High Temperature Fracture Toughness Tests on P91 Steel for Fusion Applications

Steels used in fusion reactors must maintain structural integrity and safety under extreme conditions, including high temperatures and 14MeV neutron irradiation. Fracture toughness is influenced by geometry, temperature, and loading conditions. Due to the limited availability of irradiated materials, using small-scale samples is desirable. In this work, fracture toughness tests were performed on three compact tension samples (50 mm wide and 25 mm thick) made from Grade P91 steel, a material similar to EUROFER97, at 600°C. Crack length was monitored using Direct Current Potential Drop method, while surface strains were measured via Digital Image Correlation. The average J IC value was found to be equal to 321 kPa·m, though significant plastic deformation around the crack tip was observed. This deformation, combined with the non-rectilinear crack propagation, led to questioning the predictions of the standards, with a final J IC value of 800 kPa·m based on physical crack extension measurements. The influence of sample size was also explored, revealing that larger specimens are more likely to achieve plane strain conditions, while miniature ones tend to exhibit plane stress dominance at the crack tip, leading to lower toughness values.