Environmentally Assisted Cracking: Theory and Practice: Stress Corrosion Cracking I
Sponsored by: TMS Structural Materials Division, TMS: Corrosion and Environmental Effects Committee, TMS: Mechanical Behavior of Materials Committee
Program Organizers: Bai Cui, University of Nebraska–Lincoln; Raul Rebak, GE Global Research; Sebastien Dryepondt, Oak Ridge National Laboratory; Srujan Rokkam, Advanced Cooling Technologies
Monday 2:00 PM
February 27, 2017
Location: San Diego Convention Ctr
Session Chair: Gary Was, University of Michigan; Sergei Shipilov, Oak Ridge National Laboratory
2:00 PM Invited
The Importance of Radiation and Deformation in Environmentally Assisted Cracking: Gary Was1; Drew Johnson1; Ian Robertson2; Diana Farkas3; 1University of Michigan; 2University of Wisconsin; 3Virginia Tech
While irradiation is known to have profound consequences on the microstructure and mechanical properties of metals and alloys, its impact on environmental degradation is equally important. The combination of radiation and a chemically aggressive environment gives rise to unique degradation modes such as irradiation assisted stress corrosion cracking. Recent evidence suggests that localized deformation as a potential factor driving the degradation. More specifically, it is the high local elastic stress at dislocation channel-grain boundary intersections that is believed to be the key factor in crack nucleation. This talk will examine the response of irradiated (in reactor or with protons) commercial and high purity austenitic stainless steels to stress in high temperature water. The ways in which dislocation channels interact with grain boundaries is described along with data, modeling and calculations that provide evidence that high local stress is the key factor in initiating IG cracks.
Correlating Grain Boundary Microchemistry in Austenitic Stainless Steels with Their Susceptibility to Irradiation-assisted Stress Corrosion Cracking: Mo-Rigen He1; Drew Johnson2; Bai Cui3; Gary Was2; Ian Robertson1; 1University of Wisconsin-Madison; 2University of Michigan; 3University of Nebraska-Lincoln
Irradiation-assisted stress corrosion cracking (IASCC) is a primary cause of failure for the austenitic stainless steels (SS) used in light-water nuclear reactors. However, it remains unclear why intergranular cracking is only initiated at a limited fraction of discontinuous intersections between dislocation channels (DCs) and grain boundaries (GBs). Here we present electron microscopy characterization of several sites of discontinuous DC-GB intersections in a 13Cr15Ni SS test bar after proton irradiation and straining in high-temperature water. A strong segregation of Ni and weak depletion of Cr are observed at random high-angle GBs, both enhanced in the vicinity of surface oxides or crack tip. Moreover, such radiation-induced segregation (RIS) can be spatially inhomogeneous, the cracked GB sites show significantly weaker RIS than un-cracked GBs. The critical role of GB microchemistry (specifically, Ni segregation) in the susceptibility to IASCC may be attributed to the modification of GB microstructure and the mechanisms of slip transfer.
Fundamental Mechanisms of Mitigating Stress Corrosion Cracking of Austenitic Stainless Steels by Laser Shock Peening: Bai Cui1; Fei Wang1; Xiaoxing Qiu1; Chenfei Zhang1; Yongfeng Lu1; Michael Nastasi1; 1University of Nebraska–Lincoln
Austenitic stainless steels are susceptible to stress corrosion cracking (SCC) in hot and corrosive water environments, which has become a significant limit for the lifetime of these alloys in petroleum, chemical, and nuclear industries. The existing methods for controlling SCC have many limitations and problems. A laser shock peening (LSP) process has been identified as a new approach that can effectively mitigate SCC of austenitic stainless steels. However, the fundamental mechanisms by which this occurs remain poorly understood. In the LSP process, laser-driven shock waves are generated which can penetrate to a depth of more than 1 mm from the surface, inducing significant compressive residual stresses and plastic deformation in the sub-surface. This research will use experimental approaches to understand the microstructural evolution generated by LSP, determine the processing-microstructure-property relationship in the LSP process, and elucidate the mechanisms that enable LSP to mitigate SCC of austenitic stainless steels.
Modeling Corrosion Damage and Crack Propagation Using Novel Meshless Peridynamics Framework: Srujan Rokkam1; Michael Brothers1; Max Gunzburger2; Kishan Goel3; 1Advanced Cooling Technologies, Inc.; 2Florida State University; 3Naval Air Systems Command
Understanding the effects of corrosion damage, its influence on environmentally assisted crack propagation and resulting failure is of huge interest for estimating behavior of structural components in several engineering applications. In this presentation, we discuss a recently developed meshless approach for modeling corrosion damage and crack propagation under synergistic effects of corrosion and mechanical loading. The approach is based on non-local peridynamics theory that replaces governing equations which are typically partial differential equations (as in classical continuum mechanics) with integro-differential equations that are easy to solve across discontinuities like crack. The resulting framework is well suited for modeling crack propagation and failure, without the need to re-mesh the domain or the need for complicated crack path algorithms like that of XFEM or cohesive element method. Here we summarize the development of the peridynamics theory for corrosion damage and illustrate its working for corrosion induced crack nucleation and growth in metals.
3:40 PM Break
Peridynamic Modeling of Autonomous Lacy Cover Formation and of SCC: Siavash Jafarzadeh1; Ziguang Chen1; Florin Bobaru1; 1University of Nebraska-Lincoln
Pits covered with lacy structure in stainless steel are dangerous because they may not be easily spotted, and catastrophic cracks can grow from them. Partial passivation of the pit walls is considered to be the main reason for the formation of lacy covers. In this study, a previously introduced peridynamic corrosion model is improved with a concentration-based passivation criterion. Having material properties, kinetics of the corresponding anodic reaction, and the applied potential as inputs, this model simulates pitting corrosion and the Diffusion-based Corrosion Layer (DCL). DCL can contribute to stress corrosion cracking (SCC). The computational results under potentiostatic conditions are in perfect agreement with the experiments. Evolution of electrolyte IR-drop is then taken into account. The effect of solution concentration and potential on pit ratio, corrosion rate, and the lacy cover structure were investigated. The model is also used to investigate the effect stresses have on corrosion rates and SCC.
Crack Growth Prediction for Stress Corrosion Cracking and Corrosion Fatigue of Irradiated Stainless Steels: Robert Fuller1; Jutima Simsiriwong2; Nima Shamsaei2; 1Entergy Operations; 2Mississippi State University
Due to its high strength, ductility, and fracture toughness, austenitic stainless steel (SS) 304 is commonly used as a structural alloy in internal components of boiling water reactors (BWRs). In this study, a crack growth model that accounts for stress corrosion cracking and corrosion fatigue for unirradiated SS 304 is analytically investigated. The model is extended to include the effects of test temperature, radiation dose, and volumetric swelling for irradiated SS 304 based on simple monotonic properties such as ultimate tensile strength and yield strength. Crack growth rate data of irradiated SS 304 under various test conditions, resembling the BWR environment with operating temperature of core internal components, are employed to validate the proposed model. The applicability of the proposed approximation model based on simple material properties is discussed.