573 research outputs found
Basic Theory of Ultrasonic Scattering by Defects: Numerical Studies and Features for Experimental Application
Well, the theory of scattering of elastic waves is difficult. I don\u27t want to minimize that, but I thought I might make a few diversionary introductory remarks. My first reference will be the Los Angeles Times of today, the astrology column. It advises those born under the sign of Leo to choose their words carefully to avoid trouble. That happens to be applicable to yours truly. I hope I do it
Retrospective Comments on the Elastic Wave Scattering Problem
Developments over the past several years have led to renewed study of theoretical methods for treating scattering of ultrasound by defects in elastic solids. Characteristically, many theories of scattering, until a few years ago, dealt with scalar waves and simple obstacles. Within that context two distinct regimes were apparent -- the long wave length, and the short wave or imaging regime. The treatment of vector fields in elastic solids is considered the more cumbersome, but we now have made progress -- still for idealized configurations. Specifically, the talk will deal with hypotheses on the types of problems which face us in the next generation of situations
A Technique for Determining Flaw Characteristics from Ultrasonic Scattering Amplitudes
We report an approximate technique for determining the characteristics of flaws in elastic media from a knowledge of their ultrasonic scattering amplitudes. The technique is rigorously valid in the weak scattering limit. Good results have been obtained for strongly scattering flaws. In particular, we tested the technique for a 2-1 oblate spheroidal void in Ti, and for various strongly scattering spherical defects. For these.tests the technique yields good results for the volume of the flaws. In the case of the oblate spheroid, satisfactory results were obtained for the calculated ratio of major to minor axis, indicating that the technique is sensitive to the shape of the flaw
Inhomogeneity-Induced Superconductivity?
A t-J-like model for inhomogeneous superconductivity of cuprate oxides is
presented, in which local anisotropic magnetic terms are essential. We show
that this model predicts pairing, consistent with experiments, and argue how
the macroscopic phase-coherent state gradually grows upon lowering of the
temperature. We show that appropriate inhomogeneities are essential in order to
have significant pair binding in the thermodynamic limit. Particularly, {\it
local} breaking of SU(2) spin symmetry is an efficient mechanism for inducing
pairing of two holes, as well as explaining the magnetic scattering properties.
We also discuss the connection of the resulting inhomogeneity-induced
superconductivity to recent experimental evidence for a linear relation between
magnetic incommensurability and the superconducting transition temperature, as
a function of doping.Comment: 4 pages, REVTEX, 4 jpeg figures. To appear in Europhys. Let
Temperature dependent characterization of optical fibres for distributed temperature sensing in hot geothermal wells
This study was performed in order to select a proper fibre for the
application of a distributed temperature sensing system within a hot geothermal
well in Iceland. Commercially available high temperature graded index fibres
have been tested under in-situ temperature conditions. Experiments have been
performed with four different polyimide coated fibres, a fibre with an aluminum
coating and a fibre with a gold coating. To select a fibre, the relationship
between attenuation, temperature, and time has been analyzed together with SEM
micrographs. On the basis of these experiments, polyimide fibres have been
chosen for utilisation. Further tests in ambient and inert atmosphere have been
conducted with two polyimide coated fibres to set an operating temperature
limit for these fibres. SEM micrographs, together with coating colour changes
have been used to characterize the high temperature performance of the fibres.
A novel cable design has been developed, a deployment strategy has been worked
out and a suitable well for deployment has been selected.Comment: PACS: 42.81.Pa, 93.85.Fg, 47.80.Fg, 91.35.Dc, 07.20.Dt, 07.60.V
Premartensitic transition driven by magnetoelastic interaction in bcc ferromagnetic
We show that the magnetoelastic coupling between the magnetization and the
amplitude of a short wavelength phonon enables the existence of a first order
premartensitic transition from a bcc to a micromodulated phase in .
Such a magnetoelastic coupling has been experimentally evidenced by AC
susceptibility and ultrasonic measurements under applied magnetic field. A
latent heat around 9 J/mol has been measured using a highly sensitive
calorimeter. This value is in very good agreement with the value predicted by a
proposed model.Comment: 4 pages RevTex, 3 Postscript figures, to be published in Physical
Review Letter
Interpretation of Ultrasonic Scattering Measurements by Various Flaws from Theoretical Studies
Let me begin by saying that this is part of a program which is very much complementary to and is helped by the experimental program; so I\u27ll repeat, in part, some of the overview comments that Bruce Thompson made this morning. We\u27ve regarded our role as one of trying to plug into the overall program those aspects of the theory of elastic wave scattering which can be developed in a utilitarian way in terms of modern analytical and computational techniques, in a form which I hope can be useful in signal processing, interpretation, design of experiment, etc. To outline our point of view, I\u27m going to first give a survey of what we have been doing and then give some of the results. It\u27s rather shocking to think of the amount of money that goes into an experimental program and its interpretation, if the interpretation is done in terms of acoustic or scalar wave scattering theories, when the differences from proper elastic theory can be as great as those which Bruce Thompson pointed to this morning
Three-Particle Correlations in Simple Liquids
We use video microscopy to follow the phase-space trajectory of a
two-dimensional colloidal model liquid and calculate three-point correlation
functions from the measured particle configurations. Approaching the
fluid-solid transition by increasing the strength of the pair-interaction
potential, one observes the gradual formation of a crystal-like local order due
to triplet correlations, while being still deep inside the fluid phase.
Furthermore, we show that in a strongly interacting system the Born-Green
equation can be satisfied only with the full triplet correlation function but
not with three-body distribution functions obtained from superposing
pair-correlations (Kirkwood superposition approximation).Comment: 4 pages, submitted to PRL, experimental paper, 2nd version: Fig.1 and
two new paragraphs have been adde
Exact Thermodynamics of the Double sinh-Gordon Theory in 1+1-Dimensions
We study the classical thermodynamics of a 1+1-dimensional double-well
sinh-Gordon theory. Remarkably, the Schrodinger-like equation resulting from
the transfer integral method is quasi-exactly solvable at several temperatures.
This allows exact calculation of the partition function and some correlation
functions above and below the short-range order (``kink'') transition, in
striking agreement with high resolution Langevin simulations. Interesting
connections with the Landau-Ginzburg and double sine-Gordon models are also
established.Comment: 4 pages, 3 figures (embedded using epsf), uses RevTeX plus macro
(included). Minor revision to match journal version, Phys. Rev. Lett. (in
press
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