Prediction of Fatigue Life Induced by Defects Considering Crack Initiation

Abstract

Defects such as inclusions and void can be the origin of fatigue failure particularly in welding and casting materials. The fatigue life is influenced by the size, direction, shape and location of the defects. Therefore, many fatigue tests are necessary to obtain the fatigue properties. On the other hand, the prediction of fatigue life by representing characteristic variations of defects with probability distribution functions has been investigated by using several physical models and empirical formulae. However, most of the prediction methods of fatigue life arising from defects have not included the crack initiation. In the present study, the prediction was conducted by dividing the process into crack initiation and crack propagation. Voids, hard inclusions (Al2O3) and soft inclusions (MnS) were supposed as defects and two prediction models were proposed. Only the life of crack propagation was predicted by Paris law in one model (model A) while the life of crack initiation as well as propagation was predicted by Tanaka and Mura model in the other model (model B). The stress intensity factor using √area (projected square root area of defects) proposed by Murakami et al. was applied to Paris law in both models. The stress concentration and Taylor factor were applied to Tanaka and Mura model in the model B. In case of casting materials including voids, the fatigue life predicted by both models was within the range of the experimental scattering. Although the fatigue life predicted by model A was not consistent with the experimental results under high and low stress in case of high strength steel including MnS, the fatigue life predicted by model B mostly showed a good agreement with experimental results. Therefore, the present result suggested that the fatigue life prediction considering crack initiation showed higher precision than the prediction without crack initiation.