Effect of Pre-accumulated Plastic Strain on Stress Corrosion Cracking and Fatigue Life of Steels – Experiment and Modeling

Steel structures may experience localized plastic strains arising from a wide range of service anomalies. Regions of accumulated plastic strain are more prone to accelerated stress corrosion cracking and reduced fatigue life. In this work, we systematically analyzed intergranular corrosion (IGC) under combined oscillatory mechanical loading and active electrochemical environment in a specially designed experimental apparatus. Loading cycles were designed to mimic both the low-amplitude high-frequency vibration loads and the low-frequency high-amplitude structural duty cycles. Electrochemical potentials were maintained for active dissolution in moderately alkaline carbonate-bicarbonate solutions and under pre-accumulated plastic strain ranging from 0% to 4.0%. We report the strain-dependent morphological evolution during the initial stage of IGC of X70 steel in sodium bicarbonate solution in the potential range of high-stress corrosion cracking (SCC) susceptibility. At potentials in the range of SCC susceptibility, IGC creates triangular wedges of porous corrosion products centered at grain boundary triple junctions. The wedge shapes and the total integrated charge from the polarization curves were greatly affected and correlated with the level of the accumulated plastic strains and the load profile. To model these interactive threats on the remaining fatigue life of the structure, a three-dimensional elastoplastic continuum damage mechanics model for multiaxial fatigue is developed. The modeling framework employs the thermodynamic formulation for the elastic and plastic continuum damage evolution laws proposed by LeMaitre and Chaboche and is implemented into an ABAQUS user material subroutine (UMAT). The model accounts for both the pre-accumulated plastic strain and the induced elastoplastic fatigue strains to accelerate the evolution of damage accumulation. The experimentally observed acceleration of wedge propagation under different electrochemical rates is also integrated into the damage evolution equation. The initial model predictions show up to 95% of life reduction for a pre-accumulated plastic strain of up to 5%. These findings can be used to advance the understanding of the combined effect of damage and corrosion on the remaining fatigue life of energy materials.