On Issue of Comparison of the Unified Curve and Master Curve Methods and Application for RPV Structural Integrity Assessment

Application of two engineering methods, the Master Curve and the Unified Curve, has been analyzed for structural integrity assessment of reactor pressure vessels. These methods have been compared using the fracture toughness data base consisting of 44 sets for ferritic steels and their welds with various degrees of embrittlement. As the comparison tests of the Master Curve and the Unified Curve, three statistical parameters have been used. Treatment of the fracture toughness data base has shown the advantage of the Unified Curve over the Master Curve when using all three parameters. Possible reasons of illusory advantage of the Master Curve over the Unified Curve that is revealed sometime when comparing these methods have been analyzed, and the main requirements have been formulated and substantiated for fully objective comparison of the Master Curve and the Unified Curve. It has also been explained with the Prometey brittle fracture model and the experimental data on the temperature dependence of thermal part of yield stress why the K JC (T) dependencies for different ferritic steels with small degrees of embrittlement may be approximated by the same exponential function as taken for the Master Curve. The correlation has been found between the temperature dependence of thermal part of yield stress and the Master Curve.