A z-transform technique for thin-layer reverberation cancellation applied to ultrasonic NDT of adhered structures

This paper describes a z-transform technique to enhance ultrasonic compression wave NDT signals obtained from thin-layer bonded structures. Often diagnostic components in the received signals are masked by reverberations excited elsewhere in the structure. This paper outlines some of the problems in detecting void disbonds in these structures. A wave system model has been used to simulate acoustic wave propagation in layered structures and the simulations have been used as an aid to characterizing diagnostic signal components. By considering the bonded structure as an acoustic filter we can describe some of its effects on the interrogating NDT signal by z-transforms. Inverse filters can then be generated which operate as digital filters and these are used to adaptively cancel out unwanted reverberations in the received signals. Preliminary results are presented for steel-adhesive-steel bonded systems.