In United States and German licensing rules according to American Society of Mechanical Engineers (ASME) Code and KTA-Regel for safety against brittle fracture of light-water reactor (LWR) pressure vessel steels, the nil ductility transition temperature plays an important role. It is based on the drop-weight test after Pellini. Together with the results from the Charpy test, the nil ductility transition (NDT) temperature is used to determine the reference temperature R T N D T . This temperature is necessary for the modified porse-diagram, the (FAD) diagram and also for the fracture mechanics diagram. The NDT temperature of unirradiated specimens can be determined without any difficulties, whereas the irradiation and testing of such specimens, mainly for surveillance purposes, is usually not possible. For these reasons the temperature shift Δ T 41 J as measured in the Charpy test is normally used.

Numerous tests with Charpy and drop-weight specimens irradiated at the GKSS-Research Center have shown that the measured Δ T 41 J shift gives very conservative results. Earlier results from instrumented Charpy and drop-weight tests have demonstrated that the temperature shift of the crack arrest load can be correlated very well with that measured in the drop-weight test.

Because the crack-arrest load is a function of the proportion of cleavage in the broken Charpy specimen (fast running crack length in the load-deflection curve), it could be shown, that the NDT temperature in the drop-weight test can be correlated with that temperature, where the portion of cleavage in the Charpy fracture surface has a value of about two thirds of the broken area T 66 % . Consequently the same is valid for the temperature shifts caused by irradiation. Therefore, it should be possible to determine ΔNDT in an uninstrumented Charpy test. These relations have been obtained for forged pressure vessel steels and weld material of the types 22NiMoCr37 (ASTM A 508, Cl 2) and 20MnMoNi55 (ASTM A533 B).