  A non contact technique using Zero-Group Velocity (ZGV) Lamb modes is
developed to probe the bonding between two solid plates coupled by a thin
layer. The layer thickness is assumed to be negligible compared with the plate
thickness and the acoustic wavelength. The coupling layer is modeled by a
normal and a tangential spring to take into account the normal and shear
interfacial stresses. Theoretical ZGV frequencies are determined for a
symmetrical bi-layer structure and the effect of the interfacial stiffnesses on
the cut-off and ZGV frequencies are evaluated. Experiments are conducted with
two glass plates bonded by a drop of water, oil, or salol, leading to a few
micrometer thick layer. An evaluation of normal and shear stiffnesses, is
obtained using ZGV resonances locally excited and detected with laser
ultrasonic techniques.
