The inverse problem concerning the ultrasonic NDE of a layered medium

For a three layered medium immersed in water, the transfer function is written in terms of three travel times, three acoustic impedance ratios, and a reference time constant, tau. Using a measured value for tau, travel times and acoustic impedance ratios are inverted using the peaks of the magnitude of the transfer function. Peak frequencies ranging from 1-20 MHz are used for the following specimens: 304SS/Al/304SS, 436SS/Al/304SS, and Ag/Cu/Ni. The nominal layer thickness varies from about 0.3 mm to 2 mm. The ultrasonically measured total thickness agrees to within 13 mu m with direct micrometer measurements. The errors for the acoustic impedances are less than 3%.