Use of shear horizontal waves to distinguish adhesive thickness variation from reduction in bonding strength. 
The capability of shear horizontal (SH) guided waves, to evaluate geometrical imperfections in a bonding layer, is investigated.  SH waves are used in a three-layer structure in which the adhesive layer has variable thickness.  It is proven that the SH waves are adapting to the local thickness of the adhesive layer (adiabatic waves).  This is particularly useful in case of small thickness variations, which is of technical interest.  The influence of thickness and stiffness of the adhesive layer on the wavenumbers are investigated.  The selected SH2 mode is proven to be very sensitive to the adhesive layer thickness variation in the given frequency range and considerably less sensitive to the adhesive stiffness variation.  This property is due to its specific displacement field and is important in practical applications, such as inspection techniques based on SH waves, in order to avoid false alarms.