Comparison of the Mechanical Behaviour of Standard and Auxetic Foams by X‐ray Computed Tomography and Digital Volume Correlation

The tensile behaviour of standard and auxetic polyurethane foams are contrasted by digital volume correlation of 3D images collected by in situ X‐ray computed tomography (CT). It was found that subset sizes of 32 and 64 voxels for the auxetic and standard foams were optimal for strain resolutions in the order of 0.1%. For the standard foam, good uniformity of strain was observed at low strains giving a tangent Poisson's ratio of 0.5. Some heterogeneity of strain was observed at higher strains, which may be related to the fixtures. The behaviour of the auxetic foam was totally different, with strain being spatially heterogeneous with transverse strains both positive and negative but giving a negative Poisson's ratio on average. This suggests that the unfolding tendency of some groups of cells was higher than others because of the complex frozen starting microstructure. Further different methods of deriving Poisson's ratio gave different results. Besides revealing interesting microstuctural mechanisms of transverse straining, the study also shows digital volume correlation of tomography sequences to be the perfect tool to study complex mechanical behaviour of cellular materials.     

Tensile-force-induced delamination of polymeric coatings in tightly jacketed double-coated optical fibers

To maintain the mechanical strength, the glass fiber of optical fibers is coated by polymeric materials during the fabrication process. However, when the external tensile-force-induced shear stress at the interface of the glass fiber and primary coating is larger than its adhesive stress, the polymeric coatings will be delaminated from the glass fiber and optical fiber will lose its mechanical strength. In this article, the tensile-force-induced delamination of polymeric coatings in tightly jacketed double-coated optical fibers is investigated. To minimize the coating's delamination, the tensile-force-induced shear stress at the interface of the glass fiber and primary coating should be reduced. The method to minimize such a shear stress is to select suitable polymeric coatings as follows. The Poisson's ratio of the primary coating and the Young's moduli of the secondary coating and jacket should be increased, but the Young's modulus of the primary coating and thickness of the secondary coating should be decreased. On the other hand, the thickness of the primary coating has an optimal value. The selection of the adhesive shear stress between the glass fiber and primary coating in the minimization of the coating's delamination is also discussed.     

A Decomposition Method for the Analysis of Viscoelastic Structural Dynamics with Time‐Dependent Poisson's Ratio

Abstract: This paper presents a decomposition method for the dynamic analysis of elastic–viscoelastic composite (EVC) structures with time‐dependent Poisson's ratio. The analysis splits the viscoelasticity matrix with time‐dependent Poisson's ratio into two matrices in a simple form in which the time‐dependent Poisson's ratio does not appear. The decomposition simplifies the process of dynamical analysis for EVC structures with time‐dependent Poisson's ratio. The approach also makes it possible to apply existing analysis methods for constant Poisson's ratio structures directly to structures with time‐dependent Poisson's ratio. Based on the numerical results of three case studies, it is found that the time‐dependent Poisson's ratio has little influence on the structure's natural frequencies and damping properties. Therefore, it could be concluded that the effect of time‐dependency in Poisson's ratio may be ignored in the EVC structural dynamic analysis without introducing notable errors.     

A generalized complex eigenvector method for dynamic analysis of heterogeneous viscoelastic structures

A generalized complex eigenvector method which can be used to a linear dynamic analysis of viscoelastic structures is described. Here dynamic analysis is understood as transient analysis and frequency response analysis. The generalized complex eigenvector method is based on finite element discretization of structure, approximation of viscoelastic properties by differential operators and mode superposition technique. Coefficients of differential operator are defined from the condition of best coincidence of complex characteristic of viscoelastic material and complex characteristic of differential operator in preset frequency range. Advantage of this method is that it allows to take into account the real changes of the viscoelastic property in frequency range. Also, the generalized complex eigenvector method permit to describe a viscoelastic properties by two functions (complex Young's modulus, complex Poisson's ratio). The method is verified with the help of comparing with solutions obtained by complex modulus method. An influence of viscoelastic Poisson's ratio on transient and frequency responses of structure is demonstrated. Copyright © 2001 John Wiley & Sons, Ltd.     

Negative Poisson's Ratio in Modern Functional Materials

Materials with negative Poisson's ratio attract considerable attention due to their underlying intriguing physical properties and numerous promising applications, particularly in stringent environments such as aerospace and defense areas, because of their unconventional mechanical enhancements. Recent progress in materials with a negative Poisson's ratio are reviewed here, with the current state of research regarding both theory and experiment. The inter‐relationship between the underlying structure and a negative Poisson's ratio is discussed in functional materials, including macroscopic bulk, low‐dimensional nanoscale particles, films, sheets, or tubes. The coexistence and correlations with other negative indexes (such as negative compressibility and negative thermal expansion) are also addressed. Finally, open questions and future research opportunities are proposed for functional materials with negative Poisson's ratios.     

A variational discrete element method for quasistatic and dynamic elastoplasticity

We propose a new discrete element method supporting general polyhedral meshes. The method can be understood as a lowest-order discontinuous Galerkin method parametrized by the continuous mechanical parameters (Young's modulus and Poisson's ratio). We consider quasistatic and dynamic elastoplasticity, and in the latter situation, a pseudoenergy conserving time-integration method is employed. The computational cost of the time-stepping method is moderate since it is explicit and used with a naturally diagonal mass matrix. Numerical examples are presented to illustrate the robustness and versatility of the method for quasistatic and dynamic elastoplastic evolutions.     

Poisson's ratio as a sensitive indicator of (fatigue) damage in fibre-reinforced plastics

Even if the extent of damage in fibre‐reinforced plastics is limited, it already affects the elastic properties. Therefore, the damage initiation and propagation in composite structures is monitored very carefully. Beside the use of nondestructive testing methods (ultrasonic inspection, optical fibre sensing), the follow‐up of the degradation of engineering properties such as the stiffness is a common approach. In this paper, it is investigated if the Poisson's ratio can be used as a sensitive indicator of (fatigue) damage in fibre‐reinforced plastics. Static, cyclic and fatigue tests have been performed on [0°/90°]_2s glass/epoxy laminates, and axial and transverse strain were measured continuously. The evolution of the Poisson's ratio ν_xy versus time and axial strain ɛ_xx is studied. It is concluded that the degradation of the Poisson's ratio can be a valuable indicator of damage, in combination with the stiffness degradation.     

考虑泊松效应的材料/结构一体化设计方法

为实现含有不同泊松比组分复合材料的优化设计,并考虑宏观结构及复杂的边界条件,提出了考虑泊松效应的材料/结构一体化设计方法,其显著特征在于不同组分材料中引入了泊松比插值,假设宏观结构由周期性排列的复合材料组成,复合材料含两种各向同性且泊松比不同的组分材料,以静态问题中柔顺度最小化或动态问题中特征值最大化为目标以及宏微观体积比为约束建立了拓扑优化模型。采用均匀化理论预测了复合材料等效性能,推导了目标函数对宏微观密度变量的敏度表达式。分别采用密度过滤和敏度过滤来消除宏微观拓扑优化中的不稳定性现象。采用优化准则法分别更新宏观、微观密度变量,考察了微观体积比和组分材料泊松比参数对优化结果的影响。三维数值算例结果表明所提出的一体化方法具有可行性和优越性。     

Design,Preparation, and Mechanical Property Investigation of Ti–Ta 3D-Auxetic Structure by Laser Powder Bed Fusion

In recent years, Ti–Ta alloy has become a hot material in orthopedic implants. At the same time, the metal lattice structure with a negative Poisson's ratio has a great prospect in the application of implants due to its unique mechanical properties. Herein, an antichiral three-dimensional polyhedron (A3P) lattice structure with negative Poisson's ratio is designed. This lattice structure is prepared by laser powder bed fusion (LPBF) technology using Ti–Ta alloy as the feedstock material. The surface morphology and relative density of the Ti–Ta samples built with different laser scanning speeds are compared for the optimization of the Ti–Ta's LPBF parameter. Then, the A3P structure of the Ti–Ta alloy is fabricated using the optimized LPBF parameters. The negative Poisson's ratio effect of the A3P structure is verified by finite element analysis (FEA) and compared with experimental results. The compression test results show that the yield strength and elastic modulus of the A3P structure are 9.2 MPa and 0.14 GPa, respectively, which is suitable for the orthopedics application. A3P structure has higher fracture energy compared with diamond structure, indicating it can absorb more energy in the fracture process. A3P shows a promising prospect in the application of orthopedics implantation.     

Characterization of single crystal silicon and electroplated nickel films by uniaxial tensile test with in situ X-ray diffraction measurement

In this study, mechanical properties of micron‐thick single crystalline silicon (Si) and electroplated nickel (Ni) films at intermediate temperatures are investigated by means of X‐ray diffraction (XRD) tensile testing. The developed tensile test technique enables us to directly measure lateral (out‐of‐plane) elastic strain of microscale crystalline specimen using XRD during tensile loading, and determines Young's modulus, Poisson's ratio and tensile strength of the Si and Ni specimens. The specimens, measuring 10 μm thick, 300 μm wide and 3 mm long, are prepared through a conventional micro‐machining process, and the ultraviolet lithographie galvanoformung abformung (UV‐LIGA) process including a molding and an electroplating. The Si specimens, showing brittle fracture at room temperature (R.T.), have average Young's modulus and Poisson's ratio of 169 GPa and 0.35, respectively, in very good agreement with analytical values. The Ni specimens, showing ductile fracture, have those of 190 GPa and 0.24, lower than bulk coarse grained Ni. Young's moduli of both the Si and Ni specimens decrease with increasing temperature, but Poisson's ratios are independent of temperature. The influence of specimen size on elastic‐plastic properties of the specimens is discussed.     

A simplified micromechanical model for predicting effective mechanical behaviors of continuous bidirectional-fiber-reinforced composites

A simplified micromechanical model is proposed to estimate the macroscopic mechanical properties of continuous bidirectional-fiber-reinforced composites (CBFRCs) by ignoring Poisson's effect. The model is validated by results from a homogenized finite element approach. Based on the proposed analytical model, the influences of the ratios of fiber/matrix modulus and the fiber volume ratio on the effective modulus and the tensile strength are specifically investigated. The suggested theoretical method provides a convenient tool for estimating the effective mechanical behaviors of CBFRCs, which can be expressed as a function of fiber volume fraction and material parameters.     

Improvement of pressure distribution to arbitrary geometry with boundary condition represented by polygons in particle method

The boundary condition represented by polygons in the moving particle semi‐implicit method can accurately represent geometries and treat complex geometry with high efficiency. However, inaccurate wall contribution to the Poisson's equation leads to drastic numerical oscillation. To address this issue, in this research, we analyzed the problems of the Poisson's equation used in the boundary condition represented by polygons. The new Poisson's equation is proposed based on the improved source term (Tanaka and Masunaga, Trans Jpn Soc Comput Eng Sci, 2008). The asymmetric gradient model (Khayyer and Gotoh, Coastal Engineering Journal, 2008) is also adopted to further suppress the numerical oscillation of fluid particles. The proposed method can dramatically improve the pressure distribution to arbitrary geometry in three dimensions and keep the efficiency. Four examples including the hydrostatic simulation, dam break simulation, and two complex geometries are verified to show the general applicability of the proposed method. Copyright © 2017 John Wiley & Sons, Ltd.     

玻璃微珠含量及粒径对填充聚丙烯复合材料动态力学性能的影响

应用动态力学分析仪,在－１５０￣１０１０℃的温度范围内,考察了玻璃微珠填充聚丙烯中微珠的含量及其粒径对复合材料动态力学性能的影响。结果表明,室浊下的贮能模量和 损耗模量随着微珠体积分数Φｆ的增加而呈非线性形式增大;在相同条件下,最大粒径微珠填充体系的动态模量高于较小粒径微珠填充体系;微珠含量和粒径对复合材料的阻尼的影响不明显;在Φｆ５％￣１５％范围内,玻璃化转变温度随着Φｆ的增加而增大,然后随之下     

Experimental characterisation of the drying effect on uniaxial mechanical behaviour of mortar

The main objective of this present study was to evaluate, for a standard mortar, the drying effect on its mechanical behaviour. Numerous uniaxial compression tests were thus performed with loading-unloading cycles. They were carried out on different samples previously preserved under various conditions of conservation: preserved from desiccation, air drying and rapid drying at 60°C. The obtained results showed significant influences of these conditions on the material behaviour (increase in strength, decrease in Young's modulus and Poisson's ratio) and the necessity of taking into account the coupling effects between mechanical—poromechanical behaviours and drying.     

Thermal Stresses in Auxetic Plates and Shells

Solids that possess negative Poisson's ratio are termed auxetic materials. This article considers the effect of auxeticity, i.e., the negative extent of Poisson's ratio on thermal stresses in plates, shells, and other solids. Results show that in cases where temperature gradient is in the through-thickness of a fully-clamped plate or when the temperature gradient is in the radial direction of fully-clamped shells, the thermal stresses can be significantly reduced by using auxetic materials. The same result applies to spheres and cylinders with radial temperature gradient. The maximum thermal stress is minimized if the selected material possesses Poisson's ratio of –1. However, the use of auxetic material is not always advantageous, nor does the use of materials with Poisson's ratio of –1 always minimize the maximum thermal stresses. It is herein suggested that, in addition to the use of materials with lower modulus and lower coefficient of thermal expansion, the use of auxetic materials offers an alternative route for lowering thermal stresses in some cases.     

Variability of Poisson's Ratio and Enhanced Ductility in Amorphous Metal

Ductile bulk metallic glass of composition 53.0Zr–18.7Cu–12.0Ni–16.3Al (at%) is plastically deformed under uniaxial compression and observed in situ by synchrotron high‐energy X‐ray diffraction. The diffraction patterns reveal the induced atomic strain is orientation dependent. At the onset of plastic deformation, the atomic strain in the compression direction saturates to a close‐nearest‐neighbor distance while atoms relax in the transverse direction. The ever increasing transverse atomic strain expresses in an augmentation of the apparent Poisson's ratio up to ν = 0.5, which is consistent with volume conservation. Contradicting phenomena from linear mechanics, such as the non‐vanishing shear modulus at ν = 0.5 can be explained by the non‐affine character of the deformation, giving rise to characteristics of a localized martensitic phase transformation. The findings explain the often‐reported phenomena such as, the high Poisson's ratio values found in metallic glasses, the partially liquid character of the structure, the free volume increase and the Bauschinger effect.     

TC4负泊松比复合结构的人工骨力学性能调控研究

目的 基于不同变形机制的负泊松比结构优化设计新型复合多孔结构样件,增加力学性能的调控维度,以满足人体骨低弹性模量的匹配要求。方法 用内凹多边形替代手性结构的圆环,以获得新型的复合胞元结构。利用选区激光熔化成形技术制备负泊松比多孔人工骨样件,通过压缩实验揭示胞元结构类型、结构参数、孔隙率对屈服强度、弹性模量的影响规律,评测不同结构样件与人体骨间的力学性能匹配程度。结果 当孔隙率为65%~85%时,复合结构样件的成形质量、力学性能基本介于手性结构的和内凹结构的之间,且与孔隙率密切相关。手性结构、内凹结构和复合结构的弹性模量分别为2.39~4.64、1.12~3.77、1.01~3.47 GPa,屈服强度分别为65.19~223.06、45.25~195.81、26.54~143.58 MPa。复合结构的弹性模量随环径和内凹角度的增大而减小。当孔隙率为75%时,环径由2.4 mm变至2.0 mm,弹性模量由2.651 GPa降低至2.082 GPa。当内凹角度由85°变至65°时,弹性模量则由3.566 GPa降低至1.982 GPa。结论 复合胞元结构可以融合材料特性,增加调控维度,进而匹配人工骨结构的低弹性模量要求。     

Viscoelastic properties of silicone rubber with admixture of SiO2 nanoparticles

Measurements have been carried out to investigate the change of dynamic viscoelastic properties of silicone rubber with the admixture of small amount SiO₂ nanoparticles. A novel measurement method for the dynamic viscoelastic properties including the modulus of elasticity, the loss factor and the Poisson's ratio is employed. It was found that an admixture of rather small amount of SiO2 nanoparticles can substantially affect the viscoelastic properties of the silicone rubber. It is observed that the modulus of elasticity is increased, while the loss factor and the Poisson's ratio are reduced.     

Biaxial monotonic behavior of a multidirectional glass fiber epoxy pipe

This paper presents the results of an experimental investigation into the biaxial monotonic behavior of a filament wound multidirectional glass-fiber/epoxy pipe. Tests were conducted to observe the leakage (functional) failure of the composite pipe subjected to various hoop to axial stress ratios. The change in stiffness in the axial and the hoop directions and change in the Poisson's ratio are discussed. The results show that there is a significant nonlinear change in the stiffness and the Poisson's ratio prior to leakage. The nonlinear change in the Poisson's ratio proved to be a useful indicator of damage, independent of the tested stress rate. Macro and micro observations indicate that a dominant axial tension gave a uniform type of matrix cracking within the specimen, where a dominant hoop stress induced delamination within the laminate.     

A bond-based peridynamic model considering effects of particle rotation and shear influence coefficient

The classical bond-based peridynamic (BPD) model is limited with a fixed Poisson's ratio. To overcome this limitation, an improved BPD model is proposed to analyze the deformation and crack propagation of microelastic brittle materials with emphasis on varying Poisson's ratios. In the proposed model, the bond is subjected to axial and transverse pairwise forces, and the particle rotation angle is added to eliminate the additional bending moment caused by transverse forces, which is a key factor to satisfy the balance of angular momentum exactly. As a result, the bond not only has axial displacement but also has transverse displacement and particle rotation. This extension in the displacement mode overcomes the limitation of the fixed Poisson's ratio in the classical BPD model. The simulation results demonstrate the precision of the improved BPD model and prove its ability to predict deformations and crack propagations.