PhDKissing 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
