High Refractive Index Silicate Fibres for Cure Monitoring of Epoxy Matrix Composites

Fibre evanescent wave spectroscopy (FEWS) cure monitoring has previously been demonstrated with reinforcing fibres using low refractive index epoxy systems. To enable sensing of a range of aerospace grade epoxy systems, chalcogenide fibres with high refractive index (RI) and a large infra-red (IR) transmission were used for FEWS cure monitoring. However, chalcogenides have low chemical durability and poor mechanical properties resulting in fibre fractures and degradation of the optical properties with time. The aim of the project was to produce a high RI silicate glass, suitable for near infra-red (1000-2500nm) FEWS cure monitoring all possible epoxy resin systems throughout the entire curing process, whilst maintaining mechanical properties suitable for reinforcement.
A range of alkali and alkali free silicate glasses and epoxy systems were produced and their refractive indices optically characterised using ellipsometry. The glasses were thermally analysed for the liquidus and fibre forming temperatures for fiberisation. A fibre up-drawing jig was constructed and custom glass fibres were drawn. The composition was then mechanically characterised, demonstrating mechanical properties similar to reinforcing fibres.
Subsequently, the fibres were embedded into sunflower oil, liquid epoxy systems and epoxy pre-preg systems. Sunflower oil systems demonstrated proof of FEWS sensing showing excellent peak correlation. Liquid epoxy systems were successfully cure monitored by FEWS. Pre-preg systems did not display FEWS peaks, but were monitored using light intensity.