Weak Bonds Characterization and Detection in Composite Structures

Adhesive bonding is largely used at Bell Helicopter Textron and throughout the aerospace industry. Weak bonds, also commonly referred to kissing or slick bonds, are adhesive bonds exhibiting a low cohesion interface between the adhesive and the substrate made of metal or composite. The occurrence of this condition is one of the many reasons why mechanical fasteners are still required by aviation authorities to insure a second load path. This type of condition is still not well understood and its detection remains a challenge for any NDI group. Kissing bonds are often not detected under classical NDI inspection techniques (i.e.: Tap testing, Pulse-echo or through transmission C-Scan, A and B scan) since there is no noticeable separation between the substrate and the adhesive surfaces. Factors influencing the strength of a bond include surface contaminants, incorrect anodization or priming, adhesive chemistry, cure cycle, residual stresses, moisture ingress or any combination of these or other factors. Silicone based oils and release agents are known to contaminate adhesive bonds and therefore affect the integrity and strength of the joint. Literature and industry do not agree on an exact definition or criteria to classify a weak, slick or kissing bond. Therefore, this issue needs to be addressed. The research effort first consisted in developing a methodology to consistently reproduce conditions leading to a controlled slick bond presence in adhesively bonded test samples. Once a consistent and repeatable process was developed, efforts were directed into the characterization of this type of condition with the use of non-destructive and destructive inspection. Benefiting of this research, an array of NDI techniques have been evaluated to detect those types of conditions and to pinpoint the most suited non-destructive technique to be employed in a manufacturing environment. With the data gathered from the experimental results, recommendations and general criteria defining slick, kissing or weak bonds will be enumerated and discussed.