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Spatiotemporal correlations between plastic events in the shear flow of athermal amorphous solids
The slow flow of amorphous solids exhibits striking heterogeneities: swift
localised particle rearrangements take place in the midst of a more or less
homogeneously deforming medium. Recently, experimental as well as numerical
work has revealed spatial correlations between these flow heterogeneities.
Here, we use molecular dynamics (MD) simulations to characterise the
rearrangements and systematically probe their correlations both in time and in
space. In particular, these correlations display a four-fold azimuthal symmetry
characteristic of shear stress redistribution in an elastic medium and we
unambiguously detect their increase in range with time. With increasing shear
rate, correlations become shorter-ranged and more isotropic. In addition, we
study a coarse-grained model motivated by the observed flow characteristics and
challenge its predictions directly with the MD simulations. While the model
captures both macroscopic and local properties rather satisfactorily, the
agreement with respect to the spatiotemporal correlations is at most
qualitative. The discrepancies provide important insight into relevant physics
that is missing in all related coarse-grained models that have been developed
for the flow of amorphous materials so far, namely the finite shear wave
velocity and the impact of elastic heterogeneities on stress redistribution
Effects of inertia on the steady-shear rheology of disordered solids
We study the finite-shear-rate rheology of disordered solids by means of
molecular dynamics simulations in two dimensions. By systematically varying the
damping magnitude in the low-temperature limit, we identify two well
defined flow regimes, separated by a thin (temperature-dependent) crossover
region. In the overdamped regime, the athermal rheology is governed by the
competition between elastic forces and viscous forces, whose ratio gives the
Weissenberg number (up to elastic parameters); the
macroscopic stress follows the frequently encountered Herschel-Bulkley
law , with yield stress
\Sigma\_0\textgreater{}0. In the underdamped (inertial) regime, dramatic
changes in the rheology are observed for low damping: the flow curve becomes
non-monotonic. This change is not caused by longer-lived correlations in the
particle dynamics at lower damping; instead, for weak dissipation, the sample
heats up considerably due to, and in proportion to, the driving. By suitably
thermostatting more or less underdamped systems, we show that their rheology
only depends on their kinetic temperature and the shear rate, rescaled with
Einstein's vibration frequency.Comment: Accepted for publication in Phys. Rev. Let
Deformation-induced accelerated dynamics in polymer glasses
Molecular dynamics simulations are used to investigate the effects of
deformation on the segmental dynamics in an aging polymer glass. Individual
particle trajectories are decomposed into a series of discontinuous hops, from
which we obtain the full distribution of relaxation times and displacements
under three deformation protocols: step stress (creep), step strain, and
constant strain rate deformation. As in experiments, the dynamics can be
accelerated by several orders of magnitude during deformation, and the history
dependence is entirely erased during yield (mechanical rejuvenation). Aging can
be explained as a result of the long tails in the relaxation time distribution
of the glass, and similarly, mechanical rejuvenation is understood through the
observed narrowing of this distribution during yield. Although the relaxation
time distributions under deformation are highly protocol specific, in each case
they may be described by a universal acceleration factor that depends only on
the strain.Comment: 15 pages, 15 figure
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