INFLUENCE OF HYDROSTATIC STRESS DISTRIBUTION ON THE MODELLING OF HYDROGEN ASSISTED STRESS CORROSION CRACK GROWTH

This study on stable crack growth under hydrogen assisted stress corrosion cracking (HASCC) is concerned with modelling of two interdependent processes: diffusion of hydrogen into material, and the resulting advancement at crack tip. The concentration of diffused hy- drogen ahead of the crack tip is influenced by the local distribution of hydrostatic stresses and plastic strains. The influence of hydrostatic stress may become predominant at lower plastic strain levels for hydrogen distribution and crack growth but, to the best of authors' knowledge, this has not been substantiated. The present study deals with modelling of the interdependent processes and examines the issue. The crack growth has been studied in compact tension specimens of a structural steel under HASCC conditions at three levels of plastic strain. A hydrogen concentration dependent cohesive zone model (HCDCZM) has been employed. The distribution of diffused hydrogen is obtained by solving the governing non-linear equation of diffusion which includes the hydro- static stress effect using a numerical scheme based on finite difference method. The hydrogen distribution is then used to predict crack propagation using a finite element based cohesive zone model. This paper presents details of the modelling and results concerning the crack tip hydro- gen concentration and comparison of predicted and experimental variation of crack opening displacement with crack extension. The influence of hydrostatic stress on the hydrogen distribu- tion is noticeable for the two slower loading rates where plastic strain levels are below 0.05. For all three loading rates, the average hydrogen concentration near the crack tip is higher in the present study than the results obtained in an earlier study by excluding the effect of hydrostatic stress. This difference calls for a decrease in cohesive strength reduction factor to maintain an equal level of crack extension rate as in the earlier study. The results are acceptable without any need to change the effective diffusivity.