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, keeping the ratio of the bulk longitudinal to bulk sheer wave attenuation constant at the value measured in high-density polyethylene, the degree of coupling between the shear and longitudinal partial waves decreases and the phase velocity dispersion curves for different modes of the same symmetry can cross; this is not possible for an elastic plate. With increasing attenuation, some modes become asymptotic to the bulk longitudinal velocity at high frequency, rather than to the bulk shear velocity. At high values of attenuation, there is minimal coupling between the longitudinal and shear partial waves and the behavior of the longitudinal modes is analogous to that of a "fluid plate." The predicted group velocities and attenuations of selected modes in a high-density polyethylene plate have been checked experimentally, and good agreement was obtained. (C) 1998 Acoustical Society of America. [S0001-4966(98)02508-9].