Radiation Damage Assessment by the Use of Dynamic Toughness Measurements on Pre-Cracked Charpy-V Specimens

The small size of Charpy-type surveillance specimens can make it difficult to obtain fracture toughness measurements which are directly transferable to real-life structures. FEM calculations show that the pre-cracked Charpy specimen (PCCv) tends to lose constraint before the onset of cleavage. However, if the PCCv specimen is loaded under impact conditions, even for moderately strain-rate sensitive materials the deformation level at which loss of constraint occurs is significantly higher, due to the increase in flow properties. This implies that the ductile-to-brittle transition temperature measured under high rate conditions is shifted to higher temperatures with respect to quasi-static toughness tests. This circumstance has been verified through instrumented impact tests on PCCv specimens of two reactor pressure vessel steels, in both baseline and irradiated conditions. The dynamic reference temperature shift due to irradiation has been evaluated and compared with its static counterpart and with index temperature shifts based on conventional impact parameters. Furthermore, the possibility of predicting the dynamic T₀ from the knowledge of the static values, the loading rate and the material's yield strength, has been investigated.