Post buckling behaviour of stiffened composite panels loaded in cyclic compression and shear

This paper presents the summary of a multiphase experimental - analytical study of the post-buckling mechanical behavior of a graphite/epoxy integrally stiffened panel. The first phase of the study included cyclic compression tests in the post-buckling regime of flat panels stiffened by either 'I' or 'J' shaped stiffeners. Static residual strength of the panels after 250,000 cycles was greater than the reference strength; however, some stiffness loss was observed. A series of tests of individual stiffeners, identical to those in the panels, was also carried out and the results showed the same trends as had been observed in the panels. There were no cases of early failure during the cyclic tests. The second phase included an experimental study of the post-buckling behavior of cylindrical panels integrally stiffened in the axial and transverse directions. Panels were tested in cyclic compression, cyclic torsion, and in combinations of both. The panels were stressed through 40,000 cycles, damage was inflicted, and an additional 40,000 cycles were imposed. No damage development was observed. The third phase of the study included an analytical effort for the development of a computer code, PBCOMP, for the buckling and post-buckling analysis of stiffened laminated flat and curved panels. The results of this study clearly show a great potential for the safe use of stiffened graphite/epoxy panels in aircraft structures.