Kissing bonds in adhesive joints: a holistic approach for surface chemistry and joint mechanics

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

A novel method to produce kiss-bonds in composites components for NDI and characterisation purposes

Kiss-bonds (kissing bonds) are a defect type that feature a localised loss of structural continuity within the material, yet the material remains in intimate contact across the defect. Typically, shear and normal tensile stresses cannot be conducted across such defects (although, pure compressive stresses are possible). Kiss-bond defects are difficult to detect reliably – both within the bulk of the material (interlaminar) and within bond-lines of adhesively bonded joints or repairs – using conventional Non-Destructive Inspection (NDI) techniques. Compounding this issue is the lack of a reliable technique to create representative kiss-bond defects in a controlled fashion for the purpose of NDI equipment calibration or development, or scientific investigation. A novel method for manufacturing composite material test panels with kiss-bond defects (for research or NDI calibration, for example) in a controlled and repeatable fashion has been developed. Small areas of two adjacent pre-preg plies were pre-cured before being incorporated within a laminate. During consolidation, no bonding occurs between the pre-cured areas, thus creating a kiss-bond defect of known geometry. Test panels with 6 × 6 mm and 10 × 10 mm kiss-bond defects were manufactured. The robustness of the technique was verified using ultrasonic and laser shearography NDI methods; 7 of the 10 manufactured defects were classified as kiss-bonds, with the remaining 3 identified as dis-bonds