Abstract
The fatigue limit of high strength steels does not increase linearly with hardness, and moreover the scatter of the fatigue limit is very large. Spring steels are a typical material which shows such kinds of fatigue characteristics. The fatigue fracture origin of high strength steels is not slip band cracks or grain boundary cracks but usually nonmetallic inclusions contained in the materials.
In this paper, the combined effects of nonmetallic inclusions with the hardness of a matrix on fatigue strength is considered by the fracture mechanics and an equation to predict quantitatively the fatigue strength of high strength steels is proposed. The crucial geometrical parameter of inclusions which controls fatigue strength is the square root of the area (_??_area) of the inclusion projected onto a plane perpendicular to the direction of the maximum tensile stress and the crucial material parameter is the Vickers hardness (Hv). The prediction method is applied to evaluate the fatigue limit of an automobile suspension steel (SUP12) and it is shown that the predictions of the fatigue strength reduction caused by inclusions are in good agreement with the experimental results.
