Real-time Detection of Debonding between Honeycomb Core and Facesheet using a Small-diameter FBG Sensor Embedded in Adhesive Layer

The authors have developed a novel technique to detect debonding in honeycomb sandwich structures using small-diameter fiber Bragg grating (FBG) sensors. A small-diameter FBG sensor is embedded in the adhesive layer between the core and the facesheet during the curing process of the adhesive layer. After the curing process, the reflection spectrum from the small-diameter FBG sensor is distorted because the formation of fillets induces a non-uniform strain distribution in the adhesive layer. However, after debonding, the core with the fillets peels off the facesheet, and, as a result, the reflection spectrum recovers its original shape due to the release of the non-uniform strain. Debonding can thus be detected with high sensitivity in real time from the recovery in the shape of the reflection spectrum. In addition, a more reliable debonding detection technique using a small-diameter chirped FBG sensor is proposed, in which two indicators in the reflection spectrum have been successfully used for a more reliable judgment of debonding.