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NDE of zinc layer on steel substrate using laser-ultrasonic SAW

Laser-ultrasound spectroscopy, a non-contact ultrasonic technique was used to characterize the Lame coefficients ((lambda) ,(mu) ) and thickness (h) of a zinc layer on a steel substrate. This characterization is based on fitting the measured velocity dispersion curve of surface acoustic waves (SAW) to the dispersion calculated one using the conjugates gradients algorithm (C.G). A short laser pulse was used to generate a wideband pulse of ultrasound and a laser interferometer was used for its detection. From a large number of echoes we identified the one corresponding to the SAW. Furthermore other useful information were obtained from these data like attenuation and surface skimming longitudinal wave. Measurements of the velocity dispersion of the Rayleigh wave were achieved up to 50 MHz. The evaluation of layer's parameters performed for similar cases, on a pseudo-experimental model, were obtained with accuracy better then 1% for (h, (mu) ) and about 4% to 6% for (lambda) .