Abstract:
Bending (flexing) modes are excited when signals scatter at oblique incidence from spheroidal shells [M. Werby et al., J. Acoust. Soc. Am. 85, 2365 (1989)]. This was inferred by comparing the exact T-matrix resonance calculations with those predicted from Timoshenko beam theory. A more sophisticated approach was also developed using the general phase- integral method that allowed for nonuniform shapes. The possibility of the existence of such modes not only for fairly thick shells but also for thin shells is considered. Acoustical signals scattered by elastic shells of various materials, aspect ratios and shell thickness are investigated. The presence of bending modes even for very thin shells is demonstrated. It is showed that for thick shells the resonances manifest themselves as maximum amplitude returns while for thin shells they manifest themselves as minimum amplitude returns. The effect is associated with the transitional nature of a rigid-like background to a soft-like background for the two extremes so that the return signals vary in their coherence from adding constructively to adding destructively over the thickness variation. The sensitivity of resonance locations as a function of the elastic parameters is also presented. [Work supported by Naval Research Laboratory and Office of Naval Research.]