Assessment of the mechanical properties of fusion materials by small specimen testing
Future fusion reactors such as ITER and DEMO will expose materials to extreme conditions including heat loads and high-energy neutron irradiation. Testing these materials is challenging, because of limited volume available to irradiate specimens and because of highly radioactive resulting waste, making conventional testing impractical.
These challenges are also central to advanced irradiation facilities at SCK CEN such as BR2 and MYRRHA in the future, where limited irradiation space, high damage rates, and post-irradiation handling constraints strongly motivate the use of small specimen test techniques. In such facilities, specimen miniaturization is essential to maximize the use of available irradiation volume and to enable efficient post-irradiation mechanical characterization.
This thesis develops and validates a methodology based on small specimen test techniques, combined with finite element modelling, to extract reliable mechanical properties that can be then transferred to real components. Two key fusion classes of materials are investigated: several tungsten grades used as plasma-facing components, and the reduced-activation ferritic-martensitic steel EUROFER97 which is the reference structural material for test blanket modules. The work combines micro-hardness, miniaturized tensile tests on flat and cylindrical samples, and fracture toughness tests on miniaturized disk compact tension specimens and standard compact tension specimens. Damage and fracture modelling is performed using the Gurson-Tvergaard-Needleman constitutive law as well as cohesive zone model.
The results link tungsten’s initial microstructure to irradiation-induced hardening, and show that mini flat tensile specimens can provide hardening laws that accurately predict the behaviour of conventional cylindrical samples, even after neutron irradiation. Most importantly, the study demonstrates that the fracture toughness measured on a 4 mm-thick mini specimen can be transferred to a 20 mm-thick standard specimen, relying on a micromechanics-informed cohesive zone model. Overall, the thesis proposes a validated route to characterize fusion materials more efficiently and to support the design and safety assessment of future fusion reactors.
Promoter:
- Thomas Pardoen (UCLouvain)
SCK CEN mentors:
- Dmitry Terentyev
- Wouter Van Renterghem