LAMB WAVES IN HIGHLY ATTENUATIVE PLASTIC PLATES

The influence of material attenuation on Lamb wave dispersion behavior has been studied analytically. As the attenuation is increased, keepi ng the ratio of the bulk longitudinal to bulk sheer wave attenuation c onstant at the value measured in high-density polyethylene, the degree of coupling between the shear and longitudinal partial waves decrease s and the phase velocity dispersion curves for different modes of the same symmetry can cross; this is not possible for an elastic plate. Wi th increasing attenuation, some modes become asymptotic to the bulk lo ngitudinal velocity at high frequency, rather than to the bulk shear v elocity. At high values of attenuation, there is minimal coupling betw een the longitudinal and shear partial waves and the behavior of the l ongitudinal modes is analogous to that of a ''fluid plate.'' The predi cted group velocities and attenuations of selected modes in a high-den sity polyethylene plate have been checked experimentally, and good agr eement was obtained. (C) 1998 Acoustical Society of America. [S0001-49 66(98)02508-9].