Task 2: Fracture-Toughness Transition and Master-Curve Methodology

Fracture-toughness transition and master-curve (MC) methodology is being broadly explored for pressure-vessel applications through a series of experiments, analyses, and evaluations in eight Subtasks. For example, pertinent fracture-toughness data needed to assess the shift and potential change in shape of the fracture-toughness curves due to neutron irradiation are being collected and statistically analyzed. The effects of irradiation on fracture-toughness curve shape for highly embrittled RPV steels, dynamic effects, crack arrest, intergranular fracture, and subsized specimens are also being explored. Finally, guidelines for the application of "surrogate materials" to the assessment of fracture toughness of RPV steels will be evaluated.

In most of the surveillance capsules in currently operating nuclear reactors, only Charpy V-Notch (CVN) specimens are available. These specimens are now being considered for the development of fracture-mechanics related data. The CVN specimen, converted into a fatigue precracked specimen, is extremely small for fracture-mechanics-related toughness evaluation. Nevertheless, the precracked CVN (PCVN) is being incorporated into experiments and round robin activities that are world-wide in scope. This data is being used to develop a reference temperature, T₁₀₀, that positions a universal fracture-toughness transition curve. The plot shown in Figure 1 compares the T₁₀₀ derived from PCVN to T₁₀₀ determined using conventional fracture-mechanics-type specimens. The plot is preliminary since considerably more data are needed to evaluate accuracy and precision.