Direct Use of Fracture Toughness for Flaw Evaluations of Pressure Boundary Materials in Section XI, Division 1, Class 1 Ferritic Steel Components

The ASME Boiler and Pressure Vessel Code; Section XI provides Rules for inspection and fracture safety assessment of nuclear plant pressure boundary components. This Code provides methods for assessing the stresses and moments contributing to the forces available to drive crack growth in a component as described by stress intensity factors as well as the measures of material resistance to crack extension, measured by fracture toughness. Much of the current Code is based on linear elastic fracture mechanics methodologies developed 40 years ago [1], or more, at a time when drop weight tear tests [2] and Charpy V-notch impact tests [3] were the accepted standards used for characterizing a material’s resistance to brittle fracture. Ensuing research produced experimental methods to directly measure a material’s resistance to both brittle and ductile fracture. Data from such experiments provided the evidence supporting a suite of best estimate models describing fracture toughness behavior across a range of temperatures and strain rates. These models include cleavage crack initiation and crack arrest fracture toughness (K_Jc and K_Ia behavior, respectively) on the lower shelf and through transition, and also ductile crack initiation and crack growth resistance (J_Ic, J_0.1, and J–R behavior) on the upper shelf. Best-estimate models provide a more accurate means of assessing a material’s expected behavior under all loading and temperature conditions; they also enable an explicit characterization of uncertainties. For these reasons, there is a growing advocacy within ASME Code groups for incorporating these best estimate toughness models into Sections III and XI of the Boiler and Pressure Vessel Code.

The first direct implementation of the K_Jc best-estimate model in the ASME Code was in Code Case (CC) N-830, which was adopted by the ASME Code in 2014. N-830 states that the 5^th percentile lower bound of the K_Jc Master Curve [4], indexed by T₀, can be used as an alternative to the ASME RT_NDT-indexed K_Ic curve in a flaw evaluation performed using Non-Mandatory Appendix A to Section XI. Since that time, work has progressed within the Working Group on Flaw Evaluation (WGFE) to further improve the CC. The proposed Revision 1 of CC N-830 incorporates a complete and self-consistent suite of models that completely describe the temperature dependence, scatter, and interdependencies (such as those resulting from irradiation or other hardening mechanisms) between all fracture toughness metrics (i.e., K_Jc, K_Ia, J_Ic, J_0.1, and J–R) from the lower shelf through the upper shelf. By incorporating both a statistical characterization of fracture toughness as well as the ability to estimate a bounding curve at any percentile, the revised CC provides a consistent basis for the conduct of both conventional deterministic flaw evaluations as well as probabilistic evaluations that may be pursued in certain circumstances. Additionally, for the first time within ASME Section XI, both transition and upper shelf toughness properties are provided in a consistent manner in the same document, which provides the analyst an easy means to determine what fracture behavior (i.e., transition or upper shelf) can be expected for a particular set of conditions.

The WGFE conducted round-robin assessments of the proposed CC N-830-R1 equations and their use in flaw evaluations, and is supporting documentation of the technical basis supporting the development and implementation of N-830-R1. This paper summarizes that technical basis report. A companion paper presented at this meeting describes the round-robin assessments.