Built-in real-time sensors of degradation

The use of fibre optic sensors for SHM has increased rapidly in recent years to obtain real-time data on stress or strain in structures such as buildings, bridges and pipelines. Adapting the best of these technologies for flight worthiness and aircraft certification was the goal of the EU-funded project 'Fiber optic sensors application for structural health monitoring' (FOSAS).

Scientists sought to exploit fibre optic Bragg grating (FOBG) sensors for strain evaluation to predict initiation of corrosion. Fatigue corrosion is a complex phenomenon of degradation due to both mechanical cyclic stress and a corrosive environment. Although the individual effects have been studied extensively, their synergistic effects are not well understood. The team investigated both the electrochemical behaviour and mechanical strain of a metal coupon during fatigue testing in an aggressive environment. A FOBG sensor was used to monitor strain.

Development of improved SHM methods requires a suitable test apparatus. FOSAS also developed a tool to support development and certification of new SHM technologies for avionic materials and structures. It uses fibre optic sensors in place of traditional strain gauges. As a result of development work, the team has delivered a range of fibre optic SHM sensors. A single FBG sensor monitors quasi-static strain and high-frequency damage signals simultaneously, and another FBG sensor measures tri-axial strain and temperature simultaneously.

Finally, the team targeted a novel distributed temperature-sensing system with ultra-high resolution based on standard optical fibres and swept wavelength interferometry. The technique converts a spectral shift to a temperature change. After a market analysis to see which technologies on the market were affordable and had the potential for aircraft certification, a distributed sensing technique was developed. The prototype system was tested in a ground test campaign and has been certified for laboratory use.

FOSAS has adapted numerous advanced fibre optic SHM technologies for aeronautical certification and use, and delivered tools for further development. Adoption of these advanced, low-cost technologies will significantly enhance the safety of the aircraft industry while reducing costs associated with operation and maintenance. Enhancing the competitiveness of the EU aerospace sector will have important benefits for the European economy.