Identification of impact damage in sandwich structures by application of high speed MEMS-OSA to FBG sensors

In this research, the authors developed various detection techniques for particular damages, such as debonding and impact damage, in sandwich panels consisting of composite face-sheets and aluminum honeycomb core with small-diameter optical fiber sensors. First, two methods for debonding detection were established taking advantage of the behavior of fillets formed on the adhesive layer between the core and the skin. One method uses the fracture of optical fibers, and the other one uses the shape recovery of the reflection spectrum from a fiber Bragg grating (FBG) sensor because of the release of thermal residual stress in the fillets. Secondly, as for impact damages, chirped FBG sensors were applied to monitor the change in strain distribution of the face-sheet due to the dent caused by the impact loadings. Furthermore, a newly developed MEMS-optical spectrum analyzer (MEMS-OSA) was introduced to identification of impact points and damages. This system could measure the reflection spectrum at very high speed. The change in the form of the reflection spectrum during the impact loading was found to be different depending on the impact energy and the impact location, and this tendency was confirmed by theoretical simulations using the change in the strain distribution obtained by foil strain gauges. These results show that the high speed measurement of the reflection spectrum by MEMS-OSA has a potential to identify the impact location and damage magnitude through the comparison with theoretical simulations.