Detection of disbonds in secondary bonded structures using embedded Bragg grating optical fibre sensors

The use of Bragg grating optical fibre sensors for damage detection and health monitoring of highly loaded structural features in diverse stress fields is evaluated numerically. A composite patch bonded to a metallic 'crotch' specimen is considered. The specimen is representative of both high and low load transfer regions, with areas of high peel and shear. Sensors distributed along two optical fibres embedded in the composite patch, one near the surface and the other near the bond-line, are capable of detecting disbond damage between the substrate plate and the patch under tensile and residual (thermal) loading. Damage detection sensitivity is investigated in various regions along the bond-line. The axial strains in the optical fibre are used as the primary indicator of damage but shear strains can be equally effective, given the availability of suitable sensor technology. The presence of damage is indicated by discrepancies between the strain readings across the thickness of the patch (patch bending) or by discontinuities in strain distribution along the fibre. Also the response of the Bragg grating to the various strain gradients is also numerically modelled in order to understand the effects of the severe strain gradients on the reflected Bragg grating optical spectrum.