Ultrasonic compression wave NDT of adhered metal lap joints of uncertain dimensions

This paper is concerned with non-destructive testing of adhered lap joints using simple compression wave pulse-echo apparatus applied to bondlines in which the thickness of the adhesive layer is not known, and to which physical access is only available on one side of the structure. We consider in detail the nature of the time domain reflection response 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 reverberations in the front adherend to be estimated. This parameter can be used as the basis of front adherend disbond detection, and possibly to detect wrong material as distinct from cured adhesive, in contact with the adherend. A discussion is included which assesses the computation errors that would be involved in using the reverberation decay rate 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 ofa void-disbond between the adhesive and the rear adherend, furthest from the interrogating transducer. Whether or not such a disbond is detectable is determined for a range of adhesive layer and adherend thicknesses. For the purpose of comparison with the time domain analysis we include a briefdiscussion on the use of the frequency domain representation of the bond reflection response for similar detection tasks ; it is shown that the frequency domain can be employed in situations when the bond layer thicknesses are known, but would not be appropriate if these dimensions are not known a priori.