Robust knockdown factors for the design of axially loaded cylindrical and conical composite shells – Development and Validation

The stability failure of the axially loaded cylindrical shell is considered as the last unresolved classical stability problem, although it has been investigated for over 100 years. Therefore designers rely on the application of empirical knockdown factors from the 1960s like the NASA SP-8007 for cylindrical shells and the NASA SP-8019 for truncated conical shells which are very conservative for modern shell structures. Perturbation approaches for the design of axially loaded cylindrical and conical shells are presented in this paper. These approaches deliver knockdown factors for a physical based estimation of the lower-bound buckling load and are suitable for research and industrial applications as they are independent from imperfection measurements and easy to implement. The corresponding numerical models are validated by means of high-fidelity buckling experiments and it shows that experimental buckling loads can be calculated very precisely in contrast to the previous methodology. Additionally, new robust knockdown factors are proposed for preliminary shell design which are based on curve fitting of numerical knockdown factors of the perturbation approaches. Thus, it is possible to utilize the load bearing capability of launch-vehicle primary structures up to 40% more effectively, resulting in considerable weight saving potentials for composite shell structures