Delamination　initiation　and　propagation　behavior　of　stiffened　composite　plates　was　analyzed　using　a　novel　propagation　criterion　based　on　mode-mixity.　Moreover,　the　effect　of　ply　sequence　on　the　mechanical　performance　of　a　skin-stringer　composite　laminates　was　investigated.　Mode-mixity　was　obtained　using　strain　energy　release　rate　components,　which　was　used　then　to　get　delamination　propagation　length　and　fracture　toughness　for　a　given　delamination　length.　The　method　was　certified　by　comparing　the　interfacial　mechanical　behavior　of　two　standard　ply　sequences　with　those　used　for　V-22　airplane　structures.