Abstract: The post-buckling induced failure process was simulated by using the cohesive element in the commercial software ABAQUS. The internal forces on the flange/skin interface were extracted from FEM results, and then their features were studied. The flange thickness was considered as a parameter to build a parameterized model for studying the effect of stiffness ratios of the flange to the skin on the interface debonding. The relationship among each component of the internal forces was also investigated. Failure criteria and failure envelops were developed for the flange/skin interface debonding. Finally the design parameters of the thickness ratio and the stiffness ratio between the flange and the skin were suggested for the design.