Abstract
Rotary bending fatigue tests were carried out on a series of spheroidal graphite cast irons with various matrix hardness ranging from about 170 to 640 in Vickers hardness. Fatigue crack initiation and propagation were observed at the root of the notched test pieces. Many cracks were observed to initiate at the edges of graphite nodules by stress concentrating effect of graphite nodules. These cracks propagated to the final failure by connecting the graphite nodules when the applied stress was higher than fatigue limit. On the other hand, non-propagating cracks were found when the stress was lower than fatigue limit. By analyzing the fatigue test data using the theory of fracture mechanics, a few equations were derived, which relate the fatigue limit and the notch effect of graphite nodules to the length of non-propagating crack. These equations indicate that the notch effect of graphite nodules on the fatigue limit may be ascribed to the size and is not related to the stress concentration.
