Experimental validation of the Charpy-impact based reference temperature T0 of SA 508 heavy forgings of nuclear components and development of a Charpy-based fracture toughness lower bound curve

A reliable estimate of fracture toughness has recently been achieved using Charpy impact tests and a semi-empirical correlation that was successfully applied to various reactor pressure vessel materials. Moreover, the number of input parameters was reduced to the ductile-to-brittle transition temperature and upper shelf energy to derive a meaningful reference temperature T₀ that is found to be in close agreement with the reference temperature determined from actual fracture toughness tests according to E1921 standard. However, the number of tested specimens for each material and condition is usually small, typically about 12 specimens each for the Charpy impact and fracture toughness tests, leading to a significant scatter band, of the order of ±43 °C assuming a 95 %-confidence level. In the present work, the correlation is applied to a series of 24 A508 Cl.3 heavy forgings for which a much larger number of test specimens is available. The results confirm the performance of the correlation with respect to fracture toughness but also with respect to the reference temperature derived from the Charpy impact tests and the scatter band is reduced to ±21 °C, showing that the applied formulae are very suitable for estimating the reference temperature for large inhomogeneous forgings. Moreover, the fracture toughness lower bound curve could also be derived based solely on T_41J and envelops reasonably well the experimentally-measured fracture toughness data, covering more than 99 % of the data.