Assessment of the mechanical properties of fusion materials by small specimen testing

For the development of future nuclear fusion reactors, such as ITER and DEMO, it is crucially dependent on the performance and structural integrity of materials subjected to extreme operational conditions, including high-energy neutron irradiation and intense thermal loads. The qualification of these materials requires extensive mechanical property data, yet testing is extremely limited by the volume available in material test reactors and the challenges of handling highly radioactive components. This thesis addresses this challenge by developing and validating an integrated framework of Small Specimen Test Technique (SSTT) combined with computational modeling to evaluate the mechanical properties of key fusion materials. The materials investigated in this thesis are candidate plasma-facing tungsten grades with varied microstructures and the reduced-activation ferritic-martensitic (RAFM) steel, EUROFER97, the primary structural material for the test blanket module. Experimental methods include micro-hardness, miniaturized uniaxial tensile tests on both flat and cylindrical geometries, and fracture toughness tests on miniaturized disk compact tension (mini-DCT) and standard compact tension (CT) specimens. The computational framework uses Finite Element Method (FEM) simulations coupled with the Gurson-Tvergaard-Needleman (GTN) model for ductile damage and a Cohesive Zone Model (CZM) for crack propagation. For tungsten, a correlation between initial microstructural sink density and irradiation-induced hardening was established, providing a basis for designing radiation-tolerant alloys. For EUROFER97, a simplified method for extracting the post-necking hardening law from mini-flat tensile specimens was developed and validated. The constitutive laws derived from these space-efficient flat specimens were shown to accurately predict the tensile behavior of cylindrical specimens with over 90% accuracy in both non-irradiated and irradiated state. Most significantly, this work demonstrates that fracture toughness properties can be reliably transferred from a 4 mm thick, non-standard mini-DCT specimen to a 20 mm thick, ASTMvalid CT specimen. By calibrating the cohesive parameters using data obtained from the miniaturized test, the model was capable of predicting the fracture behaviors such as J-R curve and crack pattern of the larger specimen. In conclusion, this thesis establishes a validated and efficient methodology to characterize the mechanical properties for fusion material research. With the demonstrated transferability of tensile and fracture properties from miniaturized specimens to standard-sized specimens, it provides a prototype pathway to accelerate material development, enhance the statistical confidence of data for design codes like RCC-MRx, and ultimately improve the safety and feasibility of future fusion energy systems.