This paper presents a general overview of a number of optical fibre sensor systems which have been developed and used in advanced fibre-reinforced composites for in-situ process and condition monitoring. The in-situ process monitoring techniques were optical-fibre-based evanescent wave spectroscopy, transmission near-infrared spectroscopy and refractive index monitoring. The optical fibre sensors were successful in tracking the cure reaction. The condition monitoring of advanced fibre-reinforced composites was carried out using two intensity-based optical fibre sensor systems: an extrinsic multi-mode Fabry-P érot sensor and Bragg gratings. In addition to this, the feasibility of using the reinforcing fibre as a light guide was demonstrated. These sensor systems were evaluated under quasi-static, impact and fatigue loading. The test specimens consisted of prepreg-based carbon-fibre-reinforced epoxy and glass-fibre-reinforced epoxy filament-wound tubes. Excellent correlation was obtained between surface-mounted strain gauges and the embedded optical fibre sensors. The feasibility of using these sensor systems for the detection of impact damage and stiffness reduction in the composite due to fatigue damage was successfully demonstrated.

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This paper describes a comparative study of in situ cure monitoring by three methods: (i) evanescent wave spectroscopy; (ii) refractive index change; and (iii) near-infrared spectroscopy.The cure characteristics of an epoxy/amine reaction were followed in real-time during the crosslinking reaction via the above-mentioned techniques. The evanescent wave spectroscopy technique was based on monitoring the characteristic infrared absorption bands of the resin system to compute the concentration of the amine hardener as a function of cure time. Good correlation was obtained between the evanescent wave spectroscopy data and a conventional method of studying cure reactions, i.e. infrared spectroscopy.During the cure reaction, the refractive index of the resin system increases as a function of the crosslink density. This increase in the refractive index was monitored using two optical fibre techniques. In the first case, a declad region of the optical fibre was immersed in the resin system and in the second method an optical fibre reflectometer was used to track the changes in the refractive index. Once again, good correlation was obtained between the optical fibre techniques and infrared spectroscopy cure data. The results obtained from the optical fibre sensor experiments were used to model the cure kinetics of the resin system. The cure kinetic models were found to predict the cure reaction up to approximately 60% of the reaction.

A new in-situ technique for the measurement of photoviscosity effects in polymer solutions has been developed using a custom-modified, cone-and-plate rheometer. The new technique permits simultaneous irradiation of the polymer solution and continuous measurement of its viscosity in a temperature-controlled environment. An additional benefit of the technique is the greatly reduced sample volume compared to traditional capillary viscometers. However, adequate measures have to be taken to minimize the evaporation of the sample solution and to maintain the required temperature. Copolymers of methyl methacrylate and azobenzene monomers with a chromophore in the side chain have been synthesized and characterized using the new technique. This polymer exhibits a photoviscosity effect when exposed to UV irradiation (λ = 365 nm), with the reduced specific viscosity (ηsp/c) of the polymer being up to 77% lower than in the dark. However, the magnitude of the photoinduced change in ηsp/c was found to increase with the number of azobenzene units in the polymer chain, and the greatest effect was shown by a polymer with 42 mol % azobenzene loading.