Rational Determination of Lower Bound Fracture Toughness Curves Using Master Curve Approach

Abstract

The Master Curve gives the relation between the median of fracture toughness of ferritic steels and the temperature in the ductile–brittle transition temperature region. The procedure used to determine the Master Curve is provided in the current American Society for Testing and Materials (ASTM) E1921 standard. By considering the substitution of the alternative lowerbound curves based on the Master Curve approach for the KIc curves based on reference data sets in the present codes such as ASME Code Cases N629 and N631, the statistical characteristic should be well incorporated in the determination of the lowerbound curves. Appendix X4 in the ASTM standard describes the procedure used to derive the lowerbound curves; however, it appears to be addressed without sufficient consideration of the statistical reliability. In this study, we propose a rational determination method of lowerbound fracture toughness curves using the Master Curve approach. The method considers the effect of sample size in the determination of the tolerancebound curve. The adequacy of the proposed method was verified by comparing the tolerancebound curve with the fracture toughness database for national reactor pressure vessel (RPV) steels including plate and forging obtained from 4 T to 0.4 T C(T) specimens and 0.4 T SE(B) specimens. The method allows the application of the Master Curve using fewer specimens, which can coexist with the present surveillance program.