Damage tolerance investigations of innovative metallic airframe structures

The continuously expanding commercial air traffic of the last decades steadily increased
the demand for highly efficient aircraft which offer extended operation times
while reducing costs and environmental impact at the same time. The associated design
requirements for reduced structural weight and improved fatigue life represent
the major challenges for todays aircraft structures and have significantly intensified
the competition between metallic and composite airframe applications. New metallic
design concepts try to face this competition by combining latest materials and
innovative manufacturing methods, like high speed machining, laser beam welding
or friction stir welding, which allows for possible savings with respect to structural
weight and manufacturing costs. However, due to their integral characteristics, the
damage tolerance behaviour of these new designs is generally inferior to the common
differential design. Reliable estimations on the fatigue life of integrally stiffened
structures consequently necessitate assessment methodologies that are capable to include
additional manufacturing influences and offer numerical efficiency in order to
be practical for parametric studies during airframe design.
Therefore, the development and enhancement of simulation methods for efficient and
reliable evaluation of cracks and crack growth represents the main objective of this
thesis. Two simulation methods are implemented and investigated for this purpose,
that are based on different approaches and intended for distinct applications. One
method is based on analytical stress function expressions and enables a very efficient
evaluation of the complete fatigue crack growth life of cracked airframe structures.
The proposed approach in this context is generally based on plane assumptions and
limited to pure mode I crack loading. In order to be able to additionally consider
crack turning under mixed mode loading, a second simulation method is