New Achievements in Stability of Composite Aerospace Structures

European aircraft and space industry demands for reduced development and operating costs. Structural weight reduction by exploitation of structural reserves in composite aerospace structures contributes to this aim, however, it requires accurate and experimentally validated stability analysis of real structures under realistic loading conditions. This paper presents new achievements from the area of computational and experimental stability research of composite aerospace structures which contribute to that field. The topics presented are highlights from the finished EU project DESICOS (New Robust DESIgn Guideline for Imperfection Sensitive COmposite Launcher Structures, cf. [1], [2]), which was running from February 2012 until July 2015. The main objective was the development of a future design scenario for imperfection sensitive CFRP structures. To reach the objectives, improved design methods, experimental data bases as well as design guidelines for imperfection sensitive structures are needed. The experimental data bases are indispensable for validation of the analytically developed methods. Reliable fast methods allow for an economic design process. Industry brings in experience with the design and manufacture of real shells; research contributes knowledge on testing and on development of design methods. Design guidelines are defined in common, and the developed methods are validated by industry. The results of DESICOS comprise: - Material properties - Method for the design of buckling critical fibre composite launcher structures, based on the combined SPLA and stochastic procedures, validated by experiments - Experimental results of buckling tests including measured imperfections, buckling and postbuckling deformations, load shortening curves, buckling loads - Guidelines how to design composite cylindrical shells to resist buckling - Reliable procedure how to apply the Vibration Correlation Technique (VCT) in order to predict buckling loads non-destructively by experiments - Handbook including all the results - Demonstration of the potential with different industrially driven use cases. More details including all publications from the project can be found at www.desicos.eu. The presentation will give at the beginning in Section 1 an overview of the project. The following three sections will highlight selected topics. Section 2 presents an overview of 15 buckling experiments on different type of structures (monolithic cylinders and cones, sandwich) and loadings (axial compression, perturbation loads) performed by the partners DLR, TECHNION, Politecnico di Milano and Riga Technical University. Section 3 demonstrates a new Ritz approach for the fast calculation of cylindrical and conical shells under axial compression and perturbation loads (cf. [3]). Section 4 presents a new empirical approach of the Vibration Correlation Technique (VCT) in order to predict buckling loads non-destructively by means of experiments (cf. [4]). References 1. www.desicos.eu 2. Degenhardt R., Kling A., Bethge A., Orf J., Kärger L., Rohwer K., Zimmermann R., Calvi A., “Investigations on Imperfection Sensitivity and Deduction of Improved Knock-Down Factors for Unstiffened CFRP Cylindrical Shells”, Composite Structures Vol. 92 (8), (2010), pp. 1939–1946 3. Castro S., Mittelstedt C., Monteiro F., A. Arbelo M., Ziegmann G., Degenhardt R., Linear buckling predictions of unstiffened laminated composite cylinders and cones under various loading and boundary conditions using semi-analytical models, Int. Journal of Composite Structures, Vol. 118 (2014), pp. 303-315 4. Arbelo M., Almeida S., Danadon M., Rett S., Degenhardt R., Castro S., Kalnins K., Ozolins O., “Vibration correlation technique for the estimation of real boundary conditions and buckling load of unstiffened plates and cylindrical shells“, Int. Journal of Thin-Walled Structures, Vol. 79, (2014), pp.119-12