1,468 research outputs found
Dislocation core field. I. Modeling in anisotropic linear elasticity theory
Aside from the Volterra field, dislocations create a core field, which can be
modeled in linear anisotropic elasticity theory with force and dislocation
dipoles. We derive an expression of the elastic energy of a dislocation taking
full account of its core field and show that no cross term exists between the
Volterra and the core fields. We also obtain the contribution of the core field
to the dislocation interaction energy with an external stress, thus showing
that dislocation can interact with a pressure. The additional force that
derives from this core field contribution is proportional to the gradient of
the applied stress. Such a supplementary force on dislocations may be important
in high stress gradient regions, such as close to a crack tip or in a
dislocation pile-up
Generalized Stacking Fault Energy Surfaces and Dislocation Properties of Silicon: A First-Principles Theoretical Study
The generalized stacking fault (GSF) energy surfaces have received
considerable attention due to their close relation to the mechanical properties
of solids. We present a detailed study of the GSF energy surfaces of silicon
within the framework of density functional theory. We have calculated the GSF
energy surfaces for the shuffle and glide set of the (111) plane, and that of
the (100) plane of silicon, paying particular attention to the effects of the
relaxation of atomic coordinates. Based on the calculated GSF energy surfaces
and the Peierls-Nabarro model, we obtain estimates for the dislocation
profiles, core energies, Peierls energies, and the corresponding stresses for
various planar dislocations of silicon.Comment: 9 figures (not included; send requests to [email protected]
Mesoscopic Analysis of Structure and Strength of Dislocation Junctions in FCC Metals
We develop a finite element based dislocation dynamics model to simulate the
structure and strength of dislocation junctions in FCC crystals. The model is
based on anisotropic elasticity theory supplemented by the explicit inclusion
of the separation of perfect dislocations into partial dislocations bounding a
stacking fault. We demonstrate that the model reproduces in precise detail the
structure of the Lomer-Cottrell lock already obtained from atomistic
simulations. In light of this success, we also examine the strength of
junctions culminating in a stress-strength diagram which is the locus of points
in stress space corresponding to dissolution of the junction.Comment: 9 Pages + 4 Figure
T Pyxidis: The First Cataclysmic Variable with a Collimated Jet
We present the first observational evidence for a collimated jet in a
cataclysmic variable system; the recurrent nova T Pyxidis. Optical spectra show
bipolar components of H with velocities , very similar
to those observed in the supersoft X-ray sources and in SS 433. We argue that a
key ingredient of the formation of jets in the supersoft X-ray sources and T
Pyx (in addition to an accretion disk threaded by a vertical magnetic field),
is the presence of nuclear burning on the surface of the white dwarf.Comment: 10 pages 2 figures to appear in ApJ Letter
Theoretical study of dislocation nucleation from simple surface defects in semiconductors
Large-scale atomistic calculations, using empirical potentials for modeling
semiconductors, have been performed on a stressed system with linear surface
defects like steps. Although the elastic limits of systems with surface defects
remain close to the theoretical strength, the results show that these defects
weaken the atomic structure, initializing plastic deformations, in particular
dislocations. The character of the dislocation nucleated can be predicted
considering both the resolved shear stress related to the applied stress
orientation and the Peierls stress. At low temperature, only glide events in
the shuffle set planes are observed. Then they progressively disappear and are
replaced by amorphization/melting zones at a temperature higher than 900 K
Hydrodynamical Models of Outflow Collimation in YSOs
We explore the physics of time-dependent hydrodynamic collimation of jets
from Young Stellar Objects (YSOs). Using parameters appropriate to YSOs we have
carried out high resolution hydrodynamic simulations modeling the interaction
of a central wind with an environment characterized by a moderate opening angle
toroidal density distribution. The results show that the the wind/environment
interaction produces strongly collimated supersonic jets. The jet is composed
of shocked wind gas. Using analytical models of wind blown bubble evolution we
show that the scenario studied here should be applicable to YSOs and can, in
principle, initiate collimation on the correct scales (R ~ 100 AU). The
simulations reveal a number of time-dependent non-linear features not
anticipated in previous analytical studies including: a prolate wind shock; a
chimney of cold swept-up ambient material dragged into the bubble cavity; a
plug of dense material between the jet and bow shocks. We find that the
collimation of the jet occurs through both de Laval nozzles and focusing of the
wind via the prolate wind shock. Using an analytical model for shock focusing
we demonstrate that a prolate wind shock can, by itself, produce highly
collimated supersonic jets.Comment: Accepted by ApJ, 31 pages with 12 figures (3 JPEG's) now included,
using aasms.sty, Also available in postscript via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/SFIC1/SFIC.tar.g
Whole-organism concentration ratios in wildlife inhabiting Australian uranium mining environments
Wildlife concentration ratios for 226Ra, 210Pb, 210Po and isotopes of Th and U from soil, water, and sediments were evaluated for a range of Australian uranium mining environments. Whole-organism concentration ratios (CRwo-media) were developed for 271 radionuclide-organism pairs within the terrestrial and freshwater wildlife groups. Australian wildlife often has distinct physiological attributes, such as the lower metabolic rates of macropod marsupials as compared with placental mammals. In addition, the Australian CRswo-media originate from tropical and semi-arid climates, rather than from the temperate-dominated climates of Europe and North America from which most (>90%) of internationally available CRwo-media values originate. When compared, the Australian and non-Australian CRs are significantly different for some wildlife categories (e.g. grasses, mammals) but not others (e.g. shrubs). Where differences exist, the Australian values were higher, suggesting that site-, or region-specific CRswo-media should be used in detailed Australian assessments. However, in screening studies, use of the international mean values in the Wildlife Transfer Database (WTD) appears to be appropriate, as long as the values used encompass the Australian 95th percentile values. Gaps in the Australian datasets include a lack of marine parameters, and no CR data are available for freshwater phytoplankton, zooplankton, insects, insect larvae or amphibians; for terrestrial environments, there are no data for amphibians, annelids, ferns, fungi or lichens & bryophytes. The new Australian specific parameters will aide in evaluating remediation plans and ongoing operations at mining and waste sites within Australia. They have also substantially bolstered the body of U- and Th-series CRwo-media data for use internationally
Optical metrology alignment and impact on the measurement performance of the LISA Technology Package
Edge dislocations in crystal structures considered as traveling waves of discrete models
The static stress needed to depin a 2D edge dislocation, the lower dynamic
stress needed to keep it moving, its velocity and displacement vector profile
are calculated from first principles. We use a simplified discrete model whose
far field distortion tensor decays algebraically with distance as in the usual
elasticity. An analytical description of dislocation depinning in the strongly
overdamped case (including the effect of fluctuations) is also given. A set of
parallel edge dislocations whose centers are far from each other can depin
a given one provided , where is the average inter-dislocation
distance divided by the Burgers vector of a single dislocation. Then a limiting
dislocation density can be defined and calculated in simple cases.Comment: 10 pages, 3 eps figures, Revtex 4. Final version, corrected minor
error
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