The use of adhesive bonding to join metals, composites, plastics and other materials is increasing across many industries. The aircraft industry widely adopted metal-to-metal bonding after the second world war and this has since spread to different material combinations and other industrial sectors including modern aerospace, automotive, marine and wind energy.
Bonded joints offer many advantages over conventional methods such as bolting, riveting and welding. Weight saving is key but other benefits include an improved distribution of stresses by eliminating the need for holes and welds, easier design and assembly of large area joints or curved external surfaces, and an integrally sealed joint, often using dissimilar materials, that can avoid electrolytic corrosion and is less sensitive to crack propagation.
The integrity of these bonds is critical to the quality of the final product and so techniques to assess the quality of the bond have seen a resurgence. With new material combinations come new possible flaw types and the challenge is to develop inspection techniques to detect these flaws before they cause problems. For example, a modern composite sandwich construction made with two skins and a core material may contain flaws like cracks, disbonds, delaminations, voids, and crushed core but conventional inspection techniques can be of limited use because of the multiple glue lines and attenuative materials that are inherent in the lay-up.
With such a range of materials and configurations used in multi-layered bonded structures, different types of testing methods have been developed to attempt to cover all combinations. Bond testing is already established as a requirement in maintenance manuals and technical directives for multiple commercial and private aircraft types but is now seeing its use expanding.
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