Local Fracture And Mechanical Bahavior In A Safe-end Dissimilar Metal Welded Joint In AP1000Nuclear Power Plant

In this paper, three-dimensional finite element analysis models were built for a dissimilar metal weld joints (DMWJ) in AP1000nuclear pressure vessels. A hypothetical internal-suface crack was set in the DMWJ. The numerical simulations are based on Gurson-Tvergaard-Needleman model for ductile damage. Local fracture and macroscopical mechanical behavior of the crack with different locations and various sizes in the DMWJ were investigated. The bending moment-rotation curves (M-θ curves) and crack extension resistance curves (CTOD-Δa curves) were calculated. The characteristic loads (limit load, maximum load, penetrating load and ultimate load) were defined and determined. Crack growth paths and local stress-strain distributios in front of the crack tips have been simulated. The effects of crack locations, material property mismatch, initial crack size, structural geometric size, local material mechanics performance of heat-affected-zone (HAZ) and fusion zone (FZ) on local fracture and mechanical behavior of the DMWJ have been analyzed. The main results obtained are as follows:(1) The effects of crack locations on plastic deformation and crack extension resistance curves are significant. In general, the interface crack growth resistance is lower than those of the homogenous materials on both sides of the interface, and the corresponding characteristic loads and carrying capacity of the structure are lower for interface cracks. For interface cracks and near interface cracks, the distribution of the plastic doformation and triaxial stress in front of cracks are always asymmetric, and higher values always tend to occur in the materials with lower yield stress (soft material), and the cracks also tend to extend in soft material. Fracture process and mechanical behavior of DMWJ are mainly dominated by tensile mechanical properties of local material in front of crack, ductile crack extention resistance and local strength mismatch.(2) For A508/Alloy52M interface crack, the effects of initial crack sizes on macroscopic moment-rotation cuves, characteristic loads, crack extension resistance and key parameters of leak-before-leak (LBB) after large plastic deformation are significant. The crack extension resistance of deep and long cracks is low due to the large geometric constraint. The degree of safety is reduced when the cracks with certain value of a/t grow longer.(3) The width of weld and buttering has a significant influence on macroscopic mechanical behavior of the DMWJ. Instability load and corresponding rotation angle are reduced due to decreasing the width of weld and buttering, which also resulted in the reduction of load-bearing capacity of structure and crack extension resistance curves. Therefore, the width of weld and buttering should not be too small in design of DMWJ.(4) When the cracks were located in the A508#Alloy52Mb zone and the local material property in HAZ and interface zone were considered, the areas under the moment-rotation curves and the characteristic loads decreased with ring. This means that the load-bearing capacity of the DMWJ was reduced. Therefore, ignoring local mechanical and fracture properties of materials in HAZ and FZ would result in nonconservative results in safty assessment of dissimilar metal weld joint.