Structural and Thermal Prognostics and Diagnostics with Fiber Bragg Gratings

Aeronautical industry’s needs to have light and high performance structures has given a strong impetus to the development of technologies able to reduce the overall weight of the machine in order to increase its efficiency and to reduce cost. The research activity carried out in this thesis is aimed at the study and optimization of some locking systems for the use of optical fibers as sensors and for the validation of the sensors themselves. They have many distinctiveness that make them ideal candidates for aeronautical applications, among which first the weight and the reduced dimensions that allow a comfortable installation without invasiveness. Added to these qualities is the ability to multiplexing multiple sensors on the same fiber, to use the same fiber for data transmission and immunity to electromagnetic interference. There are several types of sensors that use optical fibers. In our case we used strain sensors based on Bragg gratings. These sensors consist of a photoengraved grid within the fiber that can reflect only a few wavelengths of the incident light. If the geometric properties change under the influence of an external factor, the shape of the reflected spectrum also changes. In the first part of the work we have described the theoretical basis necessary to achieve the objectives initially set. In particular, we analyzed: - The main characteristics of optical fibers and the related physical laws that govern them; - The main characteristics of the sensors, their realization technology and the laws that link the variation in length with the various quantities that they can analyze; - The theory of mechanical measurements that allows the identification of the various types of errors committed during measurement; - The statistical theory to be used for the evaluation of the error. In the second part the design and construction choices taken during the test bench are exposed. The "propaedeutic" nature of our research has forced us to choose an equipment that can be adapted to the different situations that may arise in the future. Interchangeable locking systems have been developed that allow the replacement of the blocking element and of the variable height thermal elements. All elements must also be able to function in combination with each other. It is also briefly described the calculation code developed for processing the data obtained from the interrogation system connected to the test bench. In the third and last part, the tests performed with the selected locking means and the results obtained are described.