Bonded hard-patch composite scarf repairs: Influence of machining techniques and surface treatment

Advanced composites are widely being used in the design of aircraft structures to reduce weight, improve fuel efficiency and consequently reduce emissions. However, when damaged in service, composite components (e.g. fuselage or wing skins) require robust repair methods to restore structural integrity (i.e. strength and stiffness) as well as aerodynamics. In this context, adhesively bonded composite scarf repairs can offer high joint efficiency and also aerodynamic surface finish. However, to produce reliable bonded scarf repairs, two fabrication steps are critical: (a) accurate machining to achieve designed patch geometry and (b) appropriate surface treatment prior to adhesive bonding. In this work, hard-patch scarf repairs (i.e. secondary bonding of a cured patch with a structural adhesive) were investigated for understanding the influence of different machining techniques and surface treatments on structural repair efficiency (i.e. tensile strength). The effect of (a) manual sanding, (b) manual sanding after CNC machining and (c) laser ablation after CNC machining were investigated and compared. Representative bonded scarf repairs (i.e. scarf joints) were produced by a hard-patch approach using carbon-fiber epoxy pre-preg laminates (HTA 6376 supplied by Hexcel) and a structural film adhesive (FM300-2 supplied by Cytec). Pre-cured parent and patch laminates were bonded in a hot drape former (an out-of-autoclave approach). Quasi-static tensile tests were performed on scarf joints with different surface treatments. Moreover, fractography studies were conducted using optical and scanning electron microscopy to investigate failure mechanisms. The joint strengths and failure mechanisms were compared and the key observations made were discussed.