Proposal for Predictions of Gigacycle Fatigue Strength in High-strength Steel

Predictions of gigacycle fatigue strength in high-strength steel were derived by using previously proposed method and past fatigue test results. The predictions were proposed for 5 grades of high-strength steel mainly under R = -1. SUPT then had 2 heat treatment conditions and predictions for SCM440 were not only under R = -1 but also under R = 0. Accuracy of the predictions was mostly good, while the predictions for S40C, SUJ2 and SCM440 under R = -1 showed a little bit inferior accuracy to others. Although the accuracy for S40C was the lowest, this was perhaps attributable to large scattering of the fatigue test results caused by poor hardenability. In these analysis, existence of fatigue limits was suggested in case of the internal fracture. The new fatigue limits could probably be confirmed by conducting 10(11) cycles fatigue tests in future. Temporary predictions of the fatigue limits were derived in this report. Predicted S-N curves showed large difference among the steel grades in a short life region, while the difference was small in a gigacycle region. Although the predicted gigacycle fatigue strength were reduced according to increase of the inclusion size, the reduction became gentle for large inclusions. Accordingly, terribly low fatigue strengths were not predicted even for huge inclusions. Mean stress effects showed good agreements with modified Goodman's rule. However, general predictions regardless of the steel grades were difficult to derive in this study, so analogy or additional fatigue tests were necessary to predict the gigacycle fatigue strength of unlisted steels.