Recent Results on Master Curve Assessment of Ferritic Ductile Cast Iron Dynamic Fracture Toughness Data and Fracture Mechanisms

ABSTRACT

The current framework for the safety assessment of ferritic ductile cast iron (DCI) containers used for the transport and storage of radioactive materials is based on the principles of fracture mechanics. Advisory Material for the IAEA Regulations for the Safe Transport of Radioactive Materials (2018 Edition), Appendix V: Guideline for the Safe Design of Packages against Brittle Fracture, the primary criterion is the prevention of crack initiation, and the design should not rely on any predicted ductile tearing resistance. Similarly, the ASME Code, Sect. III, Div. 3, explicitly deals with DCI and requires safety proof for dynamic loading conditions. However, the information provided in the guidelines does not appear sufficient for appropriately performing such dynamic fracture mechanics safety assessments. The ongoing German research project MCGUSS, Investigation of the Master Curve Concept for Ferritic Ductile Cast Iron, Subproject BAM Berlin: Investigations Using SE(B) Specimens (Contract No. 1501651), Subproject MPA Stuttgart: Investigations Using C(T) and DC(T) Specimens (Contract No. 1501650), is designed to address this issue. MCGUSS systematically investigates the application potential of the probabilistic fracture mechanics master curve (MC) concept according to ASTM E1921, Standard Test Method for Determination of Reference Temperature, T0, for Ferritic Steels in the Transition Range, and identifies potential modifications specific to the dynamic brittle fracture of DCI. Although MCGUSS covers testing of a large number of SE(B)- and C(T)-type specimens, this paper initially focuses on SE(B) results only. It presents the production of the test material and its properties. Particular focus is given to the fracture mechanics test facilities for dynamic small- and large-scale testing. The obtained data were statistically analyzed, and the results of the MC analyses are discussed. Particular emphasis is placed on using optical and scanning electron microscopy to link microstructural damage and failure processes to the fracture toughness data obtained. For the description of the established dynamic toughness data, a fracture mechanism called “specimen size-dependent arrest of local brittle fractures before global brittle failure by the weakest link” is proposed. Regarding the MC concept, it was demonstrated that the ASTM E1921 procedure cannot be directly applied to dynamic DCI toughness data. Material-specific modifications are being investigated, and the DCI dynamic fracture toughness database is being expanded.