Evaluation of a distributed fibre optic strain sensing system for full-scale fatigue testing

The Defence Science and Technology Group has been conducting full-scale fatigue tests of ex-service F/A-18 Hornet centre fuselages, in support of the Royal Australian Air Force's structural integrity management programs for its frontline fighter fleet for over 12 years. Historically, conventional electrical resistance foil strain gauges have been used extensively on these tests to monitor and record the structural response to loading; however, there are limitations with these in terms of cost, installation times and physical complexity. Developments in commercially available, distributed fibre optic strain measurement systems presents the opportunity to overcome these limitations, as demonstrated on the most recent centre fuselage test article. Based on Rayleigh scattering, the system ('ODiSI B' by Luna Innovations) was trialled which allowed comparisons of strain response, spatial resolution and noise levels with conventional foil gauges. Comparisons were also made of the full-field strain mapping capability of the system with full-field stress mapping by thermoelastic stress analysis. Furthermore, the distributed fibre optics demonstrated their potential to detect crack propagation on a coupon with induced crack growth.