Defect-sensitive fatigue assessment of heavy-section ductile cast irons: a comparative study of pearlitic and high-silicon ferritic grades

Heavy-section castings of ductile cast iron (DCI) unavoidably contain micro shrinkage porosity due to non-uniform, slow cooling, and service components also feature geometric stress raisers. This study quantifies how these two realities—intrinsic defects and notches—jointly control fatigue resistance and formalizes a design approach that accounts for their interaction. We compare a pearlitic EN-GJS-600–3 (GJS-600–3) and a high-silicon solid solution strengthened ferritic (HSi) DCI, which exhibit different matrix ductility and distinct pore populations. Pore size distributions are characterized (via X-ray computed tomography, CT), and extreme-value statistics are used to estimate the most critical defect expected in the highly stressed region of notched specimens. This defect measure is then coupled to a strain energy density (SED) criterion to predict fatigue limits. Fatigue tests on plain and V-notched specimens with varying notch severity reveal a systematic transition from pore-dominated initiation (plain and mildly notched) to notch-dominated initiation (severe notches). The proposed CT–statistics–SED framework reproduces both the fatigue limits and the observed switch in the governing initiation site. Compared with GJS-600–3, the HSi grade shows lower intrinsic fatigue strength but greater tolerance to distributed microporosity, leading to improved reliability in geometries with large highly stressed volumes. The approach provides a practical route to defect-aware fatigue design of DCI components, suggesting material-and-geometry selection: pearlitic grades for smaller, sharper features where notch control prevails; high-silicon ferritic grades for large, blunt features where defect tolerance is paramount. Overall, the method supports lighter, more reliable cast designs without resorting to overly conservative safety factors.