Test results show that separation failure in co - cured skin / stiffener interfaces is characterized by dense networks of interacting cracks and crack path migrations that are not present in standard characterization tests for delamination . These crack networks result in measurable large - scale and sub - ply - scale R curve toughening mechanisms , such as fiber bridging , crack migration , and crack delving . Consequently , a number of unknown issues exist regarding the level of analysis detail that is required for sufficient predictive fidelity . The objective of the present paper is to examine some of the difficulties associated with modeling separation failure in stiffened composite structures . A procedure to characterize the interfacial material properties is proposed and the use of simplified models based on empirical interface properties is evaluated .