title: Optical fibre sensors for process and structural health monitoring of advanced composite materials
Creators: name: Kuang, Kevin
Publisher: name: University of Liverpool Repository
Year: 2010

description: The concept of smart structures with integrated optical fibre sensors capable of performing real-time structural health-monitoring has attracted much attention in recent years due to the potential safety and economic benefits they offer. In addition, optical fibres have also been employed as in-situ sensors to monitor the fabrication cycle of polymer composites. The aim of the present research study is to explore the concept of smart composite structures for damage detection and strain monitoring using optical fibre technology. The first part of this thesis summarises a study of the use of two types of optical fibre systems for realtime health-monitoring of advanced composite structures based on a conventional multi-mode optical fibre and a Bragg grating sensor. These sensors have either been embedded or surface-bonded to the host material for health assessment of the structure. The second part of the thesis outlines an investigation which has been carried out to assess the potential of an intensity-based optical fibre sensing system to monitor the chemo-rheological changes which take place during the processing of a thermoplastic composite. The study has demonstrated the potential of the system and has shown that this technique offers a cost-effective means of monitoring the processing cycle of a polymer composite material. Regarding the health-monitoring aspect of the research, intensity-based optical fibre sensors have been embedded in a thermoplastic composite and a fibre-metal laminate system based on the same composite constituent. Impact tests and quasi-static three-point bend tests have been conducted to investigate the ability of these optical fibres to detect damage induced by these loading types. The embedded UAT-type optical fibre was found to be sensitive to impact energies as low as 1 Joule. The hard-clad EMT-type optical fibre on the other hand was shown to be capable of surviving extensive sub-perforation-type impact damage and could be used for detecting ballistic damage in aircraft combat situations. In the three-point bend tests, embedded UAT-type optical fibres were observed to be sensitive to flexural failure and were shown to be capable of detecting damage in the host material. Fibre Bragg grating sensors were embedded in a number of thermosetting composites and a thermoplastic fibre-metal laminate to examine their potential for strain monitoring and damage detection. Uniaxial tests have demonstrated that specimens exhibiting a single peak spectrum show excellent linearity. In contrast, specimens with multiple-peak spectra were shown to exhibit strain anomalies. In impact tests, the specimens were impacted repeatedly and, although in each case the spectrum underwent a significant change in shape, the sensor showed excellent survivability. Postimpact fatigue tests have demonstrated that the linearity of the FBG was maintained. The results also highlighted the potential of the sensor to detect impact event and damage propagation. In addition, the study has demonstrated the ability of fibre Bragg gratings to measure post-processing residual strains within a multi-material structure. Multi-mode optical fibres were used as sensors in an intensity-based optical fibre system to obtain real-time information during the processing of thermoplastic-based glass fibre-reinforced polypropylene (GFPP). The technique used to monitor the melting and solidification process in this study is based on monitoring the modulation of the refractive index of the polymer matrix. The intensity modulation of the signal was monitored during the composite processing cycle. Differential scanning calorimetry (DSC) was performed to provide a reference to evaluate the validity of the optical fibre data. The results have demonstrated the potential of this method in achieving a repeatable and accurate indication of the melting and recrystallisation temperatures within the semicrystalline matrix.