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

Significance Statement

Thin-walled shells like cylinders or truncated cones are important primary structures for launch-vehicle systems the Ariane 5 or SpaceX’s Falcon-9. These shells are subjected to axial compression due to weight of the upper structural elements and propulsive loads during launch.

Thin-walled shells are considered as imperfection sensitive which means that their maximum load carrying capacity under axial compression, the buckling load, is reduced significantly by assembly or manufacturing based deviations of the nominal shell geometry.

Due to significant differences between linear buckling theory and corresponding experimental data, the design of thin-walled shell relies on the application of empirical knockdown factors such as the NASA SP-8007 for cylindrical and the NASA SP-8019 for conical shells. These knockdown factors are based on experimental data from the beginning of the 20th century and have been shown to be very conservative for modern shell structures.

Ronald Wagner and colleagues at the German aerospace center (DLR) developed new innovative numerical design approaches for axially loaded cylindrical and conical shells, the single dimple perturbation approaches. These methods offer a physical based estimation of the lower-bound buckling load and are independent from costly imperfections measurements as well as easy to implement. Their work is now published in Composite Structures.

The research team validated that experimental buckling loads can be predicted very precisely in contrast to other established methods. In addition new physical based knockdown factors for the preliminary design of launch-vehicle primary structures were developed and validated. The new knockdown factors for short and thin shell are significantly higher than those of the NASA SP-8007 or NASA SP-8019. In consequence, it is possible to utilize the load bearing capacity of launch-vehicle primary structures more effectively, resulting on considerable weight saving potentials.

axially loaded cylindrical and conical composite shells (Advances in Engineering)

axially loaded cylindrical and conical composite shells (Advances in Engineering)

About the author

Prof. Dr.-Ing. Christian Hühne has 18 years of experience in developing methods to determine the load carrying capability of aeronautical structures and in realization of several projects dealing with aircraft fuselage structures and rocket structures made from composite material.
In 2008 he became Head of Department Composite Design at the DLR Institute of Composite Structures and Adaptive Systems, Brunswick. In 2014 he became Professor of Function Integrated Structures at the Technical University of Brunswick, Germany.

About the author

Steffen Niemann has 10 years of experience in design, manufacturing and testing of composite aircraft structures at the DLR Institute of Composite Design and Adaptive Systems in Braunschweig. He is participating in national and European projects and is working on his PhD thesis.

About the author

Ronald Wagner has 5 years of experience in design of composite shells and probabilistic methods at the DLR Institute of Composite Design and Adaptive Systems in Braunschweig. He is participating in national and European projects and is finishing his PhD thesis.

Reference

H.N.R. Wagner, C. Hühne, S. Niemann. Robust knockdown factors for the design of axially loaded cylindrical and conical composite shells –  Development and Validation. Composite Structures volume 173 (2017) pages 281–303

Institute for Composite Structures and Adaptive Systems, German Aerospace Center (DLR), Lilienthalplatz 7, 38108 Braunschweig, Germany.

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