Time-Domain reflection field analysis for evaluation of thin layered medium

In the conventional ultrasonic testing of a thin layer with the thickness less than wavelength, successive echo signals become inseparable and to 'see' directly the time interval between them become impossible. Also, frequency domain techniques such as frequency spectroscopy is not applicable in high frequency region beyond about twice of frequency band width of a ultrasonic transducer. Recently, a new time-domain analysis technique to determine the thickness(or the wave velocity) of a thin plate and through-transmission experiments were reported. We have studied to know the validity of the technique and applicability to contact testing under a reflection system which is easier to access and more generally used. We drived a reconstructed reflection field equation. We performed experiments in reflection field system using contact type transducers of center frequency 1, 2.25, and 5 MHz with water layers of thickness ranging from 0.055 mm to 1.44 mm(0.037) larger than about 0.4, 3% for h*/λ larger than 0.8, and 1% for h*/λ larger than 2. High frequency elastic wave was effective for more accurate measurement of thickness or wave velocity in our reflection system. The time domain technique was applicable to evaluation of a thin layered medium under reflection system using contact type transducers.