Fatigue crack growth in ductile materials like aluminum alloys is mainly driven by the interaction between intrinsic (damaging) and extrinsic (shielding) mechanisms. Environmental conditions, loading characteristics and the microstructure of the material correlate with each other in terms of crack growth. For a fatigue crack under pure Mode 1 loading, the anisotropy of the microstructure leads to microscopic crack deflections, crack branching and crack closure effects resulting in change of crack driving force, growth rate and crack propagation direction. All of these effects (damage and shielding) are closely interrelated, making it difficult to distinguish their individual effects on crack propagation. From an experimental point of view, these effects are also difficult to access due to their microscopic scale and their randomness.

In this work, we examine the fatigue crack propagation behavior of an AA2024-T3 aluminum alloy using MT160 specimen under constant amplitude analyzing the impact of different rolling orientations (L-T and T-L) and loading ratios of R= 0.1, 0.3, 0.5. A servo-hydraulic test stand in combination with a robot supported multiscale digital image correlation (DIC) system is used to capture microscopic deformation fields along the entire crack propagation process. This allows to evaluate cause-effect relationships between microstructural characteristics and fatigue crack mechanics on a microscopic length scale.   
The experimental results verify that the fatigue crack propagation rate is higher in the T-L direction than in L-T direction. Evaluation of the DIC data reveals a zig-zag propagating crack in L-T direction with asymmetric plastic zones. In comparison, the crack path of the T-L specimen is straighter. Both effects have an impact on the crack tip loading conditions and crack closure behavior that were quantified by our DIC evaluation algorithms. Finally, these advantageous evaluation methods allow mechanism-based separations of relevant crack propagation mechanisms.