6,479 research outputs found
Estimation of Effective Elastic Properties of General Multifunctional Honeycomb Structures Using a Unit Cell Method
Sandwich composite structures are ideal configurations in which to incorporate additional functionality beyond load carrying capabilities. The inner core can be layered to facilitate other functions such as power storage for a battery. In this work we investigate an assemblage of analytical tools to compute effective properties that allow complex layered core architectures to be homogenized into a single continuum layer. This provides a great increase in computational efficiency to numerically simulate the structural response of multifunctional sandwich structures under applied loads
A boundary element alternating method for two-dimensional mixed-mode fracture problems
A boundary element alternating method, denoted herein as BEAM, is presented for two dimensional fracture problems. This is an iterative method which alternates between two solutions. An analytical solution for arbitrary polynomial normal and tangential pressure distributions applied to the crack faces of an embedded crack in an infinite plate is used as the fundamental solution in the alternating method. A boundary element method for an uncracked finite plate is the second solution. For problems of edge cracks a technique of utilizing finite elements with BEAM is presented to overcome the inherent singularity in boundary element stress calculation near the boundaries. Several computational aspects that make the algorithm efficient are presented. Finally, the BEAM is applied to a variety of two dimensional crack problems with different configurations and loadings to assess the validity of the method. The method gives accurate stress intensity factors with minimal computing effort
Doping and Field-Induced Insulator-Metal Transitions in Half-Doped Manganites
We argue that many properties of the half-doped manganites may be understood
in terms of a new two-(eg electron)-fluid description, which is energetically
favorable at intermediate Jahn-Teller (JT) coupling. This emerges from a
competition between canting of the core spins of Mn promoting mobile carriers
and polaronic trapping of carriers by JT defects, in the presence of CE,
orbital and charge order. We show that this explains several features of the
doping and magnetic field induced insulator-metal transitions, as the
particle-hole asymmetry and the smallness of the transition fields.Comment: 4 pages, 4 figure
Instabilities and Insulator-Metal transitions in Half-Doped Manganites induced by Magnetic-Field and Doping
We discuss the phase diagram of the two-orbital model of half-doped
manganites by calculating self-consistently the Jahn-Teller (JT) distortion
patterns, charge, orbital and magnetic order at zero temperature. We analyse
the instabilities of these phases caused by electron or hole doping away from
half-doping, or by the application of a magnetic-field. For the CE insulating
phase of half-doped manganites, in the intermediate JT coupling regime, we show
that there is a competition between canting of spins (which promotes mobile
carriers) and polaronic self-trapping of carriers by JT defects. This results
in a marked particle-hole asymmetry, with canting winning only on the electron
doped side of half-doping. We also show that the CE phase undergoes a
first-order transition to a ferromagnetic metallic phase when a magnetic-field
is applied, with abrupt changes in the lattice distortion patterns. We discuss
the factors that govern the intriguingly small scale of the transition fields.
We argue that the ferromagnetic metallic phases involved have two types of
charge carriers, localised and band-like, leading to an effective two-fluid
model.Comment: 22 pages, 28 figure
Can correlations drive a band insulator metallic?
We analyze the effects of the on-site Coulomb repulsion U on a band insulator
using dynamical mean field theory (DMFT). We find the surprising result that
the gap is suppressed to zero at a critical Uc1 and remains zero within a
metallic phase. At a larger Uc2 there is a second transition from the metal to
a Mott insulator, in which the gap increases with increasing U. These results
are qualitatively different from Hartree-Fock theory which gives a
monotonically decreasing but non-zero insulating gap for all finite U.Comment: 4 pages, 5 figure
The Exotic Barium Bismuthates
We review the remarkable properties, including superconductivity,
charge-density-wave ordering, and metal-insulator transitions, of lead- and
potassium-doped barium bismuthate. We discuss some of the early theoretical
studies of these systems. Our recent theoretical work, on the negative-U\/,
extended-Hubbard model for these systems, is also described. Both the large-
and intermediate-U\/ regimes of this model are examined, using mean-field and
random-phase approximations, particularly with a view to fitting various
experimental properties of these bismuthates. On the basis of our studies, we
point out possibilities for exotic physics in these systems. We also emphasize
the different consequences of electronic and phonon-mediated mechanisms for the
negative U.\/ We show that, for an electronic mechanism, the \secin
\,\,phases of these bismuthates must be unique, with their transport properties
{\it dominated by charge Cooperon bound states}. This can explain the
observed difference between the optical and transport gaps. We propose other
experimental tests for this novel mechanism of charge transport and comment on
the effects of disorder.Comment: UUencoded LaTex file, 122 pages, figures available on request To
appear in Int. J. Mod. Phys. B as a review articl
Frequency Response of an Aircraft Wing with Discrete Source Damage Using Equivalent Plate Analysis
An equivalent plate procedure is developed to provide a computationally efficient means of matching the stiffness and frequencies of flight vehicle wing structures for prescribed loading conditions. Several new approaches are proposed and studied to match the stiffness and first five natural frequencies of the two reference models with and without damage. One approach divides the candidate reference plate into multiple zones in which stiffness and mass can be varied using a variety of materials including aluminum, graphite-epoxy, and foam-core graphite-epoxy sandwiches. Another approach places point masses along the edge of the stiffness-matched plate to tune the natural frequencies. Both approaches are successful at matching the stiffness and natural frequencies of the reference plates and provide useful insight into determination of crucial features in equivalent plate models of aircraft wing structures
Static and Dynamic Structural Response of an Aircraft Wing with Damage Using Equivalent Plate Analysis
A process to generate an equivalent plate based on an optimization approach to predict the static and dynamic response of flight vehicle wing structures is proposed. Geometric-scale and frequency-scale factors are defined to construct an equivalent plate with any desired scale to use in simulation and wind tunnel experiments. It is shown that the stiffness and the displacements are scaled linearly with the geometric-scale factor, whereas the load is scaled as the square of the geometric-scale factor. The scaled stiffness of the reference flight vehicle is matched first to construct the equivalent plate. Then the frequency-scale factor is defined to scale the flight vehicle frequencies. The scaled flight vehicle frequencies are matched by placing arbitrary point masses along the equivalent plate geometry. Two simple stiffened-plate examples, one with damage and another without damage, were used to demonstrate the accuracy of the optimization procedure proposed. Geometric-scale factors ranging from 0.2 to 1.0 were used in the analyses. In both examples, the static and dynamic response of the reference stiffened-panel solution is matched accurately. The scaled equivalent plate predicted the first five frequencies of the stiffened panel very accurately. Finally, the proposed equivalent plate procedure was demonstrated in a more realistic typical aircraft wing structure. Two scale equivalent plate models were generated using the geometric-scale factors 1.0 and 0.2. Both equivalent plate models predicted the static response of the wing structure accurately. The equivalent plate models reproduced the first five frequencies of the wing structure accurately
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