Kissing bonds in adhesive joints: a holistic approach for surface chemistry and joint mechanics
Kissing bonds (KBs) refer to the situation where two surfaces are only partially bonded or are debonded but touching or in very close proximity. This may be the consequence of poor adhesion, environmental degradation or impact damage. This defect is not visible macroscopically and because of their intimate contact which makes it more difficult to detect using a non destructive technique (NDT) than conventional defects such as voids or cracks etc. The success of NDT evaluation and widespread use of adhesive bonding rely greatly upon comprehensive knowledge of morphology, surface chemistry and mechanics associated with KBs. Two approaches were successfully taken to produce reliable and repeatable KBs: by surface contamination using a mould release agent (Frekote®700-NC); and by weakening the electrically-debonding adhesive, ElectRelease™, with a low voltage. Significant changes in morphology and elemental distribution of the contaminant at/near the Frekote contaminated interfaces were found. Some morphological and chemical changes at/near the anodic metal/ElectRelease™ interface were also evident. Additional information about chemical interactions at/or near the contaminated interface due to the presence of Frekote and the application of the electric field confirmed the changes in morphology and elemental distribution. Double-lap joints with KBs were tested in tension with local strains captured by strain gauges and extensometer. Significant reduction in failure strength was apparent when using Frekote and ElectRelease™ subjected to the electric field. The tests were simulated using finite element analysis. Cohesive elements were introduced along the predicted failure interfaces taking into account the adhesion loss associated with KBs. The experimental failure load and local strain results were in good agreement with the finite element predictions. The ways that KBs were produced and the understandings in morphology, surface chemistry and their failure mechanisms contributed to the modified criteria of KBs and the development of the non-linear ultrasonic technique investigated by the NDT group at the University of Bristol. The morphology, surface chemistry and failure mechanisms of KBs in adhesive joint are now better understood.
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