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The present study develops an auxetic meta-structure booster to advance piezoelectric energy harvesting (PEH). The model comprised a three-dimensional (3D) cantilever beam equipped with an auxetic substrate bonded to a piezoelectric (PZT) point defect using an epoxy layer. The substrate used in this proposed energy harvester possesses an auxetic region responsible for generating auxetic behavior and concentrating stress in the PZT layer. The auxetic model called auxetic structure (AS)-II is inspired by ancient motifs and is designed based on the combination of several promising mechanisms, including reentrant, chiral, perforation, and honeycomb supercell. The finite element method is employed to create numerical models whose accuracy is confirmed by mesh convergence and available experimental data. A parametric study is conducted on the AS-II harvester to examine the influence of geometry and PZT material type on the harvested electrical power. Our research efforts stem from both mechanical and electrical circuit fields to fulfill the broadband design target within the desired ultra-low-frequency range of 1 – 20 Hz , regardless of the resonance condition. As a result, the impact of other factors, such as load resistance, frequency, load amplitude, and bonding stiffness, on the PEH performance is further investigated. The reason behind such outstanding performance of AS-II is explained in terms of geometrical physics and stress distribution. The effect of geometrical dimensions on the auxetic behavior is also analyzed using Poisson's ratio (PR) study. Subsequently, we demonstrate that this innovative adapter can advance the system's performance in terms of harvested power and lightweight by a magnification factor of 14.61 and 5.8 times that of a plain substrate and 2.25 and 3.16 times that of a conventional auxetic harvester, respectively. The present harvester is strain controllable, where it can set PR between − 2 and 0 , resulting in robust PEH even if switching input vibration from high to low levels. This new idea enables us to serve cantilever-type resonators at frequencies far lower than their resonant frequencies with small portions of incident excitement. [ABSTRACT FROM AUTHOR]

AbstractThis article proposes a kind of double concave double-arrow cell. The equivalent thermal expansion coefficient is obtained by using the geometric relation of deformation. The theoretical results agree well with the finite element results. The effect of material matching and geometry size on the equivalent thermal expansion coefficient is discussed. Based on the proposed two-material double-arrow cell, a two-material honeycomb structure is constructed. The thermal expansion coefficient, equivalent elastic modulus and Poisson’s ratio of the structure under thermal load are numerically discussed. It is found that the thermal expansion coefficient of the structure increases with the increase of the thermal expansion coefficient of the material, and the thermal expansion coefficient of the up arrow has a greater effect on the thermal expansion performance of the structure than that of the down arrow. Compared with the single material honeycomb structure, the double-material concave honeycomb structure does not produce the deformation mode of surrounding diffusion, and its longitudinal expansion is smaller. It shows that zero expansion can be achieved by adjusting the material and geometric parameters, and this structure provides a new idea for the design of other zero expansion structures. [ABSTRACT FROM AUTHOR]

In this study, we designed improved cellular materials (CMs) with good energy absorption capacities, proposing new types of CMs with negative Poisson ratios by introducing sinusoidal cell walls and three different gradients. The amplitude of the sinusoidal wall, cell periodicity, and cell wall thickness were modulated in three layers across the structure (denoted as AG-CM, PG-CM, and WTG-CM, respectively). The influence of the different gradient parameters on the dynamic response of the CMs was analysed. The finite element method was used to explicitly simulate the deformation behaviour, dynamic shock stress, and energy absorption characteristics of the proposed structures at different impact speeds and directions. The initial peak stress in the face of the impact of the CM with a reasonable gradient setting was lower than that of the non-gradient CM. In contrast, its energy absorption was higher than that of the non-gradient CM. WTG-CM exhibited the highest energy absorption for a low-speed impact (7 m/s). For a medium and high speed impacts (35 m/s and 70 m/s), AG-CM exhibited the highest energy absorption in the face of positive gradient impact, whereas WTG-CM exhibited the highest energy-absorbing energy when impacting against the gradient. Under similar impact rates, the energy absorption energy of the different CMs varied by 32.1%. For the same CM, the maximum difference in absorption energy between shock modes was 33.8%. [ABSTRACT FROM AUTHOR]

Copyright of Bulletin of the Chinese Ceramic Society is the property of Bulletin of the Chinese Ceramic Society Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

AbstractA star-shaped auxetic cellular metamaterial with a negative Poisson’s ratio (NPR) was employed in this study to model lightweight sandwich panels, and an analysis of its sound transmission characteristics was conducted. Utilizing the first-order shear deformation theory (FSDT) and the Hamiltonian principle, the free vibration equation for the sandwich plate is derived. Subsequently, leveraging the fluid-structure coupling condition and incorporating the Navier method, the sound transmission loss (STL) is analytically described. The efficacy of the proposed method is validated through numerical analysis and an acoustic insulation experiment using an impedance tube. Based on the theoretical modeling, it is calculated that the sound insulation effect of the star-shaped cellular core exceeds that of the traditional reentrant cellular with NPR. A systematic analysis is subsequently conducted to explore the impact of star-shaped cellular structure parameters on the STL of the sandwich panel. It can be obtained that the increase in core layer thickness leads to a reduction in the first-order resonance frequency of the sandwich panel, but concurrently diminishing its sound insulation performance. The change of the connecting rib length exerts a more considerable influence on the structure’s STL compared to the side rib length. While as the cell wall thickness reduces from 2.0 to 1.0 mm, the average STL of the sandwich plate across the analysis band decreases by 8.43%, from 46.3 to 42.7 dB. [ABSTRACT FROM AUTHOR]

Compared to the conventional cellular cores, the cellular cores auxetic metamaterials with negative Poisson's ratios have unique mechanical deformation characteristics, superior strength to density ratio and high stiffness, both of which can be used to build lightweight sandwich constructions. The purpose of this study is to investigate the vibration acoustic response of porous functionally graded honeycomb sandwich plates with a negative Poisson's ratio (NPR). Under simply supported boundary conditions, the dynamic equation of the sandwich plate is derived based on the Hamilton principle, and the eigenvalue of the modal function matrix is coupled with the Navier method to obtain the natural frequency. The theoretical calculation results are compared with the finite element software COMSOL simulation results, so as to verify the validity and correctness of the theoretical model proposed in this paper. The far-field radiation sound pressure level of sandwich plate under point force loading is derived by Rayleigh integral theory. The correctness of the proposed model is verified by comparing the radiation sound pressure level calculated by the theoretical model with the results obtained by the finite element software simulation. The effects of porous functionally graded materials (FGM) and cellular structure parameters on the acoustic radiation characteristics of sandwich plates are discussed in detail. The results show that: The relative error between the theoretical results and the literature results is 5.43 %. Under the same conditions, the amplitude of the radiation sound pressure level (SPL) decreases by about 29.48 % in the low frequency stage, 28.9 % in the medium frequency stage and 11.08 % in the high frequency stage under the point excitation when the cellular side length is 6 mm compared with 8 mm in the low frequency stage. The movement trend and amplitude of the sandwich plate's emitted SPL are significantly impacted by changes in the characteristics of the cellular structure. [ABSTRACT FROM AUTHOR]

The direct measurement of Poisson ratios via the impulse excitation technique (IET) is based on determining the frequency ratio between the resonant frequencies of flexural and torsional vibrations of circular discs. However, the direct measurement of negative Poisson ratios, which occur in auxetic materials, has been impeded until now by the absence of numerical values of Martinček's solution for frequency ratios smaller than 1.35, because the relevant ASTM standards have not considered the possibility of negative Poisson ratios so far. Here we provide this missing link by reconsidering Martinček's solution and presenting the necessary values in the forms of graphs and fit relations from which the Poisson ratio, including negative ones (down to −0.273), can be determined from frequency ratios down to 1.1 for a wide range of specimen aspect ratios (down to 4). [ABSTRACT FROM AUTHOR]

Background: Today, energy harvesting is a hot topic in the scientific community because of the scarcity and insufficiency of energy resources. Piezoelectric systems have been proven by many studies to be very efficient in energy harvesting. In addition, an increase in efficiency has been observed by using auxetic materials in piezoelectric systems due to their extraordinary properties. Because of its capability to sustainably provide wireless sensors and portable electronic devices, a worldwide effort is being made to capture energy using the mechanical vibrations of the environment. Purpose: The main purpose was to understand auxetic materials and piezoelectric systems more clearly by making a comprehensive compilation. Methods: This review article investigated an auxetic piezoelectric energy harvester (APEH) system focusing on the structure of auxetic materials along with their behavior and efficiency in the system. Results: As a result of general examinations, it was seen that structures with negative Poisson ratios were much more efficient in energy harvesting in this type of system compared to conventional structures. Conclusion: Due to APEH's advantages such as simplicity, scalability, and high-power production, piezoelectric energy harvesting has gained more popularity than other vibration-based energy harvesting methods. [ABSTRACT FROM AUTHOR]

Auxetic structures are special structures with negative Poisson's ratio due to their geometric shapes in their internal structure. When tensile force is applied to these structures, transverse and longitudinal expansions are observed, while transverse and longitudinal contractions are observed when compressive force is applied. There are many different unit cell designs such as chiral, arrowhead, re-entrant in auxetic structures. In this study, a unique cell design was studied and the behavior of an auxetic lattice structure was investigated. The lattice structure designed with different geometric thicknesses and different matrices was analyzed with the finite element analysis program and found to have a negative Poisson's ratio. In the designed lattice structure, the structure with 4×4 matrix and 3 mm geometric thickness was found to have the best negative Poisson's ratio.

Due to the influence of mass law, traditional lightweight sandwich structures have struggled to surpass solid structures in sound insulation performance. To this end, we propose an acoustic metamaterial structure with a sandwich configuration based on the re-entrant negative Poisson's ratio (NPR) structure and systematically investigate its sound transmission loss (STL) performance under incident plane wave conditions. We used the acoustic impedance tube method to experimentally study the sound insulation performance of the re-entrant NPR sandwich structure under free boundary conditions, and then established an acoustic analysis simulation model based on COMSOL Multiphysics software, which verified that the results obtained by the experiment and the numerical simulation were in good agreement. The results show that the sandwich structure exhibits excellent sound transmission loss performance in the studied frequency range (250–4000 Hz), and the overall sound insulation performance exceeds the curve of the mass theorem, basically achieving more than 20 dB when the sandwich thickness is 2 cm. Finally, we conduct parametric studies to establish a correlation between the geometric design of NPR sandwich structures and their sound transmission loss performance. The research shows that the changes of the length of the ribs, the distance from the ribs to the center of the unit, and the length of the upper wall and the lower wall have a significant impact on the sound insulation performance of the re-entrant NPR sandwich structure, while the change of the wall thickness basically will not affect the sound insulation performance of the sandwich structure. This research can provide practical ideas for the engineering application of noise suppression designs of lightweight structures. [ABSTRACT FROM AUTHOR]

The work presents and describes distinctive properties and classification of most-known metamaterial. The principle of physical preconditions has been described together with their mechanical behaviour in the implementation of vision applications in the additive manufacturing industry. In addition, this paper presents an overview of the recent developments in the area of mechanical metamaterials. [ABSTRACT FROM AUTHOR]

Textile auxetic materials with negative Poisson’s ratio (NPR) have potential applications in protective textile, filtration, and energy absorbance. However, cotton core-spun based auxetic textile is rarely investigated. In this study, cotton core-spun (Lycra)-based helical auxetic yarn (HAY) was developed, and the effects of core filament linear density on the auxetic behavior were studied. Core-spun cotton yarns of 8/1 Nec containing core filament (180, 260, and 420 D) were developed. Then, these core-spun yarns were wrapped with 50 D Dyneema filament at 393 twist/m to make the HAY. The diameter, NPR, and tensile strength of HAY were increased with increasing linear density of Lycra filament. Image analysis of HAY containing 420 D Lycra showed 57% diameter increase with the change in axial strain from 0 to 15 mm. The maximum auxetic behavior with NPR of −3.97 and tensile strength of 26.71 cN/tex was observed in 420 D Lycra-based HAY. This study proved that the cotton core spun-based HAY can be produced with significant auxetic properties for practically potential use in wearable protective textiles.

Copyright of Bulletin of the Chinese Ceramic Society is the property of Bulletin of the Chinese Ceramic Society Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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

Copyright of Journal of Natural Fibers is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The concave corner structure has excellent vibration and noise reduction characteristics to effectively attenuate the structural vibration response, honeycomb structure with excellent mechanical properties has been more commonly used in engineering, so the composite structure of concave corner honeycomb has attracted the attention of scholars. A novel phononic crystal model was created using the inner concave honeycomb structure' s fractal design. Based on the finite element method, the fractal concave angle honeycomb structure was analyzed to calculate the energy band structure and vibration transmission characteristics, as well as to examine the negative Poisson ratio characteristics of the structure and the effect of the structural fractal on the vibration band gap. By varying parameters such as wall thickness and the angle of the inner concavity, filling the steel at the apex of the fractal structure can provide better suppression of certain frequency bands of vibration. The results show that the fractal structure still has negative Poisson ratio characteristics, the fractal structure produces a wider band gap in the second-order structure, the increase in wall thickness and concave angle causes the structural vibration band gap to shift to the high-frequency region, the filled steel causes the band gap to widen. [ABSTRACT FROM AUTHOR]

At present, the direct preparation of polymer foam with negative Poisson ratio (auxetic foam) from nonpolar polymer resins is still a challenge. Herein, a one‐pot CO2 foaming method is established to directly prepare closed‐cell auxetic foams with different cellular structures (bimodal cell structure and homogeneous cell structure) from polypropylene (PP) resin by introducing hydrophilic additives (NaCl) into PP and regulating foaming conditions. The effects of foaming temperature, saturation time, and size of added NaCl on Poisson's ratio are studied. The comparison with PP/talc composites shows that the presence of NaCl plays a crucial role in the formation of the negative Poisson ratio of foam. Furthermore, the regulation of the size of negative Poisson ratio is achieved by controlling the amount of NaCl in PP/NaCl composites. This study has important reference significance for the preparation of nonpolar polymer auxetic foams. [ABSTRACT FROM AUTHOR]

The novel cellular cores auxetic metamaterials with negative Poisson's ratio (NPR) have unique mechanical deformation characteristics compared to conventional cellular cores, which can be further applied to modeling lightweight sandwich structures. The Arc-type cellular core is based on the traditional re-entrant cellular core and incorporates a bending configuration, facilitating a smooth transition of the cell elements. Based on that, the sound transmission characteristics of sandwich doubly-curved shells with NPR arc-type auxetic cellular core layer is studied in this article. The Hamilton's principle is applied to derive the governing equations and considers fluid-structure coupling by applying normal velocities continuity conditions at the fluid-structure interface, which is further analytically solved using Navier's method, and the sound transmission loss (STL) is described analytically. The correctness of the proposed formula is verified by comparing the theoretical solution with the calculation results of the commercial software COMSOL and the experimental results of impedance tube sound insulation. Based on the theoretical model, it is calculated that the average STL of the arc-type cellular is 6.61% higher than that of the traditional re-entrant cellular within the broad low-frequency range of 100–2448Hz. Then the effect of key parameters on the STL of sandwich structure is systematically analyzed, and the results show that the first frequency of the sandwich shell decreases with the increase of the rib thickness, but increases with the increase of the arc radius. Notably, a smaller arc radius and a larger rib thickness exhibit superior sound insulation properties across a broad frequency range. In addition, when the core layer thickness increases from 0.007 m to 0.009 m , the average STL of sandwich structures is reduced by 7.38% in a broad low-frequency range of 100–2448Hz. • An innovative stiffened sandwich doubly-curved shell with arc-type auxetic cellular cores is proposed. • Analytical models are established and validated by experimental and the simulated results. • The sound performance of sandwich shell structure with arc-type cellular cores was systematically compared. • The average STL of the arc-type cellular core is higher than that of the traditional re-entrant cellular core. [ABSTRACT FROM AUTHOR]

An analytical model is formulated for 2-D periodic negative honeycomb thicken strut re-entrant lattice structures and a modified rounded corner negative honeycomb structure that shows negative Poisson's ratios (NPR). Analytical modeling is done using Castigliano's second theorem, where each beam is modeled using Timoshenko beam theory, considering bending, stretching, and transverse shearing. Elastic modulus and Poisson's ratio have been formulated for both structures in the form of non-dimensional geometrical characteristics, such as length ratios, angles of re-entrant arms, shear correction factor, and the material's Young's modulus and Poisson's ratios. Numerical simulations conducted in ABAQUS-CAE explicit solver validate the analytical model. The effect of the non-dimensional parameters on the qualities of the developed structure is demonstrated. It is observed that the structures with a low curvature ratio have a high fluctuation of Poisson's ratio and Elastic constant when plotted against the other parameters. The slenderness ratio has little impact on Poisson's ratio but significantly influences elastic modulus. It is shown that various needs can be satisfied by customizing the Poisson's ratios and elastic constant of both forms of lattice construction over an extensive range by carefully choosing the geometrical parameters and material. [ABSTRACT FROM AUTHOR]

The structural and mechanical properties of various types of poly(phenylacetylene) crystalline networked polymers which stack in the third direction in a graphite-like manner have been the subject of extensive research over the past few years. These studies have suggested that depending on the particular manner of substitution of the phenyls, it is possible to achieve some very interesting mechanical properties, which include, in some cases, negative Poisson’s ratio (NPR) and/or negative linear compressibility (NLC). The current study investigated how alternatives to these systems can be designed, and specifically tailor-made to exhibit desirable anomalous properties, such as NPR and NLC, through the replacement of the acetylene chains with ethylene chains, so as to produce the poly(phenylethylene) equivalents. Using force field-based simulations, via the use of the polymer consistent force-field (PCFF) it was noted that, to a first approximation, these systems mirror some of the analogous properties exhibited by their poly(phenylacetylene) counterparts. In particular, poly(phenylethylene) systems built from 1234- and 1245-substituted phenyls exhibited negative Poisson’s ratios, with the latter also exhibiting negative linear compressibility. This anomalous behaviour, mirrors, to some extent, that exhibited by their poly(phenylacetylene) counterparts, albeit some differences were noted, such as a reduction in the degrees of auxeticity. It was also noted that the poly(phenylethylene) systems modelled here tend to stack in the third direction, in a different manner than their poly(phenylacetylene) analogues, which difference is likely to be the factor for such reduction in auxeticity.

Featured Application: In this work the anomalous mechanical behavior of uranyl carbonate mineral rutherfordine is unveiled by performing high-quality theoretical solid-state calculations. This study provides a theoretical framework which may be utilized to study the mechanical behavior of all types of solid materials. In the context of Radioactive Nuclear Waste Disposal, this framework provides in a very detailed, safe, and cheap way, of obtaining these properties which may be applied to the study of the main components of the spent nuclear fuel. The mechanical behavior of the uranyl carbonate mineral, rutherfordine, UO2CO3, was studied by means of theoretical solid-state methods based in Density Functional Theory using plane waves and pseudopotentials. The results of the computations reported in this work show that this mineral exhibits the important negative Poisson ratio (NPR) phenomenon. In order to show that this feature is not an artifact associated to the theoretical treatment employed, additional calculations were carried out using very large calculation parameters. These calculations improved the mechanical description of this mineral and confirmed its auxeticity, i.e., it shows NPR values. Rutherfordine is a highly anisotropic material showing a maximum value of the NPR of the order of −0.3 ± 0.1 for applied stresses directed along the X axis, the transverse direction being the Y axis perpendicular to the structural sheets in rutherfordine structure. The underlying reason for this observation is that under the effect of applied positive pressures, the interlayer space between the sheets of rutherfordine vary in the opposite way to the expected behavior; that is, it decreases instead of increasing. [ABSTRACT FROM AUTHOR]

Focusing on planar isotropic petal-shaped auxetics, an isogeometric design framework is presented to achieve tunable effective properties. Specifically, the design framework includes (i) a NURBS-based parametric modelling scheme that characterizes petal-shaped auxetics with a small number of design variables; (ii) a systematic consideration of petal form, component widths and base material properties; (iii) a semi-analytical sensitivity analysis method based on material derivatives; and (iv) constraints for effective stiffness and target Poisson ratio. Three cases are considered: Case A with the same component width, Case B with different component widths, and Case C for composite designs with multiple base materials. For each case, a design limit curve is obtained for the effective Poisson ratio over a range of effective stiffness constraints, to give a quick overview on the properties attainable for each design setting. The optimization framework is next demonstrated for designing composite petal-shaped auxetics with target effective properties. [ABSTRACT FROM AUTHOR]

Carbon fiber-reinforced polymer (CFRP) structures are widely used in aviation industry, which can sustain damage during assembly from low-velocity impact events, decreasing residual stress and bearing capacity. A method of negative Poisson ratio rubber protective layer placing on the CFRP structures has been proposed to address this. The influence of cell angle on the impact response and protection effect were first investigated in this study. The experimental results indicate that the negative Poisson ratio rubber protective layer enhanced the impact resistance of the CFRP laminates, which reduces 33.1% the impact peak force and 17.5% the absorbed energy. Moreover, the impact resistance of CFRP with increases with the increasement of the cell angle. A low-velocity impact finite element model was developed with mixed-mode damage criterion for laminates and the hyperelastic model for rubber. The simulation results were validated with the experiment results in impact response and delamination damage. Additionally, the impact response and resistance were analyzed by varying seven different structural parameters of the protective layer to reveal the influence of relative volume. The experiment and simulation results demonstrated that the impact resistance increased with increasing cell angle and relative volume of the negative Poisson ratio rubber protective layer. • The influence of structural parameters on the impact resistance of rubber negative Poisson ratio protective layer was revealed by low-velocity impact tests. • The internal damage and damage mechanism of composite laminates were simulated with a mixed-mode damage criterion. • The relation between the relative volume of rubber negative Poisson ratio protective layer and its impact resistance was discussed by simulation results. • The low-velocity impact process of the composite laminate with the rubber negative Poisson ratio protective layer was detailed. [ABSTRACT FROM AUTHOR]

A contrast in Poisson ratio is a possible strategy to enhance the stiffness of composite structures. In solid materials Poisson ratio is hardly tailorable unless cellular architectures are considered. Here, we first investigated the effect of a single re-entrant inclusion acting as a defect into a regular (non-re-entrant) honeycomb lattice. Building on this, we generated regular patterns of re-entrant inclusions into a regular hexagonal cellular matrix and we characterized the apparent stiffness and Poisson ratio of the obtained structures. We also explored the role of the intrinsic material properties of the inclusion as well as of its closest environment on the interplay between the deformations of different phases in the lattice. Our main finding is that a small fraction of re-entrant inclusions (around 12%) is sufficient to generate a substantial augmentation in stiffness (300%) at constant overall relative density and without inducing strong anisotropy. Eventually, we fabricated by 3D polyjet printing bi-material composite architectures to demonstrate the superior mechanical behavior obtained exploiting the Poisson effect. [ABSTRACT FROM AUTHOR]

The mechanical behavior of the uranyl carbonate mineral, rutherfordine, UO2CO3, was studied by means of theoretical solid-state methods based in Density Functional Theory using plane waves and pseudopotentials. The results of the computations reported in this work show that this mineral exhibits the important negative Poisson ratio (NPR) phenomenon. In order to show that this feature is not an artifact associated to the theoretical treatment employed, additional calculations were carried out using very large calculation parameters. These calculations improved the mechanical description of this mineral and confirmed its auxeticity, i.e., it shows NPR values. Rutherfordine is a highly anisotropic material showing a maximum value of the NPR of the order of −0.3 ± 0.1 for applied stresses directed along the X axis, the transverse direction being the Y axis perpendicular to the structural sheets in rutherfordine structure. The underlying reason for this observation is that under the effect of applied positive pressures, the interlayer space between the sheets of rutherfordine vary in the opposite way to the expected behavior; that is, it decreases instead of increasing.

Using first-principles calculations, we investigate the fully oxidized silicene and germanene with stoichiometric ratio Si:O/Ge:O = 1:1. For both compounds, the zigzag ether-like conformation (z-sSiO/z-sGeO) is found to be the most energetically favorable structure. These z-sSiO and z-sGeO nanosheets have prominent elastic characteristics, which even exhibit an unconventional auxetic behavior with negative Poisson ratios. After oxidation, the semi-metallic nanosheets are transformed into semiconductors with narrow direct band gaps. Due to the anisotropic mechanical and electronic properties, the z-sSiO and z-sGeO possess an axially high intrinsic charge mobility up to the order of 104 cm2/Vs, which is comparable to that of graphene nanoribbons. Our studies demonstrate that the silicene and germanene oxides have peculiar mechanical and electronic properties, which endow these nanostructures for potential applications in nanoelectronics and devices. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

A continuum-based micromechanical model is employed for the prediction of the elasto-plastic behavior of periodic microstructural arrays that can generate negative values of Poisson's ratios. The combined effects of the negative Poisson's ratio generated by the array microstructure and the elastoplastic behavior of the constituents are studied. A design methodology for the determination of the constituents' properties of two-phase arrays that generate required values of negative Poisson's ratio is considered. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the behavior of auxetic composites is interpreted in the light of Cosserat elasticity which admits degrees of freedom not present in the classical elasticity: The rotation of points in the material, and a couple per unit area or the couple stress. The prediction of the Young’s modulus is developed for a laminated periodic material made up of alternating aluminum and auxetic material, by using the Bécus homogenization technique. [Copyright &y& Elsevier]

We review the results obtained for the 2D microscopic model of crystal having finite size particles proposed by Ishibashi and Iwata and later generalized in order to take into account deformability of particles. We focus on the elastic properties of the discrete model and also discuss various continuum analogues to the model, including the multi-field continuum approximation. Several new results are reported, e.g., we describe the conditions of thermodynamic stability of the model and study the variation of Poisson ratio of the model as the function of model parameters in single-crystal and poly-crystal states. The role of the rotational degrees of freedom pertaining to the finite size particles is highlighted. Particularly we demonstrate that the microscopic rotations are responsible for auxetic behavior (i.e., exhibiting negative Poisson ratio) of the macroscopically isotropic poly-crystal. [ABSTRACT FROM AUTHOR]

Dynamic simulations under stress were carried out on a series of main-chain polymers containing two types of mesogen. One mesogen is orientated in-line with the chain (end-to-end) and the second (swivelling) has the main chain passing at right angles through it. Under strain the swivelling mesogens are expected to rotate, pushing apart the neighbouring chains. Polymers incorporating 50% of both mesogens showed a reduced Poisson's ratio. In particular, swivelling mesogens of seven rings length produced a negative Poisson's ratio, and thus should be auxetic. In contrast, polymers containing only end-to-end mesogens had relatively high Poisson's ratios of 0.53–0.59. [ABSTRACT FROM AUTHOR]

Abstract: A series of liquid crystalline copolyesters, possibly having auxetic behaviour, were designed and synthesized by solution polycondensation from 4,4′-(decanoxy) dibenzoyl chloride with 2,5-bis(p-alkoxy benzoyloxy) hydroquinone and 4,4′-dihydroxy-α,ω-diphenoxydecane in different ratios. All the polymers have lower melting point ranging from 80 to 140°C, which is beneficial to the liquid crystalline state formation and the thermoplastic processing. The lateral rods derived from 2,5-bis(p-alkoxy benzoyloxy) hydroquinone monomer gave an obvious influence on properties of the resulting polymers, in which the liquid crystalline state stability and the solubility in common organic solvents were enhanced. Observations from POM and DSC indicated that all the polymers are thermotropic liquid crystalline with nematic characteristics. [Copyright &y& Elsevier]

We study hexagonal materials of MgB ( h -BN) and related compounds under various compression conditions. We found the anomalous behavior of lattice constants a ( b ) for MgB ( h -BN) and HBC. Lattice constants a of MgB ( h -BN) and HBC contract under c -axis compression. This means that the Poisson ratio is negative. The lattice contraction of a -axis is 0.0017 nm for MgB ( h -BN) when P z =50 GPa, 0.0002 nm for HBC when P z =20 GPa. The contraction of HBC is quite small. [ABSTRACT FROM AUTHOR]

Three new kinds of molecular networks are designed and predicted to exhibit negative Poisson ratios. Molecular mechanics calculations on these networks show that the magnitude of Poisson ratios depends on the relative flexibility of beam and arm structures. Several new kinds of auxetic polymers, whose successful synthesis should be easier than that of the corresponding auxetic networks, are then proposed. It is found that the kabob-like polymers with auxegens lying vertically on the main chain can acquire auxeticity while those with auxegens lying horizontally on the main chain cannot. Besides, a half kabob-like o pseudo-ladder polymer with auxegens linked at the intersection of the beam and the arm does show auxeticity when adopting constrictive conformers. It is, however, worthwhile noting that the origins of auxeticity still await and strongly deserve further experimental and theoretical investigations. [ABSTRACT FROM AUTHOR]

Metamaterials are synthetic materials designed to have unique properties like negative Poisson ratio (NPR). NPR metamaterials, also known as auxetics, offer significant value in applications that require high energy absorption, e.g., packing materials, medical knee pads, footwear. However, material uncertainty arising out of manufacturing tolerance, inhomogeneity of material properties, and others could lead to significant variations in the response of the metamaterials. Thus, a SIMP based robust topology optimization (RTO) design for the NPR metamaterials under material uncertainty is investigated. The weighted mean and variance of the deterministic objective function is utilized to form a robust objective function. The variation in effective Poisson's ratio with respect to the lower bound goes from 15.40% to 105% with deterministic topology optimization. In contrast, RTO produces more stable designs and shows the variation of only 1.72% to 2.54%. Several parametric studies are used to demonstrate the feasibility of the proposed RTO methodology. • Uncertainty in base material properties is incorporated by considering them as interval variables. • The robust objective function based on SIMP is solved using the FEniCS python interface. • Reliable results are achieved even with a considerable variation in the input material properties. • Implementation of the robust algorithm does not require significant changes to deterministic code. [ABSTRACT FROM AUTHOR]

A novel two-dimensional (2D) semiconducting carbon allotrope is proposed by inserting alkyne into tetrahexcarbon. This new carbon allotrope consists of tetra-rings and acetylenic linkages and is called TAL-carbon. To the best of our knowledge, this is the first 2D carbon allotrope composed of sp - sp 2- sp 3 hybridized carbon atoms. Due to its unique puckered structure, TAL-carbon is the fourth 2D carbon material with an intrinsic in-plane negative Poisson ratio (NPR) reported after graphene, penta-graphene, and tetrahexcarbon. Materials with NPR exhibit a wide range of potential applications in biomedicine, aerospace industries, and precision instruments. TAL-carbon has an indirect band gap of 1.614 eV. At room temperature, it exhibits high in-plane electron mobility of up to ~3.5 × 104 cm2V−1s−1, which is 30 times higher than monolayer black phosphorus and two orders of magnitude higher than monolayer MoS 2. The phonon dispersion curves and in-plane elastic constants indicate that TAL-carbon is dynamically and mechanically stable. Its equibiaxial strain limit reaches 13.7%, similar to that of graphene. An ab initio molecular dynamics (AIMD) simulation demonstrates that TAL-carbon is thermally stable at 300 K and 1000 K. The results show that TAL-carbon is a promising 2D auxetic material for applications in nanomechanics and nanoelectronics. [ABSTRACT FROM AUTHOR]

Entropic surfaces represented by fluctuating two-dimensional (2D) membranes are predicted to have desirable mechanical properties when unstressed, including a negative Poisson's ratio ("auxetic" behavior). Herein, we present calculations of the strain-dependent Poisson ratio of self-avoiding 2D membranes demonstrating desirable auxetic properties over a range of mechanical strain. Finite-size membranes with unclamped boundary conditions have positive Poisson's ratio due to spontaneous non-zero mean curvature, which can be suppressed with an explicit bending rigidity in agreement with prior findings. Applying longitudinal strain along a singular axis to this system suppresses this mean curvature and the entropic out-of-plane fluctuations, resulting in a molecular-scale mechanism for realizing a negative Poisson's ratio above a critical strain, with values significantly more negative than the previously observed zero-strain limit for infinite sheets. We find that auxetic behavior persists over surprisingly high strains of more than 20% for the smallest surfaces, with desirable finite-size scaling producing surfaces with negative Poisson's ratio over a wide range of strains. These results promise the design of surfaces and composite materials with tunable Poisson's ratio by prestressing platelet inclusions or controlling the surface rigidity of a matrix of 2D materials.