Achievements in the design of thin-walled composite structures

Kurzfassung

Aerospace and space industry strives for significantly reduced development and operating costs. Reduction of structural weight at safe design is one possibility to reach this objective. Another one is the use of reliable simulation methods in order to minimize expensive and time consuming experimental design studies. The presentation shows examples on new achievements in the design of thin-walled composite structures of the buckling and the postbuckling behaviour of thin-walled structures up to collapse, respectively, which allow the exploitation of considerable reserves in primary fibre composite structures in aerospace applications. Investigation of the postbuckling behaviour of stiffened composite aerospace structures This part demonstrates experimental and numerical studies of axially loaded stringer-stiffened thin-walled composite structures in the postbuckling area. The structures considered are undamaged and pre-damaged. They are loaded in a first step by several cyclic loading just before collapse to investigate the damage growth of such structures. In the last step the structures are loaded in one step until collapse. During the experiments advanced measurement systems [1] with high-speed cameras (ARAMIS), thermography and lamp-waves were applied to take as much information as possible from the experiments. The presentation gives examples from various test results and comparison numerical simulations [2,3,4]. Fast simulation tool for the simulation of unstiffened cylindrical and conical composite structures A new semi-analytical method capable to predict the static and the instability response of the non-linear buckling of unstiffened laminated composite cones and cylinders under various loads and boundary conditions is presented. The tool considers geometric and load imperfections. The Ritz method is selected to solve the non-linear set of equations and a new set of appropriate approximation functions for the displacement field is proposed, in order to simulate axial compression, torsion, pressure, load asymmetry, any arbitrary surface or concentrated loads, and any load case combining these loads. Elastic constraints are used to produce a wide range of boundary conditions, covering the four types of boundary conditions commonly used in the literature. For conical shells a novel approximation is proposed in order to efficiently perform the analytical integration of the linear stiffness matrices. A detailed convergence analysis is presented and the proposed models are verified with finite element results, models available from the literature and test results [5-7]. Numerical assessment of existing vibration correlation techniques The buckling of real space structures is challenging to validate as the first failure in the experiment might be material and not buckling as predicted. One reason is the high imperfection sensitivity of such thin-walled structures. There is therefore a high need for non-destructive methods to calculate the buckling. The Vibration Correlation Technique (VCT) allows determining the buckling load without reaching the instability point. Here, the structures will be loaded at different load steps. In each step the structures is broad into vibrations and the eigenfrequencies are measured. The relation between the load and the first (or higher) eigenfrequency is nonlinear. There exist several methods to predict the buckling of different type of structure (beam, plate, shell) or material based on this nonlinear dependence. In 2014, Arbelo developed a promising method for cylindrical shells [8]. This presentation explores further the novel approach and compares the results with other available VCT methods through numerical Finite Element (FE) simulations. [1]. Degenhardt R., Kling A., Klein H., Hillger W., Goetting Ch., Zimmermann R., Rohwer K., Gleiter A., “Experiments on Buckling and Postbuckling of Thin-Walled CFRP Structures using Advanced Measurement Systems”, International Journal of Structural Stability and Dynamics, Vol. 7, No. 2 (2007), pp. 337-358 [2]. Degenhardt R., Kling A., Rohwer K., Orifici A. C., Thomson R. S., “Design and Analysis of Stiffened Composite Panels Including Postbuckling and Collapse”, Computers and Structures Vol. 86 (2008), pp. 919-929 [3]. Degenhardt R., Wilckens D., Klein H., Kling A., Hillger W., Goetting Ch., Rohwer K., Gleiter A., “Experiments to detect the damage progress of axially compressed CFRP panels under cyclic loading”, Special volume of Key Engineering Materials Vol. 383 (2008) pp 1-24 [4]. Wilckens D., Degenhardt R., Rohwer K., R., Zimmermann R., Kepke M., Hildebrandt B., Zipfel A., “Cyclic buckling tests of pre-damaged CFRP stringer stiffened panels”, International Journal of Structural Stability and Dynamics, Vol. 10, No. 4 (2010), pp. 827-852 [5]. 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 [6]. Castro S., Mittelstedt C., Monteiro F., A. Arbelo M., Degenhardt R., “A semi-analytical approach for the linear and non-linear buckling analysis of imperfect unstiffened laminated composite cylinders and cones under axial, torsion and pressure loads “, Int. Journal of Thin-Walled Structures, Vol. 90 (2015) pp. 61–73 [7]. Castro S., Mittelstedt C., Monteiro F., Ziegmann G., Degenhardt R., Evaluation of non-linear buckling loads of geometrically imperfect composite cylinders and cones with the Ritz method, Int. Journal of Composite Structures, 10.1016/j.compstruct.2014.11.050 [8]. M. A. Arbelo, S. F. M. de Almeida, M. V. Donadon, S. R. Rett, R. Degenhardt, S. G. P. Castro, K. Kalnins und O. Ozoliņš, „Vibration correlation technique for the estimation of real boundary conditions and buckling load of unstiffened plates and cylindrical shells,“ Thin-Walled Structures, Bd. 79, pp. 119-128, 2014.