Prediction of Fatigue Life of Steels in Consideration of Defect-induced Crack Initiation and Propagation

In the present study, the prediction of fatigue life by representing characteristic variations of defects with probability distribution functions was conducted by dividing the fatigue process into the crack initiation and crack propagation. Voids, hard inclusions (Al2O3) and soft inclusions (MnS) in steels were supposed as defects and two prediction models were proposed. Only the life of crack propagation was predicted by Paris law in initial defects model (model A) while the life of crack initiation as well as propagation was predicted by Tanaka-Mura model in crack initiation model (model B). The stress intensity factor using root area (projected square root area of defects) proposed by Murakami et al. was applied to Paris law in both models. The stress concentration due to defects and Taylor factor were applied to Tanaka-Mura model in the model B. These models were applied to four types of steels and the fatigue life was compared with the experimental results. In case of ductile cast iron including voids, the fatigue life predicted by both models was within the range of the experimental scattering. Although the fatigue life predicted by the model A was not consistent with the experimental results under high and low stress levels in case of Cr-Mo steel including MnS inclusions, the fatigue life predicted by the model B mostly showed a good agreement with experimental results, Therefore, it was demonstrated that the fatigue life prediction considering crack initiation showed higher precision than the prediction without crack initiation.