22,762 research outputs found
Deflections of beam columns on multiple supports
Lateral deflections of beam columns on multiple equally spaced supports are calculated using the STAGS nonlinear structural analysis computer program. Three lateral loadings are considered, uniform, linear, and uniform over only the center bay. Two types of boundary conditions are considered at the end supports, clamped, and simple support. The effect of an initial sinusoidal imperfection are considered. Deflections in the center and end bays of the beam columns are presented as a function of applied axial compressive load. As the number of bays becomes large, the effect of boundary conditions on the deflections in the center bays diminishes. For cases involving a uniform or linearly varying load, imperfections can have a much larger effect on deflections in the center bays than can lateral pressure
Buckling loads for stiffened panels subjected to combined longitudinal compression and shear loadings: Results obtained with PASCO, EAL, and STAGS computer
The shear buckling analyses used in PASCO are summarized. The PASCO analyses include the basic VIPASA analysis, which is essentially exact for longitudinal and transverse loads, and a smeared orthotropic solution which was added to alleviate a shortcoming in the VIPASA analysis. Buckling results are presented for six stiffened panels loaded by combinations of longitudinal compression and shear. The buckling results were obtained with the PASCO, EAL, and STAGS computer programs. The EAL and STAGS solutions were obtained with a fine finite element mesh and provide calculations for the entire range of combinations of longitudinal compression and shear loadings
Current research on shear buckling and thermal loads with PASCO: Panel Analysis and Sizing Code
The PASCO computer program to obtain the detailed dimensions of optimum stiffened composite structural panels is described. Design requirements in terms of inequality constraints can be placed on buckling loads or vibration frequencies, lamina stresses and strains, and overall panel stiffness for each of many load conditions. General panel cross sections can be treated. An analysis procedure involving a smeared orthotropic solution was investigated. The conservatism in the VIPASA solution and the danger in a smeared orthotropic solution is explored. PASCO's capability to design for thermal loadings is also described. It is emphasized that design studies illustrate the importance of the multiple load condition capability when thermal loads are present
Buckling loads of stiffened panels subjected to combined longitudinal compression and shear: Results obtained with PASCO, EAL, and STAGS computer programs
Buckling analyses used in PASCO are summarized with emphasis placed on the shear buckling analyses. The PASCO buckling analyses include the basic VIPASA analysis, which is essentially exact for longitudinal and transverse loads, and a smeared stiffener solution, which treats a stiffened panel as an orthotropic plate. Buckling results are then presented for seven stiffened panels loaded by combinations of longitudinal compression and shear. The buckling results were obtained with the PASCO, EAL, and STAGS computer programs. The EAL and STAGS solutions were obtained with a fine finite element mesh and are very accurate. These finite element solutions together with the PASCO results for pure longitudinal compression provide benchmark calculations to evaluate other analysis procedures
Precision determination of absolute neutron flux
A technique for establishing the total neutron rate of a highly-collimated
monochromatic cold neutron beam was demonstrated using a method of an
alpha-gamma counter. The method involves only the counting of measured rates
and is independent of neutron cross sections, decay chain branching ratios, and
neutron beam energy. For the measurement, a target of 10B-enriched boron
carbide totally absorbed the neutrons in a monochromatic beam, and the rate of
absorbed neutrons was determined by counting 478keV gamma rays from neutron
capture on 10B with calibrated high-purity germanium detectors. A second
measurement based on Bragg diffraction from a perfect silicon crystal was
performed to determine the mean de Broglie wavelength of the beam to a
precision of 0.024 %. With these measurements, the detection efficiency of a
neutron monitor based on neutron absorption on 6Li was determined to an overall
uncertainty of 0.058 %. We discuss the principle of the alpha-gamma method and
present details of how the measurement was performed including the systematic
effects. We also describe how this method may be used for applications in
neutron dosimetry and metrology, fundamental neutron physics, and neutron cross
section measurements.Comment: 44 page
Families of Quintic Calabi-Yau 3-Folds with Discrete Symmetries
At special loci in their moduli spaces, Calabi-Yau manifolds are endowed with
discrete symmetries. Over the years, such spaces have been intensely studied
and have found a variety of important applications. As string compactifications
they are phenomenologically favored, and considerably simplify many important
calculations. Mathematically, they provided the framework for the first
construction of mirror manifolds, and the resulting rational curve counts.
Thus, it is of significant interest to investigate such manifolds further. In
this paper, we consider several unexplored loci within familiar families of
Calabi-Yau hypersurfaces that have large but unexpected discrete symmetry
groups. By deriving, correcting, and generalizing a technique similar to that
of Candelas, de la Ossa and Rodriguez-Villegas, we find a calculationally
tractable means of finding the Picard-Fuchs equations satisfied by the periods
of all 3-forms in these families. To provide a modest point of comparison, we
then briefly investigate the relation between the size of the symmetry group
along these loci and the number of nonzero Yukawa couplings. We include an
introductory exposition of the mathematics involved, intended to be accessible
to physicists, in order to make the discussion self-contained.Comment: 54 pages, 3 figure
Near threshold rotational excitation of molecular ions by electron-impact
New cross sections for the rotational excitation of H by electrons are
calculated {\it ab initio} at low impact energies. The validity of the
adiabatic-nuclei-rotation (ANR) approximation, combined with -matrix
wavefunctions, is assessed by comparison with rovibrational quantum defect
theory calculations based on the treatment of Kokoouline and Greene ({\it Phys.
Rev. A} {\bf 68} 012703 2003). Pure ANR excitation cross sections are shown to
be accurate down to threshold, except in the presence of large oscillating
Rydberg resonances. These resonances occur for transitions with
and are caused by closed channel effects. A simple analytic formula is derived
for averaging the rotational probabilities over such resonances in a 3-channel
problem. In accord with the Wigner law for an attractive Coulomb field,
rotational excitation cross sections are shown to be large and finite at
threshold, with a significant but moderate contribution from closed channels.Comment: 3 figures, a5 page
Role of oxygen in the electron-doped superconducting cuprates
We report on resistivity and Hall measurements in thin films of the
electron-doped superconducting cuprate PrCeCuO.
Comparisons between x = 0.17 samples subjected to either ion-irradiation or
oxygenation demonstrate that changing the oxygen content has two separable
effects: 1) a doping effect similar to that of cerium, and 2) a disorder
effect. These results are consistent with prior speculations that apical oxygen
removal is necessary to achieve superconductivity in this compound.Comment: 5 pages, 5 figure
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