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Parameter measurement of thin elastic layers using low-frequency multi-mode ultrasonic lamb waves

A low-frequency ultrasonic method presented in this paper is suitable for measuring any one of the four characteristic parameters, i.e., thickness, density, shear, and longitudinal wave velocities, of a thin elastic layer given the remaining three. Thickness of the thin layer is much smaller than the longitudinal wavelength of the ultrasonic wave used in the experiments. The method employs the water immersion pitch-catch technique, and is based on the dispersion properties of the first-order symmetric and anti-symmetric modes of a Lamb waveguide. The unknown parameter is estimated by minimizing the mean square error obtained by comparing theoretical dispersion curves with experimental data. A wide-band excitation technique is used to reduce measurement time, by which a frequency range of possible guided Lamb waves is simultaneously explored. The sensitivity and accuracy of the proposed technique for different parameters in different thicknesses are analyzed. Using the present technique, a thin layer with thickness down to one percent of the longitudinal wavelength can be successfully measured with an average error of about two-percent. But the method fails to evaluate the density of the thin layer if the thickness is less than five percent of longitudinal wavelength.