18,822 research outputs found
Aluminum alloys with improved strength
Mechanical strength and stress corrosion of new BAR and 7050 alloys that include Zn instead of Cr have been studied and compared with those of 7075 aluminum alloy. Added mechanical strength of new alloys is attributed to finer grain size of 5 to 8 micrometers, however, susceptibility to stress corrosion attack is increased
Avoidance of stress corrosion susceptibility in high strength aluminum alloys by control of grain boundary and matrix microstructure
The relation of microstructure to the mechanical strength and stress corrosion resistance of highest strength and overaged tempers of BAR and 7050 aluminum alloys was investigated. Comparison is made with previously studied 7075 aluminum alloy. Optical microscopy, transmission electron microscopy, and differential scanning calorimetry were used to characterize the grain morphology, matrix microstructure, and grain boundary microstructure of these tempers. Grain boundary interparticle spacing was significant to stress corrosion crack propagation for all three alloys; increasing interparticle spacing led to increased resistance to crack propagation. In addition, the fire grain size in Bar and 7050 appears to enhance crack propagation. The highest strength temper of 7050 has a comparatively high resistance to crack initiation. Overall stress corrosion behavior is dependent on environment pH, and evaluation over a range of pH is recommended
The role of Lambda in the cosmological lens equation
The cosmological constant Lambda affects cosmological gravitational lensing.
Effects due to Lambda can be studied in the framework of the Schwarzschild-de
Sitter spacetime. Two novel contributions, which can not be accounted for by a
proper use of angular diameter distances, are derived. First, a term 2m b
Lambda/3 has to be added to the bending angle, where "m" is the lens mass and
"b" the impact parameter. Second, Lambda brings about a difference in the
redshifts of multiple images. Both effects are quite small for real
astrophysical systems (contribution to the bending < 0.1 microarcsec and
difference in redshift < 10^{-7}).Comment: 4 pages. (Univ. Zuerich); v2: presentation improved, discussion
extended, references to papers posted after the v1-version added. In press on
Phys. Rev. Let
On the polar decomposition of right linear operators in quaternionic Hilbert spaces
In this article we prove the existence of the polar decomposition for densely
defined closed right linear operators in quaternionic Hilbert spaces: If is
a densely defined closed right linear operator in a quaternionic Hilbert space
, then there exists a partial isometry such that . In
fact is unique if . In particular, if is separable
and is a partial isometry with , then we prove that
if and only if either or .Comment: 17 page
No Eigenvalue in Finite Quantum Electrodynamics
We re-examine Quantum Electrodynamics (QED) with massless electron as a
finite quantum field theory as advocated by Gell-Mann-Low, Baker-Johnson,
Adler, Jackiw and others. We analyze the Dyson-Schwinger equation satisfied by
the massless electron in finite QED and conclude that the theory admits no
nontrivial eigenvalue for the fine structure constant.Comment: 13 pages, Late
Rain estimation from satellites: An examination of the Griffith-Woodley technique
The Griffith-Woodley Technique (GWT) is an approach to estimating precipitation using infrared observations of clouds from geosynchronous satellites. It is examined in three ways: an analysis of the terms in the GWT equations; a case study of infrared imagery portraying convective development over Florida; and the comparison of a simplified equation set and resultant rain map to results using the GWT. The objective is to determine the dominant factors in the calculation of GWT rain estimates. Analysis of a single day's convection over Florida produced a number of significant insights into various terms in the GWT rainfall equations. Due to the definition of clouds by a threshold isotherm the majority of clouds on this day did not go through an idealized life cycle before losing their identity through merger, splitting, etc. As a result, 85% of the clouds had a defined life of 0.5 or 1 h. For these clouds the terms in the GWT which are dependent on cloud life history become essentially constant. The empirically derived ratio of radar echo area to cloud area is given a singular value (0.02) for 43% of the sample, while the rainrate term is 20.7 mmh-1 for 61% of the sample. For 55% of the sampled clouds the temperature weighting term is identically 1.0. Cloud area itself is highly correlated (r=0.88) with GWT computed rain volume. An important, discriminating parameter in the GWT is the temperature defining the coldest 10% cloud area. The analysis further shows that the two dominant parameters in rainfall estimation are the existence of cold cloud and the duration of cloud over a point
Schwinger Algebra for Quaternionic Quantum Mechanics
It is shown that the measurement algebra of Schwinger, a characterization of
the properties of Pauli measurements of the first and second kinds, forming the
foundation of his formulation of quantum mechanics over the complex field, has
a quaternionic generalization. In this quaternionic measurement algebra some of
the notions of quaternionic quantum mechanics are clarified. The conditions
imposed on the form of the corresponding quantum field theory are studied, and
the quantum fields are constructed. It is shown that the resulting quantum
fields coincide with the fermion or boson annihilation-creation operators
obtained by Razon and Horwitz in the limit in which the number of particles in
physical states .Comment: 20 pages, Plain Te
Black Extended Objects, Naked Singularities and P-Branes
We treat the horizons of charged, dilaton black extended objects as quantum
mechanical objects. We show that the S matrix for such an object can be written
in terms of a p-brane-like action. The requirements of unitarity of the S
matrix and positivity of the p-brane tension equivalent severely restrict the
number of space-time dimensions and the allowed values of the dilaton parameter
a. Generally, black objects transform at the extremal limit into p-branes.Comment: 9 pages, REVTE
Implications of Weak-Interaction Space Deformation for Neutrino Mass Measurements
The negative values for the squares of both electron and muon neutrino masses
obtained in recent experiments are explained as a possible consequence of a
change in metric within the weak-interaction volume in the energy-momentum
representation. Using a model inspired by a combination of the general theory
of relativity and the theory of deformation for continuous media, it is shown
that the negative value of the square of the neutrino mass can be obtained
without violating allowed physical limits. The consequence is that the negative
value is not necessary unphysical.Comment: 12 pages, 5 figures, LaTe
Breaking quantum linearity: constraints from human perception and cosmological implications
Resolving the tension between quantum superpositions and the uniqueness of
the classical world is a major open problem. One possibility, which is
extensively explored both theoretically and experimentally, is that quantum
linearity breaks above a given scale. Theoretically, this possibility is
predicted by collapse models. They provide quantitative information on where
violations of the superposition principle become manifest. Here we show that
the lower bound on the collapse parameter lambda, coming from the analysis of
the human visual process, is ~ 7 +/- 2 orders of magnitude stronger than the
original bound, in agreement with more recent analysis. This implies that the
collapse becomes effective with systems containing ~ 10^4 - 10^5 nucleons, and
thus falls within the range of testability with present-day technology. We also
compare the spectrum of the collapsing field with those of known cosmological
fields, showing that a typical cosmological random field can yield an efficient
wave function collapse.Comment: 13 pages, LaTeX, 3 figure
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