A strategy to reduce delamination of adhesive joints with composite substrates

The use of bonding for joining composite materials in high-performance structures has increased significantly, as this joining method offers improved stress distributions and capability of joining dissimilar materials. However, the use of adhesive bonding for this purpose might lead to delamination failure, caused by peel stresses acting on the generally weaker transverse direction of the composite adherends. This work focused on improving the resistance to delamination of composite adhesive joints by using a novel composite with a reinforced high toughness resin on the surfaces. Single-lap joints using the novel composite material as adherends, were found to have 22% higher failure loads when compared with the specimens using carbon fiber reinforced polymer only adherends, with the failure mode changing from delamination of the adherends to cohesive failure in the adhesive. The lap shear strength was also close to that attained when using high strength steel adherends. A finite element analysis, using cohesive elements, was performed with the objective of reproducing the experimental results and better understanding the failure mechanism. Using this model, it has been determined that the change of failure mode and the plasticity on the surface layers are the two key factors underlying the increase in strength obtained with the novel adherends.