Evaluation of debonding progress in composite bonded structures using ultrasonic waves received in fiber Bragg grating sensors

We have developed a damage detection system that generates ultrasonic waves with a piezo-ceramic actuator and receives them in a fiber Bragg grating ( FBG) sensor. In this research, this system was applied to evaluate the debonding progress in carbon fiber reinforced plastic ( CFRP) bonded structures. First, small-diameter FBG sensors were embedded in adhesive layers of a double-lap-type coupon specimen consisting of CFRP quasi-isotropic laminates bonded with epoxy adhesive films. Then, an ultrasonic wave at 300 kHz was propagated through the debonded region, and the wavelet transform was applied to the received waveform. The obtained results showed clear differences depending on the debonding length. Hence, a new damage index was proposed using the difference in the distribution of the wavelet transform coefficient. The damage index increased with an increase in the debonded area. Furthermore, this system was applied to a skin/stringer structural element of airplanes made of CFRP laminates. In this case, a correlation coefficient was also calculated from the results of the wavelet transform. As a result, the damage index increased and the correlation coefficient decreased with an increase in the debonded area. Hence the length of the debonding between the skin and the stringer could be easily evaluated.