8,520 research outputs found
Thermal activation of rupture and slow crack growth in a model of homogenous brittle materials
Slow crack growth in a model of homogenous brittle elastic material is
described as a thermal activation process where stress fluctuations allow to
overcome a breaking threshold through a series of irreversible steps. We study
the case of a single crack in a flat sheet for which analytical predictions can
be made, and compare them with results from the equivalent problem of a 2D
spring network. Good statistical agreement is obtained for the crack growth
profile and final rupture time. The specific scaling of the energy barrier with
stress intensity factor appears as a consequence of irreversibility. In
addition, the model brings out a characteristic growth length whose physical
meaning could be tested experimentally.Comment: To be published in : Europhysics Letter
Hubble Space Telescope Spectroscopy of Brown Dwarfs Discovered with the Wide-field Infrared Survey Explorer
We present a sample of brown dwarfs identified with the {\it Wide-field
Infrared Survey Explorer} (WISE) for which we have obtained {\it Hubble Space
Telescope} ({\it HST}) Wide Field Camera 3 (WFC3) near-infrared grism
spectroscopy. The sample (twenty-two in total) was observed with the G141 grism
covering 1.101.70 m, while fifteen were also observed with the G102
grism, which covers 0.901.10 m. The additional wavelength coverage
provided by the G102 grism allows us to 1) search for spectroscopic features
predicted to emerge at low effective temperatures (e.g.\ ammonia bands) and 2)
construct a smooth spectral sequence across the T/Y boundary. We find no
evidence of absorption due to ammonia in the G102 spectra. Six of these brown
dwarfs are new discoveries, three of which are found to have spectral types of
T8 or T9. The remaining three, WISE J082507.35280548.5 (Y0.5), WISE
J120604.38840110.6 (Y0), and WISE J235402.77024015.0 (Y1) are the
nineteenth, twentieth, and twenty-first spectroscopically confirmed Y dwarfs to
date. We also present {\it HST} grism spectroscopy and reevaluate the spectral
types of five brown dwarfs for which spectral types have been determined
previously using other instruments.Comment: Accepted for publication in the Astrophysical Journal. 20 pages, 18
figures, 7 table
The Australian Orthopaedic Association National Joint Replacement Registry
The document attached has been archived with permission from the editor of the Medical Journal of Australia. An external link to the publisherâs copy is included.In the financial year ending June 2002, 26 689 hip replacements and 26089 knee replacements (total, 52778) were performed in Australia. Hip and knee replacement procedures have increased between 5%-10% each year for the past 10 years, with a combined increase in hip and knee replacement of 13.4% in the past year. The revision rate for hip replacement surgery in Australia is unknown but is estimated to be 20%-24%; the revision rate for hip replacement surgery in Sweden is 7%. Although data collection for the Registry is voluntary, it has 100% compliance from hospitals undertaking joint-replacement surgery.Stephen E Graves, David Davidson, Lisa Ingerson, Philip Ryan, Elizabeth C Griffith, Brian F J McDermott, Heather J McElroy and Nicole L Prat
On the chordae structure and dynamic behaviour of the mitral valve
We develop a fluid-structure interaction (FSI) model of the mitral valve (MV) that uses an anatomically
and physiologically realistic description of the MV leaflets and chordae tendineae. Three different
chordae models â complex, âpseudo-fibreâ, and simplified chordae â are compared to determine how
different chordae representations affect the dynamics of the MV. The leaflets and chordae are modelled as
fibre-reinforced hyperelastic materials, and FSI is modelled using an immersed boundary-finite element
(IB/FE) method. The MV model is first verified under static boundary conditions against the commercial
FE software ABAQUS, and then used to simulate MV dynamics under physiological pressure conditions.
Interesting flow patterns and vortex formulation are observed in all three cases. To quantify the highly
complex system behaviour resulting from FSI, an energy budget analysis of the coupled MV FSI model
is performed. Results show that the complex and pseudo-fibre chordae models yield good valve closure
during systole, but that the simplified chordae model leads to poorer leaflet coaptation and an unrealistic
bulge in the anterior leaflet belly. An energy budget analysis shows that the MV models with complex
and pseudo-fibre chordae have similar energy distribution patterns, but the MV model with the simplified
chordae consumes more energy, especially during valve closing and opening. We find that the complex
chordae and pseudo-fibre chordae have similar impact on the overall MV function, but that the simplified
chordae representation is less accurate. Because a pseudo-fibre chordal structure is easier to construct
and less computationally intensive, it may be a good candidate for modelling MV dynamics or interaction
between the MV and heart in patient-specific applications
Immersed boundary-finite element model of fluid-structure interaction in the aortic root
It has long been recognized that aortic root elasticity helps to ensure
efficient aortic valve closure, but our understanding of the functional
importance of the elasticity and geometry of the aortic root continues to
evolve as increasingly detailed in vivo imaging data become available. Herein,
we describe fluid-structure interaction models of the aortic root, including
the aortic valve leaflets, the sinuses of Valsalva, the aortic annulus, and the
sinotubular junction, that employ a version of Peskin's immersed boundary (IB)
method with a finite element (FE) description of the structural elasticity. We
develop both an idealized model of the root with three-fold symmetry of the
aortic sinuses and valve leaflets, and a more realistic model that accounts for
the differences in the sizes of the left, right, and noncoronary sinuses and
corresponding valve cusps. As in earlier work, we use fiber-based models of the
valve leaflets, but this study extends earlier IB models of the aortic root by
employing incompressible hyperelastic models of the mechanics of the sinuses
and ascending aorta using a constitutive law fit to experimental data from
human aortic root tissue. In vivo pressure loading is accounted for by a
backwards displacement method that determines the unloaded configurations of
the root models. Our models yield realistic cardiac output at physiological
pressures, with low transvalvular pressure differences during forward flow,
minimal regurgitation during valve closure, and realistic pressure loads when
the valve is closed during diastole. Further, results from high-resolution
computations demonstrate that IB models of the aortic valve are able to produce
essentially grid-converged dynamics at practical grid spacings for the
high-Reynolds number flows of the aortic root
The DEEP3 Galaxy Redshift Survey: The Impact of Environment on the Size Evolution of Massive Early-type Galaxies at Intermediate Redshift
Using data drawn from the DEEP2 and DEEP3 Galaxy Redshift Surveys, we
investigate the relationship between the environment and the structure of
galaxies residing on the red sequence at intermediate redshift. Within the
massive (10 < log(M*/Msun) < 11) early-type population at 0.4 < z <1.2, we find
a significant correlation between local galaxy overdensity (or environment) and
galaxy size, such that early-type systems in higher-density regions tend to
have larger effective radii (by ~0.5 kpc or 25% larger) than their counterparts
of equal stellar mass and Sersic index in lower-density environments. This
observed size-density relation is consistent with a model of galaxy formation
in which the evolution of early-type systems at z < 2 is accelerated in
high-density environments such as groups and clusters and in which dry, minor
mergers (versus mechanisms such as quasar feedback) play a central role in the
structural evolution of the massive, early-type galaxy population.Comment: 11 pages, 5 figures, 2 tables; resubmitted to MNRAS after addressing
referee's comments (originally submitted to journal on August 16, 2011
The effect of electromagnetic fields on postoperative pain and locomotor recovery in dogs with acute, severe thoracolumbar intervertebral disc extrusion: a randomized placebo-controlled, prospective clinical trial.
Spinal cord injury (SCI) due to acute intervertebral disc extrusions (IVDE) is common in dogs and is treated by surgical decompression. Dogs with sensorimotor complete injuries have an incomplete recovery. Pulsed electromagnetic fields (PEMF) reduce postoperative pain through anti-inflammatory effects and there is growing evidence for neuroprotective effects. This randomized, controlled clinical trial evaluated the effect of PEMF on post-operative pain and neurologic recovery in dogs with surgically treated sensorimotor complete SCI due to acute IVDE.
Sixteen dogs with surgically treated complete thoracolumbar SCI were randomized to receive PEMF (15 minutes every 2 hours for 2 weeks then twice daily for 4 weeks) or placebo starting immediately after diagnosis. The primary outcome was gait score at 2 weeks. Secondary measures of gait, pain perception and proprioceptive function were evaluated at 2 and 6 weeks. Plasma GFAP concentration was measured as a SCI biomarker. Post-operative pain was quantified by measuring mechanical sensory thresholds (MST) at control and surgical sites.
There was no significant difference in demographics or GFAP concentration between the 2 groups at trial entry. There was no difference in primary outcome or in secondary measures of gait, but proprioceptive placing was significantly better at 6 weeks and GFAP concentrations were significantly lower at 2 weeks in the PEMF group. MSTs were significantly higher in the PEMF treated group. We conclude that PEMF reduced incision-associated pain in dogs following surgery for IVDE and may reduce extent of spinal cord injury and enhance proprioceptive placing. Larger clinical trials are warranted
Mechanisms of Manganese-Assisted Nonradiative Recombination in Cd(Mn)Se/Zn(Mn)Se Quantum Dots
Mechanisms of nonradiative recombination of electron-hole complexes in
Cd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations
of Mn ions are analyzed within the framework of single electron model of
deep {\it 3d}-levels in semiconductors. In addition to the mechanisms caused by
Coulomb and exchange interactions, which are related because of the Pauli
principle, another mechanism due to {\it sp-d} mixing is considered. It is
shown that the Coulomb mechanism reduces to long-range dipole-dipole energy
transfer from photoexcited quantum dots to Mn ions. The recombination
due to the Coulomb mechanism is allowed for any states of Mn ions and
{\it e-h} complexes. In contrast, short-range exchange and
recombinations are subject to spin selection rules, which are the result of
strong {\it lh-hh} splitting of hole states in quantum dots. Estimates show
that efficiency of the {\it sp-d} mechanism can considerably exceed that of the
Coulomb mechanism. The phonon-assisted recombination and processes involving
upper excited states of Mn ions are studied. The increase in PL
intensity of an ensemble of quantum dots in a magnetic field perpendicular to
the sample growth plane observed earlier is analyzed as a possible
manifestation of the spin-dependent recombination.Comment: 14 pages, 2 figure
Orbital Kondo Effect in CeLaB: Scaling Analysis
Peculiarity of the Kondo effect in CeLaB is investigated on
the basis of the scaling equations up to third order. For the case where the
- charge fluctuation enters in addition to the - one, the
effective exchange interaction becomes anisotropic with respect to the orbital
pseudospins which represent the two different orbitals in the ground
state. Because of different characteristic energies for electric and magnetic
tensors, scaling with the single Kondo temperature does not apply to physical
quantities such as the resistivity and magnetic susceptibility. Possibility of
a bizzare phase is pointed out where the RKKY interaction leads to the spin
ordering without orbital ordering. This phase serves as a candidate of the
phase IV which is observed to be isotropic magnetically.Comment: 10 pages, 4 eps figures, submitted to PR
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