Jan Teßmer

Probabilistic Design of Axially Compressed Composite Cylinders with Geometric and Loading Imperfections

The discrepancy between the analytically determined buckling load of perfect cylindrical shells and experimental test results is traced back to imperfections. The most frequently used guideline for design of cylindrical shells, NASA SP-8007, proposes a deterministic calculation of a knockdown factor with respect to the ratio of radius and wall thickness, which turned out to be very conservative in numerous cases and which is not intended for composite shells. In order to determine a lower bound for the buckling load of an arbitrary type of shell, probabilistic design methods have been developed. Measured imperfection patterns are described using double Fourier series, whereas the Fourier coefficients characterize the scattering of geometry. In this paper, probabilistic analyses of buckling loads are performed regarding Fourier coefficients as random variables. A nonlinear finite element model is used to determine buckling loads, and Monte Carlo simulations are executed. The probabilistic approach is used for a set of six similarly manufactured composite shells. The results indicate that not only geometric but also nontraditional imperfections like loading imperfections have to be considered for obtaining a reliable lower limit of the buckling load. Finally, further Monte Carlo simulations are executed including traditional as well as loading imperfections, and lower bounds of buckling loads are obtained, which are less conservative than NASA SP-8007.

MODELS AND TOOLS FOR HEAT TRANSFER, THERMAL STRESSES, AND STABILITY OF COMPOSITE AEROSPACE STRUCTURES

An overview of DLRs current research activities and results in the fields of heat transfer, thermal stresses, and thermally induced buckling in composite aerospace structures is given. Novel theories and finite element formulations are presented, realistic modeling of boundary conditions and junction areas is highlighted, experimental validation is discussed, and design guidelines are given.

Strength of Textile Composites — A Voxel Based Continuum Damage Mechanics Approach

Industrialised infusion processes enable a cost-effective possibility to produce textile composite structures compared to pre-impregnated composite systems (Prepregs). Particularly with regards to high performance structures one has to be familiar with the material behaviour and the failure characteristic to apply fibre reinforced composites profitable. In this publication a brief overview of the current research activities to characterise the mechanical behaviour, failure and strength of textile composites compared with prepreg systems is presented.