With the increased emphasis on reducing the cost of manufacturing composite structures, secondary bonding or co-curing is an attractive option to eliminate the need for mechanically fastened subassemblies. Many composite components in aerospace structures consist of flat or curved panels with co-cured frames and stiffeners. Out-of-plane loading such as internal pressure in a composite fuselage or out-of-plane deformations in compression-loaded post-buckled panel may lead to debonding of the frame or stiffener from the panel [1]. In polymer matrix composites, progressive type failures are often observed where catastrophic failure is generally preceded by constituent level damage accumulation. Some investigations efforts propose progressive failure algorithms aimed at capturing the progressive failure process from the initial to final failure [2]. The possibility of accurately predicting the damage and progressive failure in structures produced from composite materials is an important task in the design of lightweight structures. The interlaminar delamination fracture toughness of composite rib-stiffened-shell elements made of carbon/epoxy materials has been investigated. Double cantilever beam (DCB) tests and pull-off tests were carried out for this purpose. The knowledge of experimentally observed failure mechanisms is needed to determine the constituent ultimate strength from the micromechanics-based volume averaged stress values