Strain Measurements Using Fibre Bragg Gratings during Full-Scale Structural Testing of an F/A-18 Centre Barrel

This paper reports on an experimental program of work which investigates the use of fibre Bragg gratings (FBGs) for the measurement of surface strains during full-scale structural testing. The test article was an ex-service F/A-18 wing attachment centre barrel (CB). The CB has three main structural bulkheads whose function is to carry wing loads into the fuselage. Two FBGs were surface mounted to each bulkhead on the starboard side with electrical resistance foil strain gauges bonded to symmetric locations on the port side of the structure for comparison. The FBGs were fabricated during the fibre draw process and were then packaged in a composite tape for secondary bonding to the structure under test using a standard aerospace qualified strain gauge adhesive. The centre barrel was subjected to a series of load surveys comprising 50%, 70% and 100% of the maximum spectrum load amplitude and low constant amplitude (8% of maximum spectrum load) cycling during thermoelastic stress analysis surveys prior to the initiation of variable amplitude accelerated fatigue spectrum loading until structural failure. The experimental data shows good agreement between the electrical-resistance foil and optical gauges when the loading is symmetric with additional detail provided by the optical gauges which had a higher sampling rate. These results confirm the potential of FBGs to provide detailed experimental strain data from across the structure without the associated complexity of wiring and installation often associated with electrical foil gauges.