197 research outputs found
Universal shapes formed by two interacting cracks
We investigate the origins of the widely-observed "en passant" crack pattern
which forms through interactions between two approaching cracks. A rectangular
elastic plate is notched on each long side and then subjected to quasistatic
uniaxial strain from the short side. The two cracks propagate along
approximately straight paths until they pass each other, after which they curve
and release a lenticular fragment. We find that for materials with diverse
mechanical properties, the shape of this fragment has an aspect ratio of 2:1,
with the length scale set by the initial crack offset and the time scale
set by the ratio of to the pulling velocity. The cracks have a universal
square root shape which we understand using a simple geometric model of the
crack-crack interaction
Correlation between Voronoi volumes in disc packings
We measure the two-point correlation of free Voronoi volumes in binary disc
packings, where the packing fraction ranges from 0.8175 to
0.8380. We observe short-ranged correlations over the whole range of and anti-correlations for . The spatial extent of
the anti-correlation increases with while the position of the
maximum of the anti-correlation and the extent of the positive correlation
shrink with . We conjecture that the onset of anti-correlation
corresponds to dilatancy onset in this system
Local origins of volume fraction fluctuations in dense granular materials
Fluctuations of the local volume fraction within granular materials have
previously been observed to decrease as the system approaches jamming. We
experimentally examine the role of boundary conditions and inter-particle
friction on this relationship for a dense granular material of bidisperse
particles driven under either constant volume or constant pressure. Using a
radical Vorono\"i tessellation, we find the variance of the local volume
fraction monotonically decreases as the system becomes more dense,
independent of boundary condition and . We examine the universality and
origins of this trend using experiments and the recent granocentric model
\cite{Clusel-2009-GMR,Corwin-2010-MRP}, modified to draw particle locations
from an arbitrary distribution of neighbor distances . The
mean and variance of the observed are described by a single
length scale controlled by . Through the granocentric model, we
observe that diverse functional forms of all produce the trend of
decreasing fluctuations, but only the experimentally-observed
provides quantitative agreement with the measured fluctuations. Thus, we
find that both and encode similar information
about the ensemble of observed packings, and are connected to each other by the
local granocentric model
"Barchan" dunes in the lab
We demonstrate the feasibility of studying dunes in a laboratory experiment.
It is shown that an initial sand pile, under a wind flow carrying sand,
flattens and gets a shape recalling barchan dunes. An evolution law is proposed
for the profile and the summit of the dune. The dune dynamics is shown to be
shape invariant. The invariant shape, the ``dune function'' is isolated.Comment: 3 pages, 4 figure
Meria – San Paolo
Le site de San Paolo est implanté sur une butte de faible hauteur (60 m d’altitude en moyenne). La colline se dresse à mi-chemin entre la mine d’antimoine de Meria et la marine, située à 1,2 km. Le ruisseau de Meria s’écoule en contrebas jusqu’à la mer. Le site de San Paolo bénéficie d’une vue imprenable sur l’archipel toscan et en particulier sur les îles de Capraia et d’Elbe. En 2013, une prospection a mis en évidence l’intérêt du site dans le cadre d’une problématique centrée sur la défini..
Super-diffusion around the rigidity transition: Levy and the Lilliputians
By analyzing the displacement statistics of an assembly of horizontally
vibrated bidisperse frictional grains in the vicinity of the jamming transition
experimentally studied before, we establish that their superdiffusive motion is
a genuine Levy flight, but with `jump' size very small compared to the diameter
of the grains. The vibration induces a broad distribution of jumps that are
random in time, but correlated in space, and that can be interpreted as
micro-crack events at all scales. As the volume fraction departs from the
critical jamming density, this distribution is truncated at a smaller and
smaller jump size, inducing a crossover towards standard diffusive motion at
long times. This interpretation contrasts with the idea of temporally
persistent, spatially correlated currents and raises new issues regarding the
analysis of the dynamics in terms of vibrational modes.Comment: 7 pages, 6 figure
Kinetic Heterogeneities at Dynamical Crossovers
We perform molecular dynamics simulations of a model glass-forming liquid to
measure the size of kinetic heterogeneities, using a dynamic susceptibility
that quantifies the number of particles whose dynamics
are correlated on the length scale and time scale . By measuring
as a function of both and , we locate local maxima
at distances and times . Near the dynamical
glass transition, we find two types of maxima, both correlated with crossovers
in the dynamical behavior: a smaller maximum corresponding to the crossover
from ballistic to sub-diffusive motion, and a larger maximum corresponding to
the crossover from sub-diffusive to diffusive motion. Our results indicate that
kinetic heterogeneities are not necessarily signatures of an impending glass or
jamming transition.Comment: 6 pages, 4 figure
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