Time-domain reflection field analysis for ultrasonic evaluation of thin layered media

In conventional ultrasonic testing of a thin layer with thickness less than about twice the wavelength, successive echo signals become inseparable. Recently, a new time-domain analysis technique to determine the thickness or the wave velocity of a thin plate was developed and through-transmission experiments were reported. We have studied the validity of the technique and its applicability to contact testing under a reflection field system. We found a mistake in the derivation of the reconstructed reflection field equation reported, and a correction has been made. We performed the experiment in a reflection field system using contact type transducers of centre frequency 1, 2.25, and 5 MHz with water layers of thickness ranging from 0.06 to 1.44 mm (0.04 less than or equal to thickness/wavelength < 5). Thickness and wave velocity were determined within 7% error for the ratio of thickness to wavelength (h*/lambda) larger than about 0.4, 3% for h*/lambda larger than 0.8, and 1% for h*/lambda larger than 2. The time domain technique with our corrected equation was applicable to the evaluation of a thin layered medium under reflection system using contact type transducers. Copyright (C) 1996 Elsevier Science Ltd.