Effect of Cyclic Buckling Under Combined Loading on Pre-Damaged Composite Stiffened Box

The results obtained on two pre-damaged closed boxes composed by four graphite-epoxy curved stiffened panels, and manufactured by Agusta/Westland, are presented. The closed box configuration allows to apply shear load on each panel by subjecting the box to torque. The pre-damages are obtained using Teflon inserts between three stringers and the skin, so to reproduce skin-stringer disbonding. The boxes are tested under axial compression and torque, applied individually and in combination, both statically and cyclically. The static tests allow to compare different procedures of loading application and to investigate the effect of post-buckling combined loads on the damage propagation. The cyclic tests allow to investigate the influence of cyclic post-buckling combined loads in terms of global behaviors, of collapse modalities and of damage propagation. The results show the reliability of these structures, even if pre-damaged, to safely operate in the post-buckling field, also when the buckling load is reached thousands of times during the operative life.