Superior resistance to the stage II fatigue crack growth was obtained on an ultra high carbon steel containing massive eutectic carbide extended in the direction perpendicular to the original crack growth direction by forging. The fatigue crack growth rate obtained in this study was about 1/4 times as low as that of general mild steel without eutectic carbides. Microscopic observations revealed that the fatigue crack path was bent or branched when it encountered the massive carbide. Taking the actual crack configurations affected by the carbide into consideration, the “Effective stress intensity factor (SIFeff)” for fatigue crack growth was estimated, in which significant reduction of the SIFeff was obtained.
An analytical study showed that the SIFeff of the bent crack would decrease significantly with increasing the angle θ between the carbide extention direction and the loading direction. The relation between the growth rate and SIFeff was examined on various specimens that have different carbide orientation on the basis of the above analysis. It was found that the growth rate drastically changed in accordance with θ and also found that the dependense of the growth rate on θ was in good agreement with the SIFeff-θ relation. It was made clear that crack growth retardation was due to the reduction of SIFeff resulting from the complicated crack morphology affected by massive carbide.
