Mechanical Field At The Tip Of Stress Corrosion Cracking In Safety End Dissimilar Metal Welded Joint Of Nuclear Power Plant

The Stress Corrosion Cracking(SCC)in weldments of Light Water Reactor(LWR)is a slow propagation procedure.The SCC crack tips are affected by the interactive effects of material,loading and hydrochemistry environment.SCC can introduce considerable hazard to the long term safe operation of nuclear power plants.Due to the heterogeneity of material and mechanical properties in safety end dissimilar weldments and unevenly distributed residual stress,the quantitative prediction of SCC propagation behavior under the interactive effects of which became complicated.In order to solve this problem,theoretical analyses,experiments and numerical simulations are combined to establish the method for the acquirement of local material properties and residual stress in safety end heterogeneous weldments.The method for analyzing the SCC crack tip field under the interactive effect of material heterogeneity and residual stress field in safety end weldment was studied.The main research content and results acquired are shown as follows:By means of the combination of press-in tests and finite element numerical simulation,the relationship between Vickers Hardness and mechanical properties of the main component materials of safety end are established.A low alloy steel SA508 weldment specimen was selected to acquire its mechanical properties by respectively using tensile tests and the reflection relationship of Vickers hardness.The discrepancy between the two methods is under 1.5%,implying the feasibility of the local material property acquisition method of weldments by combining press-in tests and numerical simulations.Using press-in tests to acquire the Vickers hardness distributions in the welding seam area of safety end weldment specimens,and the according material mechanical properties analyses were practiced.The distribution of material properties in the welding seam area were fitted into yield stress approximate curves which are continuously changing along the axial direction.The method of pre-defined field was used to realize the unevenly distribution of material property heterogeneity field of the dissimilar weldments in the numerical simulation experiment.A small stress maintaining device was developed to obtain the relationships between the residual stress and the Vickers hardness value and the indentation appearance of the metal specimens by applying the press-in test under loading(residual stress)combining with corresponding finite element numerical simulation.According to which,the local surface residual stress acquisition method using press-in tests was established.After the comparison of the boundary force loading method,the boundary displacement loading method,the predefined temperature field method and the predefined stress field import method,the predefined stress field import method is regarded the best to characterize the residual stress field of the safety end weldment.By means of pre-defined temperature field and pre-defined stress field import techniques,weldment models with non-uniform mechanical property and residual stress fields were established.The interactive effect of material heterogeneity and residual stress field was realized,as well as the influence of the interactive effect of those two fields on the driving force of crack propagation.By introducing two-dimensional growing crack and three-dimensional static crack into the weldment model,the crack propagation behavior in the welded joint under the interactive effect of unevenly distributed mechanical property and residual stress fields was studied.The finite element model of SS304 cladding layer with heterogeneous material properties was established,and the crack tip field distribution of SCC crack propagating from SS304 cladding layer to parental metal SA508 was studied.The SCC two-dimensional propagating crack and three-dimensional static crack were introduced separately into the central welding seam and the thermal affected zone on both sides of the weldment model to obtain the influence of the interactive effects of mechanical properties heterogeneity and residual stress field on the mechanical fields at SCC crack tips.