Study on Ductile Crack Extension and Fracture Behavior in Plate Specimen With a Semi-Elliptical Surface Crack Using a BWR Reactor Pressure Vessel Material

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. In the previous study, the fracture mode of dissimilar metal welds (DMWs) of LAS and Ni-base alloy weld has been investigated by using a large-scale, heavy forged steel part named “bottom head ring” of the RPV, manufactured for a recent boiling water reactor (BWR). 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. All the fracture tests have successfully demonstrated the applicability of the fracture assessment methodology based on EPFM to the DMWs of RPV components. In the fracture tests on the plate specimens, a periodic unloading condition was applied in order to obtain specimen compliance: inverse of the gradient in the load-displacement relationship under unloading. The compliances were evaluated from the crack mouth opening displacement (CMOD) data that have been obtained for large CMOD of more than 10 mm at fracture. The results of detailed evaluations of the ductile crack extension behavior based on the evaluated compliance indicated that the ductile crack extension occurred near the maximum load. Evaluations of the J–Δa relationships of the plate specimens based on the CMOD data were also performed by finite element analyses (FEA) according to the proposed method. The J–R curves obtained for the plate specimens commonly showed similar behaviors of having significantly higher J for the small Δa range from 0 to about 2 mm than that for the compact tension (C(T)) specimen, indicating a difference in the plastic constraint in the crack tip between the two types of specimens. These results demonstrate that the method proposed in this study is highly useful for evaluating the J–Δa relationship of the fracture test specimen. Fracture assessments using the J–R curves of the plate specimens provided a better prediction of the fracture load than that using the J–R curve of the conventional C(T) specimen. All the results supported the applicability of the EPFM methodology in the fracture evaluation, shown in the previous work.