Anisotropy-induced spiral buckling in compression-loaded cylindrical shells

The study of axial compression buckling of isotropic cylinders has received much attention by various researchers over the years. It is commonly acknowledged that the presence of minute imperfections reduces potential buckling loads significantly in comparison with classical linear predictions. This approach has been extended by a significant, yet fewer, number of researchers to composite cylindrical shells. It is shown that imperfections may 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 significant role in reducing buckling loads from those classically predicted. Indeed, the assumption of deflections 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 flexural/twist coupling is developed. Backed up by detailed comparison with finite 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.