Environmental effects on the fractographic analysis of Mode I delamination on secondary and co-bonded composite adhesive joints

Adhesive bonding technologies for thermoset polymer composites have been used in marine, automotive, construction and aerospace industries due to their superior mechanical behaviour (high strength-to-weight ratio, damage tolerance and fatigue resistance) compared to conventional joining methods. The main disadvantage of this joining technology is its susceptibility to delamination due to disbonding during use. Loading conditions, adhesive type, ageing effects and lack of inspection procedures are just some of the elements that affect the overall structural performance of the composite joint during the manufacturing process. A deeper understanding of how these elements affect joint behaviour is required to improve joint performance and design. This work provides a comparative fractographic analysis for two different joint types: co-bonded (CB) and secondary bonded (SB) joints, under Mode I delamination at elevated temperature and high humidity conditions. Fractographic analysis was used to compare the two joint technologies and explain the differences in toughness values and fracture behaviour, revealing crack propagation mechanisms in composite joints. While the CB and SB joints have comparable fracture toughness (G_IC) values, different fracture characteristics and bonding methods can discern these two bonding technologies, indicating that SB joints are more susceptible to environmental conditioning.