Mechanism Study Of Pit Evolution And Crack Propagation For Corrosion Fatigue

The corrosion fatigue is a fracture process of metal material under the combined effect ofthe alternating stress and corrosive medium. A large number of engineering structures, such asmarine structures, petrochemical equipments and aircraft structures, suffer cyclic loadings incorrosive environments. Corrosive environments reduce the fracture toughness, speed up theformation and propagation of cracks, and greatly reduce the service life of metal structure.Corrosion fatigue has now become a serious problem of the structure withstanding cyclicloading, and corrosion fatigue properties research has been updated to an important part of thedesign of the durability and integrity design for engineering structures. Up to now the studiesabout corrosion fatigue in theory and practical applications have been developed rapidly. Butcorrosion fatigue process is very complex and involves many details in mechanics, chemistry,electrochemistry, materials science and metallurgy, there are many issues still could not getgood solutions. The majority of researches actually only stay in trying to find out the laws ofexperimental results, as a result, the evaluation methods based on corrosion fatiguemechanism and thermodynamic equilibrium theory are very scarce, although there are a lot ofrelative studies. Meanwhile, with the rapid development of the aerospace and marineengineering, corrosion fatigue problems become more prominent, while the results of relatedresearches are far to meet the needs of engineering applications.In this paper, based on the principles of corrosion electrochemics and thermodynamics,the pit evolution, corrosion fatigue crack nucleation and corrosion fatigue crack propagationduring corrosion fatigue process of metal materials are discussed. Then corrosion fatiguedamage theory is proposed and corrosion fatigue damage evolution law is established. Themain contents of this paper are as follows:(1) According to the principles of thermodynamics, the basic thermodynamic equationduring pit evolution and crack propagation is established, from which the thermodynamiccriterion that controls the direction of pit evolution and crack propagation is obtained.(2) The two-dimensional model of corrosion fatigue pit evolution and crack nucleation is established. According to the two-variable semi-elliptical model of corrosion pit, thethermodynamic potential of the elastic solid with an evolving pit is derived, from which theevolving law of shape parameter with corrosion pit depth is deduced. The critical condition ofcorrosion fatigue crack nucleation is established, according to the stress intensity factorcriterion and dislocation theory of crack nucleation, consequently qualitative analysis of thecorrosion fatigue crack nucleation life is carried on. Similarly, the three-dimensional modelanalysis of pit evolution and crack nucleation is studied.(3) According to the mechanism that corrosive environment promotes crack propagationby anodic dissolution at crack tip and reduction of free energy at crack surface, the energyconservation equation during corrosion fatigue crack propagation process is established, fromwhich the explicit equation of corrosion fatigue crack propagation rate is derived. Then theeffects of anodic dissolution, free energy reduction at crack surface and cyclic loadingparameters on corrosion fatigue crack propagation rate and crack propagation life arequalitative analyzed.(4) The three-point bending corrosion fatigue crack propagation experiments of LY12CZaluminum alloy is designed, and the influences of corrosion environment, cyclic loadingfrequency, stress ratio and pH value of solution on the corrosion fatigue crack propagationrate are studied.(5) Based on the damage mechanics theory, the corrosion fatigue damage is studied.Corrosion fatigue damage is treated as non-linear cumulative outcome of stress corrosiondamage and fatigue damage. The relative damage parameters are determined by theexperimental results of LY12CZ aluminum alloy, and corrosion fatigue damage evolution lawis established. Afterwards, the corrosion fatigue life normalized evaluation based on thedamage evolution law is formed.