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Optical NDT of Adhesively Bonded Joints

The basic failure modes of adhesively bonded joints are either in the adhesive (cohesive failure) or along the bondline interface (adhesive failure). The cohesive failure mode results in either shear or peeling failure of the adhesive and is associated with the creation of debonds. The adhesive structural performance is also influenced by voids and porosity in the adhesive layer. The bondline interface adhesive failure condition will also produce debonds, but debonds are typically driven by inadequate surface preparation, which is associated with weak bondlines. The difficulty in finding weak bondlines is that the bondline is intact and no free surfaces have been created. However, there is a change in the local load transfer characteristic, which is a result of changes in the local joint stiffness. Changes in bondline stiffness will enhance the out-of-plane deformation of the joint. Observation of the out-of-plane deformation is a key in the identification of a weakened bondline, and represents an indication that either poor surface preparation or aging effects have occurred. Optical NDT techniques, such as holographic interferometry, have been used to better understand the structural response of bondline discontinuities, both debonds and weak bonds. The interferogram fringe patterns show the structural response of the bondline. These fringe patterns clearly highlight whether the bondline has fractured or merely degraded. The significance of these studies is that weak bonds influence the structural load response of the bondline.

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