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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.

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The authors are constructing a damage detection system using ultrasonic waves. In this system, a piezo-ceramic actuator generates ultrasonic waves in a carbon fiber reinforced plastic (CFRP) laminate. After the waves propagate in the laminate, transmitted waves are received by a fiber Bragg grating (FBG) sensor using a newly developed high-speed optical wavelength interrogation system. In this research, this system was applied to the evaluation of debonding progress in CFRP bonded structures. At first, small-diameter FBG sensors, whose cladding diameter is about 1/3 of common optical fibers, were embedded in an adhesive layer of a double-lap type coupon specimen consisting of CFRP quasi-isotropic laminates, and the ultrasonic wave was propagated through the debonded region. After that, the wavelet transform was applied to the received waveforms and the results showed clear difference depending on the debonding length. Hence, a new damage index was proposed, which could be obtained from the difference in the distribution of the wavelet transform coefficient. As a result, the damage index increased with an increase in the debonded area. Furthermore this system was applied to the skin/stringer structural element of airplanes made of CFRP laminates. Both of the waves received by a bonded FBG and by an embedded FBG changed sensitively to the debonding progress. Also, the damage index could evaluate the length of the debonding between the skin and the stringer.