Effect of grain size on crack growth resistance and notch sensitivity in fatigue of carbon steel

Abstract

The effect of grain size on the crack growth resistance and the notch sensitivity in fatigue of commercial carbon steels with grain sizes of 6.5 and 20 micrometers was investigated using smooth specimen and V-groove notched specimens with various notch radii under rotating bending. A crack initiated at the early stage of stress repetitions and most of fatigue life was occupied by the growth life of a crack smaller than 1-2 mm. The crack growth rate was determined by the term σ(sub a)(exp n)l, uniquely in both steels, where σ(sub a) and l are the stress amplitude and the crack length and n is constant. The fine-grained steel has an excellent resistance to crack growth in comparison with many annealed carbon steels. This was explained by the barrier effect of grain boundary. Consequently, the notch sensitivity is higher in the fine-grained steel than in coarse-grained not only for crack initiation but also crack propagation.