and desirable mechanical properties. Key areas discussed include the fabrication method, the effects played by different parameters (temperature, heating time, cell shape and size and volumetric compression ratio), microstructural models, mechanical properties and potential applications. [ABSTRACT FROM AUTHOR]

The subject of thermal expansion, particularly negative thermal expansion, is a topic of great practical importance in view of its relevance in many everyday applications and the consequences which may arise when the thermal expansion if not controlled. This paper presents a detailed analytical model of a composite system, which enables control of the thermal expansion through the use of the thinning that is observed when a conventional material is mechanically stretched (the Poisson's effect). It is shown that the model can predict the thermal expansion of such systems to an extent comparable to more complex finite element simulations. The proposed model also permits optimization of the system to exhibit maximum negative thermal expansion, or, thermal expansion of a desired magnitude. [ABSTRACT FROM AUTHOR]

Auxetic materials exhibit counterintuitive and interesting properties, which can be utilized to develop new families of products, which can meet sophisticated needs. Study presented within this paper is focused on manufacturing and testing auxetic polyurethane foams for potential introduction for seats in military helicopters. Firstly, a manufacturing process was developed which benefits from the well-known methods of processing conventional foams and allows preparing necessary test specimens for further tests. Secondly, a series of static and dynamic tests was performed during which the market available conventional foams used in transport are compared with processed auxetic foams. [ABSTRACT FROM AUTHOR]

This work describes a series of investigations carried out on sets of pristine and auxetic (negative Poisson's ratio (NPR)) open‐cell polyurethane foams subjected to relative humidity (RH) conditioning ranging from 9% to 92% at room temperature. The foams have been produced using a uniaxially thermoforming process. Pristine and auxetic foams have then been subjected to quasi‐static compressive cyclic loading (with maximum strains of 10% and 80%, respectively), as well as vibration transmissibility tests with base accelerations up to 2.29 g. Increasing levels of RH do not seem to statistically affect the moduli and PRs of foams subjected to lower maximum strains, however, especially the auxetic foams at higher compressive deformations show a decrease in the stiffness with the increase of the RH. The vibration tests show an increasing trend of the dynamic modulus of the foams with the increase of the RH. The results indicate the complexity of the interaction between foam architecture and absorption/desorption mechanisms occurring inside these porous auxetic materials. [ABSTRACT FROM AUTHOR]

Cellular solids whose ligaments are made of two distinct materials are constructed for studying their auxeticity and negative thermal expansion via 2D models with the methodology of finite‐element numerical analysis. Three bimaterial cellular models with various radii of ligament curvature (Rc = 5, 8, 11 cm) are analyzed. It is found that when the ligaments comprise aluminum and titanium alloy with Rc = 5 cm, the cellular solid exhibits simultaneous negative effective Poisson's ratio (−0.418) and negative thermal expansion coefficient (−20.82 × 10−6 K−1). When the radius of ligament curvature increases, auxeticity is less pronounced. Positive and negative thermal expansion can be tuned solely by the coefficients of thermal expansion of ligaments' constituents. Furthermore, smaller radii of curvature give rise to larger negative thermal expansion. In addition, modulus mismatch in the bimaterial ligament can be used to tune overall properties. Our findings herein may serve as foundations for the future quantitative design of such cellular materials. In addition, effective Poisson's ratio and coefficient of thermal expansion can be positively or negatively enhanced due to the interplay between the positive and negative phases. Interactions between negative stiffness and larger curvatures may make the system less unstable. [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]

An auxetic metamaterial based on a semirotation system is designed herein by combining rigid units of two different shapes, such that each unit cell consists of one nonrotating rhombus and four rotating right triangles. The on‐axes Poisson's ratios are obtained for both infinitesimal and finite deformations by geometrical consideration. The on‐axes Young's moduli of this metamaterial are established by matching the spring potential energy stored at the hinged joints with the strain energy of deformation for the homogenized continuum of the metamaterial. Plotted results describe the interlacing effect of the shape descriptor, the original separation angles, and the change in the separation angle on the on‐axes Poisson's ratio and Young's modulus. The latter is directly proportional to the equivalent spring stiffness per unit cell. In addition to attaining auxetic behavior, this metamaterial can also be designed to achieve specific Young's modulus behavior including extreme stiffness in one of the directions either at infinitesimal or finite deformation. [ABSTRACT FROM AUTHOR]

Life‐changing illnesses and fatalities caused by inefficient personal protective clothing and equipment are increasing day by day. The discrepancy between the test standards, environmental conditions, and infield collision is the reason why personal protection equipment and clothing have not yet decreased the number of injuries in various fields such as military and sports. To further increase the efficiency of personal protection materials, auxetic materials are suggested because of their high energy absorption, good permeability, form‐fitting ability, and high indentation resistance. Auxetic materials are nonconventional materials with a negative Poisson's ratio and can shrink under compression and expand when subjected to stretching. In comparison to nonauxetic conventional materials, auxetic materials have improved properties that can be useful for personal protection applications. This review focuses on the importance of auxetic materials for personal protection, focusing on reducing the chances of injury and what possible ideas can further benefit the protection products. The first part of the review discusses the auxetic structures and their protection applications followed by the fabrication of auxetic structures. The review will conclude with limitations, economic prospects, and what possible work can further improve the potential use of auxetic materials in personal protection. [ABSTRACT FROM AUTHOR]

Auxetics with negative Poisson's ratio (NPR) can expand dimensions along the direction perpendicular to the tensile direction, such as rigid rotation, reentrant and chiral structures, etc. The mechanical performances of chiral metastructures are closely related to internal defects and interactions between defects. In this paper, two types of irregular chiral structures with Gaussian dispersion of circular node positions and missing ligaments are designed, namely: anti‐tetrachiral metastructure and tetrachiral hybrid metastructure. These two types of chiral metastructures are characterized by node radius, ligament thickness, ligament length, the circular node center dispersion range, and missing ligament percentages. Effects of random node distribution range and cell wall missing percentage on the elastic modulus and Poisson's ratio of chiral metastructures are investigated through experimental tests and finite element (FE) simulation systematically. Through dimensionless analysis, it is found that ligament thickness has remarkable influence on the negative Poisson's ratio. Effect of missing ligament on the mechanical properties of the structure is greater than effect of random node center dispersion. With the increase of ligament missing percentage, the resultant modulus of chiral metastructures decreases, and the corresponding NPR increases. [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]

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]

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]

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]

Effective properties and dynamic response of a sandwich panel made of two face sheets and auxetic core are analyzed in this study by computer simulations. The inner composite layer is made of a cellular auxetic structure immersed in a filler material of a given Poisson's ratio (filler material fills the voids in structure). Each cell is composed of an auxetic structure (re-entrant honeycomb or rotating square), i.e., exhibiting negative Poisson's ratio without any filler. Influence of filler material on the effective properties of the sandwich panel is investigated. The proposed structure shows interesting structural characteristics and dynamic properties. Our results clearly show that it is possible to create auxetic sandwich panels made of two solid materials with positive Poisson's ratio. This is even possible if the filler material is nearly incompressible, but can move in out-of-plane direction. Moreover, effective Young's modulus of such sandwich panels becomes very large if the Poisson's ratio of the filler material tends to −1. [ABSTRACT FROM AUTHOR]

Negative Poisson's ratio (NPR) structures, also known as auxetics, are attracting growing interest due to their unique mechanical properties and a wide range of potential deployment. More attention has been paid to explore novel auxetic structures through bionics, lattice cells, or origami. However, the systemic design method of NPR structures has not yet matured. Herein, a design framework of NPR structures is proposed by integrating freedom, actuation, and constraint topologies (FACT) theory and isogeometric analysis. Based on the isogeometric method, the shape and mechanical properties of the curved beam can be changed by adjusting the control points of the curved beam. The configuration library is established by encapsulating curved beams to the straight beam. FACT theory is used to synthesize the cellular structure whose compliant elements are chosen from the curved beam configuration library. In this way, two NPR structures are designed. Simulation and experiments demonstrate the feasibility of the proposed design framework of NPR structure. The results of this study have a wide range of potential applications for designing future NPR structures. A new idea to design architected metamaterial systems exhibiting other properties is provided. [ABSTRACT FROM AUTHOR]

This study puts forward a method of reconstruction of the suboptimal isotropic truss microstructures for the known distribution of Young's modulus and Poisson's ratio predicted by the Isotropic Material Design (IMD) method of minimizing the compliance (maximizing the stiffness) of a 2D elastic body. The varying underlying microstructures corresponding to the optimal designs are recovered by matching the values of the optimal elastic moduli with the values of the effective moduli of the representative volume elements (RVE) computed by the asymptotic homogenization method. The used hexagonal shape of the RVE with its specific internal symmetry provides the assumed isotropy of the periodic body. Hexagonal periodic cells are made up of 3 or 12 pin‐jointed families of bars with different stiffness characteristics. Combining these families into two or three groups with identical bar stiffnesses in a group allows for the exact reproduction of Poisson's ratio for each point of the body. The ability to model an auxetic behavior within the subdomains where the optimal Poisson's ratio assumes negative values is also shown. The effective value of the Young's modulus is scaled independently of the value of the effective Poisson's ratio by selecting the cross section of the bars in the group. [ABSTRACT FROM AUTHOR]

Boron arsenate (BAsO4) is a β‐cristobalite‐like material, which crystallizes in the I4¯ space group. However, unlike β‐cristobalite itself, BAsO4 is well characterized and known to be stable and at ambient conditions. This work presents the results of extensive density functional theory (DFT)‐based simulations aimed at elucidating the nanoscale deformations that such crystals undergo when subjected to uniaxial tensile and compressive loads along their crystallographic [001] c‐direction. It is shown that such uniaxial loading results primarily in intra‐tetrahedral deformations rather than their tilting/rotations (which causes the auxeticity in the (001) plane). Through their analysis in isolation from other modes of deformation, this study gives a much better insight into such intra‐tetrahedral deformations and helps explain better the cause of the measured negative linear compressibility in the c‐direction at elevated pressures, which recently has been attributed to a "demi wine‐rack" mechanism caused by the same intra‐tetrahedral deformations. [ABSTRACT FROM AUTHOR]

A stiff tubular cellular structure topology is designed by alternatively indenting rectangular contour patterns on the cell ribs. Modified topologies can be obtained by tailoring the geometric parameters. The Poisson's ratios and elastic moduli of the proposed tubular structures with various geometric parameters are determined using finite element method (FEM). Results show that the minimum Poisson's ratio of the proposed structures can reach −0.28. The unit cell topology and orientation can significantly affect the structures deformation behaviors. The structures exhibit mixed mode of bending‐ and stretching‐dominated deformation responses, and show improved specific elastic moduli compared with traditional open‐cell stochastic foams with positive Poisson's ratios. 3D‐printed samples with identical geometric variables to those of the FE models are fabricated, and their Poisson's ratios and stress–strain relationships are determined experimentally, and compared with simulation results. Excellent agreement is achieved between measurements and simulations. The design concept proposed here can be optimized for specific applications via geometric parameters manipulation. [ABSTRACT FROM AUTHOR]

Herein, the structural, mechanical, and electronic properties of Pd doped with Ti under different pressures are studied, using first‐principles calculations. The formation enthalpy is negative, demonstrating that the structure of the Pd–Ti alloy is thermodynamically stable. The corresponding mechanical stability conditions prove that the Pd–Ti alloy is mechanically stable. Moreover, the shear modulus, bulk modulus, ratios of bulk modulus to shear modulus, hardness, and Poisson's ratio are calculated. The results indicate that the hardness, ductility, and plasticity of the Pd–Ti alloy improve. In addition, the density of states, bond population, and Debye temperature as functions of pressure are investigated, revealing that the Pd–Ti alloy is metallic and the pressure does not make a huge change to the structure of the Pd–Ti alloy and makes the thermal conductivity and interatomic bonding of the Pd–Ti alloy improved. Theoretical basis is also provided for further research on Pd‐based alloys which probably have good prospects for superalloys. [ABSTRACT FROM AUTHOR]

The behavior of the chiral S‐shaped structure subjected to large tensile loads in both X and Y directions is reported herein. The physical samples of stainless steel 316L are fabricated by the selective laser melting technique and experimental tests are conducted on a universal tensile testing machine. The results of tensile tests in the form of deformation patterns, tensile stress–strain curves, and Poisson's ratios are shown pictorially and graphically. The orientation of the S‐shaped structure with X‐direction loading is found to exhibit higher auxeticity over the large range of tensile loading. Nonetheless, the structure is also found auxetic when loaded from Y‐direction albeit the values of negative Poisson's ratios are not as high as of the X‐directional loading. [ABSTRACT FROM AUTHOR]

Recently, the auxetic fabrics have gained much importance in the scientific and industrial community due to their excellent impact‐resistance property. Due to this property, they have several applications, including automotive, aerospace, and ballistic areas. The auxetic three‐dimensional (3D) woven fabrics are a less explored domain in the auxetic community. Herein, special attention is given to the numerical analysis of the 3D auxetic structure. The negative Poisson's ratio of 3D auxetic structures is studied using ANSYS workbench structural analysis module. The effect of binding yarn float length on negative Poisson's ratio is tested on ten different 3D orthogonal through the thickness structures with binding yarn float length of 1:1, 2:1, and 3:1. Furthermore, the effect of the number of binding yarns and the number of layers is also studied numerically. The results show that the auxeticity of the woven structure increases with increasing the number of binding yarns and their float length. Moreover, decreasing the number of layers from 3 to 1 increases the auxeticity. [ABSTRACT FROM AUTHOR]

A general expression for the Poisson's ratio (PR) of square or 2D cubic materials is discussed in terms of the elastic moduli ratios, X=G/K and Y=G/W, where G and W are the shear moduli, and K is the bulk modulus. The entire XY‐plane is partitioned into nonauxetic, partially auxetic, and auxetic regions by two straight lines. A representation of the XY‐plane with the elastic constant ratio λ=C12/C44 is exploited to investigate 2D square materials under nonzero hydrostatic pressure. It is demonstrated that 2D square materials with λ>0 can be auxetic only under negative pressure. Similarities and differences between the corresponding descriptions of auxetic behavior of cubic 3D materials are pointed out. [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]

Chiral materials may exhibit negative Poisson's ratio and deformation‐mode coupling phenomena. The finite element numerical method is adopted to analyze a class of 2D chiral and nonchiral materials and to show the effects of microstructural geometry on their effective elastic properties and coupling between tension/compression and bending. With the same area fraction (AF), nonchiral samples show larger effective moduli than chiral ones. The number of unit cells may reduce negativity in effective Poisson's ratio of the chiral samples due to nonuniform lateral deformation under uniaxial straining. Increasing AF in a hierarchical pattern in the chiral samples makes their Poisson's ratio more negative. Bending occurs in the chiral samples when they are under uniform, uniaxial, tensile, or compressive straining due to the coupling of deformation modes. The sensibility of tension–bending coupling may be controlled by the chiral microstructure. Optimization of the coupling sensitivity may help develop novel mechanical sensors. [ABSTRACT FROM AUTHOR]

Herein, the deformation and properties of cellular structures of missing‐rib (MR) model and mixed structures (MSs) are studied experimentally and numerically. Samples with different beam angles and lattice patterns 3D printed with thermoplastic elastomer (TPE) are tested in uniaxial compression. Parametric finite‐element models are developed and used to simulate structures of different sample dimensions and lattice patterns with a particular focus on beam–wall contacts. Structures with different sizes and aspect ratios, beam angles, and lattice patterns are comparatively studied. The numerical results show a good agreement with the experimental data on samples of various sizes/patterns. The main deformation modes and key stages of the corner edge–wall contact and structure deformation are presented, and the key mechanism is analyzed. Poisson's ratio and stability of the auxeticity of different structures are established. The link between the structural and auxeticity stabilities and the characteristic of the corner edge–wall contact in MR and MSs are discussed. The MSs with alternative MR columns at opposite orientations show a better shape stability and higher critical strain for a stable auxeticity. [ABSTRACT FROM AUTHOR]

Auxetic materials exhibit a negative Poisson's ratio. Herein, the performance of rectangular auxetic membranes, under uniform load, is evaluated in terms of the maximum stress encountered. To facilitate convenient calculation, empirical modeling is conducted on the exact solution to reproduce displacement coefficients that are sufficiently accurate. A dimensionless maximum stress is then introduced to allow the effect from the membrane's Poisson's ratio and aspect ratio to be directly observed. Results reveal that 1) the dimensionless maximum stress reduces monotonically and nonmonotonically for conventional and auxetic membranes, respectively, 2) for the same Poisson's ratio magnitude the stresses in auxetic membranes are lower than those from conventional ones, 3) the optimal Poisson's ratio, on the basis of dimensionless maximum stress minimization, falls within the negative range, and 4) the dimensionless maximum stress minimization is especially effective for square membranes as well as very long and narrow membranes. It is therefore recommended that material auxeticity be taken into account—alongside geometrical and other material properties—in designing membranes and highly compliant very thin plates. [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]

Mechanical metamaterials represent a class of deformable systems, which exhibit macroscopic deformations, mechanical, and/or thermal properties. These emerge due to the structure of their subunits rather than their materials composition and typically exhibit anomalous (normally negative) macroscopic structural, mechanical, or thermal property/properties caused by a change in shape/size of the system. Herein, a class of honeycombs is discussed, which push to the extreme the classical definition of "mechanical metamaterials," exhibiting temperature‐tunable Poisson's ratio properties. More specifically, centrosymmetric honeycombs with T‐shaped joints constructed from different materials are shown to exhibit temperature‐dependent Poisson's ratio values, which can be either positive or negative (auxetic) depending on the external stimulus the system is subjected to. The sign and magnitudes of the Poisson's ratio values are explained in terms of particular geometries that these composite honeycomb systems attain at different temperature conditions. In particular, auxeticity is attributed to the transformation of the T‐shaped units to re‐entrant units. Practical aspects on how these properties may be achieved are discussed, including the possibility that the changes in shape, and hence Poisson's ratios, are induced via different extents of dryness on opposite surfaces of ligaments made from absorbent materials. [ABSTRACT FROM AUTHOR]

Herein, the effect of symmetric carbon fiber composites laminates with angle‐ply stacking sequences ([±β/±θ]S) in the design of composite piezoelectric energy harvesters (PEHs) is evaluated. Some of those specific stacking sequences also feature negative Poisson's ratio (NPR) or near zero Poisson's ratio (NZPR) through the thickness for ply angles θ < 45°. Six different architectures with similar in‐plane elastic modulus E1 and different positive Poisson's ratio and NPR v13 values are considered here. Finite element models are developed to understand the distribution of the voltage density of the laminates with the different stacking sequences and their bending properties. Experimental tests (three‐point bending and vibration) are also performed. Both the simulations and the experimental results show that the PEH with the stacking sequences of near zero v13 generate the highest power compared with the other composite energy harvesters. The maximum voltage frequency response function (FRF) happens at the fundamental resonance, and the PEH with near zero v13 has also the lowest resonance frequency compared with the other stacking sequences. [ABSTRACT FROM AUTHOR]

The article discusses the 2D problem of manufacturability of the minimum compliance designs of the structural elements made of inhomogeneous materials of local isotropic or square symmetry properties. The available isotropic material design (IMD) and cubic material design (CMD) methods deliver the optimal distribution of the elastic moduli within the design domain. Within the 2D setting, the cubic symmetry reduces to the symmetry of a square. The varying underlying microstructures corresponding to the optimal designs are recovered by matching the values of the optimal moduli with the values of the effective moduli of the representative volume elements (RVEs) computed by the asymptotic homogenization method for periodic media. The shape of the RVE and its internal symmetries are properly selected providing assumed isotropy or symmetry of a square of the homogenized constitutive tensor. The microstructure topology is described by parametric description of single (or several) fibers in RVE. The periodicity of the structure and the final topology of the material within RVE is ensured by multiplication of a single fiber in accordance with the symmetries adopted for RVE. The ability to model an auxetic behavior within the subdomains where the optimal Poisson's ratio assumes negative values is shown. [ABSTRACT FROM AUTHOR]