Study On The Corrosion Fatigue Crack Growth Of Offshore Structure Steel

The study on corrosion fatigue crack growth (CFCG) of offshore structure steel is significant in practical engineering. The yielding strength of offshore structure steel is generally form 300 MPa to 600 MPa. The mechanism of fatigue crack growth in corrosion environment is anodic dissolution. So far most of research on CFCG of offshore structure steel has carried out experiments to fit crack growth curve, however, few research has been concerned about anodic dissolution. In this article, the CFCG of offshore structure steel is investigated with crack growth experiments in which the anodic dissolution is considered.First, Fatigue crack growth experiment for common offshore structure steel D36 was performed. To determine the crack length accurately, the method of CTOD was used to get fatigue crack growth rate curve. The effect of stress ratio and loading frequency was discussed. The fatigue crack growth rate model for steel D36 in air was fitted based on Walker formula.Then, The concept of anodic dissolution was introduced:local plastic deformation happens near crack tip because of stress concentration in corrosion environment. So electrode potential decreases and the anode oxidation occurs. Using electrochemical equivalent as transition parameter, crack growth rate of anodic dissolution in a loading period was known. The relationship between current density and strain near crack tip was discussed. And the rate formula of anodic dissolution was built.Finally, corrosion fatigue crack growth experiment of steel D36 was carried out in artificial seawater. The effect of stress ratio and loading frequency was also discussed. Combining the fatigue crack growth rate model in air and the rate formula of anodic dissolution, the CFCG model for steel D36 in seawater was built.