RESONANCE TECHNIQUES AND APPARATUS FOR ELASTIC-WAVE VELOCITY DETERMINATION IN THIN METAL PLATES

The principles of operation of a new pulsed ultrasonic resonance spectrometer are presented. The system incorporates: (1) the use of electromagnetic-acoustic transducers (EMATs), (2) a method of obtaining acoustic signals similar to the sampled continuous-wave scheme described by Bolef and Miller, (3) the Clark method of recording line shapes, and (4) a high power gated rf amplifier and a unique implementation of superheterodyne and quadrature phase sensitive detection circuitry. The result is a spectrometer capable of measuring both shear and longitudinal ultrasonic velocities in very thin samples with much greater reproducibility than other pulse methods. It also has applicability in measuring attenuation and other physical properties such as composite material bonding that can be accessed using ultrasound. Experimental results of tests using EMATs in the 0.1-20 MHz range on thin (1.27 and 2.49 mm thick) aluminum sheets are presented.