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From:
Richard Degenhardt (1, 2), Alexander Kling (1), Rolf Zimmermann (1), Falk Odermann (1) and F.C. de Araújo (3)
(1) DLR, Institute of Composite Structures and Adaptive Systems, Braunschweig, Germany
(2) PFH, Private University of Applied Sciences Göttingen, Composite Engineering Campus Stade, Germany
(3) Dept Civil Eng, UFOP, Ouro Preto, MG, Brazil
“Dealing with Imperfection Sensitivity of Composite Structures Prone to Buckling”, DOI: 10.5772/45810, Chapter 1 in Advances in Computational Stability Analysis, edited by Safa Rozkurt Coskun, ISBN 978-953-51-0673-9, INTECH open science/open minds, August 1, 2012
INTRODUCTION: Currently, imperfection sensitive shell structures prone to buckling are designed according the NASA SP 8007 guideline using the conservative lower bound curve. This guideline dates from 1968, and the structural behaviour of composite material is not appropriately considered, in particular since the imperfection sensitivity and the buckling load of shells made of such materials depend on the lay-up design. This is not considered in the NASA SP 8007, which allows designing only so called "black metal" structures. There is a high need for a new precise and fast design approach for imperfection sensitive composite structures which allows significant reduction of structural weight and design cost. For that purpose a combined methodology from the Single Perturbation Load Approach (SPLA) and a specific stochastic approach is proposed which guarantees an effective and robust design. The SPLA is based on the observation, that a large enough disturbing load leads to the worst imperfection; it deals with the traditional (geometric and loading) imperfections [1]. The stochastic approach considers the non-traditional ones, e.g. variations of wall thickness and stiffness. Thus the combined approach copes with both types of imperfections. A recent investigation demonstrated, that applying this methodology to an axially loaded unstiffened cylinder is leading directly to the design buckling load 45% higher compared with the respective NASA SP 8007 design [2]. This chapter presents in its first part the state-of-the-art in buckling of imperfection sensitive composite shells. The second part describes current investigations as to the SPLA, the stochastic approach and their combination. In a third part an outlook is given on further studies on this topic, which will be performed within the framework of the running 3-year project DESICOS (New Robust DESIgn Guideline for Imperfection Sensitive COmposite Launcher Structures) funded by the European Commission; for most relevant architectures of cylindrical and conical launcher structures (monolithic, sandwich - without and with holes) the new methodology will be further developed, validated by tests and summarized in a handbook for the design of imperfection sensitive composite structures. The potential will be demonstrated within different industrially driven use cases.
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