HIGH-FREQUENCY BACKSCATTERING ENHANCEMENTS BY THICK FINITE CYLINDRICAL-SHELLS IN WATER AT OBLIQUE-INCIDENCE - EXPERIMENTS, INTERPRETATION, AND CALCULATIONS

Impulse response backscattering measurements are presented and interpr eted for the scattering of obliquely incident plane waves by air-fille d finite cylindrical shells immersed in water. The measurements were c arried out to determine the conditions for significant enhancements of the backscattering by thick shells at large tilt angles. The shells i nvestigated are made of stainless steel and are slender and have thick ness to radius ratios of 7.6% and 16.3%. A broadband PVDF (polyvinylid ene fluoride) sheet source is used to obtain the backscattering spectr al magnitude as a function of the tilt angle (measured from broadside incidence) of the cylinder. Results are plotted as a function of frequ ency and angle. These plots reveal large backscattering enhancements a ssociated with elastic excitations at high tilt angles, which extend t o end-on incidence in the coincidence frequency region. Similar featur es are present in approximate calculations for finite cylindrical shel ls based on full elasticity theory and the Kirchhoff diffraction integ ral. One feature is identified as resulting from the axial (meridional ray) propagation of the supersonic a(0) leaky Lamb wave. A simple app roximation is used to describe circumferential coupling loci in freque ncy-angle space for several surface waves. The resulting loci are used to identify enhancements due to the helical propagation of the subson ic a(0-) Lamb wave. (C) 1998 Acoustical Society of America. [S0001-496 6(98)04302-1].