Bøger af Sascha Häusler

The continuously expanding commercial air traffic of the last decades steadily increasedthe demand for highly efficient aircraft which offer extended operation timeswhile reducing costs and environmental impact at the same time. The associated designrequirements for reduced structural weight and improved fatigue life representthe major challenges for todays aircraft structures and have significantly intensifiedthe competition between metallic and composite airframe applications. New metallicdesign concepts try to face this competition by combining latest materials andinnovative manufacturing methods, like high speed machining, laser beam weldingor friction stir welding, which allows for possible savings with respect to structuralweight and manufacturing costs. However, due to their integral characteristics, thedamage tolerance behaviour of these new designs is generally inferior to the commondifferential design. Reliable estimations on the fatigue life of integrally stiffenedstructures consequently necessitate assessment methodologies that are capable to includeadditional manufacturing influences and offer numerical efficiency in order tobe practical for parametric studies during airframe design.Therefore, the development and enhancement of simulation methods for efficient andreliable evaluation of cracks and crack growth represents the main objective of thisthesis. Two simulation methods are implemented and investigated for this purpose,that are based on different approaches and intended for distinct applications. Onemethod is based on analytical stress function expressions and enables a very efficientevaluation of the complete fatigue crack growth life of cracked airframe structures.The proposed approach in this context is generally based on plane assumptions andlimited to pure mode I crack loading. In order to be able to additionally considercrack turning under mixed mode loading, a second simulation method is presentedwhich implements an extended finite element framework for a mesh independentrepresentation of cracks in two dimensions. The additional combination with thematerial force concept, as alternative crack state parameter, allows for automatedsimulations of crack growth under mixed mode loading without any need for remeshingoperations.Both simulation methods are validated based on different crack configurations andare applied for crack growth investigations of varying configurations of integrallystiffened panels under pure mode I and mixed mode loading conditions. In thiscontext, a special focus is set on the influences of additional internal stresses thatfollow either from the applied manufacturing processes or an intentional prestressingof the stiffeners. Despite the general limitation to plane considerations, the proposedmethods show a good accordance with experimental, theoretical and alternativenumerical results. This demonstrates their capabilities to simulate fatigue crackgrowth and crack turning in integrally stiffened airframe structures and motivatesfurther research with respect to a possible extension to three-dimensional problems.