Thermomechanical calibration of FBG sensors integrated in aerospace structures

Optical fibers have found widespread use in a multitude of areas over the last decades. Their main application field is communication, but they are gaining interest and their implementation is spreading even in the medical sector, as well as the infrastructure and engineering. Other than being utilized for signal transmission, optical fibers can also be used to house specific types of sensors, such as Fiber Bragg Grating (FBG) sensors. This type of sensor is capable of reflecting a wavelength, known as Bragg wavelength, which can be influenced by the physical deformations of the sensor itself and by variations in ambient conditions, allowing FBGs to be used for the measurement of deformations, vibrations, temperature and humidity variations. These features, combined with their reduced size and weight, insensitivity to electromagnetic interference, and the possibility of being easily integrated into composite structures, make them great candidates as sensing elements also for aerospace applications. The investigation of the behavior of FBGs integrated in composite material structures as temperature and mechanical strain sensors is the main subject of this work. The activity discussed in this project starts with the experimental determination of the K_T and K_epsilon coefficients of the sensors. Then, in order to isolate the thermal effects from the mechanical ones, alternative approaches in this regard will be proposed and experimentally tested. The issue of thermal decoupling is addressed, first, using a hybrid system of digital and optical sensors, then with an intermediate solution composed by two optical-only systems, mechanically decoupled from each other, and lastly, a single, completely autonomous optical configuration is analysed. Finally, the experimental results will be compared to reference values to verify the validity and practical application possibilities of FBGs adopting such thermal decoupling methodologies.