813 research outputs found
Computational modelling and experimental characterisation of heterogeneous materials
Heterogeneous materials can exhibit behaviour under load that cannot be described by classical continuum elasticity. Beams in bending can show a relative stiffening as the beam depth tends to zero, a size effect. Size effects are recognised in higher order continuum elastic theories such as micropolar elasticity. The drawback of higher order theories is the requirement of addition constitutive relations and associated properties that are often difficult to establish experimentally. Furthermore the finite element method, of great benefit in classical elasticity, has shown limitations when applied to micropolar elasticity. The determination of additional constitutive properties and the computational modelling of micropolar elasticity will be discussed in the context of a model heterogeneous material loaded in simple 3 point bending. The model material was created by drilling holes in aluminium bar in a regular pattern, with the hole axis normal to the plane of bending. The bending tests show that a size effect is present. These results are compared against modelling the detailed beam geometries in the finite element package ANSYS, which again shows the size effect. These two bending test are used to extract the additional micropolar elastic material properties. A comparison is then made against analytical solutions,numerical solutions using a micropolar beam finite element and a micropolar plane stress control volume method.It will be shown that the need for extensive experimental testing to determine the additional constitutive properties may not be necessary with the appropriate use of numerical methods
Viscoelasticity of 2D liquids quantified in a dusty plasma experiment
The viscoelasticity of two-dimensional liquids is quantified in an experiment
using a dusty plasma. An experimental method is demonstrated for measuring the
wavenumber-dependent viscosity, , which is a quantitative indicator of
viscoelasticity. Using an expression generalized here to include friction,
is computed from the transverse current autocorrelation function
(TCAF), which is found by tracking random particle motion. The TCAF exhibits an
oscillation that is a signature of elastic contributions to viscoelasticity.
Simulations of a Yukawa liquid are consistent with the experiment.Comment: 5 pages text, 3 figures, 1 supplementary material, in press Physical
Review Letters 201
Biomimetic spatial and temporal (4D) design and fabrication
We imagine the built environment of the future as a ābio-hybrid machine for living inā that will sense and react to activities within the space in order to provide experiences and services that will elevate quality of life while coexisting seamlessly with humans and the natural environment. The study of Hierarchical design in biological materials has the potential to alter the way designers/ engineers/ crafts-men of the future engage with materials in order to realise such visions. We are ex-ploring this design approach using digital manufacturing technologies such as jac-quard weaving and 3D printing
Indentability of conventional and negative Poisson's ratio foams
The indentation resistance of foams, both of conventional structure and of re-entrant structure giving rise to negative Poisson's ratio, is studied using holographic interferometry. In holographic indentation tests, re-entrant foams had higher yield strengths sigma(sub y) and lower stiffness E than conventional foams of the same original relative density. Calculated energy absorption for dynamic impact is considerably higher for re-entrant foam than conventional foam
Viscoelastic Properties of Crystals
We examine the question of whether fluids and crystals are differentiated on
the basis of their zero frequency shear moduli or their limiting zero frequency
shear viscosity. We show that while fluids, in contrast with crystals, do have
a zero value for their shear modulus, in contradiction to a widespread
presumption, a crystal does not have an infinite or exceedingly large value for
its limiting zero frequency shear viscosity. In fact, while the limiting shear
viscosity of a crystal is much larger than that of the liquid from which it is
formed, its viscosity is much less than that of the corresponding glass that
may form assuming the liquid is a good enough glass former.Comment: 13 pages, 3 figure
Damping Composite Materials: Effect of Structural Hierarchy
ABSTRACT: A combination of stiffness and loss (the product Etan ) is desirable in damping layer and structural damping applications. Composite materials of structure which gives rise to Reuss or Hashin-Shtrikman lower bound behavior can give rise to such properties. Hierarchical particulate morphologies attain the HashinShtrikman curve. We show that hierarchical composites give rise to complex Poisson's ratios which, however, have minimal effect on the stiffness-map. We show that structural hierarchy is useful in viscoelastic composites in that it enables the attainment of high concentrations of spherical inclusions, and that it facilitates the attainment of both stiffness and damping. A damping layer upon a substrate is considered as the top level of the structural hierarchy. We demonstrate that if the layer itself is a relatively stiff composite, the penalty usually associated with such a geometry for compliant layers is ameliorated
Negative stiffness and enhanced damping of individual multiwalled carbon nanotubes
The mechanical instabilities and viscoelastic response of individual multiwalled carbon nanotubes and nanofibers (MWCNTs/Fs) under uniaxial compression are studied with atomic force microscopy. Specific buckling events are evident by regimes of negative stiffness, i.e., marked drops in force with increasing compression. Uniaxial cyclic loading can be repeatedly executed even in initially postbuckled regimes, where the CNTs/Fs display incremental negative stiffness. Increases in mechanical damping of 145ā600 % in these initially postbuckled regimes, as compared to the linear prebuckled regimes, are observed. Increased damping is attributed to frictional energy dissipation of walls in buckled configurations of the MWCNTs/Fs. This represents the extension of the concept of negative stiffness to the scale of nanostructures and opens up possibilities for designing nanocomposites with high stiffness and high damping simultaneously
Relative Costs of Achieving Various Levels of Phosphorus Control at Municipal Wasterwater Treatment Plants in the Great Lakes Basin: A Technical Report
Workshop on Environmental Mapping of the Great Lakes: Proceedings of a Symposium Held in Windsor, Ontario, November 8-10, 1976
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