Toughening boron/epoxy bonded joints using the resin film infusion technique

Bonded boron/epoxy repairs to aircraft can be susceptible to fatigue damage and crack propagation at high levels of in-service loading. In an effort to improve the fatigue tolerance of these repairs, an attempt has been made to improve the mode I fracture toughness. Two techniques for toughening boron/epoxy plies for use in bonded composite repairs were investigated. Firstly, the resin film infusion technique was used with dry boron fibres and the rubber-toughened film adhesive FM73 to produce a toughened boron/epoxy ply. The second technique involved co-curing a boron/epoxy laminate with FM73 adhesive. Bonded joints, comprising various toughened and un-toughened boron/epoxy adherends and FM73 adhesive, were made into fracture toughness specimens and tested. The fracture toughness for crack initiation increased from 328 j/m(2) for the un-toughened specimens to 1600 and 3 100 j/m(2), respectively, for the two toughened specimens. The second technique, however, produced unstable fractures. Fractography revealed that the boron fibre-to-resin interface was the preferred failure path in all cases. (C) 2003 Elsevier Science Ltd. All rights reserved.