858 research outputs found
Hydrostatic compression on polypropylene foam
Models currently used to simulate the impact behaviour of polymeric foam at high strain rates use data from mechanical tests. Uniaxial compression is the most common mechanical test used, but the results from this test alone are insufficient to characterise the foam response to three-dimensional stress states. A new experimental apparatus for the study of the foam behaviour under a state of hydrostatic stress is presented. A flywheel was modified to carry out compression tests at high strain rates, and a hydrostatic chamber designed to obtain the variation of stress with volumetric strain, as a function of density and deformation rate. High speed images of the sample deformation under pressure were analysed by image processing. Hydrostatic compression tests were carried out, on polypropylene foams, both quasi statically and at high strain rates. The stressâvolumetric strain response of the foam was determined for samples of foam of density from 35 to 120 kg/m3, loaded at two strain rates. The foam response under hydrostatic compression shows a non-linear elastic stage, followed by a plastic plateau and densification. These were characterised by a compressibility modulus (the slope of the initial stage), a yield stress and a tangent modulus. The foam was transversely isotropic under hydrostatic compression
Power law in the angular velocity distribution of a granular needle
We show how inelastic collisions induce a power law with exponent -3 in the
decay of the angular velocity distribution of anisotropic particles with
sufficiently small moment of inertia. We investigate this question within the
Boltzmann kinetic theory for an elongated granular particle immersed in a bath.
The power law persists so long as the collisions are inelastic for a large
range of angular velocities provided the mass ratio of the anisotropic particle
and the bath particles remains small. Suggestions for observing this peculiar
feature are made.Comment: 8 pages, 4 figure
Aging and response properties in the parking-lot model
An adsorption-desorption (or parking-lot) model can reproduce qualitatively
the densification kinetics and other features of a weakly vibrated granular
material. Here we study the the two-time correlation and response functions of
the model and demonstrate that their behavior is consistent with recently
observed memory effects in granular materials. Although the densification
kinetics and hysteresis are robust properties, we show that the aging behavior
of the adsorption-desorption model is different from other models of granular
compaction. We propose an experimental test to distinguish the possible aging
behaviors.Comment: 9 pages, 7 figures, to appear in Eur. Phys. Jour.
Comment on ``Stripe Glasses: Self-Generated Randomness in a Uniformly Frustrated System''
comment on J. Schmalian and P. Wolynes, Phys. Rev. Lett. {\bf 85}, 836
(2000).Comment: 1 page, 1 Figure, accepted in Phys. Rev. Letter
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