He Xue

Effects of Stress Intensity Factor on Electrochemical Corrosion Potential at Crack Tip of Nickel-Based Alloys in High Temperature Water Environments

Abstract Stress will enhance the local anodic dissolution rate of nickel-based alloys at crack tip and accelerate the crack propagation in high temperature water environments. The relationship between stress and corrosion was studied by the elastic plastic finite element method. The effects of stress intensity factor on the surface electrochemical corrosion potential (ECP) at the crack tip of alloys 600 with a 1T-CT specimen in simulated boiling water reactor environment were analyzed. The effects of plastic deformation and elastic deformation on the changes of the electro-chemical corrosion potential around the crack tip were also discussed. The results indicate that the effects of stress intensity factors on ECP changes of the crack tip surface have the maximum values in the front of the crock propagation, and they decrease gradually towards the sides of the crack. The values of ECP changes increase with the increase of stress intensity factor. The effect of plastic deformation is more obvious than that of elastic deformation.

Numerical investigation on stress corrosion cracking behavior of dissimilar weld joints in pressurized water reactor plants

There have been incidents recently where stress corrosion cracking (SCC) observed in the dissimilar metal weld (DMW) joints connecting the reactor pressure vessel (RPV) nozzle with the hot leg pipe. Due to the complex microstructure and mechanical heterogeneity in the weld region, dissimilar metal weld joints are more susceptible to SCC than the bulk steels in the simulated high temperature water environment of pressurized water reactor (PWR). Tensile residual stress (RS), in addition to operating loads, has a great contribution to SCC crack growth. Limited experimental conditions, varied influence factors and diverging experimental data make it difficult to accurately predict the SCC behavior of DMW joints with complex geometry, material configuration, operating loads and crack shape. Based on the film slip/dissolution oxidation model and elastic-plastic finite element method (EPFEM), an approach is developed to quantitatively predict the SCC growth rate of a RPV outlet nozzle DMW joint. Moreover, this approach is expected to be a pre-analytical tool for SCC experiment of DMW joints in PWR primary water environment.

Calculations and modeling of material constants in hyperbolic-sine creep model for 316 stainless steels

The material constants calculation models for hyperbolic-sine creep model were proposed. The material constants used in hyperbolic-sine creep model for 316 stainless steel were calculated due to the models proposed and experimental data in the temperature range from 873K to 1023K. The relationships between material constants of 316 stainless steel creep model and temperature were obtained by curve fitting. The creep rate predict model of 316 stainless steel with only stress and temperature was also developed, the creep rates predicted were in good agreement with experimental data.

Numerical Analysis of the Mesoscale Mechanical Field at the Intergranular Crack Tip

Austenitic stainless steel components operated in the light-water reactor (LWR) environment are susceptible to intergranular corrosion cracking (IGSCC). It is well known that the environment and materials near the crack tip are the most important factors affecting the IGSCC crack growth rate in the high-temperature aqueous environments. An intergranular crack growth model based on the multi-scale method is proposed in this study to accurately capture the stress-strain state at the crack tip of IGSCC. This model is implemented in ABAQUS via a sub-model technique. The effect of the extension direction of the kinking cracks on IGSCC behavior is studied for better understanding the effect of stress-strain field at crack tip on the crack growth rate in LWR.Copyright

Effects of Grain Orientation on Stress State near Grain Boundary of Austenitic Stainless Steel Bicrystals

The stress state at the crack tip of structural components in nuclear power plants used in the SCC quantitative prediction models is based on the assumption that the polycrystalline material is isotropic and homogeneous at present. However, the crystals in polycrystalline materials are anisotropic with different orientations, which would induce a nonuniform stress to cause the initiation and propagation of SCC. By using a finite element method, the elastic responses of anisotropic behaviors of austenitic stainless steel bicrystals are studied. The results indicate that the stress distribution near GBs depends strongly on the crystal orientation. A larger Mises stress concentration exists on the GB with larger stiffness along the load direction. The Mises stress difference is higher in the bicrystal with bigger elastic modulus difference of two neighboring grains along the tensile axis. In the bicrystal with GB perpendicular to the tensile axis, the grain orientation has little effects on the Mises stress far from the GB in both grains. The strain inconsistency in bicrystals is affected by the mismatch of two neighboring grains. The larger the elastic modulus differences between two neighboring grains caused by misorientation, the larger the strain inconsistency in the bicrystal.

Micro-mechanical State at Tip of Environmentally Assisted Cracking in Nickel-based Alloy

Abstract Nickel-based alloys and austenitic stainless steels are widely used in the structures of primary circuit of nuclear power plants. Environmentally assisted cracking (EAC) of these materials is one of the most significant potential safety hazards in the primary circuit of nuclear power plants. Researches show that EAC in nickel-based alloy is a process of oxide film rupture and reform at the tip of EAC in the high temperature water environment of nuclear power plants. To understand the micro-mechanical state at the tip of EAC, the stress-strain in the oxide film and the base metal at the EAC tip was simulated and discussed using a commercial finite element analysis code, which provides a foundation to improve the quantitative predication accuracy of EAC growth rate of nickel-based alloys and austenitic stainless steels in the important structures of nuclear power plants.

Analysis of Safety Hazard Factors and Fuzzy Comprehensive Evaluation of Mine Main Ventilator

Forecasting and evaluating the safety of mine main ventilator are helpful to prevent mine ventilation safety accidents from happening. First, on the basis of collecting and analyzing a large quantity of fault information of main ventilator in the nationwide coal mine, the index system was established for safety evaluation of coal mine ventilator. Second, the remark set was set up. Then, weights of safety estimation index of mine main ventilator was determined by using Analytic Hierarchy Process(AHP). With the experts grading method, single-factor matrix of fuzzy evaluation was established. Last, mathematical model of fuzzy comprehensive evaluation was settled for mine main ventilator based on fuzzy theory and used to evaluate the safety of the coal mine main ventilator. The current safety assessment level of coal mine ventilator is “basically safe” and the result can be used to make decisions.

Effects of creep on mechanical fields nearby crack tip in weld joints

Abstract Stress and strain differences nearby crack tip caused by material heterogeneity of welded joints will eventually affect the stress corrosion cracking growth rate. By using 316 stainless steel and Inconel 82 as the base metal and weld metal, the effects of creep on mechanical fields nearby crack tip in weld joints were studied by finite element method with one inch compact tension specimen. The results indicate that yield strength has greater impact on the mechanical field nearby crack tip; the material which has lower yield stress and larger creep rate will lead to lower stress and larger strain at crack tip. When two different metals are welded together, the stress and strain in weld fusion line specimen is mainly dominant by creep of the metal with larger creep strain rate. Moreover, the creep effects on equivalent plastic strain and plastic zone are insignificant no matter where the crack tip locates in weld joints.

Effect of Welded Mechanical Heterogeneity on Fracture Parameters at the Crack Tip

The welded joint is the critical area in the structure integrity investigation. To understand the influence of the mechanical heterogeneous on the fracture parameters in welded joint, the represented way of the welded mechanical heterogeneity and its effect on the mechanical parameters at the crack tip are analyzed by using an elastic-plastic finite element analysis software in this paper. The investigated results indicate that continuous change of material mechanical properties could express the effect of the actual welded mechanical heterogeneity on the fracture mechanical parameters change in the welded joint more accurately, which provide a new approach on the fracture problem investigation in welded joints.

Effect of Creep on Stress and Strain at the Tip of Stress Corrosion Cracking in High Temperature Water Environments

Stress and strain at the crack tip are main mechanical parameters which estimate the stress corrosion cracking rate in metals, and the creep of metals in high temperature and high pressure environment will lead to the redistribution of stress and strain nearby the crack tip. The effects of creep on stress and strain nearby the crack tip are discussed by using 1T-CT specimen and finite element method in this study. The investigated results indicate that both increasing of temperature and stress intensity factor would induce the equivalent creep strain increases at the crack tip.