Study On The Effect Of Material Inhomogeneity On Mechanical State At Crack Tip Of The Safety End Welded Joint In Nuclear Power Plant

The safety end dissimilar metal weld joint(DMWJ)in the primary circuit of the pressurized water reactor(PWR)nuclear power plants is a special structure used to connect the reactor pressure vessel and the primary circuit pipeline,which is usually employed in the demanding environment with high temperature and high pressure water.Because of the difference in the properties of the dissimilar metal and the influence of the welding process on structures,the safety end DMWJ not only exhibit a heterogeneity microstructure and mechanical properties,but also may have various crack defects,making it a weak area that is prone to failure or fracture in service.Therefore,it is significant to study the mechanical state of the defect-containing welded structures in demanding environments.Using a combination of theoretical analysis,mechanical test,and elastic-plastic finite element method,a typical example SA508-52M-316L DMWJ was taken as the research object.The distribution of mechanical properties of SA508-52M-316L DMWJ was analyzed,and the changes in the stress state of the environmental assisted cracking(EAC)crack tip area under the interaction of complex external loads and inhomogeneous mechanical properties were investigated in this dissertation.The main research content and conclusions acquired are presented as follows:(1)Based on the traditional indentation test theory,an indentation test platform was designed and built to obtain the mechanical properties of the local region of the DMWJ.At the same,the finite element model for describing the indentation test process was established.The measurement accuracy of the self-built indentation test platform was verified by comparing the indentation test and finite element simulation results.(2)The relationship between the indentation response and the mechanical parameters of three different materials used in PWR were analyzed by using the indentation test combined with the finite element inversion method.The method for calculating the material mechanical parameters through the indentation parameter was established,and the reliability was verified by numerical simulation.By using combined elastoplastic finite element analysis(EPFEA)and the indentation test,the distribution law of the mechanical properties in the local region near the weld-fusion line of the DMWJ was obtained.(3)The DMWJ model with continuous transition mechanical properties was established by using the predefined temperature field method with the actual mechanical properties of DMWJ obtained from the indentation test.On this basis,the difference between the continuous transition model and the "sandwich" model in characterizing the stress-state distribution of DWMJ was analyzed by using numerical simulation.(4)The microstructure of the typical DMWJ was analyzed by a couple of techniques,such as optical microscope(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS),electron backscatter diffraction(EBSD),and Vickers hardness.The effect of inhomogeneous microstructure and element distribution on mechanical properties of welded joints was studied,and the potential cause of stress corrosion cracking in the DMWJ was analyzed.On this basis,the relatively weak position in the DMWJ was determined.(5)Combined with the results of microstructure analysis,the interface and sub-interface crack were introduced into the DMWJ with continuous transition model and the "sandwich"model,respectively.The difference of the stress state at the crack tip of stress corrosion cracking(SCC)between two models(i.e.continuous transition model and "sandwich" model)were analyzed under the coupling effect of internal pressure and external load.The variation of stress and strain on the crack tip with different crack lengths and positions were investigated.(6)A full-scale three-dimensional model of safety end DMWJ in the primary circuit of PWR with axial inner surface cracks was established under the consideration of the complex external load and inhomogeneous distribution of mechanical properties.Based on the consideration of the distribution profile of the Mises stress,plastic strain,and stress triaxiality as the crack tip mechanical state along crack front,the crack growth stability with different crack shapes and positions were analyzed.