In carbon fiber reinforced plastic (CFRP) skin-stringer structures, although adhesive bonding between skin and stringer parts is considered as an effective joint method, there is a possibility that debonding damages occur under cyclic fatigue loadings and these damages may decrease the bending stiffness of the structures. In this research, therefore, a structural health monitoring technique using a broadband ultrasonic wave propagation system to detect debonding in the adhesive layer of CFRP bonding structures was tested. By using this system, broadband modal dispersion characteristics of Lamb waves propagating in CFRP bonding structures were observed experimentally. The dispersion characteristics change depending on the thickness of the wave propagation path. Since the thickness of the intact bonding area and that of the debonded area were clearly different in the bonding structures, it was expected that the debonding damages will be detected from the changes of the dispersion characteristics. Therefore, we manufactured simple specimens to simulate the skin-stringer bonding structures by bonding a 180^L×25^W×3.4^T mm^3 strip laminate on a 180^L×60^W×3.4^T mm^3 skin panel. Four specimens with various artificial debonding lengths in the adhesive layer were prepared. Through the tests, the relationship between the changes of the modal dispersion characteristics and the debonding lengths were investigated.
