Optical diagnostics for structural health monitoring of inaccessible systems

Abstract

The work in this thesis is concerned with the development of optical sensors for the structural health monitoring of systems with inaccessible components in the presence of ionising radiation. Two primary approaches to determine structural health are investigated, these being the implementation of distance measurement sensors (to assess, for example, the occurrence of creep/cracking) and gas sensors (to assess the occurrence of outgassing which can be indicative of chemical ageing). Distance measurement sensors are developed using fibre optic Fabry-Pérot interferometry (FPI) measured in reflection. A fast-Fourier transform (FFT) of the reflected interference spectrum is incorporated to demodulate the signal and extract cavity length information. To mitigate noise and enhance measurement accuracy and sensitivity, spectrum reconstruction in the form of a function-fitting algorithm (FFA) is developed, the input of which is fed by the FFT output. The FFA demonstrates measurement improvements of approximately one-and-a-half orders of magnitude. To situate the fibre FPI sensors within spatially confined, closely positioned components, turning mirrors are fabricated on two cores of multi-core fibre (MCF) which redirect the fibre-guided light perpendicular to the fibre axis in opposite directions. This allows for the absolute distance between component parts to be determined as Fabry-Pérot cavities are formed on both sides of the MCF. Three different turning-mirror fabrication processes are developed, the results from each are compared and discussed. The distance measurement capabilities are expanded to that of a two-point measurement system to allow for tilt measurement competencies. This facilitates further comprehensiveness with regards to structural health monitoring. Further, two optical gas sensors are investigated, both based on the principle of absorption spectroscopy. One consists of evanescent wave generation in a tapered fibre and the other of incoherent broadband cavity-enhancement. Modelled and experimental results are presented and limitations are discussed.