Influence of fatigue precracking and specimen size on Master Curve fracture toughness measurements of EUROFER97 and F82H steels

Abstract

EUROFER97 and F82H are two reference reduced-activation ferritic-martensitic (RAFM) steels from Europe and Japan, respectively. Both steels have favorable properties for fusion first wall and blanket structure applications [1], [2], [3], [4], [5], such as reduced activation, superior swelling resistance, good thermal conductivity, and favorable fracture toughness. However, exposing materials to the harsh environment of a fusion reactor, characterized by 14 MeV neutrons, will result in both irradiation damage and He/H transmutation in the material [6], which could result in a significant degradation of material fracture toughness properties. Therefore, the post-irradiation evaluation of RAFM steel fracture toughness is critical to understand the material degradation behavior to ensure the safe long-term operation of a fusion reactor [7]. Due to the space constraints of existing test reactors and future fusion neutron sources, as well as higher costs and challenges in controlling and maintaining a uniform irradiation condition for large size specimens, the development of small specimen test techniques (SSTT) is indispensable to evaluate the performance of irradiated materials. Under the auspices of the International Atomic Energy Agency (IAEA), a Coordinated Research Project (CRP) titled “Towards the Standardization of Small Specimen Test Techniques for Fusion Applications” has started since 2017. The overall objective of the project is to provide a set of guidelines for SSTT based on commonly agreed best practices for five mainstream test techniques: tensile, creep, low cycle fatigue, fracture toughness, and fatigue crack growth rate. The project intends to act as a first step towards a full standardization of SSTT for testing and qualifying fusion structural materials. As participants in this project, Oak Ridge National Laboratory (ORNL) and the Centre for Energy, Environmental and Technological Research (CIEMAT) performed interlaboratory fracture toughness testing based on the Master Curve method in the ASTM E1921-21 standard [8]. This paper summarizes our key findings concerning fatigue precracking and specimen size effects on Master Curve fracture toughness characterization of EUROFER97 and F82H steels. We also evaluate and provide recommendations on the minimum number of specimens needed for each specimen type for obtaining reliable T0 values.