Plateau lower-bounds to the imperfection sensitive buckling of composite shells

The buckling of thin FRP laminated shells are sensitive to initial geometric imperfections. A large number of geometric and material variables prohibit the traditional experiment-based lower-bound design method for metallic shells from being extended to composite ones. As an alternative, the so-called reduced stiffness method (RSM) has been applied to the lower bound buckling of FRP laminated shells. It has been shown for shorter shells the method predicted excellent lower bounds to the nonlinear buckling loads. This paper aims to extend the study to longer shells. It is shown for longer composite shells the lower-bound buckling modes generally occur in the long axial wave mode having one axial halfwave but require imperfection amplitudes of impractical largeness. Further studies of the geometric parameters identify the existence of intermediate and significant plateaus to the lower bounds. These plateau lower-bounds demonstrate the importance of the short axial wave modes having more than one axial halfwave, associated with the imperfection amplitudes of practical smallness. Using the plateau values and modes to predict lower bounds is suggested to provide an important alternative for improving shell buckling design.