Damage detection in composites using e-glass fibre and small diameter optical fibre

Abstract

The primary aim of this study was to demonstrate that E-glass and custom-made small diameter optical fibres, with an outer diameter of 12 micrometres, can be used to monitor the following: (i) In-situ monitoring of the partial dissolution of the cladding on the small-diameter optical fibres in hydrofluoric acid. This was necessary for the small-diameter optical fibres as the thickness of the original cladding was not appropriate to enable evanescent wave spectroscopy. (ii) In-situ monitoring of the impregnation process. Since the cladding was etched to access the evanescent field in the optical fibres, it was demonstrated that the impregnation of the fibres by the resin could be monitored. (iii) In-situ cure monitoring. After impregnation, the cross-linking reactions taking place at the surface of the glass fibres were monitored using near-infrared spectroscopy. The feasibility of using the glass fibres for monitoring temperature was also demonstrated. (iv) Finally, after the composite was cured, it was tensile tested to failure whilst monitoring the transmitted light intensity through the optical fibres. The un-impregnated bundles were also tensile tested to failure. The failure of the individual filament in the bundle (un-impregnated) and composites were monitored by tracking the intensity of the transmitted light through each filament. Conventional acoustic emission was used to cross-correlate the fracture of the individual filaments. Conventional E-glass fibres can be used as light guides if the conditions for total internal reflection is enabled. In this current study, the matrix served as the cladding.

This study has developed a range of techniques that can potentially facilitate the full life cycle monitoring of glass fibre composites. In other words, the same test specimen can be used to monitoring the surface treatment, temperature during drying or heat treatment, cross-linking kinetics and damage during mechanical loading. The self-sensing technique developed in this study can also be used as a tool to study the degradation of properties when the fibres are recycled and reused.