265 research outputs found
Real-time video correlator
Device provides two-dimensional correlation of video data. Operation is reliable, accurate, and predictable
Making chiral fermion actions (almost) gauge invariant using Laplacian gauge fixing
Straight foreward lattice descriptions of chiral fermions lead to actions
that break gauge invariance. I describe a method to make such actions gauge
invariant (up to global gauge transformations) with the aid of gauge fixing. To
make this prescription unambiguous, Laplacian gauge fixing is used, which is
free from Gribov ambiguities.Comment: 3 p., Latex, (proc. Lattice '93, Dallas), 2 figs. appended, UCSD/PTH
93-4
Ultrasonic metal sheet thickness measurement without prior wave speed calibration
Conventional ultrasonic mensuration of sample thickness from one side only requires the bulk
wave reverberation time and a calibration speed. This speed changes with temperature, stress,
and microstructure, limiting thickness measurement accuracy. Often, only one side of a
sample is accessible, making in situ calibration impossible. Non-contact ultrasound can
generate multiple shear horizontal guided wave modes on one side of a metal plate. Measuring
propagation times of each mode at different transducer separations, allows sheet thickness to
be calculated to better than 1% accuracy for sheets of at least 1.5 mm thickness, without any
calibration
Shear horizontal (SH) ultrasound wave propagation around smooth corners
Shear horizontal (SH) ultrasound guided waves are being used in an increasing number of non-destructive testing (NDT) applications. One advantage SH waves have over some wave types, is their ability to propagate around curved surfaces with little energy loss; to understand the geometries around which they could propagate, the wave reflection must be quantified. A 0.83 mm thick aluminium sheet was placed in a bending machine, and a shallow bend was introduced. Periodically-poled magnet (PPM) electromagnetic acoustic transducers (EMATs), for emission and reception of SH waves, were placed on the same side of the bend, so that reflected waves were received. Additional bending of the sheet demonstrated a clear relationship between bend angles and the reflected signal. Models suggest that the reflection is a linear superposition of the reflections from each bend segment, such that sharp turns lead to a larger peak-to-peak amplitude, in part due to increased phase coherence
Deciding Joinability Modulo Ground Equations In Operational Type Theory
Operational Type Theory (OpTT) can be used to construct and check proofs related to programs, but the development of these proofs can be somewhat tedious. An algorithm is presented that can be used to automatically generate proofs of equality in OpTT. The algorithm takes as input a set of ground equations and two terms that should be tested for joinability modulo the supplied ground equations. The algorithm will equate the terms if and only if there exists an OpTT proof that can equate the two terms using only the proof rules related to evaluation under the operational semantics, symmetry, transitivity, and congruence with respect to the supplied ground equations. The description of this algorithm is accompanied by a proof that the algorithm is partially correct
Weld defect detection using PPM EMAT generated shear horizontal ultrasound
AbstractAustenitic welds are inspected using PPM EMAT generated shear horizontal (SH) waves. Results are compared to measurements taken using a 1D piezoelectric phased array using the total focusing method (TFM). For the first time there is clear experimental evidence of the SH wave method demonstrating higher sensitivity to defect detection. SH waves suffer less beam steering in a weld than either compression or SV waves, which can miss defects due to weld microstructure anisotropy and attenuation. All defects were identified from every side of the weld/plate using the SH waves, but this was not possible using the piezoelectric transducer
Manifestly Gauge Invariant Models of Chiral Lattice Fermions
A manifestly gauge invariant lattice action for nonanomalous chiral models is
proposed which leads in the continuum limit to the theory free of doublers.Comment: 9 pages, LaTeX. Revised version with an extended discussion of the
role of higher derivative regulators. Submitted to Phys.Lett.B. Preprint
SMI-9-9
Time of flight diffraction and imaging (TOFDI)
Time of flight diffraction and imaging (TOFDI) is based on time of flight diffraction (TOFD),
adding cross-sectional imaging of the sample bulk by exploiting the scattering of ultrasonic waves
from bulk defects in metals. Multiple wave modes are emitted by a pulsed laser ultrasound ablative
source, and received by a sparse array of receiving electromagnetic acoustic transducers (EMATs),
for non-contact (linear) scanning, with mode-conversions whenever waves are scattered.
Standard signal processing techniques, such as band-pass filters, reduce noise. A B-scan is
formed from multiple data captures (A-scans), with time and scan position axes, and colour
representing amplitude or magnitude. B-scans may contain horizontal lines from surface waves
propagating directly from emitter to receiver, or via a back-wall, and angled lines after reflection
off a surface edge. A Hough transform (HT), modified to deal with the constraints of a B-scan,
can remove such lines. A parabola matched filter has been developed that identifies the features
in the B-scan caused by scattering from point-like defects, reducing them to peaks and minimising
noise. Multiple B-scans are combined to reduce noise further. The B-scan is also processed to form
a cross-sectional image, enabling detection and positioning of multiple defects.
The standard phase correlation technique applied to camera images, has been used to track
the relative position between transducer and sample. Movement has been determined to sub-pixel
precision, with a median accuracy of 0.01mm of linear movement (0.06 of a pixel), despite uneven
illumination and the use of a basic low resolution camera.
The prototype application is testing rough steel products formed by continuous casting, but
the techniques created to facilitate operation of TOFDI are applicable elsewhere
The Chiral Extension of Lattice QCD
The chiral extension of Quantum Chromodynamics (XQCD) adds to the standard
lattice action explicit pseudoscalar meson fields for the chiral condensates.
With this action, it is feasible to do simulations at the chiral limit with
zero mass Goldstone modes. We review the arguments for why this is expected to
be in the same universality class as the traditional action. We present
preliminary results on convergence of XQCD for naive fermions and on the
methodology for introducing counter terms to restore chiral symmetry for Wilson
fermions.Comment: 7 pages, LATTICE 94 talk by R. Brower: Latex file with 2 postscript
figures for encapsulatio
Mode mixing in shear horizontal ultrasonic guided waves
SH guided waves are used increasingly for non-destructive testing (NDT) applications, particularly for pipes and pipe supports using circumferentially guided wave modes. In practical implementations, it is not always straightforward to ensure single-mode operation and this requires consideration when interpreting results. During shear horizontal (SH) wave generation or SH guided wave interaction with geometrical changes or defects, multiple SH guided wave modes may be produced, depending on the shear wave speed, the frequency of operation, the thickness of the sample and the transducer characteristics. This paper discusses the interference patterns created as the multiple SH modes mix (for both continuous tone generation and short bursts), and the problems caused by the interference patterns on applications such as NDT. In particular, the patterns can lead to defects being missed during an NDT inspection using SH waves, and a way to circumvent this problem is suggested
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