UNSPECIFIED (1991) LASER-GENERATED SURFACE-WAVES ON PLATES OF VARYING THICKNESS. BRITISH JOURNAL OF NON-DESTRUCTIVE TESTING, 33 (4). pp. 177-182. ISSN 0007-1137

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Abstract

Ultrasonic waves which propagate along the surface of a plate can be described by the Rayleigh-Lamb frequency equation. Considerable simplification of the theoretical analysis occurs when the plate thickness is either much greater or much smaller than the ultrasonic wavelengths. In the short wavelength case, ultrasound propagates as a non-dispersive, surface-travelling (Rayleigh) wave. When all the wavelength components are much greater than the plate thickness, the zero-order Lamb wave modes are the most prominent features. In this paper; we present the results of a series of experiments on aluminium plate samples, examining the transition between the two extremum forms as the plate thickness was decreased. The broad bandwidth (10ns rise-time) surface-travelling waves were generated using a Nd:YAG laser. They were detected, 20 mm from the source, by broadband electromagnetic acoustic transducers (EMATs), sensitive either to the normal, or the tangential, velocity of the sample surface. On thin foils, the prominent antisymmetric zero-order Lamb mode is highly dispersive, the high-frequency components of the signal arriving first. Effects observed at the front of the signal seem to derive from the finite (1 mm) width of the detector coil. Hence, for comparison, a homodyne laser interferometer approximating to a point detector was also used. For aluminium plate with thickness greater than 6 mm, the non-dispersive Rayleigh wave was clearly distinguishable from the reverberating bulk waves. In plates less than 0.5 mm thick the lower order Lamb modes were dominant and individual bulk waves were not observed. On intermediate plates, the waveforms appeared inherently complicated, comprising an inextricable combination of bulk-reverberating and Lamb waves.

Item Type:	Journal Article
Subjects:	T Technology > TA Engineering (General). Civil engineering (General)
Journal or Publication Title:	BRITISH JOURNAL OF NON-DESTRUCTIVE TESTING
Publisher:	BRITISH INST NON-DESTRUCTIVE TESTING
ISSN:	0007-1137
Date:	April 1991
Volume:	33
Number:	4
Number of Pages:	6
Page Range:	pp. 177-182
Publication Status:	Published
URI:	http://wrap.warwick.ac.uk/id/eprint/22776

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