838 research outputs found
Dislocation Core Energies and Core Fields from First Principles
Ab initio calculations in bcc iron show that a screw dislocation
induces a short-range dilatation field in addition to the Volterra elastic
field. This core field is modeled in anisotropic elastic theory using force
dipoles. The elastic modeling thus better reproduces the atom displacements
observed in ab initio calculations. Including this core field in the
computation of the elastic energy allows deriving a core energy which converges
faster with the cell size, thus leading to a result which does not depend on
the geometry of the dislocation array used for the simulation.Comment: DOI: 10.1103/PhysRevLett.102.05550
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
Recognizing flu-like symptoms from videos
© 2014 Hue Thi et al.; licensee BioMed Central Ltd. Background: Vision-based surveillance and monitoring is a potential alternative for early detection of respiratory disease outbreaks in urban areas complementing molecular diagnostics and hospital and doctor visit-based alert systems. Visible actions representing typical flu-like symptoms include sneeze and cough that are associated with changing patterns of hand to head distances, among others. The technical difficulties lie in the high complexity and large variation of those actions as well as numerous similar background actions such as scratching head, cell phone use, eating, drinking and so on. Results: In this paper, we make a first attempt at the challenging problem of recognizing flu-like symptoms from videos. Since there was no related dataset available, we created a new public health dataset for action recognition that includes two major flu-like symptom related actions (sneeze and cough) and a number of background actions. We also developed a suitable novel algorithm by introducing two types of Action Matching Kernels, where both types aim to integrate two aspects of local features, namely the space-time layout and the Bag-of-Words representations. In particular, we show that the Pyramid Match Kernel and Spatial Pyramid Matching are both special cases of our proposed kernels. Besides experimenting on standard testbed, the proposed algorithm is evaluated also on the new sneeze and cough set. Empirically, we observe that our approach achieves competitive performance compared to the state-of-the-arts, while recognition on the new public health dataset is shown to be a non-trivial task even with simple single person unobstructed view. Conclusions: Our sneeze and cough video dataset and newly developed action recognition algorithm is the first of its kind and aims to kick-start the field of action recognition of flu-like symptoms from videos. It will be challenging but necessary in future developments to consider more complex real-life scenario of detecting these actions simultaneously from multiple persons in possibly crowded environments
Primary and secondary electrical space power based on advanced PEM systems
For new space ventures, power continues to be a pacing function for mission planning and experiment endurance. Although electrochemical power is a well demonstrated space power technology, current hardware limitations impact future mission viability. In order to document and augment electrochemical technology, a series of experiments for the National Aeronautics and Space Administration Lewis Research Center (NASA LeRC) are underway at the Los Alamos National Laboratory that define operational parameters on contemporary proton exchange membrane (PEM) hardware operating with hydrogen and oxygen reactants. Because of the high efficiency possible for water electrolysis, this hardware is also thought part of a secondary battery design built around stored reactants - the so-called regenerative fuel cell. An overview of stack testing at Los Alamos and of analyses related to regenerative fuel cell systems are provided in this paper. Finally, this paper describes work looking at innovative concepts that remove complexity from stack hardware with the specific intent of higher system reliability. This new concept offers the potential for unprecedented electrochemical power system energy densities
Nonequilibrium brittle fracture propagation: Steady state, oscillations and intermittency
A minimal model is constructed for two-dimensional fracture propagation. The
heterogeneous process zone is presumed to suppress stress relaxation rate,
leading to non-quasistatic behavior. Using the Yoffe solution, I construct and
solve a dynamical equation for the tip stress. I discuss a generic tip velocity
response to local stress and find that noise-free propagation is either at
steady state or oscillatory, depending only on one material parameter. Noise
gives rise to intermittency and quasi-periodicity. The theory explains the
velocity oscillations and the complicated behavior seen in polymeric and
amorphous brittle materials. I suggest experimental verifications and new
connections between velocity measurements and material properties.Comment: To appear in Phys. Rev. Lett., 6 pages, self-contained TeX file, 3
postscript figures upon request from author at [email protected] or
[email protected], http://cnls-www.lanl.gov/homepages/rafi/rafindex.htm
Screw dislocation in zirconium: An ab initio study
Plasticity in zirconium is controlled by 1/3 screw dislocations
gliding in the prism planes of the hexagonal close-packed structure. This
prismatic and not basal glide is observed for a given set of transition metals
like zirconium and is known to be related to the number of valence electrons in
the d band. We use ab initio calculations based on the density functional
theory to study the core structure of screw dislocations in zirconium.
Dislocations are found to dissociate in the prism plane in two partial
dislocations, each with a pure screw character. Ab initio calculations also
show that the dissociation in the basal plane is unstable. We calculate then
the Peierls barrier for a screw dislocation gliding in the prism plane and
obtain a small barrier. The Peierls stress deduced from this barrier is lower
than 21 MPa, which is in agreement with experimental data. The ability of an
empirical potential relying on the embedded atom method (EAM) to model
dislocations in zirconium is also tested against these ab initio calculations
Dynamic ductile to brittle transition in a one-dimensional model of viscoplasticity
We study two closely related, nonlinear models of a viscoplastic solid. These
models capture essential features of plasticity over a wide range of strain
rates and applied stresses. They exhibit inelastic strain relaxation and steady
flow above a well defined yield stress. In this paper, we describe a first step
in exploring the implications of these models for theories of fracture and
related phenomena. We consider a one dimensional problem of decohesion from a
substrate of a membrane that obeys the viscoplastic constitutive equations that
we have constructed. We find that, quite generally, when the yield stress
becomes smaller than some threshold value, the energy required for steady
decohesion becomes a non-monotonic function of the decohesion speed. As a
consequence, steady state decohesion at certain speeds becomes unstable. We
believe that these results are relevant to understanding the ductile to brittle
transition as well as fracture stability.Comment: 10 pages, REVTeX, 12 postscript figure
The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Spectral Properties and Energetics
Observations of gamma ray bursts (GRBs) with Swift produced the initially
surprising result that many bursts have large X-ray flares superimposed on the
underlying afterglow. The flares were sometimes intense, had rapid rise and
decay phases, and occurred late relative to the ``prompt'' phase. Some
remarkable flares are observed with fluence comparable to the prompt GRB
fluence. Many GRBs have several flares, which are sometimes overlapping. Short,
intense, repetitive, and late flaring can be most easily understood within the
context of the standard fireball model with the internal engine that powers the
prompt GRB emission in an active state at late times. However, other models for
flares have been proposed. Flare origin can be investigated by comparing the
flare spectra to that of the afterglow and the initial prompt emission. In this
work, we have analyzed all significant X-ray flares from the first 110 GRBs
observed by Swift. From this sample 33 GRBs were found to have significant
X-ray flares, with 77 flares that were detected above the 3 level. In
addition to temporal analysis presented in a companion paper, a variety of
spectral models have been fit to each flare. In some cases, we find that the
spectral fits favor a Band function model, which is more akin to the prompt
emission than to that of an afterglow. We find that the average fluence of the
flares is 2.4e-7 erg/cm^2/s in the 0.2-10 keV energy band, which is
approximately a factor of ten below the average prompt GRB fluence. These
results, when combined with those presented in the companion paper on temporal
properties of flares, supports the hypothesis that most X-ray flares are
late-time activity of the internal engine that spawned the initial GRB; not an
afterglow related effect.Comment: accepted by ApJ; 39 pages with 14 figures and 7 table
The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Temporal Properties and Morphology
We present the first systematic investigation of the morphological and timing
properties of flares in GRBs observed by Swift/XRT. We consider a large sample
drawn from all GRBs detected by Swift, INTEGRAL and HETE-2 prior to 2006 Jan
31, which had an XRT follow-up and which showed significant flaring. Our sample
of 33 GRBs includes long and short, at low and high redshift, and a total of 69
flares. The strongest flares occur in the early phases, with a clear
anti-correlation between the flare peak intensity and the flare time of
occurrence. Fitting each X-ray flare with a Gaussian model, we find that the
mean ratio of the width and peak time is = 0.13+/-0.10, albeit
with a large scatter. Late flares at times > 2000 seconds have long durations,
Delta t>300 s, and can be very energetic compared to the underlying continuum.
We further investigated if there is a clear link between the number of pulses
detected in the prompt phase by BAT and the number of X-ray flares detected by
XRT, finding no correlation. However, we find that the distribution of
intensity ratios between successive BAT prompt pulses and that between
successive XRT flares is the same, an indication of a common origin for
gamma-ray pulses and X-ray flares. All evidence indicates that flares are
indeed related to the workings of the central engine and, within the standard
fireball scenario, originate from internal shocks rather than external shocks.
While all flares can be explained by long-lasting engine activity, 29/69 flares
may also be explained by refreshed shocks. However, 10 can only be explained by
prolonged activity of the central engine.Comment: submitted to Ap
Interaction Effects in a One-Dimensional Constriction
We have investigated the transport properties of one-dimensional (1D)
constrictions defined by split-gates in high quality GaAs/AlGaAs
heterostructures. In addition to the usual quantized conductance plateaus, the
equilibrium conductance shows a structure close to , and in
consolidating our previous work [K.~J. Thomas et al., Phys. Rev. Lett. 77, 135
(1996)] this 0.7 structure has been investigated in a wide range of samples as
a function of temperature, carrier density, in-plane magnetic field
and source-drain voltage . We show that the 0.7
structure is not due to transmission or resonance effects, nor does it arise
from the asymmetry of the heterojunction in the growth direction. All the 1D
subbands show Zeeman splitting at high , and in the wide channel
limit the -factor is , close to that of bulk GaAs.
As the channel is progressively narrowed we measure an exchange-enhanced
-factor. The measurements establish that the 0.7 structure is related to
spin, and that electron-electron interactions become important for the last few
conducting 1D subbands.Comment: 8 pages, 7 figures (accepted in Phys. Rev. B
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