Effects of bondline flaws on the damage tolerance of composite scarf joints

Scarf repairs to aircraft structures need to sustain design ultimate load in the presence of flaws due to manufacturing and impact by foreign objects, in order to demonstrate compliance with airworthiness regulations. This paper presents an investigation into the effect of disbonds on the load-carrying capacity of adhesively bonded scarf joints. Experiments were conducted on scarf joints containing disbonds of varying lengths. The results showed that the load-carrying capacity of scarf joints decreases with the size of the bondline flaw at a faster rate than the reduction in the effective bond area. Fractographic analysis showed that the fracture occurred in the composite matrix adjacent to the adhesive-composite interface, at a distance equal to a small fraction of ply thickness. Computational analyses using the virtual crack closure technique (VCCT) and the cohesive zone model (CZM) confirmed these experimental observations: model predictions using composite material properties were in better correlation with experimental results than those using adhesive properties. Furthermore, CZM is capable of predicting the effects of flaws of all sized being considered, while the VCCT model is only applicable to joints containing flaws greater than a certain size.