1,290 research outputs found
Elastic moduli of model random three-dimensional closed-cell cellular solids
Most cellular solids are random materials, while practically all theoretical
results are for periodic models. To be able to generate theoretical results for
random models, the finite element method (FEM) was used to study the elastic
properties of solids with a closed-cell cellular structure. We have computed
the density () and microstructure dependence of the Young's modulus ()
and Poisson's ratio (PR) for several different isotropic random models based on
Voronoi tessellations and level-cut Gaussian random fields. The effect of
partially open cells is also considered. The results, which are best described
by a power law (), show the influence of randomness
and isotropy on the properties of closed-cell cellular materials, and are found
to be in good agreement with experimental data.Comment: 13 pages, 13 figure
Elastic properties of model porous ceramics
The finite element method (FEM) is used to study the influence of porosity
and pore shape on the elastic properties of model porous ceramics. The Young's
modulus of each model was found to be practically independent of the solid
Poisson's ratio. At a sufficiently high porosity, the Poisson's ratio of the
porous models converged to a fixed value independent of the solid Poisson's
ratio. The Young's modulus of the models is in good agreement with experimental
data. We provide simple formulae which can be used to predict the elastic
properties of ceramics, and allow the accurate interpretation of empirical
property-porosity relations in terms of pore shape and structure.Comment: 17 pages, 13 figure
Dimer percolation and jamming on simple cubic lattice
We consider site percolation of dimers (``neadles'') on simple cubic lattice.
The percolation threshold is estimated as . The jamming threshold is estimated as .Comment: 3 pages, 4 figures, submitted to EPJ
A reaction-diffusion model for the hydration/setting of cement
We propose a heterogeneous reaction-diffusion model for the hydration and
setting of cement. The model is based on diffusional ion transport and on
cement specific chemical dissolution/precipitation reactions under spatial
heterogeneous solid/liquid conditions. We simulate the spatial and temporal
evolution of precipitated micro structures starting from initial random
configurations of anhydrous cement particles. Though the simulations have been
performed for two dimensional systems, we are able to reproduce qualitatively
basic features of the cement hydration problem. The proposed model is also
applicable to general water/mineral systems.Comment: REVTeX (12 pages), 4 postscript figures, tarred, gzipped, uuencoded
using `uufiles', coming with separate file(s). Figure 1 consists of 6 color
plates; if you have no color printer try to send it to a black&white
postscript-plotte
Elastic properties of a tungsten-silver composite by reconstruction and computation
We statistically reconstruct a three-dimensional model of a tungsten-silver
composite from an experimental two-dimensional image. The effective Young's
modulus () of the model is computed in the temperature range 25-1060^o C
using a finite element method. The results are in good agreement with
experimental data. As a test case, we have reconstructed the microstructure and
computed the moduli of the overlapping sphere model. The reconstructed and
overlapping sphere models are examples of bi-continuous (non-particulate)
media. The computed moduli of the models are not generally in good agreement
with the predictions of the self-consistent method. We have also evaluated
three-point variational bounds on the Young's moduli of the models using the
results of Beran, Molyneux, Milton and Phan-Thien. The measured data were close
to the upper bound if the properties of the two phases were similar ().Comment: 23 Pages, 12 Figure
Quantifying Shape of Star-Like Objects Using Shape Curves and A New Compactness Measure
Shape is an important indicator of the physical and chemical behavior of natural and engineered particulate materials (e.g., sediment, sand, rock, volcanic ash). It directly or indirectly affects numerous microscopic and macroscopic geologic, environmental and engineering processes. Due to the complex, highly irregular shapes found in particulate materials, there is a perennial need for quantitative shape descriptions. We developed a new characterization method (shape curve analysis) and a new quantitative measure (compactness, not the topological mathematical definition) by applying a fundamental principle that the geometric anisotropy of an object is a unique signature of its internal spatial distribution of matter. We show that this method is applicable to âstar-likeâ particles, a broad mathematical definition of shape fulfilled by most natural and engineered particulate materials. This new method and measure are designed to be mathematically intermediate between simple parameters like sphericity and full 3D shape descriptions.
For a âstar-likeâ object discretized as a polyhedron made of surface planar elements, each shape curve describes the distribution of elemental surface area or volume. Using several thousand regular and highly irregular 3-D shape representations, built from model or real particles, we demonstrate that shape curves accurately encode geometric anisotropy by mapping surface area and volume information onto a pair of dimensionless 2-D curves. Each shape curve produces an intrinsic property (length of shape curve) that is used to describe a new definition of compactness, a property shown to be independent of translation, rotation, and scale. Compactness exhibits unique values for distinct shapes and is insensitive to changes in measurement resolution and noise. With increasing ability to rapidly capture digital representations of highly irregular 3-D shapes, this work provides a new quantitative shape measure for direct comparison of shape across classes of particulate materials
Deconfinement and Percolation
Using percolation theory, we derive a conceptual definition of deconfinement
in terms of cluster formation. The result is readily applicable to infinite
volume equilibrium matter as well as to finite size pre-equilibrium systems in
nuclear collisions.Comment: 13 pages, latex, six figures include
Correlations in the T Cell Response to Altered Peptide Ligands
The vertebrate immune system is a wonder of modern evolution. Occasionally,
however, correlations within the immune system lead to inappropriate
recruitment of preexisting T cells against novel viral diseases. We present a
random energy theory for the correlations in the naive and memory T cell immune
responses. The non-linear susceptibility of the random energy model to
structural changes captures the correlations in the immune response to mutated
antigens. We show how the sequence-level diversity of the T cell repertoire
drives the dynamics of the immune response against mutated viral antigens.Comment: 21 pages; 6 figures; to appear in Physica
Pressure dependence of the sound velocity in a 2D lattice of Hertz-Mindlin balls: a mean field description
We study the dependence on the external pressure of the velocities
of long wavelength sound waves in a confined 2D h.c.p. lattice of 3D
elastic frictional balls interacting via one-sided Hertz-Mindlin contact
forces, whose diameters exhibit mild dispersion. The presence of an underlying
long range order enables us to build an effective medium description which
incorporates the radial fluctuations of the contact forces acting on a single
site. Due to the non linearity of Hertz elasticity, self-consistency results in
a highly non-linear differential equation for the "equation of state" linking
the effective stiffness of the array with the applied pressure, from which
sound velocities are then obtained. The results are in excellent agreement with
existing experimental results and simulations in the high and intermediate
pressure regimes. It emerges from the analysis that the departure of
from the ideal Hertz behavior must be attributed primarily to the
fluctuations of the stress field, rather than to the pressure dependence of the
number of contacts
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