Challenges and opportunities for the stability design of future composite space and airframe structures

Kurzfassung

Space and aircraft industry demands for reduced development and operating costs. Structural weight reduction by exploitation of structural reserves in composite space and aerospace structures contributes to this aim, however, it requires accurate and experimentally validated stability analysis. This paper deals with challenges and advances related to stability analysis of composite structures. Thin-walled light weight structures endangered by buckling can be for instance divided into two groups: imperfection tolerant and imperfection sensitive structures. For both groups design guidelines for composites structures are still under development. This paper gives a short state-of-the-art and presents proposals for future design strategies. Imperfection tolerant structures are for instance stiffened panels as used in aeronautic applications. The maximum load is quite insensitive with respect to imperfections. Such structures are characterised by a relatively large postbuckling area which is usually used for designs in metallics but not composites. There are some finished and running European projects (e.g. COCOMAT, [1,2]) to this topic which are highlighted shortly. Imperfection sensitive structures are for instance unstiffened structures or stiffened structures with a rather dominant skin compared to the stiffeners. For such structures the maximum load is equal or close to the first buckling load and is rather imperfection sensitive. These types of structures are for instance commonly used in space applications. The currently applied design guidelines (e.g. NASA SP 8007) with respect to structural stability were developed only for metallic structures and are from 1968. For composite structures no guidelines exist. The “Single Perturbation Load Approach” (cf. [3]) is one promising concept for future applications but on the other hand it opens a lot of new questions. These are investigated within the running EU project DESICOS (New Robust DESIgn Guideline for Imperfection Sensitive COmposite Launcher Structures, [4]), which started in January 2012 and will be finished in March 2015. For most relevant architectures of cylindrical and conical launcher structures (monolithic, sandwich- without and with holes) DESICOS investigates a combined methodology from the Single Perturbation Load Approach and a specific stochastic approach which shall allow an effective and robust design. Recent investigations demonstrated, that an axially loaded unstiffened cylinder, which is disturbed by a large enough single perturbation load, is leading directly to the design buckling load 45% higher compared with the respective NASA SP 8007 design [5]. Within DESICOS the new methods are 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. This paper presents also the objectives, the main results and gives and overview of first results of the DESICOS project achieved so far.