LAMB-TYPE WAVES IN A SYMMETRICAL SOLID-FLUID-SOLID TRILAYER

Guided waves in a symmetric solid/fluid/solid tri layer are investigat ed. Derivation of the dispersion equation underlines the: existence of global symmetric and antisymmetric modes. These two families are show n to be affected by the coupling very differently. Approximate analyti cal expressions are obtained for the case of a thin and compliant flui d layer, with the formation of doublets for moderately thin liquid fil ms. The existence of a low frequency mode whose cut-off frequency is r elated 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 trans mitted spectrum. Computed and measured cut-off frequencies are in good agreement, with an error of about 1 % or less.