Vibration and damage monitoring in advanced composites using multiplexed fibre bragg grating (FBG) sensors

Abstract

In situ and real-time structural health monitoring (SHM) is essential for the utilisation of advanced composite materials. This thesis presents a feasibility study of using multiplexed fibre Bragg grating (FBG) sensors for vibration and damage monitoring in composite materials based on analytical, numerical and experimental tools. Preliminary experimental studies on the mechanical properties of glass fibre-reinforced epoxy (GF/EP) beams with embedded optical fibres were carried out for providing a guideline on the placement of embedded optical fibres. The static strain distribution, natural frequencies and strain mode shape of the beams were found to be affected significantly by delamination size and location. An analytical model was developed to investigate the influence of the delamination on the mode II energy release rate (GII) of the beams. Observation of the fracture of the beams was highly correlated with their GII. Effects of the embedded optical fibre on the natural frequency shift and the GII of the delaminated beams were examined. The natural frequency shifts of the beams with and without embedded optical fibres were comparable. No significant effect of the optical fibres on the fracture behaviours of the beams was observed if the optical fibres were located at least two layers away from the delamination. The capability of FBG sensors for average and distributive strain measurements in composite beams was demonstrated. The response from the sensors on measuring distributive strain was estimated by considering a modified stress transfer mechanism of an embedded optical fibre composite structure. Vibration monitoring, delamination characterisation and fracture process monitoring of composite beams using FBG sensors were studied. Natural frequency and strain mode shape of the beams could be determined by a multiplexed FBG sensor array. Delamination could be characterised by the FBG sensor by virtue of the Bragg wavelength shift, level of intensity, spectral span and shape of reflection spectrum from the sensor. Similarly, the fracture process of the beams could be on-line monitored by the FBG sensors associated with the load-displacement relationship. A two-step genetic algorithm (TSGA) was introduced to determine the optimal FBG sensor placement for damage detection. Optimal FBG sensor arrays were then explored to detect the delamination in composite beams based on natural frequency and strain mode shape approaches. The sensor arrays were able to quantitatively identify the delamination sizes and location of the beams by using the first three vibration modes combined with genetic algorithm (GA). Moreover, the presence of the delamination could be detected through the strain mode approach. The role of the optical fibre technology for future development of SHM system in advanced composite structures will be elucidated.