Ultrasonic Transients Research Articles

Ultrasonic generation by pulsed UV lasers

A Q‐switched, frequency doubled ruby laser has been used to generate ultrasonic transients in both metals and water. The laser source delivered multimode pulses, of 30‐ns duration and energy ⩽200 mJ, at a wavelength of 347 nm in the UV. Displacements generated in metal plates were detected by wideband capacitance transducers, and compared to wave propagation theory. Thermoelastic, ablative, and modified surface sources were examined, and a good correlation between experiment and theory obtained. In addition, wideband directivity patterns in aluminum were obtained for the thermoelastic source. Thermal generation in water with a cylindrical geometry was also examined, and the expected dipolar pressure transients recorded. It was shown that their duration was a function of the multimode laser beam diameter over the 1.5‐ to 4‐mm range.

A comparison of optically and piezoelectrically sensed acoustic emission signals

The usual sensor for acoustic emission is the piezoelectric transducer. Although this transducer is readily available, reasonably inexpensive, and very sensitive to ultrasonic transients, it has several serious drawbacks as a transducer: It distorts the signals being measured, it exhibits resonances, it has limited bandwidth, it responds differently to surface acoustic waves and bulk waves (because of its large sensitive area), and its calibration is a matter of considerable uncertainty. Essentially, it is a qualitative transducer. Furthermore, it cannot measure local effects within a millimeter of an emission source, where the mechanisms causing the ultrasonic transient are presumably most clearly distinguishable. Optical transducers, on the other hand, have the great advantage of providing accurate, quantitative, highly localized information; they do not disturb the waves being measured and are not limited by frequency response.