C. Zhu V. K. Kinra
Ctr. for Mech. of Composites, Dept. of Aerosp. Eng., Texas A&M Univ., College Station, TX 77843-3141
This paper describes a technique for ultrasonic nondestructive evaluation of a thin coating (subwavelength) on a thick substrate. A plane longitudinal wave which is normally incident upon the coating is considered. Transfer functions have been derived for both the coating-side and the substrate-side insonification. A systematic analysis of the sensitivity of the transfer functions to the thickness and wavespeed has been carried out. An inverse algorithm has been developed to reconstruct the thickness and the phase velocity through a comparison of the theoretical and the measured transfer functions. Using this technique both the thickness and the wave speed of the coating can be extracted from the same measurement, without knowing either. The technique was used to measure the thickness and wave speed of epoxy and Plexiglas coatings (50--100 (mu)m) on an aluminum substrate using low-frequency (10 and 20 MHz) transducers; the ratio of thickness/wavelength was about 1/3. The precision in the measurement of the thickness and the wave speed was found to be (plus or minus)2 (mu)m and (plus or minus)3%, respectively. [Work supported by Texas Advanced Technology Program.]