Adhesive bonding

Adhesive bonding is perhaps best suited to joining thin components where the load paths are known and understood. It is possible in these situations to design joints that are as strong as mechanical joints. Many different adhesives are available for bonding thermosetting matrix composites. Generally these adhesives must be cured under application of heat and under pressure. Thermoplastic matrix composites require additional surface treatment to promote high strength bonds and adhesive bonding is perhaps not so widely used. Alternative bonding methods for thermoplastics exist and should be investigated. Key points for bonded joints include:

• Adhesives must be selected to match substrates
• Adhesives must be selected to match expected service environment
• Surfaces must be prepared in clean conditions to avoid surface contamination
• Disassembly not possible without imparting damage into joined parts
• Inspection of bonded joints after manufacture can be carried out using NDT methods to spot bond defects, however, the bonding characteristics of the adhesive can only be determined through destructive testing

Bonding of finished (cured) composite components can be achieved using a separate structural adhesive. For all bonds, it is essential to have no surface contamination to avoid premature failure of the bonded joint. Some manufacturing options are able to utilise a peel-ply approach whereby the surface to be bonded is exposed only prior to bonding. It is also common to use grit-blast techniques to clean surfaces and to promote good surface bonding characteristics. Grit blasting is always followed by wiping over the surface using a suitable solvent selected to be compatible with the composite system.

The orientation of the loading on an adhesively bonded joint is critical. Adhesive joints must be designed so that the components to be joined overlap and place the adhesive in shear and minimise other loading orientations (tension, peel). There are a number of commonly used joint configurations including:

Selection of bonded joint configuration depends on the loading conditions on the joint and the balance between improved joint mechanical performance and manufacturing complexity and cost.