4,818 research outputs found
Ultrasonic attenuation as an indicator of fatigue life of graphite/epoxy fiber composite
The narrow band ultrasonic longitudinal wave velocity and attenuation were measured as a function of the transfiber compression-compression fatigue of unidirectional graphite/epoxy composites. No change in velocity was detected at any point in fatigue life. For specimens fatigued at 80% of static strength, there was generally a 5% to 10% increase in attenuation, however, this increase does not appear to be a satisfactory indicator of fatigue life. On the other hand, there appears to be a correlation between initial attenuation (measured prior to cycling) and cycles to fracture. Initial attenuation as measured at 1.5 MHz and 2.0 MHz appears to be a good indicator of relative fatigue life
Density functional study of the adsorption of K on the Ag(111) surface
Full-potential gradient corrected density functional calculations of the
adsorption of potassium on the Ag(111) surface have been performed. The
considered structures are Ag(111) (root 3 x root 3) R30degree-K and Ag(111) (2
x 2)-K. For the lower coverage, fcc, hcp and bridge site; and for the higher
coverage all considered sites are practically degenerate.
Substrate rumpling is most important for the top adsorption site. The bond
length is found to be nearly identical for the two coverages, in agreement with
recent experiments. Results from Mulliken populations, bond lengths, core level
shifts and work functions consistently indicate a small charge transfer from
the potassium atom to the substrate, which is slightly larger for the lower
coverage.Comment: to appear in Phys Rev
Ground state properties of heavy alkali halides
We extend previous work on alkali halides by calculations for the heavy-atom
species RbF, RbCl, LiBr, NaBr, KBr, RbBr, LiI, NaI, KI, and RbI. Relativistic
effects are included by means of energy-consistent pseudopotentials,
correlations are treated at the coupled-cluster level. A striking deficiency of
the Hartree-Fock approach are lattice constants deviating by up to 7.5 % from
experimental values which is reduced to a maximum error of 2.4 % by taking into
account electron correlation. Besides, we provide ab-initio data for in-crystal
polarizabilities and van der Waals coefficients.Comment: accepted by Phys. Rev.
Lissajous curves and semiclassical theory: The two-dimensional harmonic oscillator
The semiclassical treatment of the two-dimensional harmonic oscillator
provides an instructive example of the relation between classical motion and
the quantum mechanical energy spectrum. We extend previous work on the
anisotropic oscillator with incommensurate frequencies and the isotropic
oscillator to the case with commensurate frequencies for which the Lissajous
curves appear as classical periodic orbits. Because of the three different
scenarios depending on the ratio of its frequencies, the two-dimensional
harmonic oscillator offers a unique way to explicitly analyze the role of
symmetries in classical and quantum mechanics.Comment: 9 pages, 3 figures; to appear in Am. J. Phy
Ground-state properties of rutile: electron-correlation effects
Electron-correlation effects on cohesive energy, lattice constant and bulk
compressibility of rutile are calculated using an ab-initio scheme. A
competition between the two groups of partially covalent Ti-O bonds is the
reason that the correlation energy does not change linearly with deviations
from the equilibrium geometry, but is dominated by quadratic terms instead. As
a consequence, the Hartree-Fock lattice constants are close to the experimental
ones, while the compressibility is strongly renormalized by electronic
correlations.Comment: 1 figure to appear in Phys. Rev.
Automation of orbit determination functions for National Aeronautics and Space Administration (NASA)-supported satellite missions
The Flight Dynamics Facility (FDF) at Goddard Space Flight Center (GSFC) provides spacecraft trajectory determination for a wide variety of National Aeronautics and Space Administration (NASA)-supported satellite missions, using the Tracking Data Relay Satellite System (TDRSS) and Ground Spaceflight and Tracking Data Network (GSTDN). To take advantage of computerized decision making processes that can be used in spacecraft navigation, the Orbit Determination Automation System (ODAS) was designed, developed, and implemented as a prototype system to automate orbit determination (OD) and orbit quality assurance (QA) functions performed by orbit operations. Based on a machine-resident generic schedule and predetermined mission-dependent QA criteria, ODAS autonomously activates an interface with the existing trajectory determination system using a batch least-squares differential correction algorithm to perform the basic OD functions. The computational parameters determined during the OD are processed to make computerized decisions regarding QA, and a controlled recovery process isactivated when the criteria are not satisfied. The complete cycle is autonomous and continuous. ODAS was extensively tested for performance under conditions resembling actual operational conditions and found to be effective and reliable for extended autonomous OD. Details of the system structure and function are discussed, and test results are presented
Development of a Computerized App Based on Fitness Norms of University Students
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