CertBond // COST Action CA18120 // Reliable roadmap for certification of bonded primary structures
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Adhesive bonding technology has been present in aircraft history since the late 1940’s.

Adhesive bonding technology has been present in aircraft history since the late 1940’s. Although it has shown its potential for several decades, its application as a joining technology and assembly method has always been limited to secondary parts of the aircraft.

Conventionally in metal aircrafts different parts of aluminium sheets are assembled using fasteners, namely rivets. Riveting is a joining technology that has been streamlined over the past decades. Recent developments in the automatization have made riveting economically attractive.

Nowadays aircraft manufacturers are increasingly using Carbon Fiber Reinforced Polymers as a light and robust replacement of aluminium. Riveting such composite materials has a lot of hazards, such as fibre breaking, stress concentrations, black dust, etc. Adhesive bonding is the most promising joining technology in terms of weight and performance for assembling composite parts. However, bonded joints without additional fasteners are only certified for assembling secondary structures of an aircraft, whose failure is not detrimental for aircraft safety.

In comparison with metals, composites bring extra difficulties for bonding since the manufacturing processes use release agents which can lead to reduce adhesion strength that cannot be detected with the conventional non-destructive testing, such as ultrasound.

Therefore, the current common practice in primary bonded structures is to include fasteners, which typically can carry the full limit load, along with the bonded systems, as “back-up” in case the bond fails.

This design practice is not limited to aircraft structures. In bridges, ships, oil-platforms, industries where composites are also gaining their share, it is common practice to include fasteners and/or anchorage systems due to lack of confidence in bonded joints.

In addition to this, some chemical components used in the application of adhesive bonding represent a hazard for both human health and environmental. Examples of this are isocyanates (VOCs), which are being used in the adhesive material synthesization, and chromates, used for guaranteeing a durable and effective surface treatment prior to bonding metals. 

However, both chemical components are among the current chemicals for which industrial users must find substitutes, or request authorisation from EU regulators to continue their use to be compliance with the strict REACH EU regulations.

This state-of-the-art hinders the full potential of bonded systems (uniform distribution and no stress concentration) and moreover, it does not diminish any of the hazards caused by drilling in composites. New alternatives are urgently needed to use the full potential of bonded systems in primary composite structures.