Spheroidal graphite cast iron tends to have shrinkage cavities at the final solidification portion during cooling in casting. These cavities can significantly deteriorate the fatigue strength depending on the size and distribution. However, there are very few quantitative studies on this issue. In this study, ferritic-pearlitic spheroidal graphite cast iron cylindrical specimens, in which internal shrinkage cavities were intentionally developed, were prepared by utilizing a solidification simulation software. High cycle axial loading fatigue tests were then performed at an R of 0.1 to quantitatively investigate the influence of internal shrinkage cavities on the fatigue strength. Non-destructive measurement of size and distribution of internal shrinkage cavities was carried using an X-ray CT device. The S-N data showed a large scatter because of the different influences of shrinkage cavities in respective specimens on the fatigue crack initiation and propagation characteristics. The cluster of shrinkage cavities was approximated to an equivalent penny-shaped crack by projecting it in the loading direction. The mode I stress intensity factor (SIF) range, ΔK, estimated based on both the crack size and applied stress amplitude, described reasonably the experimental results. The fatigue limit could successfully be evaluated using the threshold SIF range, ΔKth, for a long fatigue crack obtained from compact tension specimens.
