  Fatigue crack growth is usually a three-dimensional problem, but it is often
simplified to two dimensions to reduce complexity. However, this study
investigates the relationships between microscopic effects such as crack
kinking, shear lips, and plasticity that are present in reality. Therefore,
crack propagation tests were carried out on 2-mm-thick MT-160 specimens of
AA2024-T3 sheet material in L-T and T-L orientation. Using high-resolution
digital image correlation (DIC), the plastic zone was identified and measured
on the samples surface. The fracture surfaces were then digitized and their 3D
shape characterized. Finite element simulations confirm the presence of a local
mixed-mode I/II/III state along the crack front for a slant or double
shear-fracture type. A derived mapping function enables the determination of
the fracture type from the surface plastic zone, along with the current crack
tip loading during the test. Finally, a transition of the fracture type also
leads to a short-term delay in the crack propagation rate. Based on this
information crack propagation curves are computed with regards to the local 3D
crack orientation.
