2,320 research outputs found
Finite-element analyses and fracture simulation in thin-sheet aluminum alloy
A two-dimensional, elastic-plastic finite-element analysis was used with a critical crack-tip-opening angle (CTOA) fracture criterion to model stable crack growth in thin-sheet 2024-T3 aluminum alloy under monotonic loading after precracking at different cyclic stress levels. Tests were conducted on three types of specimens: middle-crack, three-hole-crack and blunt-notch tensile specimens. An experiment technique was developed to measure CTOA during crack growth initiation and stable tearing using a high-resolution video camera and recorder. Crack front shapes were also measured during initiation and stable tearing using a fatigue marker-load technique. Three-dimensional elastic-plastic finite-element analyses of these crack shapes for stationary cracks were conducted to study the crack-front opening displacements. Predicted load against crack extension on middle-crack tension specimens agreed well with test results even for large-scale plastic deformations. The analyses were able to predict the effects of specimen size and precracking stress history on stable tearing. Predicted load against load-line displacements agreed well with test results up to maximum load bu the analyses tended to overpredict displacements as crack grew beyond the maximum load under displacement-controlled conditions. During the initiation phase, the measured CTOA values were high but decreased and remained nearly constant after a small amount of stable tearing. The constant value of CTOA agree well with the calculated value from the finite-element analysis. The larger CTOA values measured at the sheet surface during the initiation phase may be associated with the crack tunneling observed in the tests. Three-dimensional analyses for nonstraight crack fronts predicted much higher displacements near the free surface than in the interior
Antenna pattern shaping, sensing, and steering study Final report
Design of steerable satellite antenna with beam pattern sensing syste
Allochronic Speciation In Field Crickets, And A New Species, Acheta Veletis
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137466/1/evo03095.pd
Demonstration of images with negative group velocities
We report the experimental demonstration of the superluminal propagation of
multi-spatial-mode images via four-wave mixing in hot atomic vapor, in which
all spatial sub-regions propagate with negative group velocities. We
investigate the spatial mode properties and temporal reshaping of the fast
light images, and show large relative pulse peak advancements of up to 64% of
the input pulse width. The degree of temporal reshaping is quantified and
increases as the relative pulse peak advancement increases. When optimized for
image quality or pulse advancement, negative group velocities of up to
and , respectively, are
demonstrated when integrating temporally over the entire image. The present
results are applicable to temporal cloaking devices that require strong
manipulation of the dispersion relation, where one can envision temporally
cloaking various spatial regions of an image for different durations.
Additionally, the modes involved in a four-wave mixing process similar to the
present experiment have been shown to exhibit quantum correlations and
entanglement. The results presented here provide insight into how to tailor
experimental tests of the behavior of these quantum correlations and
entanglement in the superluminal regime.Comment: 9 pages, 4 figure
Controlling quasiparticle excitations in a trapped Bose-Einstein condensate
We describe an approach to quantum control of the quasiparticle excitations
in a trapped Bose-Einstein condensate based on adiabatic and diabatic changes
in the trap anisotropy. We describe our approach in the context of Landau-Zener
transition at the avoided crossings in the quasiparticle excitation spectrum.
We show that there can be population oscillation between different modes at the
specific aspect ratios of the trapping potential at which the mode energies are
almost degenerate. These effects may have implications in the expansion of an
excited condensate as well as the dynamics of a moving condensate in an atomic
wave guide with a varying width
Atmospheric frontal zone studies
The research supported by this contract and directed Activities in the inversion and interpretation of data produced by the Nimbus-7 scanning multichannel microwave radiometer (SMMR) are reported. There were five principal subjects: (1) modeling of the emissivity of foam patches on the ocean surface; (2) inversion of radiometric data by a multidimensional algorithm; (3) an operational water vapor retrieval algorithm; (4) inference of Antarctic firm accumulation rates; and (5) inference of water vapor over the Arctic sea ice
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