ASA 129th Meeting - Washington, DC - 1995 May 30 .. Jun 06

5aPA8. Lamb waves of two elastic plates coupled by a thin water film.

M. Rousseau

F. Coulouvrat

Laboratoire de Modelisation en Mecanique, Universite Pierre et Marie Curie et C.N.R.S. URA 229) Case 162, 4, place Jussieu, F-75252 Paris Cedex 05, France

O. Lenoir

H. Khelil

J. L. Izbicki

Universite du Havre and C.N.R.S. URA 1373) F-76610 Le Havre, France

This experimental and theoretical study investigates the guided waves that propagate along a multilayered structure, composed of two identical, infinite, aluminum plates coupled by a thin water film. The related dispersion equation is written under a closed form. Solutions are calculated numerically to draw the dispersion curves. The low- and high-frequency limits are calculated analytically. Special attention is devoted to the determination of the cut-off frequencies. An asymptotic analysis shows that, in most cases, a doubling of the classical Lamb waves happens. The small deviation of the cut-off frequencies from the classical ones has been evaluated. An additional, single mode is also outlined; its cut-off frequency is smaller than the Lamb waves ones and depends heavily on the water film thickness. The experiments have been performed on 3- and 5-mm aluminum plates insonated at normal incidence by short pulses. The reflected signal is then FFT-processed; depending on whether the specular echo is removed or not, a resonance or a backscattered spectrum is obtained. The doubling of the Lamb waves, and the existence of an additional low-frequency mode, have been thus confirmed. For a structure composed of three aluminum plates, the experiment now outlines families of three models, each family being closely related to one classical lamb wave.