The V(z) Inversion Technique for Evaluation of an Adhesively Bonded Structure

Based on the fact that the evolution trace in incident angle and frequency of the resonance zeros of the reflection coefficient function for a water charged layered medium is equivalent to its guided wave mode dispersion, the interfacial adhesion of a three-layer aluminum–adhesive–aluminum bonding structure is characterized nondestructively by determining the interface shear stiffness k_t associated with the interfacial strength. The resonance reflection function is obtained experimentally by the V(z) inversion technique using an ultrasonic focused transducer of wide-band and large angular aperture (up to ±45^°). The dispersion curves are numerically calculated, adjusting the parameter k_t so that the difference between the dispersion curves and the angular-frequency tracing of the reflection zeros is minimum. The parameter k_t at an interface of weakly adhered aluminum epoxy-resin is estimated to be 10¹⁴ N/m³.