Getting light through black composites: embedded triboluminescentstructural damage sensors

Abstract

Triboluminescent materials offer a viable route to real timestructural damage sensing. The sensors can be externally attached to thesurface of metals or composites, or embedded within composites. In situ monitoring of structural damage in composites is particularlyrelevant since severe internal damage can exist with little indication of thisdamage on the composite surface. The main issue related to embeddingtriboluminescent sensors within composites is how to access efficiently theoptical signal generated upon structural damage and how to guide efficientlythis optical damage signal to a remote detector. Earlier work relied onside-coupling of the triboluminescent light into a curved conventional silicafibre and/or end-coupling into the silica fibre, if the damage to the hoststructure had also broken the fibre. Both these light collection methods are,of course, inefficient and resulted in an optical damage signal with a verypoor signal-to-noise (S/N) ratio. By using novel photoluminescent polymericand silica fibres, we have shown that it is possible to efficiently captureand guide the optical damage signal from an embedded triboluminescent sensorto a remote detector. These fibres resulted in a signal with a S/N ratio thatwas orders of magnitude larger than that achieved using the conventionalsilica fibre. Furthermore, we have shown that the light collection/guidingtechnique can even be used effectively with triboluminescent sensors embeddedwithin black carbon fibre reinforced plastic (CFRP) composites. Finally, we havedemonstrated that for resins and glass fibre reinforced plastic (GFRP) composites, triboluminescentsensors act as truly global damage sensors, whereas for CFRP composites, thedamage sensing is localized to areas close to the embedded photoluminescentfibre.