Local approach to fracture based prediction of the ∆T56J and ∆TKIc 100 shifts due to irradiation for an A508 pressure vessel steel. Engineering Fracture Mechanics, Elsevier, 2006, 73, pp.

No abstract

Nuclear pressure vessel steels are subjected to irradiation embrittlement which is monitored using Charpy tests. Reference index temperatures, such as the temperature for which the mean Charpy rupture energy is equal to 56 J (T56J), are used as embrittlement indicators. The safety integrity evaluation is performed assuming that the shift of RTNDT due to irradiation is equal to the shift of T56J. In this work a material model integrating a description of viscoplasticity, ductile damage and brittle fracture is used to simulate both the Charpy test and the fracture toughness test (CT geometry). The model is adjusted on an unirradiated material. It is then applied to irradiated materials assuming that irradiation affects hardening. It is shown that irradiation probably also affects brittle failure. The shift of RTNDT and the predicted shift of T100MPam are then compared for a given level of irradiation.

The present study investigates the temperature and specimen geometry dependence of cleavage fracture micromechanisms of a French pressure vessel steel (A508 Cl.3) over a temperature range which covers the lower shelf up to the DBT fracture toughness range. Notched tensile (NT) specimens with 3 different radii and CT specimens with two different thicknesses (12.5 and 25mm) were used to study the effect of temperature and geometry (constraint) on (i) the nature of the defects involved in the cleavage triggering, (ii) the mechanical parameters (stress and strain) values (iii) the Weibull parameters. The fractographic investigations showed that Manganese sulfide clusters and/or stringers were involved in the cleavage initiation process on the notched geometry at all temperatures from ¢ 130 £ C whereas they were never encountered in the cracked geometries. It is shown that only NT tests with a mean fracture strain lower than 25% have to be considered to make sure that the same nature of defects will be involved in the cleavage initiation. The mean value of the local stress was found to be approximately constant for NT and CT25 specimens over the temperature range [¢ 150. Also a unique set of Weibull parameters was found to describe all the NT tests over this temperature range.