Fabrication and Application of Fibre Bragg Grating Sensors for Monitoring Structural Behaviour

Abstract

Optical fibre sensors with fibre Bragg grating (FBG) are used widely because of their advantages such as electrical immunity and high sensitivity. The motivation for this thesis is to use the FBG sensors to measure strain gradient and to monitor crack growth, going through the procedures of fabrication and application of fibre FBG sensors for monitoring structural behaviour. The history and advances in FBG sensors are first studied through a literature review, and it highlights several major fabrication methods for the FBG. The FBG sensors used in this study were fabricated using an improved phase mask method. The strains in tensile and bending specimens were successfully measured using both the fabricated and commercial FBG sensors. ABAQUS software was used for simulation of different strain gradient. Spectrum distortion of reflected light was identified by calculating differences in intensity. Furthermore, the coupled mode theory and T-matrix methods were applied to develop an effective model to analyse grating patterns for FBG sensors under strains. Simulation analysis based on MATLAB was performed for the FBG sensors under different strain circumstances. Some strain conditions which were not convenient for the actual test were also analysed using MATLAB and provided a useful reference for studying the FBG sensors under complex strains. Finally, a self-fabricated FBG array was used to monitor mode I interlaminar crack growth in a double cantilever beam (DCB) specimen of glass fibre-reinforced polymer (GFRP) composite. The array with four sets of FBG sensors was attached to the side of cantilever beam of the DCB specimen, the result was analysed, indicating that the array sensor was sensitive to crack growth. This result proved that both the fabrication and the testing method were successful, and attaching FBG sensors on the material surface can be a convenient method for structural health monitoring.