538 research outputs found
Plastic deformations in crystal, polycrystal, and glass in binary mixtures under shear: Collective yielding
Using molecular dynamics simulation, we examine the dynamics of crystal,
polycrystal, and glass in a Lennard-Jones binary mixture composed of small and
large particles in two dimensions. The crossovers occur among these states as
the composition c is varied at fixed size ratio. Shear is applied to a system
of 9000 particles in contact with moving boundary layers composed of 1800
particles. The particle configurations are visualized with a sixfold
orientation angle alpha_j(t) and a disorder variable D_j(t) defined for
particle j, where the latter represents the deviation from hexagonal order.
Fundamental plastic elements are classified into dislocation gliding and grain
boundary sliding. At any c, large-scale yielding events occur on the acoustic
time scale. Moreover, they multiply occur in narrow fragile areas, forming
shear bands. The dynamics of plastic flow is highly hierarchical with a wide
range of time scales for slow shearing. We also clarify the relationship
between the shear stress averaged in the bulk region and the wall stress
applied at the boundaries.Comment: 17 pages, 15 figures, to appear in Physical Review
Dynamics of Viscoplastic Deformation in Amorphous Solids
We propose a dynamical theory of low-temperature shear deformation in
amorphous solids. Our analysis is based on molecular-dynamics simulations of a
two-dimensional, two-component noncrystalline system. These numerical
simulations reveal behavior typical of metallic glasses and other viscoplastic
materials, specifically, reversible elastic deformation at small applied
stresses, irreversible plastic deformation at larger stresses, a stress
threshold above which unbounded plastic flow occurs, and a strong dependence of
the state of the system on the history of past deformations. Microscopic
observations suggest that a dynamically complete description of the macroscopic
state of this deforming body requires specifying, in addition to stress and
strain, certain average features of a population of two-state shear
transformation zones. Our introduction of these new state variables into the
constitutive equations for this system is an extension of earlier models of
creep in metallic glasses. In the treatment presented here, we specialize to
temperatures far below the glass transition, and postulate that irreversible
motions are governed by local entropic fluctuations in the volumes of the
transformation zones. In most respects, our theory is in good quantitative
agreement with the rich variety of phenomena seen in the simulations.Comment: 16 pages, 9 figure
Yield conditions for deformation of amorphous polymer glasses
Shear yielding of glassy polymers is usually described in terms of the
pressure-dependent Tresca or von Mises yield criteria. We test these criteria
against molecular dynamics simulations of deformation in amorphous polymer
glasses under triaxial loading conditions that are difficult to realize in
experiments. Difficulties and ambiguities in extending several standard
definitions of the yield point to triaxial loads are described. Two
definitions, the maximum and offset octahedral stresses, are then used to
evaluate the yield stress for a wide range of model parameters. In all cases,
the onset of shear is consistent with the pressure-modified von Mises
criterion, and the pressure coefficient is nearly independent of many
parameters. Under triaxial tensile loading, the mode of failure changes to
cavitation.Comment: 9 pages, 8 figures, revte
Nonlinear Stress Fluctuation Dynamics of Sheared Disordered Wet Foam
Sheared wet foam, which stores elastic energy in bubble deformations, relaxes
stress through bubble rearrangements. The intermittency of bubble
rearrangements in foam leads to effectively stochastic drops in stress that are
followed by periods of elastic increase. We investigate global characteristics
of highly disordered foams over three decades of strain rate and almost two
decades of system size. We characterize the behavior using a range of measures:
average stress, distribution of stress drops, rate of stress drops, and a
normalized fluctuation intensity. There is essentially no dependence on system
size. As a function of strain rate, there is a change in behavior around shear
rates of .Comment: accepted to Physical Review
The Active Jet in NGC 4258 and Its Associated Shocks
We present images and spectra of the active jet and anomalous arms on subparsec through kiloparsec scales in the LINER/Seyfert galaxy NGC 4258 (M106). New VLBA and multiconfiguration VLA images show that, on 0.3-300 pc scales, the jet in projection aligns with (1) the spin axis of the underlying accretion disk and (2) two radio hot spots 24'' S (840 pc) and 49'' N (1.7 kpc) from the nucleus. Hubble Space Telescope WFPC2 [N II] λ6583 + Hα images locate interactions between the jet and the interstellar medium (ISM). The most prominent is a pair of emission line arcs whose apices face away from the galaxy nucleus and envelop the leading edges of the radio hot spots. Ground-based (WHT) spectra with 2 Å resolution confirm that the gaseous kinematics and excitation of both arcs have the spatio-kinematic structure expected for jet working surfaces with a shock velocity 350 ± 100 km s-1. The north shock is oblique and may lie in a nuclear ionization cone. The south shock shows a detached, putative Mach disk. Models suggest that the S shock is a bow shock around a jet whose progress toward us through the galaxy ISM has stalled. This is notable because the inferred outflow axis is misaligned by ~65° (in three dimensions) with the spin axis of the accretion disk. Our emission line ratios and profiles diagnose the physical properties of the shocks, possible Mach disk, and thence the jets. The shocks lie at one end of a swath of kinematically disturbed gas that reaches back to the previously recognized spiral "anomalous arms," suggesting that they are linked dynamically by precession of the central engine; although claimed elsewhere to be bar shocks, the anomalous arms are probably a fossil record of changing jet activity in NGC 4258. Our results imply that the jet has recently moved a long way out of the plane of the galaxy. A deep Taurus Tunable Filter Hα image shows that discrete strands in the anomalous arms persist to galactocentric radii of at least 4' (>8 kpc), indicating an ongoing ISM interaction
21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies
The clumpy maser disks observed in some galactic nuclei mark the outskirts of
the accretion disk that fuels the central black hole and provide a potential
site of nuclear star formation. Unfortunately, most of the gas in maser disks
is currently not being probed; large maser gains favor paths that are
characterized by a small velocity gradient and require rare edge-on
orientations of the disk. Here we propose a method for mapping the atomic
hydrogen distribution in nuclear disks through its 21cm absorption against the
radio continuum glow around the central black hole. In NGC 4258, the 21cm
optical depth may approach unity for high angular-resolution (VLBI) imaging of
coherent clumps which are dominated by thermal broadening and have the column
density inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cm
absorption over the full rotation velocity width of the material in front of
the narrow radio jets gives a mean optical depth of ~0.1. Spectroscopic
searches for the 21cm absorption feature in other galaxies can be used to
identify the large population of inclined gaseous disks which are not masing in
our direction. Follow-up imaging of 21cm silhouettes of accelerating clumps
within these disks can in turn be used to measure cosmological distances.Comment: 4 page
Sheared Solid Materials
We present a time-dependent Ginzburg-Landau model of nonlinear elasticity in
solid materials. We assume that the elastic energy density is a periodic
function of the shear and tetragonal strains owing to the underlying lattice
structure. With this new ingredient, solving the equations yields formation of
dislocation dipoles or slips. In plastic flow high-density dislocations emerge
at large strains to accumulate and grow into shear bands where the strains are
localized. In addition to the elastic displacement, we also introduce the local
free volume {\it m}. For very small the defect structures are metastable
and long-lived where the dislocations are pinned by the Peierls potential
barrier. However, if the shear modulus decreases with increasing {\it m},
accumulation of {\it m} around dislocation cores eventually breaks the Peierls
potential leading to slow relaxations in the stress and the free energy
(aging). As another application of our scheme, we also study dislocation
formation in two-phase alloys (coherency loss) under shear strains, where
dislocations glide preferentially in the softer regions and are trapped at the
interfaces.Comment: 16pages, 11figure
Effects of crack tip geometry on dislocation emission and cleavage: A possible path to enhanced ductility
We present a systematic study of the effect of crack blunting on subsequent
crack propagation and dislocation emission. We show that the stress intensity
factor required to propagate the crack is increased as the crack is blunted by
up to thirteen atomic layers, but only by a relatively modest amount for a
crack with a sharp 60 corner. The effect of the blunting is far less
than would be expected from a smoothly blunted crack; the sharp corners
preserve the stress concentration, reducing the effect of the blunting.
However, for some material parameters blunting changes the preferred
deformation mode from brittle cleavage to dislocation emission. In such
materials, the absorption of preexisting dislocations by the crack tip can
cause the crack tip to be locally arrested, causing a significant increase in
the microscopic toughness of the crack tip. Continuum plasticity models have
shown that even a moderate increase in the microscopic toughness can lead to an
increase in the macroscopic fracture toughness of the material by several
orders of magnitude. We thus propose an atomic-scale mechanism at the crack
tip, that ultimately may lead to a high fracture toughness in some materials
where a sharp crack would seem to be able to propagate in a brittle manner.
Results for blunt cracks loaded in mode II are also presented.Comment: 12 pages, REVTeX using epsfig.sty. 13 PostScript figures. Final
version to appear in Phys. Rev. B. Main changes: Discussion slightly
shortened, one figure remove
Star Formation in the Central 400 pc of the Milky Way: Evidence for a Population of Massive YSOs
The central kpc of the Milky Way might be expected to differ significantly
from the rest of the Galaxy with regard to gas dynamics and the formation of
YSOs. We probe this possibility with mid-infrared observations obtained with
IRAC and MIPS on Spitzer and with MSX. We use color-color diagrams and SED fits
to explore the nature of YSO candidates (including objects with 4.5 micron
excesses possibly due to molecular emission). There is an asymmetry in the
distribution of the candidate YSOs, which tend to be found at negative Galactic
longitudes; this behavior contrasts with that of the molecular gas,
approximately 2/3 of which is at positive longitudes. The small scale height of
these objects suggests that they are within the Galactic center region and are
dynamically young. They lie between two layers of infrared dark clouds and may
have originated from these clouds. We identify new sites for this recent star
formation. The methanol masers appear to be associated with young, embedded
YSOs characterized by 4.5 micron excesses. We use the SEDs of these sources to
estimate their physical characteristics. Within the central 400x50 pc
(|l|<1.3\degr and |b|<10') the star formation rate based on the identification
of Stage I evolutionary phase of YSO candidates is about 0.14 solar mass/yr. We
suggest that a recent burst of star formation took place within the last 10^5
years. This suggestion is also consistent with estimates of star formation
rates within the last ~10^7 years showing a peak around 10^5 years ago. Lastly,
we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of
the Galaxy. This implies that star formation does not appear to be dramatically
affected by the extreme physical conditions in the Galactic center region.Comment: 96 pages, ten tables, 35 figures, ApJ (in press), replaced by a
revised versio
Boundary lubrication with a glassy interface
Recently introduced constitutive equations for the rheology of dense,
disordered materials are investigated in the context of stick-slip experiments
in boundary lubrication. The model is based on a generalization of the shear
transformation zone (STZ) theory, in which plastic deformation is represented
by a population of mesoscopic regions which may undergo non affine deformations
in response to stress. The generalization we study phenomenologically
incorporates the effects of aging and glassy relaxation. Under experimental
conditions associated with typical transitions from stick-slip to steady
sliding and stop start tests, these effects can be dominant, although the full
STZ description is necessary to account for more complex, chaotic transitions
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