Debonding monitoring of a composite repair patch using small-diameter FBG sensors

This paper presents the health monitoring technique of composite repair patches using small-diameter fiber Bragg grating (FBG) sensors. The composite patch structures consist of carbon fiber reinforced plastic (CFRP) patches, aluminum substrates and adhesive layers. The debonding progressed between aluminum substrate and the adhesive layers under cyclic loading. In this research, two types of the specimen were prepared for evaluation of the debonding size. One is specimen with the patches glued on the both side of the aluminum plate. The other is pre-notched specimen for investigation of the influence of the crack propagation in the substrate. The FBG sensors were embedded into the adhesive layers easily, because the diameter of the sensor was only 52 mum. Reflection spectra from the sensor were measured at various lengths of the debonding. The form of the spectrum changed sensitively with an increase in the debonding size. For confirmation of the measured results, the spectra were simulated theoretically. As a result, the change in the calculated spectrum was consistent with that in the measured spectrum, and the relation between the spectrum and the debonding was clarified. Moreover, the debonding length was evaluated quantitatively by the monitoring of the form of the reflection spectrum.