89,159 research outputs found
On numerical integration and computer implementation of viscoplastic models
Due to the stringent design requirement for aerospace or nuclear structural components, considerable research interests have been generated on the development of constitutive models for representing the inelastic behavior of metals at elevated temperatures. In particular, a class of unified theories (or viscoplastic constitutive models) have been proposed to simulate material responses such as cyclic plasticity, rate sensitivity, creep deformations, strain hardening or softening, etc. This approach differs from the conventional creep and plasticity theory in that both the creep and plastic deformations are treated as unified time-dependent quantities. Although most of viscoplastic models give better material behavior representation, the associated constitutive differential equations have stiff regimes which present numerical difficulties in time-dependent analysis. In this connection, appropriate solution algorithm must be developed for viscoplastic analysis via finite element method
Experimental evaluation of atmospheric effects on radiometric measurements using the EREP of Skylab
The author has identified the following significant results. Test sites were located near the Great Salt Lake and the Salton Sea. Calculations were performed for a set of atmospheric models corresponding to the test sites, in addition to standard models for summer and winter midlatitude atmospheres with respective integrated water vapor amount of 2.4 g/sq cm and 0.9 g/sq cm. Each atmosphere was found to contain an average amount of continental aerosol. Computations were valid for high solar elevation angles. Atmospheric attenuation quantities were computed in addition to simulated EREP S192 radiances
A Modified Optical Potential Approach to Low-energy Electron-helium Scattering
Optical potential approach to low energy electron- helium scatterin
A computer program for predicting nonlinear uniaxial material responses using viscoplastic models
A computer program was developed for predicting nonlinear uniaxial material responses using viscoplastic constitutive models. Four specific models, i.e., those due to Miller, Walker, Krieg-Swearengen-Rhode, and Robinson, are included. Any other unified model is easily implemented into the program in the form of subroutines. Analysis features include stress-strain cycling, creep response, stress relaxation, thermomechanical fatigue loop, or any combination of these responses. An outline is given on the theoretical background of uniaxial constitutive models, analysis procedure, and numerical integration methods for solving the nonlinear constitutive equations. In addition, a discussion on the computer program implementation is also given. Finally, seven numerical examples are included to demonstrate the versatility of the computer program developed
Hawking radiation of unparticles
Unparticle degrees of freedom, no matter how weakly coupled to the standard
model particles, must affect the evolution of a black hole, which thermally
decays into all available degrees of freedom. We develop a method for
calculating the grey-body factors for scalar unparticles for 3+1 and higher
dimensional black holes. We find that the power emitted in unparticles may be
quite different from the power emitted in ordinary particles. Depending on the
parameters in the model, unparticles may become the dominant channel. This is
of special interest for small primordial black holes and also in models with
low scale quantum gravity where the experimental signature may significantly be
affected. We also discuss the sensitivity of the results on the (currently
unknown) unparticle normalization.Comment: Calculations for different normalization of unparticles included,
discussion expanded, version published in Phys. Rev.
Excitons in carbon nanotubes: an ab initio symmetry-based approach
The optical absorption spectrum of the carbon (4,2) nanotube is computed
using an ab-initio many-body approach which takes into account excitonic
effects. We develop a new method involving a local basis set which is symmetric
with respect to the screw symmetry of the tube. Such a method has the
advantages of scaling faster than plane-wave methods and allowing for a precise
determination of the symmetry character of the single particle states,
two-particle excitations, and selection rules. The binding energy of the
lowest, optically active states is approximately 0.8 eV. The corresponding
exciton wavefunctions are delocalized along the circumference of the tube and
localized in the direction of the tube axis.Comment: 4 pages, 1 LaTex file + 4 eps figure
Albedo and flux extinction coefficient of impure snow for diffuse shortwave radiation
Impurities enter a snowpack as a result of fallout of scavenging by falling snow crystals. Albedo and flux extinction coefficient of soot contaminated snowcovers were studied using a two stream approximation of the radiative transfer equation. The effect of soot was calculated by two methods: independent scattering by ice grains and impurities and average refractive index for ice grains. Both methods predict a qualitatively similar effect of soot; the albedo is decreased and the extinction coefficient is increased compared to that for pure snow in the visible region; the infrared properties are largely unaffected. Quantitatively, however, the effect of soot is more pronounced in the average refractive index method. Soot contamination provides a qualitative explanation for several snow observations
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