Study On Approach Of Fatigue Life Assessment For Welded Structures Considering Corrosion Damage

Marine environment is extremely complex. Because of large amounts of chloride ions and dissolved oxygen in water, metal corrosion happened easily in the ocean. Meanwhile, the cyclic loading of winds, waves and flows on the structure may cause the fatigue problems. Ships and offshore structures generally serve about twenty years in the marine environment. So the coupling effect between the electrochemical corrosion and cyclic loading is inevitable. Corrosion fatigue is always one of the main failure modes of ships and offshore structures and the corrosion fatigue damage usually occurs in the welding position.Corrosion fatigue process is very complex and involves many details in materials science, environment and metallurgy. There is damage mechanism that still could not get good solutions. In this paper, based on the principles of mechanochemical effect, laboratory simulation experiment and finite element software, considering the coupling effect of corrosion and stress, corrosion fatigue damage mechanism of ships and offshore structures is discussed.First, we analyzed the real sea coupon corrosion data and built a power function model of the corrosion depth and the exposure time under no stress condition.Then, based on the coupling effect of corrosion and stress, we built a new corrosion model considering the mechanical factors. Because the mechanochemical working mechanism is different in elastic stage and in plastic stage, our model is divided into two parts. Through laboratory model test, we analyze the influence of mechanical factors to the corrosion rate. And get the correlation coefficient by fitting the data. Then get the new corrosion model considering the mechanical factors.Finally, finite element model of welded components is established and the thickness is reduced based on the uniform corrosion model and new model. The hot spot stress is extracted to assess the corrosion fatigue damage. The results of calculation show that corrosion fatigue damage which is considering the coupling effect is more severe than the single uniform corrosion fatigue damage. The stress of weld position can increase to plastic stage easily and the corrosion rate may increase significantly, which may lead severely corrosion fatigue damage. Therefore, the new method which considers the coupling effect of corrosion and stress can assess the corrosion fatigue life more accurately.