Anisotropy induced spiral buckling in compression-loaded composite shells

The study of axialcompression bucklingof isotropic cylinders has receivedmuch attention by various researchers over the years. It is commonly acknowledged that the presence of minute imperfections reduces potential buckling loads signiŽ cantly in comparisonwith classical linear predictions. This approachhas been extended by a signiŽ cant, yet fewer, number of researchers to composite cylindrical shells. It is shown that imperfectionsmay not be the only major factor for the discrepancy between experimentally obtained buckling loads and those predicted from linear bifurcation theory with orthotropic properties. Flexural/twist anisotropy, present in most balanced, symmetric laminates with angle ply layers is shown to play a signiŽ cant role in reducing buckling loads from those classically predicted. Indeed, the assumption of de� ections in the form of a double sine series appears to be questionable for such laminates. A previously unreported classical linear analysis including the effect of � exural/twist coupling is developed. Backed up by detailed comparison with Ž nite element studies, it is shown that buckling loads can be reduced by up to 30%for a class of quasi-isotropic laminates and is accompanied by a change in mode form from doubly periodic to spiral in nature.