Recently ASTM adopted a Test Method for the Determination of Reference Temperature, To, for Ferritic Steels in the Transition Range (E1921-97). This test method quantifies the variation of fracture toughness with temperature throughout the transition regime based on limited replicate testing at a single temperature. Advances in elastic plastic fracture mechanics enabled adoption of size requirements in E1921-97 that permit testing of specimens as small as 10 mm cross section. Thus, E1921-97 presents the first opportunity to measure directly the fracture toughness of samples removed from surveillance capsules, rather than inferring toughness using RTNDT indexed KIC curve.
We assemble a database of fracture toughness values for reactor pressure vessel steels, including over 1,600 E1921-97 valid fracture toughness values for both irradiated and unirradiated steels. In this paper, we use this database to address concerns regarding the bias and pecision of To values determined by ASTM E1921-97, and regarding the use of To as an index temperature for the ASME KIC curve. When the requirements of E1921-97 are satisfied, neither the temperature at which the fracture toughness tests are conducted nor the level of deformation prior to fracture bias the estimate of To. Furthermore, the recently proposed To-based index for the KIC curve (RTTo ≡ To+ 35°F) reduces considerably the degree of scatter in facture toughness data, and contains implicit margins on toughness that are consistent for every steel considered. RTTo is superior to RTNDT as an index temperature for the KIC curve, and also provides for a more conservative assessment of vessel safety than does RTNDT.