Non-linear properties of polymer cellular materials with a negative Poisson's ratio

Negative Poisson's ratio polymeric cellular solids (re-entrant foams) were studied to ascertain the optimal processing procedures which give rise to the smallest value of Poisson's ratio. The non-linear stress-strain relationship was determined for both conventional and re-entrant foams; it depended upon the permanent volumetric compression achieved during the processing procedure. Poisson's ratio of re-entrant foam measured as a function of strain was found to have a relative minimum at small strains. The toughness of re-entrant foam increased with permanent volumetric compression, and hence with density.     

EXPERIMENTAL STUDY ON SEDIMENT RESUSPENSION IN TAIHU LAKE UNDER DIFFERENT HYDRODYNAMIC DISTURBANCES

Contaminants resuspension in sediments induced by wind-wave could influence the water quality in shallow lakes. Resuspension of surface sediments from the Zhushan Bay, Taihu Lake was simulated under different wind forcing by using a pneumatic annular flume in this study. Acoustic Doppler Velocimeter (ADV) was used to measure flow velocity at each wind speed, and the characteristics of sediment resuspension were studied with the layered sampling technology. The experimental results show that the flow velocity increases with wind speed obviously and 6m/s is a critical wind speed which affected hydrodynamic conditions significantly. The distribution of flow velocity and water depth is different from that in ordinary open channel. With the enhanced hydrodynamic factors, the Suspended Solids Concentration (SSC) in water increases accordingly, and the incipient velocity of sediment resuspension is about 0.21 m/s. Based on the analysis of wind speed and average SSC in water column, the quantitative relationship is obtained. The SSC of the bottom layer is higher than the content of surface layer under different hydrodynamic conditions, and there are similar distributions between SSC and flow velocity in different water layers.     

HYDRAULIC RESISTANCE OF SUBMERGED VEGETATION RELATED TO EFFECTIVE HEIGHT

Submerged vegetation has a significant impact on water flow velocity. Current investigations include the impact through adding drag resistance and increasing bottom roughness coefficient, which cannot elucidate the characters of real submerged vegetation. To evaluate the effects of submerged vegetation on water currents at different velocities, a laboratory experiment was conducted using three kinds of vegetations. The effective heights of these vegetations on varying flow velocities were evaluated. An equation describing the relationship between the normalized resistance of the submerged plants and the Reynolds number based on the plant effective height was then established and used to calculate the hydraulic resistance parameters of submerged plants in different stages of growth.     

Finite element method for orthotropic micropolar elasticity

The total potential energy for a body composed of an anisotropic micropolar linear elastic material is developed and used to formulate a displacement type finite element method of analysis. As an example of this formulation triangular plane stress (and plane couple stress) elements are used to analyze several problems. The program is verified by computing the stress concentration around a hole in an isotropic micropolar material for which an exact analytical solution exists. Several anisotropic material cases are presented which demonstrate the dependence of the stress concentration factor on the micropolar material parameters.     

Elastic and viscoelastic behavior of chiral materials

Chiral materials are not invariant to inversions: there is a distinction between right- and left-handed material. Material properties such as piezoelectricity and pyroelectricity, represented by tensors of odd rank, can only occur in chiral materials. Chiral effects in elasticity cannot be expressed within classical elasticity since the modulus tensor, which is fourth rank, is unchanged under an inversion. We consider effects of chirality in elastic materials described by a generalized continuum representation, specifically Cosserat elasticity. Analysis of several configurations discloses a chiral material to generate reaction moments when compressed as a slab. A chiral plate bent to hyperbolic shape is predicted to exhibit size effects from the Cosserat characteristic length, and a shear force from the chirality. This analysis can be used for the interpretation of experiments on compliant chiral materials, in particular the evaluation of the elastic constants. Viscoelastic chiral solids are examined in the context of the correspondence principle.     

Deformation of extreme viscoelastic metals and composites

The figure of merit for structural damping and damping layer applications is the product of stiffness E and damping tan δ. For most materials, even practical polymer damping layers, E tan δ is less than 0.6 GPa. We consider several methods to achieve high values of this figure of merit: high damping metals, metal matrix composites and composites containing constituents of negative stiffness. As for high damping metals, damping of polycrystalline zinc was determined and compared with InSn studied earlier. Damping of Zn is less dependent on frequency than that of InSn, so Zn is superior at high frequency. High damping and large stiffness anomalies are possible in viscoelastic composites with inclusions of negative stiffness. Negative stiffness entails a reversal of the usual directional relationship between force and displacement in deformed objects. An isolated object with negative stiffness is unstable, but an inclusion embedded in a composite matrix can be stabilized under some circumstances. Ferroelastic domains in the vicinity of a phase transition can exhibit a region of negative stiffness. Metal matrix composites containing vanadium dioxide were prepared and studied. The concentration of embedded particles was sensitive to the processing method.     

Viscoelastic behaviour of composite materials with conventional- or negative-Poisson's-ratio foam as one phase

This article describes experimental investigations of viscoelastic properties of composites consisting of conventional and re-entrant negative-Poisson's-ratio copper foam as a matrix, with the following high-loss-filler materials: viscoelastic elastomer, solder, and indium. The viscoelastic properties of gallium and several ferrites were also determined. The loss tangent of the copper-elastomer composite substantially exceeded the (lower) Voigt limit; the loss tangent of the copper-solder and copper-indium composites were close to the (upper) Hashin limit for the two-solid phases and one-pore phase.     

Analysis of high-loss viscoelastic composites

A theoretical study of the viscoelastic properties of composites is presented with the aim of identifying structures which give rise to a combination of high stiffness and high loss tangent. Laminates with Voigt and Reuss structures, as well as composite materials attaining the Hashin-Shtrickman bounds on stiffness were evaluated by the correspondence principle. Similarly, viscoelastic properties of composites containing spherical or platelet inclusions were explored. Reuss laminates and platelet-filled materials composed of a stiff, low-loss phase and a compliant high-loss phase were found to exhibit high stiffness combined with a high loss tangent.     

Negative Poisson's ratio polymeric and metallic foams

Foam materials based on metal and several polymers were transformed so that their cellular architecture became re-entrant, i.e. with inwardly protruding cell ribs. Foams with re-entrant structures exhibited negative Poisson's ratios as well as greater resilience than conventional foams. Foams with negative Poisson's ratios were prepared using different techniques and materials and their mechanical behaviour and structure evaluated.