IRRADIATION DAMAGE EFFECTS ON THE FRACTURE-TOUGHNESS TRANSITION CURVESHAPE FOR THE REACTOR PRESSURE-VESSEL STEELS

A critical issue in the irradiation damage evaluation of reactor press ure vessel steels is the effect of irradiation damage on the fracture toughness transition curve. Normally, surveillance testing is only dir ected towards the estimation of the irradiation induced temperature sh ift in the fracture toughness transition curve. The question how and i f the irradiation damage also affects the shape of the fracture toughn ess transition curve is left open. Present assessment procedures assum e the fracture toughness transition curve shape to remain unaffected b y irradiation damage, but the assumption has until now lacked experime ntal and theoretical verification. Also, several presently applied cle avage fracture models show that the fracture toughness should be inver sely related to the material's yield stress. Thus, these models indica te that the fracture toughness transition curve shape should be affect ed by irradiation damage. In order to clarify the issue, a micromechan ism based statistical cleavage fracture model is applied to analyze ex isting fracture toughness data. It is shown that the fracture toughnes s transition curve shape is really insensitive to irradiation damage e ffects. Furthermore, it is shown that the majority of ferritic steels have similar fracture toughness transition curve shapes, thus making i t possible to describe all the steels' fracture toughness temperature dependence with a single curve. Finally, a theoretical reasoning for t he observed behavior is given, based on the cleavage fracture model.