4,154 research outputs found
The atomistic structure and energy of nascent dislocation loops
An harmonic lattice theory is used, in conjunction with Mura's theory of eigendistorsions, to study the structure and energetics of nascent dislocation loops in face-centred-cubic (FCC) crystals. An analytical expression for the activation energies of such loops is derived. The results obtained herein indicate that thermal activation of small dislocation loops is possible at high stress levels such as those found in the vicinity of a crack tip. The implications of these results in understanding phenomena such as the brittle-ductile transition are discussed
An Analysis of the Quasicontinuum Method
The aim of this paper is to present a streamlined and fully three-dimensional
version of the quasicontinuum (QC) theory of Tadmor et al. and to analyze its
accuracy and convergence characteristics. Specifically, we assess the effect of
the summation rules on accuracy; we determine the rate of convergence of the
method in the presence of strong singularities, such as point loads; and we
assess the effect of the refinement tolerance, which controls the rate at which
new nodes are inserted in the model, on the development of dislocation
microstructures.Comment: 30 pages, 16 figures. To appear in Jornal of the Mechanics and
Physics of Solid
Possible ferro-spin nematic order in NiGa2S4
We explore the possibility that the spin-1 triangular lattice magnet NiGa2 S4
may have a ferro-nematic ground state with no frozen magnetic moment but a
uniform quadrupole moment. Such a state may be stabilized by biquadratic spin
interactions. We describe the physical properties of this state and suggest
experiments to help verify this proposal. We also contrast this state with a
`non-collinear' nematic state proposed earlier by Tsunetsugu and Arikawa for
NiGa2S4
Non recurrent laryngeal nerve with right aberrant subclavian artery in recurrent case of papillary carcinoma of thyroid: an interesting clinical entity.
Background/objectives: A nonrecurrent laryngeal nerve is a rare anatomical variant and a routine preoperative imaging studies are not indicated. NRLN is related with absence of the brachiocephalic trunk and aberrant (mainly retroesophageal) course of theright subclavian artery which is evident on contrast enhanced CT scan of the chest.Case report: The patient presented with a swelling in the neck for 4 months which is slowly increasing in size. Patient gives history of a surgery done in the neck with an old midlinesuprasternal scar which was done 40 years earlier.laryngoscopic mirror examinartion was normal with moving vocal cords. Rest of the oral and oropharyngeal examination along with completeblood picture and biochemical investigations were normal. The patient was not a known diabetic or hypertensive.Conclusion: Accurate knowledge of frequent variations of the nerve along with the non recurrent variant reduces the risk of intraoperative injury. A nonrecurrent laryngealnerve(NRLN) is a rare anatomical variant and a routine preoperative imaging studies are not indicated. NRLN is related with absence of the brachiocephalic trunk and aberrant(mainly retroesophageal) course of the right subclavian artery which is evident on contrast enhanced CT scan of the chest.
Quasicontinuum Models of Interfacial Structure and Deformation
Microscopic models of the interaction between grain boundaries (GBs) and both
dislocations and cracks are of importance in understanding the role of
microstructure in altering the mechanical properties of a material. A recently
developed mixed atomistic and continuum method is extended to examine the
interaction between GBs, dislocations and cracks. These calculations elucidate
plausible microscopic mechanisms for these defect interactions and allow for
the quantitative evaluation of critical parameters such as the stress to
nucleate a dislocation at a step on a GB and the force needed to induce GB
migration.Comment: RevTex, 4 pages, 4 figure
Peeling from a patterned thin elastic film
Inspired by the observation that many naturally occurring adhesives arise as
textured thin films, we consider the displacement controlled peeling of a
flexible plate from an incision-patterned thin adhesive elastic layer. We find
that crack initiation from an incision on the film occurs at a load much higher
than that required to propagate it on a smooth adhesive surface; multiple
incisions thus cause the crack to propagate intermittently. Microscopically,
this mode of crack initiation and propagation in geometrically confined thin
adhesive films is related to the nucleation of cavitation bubbles behind the
incision which must grow and coalesce before a viable crack propagates. Our
theoretical analysis allows us to rationalize these experimental observations
qualitatively and quantitatively and suggests a simple design criterion for
increasing the interfacial fracture toughness of adhesive films.Comment: 8 pages, To appear in Proceedings of Royal Society London, Ser.
Surface pressure measurements at two tips of a model helicopter rotor in hover
Surface pressures were measured near the tip of a hovering single-bladed model helicopter rotor with two tip shapes. The rotor had a constant-chord, untwisted blade with a square, flat tip which could be modified to a body-of-revolution tip. Pressure measurements were made on the blade surface along the chordwise direction at six radial stations outboard of the 94 percent blade radius. Data for each blade tip configuration were taken at blade collective pitch angles of 0, 6.18 and 11.4 degrees at a Reynolds number of 736,000 and a Mach number of 0.25 both based on tip speed. Chordwise pressure distributions and constant surface pressure contours are presented and discussed
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