Cyclic Buckling Tests Under Combined Compression and Shear on Composite Stiffened Panels

The results obtained on two closed structural boxes composed by four graphite-epoxy curved stringer-stiffened panels, and manufactured by Agusta/Westland, are presented. The closed-box configuration allowed easily applying shear on each panel by subjecting the box to a torsion moment. The boxes were tested under combined axial compression and torque loading, both statically and cyclically. The tests allowed, on one hand, evaluating the effect of different procedures of loading application and, on the other hand, to investigate the influence of cyclic postbuckling combined loads in terms of global and local behaviors, as well as in terms of collapse modalities. The results show the reliability of these structures to safely operate in the postbuckling field, even when the buckling load is reached thousands of times during the operative life.