Validation of the Models in ASME CC N-830-1

The ASME Section XI Working Group on Flaw Evaluation (WGFE) has an ongoing effort intended to implement the K_Jc Master Curve (MC), as defined in ASTM E1921, into Section XI of the ASME Code. Code Case (CC) N-830 was approved by ASME in 2014 and was the first direct implementation of the K_Jc MC into the ASME Code. The CC used the 5^th percentile lower bound of the Wallin Master Curve as an alternative to the ASME K_Ic curve to characterize material resistance to fracture in flow evaluations. To take advantage of the best-estimate fracture toughness models developed over the last two decades, CC N-830 was later modified to include a suite of self-consistent fracture toughness models, all linked to T₀, that describe material fracture toughness metrics from lower shelf, through transition, to upper shelf behavior (i.e., K_Jc, K_Ia, J_Ic, J_0.1, and J-R). Validation efforts described in detail in EPRI MRP 418 Rev. 1 show that the best estimate models contained in CC N-830-1 provide a much more accurate and consistent representation of fracture toughness across all temperatures than do RT_NDT/K_Ic and associated J_Ic, J-R and K_Ia methods currently adopted in the Code.

Validation studies in addition to those included in MRP 418-1 were recently performed specifically focusing on materials characterized as having low upper shelf behavior to address concerns raised by Staff from the US Nuclear Regulatory Commission (NRC) regarding the validity of the CC N-830-1 model predictions for materials with upper shelf energy, or USE, < 68 J (50 ft-lbs). While data from such materials has been widely tested (e.g., irradiated Linde-80 weld) and used in the development of the J_Ic/J-R curve model, little such data was used to demonstrate the validity of the entire CC model (i.e., starting with an appropriate T₀ value that is used to predict the J-R curve within the stated CC accuracy). In response to the NRC Staff concern, the Working Group conducted a careful and statistically rigorous analysis of additional low-upper shelf datasets, to provide a more compelling demonstration that no unconservative biases exist within the CC model for such materials. This paper describes the T₀ to J-R Curve prediction model validation effort supporting consensus adoption of CC N-830-1 by ASME SC XI.