Simulation Of Crack Initiation And Propagation Behavior Of Austenitic Stainless Steel Based On XFEM

As the development of nuclear power industry in China toward the direction of integration,large-scale and longevity,the construction of a new generation of safe and efficient nuclear power plants has become important for Chinese energy industry.Catastrophic accidents caused by stress corrosion of reactor pressure vessels in primary nuclear power system have attracted wide attention.For a long time,the stress corrosion has been greatly focused and studied.As stress corrosion is a complex process influenced by the interaction of mechanics and environmental factors,so far it is still a challenging problem.The formed pit of pitting corrosion in the pipeline of the 316L stainless steel reactor pressure vessel was investigated experimentally and numerically in this paper.The influence of the shape and size of corrosion pits on the stress distribution were studied.By revealing the stress concentration position,the XFEM(extended finite element method)was used to investigate the influences of the initial crack condition,the stress,double cracks and the second corrosion pit on the initiation and the propagation of crack from corrosion pits.The conclusions were derived as follows:(1)The variations of morphologies and the sizes of pitting corrosion pits with the corrosion time were studied by the corrosion immersion experiment.Typical semi-ellipsoidal,conical and cylindrical corrosion pits were generated in 316L stainless steel that immersed in 6wt.%Fe Cl₃ solution.The longer the corrosion time was,the larger the corrosion pits were.The size of micro-pitting pits ranged from 5 to40?m.(2)Based on the pitting corrosion pits observed in the corrosion immersion experiment,the finite element method was used to numerically simulate the stress distribution of 316L stainless steel plates with different pits under 10MPa external stress.The results showed that the stress concentration on semi-ellipsoidal,conical and cylindrical corrosion pits was mainly in the mouth and the bottom of the pit.Stress concentration factor increased with the increase of the ratio of the depth to the diameter.(3)By prefabricating cracks at the stress concentrated position of pitting corrosion pits,their propagations were simulated with the XFEM.It was found that no matter whether the cracks were starting from the bottom or the mouth,their propagation directions were the same.Under the same initial conditions,the crack propagations at the mouth of semi-ellipsoidal corrosion pits and conical corrosion pits were easier than that at the bottom.With the increase of the applied external stress,the crack propagation degree of corrosion pits with three morphologies also increased gradually.When prefabricated cracks were added at the mouth and bottom of the semi-ellipsoid corrosion pits,the latter expanded earlier with a greater extension than the former.(4)A model with the secondary corrosion pit was established.The stress concentration of the pit was shifted to the mouth of the secondary pit.Under the effect of the external stress,the crack spread along the surface of the main pit,extended toward the mouth of the pit and finally permeated the surface of the material.The obtained results of this paper provide a theoretical reference for the research of the propagation characteristics of stress corrosion crack in reactor pressure vessels.