Triboluminescent materials offer a viable route to real time structural damage sensing. The sensors can be externally attached to the surface of metals or composites, or embedded within composites. In situ monitoring of structural damage in composites is particularly relevant since severe internal damage can exist with little indication of this damage on the composite surface. The main issue related to embedding triboluminescent sensors within composites is how to access efficiently the optical signal generated upon structural damage and how to guide efficiently this optical damage signal to a remote detector. Earlier work relied on side-coupling of the triboluminescent light into a curved conventional silica fibre and/or end-coupling into the silica fibre, if the damage to the host structure had also broken the fibre. Both these light collection methods are, of course, inefficient and resulted in an optical damage signal with a very poor signal-to-noise (S/N) ratio. By using novel photoluminescent polymeric and silica fibres, we have shown that it is possible to efficiently capture and guide the optical damage signal from an embedded triboluminescent sensor to a remote detector. These fibres resulted in a signal with a S/N ratio that was orders of magnitude larger than that achieved using the conventional silica fibre. Furthermore, we have shown that the light collection/guiding technique can even be used effectively with triboluminescent sensors embedded within black carbon fibre reinforced plastic (CFRP) composites. Finally, we have demonstrated that for resins and glass fibre reinforced plastic (GFRP) composites, triboluminescent sensors act as truly global damage sensors, whereas for CFRP composites, the damage sensing is localized to areas close to the embedded photoluminescent fibre.
Sage, I., Humberstone, L., Oswald, I., Lloyd, P., & Bourhill, G. (2001). Getting light through black composites: embedded triboluminescent structural damage sensors. Smart Materials and Structures, 10(2), 332-337. https://doi.org/10.1088/0964-1726/10/2/320