On the design methodology of scarf repairs to composite laminates

Scarf repairs are the preferred method of repairing composite structures, especially when externally bonded patches can no longer meet the stiffness, strength, and flushness requirements. Present designs of scarf repairs are based on two-dimensional analyses of scarf joints, assuming a uniform stress distribution along the scarf. The purpose of this investigation is to first critically assess the adequacy of the current design method, and then to develop an improved design methodology. Strain concentrations in the adhesive and the composite laminates are determined using the elastic-plastic finite element method, taking into account of the influence of stacking sequence, laminate thickness, and adhesive yielding. Three series of experiments have been carried out, involving joints between metallic and composite adherends, with matched and un-matched lay-ups. Based on the experimental findings of joints and the computational analyses of joints and repairs, an improved method has been proposed that accounts for local peak plastic strains due to non-uniform shear strain distribution. Comparison with experimental results confirmed that the new approach provides an improved first-order prediction of repair efficiency of scarf joints.