Influence of Imperfections on Axial Buckling Load of Composite Cylindrical Shells

There is a strong requirement for more robust, lighter and cheaper launch vehicle structures. Unstiffened composite cylindrical shells, which are essential to the fabrication of launch vehicle airframes, are prone to buckling and are highly sensitive to imperfections which arise during the manufacturing process. The buckling load is an important characteristic in design, and may vary drastically from the buckling load of the perfect structure when realistic imperfections are present. [1] The current design guidelines for imperfection sensitive shells are based on the NASA-SP 8007 [2] which dates from 1965. Typically, the theoretical buckling load of a given cylinder design is predicted by performing a linear bifurcation analysis using closed-form equations of the geometrically perfect structure. This theoretical buckling load is then reduced by applying an empirical knockdown factor to account for the differences between theory and test. From recent literature [3-5], the NASA-SP 8007 knockdown factors used in the design of aerospacequality shell structures were found to be overly conservative and inappropriate for shells constructed from modern manufacturing processes and materials such as composites. Such a conservative approach means that structures are therefore heavier and more costly than need be. Dependable and verified design criteria for thinwalled cylindrical shells are required, particularly for shells constructed from advanced materials and manufacturing techniques. A new design criterion using a new robust optimization methodology in conjunction with the Single Perturbation Load Approach (SPLA) and the stochastic approach is being developed by the collaborative European 7th Framework Program project: New Robust Design Guideline for Imperfection Sensitive Composite Launcher Structures (DESICOS) [6]. The design criterion will account for real behavior of composite materials including imperfections to provide less conservative and more accurate knockdown factors. This paper provides a measure of the Sensitivity and Influence of initial imperfections on the axial buckling load of 50 nominally identical composite cylinders which were numerically generated from actual shell measurements [5], as a contribution to the overall DESICOS project. The effect of initial geometric, loading, thickness and material imperfections on the axial buckling load of unstiffened composite cylindrical shells was investigated and compared using probabilistic finite element (FE) analysis in conjunction with the stochastic approach and metamodels. The magnitudes of the imperfections were plotted against the axial buckling load and the effect of each imperfection type on the overall axial buckling load determined. The results displayed may be used for the robust optimization of shells which may improve the reliability of their knock-down factors.