Relationship between fatigue crack initiation life and active slip systems in bcc Fe-Cr polycrystalline

Abstract

To clarify the controlling factors affecting fatigue crack initiation life in poly-crystal ferritic steel, we first observed the initiated fatigue crack morphology during four-point in-plane cyclic bending tests using bcc Fe-16 mass% Cr alloy with a huge grain. The experimental slip trace analyses were compared with those of the calculated shear stress among 24 slip systems based on finite element analysis (FEA). The focus was placed upon the relationship between fatigue crack initiation and the operated slip system. Under the high-cycle-fatigue condition, the inhomogeneous stress distribution was predicted by taking into account the elastic anisotropy. It was clarified that the fatigue cracks tend to initiate at sites where stress concentration was predicted by FEA, Furthermore, the slip trace near fatigue cracks was confirmed to correspond to the dominant slip systems identified by shear stress analysis. However, the fatigue initiation life depended not only on the applied stress amplitude and/or shear stress on the primary slip system, but also on the characteristics of the active slip systems, which were classified into two types. The fatigue crack initiation life, in the case of the cross slip dislocation type was longer than that in the case of the Lomer-Cottrell locking dislocation type. It was inferred that the nature in the dislocation interaction leads to the difference in the fatigue crack initiation life.