A Comparison of the tanh and Exponential Fitting Methods for Charpy V-Notch Energy Data

In the 1960s and 1970s, when the surveillance programs for currently operating commercial nuclear reactors were established, state of knowledge limitations resulted in the use of Charpy-V notch (CVN) specimens rather than fracture toughness specimens. Reasonable success has since been achieved in correlating CVN and fracture toughness parameters. Such correlations provide an important part of the technical basis for both current regulations and ASME codes. These correlations imply that trends manifest in CVN data must also appear in fracture toughness data, even though empirical evidence demonstrate that this is not always true. For example, the temperature dependence of CVN energy (CVE) in transition is thought to be a unique feature of each specific sample of ferritic steel that is tested, a view in sharp contrast with the now widely accepted view of a “master curve” for transition fracture toughness $(KJc)$⁠. Also, effects of product form on CVE temperature dependence and property correlations are widely reported, despite the fact that product form effects are absent from $KJc$ properties. These observations suggest that the mapping of CVE behavior onto fracture toughness implicit to correlation-based regulations and ASME codes may produce erroneous trends in estimated values of fracture toughness. In this paper we investigate the hypothesis that the apparent differences between CVE and fracture toughness arise due to differences in how the temperature dependence of CVE and $KJc$ data have historically been modeled. Our analysis shows that when CVE data are analyzed in a manner consistent with $KJc$ data (i.e., transition and upper shelf data are partitioned from each other and analyzed separately rather than being fit with a continuous tanh function), the apparent differences between CVE and toughness characterizations are minimized significantly, and may disappear entirely. These findings demonstrate the differences between CVE and fracture toughness data to be an artifact of the tanh analysis method rather than an intrinsic property of CVE.