Abstract The strains at which buckling and debond growth occur in adhesively bonded composite flanges containing an initial debond were experimentally measured. Test parameters including initial debond geometry, flange material stiffness, and the adhesive critical strain energy release rate ( G c) were investigated. Debond growth was found to be strongly dependent on initial debond length but weakly dependent on flange width; i.e., debonding resistance did not increase in direct proportion with the bonded overlap dimension. Flanges having higher bending stiffness exhibited significantly lower debonding strain. Finally, the effect of G c was evaluated at three levels by controlling the adhesive cure temperature and bondline thickness. Lower values of G c (207 and 552 J/m 2) allowed debond growth to occur while at the highest value of G c (1500 J/m 2), alternate failure modes occurred prior to debond growth. Ultrasonic C-scans revealed that debond growth occurred along a curved front, as dictated by the post-buckling deformation of the flanges.