Distinct Microstructural Characteristic Lengths Defining Notch Fatigue Crack Initiation and Propagation, and Defect Tolerance in Advanced Steels

This work revisits and integrates results on the fatigue behavior of advanced bainitic steels (in particular, nanostructured bainitic steels), with special emphasis on the role of the microstructure and the specimen geometry. These studies encompass the determination of intrinsic fatigue limits from notched specimens; crack growth rate measurements; fatigue crack initiation and propagation tracking in specimens containing artificial defects manufactured by focused ion beam; and microstructural examination. The novelty of the present work lies in an integral discussion that accounts for volume effects as a function of the specimen geometry and microstructural aspects. Different characteristic microstructural length scales controlling fatigue behavior have been evaluated in scenarios with variable notch (and defect) sharpness and size. The results reveal a complex, double‐fold influence of retained austenite and the fact that crack nucleation is governed by resolved shear stresses on coplanar crystal slip systems of bainitic ferrite and austenite.