Improved design methods for scarf repairs to highly strained composite aircraft structure Alex B. Harman * , Chun H. Wang Air Vehicles Division, Defence Science and Technology Organisation, PO Box 4331, Melbourne, Vic. 3001, Australia Abstract This paper presents an analytical method to optimise the profile of the scarf joint between dissimilar modulus adherends so that adhe- sive stresses are approximately uniform along the joint. The optimised scarf repair is expected to enhance joint strength and reduce the amount of material removal. Finite element analyses have been performed to both validate the optimal solution and to evaluate the use of low stiffness patch to repair carbon epoxy composites. In particular, the influence of patch lay-up on adhesive stresses has been investigated. Ó 2006 Published by Elsevier Ltd. Keywords: Scarf; Adhesive bonded joint; Composite repair; Finite element 1. Introduction Bonded repair technology is now a widely accepted alternative to mechanically fastened repairs to secondary and tertiary aircraft structure. The success of bonded repairs has led to the impetus to apply adhesive bonded repairs to primary aircraft structures, particularly compos- ite structures. It is generally accepted that overlap repairs provide the most cost effective means for repairing thin sec- tion aircraft skins, such as fuselage and some control sur- faces. However, primary aircraft structures, such as wing skins, are often quite thick in order to carry primary struc- tural loads as well as to meet the damage tolerance require- ment. Typically, a critical primary structure component, be it metallic or composite, that suffered damage above its damage tolerance limit, would either be replaced or repaired using a bolted repair. However, with the more pre- valent use of large unitised structure to construct modern aircraft, the replacement option becomes less cost effective. When a thick composite structure is repaired using the con- ventional bolted repair method, a large perturbation of the external mould line will occur. The protrusion of a thick bolted patch may severely affect the aircraft’s aerodynamic performance or the stealth characteristics of low observable aircraft. Consequently, bonded flush repairs, such as scarf repairs, may become the preferred method of repair. Scarf repairs are often designed using a two-dimensional analysis of the representative scarf joint. In terms of repair efficiency, a scarf repair can restore a much higher strength than a bonded overlap repair, because the adhesive stresses along the scarf joint do not suffer from the considerable stress concentrations present in overlap repairs. This is illustrated in Fig. 1. A well-designed scarf joint does not attract these adhesive stress concentrations. This is illus- trated in Fig. 1 showing that the strain in the bondline of a scarf joint is near constant, in contrast with that pertinent to an overlap joint. However, application of scarf repairs is more involved than the patch repair technique. First of all, for thin section structures, it is often impractical to machine shallow angles especially if the structure is not supported against out-of- plane deflection. As the parent material thickness increases, the viability of the scarf joint with its strength advantages to overlap joints improves. However, in order to restore the strength of a carbon–epoxy composite structure to 0263-8223/$ - see front matter Ó 2006 Published by Elsevier Ltd. doi:10.1016/j.compstruct.2006.04.091 * Corresponding author. Tel.: +61 3 9626 7201; fax: +61 3 9626 7174. E-mail address: Alex.Harman@dsto.defence.gov.au (A.B. Harman). www.elsevier.com/locate/compstruct Composite Structures 75 (2006) 132–144