Assessment of the Effects of Scanning Variations and Eddy Current Probe Type on Crack Detection

Eddy current procedures are currently the most capable, of the nondestructive evaluation (NDE) techniques that are being applied in industry. The performance capability of an NDE procedure is that of the probability of detection as a function of flaw size. Prediction of the performance capability of a given procedure has been inexact, due to the lack of supporting theory, and has therefore been either validated experimentally or has been assumed to be applicable to a test problem by its similarity to a “time proven” application. Rigorous experimental validation of an NDE procedure is laborious and must be repeated for each new application and/or change in NDE parameters. Attention has been focused on this problem and much of the work described in this volume is directed toward the determination of critical characteristics of NDE applications and in the generation of supporting theory to facilitate predictive modeling of NDE performance capability. The experimental work described in this paper expands on previous work on the characterization of eddy current probes, as applied to flaw detection [1,2], and is directed to support the expansion of application theory [3]

Maximizing phonon thermal conductance for ballistic membranes

At low temperatures, phonon scattering can become so weak that phonon transport becomes ballistic. We calculate the ballistic phonon conductance G for membranes using elasticity theory, considering the transition from three to two dimensions. We discuss the temperature and thickness dependence and especially concentrate on the issue of material parameters. For all membrane thicknesses, the best conductors have, counter-intuitively, the lowest speed of sound.Comment: 4 pages, 4 figures, proceedings to phonons 2007 conferenc

Characteristics of phonon transmission across epitaxial interfaces: a lattice dynamic study

Phonon transmission across epitaxial interfaces is studied within the lattice dynamic approach. The transmission shows weak dependence on frequency for the lattice wave with a fixed angle of incidence. The dependence on azimuth angle is found to be related to the symmetry of the boundary interface. The transmission varies smoothly with the change of the incident angle. A critical angle of incidence exists when the phonon is incident from the side with large group velocities to the side with low ones. No significant mode conversion is observed among different acoustic wave branches at the interface, except when the incident angle is near the critical value. Our theoretical result of the Kapitza conductance $G_{K}$ across the Si-Ge (100) interface at temperature $T=200$K is 4.6\times10^{8} {\rm WK}^{-1}{\rmm}^{-2}. A scaling law $G_K \propto T^{2.87}$ at low temperature is also reported. Based on the features of transmission obtained within lattice dynamic approach, we propose a simplified formula for thermal conductanceacross the epitaxial interface. A reasonable consistency is found between the calculated values and the experimentally measured ones.Comment: 8 figure

The Arecibo Galaxy Environments survey IV: the NGC7448 region and the HI mass function

In this paper we describe results from the Arecibo Galaxy Environments Survey (AGES). The survey reaches column densities of ~3x10^18 cm^-2 and masses of ~10^7 M_O, over individual regions of order 10 sq deg in size, out to a maximum velocity of 18,000 km s^-1. Each surveyed region is centred on a nearby galaxy, group or cluster, in this instance the NGC7448 group. Galaxy interactions in the NGC7448 group reveal themselves through the identification of tidal tails and bridges. We find ~2.5 times more atomic gas in the inter-galactic medium than in the group galaxies. We identify five new dwarf galaxies, two of which appear to be members of the NGC7448 group. This is too few, by roughly an order of magnitude, dwarf galaxies to reconcile observation with theoretical predictions of galaxy formation models. If they had observed this region of sky previous wide area blind HI surveys, HIPASS and ALFALFA, would have detected only 5% and 43% respectively of the galaxies we detect, missing a large fraction of the atomic gas in this volume. We combine the data from this paper with that from our other AGES papers (370 galaxies) to derive a HI mass function with the following Schechter function parameters alpha=-1.52+/-0.05, M^*=5.1+/-0.3x10^9 h_72^-2 M_O, phi=8.6+/-1.1x10-3 h_72^3 Mpc^-3 dex-1. Integrating the mass function leads to a cosmic mass density of atomic hydrogen of Omega_HI=5.3+/-0.8x10^-4 h_72^-1. Our mass function is steeper than that found by both HIPASS and ALFALFA (alpha=1.37 and 1.33 respectively), while our cosmic mass density is consistent with ALFALFA, but 1.7 times larger than found by HIPASS

Heat capacity of a thin membrane at very low temperature

We calculate the dependence of heat capacity of a free standing thin membrane on its thickness and temperature. A remarkable fact is that for a given temperature there exists a minimum in the dependence of the heat capacity on the thickness. The ratio of the heat capacity to its minimal value for a given temperature is a universal function of the ratio of the thickness to its value corresponding to the minimum. The minimal value of the heat capacitance for given temperature is proportional to the temperature squared. Our analysis can be used, in particular, for optimizing support membranes for microbolometers

A numerical investigation of a piezoelectric surface acoustic wave interaction with a one-dimensional channel

We investigate the propagation of a piezoelectric surface acoustic wave (SAW) across a GaAs/Al$_X$Ga$_{1-X}$As heterostructure surface, on which there is fixed a metallic split-gate. Our method is based on a finite element formulation of the underlying equations of motion, and is performed in three-dimensions fully incorporating the geometry and material composition of the substrate and gates. We demonstrate attenuation of the SAW amplitude as a result of the presence of both mechanical and electrical gates on the surface. We show that the incorporation of a simple model for the screening by the two-dimensional electron gas (2DEG), results in a total electric potential modulation that suggests a mechanism for the capture and release of electrons by the SAW. Our simulations suggest the absence of any significant turbulence in the SAW motion which could hamper the operation of SAW based quantum devices of a more complex geometry.Comment: 8 pages, 8 figure

A Self-Calibrating Eddy-Current Instrument

The calibration of eddy-current measurement systems is a long-standing problem in nondestructive evaluation. Calibration serves a number of purposes: for equipment setup and validation, for equalizing responses from different probes and instruments, for setting detection thresholds, and for quantitative flaw sizing. The most commonly used calibration method is to scan the probe to be calibrated over simulated defects such as electrical-discharge machined (EDM) slots, saw cuts, or laboratory-produced fatigue cracks. This method has the virtue of calibrating probe and instrument at the same time on the same material as that to be inspected. But it has a number of disadvantages as well. First, a large number of artifact standards must be generated, certified, and maintained in the typical inspection organization; this can result in considerable expense. Second, the signals from EDM slots and saw cuts are not equivalent to the signals from actual defects, as discussed in another paper in these proceedings [1]. Third, quantitative flaw sizing can only be accomplished over a limited range with such calibration methodology, and the accuracy of sizing flaws with this method is brought into question by the aforementioned inequality of slots and cracks. Even if laboratory-produced cracks were to be used routinely for calibration (a prohibitively expensive option), quantitative sizing could be compromised by the occurrence of crack closure effects [2]

Pulse-induced acoustoelectric vibrations in surface-gated GaAs-based quantum devices

We present the results of a numerical investigation which show the excitation of acoustoelectric modes of vibration in GaAs-based heterostructures due to sharp nano-second electric-field pulses applied across surface gates. In particular, we show that the pulses applied in quantum information processing applications are capable of exciting acoustoelectric modes of vibration including surface acoustic modes which propagate for distances greater than conventional device dimensions. We show that the pulse-induced acoustoelectric vibrations are capable of inducing significant undesired perturbations to the evolution of quantum systems.Comment: To be published in Phys. Rev.

Quantum statistical effects in nano-oscillator arrays

We have theoretically predicted the density of states(DOS), the low temperature specific heat, and Brillouin scattering spectra of a large, free standing array of coupled nano-oscillators. We have found significant gaps in the DOS of 2D elastic systems, and predict the average DOS to be nearly independent of frequency over a broad band f < 50GHz. At low temperatures, the measurements probe the quantum statistics obeyed by rigid body modes of the array and, thus, could be used to verify the quantization of the associated energy levels. These states, in turn, involve center-of mass motion of large numbers of atoms, N > 1.e14, and therefore such observations would extend the domain in which quantum mechanics has been experimentally tested. We have found the required measurement capability to carry out this investigation to be within reach of current technology.Comment: 1 tex file, 3 figures, 1 bbl fil