Evaluation of Fracture Toughness by Master Curve Approach Using Miniature C(T) Specimens

The fracture toughness Master Curve gives a universal relationship between the median of fracture toughness and temperature in the ductile-brittle transition temperature region of ferritic steels such as reactor pressure vessel (RPV) steels. The Master Curve approach specified in the ASTM standard theoretically provides the confidence levels of fracture toughness in consideration of the inherent scatter of fracture toughness. The authors have conducted a series of fracture toughness tests for typical Japanese RPV steels with various specimen sizes and shapes, and ascertained that the Master Curve can be well applied to the specimens with the thickness of 0.4-inches or larger. Considering the possible application of the Master Curve method coexistent with the present surveillance program for operating RPVs, the utilization of miniature specimens which can be taken from broken halves of surveillance specimens is quite important for the efficient determination of the Master Curve from the limited volume of the materials of concern. In this study, fracture toughness tests were conducted for typical Japanese RPV steels, SFVQ1A forging and SQV2A plate materials, using the miniature C(T) specimens with the thickness of 4 mm following the procedure of the ASTM standard. The results showed that the differences in test temperature, evaluation method, and specimen size did not affect the Master Curves, and the fracture toughness indexed by the reference temperature, T₀, obtained from miniature C(T) specimens were consistent with those obtained from standard and larger C(T) specimens. It was also found that valid reference temperature can be determined with the realistic number of miniature C(T) specimens, less than ten, if the test temperature was appropriately selected. Thus, the Master Curve method using miniature C(T) specimens could be a practical method to determine the fracture toughness of actual RPV steels.