An Improved Semi-analytical Method for the strength prediction of composite bonded scarf repairs

Scarf repairs are regarded as a highly effective repairing method to recover most of the load-carrying capabilities in primary aircraft structures. In this article, an Improved Semi-analytical Method (ISM) is proposed as a quick design tool for the strength prediction and the optimization of composite scarf repair structures made from fabric or unidirectional prepregs, including 2D scarf joints, 2D scarf repair structures, and 3D circular scarf repair structures. This approach considers the effect of the wedge-shaped area at the adhesive boundary on the maximum Shear stress Concentration Factor (SCF) of adhesive and the strength of the structure. This article proves the feasibility of applying this method to predict the tensile strength and the failure mode of 2D scarf joints and 2D scarf repair structures and shows that it can accurately predict the non-linear decrease in tensile strength of scarf repair structures with increasing scarf angle. Furthermore, the strength of 3D circular scarf repair structures with different scarf angles is measured to assess the accuracy of the ISM method in the prediction of the tensile strength for this type of repair. Due to the suppressed out-of-plane deformation of the 3D structure, the ISM method shows better strength predictions in 3D scarf repaired structures.