Mixed-mode fracture toughness of co-cured and secondary bonded composite joints

•Determined the mixed-mode fracture toughness of a co-cured and two secondary bonded joint systems.•Investigated the differences in fracture behaviour between co-cured and secondary bonded joints.•Demonstrated role of scrim cloth in fracture behaviour of FM300-2M film adhesive.•Attributed R-curve behaviour and increased toughness during mode II fracture to an evolving damage zone.

In the present study, the mixed-mode fracture toughness of three composite joint systems (2 secondary bonded and 1 co-cured) was investigated. The two secondary bonded systems predominantly resulted in cohesive failure with occasional interlaminar failure during mixed-mode loading while the co-cured system failed adhesively. This was attributed to moisture stored in the prepreg being released during the co-cure process.The mode II toughness was consistently 5–6 times that of the corresponding mode I case for each system. The increased toughness was due to a combination of an increased fracture surface area and a larger damage zone.