Evaluation of fatigue limit of spheroidal graphite cast iron

The fatigue limits of various spheroidal graphite cast irons were determined under completely reversed plane bending condition. The relationships between the fatigue limit and microstructural factors such as graphite nodule diameter and matrix hardness were clarified, and the prediction equations of the fatigue limit of the irons were determined. It is observed that the fatigue limit strongly depends on maximum nodule diameter presumed by using extreme statistics, because fatigue cracks initiate from larger graphite nodules just below the specimen surface prior to others. The fatigue limit can be significantly improved by the hardening matrix only in the specimens with a fine nodule size. Austempered cast iron has a higher fatigue limit than other irons although having the same matrix hardness. As a consequence of the quantitative analysis, the prediction equations of the fatigue limits of spheroidal graphite cast irons are given.