316 research outputs found
Pressure screening and fluctuations at the bottom of a granular column
We report sets of precise and reproducible measurements on the static
pressure at the bottom of a granular column. We make a quantitative analysis of
the pressure saturation when the column height is increased. We evidence a
great sensitivity of the measurements with the global packing fraction and the
eventual presence of shear bands at the boundaries. We also show the limit of
the classical Janssen model and discuss these experimental results under the
scope of recently proposed theoretical frameworks.Comment: 17 pages, Latex, 8 eps figures, to appear in the European Physical
Journal B (1999
Asymptotic network models of subwavelength metamaterials formed by closely packed photonic and phononic crystals
We demonstrate that photonic and phononic crystals consisting of closely
spaced inclusions constitute a versatile class of subwavelength metamaterials.
Intuitively, the voids and narrow gaps that characterise the crystal form an
interconnected network of Helmholtz-like resonators. We use this intuition to
argue that these continuous photonic (phononic) crystals are in fact
asymptotically equivalent, at low frequencies, to discrete capacitor-inductor
(mass-spring) networks whose lumped parameters we derive explicitly. The
crystals are tantamount to metamaterials as their entire acoustic branch, or
branches when the discrete analogue is polyatomic, is squeezed into a
subwavelength regime where the ratio of wavelength to period scales like the
ratio of period to gap width raised to the power 1/4; at yet larger wavelengths
we accordingly find a comparably large effective refractive index. The fully
analytical dispersion relations predicted by the discrete models yield
dispersion curves that agree with those from finite-element simulations of the
continuous crystals. The insight gained from the network approach is used to
show that, surprisingly, the continuum created by a closely packed hexagonal
lattice of cylinders is represented by a discrete honeycomb lattice. The
analogy is utilised to show that the hexagonal continuum lattice has a
Dirac-point degeneracy that is lifted in a controlled manner by specifying the
area of a symmetry-breaking defect
Super-Arrhenius dynamics for sub-critical crack growth in disordered brittle media
Taking into account stress fluctuations due to thermal noise, we study
thermally activated irreversible crack growth in disordered media. The
influence of material disorder on sub-critical growth of a single crack in
two-dimensional brittle elastic material is described through the introduction
of a rupture threshold distribution. We derive analytical predictions for crack
growth velocity and material lifetime in agreement with direct numerical
calculations. It is claimed that crack growth process is inhibited by disorder:
velocity decreases and lifetime increases with disorder. More precisely,
lifetime is shown to follow a super-Arrhenius law, with an effective
temperature theta - theta_d, where theta is related to the thermodynamical
temperature and theta_d to the disorder variance.Comment: Submitted to Europhysics Letter
Modal expansion for plasmonic resonators in the time domain
We study the electromagnetic field scattered by a metallic nanoparticle with
dispersive material parameters placed in a homogeneous medium in a low
frequency regime. We use asymptotic analysis and spectral theory to diagonalise
a singular integral operator, which allows us to write the field inside and
outside the particle in the form of a complete and orthogonal modal expansion.
We find the eigenvalues of the volume operator to be associated, via a
non-linear relation, to the resonant frequencies of the problem. We prove that
all resonances lie in a bounded region near the origin. Finally we use complex
analysis to compute the Fourier transform of the scattered field and obtain its
modal expansion in the time domain
Green's function probe of a static granular piling
We present an experiment which aim is to investigate the mechanical
properties of a static granular assembly. The piling is an horizontal 3D
granular layer confined in a box, we apply a localized extra force at the
surface and the spatial distribution of stresses at the bottom is obtained (the
mechanical Green's function). For different types of granular media, we observe
a linear pressure response which profile shows one peak centered at the
vertical of the point of application. The peak's width increases linearly when
increasing the depth. This green function seems to be in -at least- qualitative
agreement with predictions of elastic theory.Comment: 9 pages, 3 .eps figures, submitted to PR
Discrepancy between sub-critical and fast rupture roughness: a cumulant analysis
We study the roughness of a crack interface in a sheet of paper. We
distinguish between slow (sub-critical) and fast crack growth regimes. We show
that the fracture roughness is different in the two regimes using a new method
based on a multifractal formalism recently developed in the turbulence
literature. Deviations from monofractality also appear to be different in both
regimes
Confined granular packings: structure, stress, and forces
The structure and stresses of static granular packs in cylindrical containers
are studied using large-scale discrete element molecular dynamics simulations
in three dimensions. We generate packings by both pouring and sedimentation and
examine how the final state depends on the method of construction. The vertical
stress becomes depth-independent for deep piles and we compare these stress
depth-profiles to the classical Janssen theory. The majority of the tangential
forces for particle-wall contacts are found to be close to the Coulomb failure
criterion, in agreement with the theory of Janssen, while particle-particle
contacts in the bulk are far from the Coulomb criterion. In addition, we show
that a linear hydrostatic-like region at the top of the packings unexplained by
the Janssen theory arises because most of the particle-wall tangential forces
in this region are far from the Coulomb yield criterion. The distributions of
particle-particle and particle-wall contact forces exhibit
exponential-like decay at large forces in agreement with previous studies.Comment: 11 pages, 11 figures, submitted to PRE (v2) added new references,
fixed typo
Slow crack growth in polycarbonate films
We study experimentally the slow growth of a single crack in polycarbonate
films submitted to uniaxial and constant imposed stress. The specificity of
fracture in polycarbonate films is the appearance of flame shaped macroscopic
process zones at the tips of the crack. Supported by an experimental study of
the mechanical properties of polycarbonate films, an analysis of the stress
dependence of the mean ratio between the process zone and crack lengths, during
the crack growth, show a quantitative agreement with the Dugdale-Barenblatt
model of the plastic process zone. We find that the fracture growth curves obey
strong scaling properties that lead to a well defined growth master curve
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