Buckling Behavior of Composite Laminated Stiffened Panels Under Combined Shear-Axial Compression

Experimental results are reported on the behavior of four torsion boxes, each comprising two stringer-stiffened cylindrical graphite-epoxy composite panels that were subjected to torsion, axial loading, and their combinations. The buckling and postbuckling behavior of these torsion boxes demonstrated consistent results. Before performing the buckling tests, the initial geometric imperfections of the panels forming the boxes were scanned and recorded. The tests were complemented by finite element calculations that were performed for each box. These detailed calculations also assisted in identifying critical regions of the boxes, and the boxes were reinforced accordingly, to avoid their premature failure. The tests indicated that the torsion-carrying capacity was dependent on stringer geometry and layup; axial compression results were in very good agreement with previous tests carried out with single identical panels; and the boxes have a very high postbuckling-carrying capacity. Comparisons of the experimentally experienced first (skin) buckling loads and collapse torques with finite element analyses predictions were found to be in good agreement for torsion and for axial compression. The experimental results under combined loading were consistently lower than with the numerical results. This might have stemmed from repeated buckling tests used in the present test program to generate the experimental interaction curve, which could have introduced residual strains/deformations in the skin that influenced the skin buckling load capacity, thus yielding a lower first skin buckling load in the subsequent loading stages.