New Methods of Detection and Characterization of Surface Flaws

A new approach to microwave eddy current testing for surface cracks in metals involves the use of ferromagnetic resonance in a small garnet crystal placed close to the surface to be tested. It is well known in this case that the induced eddy currents on the metal surface cause a strong displacement of the ferromagnetic resonance frequency. The presence of a crack on the surface produces, by disturbing the eddy current pattern, a perturbation of the resonant frequency as the garnet sample passes over it. Theoretical detectability criteria for the garnet probe are developed on the basis of a magnetic perturbation relation and compared with calculations of Rayleigh wave and plate wave backward scattering and intermode scattering at a half-penny surface crack. Some experimental results are also given

Singlet-triplet relaxation induced by confined phonons in nanowire-based quantum dots

The singlet-triplet relaxation in nanowire-based quantum dots induced by confined phonons is investigated theoretically. Due to the quasi-one-dimensional nature of the confined phonons, the singlet-triplet relaxation rates exhibit multi-peaks as function of magnetic field and the relaxation rate between the singlet and the spin up triplet state is found to be enhanced at the vicinity of the singlet-triplet anti-crossing. We compare the effect of the deformation-potential coupling and the piezoelectric coupling and find that the deformation-potential coupling dominates the relaxation rates in most cases.Comment: 7 pages, 5 figure

Anisotropic interaction of two-level systems with acoustic waves in disordered crystals

We apply the model introduced in Phys. Rev. B 75, 064202 (2007), cond-mat/0610469, to calculate the anisotropy effect in the interaction of two level systems with phonons in disordered crystals. We particularize our calculations to cubic crystals and compare them with the available experimental data to extract the parameters of the model. With these parameters we calculate the interaction of the dynamical defects in the disordered crystal with phonons (or sound waves) propagating along other crystalographic directions, providing in this way a method to investigate if the anisotropy comes from the two-level systems being preferably oriented in a certain direction or solely from the lattice anisotropy with the two-level systems being isotropically oriented.Comment: 10 page

Resonances and Crack Roughness Effects in Surface Breaking Cracks

This study deals with two different aspects of scattering from the cracks. The first one is the measurement of crack resonances by local probes and the second is the effect of the roughness of the crack forces on the scattering data. Unlike the scattering measurement reported elsewhere, these measurements of crack resonances are to be performed by local probes. Two different types of probes are described and their problems are discussed. The effect of crack roughness on the scattering data is investigated via the reciprocity relation. The crack roughness is analyzed using perturbation theory and the equivalent boundary condition concept developed by Brekhovskikh is used to express the fields in the presence of the roughness. The effect of roughness on the scattering data is then discussed in qualitative terms

Using Capacitive Probes in Electromagnetic Nondestructive Testing

In an attempt to expand the scope of electromagnetic testing to include poorly conducting materials, we have tried using a low frequency capacitive probe. This probe has given promising performances in testing materials with resistivities of several hundred ohm-centimeters

Characterization of Surface Wave Scattering by Surface Breaking Cracks

Various analytical methods based on the real reciprocity relation are applied to the problem of Rayleigh wave scattering by a surface crack. In one formulation, the reflection coefficient observed at the transducer terminals is expressed in terms of an integral over the crack surface of the product of the perturbed and unperturbed fields. This integral is then converted to a volume integral and the Born Approximation is applied. In the other formulation a Kirchoff type approach is used such that the effect of the crack is expressed as an equivalent body force distribution. That force distribution is then approximated and normal mode techniques are used to find the scattered field amplitude. The two methods are compared with each other and with the results obtained from geometrical diffraction theory. An experimental procedure is also proposed for the inverse problem

Eddy Current Probe Design and Matched Filtering for Optimum Flaw Detection

Eddy current signals obtained from variations in the probe liftoff are in general much larger in amplitude than the useful flaw signals. Small flaw signals can, however, be detected in the presence of liftoff noise if a large enough phase angle exists between them. Figure 1(a) shows how this phase discrimination can help in liftoff noise suppression. Here, the oscilloscope traces the complex impedance of the probe. The impedance plane has been rotated so that the liftoff noise lies entirely in the horizontal channel. Now if we choose to look only at the signal in the vertical channel of the scope, or the Q channel (in phase quadrature with liftoff), there will be no liftoff noise. This, however, is not a very realistic picture. Figure 1(b) is obtained when we try to detect much smaller flaws (in this case a closed crack of 20 mils in aluminum). We see that the trace of the liftoff noise has a curvature and that there are also fluctuations along the Q channel axis. Both of these effects eventually limit the detectability of small flaws. Since this contribution of liftoff to the Q channel is in practice larger than circuit noise, we define the detection figure of merit for an EC probe as 1D=(ΔZf)sinβ(ΔZℓO)Q

On the Resonances of Surface Breaking Cracks

The resonance phenomenon observed in Rayleigh wave scattering from surface-breaking cracks has been investigated using Freund\u27s results on reflection of Rayleigh waves from an infinite crack edge. To model the crack as a section of acoustic waveguide, resonances are treated as standing waves in the length and depth directions. The model takes both faces of the crack into consideration, and this makes it possible to explain the observations of all order resonances in the length direction for excitation by a Rayleigh wave beam at normal incidence. Calculations are made for rectangular and half-penny shaped cracks and differences between the two cases are discussed