Influence Of Residual Stress And Mechanical Heterogeneity On Mechanical Field At Crack Tip In Safety End Of Nuclear Power Plant

The welded residual stress is one of the main factors lead to stress corrosion cracking(SCC) in dissimilar metal welded joints of nuclear power plant safety end. Meanwhile, the material mechanical heterogeneity of welded joints makes the SCC crack tip mechanical fields more complicated. By means of theoretical analysis and Finite Element Method, the laws of stress and strain distribution at SCC crack tip under the interactive effect of residual stress and mechanical heterogeneity in nuclear power plant safety end were simulated and analyzed. The main research works are performed in the dissertation as follows:(1) Residual stress distribution in nuclear power plant safety end welded joints were simulated by separately using predefine temperature field method and predefine stress field from output file method, which indicate the predefine stress field from output file method is a more reasonable method for welding residual stress simulation;(2) Model of dissimilar metal welded joint under fixed value residual stress field was established, influence rules of crack length to crack tip stress, strain and fracture parameter ware analyzed. The results shows that before crack length is longer than one third of pipe wall thickness, the stress, strain and fracture parameter increase with crack length, then tend to be steady;(3) Circular section model of nuclear power plant primary circuit safety end welded joint was established, changing rules of crack tip stress, strain a nd fracture parameter in different position under residual stress field were analyzed. The results shows crack tip stress and strain distribution decrease with crack length. When crack length get close to 30 mm, normal stress of crack tip turn to be negative, which may cause SCC growth stagnate.(4) Influence rules of material mechanical heterogeneity to crack tip mechanical field close to welding interface in welded joints were researched. The results indicate the higher the hardness of material area is, the higher the stress and the lower the plastic strain of crack tip area will be;(5) Stress and strain field around type I and II mixed crack in fusion boundary of dissimilar welded joint were simulated. Analyses indicate high stress and low strain area around welding interfacial crack tips tend to be in high hardness material area, which may cause crack propagation deviation.