Study on the Applicability of GTN Model on Fracture Evaluation for Plate Specimen With a Surface Crack Using a BWR Reactor Pressure Vessel Material

Abstract

Abstract Cases of stress corrosion cracking (SCC) in Ni-base alloy weld metals welded to the low alloy steel (LAS) in reactor pressure vessel (RPV) components have led to discussions on the possibility of SCC propagating into the RPV. The JSME Fitness-for-Service code provides fracture evaluation methodology based on linear elastic fracture mechanics for SCC into the RPV, assuming neutron irradiation in the RPV beltline region. It is considered, however, more rational methodology of fracture evaluation can be applied to the RPV components such as a bottom head component of boiling water reactor (BWR) since the effect of neutron irradiation is considered to be negligible. In the previous studies, the fracture mode of RPV material has been investigated by using a large-scale, heavy forged steel part named “bottom head ring”, manufactured for a recent BWR. The authors reported that the results of fracture tests showed ductile failure and the fracture loads evaluated using the elastic-plastic fracture mechanics (EPFM) methodology have showed fairly good agreement with the maximum loads in the fracture tests on plate specimens with a semi-elliptical surface crack. In the present study, the applicability of damage mechanics analysis by using Gurson-Tvergaard-Needleman (GTN) model, which can analyze ductile fracture behavior due to nucleation, growth and coalescence of voids in metallic materials, to fracture evaluation for a RPV bottom head component of BWR was investigated. GTN model parameters of the tested material were investigated by comparison of the relationship between load and load-line displacement (LLD) obtained by the JIc fracture toughness tests with that obtained by finite element analysis (FEA) using GTN model. And a set of GTN model parameters that can reproduce the load-LLD relationship by the tests were determined. Fracture analysis for the plate specimens with a semi-elliptical surface crack were performed by using the GTN model parameters determined. It was found that relationship between load and crack mouth opening displacement (CMOD) obtained by the FEA showed a good agreement with that obtained by the fracture tests of the plate specimens. Furthermore, the fracture behavior observed in the FEA using GTN model also showed good agreement with that in the fracture test, such as necking behavior on the plate specimens, ductile crack extension traces on the fracture surfaces and crack penetration. The results obtained in the present study totally supported the applicability of the GTN model to RPV bottom head component.