Smart sensing of aviation structures with fiber-optic Bragg grating sensors

We developed a surface mounting technique where fiber-optic Bragg grating (FBG) sensors are glued to the surface of structures and tested the technique on the surface of a CFRP-wing at the DASA Airbus test center Hamburg for over one year. The FBG sensors were interrogated with a measurement system capable of determining the Bragg wavelength in a few seconds over a spectral range of 60nm (around 1.53 mu m) with an absolute accuracy better than Ipm. A polarization scrambler was used to account for polarization effects. Excellent consistence between the values of electrical strain gauges and the FBG sensors were found during all measurements. However because this method shows drawbacks in a harsher environment such as a flight test, we are currently investigating the possibilities of integrating FBG sensors into the varnish of the structures. For reasons of their better mechanical performance we use FBC sensors produced on the fiber draw-tower with a special UV-curable coating. The sensors are integrated into an original Airbus varnish build-up. We observed linear strain sensitivities in a temperature range between -50 and +100 degrees C. Furthermore, at negative temperatures we found a varnish-induced polarization dependence which could be used to differentiate between strain and temperature effects.