ABSTRACT
In this paper, optical instrumentation using a non-steady state photo-emf receiver is described for the detection of relatively low frequency Lamb waves in mild steel plate. The receiver, used as the ultrasonic sensor, had the advantage of a lower frequency cut-off at 100 kHz, compared to a confocal Fabry-Perot interferometer (CFPI) of about 2 MHz. Both schemes offered greater stand-off distances (>20 cm) than was possible with EMATs, which have a stand-off distance typically less than 1.0 mm. The transient Lamb waves were laser-generated in the thermoelastic regime using single laser pulses. Mild steel plates from 0.5 mm to 5 mm thickness were examined. Typical Lamb waves had centre frequencies of 250 kHz but with frequency components that extended beyond 1 MHz in the thinner plates studied. Using the well-known Rayleigh-Lamb equations, asymptotes for the higher order modes were calculated and were not considered to be significant below a frequency thickness product of 1.6 MHz.mm for mild steel. Below this figure, associated velocity dispersion curves offered phase velocity changes that were sensitive to thickness change. A summary is presented of progress made in the non-destructive evaluation of Young’s modulus for mild steel plates.
