3,868 research outputs found
Direct evidence of plastic events and dynamic heterogeneities in soft-glasses
By using fluid-kinetic simulations of confined and concentrated emulsion
droplets, we investigate the nature of space non-homogeneity in soft-glassy
dynamics and provide quantitative measurements of the statistical features of
plastic events in the proximity of the yield-stress threshold. Above the yield
stress, our results show the existence of a finite stress correlation scale,
which can be mapped directly onto the {\it cooperativity scale}, recently
introduced in the literature to capture non-local effects in the soft-glassy
dynamics. In this regime, the emergence of a separate boundary (wall) rheology
with higher fluidity than the bulk, is highlighted in terms of near-wall
spontaneous segregation of plastic events. Near the yield stress, where the
cooperative scale cannot be estimated with sufficient accuracy, the system
shows a clear increase of the stress correlation scale, whereas plastic events
exhibit intermittent clustering in time, with no preferential spatial location.
A quantitative measurement of the space-time correlation associated with the
motion of the interface of the droplets is key to spot the long-range amorphous
order at the yield stress threshold
Modeling the mechanics of amorphous solids at different length and time scales
We review the recent literature on the simulation of the structure and
deformation of amorphous glasses, including oxide and metallic glasses. We
consider simulations at different length and time scales. At the nanometer
scale, we review studies based on atomistic simulations, with a particular
emphasis on the role of the potential energy landscape and of the temperature.
At the micrometer scale, we present the different mesoscopic models of
amorphous plasticity and show the relation between shear banding and the type
of disorder and correlations (e.g. elastic) included in the models. At the
macroscopic range, we review the different constitutive laws used in finite
element simulations. We end the review by a critical discussion on the
opportunities and challenges offered by multiscale modeling and transfer of
information between scales to study amorphous plasticity.Comment: 58 pages, 14 figure
Amorphous Systems in Athermal, Quasistatic Shear
We present results on a series of 2D atomistic computer simulations of
amorphous systems subjected to simple shear in the athermal, quasistatic limit.
The athermal quasistatic trajectories are shown to separate into smooth,
reversible elastic branches which are intermittently broken by discrete
catastrophic plastic events. The onset of a typical plastic event is studied
with precision, and it is shown that the mode of the system which is
responsible for the loss of stability has structure in real space which is
consistent with a quadrupolar source acting on an elastic matrix. The plastic
events themselves are shown to be composed of localized shear transformations
which organize into lines of slip which span the length of the simulation cell,
and a mechanism for the organization is discussed. Although within a single
event there are strong spatial correlations in the deformation, we find little
correlation from one event to the next, and these transient lines of slip are
not to be confounded with the persistent regions of localized shear --
so-called "shear bands" -- found in related studies. The slip lines gives rise
to particular scalings with system length of various measures of event size.
Strikingly, data obtained using three differing interaction potentials can be
brought into quantitative agreement after a simple rescaling, emphasizing the
insensitivity of the emergent plastic behavior in these disordered systems to
the precise details of the underlying interactions. The results should be
relevant to understanding plastic deformation in systems such as metallic
glasses well below their glass temperature, soft glassy systems (such as dense
emulsions), or compressed granular materials.Comment: 21 pages, 18 figure
A mesoscopic model for the rheology of soft amorphous solids, with application to mi- crochannel flows
We study a mesoscopic model for the flow of amorphous solids. The model is
based on the key features identified at the microscopic level, namely peri- ods
of elastic deformation interspersed with localised rearrangements of parti-
cles that induce long-range elastic deformation. These long-range deformations
are derived following a continuum mechanics approach, in the presence of solid
boundaries, and are included in full in the model. Indeed, they mediate spatial
cooperativity in the flow, whereby a localised rearrangement may lead a distant
region to yield. In particular, we simulate a channel flow and find
manifestations of spatial cooperativity that are consistent with published
experimental obser- vations for concentrated emulsions in microchannels. Two
categories of effects are distinguished. On the one hand, the coupling of
regions subject to different shear rates, for instance,leads to finite shear
rate fluctuations in the seemingly un- sheared "plug" in the centre of the
channel. On the other hand, there is convinc- ing experimental evidence of a
specific rheology near rough walls. We discuss diverse possible physical
origins for this effect, and we suggest that it may be associated with the
bumps of particles into surface asperities as they slide along the wall
Avalanches, loading and finite size effects in 2D amorphous plasticity: results from a finite element model
Crystalline plasticity is strongly interlinked with dislocation mechanics and
nowadays is relatively well understood. Concepts and physical models of plastic
deformation in amorphous materials on the other hand - where the concept of
linear lattice defects is not applicable - still are lagging behind. We
introduce an eigenstrain-based finite element lattice model for simulations of
shear band formation and strain avalanches. Our model allows us to study the
influence of surfaces and finite size effects on the statistics of avalanches.
We find that even with relatively complex loading conditions and open boundary
conditions, critical exponents describing avalanche statistics are unchanged,
which validates the use of simpler scalar lattice-based models to study these
phenomena.Comment: Journal of Statistical Mechanics: Theory and Experiment, 2015, P0201
Strain localization and anisotropic correlations in a mesoscopic model of amorphous plasticity
A mesoscopic model for shear plasticity of amorphous materials in two
dimensions is introduced, and studied through numerical simulations in order to
elucidate the macroscopic (large scale) mechanical behavior. Plastic
deformation is assumed to occur through a series of local reorganizations.
Using a discretization of the mechanical fields on a discrete lattice, local
reorganizations are modeled as local slip events. Local yield stresses are
randomly distributed in space and invariant in time. Each plastic slip event
induces a long-ranged elastic stress redistribution. Rate and thermal effects
are not discussed in the present study. Extremal dynamics allows for recovering
many of the complex features of amorphous plasticity observed experimentally
and in numerical atomistic simulations in the quasi-static regime. In
particular, a quantitative picture of localization, and of the anisotropic
strain correlation both in the initial transient regime, and in the steady
state are provided. In addition, the preparation of the amorphous sample is
shown to have a crucial effect of on the localization behavior
Strain localization and anisotropic correlations in a mesoscopic model of amorphous plasticity
A mesoscopic model for shear plasticity of amorphous materials in two
dimensions is introduced, and studied through numerical simulations in order to
elucidate the macroscopic (large scale) mechanical behavior. Plastic
deformation is assumed to occur through a series of local reorganizations.
Using a discretization of the mechanical fields on a discrete lattice, local
reorganizations are modeled as local slip events. Local yield stresses are
randomly distributed in space and invariant in time. Each plastic slip event
induces a long-ranged elastic stress redistribution. Rate and thermal effects
are not discussed in the present study. Extremal dynamics allows for recovering
many of the complex features of amorphous plasticity observed experimentally
and in numerical atomistic simulations in the quasi-static regime. In
particular, a quantitative picture of localization, and of the anisotropic
strain correlation both in the initial transient regime, and in the steady
state are provided. In addition, the preparation of the amorphous sample is
shown to have a crucial effect of on the localization behavior
- …