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 presented

which implements an extended finite element framework for a mesh independent

representation of cracks in two dimensions. The additional combination with the

material force concept, as alternative crack state parameter, allows for automated

simulations of crack growth under mixed mode loading without any need for remeshing

operations.

Both simulation methods are validated based on different crack configurations and

are applied for crack growth investigations of varying configurations of integrally

stiffened panels under pure mode I and mixed mode loading conditions. In this

context, a special focus is set on the influences of additional internal stresses that

follow either from the applied manufacturing processes or an intentional prestressing

of the stiffeners. Despite the general limitation to plane considerations, the proposed

methods show a good accordance with experimental, theoretical and alternative

numerical results. This demonstrates their capabilities to simulate fatigue crack

growth and crack turning in integrally stiffened airframe structures and motivates

further research with respect to a possible extension to three-dimensional problems.