ASA 124th Meeting New Orleans 1992 October

5aPAa10. Higher-order Lamb resonances and their critical frequencies.

M. F. Werby

NRL, Stennis Space Center, MS 39529

All the resonances generated on elastic shells are studied for a ka from 0 to 1000 for steel, aluminum, and WC for a shell thickness of 5% relative to the object radius. This study briefly illustrates the lowest-order symmetric and antisymmetric model or Lamb resonances, newly studied water borne and pseudo-Stoneley resonances, and the higher-order Lamb modes A[sub i] and S[sub i], where i=1,2,3... . This study concentrates on the higher-order Lamb resonances, derives simple expressions that predict their onset (critical frequencies), and illustrates how to classify each of the resonances for specific material. The phase velocities of some of the relevant resonances out to a ka of 1000 are plotted. For example, there are 13 resonances that can be classified for steel with a specific ordering. The ordering scheme is rather different for aluminum and WC. It is shown that the S[sub 0] Lamb mode does not asymptote to the plate velocity as predicted in thin shell theory but rather to the Rayleigh phase velocity, the same as the flexural mode. It is demonstrated by use of partial wave analysis that the new expressions that predict the onset (critical frequencies) of the higher-order Lamb modes are reliable. Further, interesting phenomena occur at the inception of some of the resonances and some of those cases are discussed.