Lamb-type waves in a symmetric solid-fluid-solid trilayer

Guided waves in a symmetric solid/fluid/solid tri layer are investigated. Derivation of the dispersion equation underlines the: existence of global symmetric and antisymmetric modes. These two families are shown to be affected by the coupling very differently. Approximate analytical expressions are obtained for the case of a thin and compliant fluid layer, with the formation of doublets for moderately thin liquid films. The existence of a low frequency mode whose cut-off frequency is related to the liquid layer thickness, the so-called "vertical mode", is proved. Theoretical results are confirmed by experiments, based on the identification of modes by analysis of the backscattered or transmitted spectrum. Computed and measured cut-off frequencies are in good agreement, with an error of about 1 % or less.