Stress-based Study On The Initiation Of Stress Corrosion Cracking Of Irradiated 304 Stainless Steel

Operation experience of in-service nuclear power plants shows that Irradiation-Assisted Stress Corrosion Cracking(IASCC)occurs in core structural materials such as austenitic stainless steel(SS)due to long-term service under intense neutron irradiation and in high temperature and high pressure corrosive water.The failure of core structural materials and components caused by this phenomenon is one of the key factors affecting the safe and efficient operation of nuclear power plants.Over the years,different IASCC models have been proposed by researchers,but limitations are shown in answering the question of when cracks initiation occurs and what conditions are required.The purpose of this study is to establish a three-dimensional(3D)model with real microscale information,to study the cracking phenomenon of austenitic SS 304 from the mechanical perspective through numerical calculation,and to explore the feasibility toward a local stress-based cracks initiation criterion.In the experimental part,serial sectioning along with Scanning Electron Microscope(SEM)observations/Electron Backscatter Diffraction(EBSD)was used to reconstruct the cracks distribution pattern and 3D microstructure of a tensile sample irradiated by protons.A methodology was developed to estimate grain boundary(GB)normal directions from the 3D microstructure.The problem of EBSD image distortion was recognized and solved by the combination of “coarse correction” and “fine correction”.In the simulation part,a Fast Fourier Transform(FFT)solver AMITEX＿FFTP was used to calculate the stress distribution in the sample during the tensile test,combined with a calibrated crystal plasticity constitutive model.The values of Intergranular Normal/Shear Stress(INS/ISS)were calculated by combining the simulation results with the estimated GB normal directions.By comparing the actual cracks distribution on the sample surface and the calculated INS/ISS,it is found that:(1)there exists obvious INS localization phenomenon,which is consistent with the observed cracks morphology;(2)compared to the non-cracked GBs,the cracked GBs have higher probability to be at high INS state,showing that the INS does play a role in the cracks initiation;(3)a predicted cracks map is obtained by considering the average INS at cracked GBs as threshold value.Most of the actual cracks are shown on the predicted map but the map has exaggerated the cracking sensitivity of the sample;(4)the ISS shows little influence on the cracks initiation.The work in this thesis provides an approach for the study of core structural material cracking.The modeling method and data processing and analysis process have been tested.Results of this thesis have certain reference value for the cracks initiation research.