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A subsurface stress/strain fatigue life approach was previously developedin order to overcome discrepancy in the life prediction of components due togeometrical differences. In the following, the subsurface approach is used to estimatethe life of common wind turbine gear box low speed shaft collar fit subject to cyclicbending. The subsurface stress gradient is obtained from a detailed 3D finite elementanalysis. For this casting application the life is in the high cycle fatigue region andhence maximum principal stress life prediction criteria is used. The stress damage issummed up along a critical subsurface path and the estimated fatigue life using thesubsurface stress approach is compared with predictions using hot spot surfaceapproach and a recently developed fatigue limit critical distance to crack propagationmethod. It is shown that the subsurface approach fatigue lives are in good agreementwith empirical based methods and can be used when the fatigue limit has beenexceeded.