The impact of hydrogen flakes on the uniaxial and biaxial fracture toughness of a forged ferritic steel

This work focuses on the effects of hydrogen flakes on the fracture toughness of forged ferritic steel under uniaxial or biaxial loading conditions. The fracture toughness under uniaxial loading was investigated on CT specimens and cruciform specimens were used for the biaxial loading conditions representative of a thermal shock in a Reactor Pressure Vessel. The observed decrease in fracture toughness in the flaked material was related to the higher carbon content near the hydrogen flake. Moreover, the nature of the initial defect (fatigue crack or hydrogen flake) did not significantly affect the fracture toughness. The cruciform specimen exhibited higher fracture toughness compared to CT specimens, even in the presence of flakes. This confirmed the conservatism of standard fracture analyses used for structure integrity assessment based on a lower bound fracture toughness curve obtained from CT specimens.