Estimation of NDT and crack-arrest toughness from Charpy force-displacement traces

A force-displacement trace of a Charpy impact test of a reactor pressure vessel (RPV) steel in the transition range has a characteristic point, the so-called `force at the end of unstable crack propagation,' F_a. A two-parameter Weibull probability function is used to model the distribution of the F_a in Charpy tests performed at Oak Ridge National Laboratory (ORNL) on different RPV steels in the unirradiated and irradiated conditions. These data have a good replication at a given test temperature, thus, the statistical analysis was applicable. It is shown that when temperature is normalized to T_NDT, using (T-T_NDT), or to T_100a, using (T-T_100a), the median F_a values of different RPV steels have a tendency to form the same shape of temperature dependence. Depending on normalization temperature, T_NDT or T_100a, the results suggest a universal shape of the temperature dependence of F_a for different RPV steels. The best fits for these temperature dependencies are presented. These dependencies are suggested for use in estimation of nil-ductility transition (NDT) or T_100a from randomly generated Charpy impact tests. The linear least squares and maximum likelihood methods are used to derive equations to estimate T_NDT and T_100a from randomly generated Charpy impact tests.