Strain and temperature measurement with fiber optic sensors in composite structures: Applications for control and health monitoring

Fiber optic Bragg grating sensors are inscribed into optical fibers. They show promising capabilities in the field of monitoring composite structures: The detection of strain, temperature and the concentration of chemicals are application possibilities for this sensor type. Each Bragg grating sensor delivers information by its Bragg wavelength shift. Using different wavelengths, multiple sensors can be read out by just one optical fiber. The optical fibers can be integrated into carbon fiber reinforced plastic (CFRP) structures. Fiber optic sensors are robust against strong electric fields and cryogenic temperatures which makes them interesting candidates for applications in space. This paper gives an overview of activities and experiments performed at the Institute for Lightweight Structures (LLB) at the Technical University of Munich (TUM). The paper demonstrates the application potential of fiber optic strain sensors for active structures and for temperature measurement at cryogenic temperatures. Different integration methods for fiber optic strain sensors were investigated and the strain measurements were compared with reference strain gauges. In the field of shape control, fiber optic strain sensors were integrated into an active CFRP rod with integrated piezo actuators. The length of the beam could be controlled by the integrated fiber optic strain sensors which was evaluated using a reference laser signal. For temperature measurement an integratable fiber optic temperature sensor was developed and calibrated down to temperatures of 20 K. The sensor was then tested and its operational performance evaluated after integration into CFRP. Simultaneous strain and temperature measurements will allow to compensate the strain error induced by temperature shifts in the fiber optic sensors. The goal of these activities is an increased accuracy for the integrated sensor system under thermal loads.