Propagation mechanisms of microstructurally short cracks—Factors governing the transition from short- to long-crack behavior

Microstructural short fatigue cracks are known to exhibit an abnormal propagation behavior as compared to long cracks, which grow by a rate that can be described by, e.g., the Klesnil–Lukáš relationship. By means of carefully recording crack length versus number of cycles in the high-cycle fatigue regime in combination with a microtexture analysis using automated electron back-scattered diffraction, the parameters determining the scatter in microcrack propagation rates and the transition from short- to long-crack behavior were identified for an austenitic–ferritic duplex steel. The three-dimensional orientation relationship of the slip planes in grains involved in the crack propagation process turned out to be most significant. This relationship determines the barrier effect of grain and phase boundaries as well as the local crack propagation mechanisms, either operating crystallographically by single slip or perpendicularly to the applied load axis operating by double/multiple slip. To predict the propagation behavior of microstructurally short cracks in a mechanism-based way, a numerical model has been developed that accounts for local interactions between the crack tip and the microstructure as the current driving force for crack advance.