Advances in developing multiscale flaw models for eddy-current NDE

The need to accurately model multiscale phenomena is ubiquitous in eddy-current nondestructive evaluation. By using volume-integral equations, we are able to develop a very simple algorithm for accurately computing the response of a very small anomaly in the presence of a much larger one. We validate the algorithm and its associated code inVIC-3D{copyright, serif} through benchmark data on two test sets: (1) a notch at a bolt hole with an upper surface coil, and (2) a notch in a bolt hole with a plate surface coil

Analytical solution for capacitance calculation of a curved patch capacitor that conforms to the curvature of a homogeneous cylindrical dielectric rod

This Letter presents an analytical expression for the capacitance of a curved patch capacitor whose electrodes conform to the curvature of a long, homogeneous, cylindrical dielectric rod. The capacitor is composed of two infinitely long curved electrodes, symmetrically placed about a diameter of the cylinder cross-section. The resulting capacitance per unit length depends on both the dielectric properties of the material under test and the capacitor configuration. A practical capacitance measurement is also presented, with appropriately guarded finite electrodes. Very good agreement between measured and theoretically predicted capacitances were observed, to within 2.4 percent. The analytical result presented in this Letter can be applied for extremely rapid evaluation of rod permittivity from measured capacitance

Requirements for the spatial storage effect are weakly evident for common species in natural annual plant assemblages

Coexistence in spatially varying environments is theorised to be promoted by a variety of mechanisms including the spatial storage effect. The spatial storage effect promotes coexistence when: (i) species have unique vital rate responses to their spatial environment and, when abundant, (ii) experience stronger competition in the environmental patches where they perform better. In a naturally occurring southwest Western Australian annual plant system we conducted a neighbour removal experiment involving eleven focal species growing in high-abundance populations. Specifically, we measured species' fecundity across a variety of environmental gradients in both the presence and absence of neighbours. For the environmental variables that we measured, there was only limited evidence for species-specific responses to the environment, with a composite variable describing overstory cover and leaf litter cover being the best predictor of fecundity for a subset of focal species. In addition, although we found strong evidence for intra-specific competition, positive environment-competition covariance was only detected for one species. Thus, positive environment-competition covariance may not be as common as expected in populations of species growing at high abundance, at least when tested in natural assemblages. Our findings highlight the inherent limitations of using natural assemblages to study spatial coexistence mechanisms, and we urge empirical ecologists to take these limitations into account when designing future experiments

Measurement and Calculation of Transient Eddy-Currents in Layered Structures

In transient eddy-current inspection, an electromagnetic field pulse is excited in a conductor by causing a step change in the current through a coil. As this pulse propagates into the material, it is broadened by dispersion and scattered by discontinuities in the conductivity and permeability of the conductor. Subsurface defects cause part of the pulse to be scattered back to the surface of the conductor where it can be observed as a transient signal in the time domain, either as an EMF across the coil, or by direct measurement of the magnetic field using, for example, a Hall sensor. The observed transient is essentially the response function of the specimen with the transducer at a particular position. It contains information over a broad spectrum in contrast to time-harmonic excitation which yields information only at a single frequency

Structure and Magnetic Fields in the Precessing Jet System SS 433 II. Intrinsic Brightness of the Jets

Deep Very Large Array imaging of the binary X-ray source SS 433, sometimes classified as a microquasar, has been used to study the intrinsic brightness distribution and evolution of its radio jets. The intrinsic brightness of the jets as a function of age at emission of the jet material tau is recovered by removal of the Doppler boosting and projection effects. We find that intrinsically the two jets are remarkably similar when compared for equal tau, and that they are best described by Doppler boosting of the form D^{2+alpha}, as expected for continuous jets. The intrinsic brightnesses of the jets as functions of age behave in complex ways. In the age range 60 < tau < 150 days, the jet decays are best represented by exponential functions of tau, but linear or power law functions are not statistically excluded. This is followed by a region out to tau ~ 250 days during which the intrinsic brightness is essentially constant. At later times the jet decay can be fit roughly as exponential or power law functions of tau.Comment: 30 Pages, 11 Figures, Submitted to Ap