Irradiation damage effects on the fracture toughness transition curve shape for reactor pressure vessel steels

A critical issue in the irradiation damage evaluation of reactor pressure vessel steels is the effect of irradiation damage on the fracture toughness transition curve. Normally, surveillance testing is only directed towards the estimation of the irradiation induced temperature shift in the fracture toughness transition curve. The question how and if the irradiation damage also affects the shape of the fracture toughness transition curve is left open. Present assessment procedures assume the fracture toughness transition curve shape to remain unaffected by irradiation damage, but the assumption has until now lacked experimental and theoretical verification. Also, several presently applied cleavage fracture models show that the fracture toughness should be inversely related to the material's yield stress. Thus, these models indicate that the fracture toughness transition curve shape should be affected by irradiation damage. In order to clarify the issue, a micromechanism based statistical cleavage fracture model is applied to analyze existing fracture toughness data. It is shown that the fracture toughness transition curve shape is really insensitive to irradiation damage effects. Furthermore, it is shown that the majority of ferritic steels have similar fracture toughness transition curve shapes, thus making it possible to describe all the steels' fracture toughness temperature dependence with a single curve. Finally, a theoretical reasoning for the observed behavior is given, based on the cleavage fracture model.