Progressive Delamination Analysis of Stiffened Composite Panels in Post-Buckling

Simple specimens in unidirectional graphite-epoxy material are investigated. They consist of a skin panel with one stiffener, where the stiffener is L-shaped and is bonded to the skin. Two undamaged specimens and three specimens with an initial separation between the stringer flange and the skin in the middle of the stringer are tested under axial compression. The compression load leads the panels to buckle, and the resulting out-of-plane deformation causes progressive debonding between the skin and the stringer. The skin-stringer separation is measured directly during the tests by means of an ultrasonic system obtaining C-scan und B-scan representations at different load steps. The tests are numerically analyzed using the finite element code ABAQUS, modeling the entire stiffened panel by four-nodes shell elements, and investigating the progressive delamination by means of the Virtual Crack Closure Technique. The results of the numerical analyses are compared to the experimental ones in terms of load-shortening curves, post-buckling shapes and progressive debonding between the skin and the stringer.