Transition Curve Fitting of Charpy Test

Abstract The Charpy V-notch (CVN) impact test is a typical method used to evaluate steel pipe toughness and resistance to fracture propagation. The CVN tests are generally conducted at multiple temperatures in order to capture the transition from the brittle regime with low impact energy to the ductile regime with higher impact energy. This transition behavior is represented by the variation with temperature in either measured impact energy or observed shear area percentage at the fracture cross section. Transition curves fitted from raw test data are used to derive key parameters, such as upper shelf impact energy (EUS) and Shear Area Transition Temperature (SATT). There are multiple procedures available to derive the transition curves, two of which are widely used. One was developed by M. Rosenfeld in 1996 by proposing the shear area as a sigmoidal function of temperature. The other is recorded in Section 9F.2.3 of API 579/ASME FFS-1 (2021), where the impact energy is expressed by a hyperbolic tangent function of temperature. The various fitting approaches may result in different derived EUS and SATT values, generating an unnecessary barrier for an operator comparing pipe properties or building up toughness distribution within a pipeline based on tests from multiple labs. For consistency, it may be necessary to repeat the transition curve fitting with a unified approach on raw test data instead of using lab reported EUS and SATT values. The derived EUS and SATT values of the tested specimen size can then be converted to the equivalent full-size values, such as by following Zhang and Rosenfeld’s (2022) approach, for the purposes of comparison, building up a database, determining distribution, or any other integrity assessment tasks. In this paper, it is first demonstrated that Rosenfeld’s sigmoidal equation and API 579/ASME FFS-1’s hyperbolic tangent equation are equivalent. Based on that similarity, a simplified fitting procedure is recommended for fitting the transition curves of shear area and impact energy over temperature. The mathematical detail of the fitting process is provided in the Appendix of the paper for implementation purposes. Finally, the performance of the procedure is demonstrated through worked examples.