Initiation of internal fatigue crack in a carbide-free bainitic steel during high cycle fatigue

The crack initiation stage is crucial to the overall fatigue behavior of the carbide-free bainitic steels. Through S-N curve testing and fracture observation, it is demonstrated that internal crack initiation is the dominant mode during high cycle fatigue. The crack initiation region exhibits a tilted micro-facet feature, which is along the direction of the maximum shear stress. Through fine characterization of crack initiation region, we proposed the crack initiation mechanism of dislocation dipoles pileup at prior-austenite grain boundaries in slip bands. Finally, the lifetime of the crack initiation stage was estimated from the deformation energy stored by dislocations pileup. It is beneficial for life prediction of the whole fatigue process for CFB steels in the future.