An experimental and numerical analysis to correlate variation in ductility to defects and microstructure in ductile cast iron components

This paper describes a statistical test plan to determine tensile properties and fracture properties for ductile cast iron considered for the Swedish nuclear waste canisters and associated analysis. Large variations were found in the ductility between tested canister inserts and between specimens taken from different locations in each insert. A large number of tested tensile specimens were subsequently analysed by fractography and metallography to relate low ductility values to size and type of casting defects. Loss of ductility could be related to slag defects and to a lesser extent to high pearlite content, low nodularity and chunky graphite. Slag defects were modelled by an elasto-plastic fracture mechanics model for penny-shaped slag defects and semi-empirical models for the other defect types. The fracture model was incorporated into a probabilistic scheme to compute distribution of elongation for the inserts and the associated defect size. The computed ductility distribution agrees very well with measured data whereas the computed defect size distribution is underestimated. By including crack growth resistance and various aspect ratios of defects a much better agreement with observed defects can be achieved.