Effects of Graphite and Artificial Small Defect on Thefatigue Strength of Current Ductile Cast Irons

Abstract

The fatigue strength of ductile cast irons is strongly influenced by small defects such as graphite, casting defects, etc. In order to investigate quantitatively the effects of these small defects, rotating bending fatigue tests of specimens containing an artificial small defect (hole or notch) as well as smooth specimens were performed on FCD400 and FCD700. The effect of the interaction between cracks or between a crack and a hole was discussed based on fracture mechanics. The microscopic observation of the surface and interior of specimens revealed that the fatigue limit is the threshold stress for propagation of a small crack emanating from a graphite or an artificial defect. Graphites distributed in the structure of current high-quality ductile cast irons are well separated so that the fatigue limit can be modelled regardless of interaction of graphites. Consequently, the fatigue limit is predicted by the Murakami and Endo's equation, which has been applied to many fatigue problems of metals containing a single defect or crack, in terms of √area for a maximum graphite or defect and the Vickers hardness Hv of matrix. © 1994, The Society of Materials Science, Japan. All rights reserved.