72 research outputs found
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
Dynamic phase diagram of plastically deformed amorphous solids at finite temperature
The yielding transition that occurs in amorphous solids under athermal
quasistatic deformation has been the subject of many theoretical and
computational studies. Here, we extend this analysis to include thermal effects
at finite shear rate, focusing on how temperature alters avalanches. We derive
a nonequilibrium phase diagram capturing how temperature and strain rate
effects compete, when avalanches overlap, and whether finite-size effects
dominate over temperature effects. The predictions are tested through
simulations of an elastoplastic model in two dimensions and in a mean-field
approximation. We find a new scaling for temperature-dependent softening in the
low-strain rate regime when avalanches do not overlap, and a
temperature-dependent Herschel-Bulkley exponent in the high strain rate regime
when avalanches do overlap.Comment: 12 pages, 10 figures, 1 table. Updated to second version June 22,
202
Free energy functionals for efficient phase field crystal modeling of structural phase transformations
The phase field crystal (PFC) method has emerged as a promising technique for
modeling materials with atomistic resolution on mesoscopic time scales. The
approach is numerically much more efficient than classical density functional
theory (CDFT), but its single mode free energy functional only leads to
lattices with triangular (2D) or BCC (3D) symmetries. By returning to a closer
approximation of the CDFT free energy functional, we develop a systematic
construction of two-particle direct correlation functions that allow the study
of a broad class of crystalline structures. This construction examines planar
spacings, lattice symmetries, planar atomic densities and the atomic
vibrational amplitude in the unit cell of the lattice and also provides control
parameters for temperature and anisotropic surface energies. The power of this
new approach is demonstrated by two examples of structural phase
transformations.Comment: 4 pages, 4 figure
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