939 research outputs found
The initiation, propagation, and effect of matrix microcracks in cross-ply and related laminates
Recently, a variational mechanics approach was used to determine the thermoelastic stress state in cracked laminates. Described here is a generalization of the variational mechanics techniques to handle other cross-ply laminates, related laminates, and to account for delaminations emanating from microcrack tips. Microcracking experiments on Hercules 3501-6/AS4 carbon fiber/epoxy laminates show a staggered cracking pattern. These results can be explained by the variational mechanics analysis. The analysis of delaminations emanating from microcrack tips has resulted in predictions about the structural and material variables controlling competition between microcracking and delamination failure modes
Localization of Two-dimensional Electron Gas in LaAlO3/SrTiO3 Heterostructures
We report strong localization of 2D electron gas in LaAlO3 / SrTiO3 epitaxial
thin-film heterostructures grown on (LaAlO3)0.3-(Sr2AlTaO3)0.7 substrates by
using pulsed laser deposition with in-situ reflection high-energy electron
diffraction. Using longitudinal and transverse magnetotransport measurements,
we have determined that disorder at the interface influences the conduction
behavior, and that increasing the carrier concentration by growing at lower
oxygen partial pressure changes the conduction from strongly localized at low
carrier concentration to metallic at higher carrier concentration, with
indications of weak localization. We interpret this behavior in terms of a
changing occupation of Ti 3d bands near the interface, each with a different
spatial extent and susceptibility to localization by disorder, and differences
in carrier confinement due to misfit strain and point defects.Comment: 12 pages, 4 figure
Coexistence of superconductivity and ferromagnetism in two dimensions
Ferromagnetism is usually considered to be incompatible with conventional
superconductivity, as it destroys the singlet correlations responsible for the
pairing interaction. Superconductivity and ferromagnetism are known to coexist
in only a few bulk rare-earth materials. Here we report evidence for their
coexistence in a two-dimensional system: the interface between two bulk
insulators, LaAlO (LAO) and SrTiO (STO), a system that has been studied
intensively recently. Magnetoresistance, Hall and electric-field dependence
measurements suggest that there are two distinct bands of charge carriers that
contribute to the interface conductivity. The sensitivity of properties of the
interface to an electric field make this a fascinating system for the study of
the interplay between superconductivity and magnetism.Comment: 4 pages, 4 figure
Master plot analysis of microcracking in graphite/epoxy and graphite/PEEK laminates
We used a variational stress analysis and an energy release rate failure criterion to construct a master plot analysis of matrix microcracking. In the master plot, the results for all laminates of a single material are predicted to fall on a single line whose slope gives the microcracking toughness of the material. Experimental results from 18 different layups of AS4/3501-6 laminates show that the master plot analysis can explain all observations. In particular, it can explain the differences between microcracking of central 90 deg plies and of free-surface 90 deg plies. Experimental results from two different AS4/PEEK laminates tested at different temperatures can be explained by a modified master plot that accounts for changes in the residual thermal stresses. Finally, we constructed similar master plot analyses for previous literature microcracking models. All microcracking theories that ignore the thickness dependence of the stresses gave poor results
Conductance asymmetry in point-contacts on epitaxial thin films of Ba(FeCo)As
Point-contact spectroscopy is a powerful tool for probing superconductors.
One of the most common observations in the point-contact spectra on the
recently discovered ferropnictide superconductors is a large conductance
asymmetry with respect to voltage across the point-contact. In this paper we
show that the antisymmetric part of the point-contact spectrum between a silver
tip and an epitaxial thin film of Ba(FeCo)As shows
certain unique features. These features have an interesting evolution with
increasing temperature up to a temperature that is 30% larger than the critical
temperature of the superconductor. We argue that this evolution can be
associated with the rich normal state properties of these materials.Comment: 4 pages, 2 figure
Calculation of a complete set of spin observables for proton elastic scattering from stable and unstable nuclei
A microscopic study of proton elastic scattering from unstable nuclei at
intermediate energies using a relativistic formalism is presented. We have
employed both the original relativistic impulse approximation (IA1) and the
generalised impulse approximation (IA2) formalisms to calculate the
relativistic optical potentials, with target densities derived from
relativistic mean field (RMF) theory using the NL3 and FSUGold parameter sets.
Comparisons between the optical potentials computed using both IA1 and IA2
formalisms, and the different RMF Lagrangians are presented for both stable and
unstable targets. The comparisons are required to study the effect of using IA1
versus IA2 optical potentials, with different RMF parameter sets, on elastic
scattering observables for unstable targets at intermediate energies. We also
study the effect of full-folding versus the factorized form of the optical
potentials on elastic scattering observables. As with the case for stable
nuclei, we found that the use of the full-folding optical potential improves
the scattering observables (especially spin observables) at low intermediate
energy (e.g. 200MeV). No discernible difference is found at a projectile
incident energy of 500 MeV. To check the validity of using localized optical
potential, we calculate the scattering observables using non-local potentials
by solving the momentum space Dirac equation. The Dirac equation is transformed
to two coupled Lippmann-Schwinger equations, which are then numerically solved
to obtain elastic scattering observables. The results are discussed and
compared to calculations involving local coordinate-space optical potentials
Effect of Soil-Structure Interaction on Nonlinear Dynamic Response of Reinforced Concrete Structures
Investigating the nonlinear dynamic response of reinforced concrete (RC) structures is of significant importance in understanding the expected behavior of these structures under dynamic loading. This becomes more crucial during the design of new or the assessment of the existing RC structures that are located in seismically active areas. The numerical simulation of this problem through the use of detailed 3D modeling is still a subject that has not been investigated thoroughly due to the significant challenges related to numerical instabilities and excessive computational demand, especially when the soil–structure interaction (SSI) phenomenon is accounted for. This study aims at presenting a nonlinear simulation tool to investigate this numerically cumbersome problem in order to provide further inside into the SSI effect on RC structures under nonlinear dynamic loading conditions. A detailed 3D numerical model of full-scale RC structures considering the SSI effect through modeling the nonlinear frame and soil domain is performed and discussed herein. The constructed models are subjected to dynamic loading conditions and an elaborate investigation is presented considering different type of structures, material properties of soil domains and depths. The RC structures and the soil domains are modeled through 8-noded hexahedral isoparametric elements, where the steel bar reinforcement of concrete is modeled as embedded beam and truss finite elements. The Ramberg–Osgood constitutive law was used for modeling the soil domain. It was shown that the SSI effect can significantly increase the flexibility of the system, altering the nonlinear dynamic response of the RC frames causing local damages that are not observed when the fixed-base model is analyzed. Furthermore, it was found that the structures founded on soft soil developed larger base-shear compared to the fixed-base model which is attributed to resonance phenomena connected to the SSI effect and the imposed accelerograms
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