This paper presents a study of guided ultrasonic waves in
a tri-layer structure composed of two aluminum sheets bonded by an adhesive
layer of resin epoxy. Numerical resolution, of high accuracy, of the
characteristic equation permitted to determine the dispersion curves and the
displacements field in the structure. A new interpretation of the dispersion
curves of the tri-layer structure is proposed highlighting the existence,
for each propagation mode, of a portion where this mode follows the mode of
an adhesive layer and another portion where it follows the mode of an
aluminum sheet. From dispersion curves, we determine the displacements field
of the tri-layer structure showing that the displacement takes place in the
adhesive layer when the mode of propagation is in the first portion and it
is dominating in the aluminum sheets if the mode is in the second portion.
In addition, we show the possibility of characterization of the quality of
bond of the two aluminum sheets from the analysis of dispersion curves of
the structure.