Degradation of meridional ray backscattering enhancements for tilted cylinders by mode conversion: wide-band observations using a chirped PVDF sheet source

Abstract

High-frequency backscattering measurements are discussed for a thick-walled hollow finite cylindrical shell. The frequency range examined is above the coincidence frequency where energy coupled to supersonic Lamb-type waves dominates the backscattering for off-beam aspect. One particular mechanism, involving a meridional leaky ray, leads to large enhancements along well-defined arcs in frequency-aspect angle space. It is shown that the backscattered levels for the flexural mode meridional ray drop dramatically above a certain frequency. This degradation of the energy in the return signal is found to be correlated with mode conversion at the truncation of the shell of the lowest order flexural mode into the next higher flexural mode. An approximate calculation utilizing simply supported boundary conditions does not display the observed degradation since it fails to allow for mode conversion.