Competition between surface defect and grain size under fatigue loading - ARMCO iron

The design of metallic cast parts requires a compromise between the fatigue resistance of the component and the allowable defect size due to the process. Treatment of defect sensitivity coupled with intrinsic length scales of grains or other microstructure attributes is ultimately necessary to form a predictive basis for defect size effects in forming and growing small defect cracks. This work presents experimental results on high cycle fatigue behavior of specimens containing a surface hemispherical defect under uniaxial tension loading for a wide range of notch size to grain size ratios, including cases where the notch size is on the order of, or even smaller, than the grain size. The influence of grain size on the fatigue strength is clearly demonstrated and the corresponding effects are evaluated. This paper shows that for the same specimen geometry, loading conditions and defect morphology, the fatigue limit is directly dependent on the relationship between the defect size and the grain size. Dimensionless Kitagawa diagram shows that the defect size which impacts the fatigue limit is greater than 0.7 times the grain size in the Armco iron and greater than three times the grain size in other steels.