Cyclic Buckling Tests Under Combined Loading on Predamaged Composite Stiffened Boxes

The results obtained on two predamaged closed boxes composed of four graphite-epoxy curved stiffened panels are presented. One panel of each box is predamaged, using Teflon inserts between three stringers and the skin to reproduce skin-stringer disbonding. The boxes are tested under axial compression and torque, applied individually and in combination, both statically and cyclically. The tests allow the investigation of the effect of static and cyclic postbuckling combined loads in terms of structural behaviors, of collapse modalities and of damage propagation. The boxes do not suffer any loss of structural performances or any predamage propagation for combined loading under small compression and torsion up to 275% of the buckling torque, which corresponds to 85% of the collapse torque.