Phase Field Modelling Of Stress Corrosion Cracking And Corrosion Fatigue In Metallic Materials

Localised corrosion is arguably considered one of the most common yet complex multi-physics phenomena of metallic materials.The combined actions of mechanical stresses and the environment during the evolution of localised corrosion,which are normally recognised as stress corrosion cracking(SCC)and corrosion fatigue(CF),can lead to catastrophic failures of engineering components and structures.Thus,the prediction of stress corrosion cracking and corrosion fatigue is of particular importance for the safety and durability of engineering structures.By considering the electrochemo-mechanical coupling phenomena,this work aims at proposing new phase field paradigms for understanding and predicting stress corrosion cracking and corrosion fatigue.The main research contents and achievements are summarised as follows:Chapter 2 presents a new theoretical and numerical framework for modelling dissolution-driven stress corrosion cracking.The model incorporates,for the first time,the role of mechanical straining as the electrochemical driving force,accelerating corrosion kinetics.Moreover,a formulation grounded upon the film rupturedissolution-repassivation mechanism is also presented to incorporate the influence of film passivation.The formulation is numerically implemented in ABAQUS UEL.Five case studies of particular interest are addressed to showcase the predictive capabilities of the model,revealing an excellent agreement with analytical solutions and experimental measurements.Specifically,the formulation can successfully predict the transition from activation-controlled corrosion to diffusion-controlled corrosion.Moreover,the sensitivity of interface kinetics to mechanical stresses and strains and the role of film rupture-dissolution-repassivation mechanism in the evolution of stress corrosion cracking are also well captured.Chapter 3 presents,for the first time,a general framework for stress corrosion cracking,incorporating both anodic dissolution and hydrogen embrittlement mechanisms,by using a multi-phase-field approach.The multi-phase-field framework is numerically implemented using the finite element method,with the displacement components,the phase field corrosion order parameter,the metal ion concentration,the phase field fracture order parameter and the hydrogen concentration being primary kinematic variables.Representative case studies are addressed to showcase the predictive capabilities of the model in various materials and environments,attaining a promising agreement with benchmark tests and experimental observations.Results show that the generalised formulation presented can capture,as a function of the environment,the interplay between anodic dissolution-and hydrogen-driven failure mechanisms;including the transition from one to the other,their synergistic action and their individual occurrence.Such a generalised framework can bring new insight into environment-material interactions and the understanding of stress corrosion cracking.Chapter 4 presents a multi-phase-field method for modelling corrosion fatigue in metallic materials.Pit evolution,pit-to-crack transition and fatigue crack propagation are all incorporated and captured in this approach.The mathematical framework is numerically implemented in ABAQUS UEL.Four representative case studies of particular interest are addressed to verify the numerical framework and investigate the interactions between pitting corrosion and fatigue growth.Fatigue limits of pre-corroded samples with different pit shapes are successfully predicted,revealing the capability of fatigue implementation in capturing S-N curves.Parametric analyses are also conducted to clarify the influences of some key electrochemical and mechanical parameters on corrosion fatigue evolution.Finally,the influence of multi-pits interplay on pitting evolution,crack propagation and fatigue limit are analysed and compared.Chapter 5 presents a new electrochemo-mechanical phase field model by combining the framework proposed in Chapter 2 with the transport of multiple species and the distribution of electrolyte potential.The new phase field model is numerically implemented in the software platform COMSOL MULTIPHYSICS.Several paradigmatic case studies are addressed to gain physical insights in electrochemical change during localised corrosion and the electrochemo-mechanical interactions.First,the benchmark test in Chapter 2 is re-formulated to demonstrate the effects of Dirichlet boundary conditions,applied potential and Na Cl bulk concentration on pit growth.Secondly,mechanically-assisted corrosion from a semi-circular pit is conducted and predicted.The roles of electrostatic potential,metal ion concentration,and FRDR mechanism on pit evolution are explored and clarified.The local solution environments inside the pit are also investigated.Moreover,the possibility of pit-to-track transition with and without Dirichlet boundary conditions is compared and discussed.