11,472 research outputs found
Dynamics of Large-Scale Plastic Deformation and the Necking Instability in Amorphous Solids
We use the shear transformation zone (STZ) theory of dynamic plasticity to
study the necking instability in a two-dimensional strip of amorphous solid.
Our Eulerian description of large-scale deformation allows us to follow the
instability far into the nonlinear regime. We find a strong rate dependence;
the higher the applied strain rate, the further the strip extends before the
onset of instability. The material hardens outside the necking region, but the
description of plastic flow within the neck is distinctly different from that
of conventional time-independent theories of plasticity.Comment: 4 pages, 3 figures (eps), revtex4, added references, changed and
added content, resubmitted to PR
Rate dependent shear bands in a shear transformation zone model of amorphous solids
We use Shear Transformation Zone (STZ) theory to develop a deformation map
for amorphous solids as a function of the imposed shear rate and initial
material preparation. The STZ formulation incorporates recent simulation
results [Haxton and Liu, PRL 99 195701 (2007)] showing that the steady state
effective temperature is rate dependent. The resulting model predicts a wide
range of deformation behavior as a function of the initial conditions,
including homogeneous deformation, broad shear bands, extremely thin shear
bands, and the onset of material failure. In particular, the STZ model predicts
homogeneous deformation for shorter quench times and lower strain rates, and
inhomogeneous deformation for longer quench times and higher strain rates. The
location of the transition between homogeneous and inhomogeneous flow on the
deformation map is determined in part by the steady state effective
temperature, which is likely material dependent. This model also suggests that
material failure occurs due to a runaway feedback between shear heating and the
local disorder, and provides an explanation for the thickness of shear bands
near the onset of material failure. We find that this model, which resolves
dynamics within a sheared material interface, predicts that the stress weakens
with strain much more rapidly than a similar model which uses a single state
variable to specify internal dynamics on the interface.Comment: 10 pages, 13 figures, corrected typos, added section on rate
strengthening vs. rate weakening material
Shear flow of angular grains: acoustic effects and non-monotonic rate dependence of volume
Naturally-occurring granular materials often consist of angular particles
whose shape and frictional characteristics may have important implications on
macroscopic flow rheology. In this paper, we provide a theoretical account for
the peculiar phenomenon of auto-acoustic compaction -- non-monotonic variation
of shear band volume with shear rate in angular particles -- recently observed
in experiments. Our approach is based on the notion that the volume of a
granular material is determined by an effective-disorder temperature known as
the compactivity. Noise sources in a driven granular material couple its
various degrees of freedom and the environment, causing the flow of entropy
between them. The grain-scale dynamics is described by the
shear-transformation-zone (STZ) theory of granular flow, which accounts for
irreversible plastic deformation in terms of localized flow defects whose
density is governed by the state of configurational disorder. To model the
effects of grain shape and frictional characteristics, we propose an Ising-like
internal variable to account for nearest-neighbor grain interlocking and
geometric frustration, and interpret the effect of friction as an acoustic
noise strength. We show quantitative agreement between experimental
measurements and theoretical predictions, and propose additional experiments
that provide stringent tests on the new theoretical elements.Comment: 12 pages, 3 figure
Stick-slip instabilities in sheared granular flow: the role of friction and acoustic vibrations
We propose a theory of shear flow in dense granular materials. A key
ingredient of the theory is an effective temperature that determines how the
material responds to external driving forces such as shear stresses and
vibrations. We show that, within our model, friction between grains produces
stick-slip behavior at intermediate shear rates, even if the material is
rate-strengthening at larger rates. In addition, externally generated acoustic
vibrations alter the stick-slip amplitude, or suppress stick-slip altogether,
depending on the pressure and shear rate. We construct a phase diagram that
indicates the parameter regimes for which stick-slip occurs in the presence and
absence of acoustic vibrations of a fixed amplitude and frequency. These
results connect the microscopic physics to macroscopic dynamics, and thus
produce useful information about a variety of granular phenomena including
rupture and slip along earthquake faults, the remote triggering of
instabilities, and the control of friction in material processing.Comment: 12 pages, 8 figure
Young red supergiants and the near infrared light appearance of disk galaxies
Disk galaxies often show prominent nonaxisymmetric features at near-infrared
wavelengths. Such features may indicate variations in the surface density of
stellar mass, contributions from young red supergiants in star forming regions,
or substantial dust obscuration. To distinguish among these possibilities, we
have searched for spatial variations in the 2.3 micron photometric CO index
within the disks of three nearby galaxies (NGC 278, NGC 2649, & NGC 5713). This
index measures the strength of the absorption bands of molecular CO in stellar
atmospheres, and is strong in cool, low surface-gravity stars, reaching the
largest values for red supergiants. We observe significant spatial CO index
variations in two galaxies (NGC 278 & NGC 5713), indicating that the dominant
stellar population in the near-infrared is not everywhere the same. Central CO
index peaks are present in two galaxies; these could be due to either
metallicity gradients or recent star formation activity. In addition,
significant azimuthal CO index variations are seen in NGC 278. Because strong
azimuthal metallicity gradients are physically implausible in disk galaxies,
these features are most naturally explained by the presence of a young stellar
population. The fraction of 2 micron light due to young stellar populations in
star forming regions can be calculated from our data. Overall, young stellar
populations can contribute ~3% of a (normal) galaxy's near infrared flux.
Locally, this fraction may rise to ~33%. Thus, young stars do not dominate the
total near infrared flux, but can be locally dominant in star forming regions,
and can bias estimates of spiral arm amplitude or other nonaxisymmetric
structures in galaxies' mass distributions.Comment: 28 pages including 3 postscript figures. A fourth figure is in jpeg
format. Uses AASTeX. Accepted for publication in The Astronomical Journa
Metastability in Two Dimensions and the Effective Potential
We study analytically and numerically the decay of a metastable phase in
(2+1)-dimensional classical scalar field theory coupled to a heat bath, which
is equivalent to two-dimensional Euclidean quantum field theory at zero
temperature. By a numerical simulation we obtain the nucleation barrier as a
function of the parameters of the potential, and compare it to the theoretical
prediction from the bounce (critical bubble) calculation. We find the
nucleation barrier to be accurately predicted by theory using the bounce
configuration obtained from the tree-level (``classical'') effective action.
Within the range of parameters probed, we found that using the bounce derived
from the one-loop effective action requires an unnaturally large prefactor to
match the lattice results. Deviations from the tree-level prediction are seen
in the regime where loop corrections would be expected to become important.Comment: 13pp, LaTex with Postscript figs, CLNS 93/1202, DART-HEP-93/0
Dynamics and Thermodynamics of the Glass Transition
The principal theme of this paper is that anomalously slow, super-Arrhenius
relaxations in glassy materials may be activated processes involving chains of
molecular displacements. As pointed out in a preceding paper with A. Lemaitre,
the entropy of critically long excitation chains can enable them to grow
without bound, thus activating stable thermal fluctuations in the local density
or molecular coordination of the material. I argue here that the intrinsic
molecular-scale disorder in a glass plays an essential role in determining the
activation rate for such chains, and show that a simple disorder-related
correction to the earlier theory recovers the Vogel-Fulcher law in three
dimensions. A key feature of this theory is that the spatial extent of
critically long excitation chains diverges at the Vogel-Fulcher temperature. I
speculate that this diverging length scale implies that, as the temperature
decreases, increasingly large regions of the system become frozen and do not
contribute to the configurational entropy, and thus ergodicity is partially
broken in the super-Arrhenius region above the Kauzmann temperature . This
partially broken ergodicity seems to explain the vanishing entropy at and
other observed relations between dynamics and thermodynamics at the glass
transition.Comment: 20 pages, no figures, some further revision
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