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Based on state-of-the-art research and regulations, the application of the fracture mechanics master curve (MC) concept to ferritic ductile cast iron (DCI) is being investigated in a recently started German joint research project between BAM Berlin and MPA Stuttgart. This paper outlines the research approach and the planned investigations and provides first results.
According to the IAEA guidelines SSG 26, the safety assessment of DCI containers for transport as well as interim and final storage of radioactive materials is based on the fracture mechanics criterion of general crack initiation exclusion. Within this context, the focus of the current research project is on the further development of brittle fracture safety assessment methods under dynamic loading conditions. Within fracture mechanics safety assessments for steels in nuclear technology, the probabilistic MC concept according to ASTM E1921 is currently used for quasi-static loading conditions as a supplement to the established deterministic ASME reference curve concept. The MC concept allows for an effective statistical consideration of the scatter of the material toughness in the transition regime. Furthermore, the MC can also be used for elevated loading rates as they occur in accident or crash scenarios. However, for DCI, a systematic review of potential modifications to the MC assumptions and procedure according to ASTM E1921 and an associated validation are still lacking. Therefore, a corresponding methodology shall be established to determine and assess dynamic fracture toughness values of DCI in the ductile-to-brittle transition regime, using samples extracted from a component.
For the experimental program, a DCI material was produced as it is generally used for transport and storage containers. A basic mechanical-technological material characterization consisting of tensile, Charpy, and Pellini tests is planned. A comprehensive fracture mechanics test program was compiled covering dynamic testing of DC(T)9, C(T)25, C(T)50, SE(B)10, SE(B)25, and SE(B)140 specimens. Moreover, numerical analyses of different specimen geometries and test setups are planned to investigate the influences of stress triaxiality and loading rate. Throughout the experimental program, the microstructure-property relationship will be investigated based on quantitative metallographic and fractographic analyses. Finally, different MC assessment strategies will be considered, and modifications will be made if necessary.
Within fracture mechanics safety assessment for steels in nuclear technology, the probabilistic master curve (MC) concept according to ASTM E1921 is currently used for quasi-static loading conditions as a supplement to the established deterministic ASME reference curve concept. However, for ductile cast iron (DCI), a systematic review of potential modifications to the assumptions and the procedure according to ASTM E1921 and an associated validation are still lacking. For this reason, the application of the fracture mechanics MC concept to ferritic ductile cast iron is being investigated in a recently started joint research project between MPA Stuttgart and BAM Berlin. According to the IAEA guidelines SSG 26, the safety assessment of DCI containers for transport as well as storage of radioactive materials is based on the fracture mechanics criterion of general crack initiation exclusion. Within this context, the focus of the current research project is on the further development of brittle fracture safety assessment methods. Therefore, a methodology shall be established to determine and assess dynamic fracture toughness values of DCI in the ductile-to-brittle transition regime, using samples extracted from a component. For the experimental program, a DCI material will be produced as it is generally used for transport and storage containers. A basic mechanical-technological material characterization consisting of tensile, Charpy and Pellini tests is planned and a comprehensive fracture mechanics test program was compiled covering dynamic testing of DC(T)9, C(T)25, C(T)50, SE(B)10, SE(B)25, and SE(B)140 specimens. Numerical analyses of different specimen geometries and test setups are planned to investigate the influences of stress triaxiality and loading rate. Throughout the experimental program, the microstructure-property relationship will be investigated based on quantitative metallographic and fractographic analyses. Finally, different MC assessment strategies will be considered and modifications will be made if necessary.
Within fracture mechanics safety assessment for steels in nuclear technology, the probabilistic master curve (MC) concept according to ASTM E1921 is currently used for quasi-static loading conditions as a supplement to the established deterministic ASME reference curve concept. However, for ductile cast iron (DCI), a systematic review of potential modifications to the assumptions and the procedure according to ASTM E1921 and an associated validation are still lacking. For this reason, the application of the fracture mechanics MC concept to ferritic ductile cast iron is being investigated in a recently started joint research project between MPA Stuttgart and BAM Berlin. According to the IAEA guidelines SSG 26, the safety assessment of DCI containers for transport as well as storage of radioactive materials is based on the fracture mechanics criterion of general crack initiation exclusion. Within this context, the focus of the current research project is on the further development of brittle fracture safety assessment methods. Therefore, a methodology shall be established to determine and assess dynamic fracture toughness values of DCI in the ductile-to-brittle transition regime, using samples extracted from a component. For the experimental program, a DCI material will be produced as it is generally used for transport and storage containers. A basic mechanical-technological material characterization consisting of tensile, Charpy and Pellini tests is planned and a comprehensive fracture mechanics test program was compiled covering dynamic testing of DC(T)9, C(T)25, C(T)50, SE(B)10, SE(B)25, and SE(B)140 specimens. Numerical analyses of different specimen geometries and test setups are planned to investigate the influences of stress triaxiality and loading rate. Throughout the experimental program, the microstructure-property relationship will be investigated based on quantitative metallographic and fractographic analyses. Finally, different MC assessment strategies will be considered and modifications will be made if necessary.
Within fracture mechanics safety assessment for steels in nuclear technology, the probabilistic master curve (MC) concept according to ASTM E1921 is currently used for quasi-static loading conditions as a supplement to the established deterministic ASME reference curve concept. However, for ductile cast iron (DCI), a systematic review of potential modifications to the assumptions and the procedure according to ASTM E1921 and an associated validation are still lacking. For this reason, the application of the fracture mechanics MC concept to ferritic ductile cast iron is being investigated in a joint research project between MPA Stuttgart and BAM Berlin. According to Hojo et al. (2008) the MC concept is already in use for the brittle fracture safety assessment of ferritic forged steel containers, which would suggest an extension to ductile cast iron containers. In compliance with IAEA SSG-26, the safety assessment of DCI containers for transport as well as storage of radioactive materials is based on the fracture mechanics criterion of general exclusion of crack initiation. Within this context, the focus of the current research project is on the further development of brittle fracture safety assessment methods. Therefore, a methodology shall be established to determine and assess dynamic fracture toughness values of DCI in the ductile-to-brittle transition regime, using only a low number of small fracture mechanics test specimens extracted from a component. For the experimental program, a DCI material of grade GJS-400 (GGG-40) has been produced as it is generally used for transport and storage containers for nuclear material. A basic mechanical-technological material characterization consisting of tensile, Charpy, and Pellini tests has been planned and a comprehensive fracture mechanics test program was compiled covering dynamic testing of DC(T)9, C(T)25, C(T)50, SE(B)10, SE(B)25, and SE(B)140 specimens at various loading rates and temperatures. The specimen testing is divided between the project partners based on specimen geometry. MPA Stuttgart has been tasked with the testing of the C(T), DC(T) and Pellini specimens, while BAM Berlin was assigned the SE(B), Charpy and tensile tests. These tests allow for the determination of the loading rate relevant for brittle fracture via C(T)25 specimens and an assessment of size effects by transferring the relevant loading rate to other specimen geometries and sizes. Numerical analyses of different specimen geometries and test setups are planned to investigate the influence of stress triaxiality and loading rate. Furthermore, the time-dependent course of the Weibull stress is assessed via these numerical analyses. Throughout the experimental program, the microstructure-property relationship will also be investigated based on quantitative metallographic and fractographic analyses like scanning electron microscope images and metallographic sections. With the acquired experimental database fundamental assumptions of the MC concept, like the weakest link model and the Weibull distribution, will be assessed for the application to DCI. In the end, the applicability of the MC concept will be evaluated and if necessary, modifications to the MC concept for DCI will be proposed. This paper will present the preliminary results of the GJS400 material characterization and the related selection of test specimen extraction points. First results of the C(T)25 and SE(B)140 specimen testing will also be discussed supported by first metallographic analyses.