Validity of the Master Curve Temperature Dependence Assumption for Highly Embrittled RPV Materials: Results From the IAEA Coordinated Research Project (CRP-8)

The fracture toughness temperature dependence (transition curve shape) has been discussed almost since the original empirical definition of the curve in 1991. The data sets showing anomalous fracture behaviour of highly irradiated VVER-1000 pressure vessel steels presented in 2000’s have further enhanced this discussion and even a special model has been proposed for highly irradiated steels, including a mathematical definition of the curve shape change. Although in most cases the standard Master Curve (MC) approach, assuming a constant transition curve shape, has proven to give a realistic description for also highly irradiated ferritic steels, there are grades which show for example abnormally weak temperature dependence. In these cases, however, an obvious reason for the behaviour may be that the material does not fail by the mechanism assumed in the MC model. The fracture toughness data collected and analysed in the CRP-8 Topic Area 3 supports the validity of the curve shape assumption of ASTM E1921 also in case of irradiated steels and gives no rise to change the present definition. The Master Curve C-parameter (the shape parameter) estimation is proposed as an appropriate analysis method when there is need to estimate also the temperature dependence, whereas the SINTAP procedure is recommended for ensuring conservative lower bound estimates when material inhomogeneity is suspected. The results show that irradiation may slightly lower the fracture toughness in the upper transition region in relation to that predicted by ASTM E1921, but the effect after moderate T₀ shift values (up to about 100°C) seems to be negligible. The investigated steels exhibit no or very weak correlation between the C-parameter and T₀.