Plastic Constraint Correction Factor on Brittle Fracture in Ductile-Brittle Transition Temperature Region (Report 2)

Abstract In order to establish a guideline for fracture evaluation considering plastic constraint in the ductile-brittle transition temperature (DBTT) region, the CAF (Constraint-Based Assessment of Fracture in Ductile-Brittle Transition Temperature Region) subcommittee has been organized in the Japan Welding Engineering Society. The CAF subcommittee conducted from 2018 to 2022 examined the applicability of fracture evaluation method considering plastic constraint effect in DBTT region. As a result, it was verified that the excessive conservatism of conventional fracture mechanics can be eliminated by conducting fracture tests of flat plate specimens with a surface flaw which have nearly the same degree of constraint as an actual reactor pressure vessel. In addition, a benchmark analyses were performed for verification of the analysis tools owned by the members of the CAF subcommittee and the results showed good coincidence among the participants. Based on these results, a draft guideline for fracture evaluation method considering plastic constraint effect in the DBTT region was produced in the report of the CAF subcommittee. In this draft guideline, a plastic constraint correction coefficient, χ was introduced in order to predict fracture behavior of a structure in the DBTT region without over-conservatism. χ was defined as a ratio of KJ of a fracture toughness specimen to KJ of a reactor pressure vessel with a specific flaw shape at the same Weibull stress level. X was formulated by parameters of m, a shape factor of the Weibull distribution for the Weibull stress, and strength ratio, RY defined by the ratio of yield strength to ultimate strength. In the succeeding CAF-II subcommittee from 2023 to 2024 fiscal year, for the purpose of codifying for the nuclear industry, additional analyses were conducted for different conditions on flaw shape, yield strength RY and m to expand the applicability of χ. In addition, for application of χ to irradiated conditions after long term operation, an estimation method of yield strength and RY for irradiated materials was also examined. This paper summarized the activity of the CAF-II subcommittee to improve the accuracy of χ with prediction of material strength by neutron irradiation from the previous report.