Research On Stiffener Runout In Stiffened Composite Panels

In an aircraft structure, it is often necessary to runout (terminate) some of the stiffeners to satisfydetailed design requirements. Depending on the structural design, stiffeners for a wing structure mayterminate at a chord-wise splice, at the forward or rear spar, at a rib, or at a structural discontinuitysuch as an access hole.But an abrupt stiffener termination can cause highly localized bendinggradients due to stiffness discontinuities and load-path centricities. In this paper, experiment testsrelating to three different stiffener runout specimens are presented and the failure modes of thesespecimens are discussed in detail. Then a robust and efficient method for quick evaluation of stringerrunout critical failure mode and failure load is presented. First the interface element based on cohesivezone model (CZM) is introduced, and detailed finite element models of the three specimens aredeveloped, and it is shouwn how such models can predict and explain the failure mode and behaviorof stiffener runout. In tension, the failure was deduced by peel stress and lognitudinal-normal shearstress. In compression, the failure was deduced by lognitudinal-normal shear stress only. Severaldesign solutions and the effects of terminating the stringer at different locations along the span havebeen researched in order to partially relieve the local stress concentration or challenge the failuremechanisms.