REINFORCEMENT OF THE F-111 WING PIVOT FITTING WITH A BORON EPOXY DOUBLER SYSTEM - MATERIALS ENGINEERING ASPECTS

Local regions in the D6ac steel F-111 wing pivot fitting suffer plastic deformation during the cold proof test employed to screen out flaws in the steel components of the airframe. This deformation produces residual stresses which can lead to a fatigue cracking problem in service. This paper describes materials engineering aspects of a programme undertaken at the Aeronautical Research Laboratory to develop an advanced fibre composite doubler system aimed at reducing the strain in these regions during the cold proof test and also to reduce cyclic strain during subsequent operation of the aircraft. The doubler system chosen is boron/epoxy (over 120 plies thick) bonded with a structural film adhesive. Aspects described include selection and characterization of composite and adhesive bonding system, doubler fabrication and application technology and evaluation of a representative doubler specimen. Use of the doubler system has been demonstrated to reduce the strain in the critical regions by over 30%, confirming design predictions.