Study On The Local Ductile Damage And Fracture Behaviour In Dissimilar Metal Welded Joint In Nuclear Power Plants

Dissimilar metal welded joints are widely used in primary water systems of pressurized water reactors in nuclear power plants, which are mainly used to join the ferritic steel pipe-nozzles of the pressure vessels to the austenitic stainless steel safe-ends. Operating experience has shown that the dissimilar metal welded joints can remarkly affect the plant availability and safety. In order to have an accurate integrity assessment for the dissimilar metal welded joints, the local ductile damage and fracture behaviour should be understood. In this paper, the finite element method based on GTN model is used to investigate the ductile crack growth behavior in the three-point bending specimens of the dissimilar metal welded joint. The local J-R resistance curves and crack growth paths are calculated for cracks with different locations and sizes in the dissimilar metal welded joint. The effects of crack locations, crack sizes and local strength mismatch on local fracture resistance, crack growth paths and integrity assessment for the dissimilar metal welded joint are analyzed. The main results obtained are summarized in following:(1) A parametric study is carried out to assess the effects of variation GTN model parameters on the J-R resistance curves, and the GTN model parameters for dissimilar metal welded joint materials are determined by a combination of numerical results and experimental data comparison. Hence, the finite element method based on GTN model is a good way to investigate the local damage and fracture behaviour for the dissimilar metal welded joint specimens and structures.(2) The initial cracks with different locations in the dissimilar metal welded joint have different crack growth resistances and growth paths. When the initial crack lies in the center of base metal or weld metal, the crack tip plastic and damage zones are symmetrical, and the crack grow path is nearly straight along the initial crack plane. But for the interface cracks and near interface cracks, the crack tip plastic and damage zones are asymmetric, and the crack growth path has significant deviation phenomenon. The crack growth tends to deviate into the material whose yield stress is lower between the two materials on both sides of the interface. The local strength mismatches make the high stress triaxiality and plastic strain distributions mainly occur in the material with lower yield stress, and lead to crack growth paths deviation and different ductile crack growth resistances. (3) The J-R resistance curve of the A508/52Mb interface crack is lowest, and it is a weak region of occurring failure and fracture in the dissimilar metal welded joint. In the absence of the local mechanical and fracture resistance properties of heat affected zone and interface region, the using of the J-R resistance curve of base metal A508or buttering metal52Mb following present codes will unavoidably produce non-conservative assessment results. Therefore, it is recommended to obtain and use local mechanical and fracture resistance properties of all regions of the dissimilar metal welded joint for accurate integrity assessment.(4) The finite element method based on GTN model can successfully calculate and predict the local fracture resistance and crack growth path in the dissimilar metal welded joint, and it can be used to predict the local ductile damage and fracture behaviour in dissimilar metal welded joint in nuclear power plants.