Changes in the reflected spectra of embedded chirped fibre Bragg gratings used to monitor disbonding in bonded composite joints

Recent work has outlined a method for health monitoring the structural integrity of adhesively bonded composite joints based on the use of a chirped fibre Bragg grating (CFBG) embedded within an adherend, but not within the glue-line. In this paper, the reflected CFBG spectra for disbonds of different lengths have been predicted with the aid of a finite-element model of the strain field resulting from a disbond and commercially available software for predicting the reflected spectra of fibre Bragg grating sensors. The modelling shows the relationship between the position of the disbond front and the dip in the reflected spectrum which moves to higher wavelengths as the disbond extends. These results enable visual observations of the disbond front in the optically transparent joint to be compared with the CFBG results, showing that the sensor can determine the disbond front position to within a few millimetres. For other bonded structures, this resolution is likely to be determined largely by the adherend materials and sensor position.