Dynamic near three-to-one internal resonance in composite stiffened panels

► We study the parametric and 3:1 internal resonance of a composite stiffened panel. ► We deal with modal interaction of two buckling mode shapes near the principal resonance. ► We consider 3:1 internal resonance between the first mode (1,3) and the second mode (3,1). ► The two mode solutions coexist and the first mode is dominant. ► The second mode can be hard or soft while the first mode is always soft.