Transition-behavior of fatigue crack with variation of stress amplitude for SAF2205 steel: Effect of dual-phase imbalance caused by welding

This work explores the influence of dual-phase imbalance on fatigue micro-scale crack for SAF2205 steel welded joints under a wide range of stress amplitudes. A series of quasi in-situ fatigue experiments and kernel average misorientation (KAM) evolution were performed. The results reveal distinct transition-behavior of crack initiation and propagation with varying stress amplitudes. At high stress amplitudes, as load increases, crack initiation shifts from austenite grain or phase boundaries to ferrite grain boundaries due to transfer of deformation-bearing phase. The crack propagation is influenced by the angle between the direction of grain orientation, grain/phase boundaries and loading axis. However, at low stress amplitudes, the predominant deformation mechanism changed from dislocation slip to strain incompatibility between grain boundary austenite (GBA) and ferrite, ulteriorly leading to crack initiation. Crack propagation is deflected when encountering intragranular austenite (IGA) due to the obstruction by high-energy phase interfaces. In addition, based on crystal plasticity finite element model (CPFEM) and extended finite element method (XFEM), a fatigue life prediction model is further developed by considering the micro- crack damage behavior, enabling precise simulation of the transition-behavior in crack initiation sites, crack propagation direction as well as S-N distribution under varying stress amplitudes.