Applicability of Fracture Toughness Prediction by Coupled Damage Mechanics Model and Brittle Fracture Model for Ductile

Brittle Transition Temperature Region for Reactor Pressure Vessel Steels

Abstract

For nuclear safety, fracture evaluation of reactor pressure vessels (RPV) under neutron irradiation is key issue. In ductile to brittle transition temperature (DBTT) region, ferritic steel which is used as material of RPV has a large scatter and it becomes important to know the accurate scatter of an irradiated material because of less margin of RPV's integrity after a long term operation. Fracture toughness from standard fracture toughness specimens such as CT specimens is often used for fracture evaluations, but it is well known that it has a large constraint, which causes lower toughness than that of flawed structures, such as a RPV with a small flaw. In this paper to establish a more precise fracture evaluation method in DBTT region considering the constraint effects for an irradiated RPV with a postulated small flaw, a coupled model of damage mechanics for ductile fracture and brittle fracture model (Beremin model) for cleavage fracture was applied for correction of the effect of a small ductile crack growth on the stress-strain field. To confirm the validity of the method, as the first trial, fracture tests using CT specimens and flat plate specimens with surface flaws were performed in several temperature conditions and the predicted fracture toughness by the model was compared with the experimental fracture toughness data.