NDT OF ADHERED LAP JOINTS - ULTRASONIC COMPRESSION WAVE NDT OF ADHERED METAL LAP JOINTS OF UNCERTAIN DIMENSIONS

This paper is concerned with non-destructive testing of adhered lap jo ints using simple compression wave pulse-echo apparatus applied to bon dlines in which the thickness of the adhesive layer is not known, and to which physical access is only available on one side of the structur e. We consider in detail the nature of the time domain reflection resp onse using a combination of results simulated using a transfer matrix wave propagation model, and results obtained from physical experiment. An adaptive filter is introduced which enables the decay rate of reve rberations in the front adherend to be estimated. This parameter can b e used as the basis of front adherend disbond detection, and possible to detect wrong material as distinct from cured adhesive, in contact w ith the adherend. A discussion is included which assesses the computat ion errors that would be involved in using the reverberation decay rat e to estimate the characteristic impedance of the adhesive layer, and thence its cure state. The adaptive filter is then used to reduce the extent to which strong adherend reverberations mask very low amplitude echoes which are diagnostic of a void-disbond between the adhesive an d the rear adherend, furthest from the interrogating transducer. Wheth er or not such a disbond is detectable is determined for a range of ad hesive layer and adherend thicknesses. For the purpose of comparison w ith the dome domain analysis we include a brief discussion on the use of the frequency domain representation of the bond reflection response for similar detection tasks; it is shown that the frequency domain ca n be employed in situations when the bond layer thicknesses are known, but would not be appropriate if these dimensions are not known a prio ri.