A Combined Model Approach For Estimating T-0

The current best-estimate model describing the fracture toughness of ferritic steels is the Master Curve methodology standardized in ASTM E1921. Shortly following standardization by ASTM, efforts were undertaken to incorporate this best-estimate model into the framework of the ASME Code to reduce the conservatisms resulting from use of a reference temperature based on the nil-ductility temperature (RTNDT) to index the plane strain fracture initiation toughness (Kit). The reference temperature RTro, which is based on the ASTM E1921-defined T-0 value, was introduced in ASME Code Cases N-629 (replaced by Code Case N-851) and N-631 to replace RTNDT for indexing the ASME KID curve. Efforts are continuing within the ASME Code to implement direct use of the Master Curve model; using the T-0 reference temperature to index an elastic-plastic, Kic fracture toughness curve. Transitioning to a direct T-0-based fracture toughness assessment methodology requires the availability of T-0 estimates for all materials to be assessed. The historical Charpy and NDT-based regulatory approach to characterizing toughness for reactor pressure vessel (RPV) steels results in a lack of To values for a large population of the US nuclear fleet. The expense of the fracture toughness testing required to estimate a valid T-0 value makes it unlikely that T-0 will ever be widely available. Since direct implementation of best-estimate, fracture toughness models in codes and regulatory actions requires an estimate of for all materials of interest it is necessary to develop an alternative means of estimating T-0. A project has been undertaken to develop a combined model approach to estimating To from data that may include limited elastic-plastic fracture toughness Kic, Charpy, tensile, ductile initiation toughness, arrest toughness, and/or nil-ductility temperature data. Using correlations between these properties and To a methodology for combining estimates of T-0 from several sources of data was developed. T-0 estimates obtained independently from the Master Curve model, the Simple T287 correlation model, and a more complex Charpy correlation model were combined using the Mixture Probability Density Function (PDF) method to provide a single estimate for T-0. Using this method, the individual To estimates were combined using weighting factors that accounted for sample size and individual model accuracy to optimize the accuracy and precision of the combined T-0 estimate. Combining weighted estimates of T-0 from several sources of data was found to provide a more refined estimate of T-0 than could be obtained from any of the models alone.