Stress concentrations around bonded repairs

Ageing aircraft fuselages require safe and damage- tolerant repair techniqu es. Bonded fibre-metal laminates (FML) combine a small mismatch in coeffici ents of thermal expansion (CTE) with the cracked skin, excellent fatigue pr operties and high strength, and are therefore promising materials for bonde d repairs. This paper focuses on local stress concentrations in the fuselag e skin at the edge of a bonded FML repair, and presents analytical, FEM and experimental results of single and multiple repairs in close proximity. Re pairs on both flat uniaxially loaded skins and curved biaxially loaded skin s were considered. The main objective of this research is the validation of the crack patching design program CalcuRep. The model on which CalcuRep is based, the 'Rose model', was extended and used for the analytical predicti on of skin stresses in the specimens. It appeared that, in order to prevent fatigue critical stresses in the skin next to the patch, one should carefu lly choose the patch material and geometry Furthermore, in a specific repai r configuration with multiple patches, a minimum separation is required to prevent high stress concentrations in the skin between the patches. It was confirmed that, in most cases, a biaxial stress field has a favourable effe ct on the stress in the skin along the patch edge, compared with the uniaxi al situation. With both the FEM and the extended Rose model, the stresses i n the skin next to the patch could be predicted with sufficient accuracy. I t is the intention to implement the extended Rose model in a new version of CalcuRep.