Lothar Staudigel

Structural Monitoring Using Fiber-Optic Bragg Grating Sensors:

For many applications in the aerospace industry, monitoring of structural performance is becoming increasingly important in order to reduce maintenance and inspection costs and enhance efficiency. One of the most attractive sensors to build large area sensor networks has become the fiber-optic Bragg grating. Despite the fact that Bragg grating sensors have been of considerable interest to the structural health monitoring community for the last few years, many challenges associated with applying Bragg grating sensors in real-world applications have not yet been solved. One of the major issues remaining is the method of attaching the sensors to the structure to be monitored. It is essential to understand the optical signal of the sensor and its relation to the actual strain fields in or on the structure. DaimlerChrysler and EADS have studied several solutions for the fiber attachment over the past few years. This paper discusses three of these methods, which are currently under investigation in both lab and field tests.

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 60 nm (around 1.53 μm) with an absolute accuracy better than 1 pm. 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 FBG 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 °C. Furthermore, at negative temperatures we found a vanish- induced polarization dependence which could be used to differentiate between strain and temperature effects.

Test of a fiber Bragg grating sensor network for commercial aircraft structures

The field test reported on in this paper has shown the great potential of fiber-optic Bragg gating sensors. They are suitable for load monitoring of aviation structures under real-world conditions. The results obtained with the Bragg gratings show excellent consistency with the strain gage results. This indicates that both the sensor installation technique of gluing the sensors to the surface and the sensor interrogation technique using a tunable laser-based system, both of which were studied in the field test, are well-suited for these measurements. In addition, the fiber-optic sensor network has several advantages: it is immune to electromagnetic interference and has high multiplexing capability, for hundreds of these sensors can be aligned along a single optical fiber, if desired. This significantly reduces the effort needed for wiring and also leads to a considerable decrease in weight compared to standard electrical sensor networks.

Lifetime of fiber Bragg gratings under cyclic fatigue

We present results on optical and mechanical stability of single-layered acrylate coated fiber Bragg gratings produced on a draw tower, and exposed to high temperature annealing and to cyclic loading. Optical stability was assessed in terms of strain response and reflectivity changes with annealing temperatures up to 400 degree(s)C. Cyclic loading of Bragg gratings with mean stresses between 2.4 GPa and 3.0 GPa was compared with predictions made by using power-law based crack growth theory with parameters obtained by dynamic tensile tests. Comparison with theory confirms predicted strength decrease and lifetime reduction.

Tunable laser-based hybrid WDM/TDM sensor system for the interrogation of low-reflective fiber bragg gratings

This paper describes a novel, patented wavelength- and time-division multiplexing sensor system based on a tunable laser source with an additional optical modulator geared for intenogating a large number of low-reflective Bragg grating sensors with high accuracy.