IAEA coordinated research project on Master Curve approach to monitor fracture toughness of RPV steels: Applicability for highly embrittled materials

In the Master Curve (MC) fracture model, a universal temperature dependence is assumed for ferritic steels, including those used in reactor pressure vessel (RPV) applications. The assumed curve shape also has been observed to be generally valid for highly irradiated or thermally aged materials that exhibit a high value of reference transition temperature, T_o. Lower than predicted fracture toughness behavior occasionally has been observed, however, in the upper transition range. It has been suggested that this behavior possibly may be associated with a lowered upper shelf toughness due to high irradiation doses. One objective of the present International Atomic Energy Agency (IAEA) Coordinated Research Project 8 (CRP-8) is to clarify the MC shape issue by collecting and analyzing relevant fracture toughness data measured on irradiated (or thermally aged) RPV and corresponding steels. For thermally aged or highly irradiated materials the fracture mode typically tends to gradually change from cleavage to one of intergranular fracture (IGF) which, if the IGF proportion is high, may significantly affect both the scatter and temperature dependence of fracture toughness. The data reviewed to date in this CRP show, in general, a very consistent fracture behavior with the basic Master Curve model that further confirms the applicability of the assumed curve shape. In cases where the basic homogeneity or fracture mode assumptions of the MC model were not satisfied due to high proportions of IGF, correspondence with the measured and predicted behavior could be markedly improved by applying available models developed to address inhomogeneous materials (e.g., SINTAP or the multi-modal model). The onset of upper shelf (T_US) and its correlation with T_o is presented as a possible approach for characterizing material behavior in the upper transition region when sufficient upper shelf fracture toughness data are available.