Abstract
The fatigue limit of hypo-eutectoid graphitic steels was studied in connection with microstructural factors such as the mean ferritic grain size dF (mm), the volume fraction of graphite f, the mean graphite nodule diameter dG (mm), and the mean ferrite path between graphite nodules λ.By the multiple regression analysis the fatigue limit σW (MPa) was expressed as follows:(This article is not displayable. Please see full text pdf.) The effect of dG was smaller than that of dF.Non-propagating microcracks were observed on the surface of smoothed specimens subjected to the repetition of 107 cycles at the stress of the fatigue limit, and furthermore, the criterion as to whether non-propagation of a microcrack can occur or not was found to be related to the microcrack length, the cyclic stress and the mean ferritic grain size. Thus it was concluded that the fatigue limit is not the critical stress for crack initiation but is the maximum stress below which the microcrack can not grow continuously.Aspects of fatigue fracture surface depended on the microstructure of graphitic steels, and the amount of facet type fractures increased with decrease in mean ferrite path between graphite nodules.The endurance ratio of the graphitic steels was in the range from 0.52 to 0.60.
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