Modelling of intergranular stress corrosion cracking mechanism

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Time: Tue 2020-04-21 10.00

Location: https://kth-se.zoom.us/webinar/register/WN_kGQFlf_7QB-7Cr1yIE9cvw, Stockholm (English)

Subject area: Engineering Mechanics

Doctoral student: Michal Sedlak Mosesson , Hållfasthetslära

Opponent: PhD Thierry Couvant, Électricité de France EDF

Supervisor: Proffesor Bo Alfredsson, Hållfasthetslära

Abstract

When assessing nuclear power plant life, stress corrosion cracking (SCC) plays an important role. Stress corrosion cracking in nuclear power plants is well recognized and heavily researched. Still due to its complicated nature it is not completely understood. There are many different damage mechanisms behind SCC. The focus in this thesis is on the slip-oxidation model. In the slip-oxidation model, the aggressive ions are diffused to the crack tip. In the crack tip the aggressive ions act as a catalyst to slow down the repassivation rate of the oxide film. At the crack tip the localized anodic dissolution occurred until an oxide film was produced to repassivate the corrosion process. Due to the constant stresses applied, the oxide film ruptured, and new virgin material was exposed to be dissolved and finally repassivated. This process is consequently repeated.   The first part of the work introduces a new formulation of a cohesive element with extended environmental degradation capability, which is essential to create the later SCC models. The new degradation method was evaluated against a Hydrogen Embrittlement (HE) experiment showing improved agreement with the experiment compared to the literature. The effect of different susceptibility zones at the crack tip was also investigated, showing that a uniform degradation throughout the susceptible zone is more influenced by the χ parameter than a triangular susceptible zone.  In the second part a phenomenological SCC model was created with the purpose to model primary water conditions in pressurized water reactors (PWR). It used the slip-oxidation model for considering SCC in boiling water reactors (BWR) under normal water chemistry (NWC).   The PWR model was implicit, coupled with a segregated solution scheme including a diffusion equation based on Fick’s second law and a cohesive zone description for the fracture mechanics part. The degradation was modelled with an anodic slip-dissolution equation that uses the effective cohesive traction and concentration as the main parameters. The model was evaluated against experiments on the effects of cold work on intergranular stress corrosion cracking (IGSCC). The model showed good agreements for both shifting amount of cold work illustrated by only changing the yield stress in the bulk material and for shifting the stress intensity factor. The model versatility was also shown by simulating IGSCC in Alloy600, also with good agreements.   The BWR model was multi-physical including a slip-oxidation, diffusion model and had adaptive oxide thickness developed into the cohesive element framework. The oxide thickness was derived from the slip-oxidation model and updated in every structural iteration to fully couple the fracture properties of the cohesive element. The cyclic physics of the slip oxidation model was replicated. The model results agreed with experiments in the literature for changes in the stress intensity factor, yield stress representing cold work and environmental factors such as conductivity and corrosion potential. The adaptive model was also expanded into a duplex oxide model with an inner and outer oxide. The model showed agreeing results with literature and the model was used to simulate different oxide growth kinetics

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