Numerical Study on the Fatigue Crack Propagation Behavior in Flattened Martensite Dual Phase Steel

The relation between the fatigue crack propagation behavior and the microscopic characteristics (the morphology, distribution, hardness of the 2nd phase) of Ferrite / Martensite Dual Phase (FMDP) steel is investigated numerically using Crystalline-Elastic-Plastic F.E. (CPFE) analysis. The calculation results lead us to a prediction that the crack growth rate of FMDP steels with polygonal and banded M-phase becomes much higher than that with flattened and banded M-phase because cracks can slip through M-phase when there exist narrow F-phase slits. The experimental results agree approximately with this prediction. This demonstrates the validity of the CPFE theory employed in this research.