Debonding monitoring of composite repair patches using embedded small-diameter FBG sensors

Small-diameter fiber Bragg grating (FBG) sensors were applied to the debonding monitoring of composite repair patches. The target of the monitoring consists of carbon fiber reinforced plastic (CFRP) patches and an aluminum substrate. The specimens were prepared through simple means: the tabular patches adhered to both sides of the aluminum substrate using epoxy adhesive films. Under cyclic loading conditions, the debonding progressed between the aluminum substrate and the adhesive layers. Before the embedding of the FBG sensors, the embedding location was determined using the calculation of the sensor signals, also called the reflection spectra. When the cyclic loading was stopped at a pre-determined numbers of cycles, the reflection spectra were measured at stress conditions of both 0 and 200 MPa. The newly appearing peak in the reflection spectrum indicated the arrival of debonding at the FBG sensors. As the debonding length increased, the intensity of the peak was found to increase. The relationship between the spectral change and the debonding length was verified by the simulated spectra and the strain distributions calculated by finite element (FE) analysis. As a result, the debonding length could be evaluated quantitatively by the spectral changes in the FBG sensors.