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Computational Science, Engineering & Technology Series
ISSN 17593158 CSETS: 15
INNOVATION IN ENGINEERING COMPUTATIONAL TECHNOLOGY Edited by: B.H.V. Topping, G. Montero, R. Montenegro
Chapter 20
Robust Design of Composite Shells: Simulation and Validation C. Hühne*, R. Rolfes* and J. Teßmer+
*Institute of Structural Analysis, University of Hannover, Germany C. Hühne, R. Rolfes, J. Teßmer, "Robust Design of Composite Shells: Simulation and Validation", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Innovation in Engineering Computational Technology", SaxeCoburg Publications, Stirlingshire, UK, Chapter 20, pp 425444, 2006. doi:10.4203/csets.15.20
Keywords: composite shells, stability, buckling, robust design.
Summary
Thinwalled shell structures like circular cylindrical shells are prone to buckling.
Imperfections which are defined as deviations from perfect shape and perfect
loading distributions can reduce the buckling load drastically compared to that of the
perfect shell. Design criteria monographs like NASASP 8007 recommend that the
buckling load of the perfect shell shall be reduced by using a knockdown factor. The
existing knockdown factors are very conservative and do not account for the
structural behaviour of composite materials. To determine an improved knockdown
factor several authors consider the realistic shape of the shell in a numerical
simulation using probabilistic methods. For this probabilistic approach a large
number of test data is needed which is often not available. Motivated by this lack of
data a new deterministic approach is presented for determining the lower bound of
the buckling load of thinwalled cylindrical composite shells. It is derived from
phenomenological test data.
Previous investigations have shown that a single buckle is a realistic, worst case and stimulating geometric imperfection of thinwalled cylindrical shells [1,2,3]. The single prebuckle is induced by a perturbation rig 1. The influence of a radial perturbation load on the structural behaviour is investigated by applying loads of different magnitudes and at different positions along the circumference. The results of one perturbation position are shown in Figure 2. Each dot marks one test and shows the buckling load as function of the perturbation load . The run of the curve is shown by three lines (Figure 2). For perturbation loads larger than the reduction of the buckling load is quite small. This means that the perturbation load has to be increased very strongly in order to reduce the buckling load any further. For perturbation loads single buckles are clearly visible, can be detected by inspection and are therefore assumed to be unrealistic. The buckling load at the intersection point of lines (a) and (b) is defined to be the new lower limit for the buckling load of realistic imperfect composite shells. Based on the test results the numerical analysis is validated. The results of the tests and the numerical analysis are compared to the values of the NASASP 8007. The new approach leads to a much less conservative, but also safe, design. The possibilities for practical use are shown using a design example. References
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