Technical Basis for Proposed Code Case of Using a Master Curve in Lieu of the Code K_Ic Curve in ASME Boiler and Pressure Vessel Code

The Master Curve method for determination of fracture toughness in the transition range in ASTM standard E1921 [1] brought an opportunity for the ASME Code to adopt a much better fracture toughness curve based on directly measured fracture toughness data. This also enables obtaining statistically based fracture toughness data. The industry, through PVRC Task Group (subsequently Section XI Task Group on Master Curve Fracture Toughness), took a two-phase approach to implement the adoption of the Master Curve method in the ASME Code. First, Phase I was completed with the issuance of ASME Code Cases N-629/N-631 [9, 10], published in 1999 which allowed the existing Code K_Ic curve to be used by means of an alternate indexing reference temperature RT_T0. This provided an important new approach to allow material specific, measured fracture toughness curves for ferritic steels in the code applications. However, this only rectified part of the shortcomings of the present Code K_Ic curve. In Phase II, it is intended to develop a direct means to utilize a tolerance bound of the Master Curve itself in place of the ASME K_Ic curve. This paper summarizes a proposal for such a procedure whereby a Master Curve fracture toughness tolerance bound is made usable in the ASME flaw evaluation processes, i.e. in Appendix A and Appendix G to Section XI of the ASME Boiler and Pressure Vessel Code. A draft code case is presented in Appendix in this paper.