S Vanaverbeke | PubMed Health

PubMed2003

The reflection of bounded inhomogeneous waves on a liquid/solid interface.

Fourier analysis and normal mode theory are used to describe the reflection of bounded inhomogeneous waves on a liquid/solid interface. Nonspecular reflection phenomena in the Rayleigh angle are studied in detail. In this way, an explanation is given for the Rayleigh dip phenomenon for positive inhomogeneity factors and the related result of a reflection coefficient larger than unity when the sign of the inhomogeneity factor is reversed. In the limit of large beamwidths, the reflection coefficient predicted by the infinite plane inhomogeneous wave theory is obtained. These results are entirely consistent with the experimental work published by Deschamps [J. Acoust. Soc. Am. 96, 2841-2848 (1994)]. The energy efficiency of Rayleigh wave excitation is investigated as well. It is shown that for large beamwidths, the energy efficiency for bounded inhomogeneous waves is considerably higher in comparison with Gaussian and square-profiled beams.

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PubMed2001

Thin coating characterization by Rayleigh waves: an analytical model based on normal-mode theory.

The acousto-optic technique to evaluate coated substrates, presented by Devolder et al. [Appl. Phys. Lett. 68, 1732 (1996)], uses the phase dependence of the reflected beam on the coating parameters. In this theoretical paper, a fully analytical model, based on normal-mode theory, is applied to this problem. It gives a transparent expression for the phase shift between the reflected and the incident field due to the presence of thin coatings (coating thickness much smaller than wavelength sound). The known numerical results from Fourier simulations, i.e. the phase dependency on the coating parameters and the advantages of using higher frequencies and wider beams, are reproduced. However, thanks to the analytical nature of the new theory, additional results are obtained. The calculation time is reduced by two orders of magnitude, which is important for the inverse problem. A technique for absolute thickness measurements is proposed. Measuring more to the right of the second reflected maximum increases the phase sensitivity. The versatility of the normal-mode approach is illustrated by applying it to other nondestructive testing (NDT) applications, such as hardness determination and adhesion testing, which are briefly discussed.

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