| D36 steel, being a material widely used in offshore structures, often suffers severe corrosion fatigue damage during a structure's service time. So, studying on the CFCG of D36 steel in seawater is significant. Among current research on the CFCG behavior of steel structures, various regression based on Paris Equation have been commonly carried out, yet very few has discussed the mechanism of crack growth. Besides, among research on the mechanism of CFCG, yet very few have given the model of crack growth. So this article studies the mechanism of the CFCG of D36 steel in seawater based on the results of CFCG test. Then, combining with the mechanism, a new model for CFCG rate is presented. At last, this article uses the new model to determine an optimum cathodic protection for D36 steel in seawater at different situations.Firstly, this article makes fatigue crack growth experiment for steel D36 and the effect of stress ratio, loading frequency and polarization potential are discussed. Then, the mechanism of the CFCG of D36 steel is studied by changing the polarization potential. The result is that the mechanism of the CFCG of D36 steel is the combined effect of anodic dissolution and hydrogen-induced cracking. In the seawater, the main mechanism is the anodic dissolution. With the reduction of polarization potential, the effect of hydrogen-induced cracking becomes more and more important.In order to get the crack growth rate caused by the anodic dissolution, the slow strain rate test is designed and the relationship between the crack tip current and strain, strain rate is fitted. Then, combining the analysis of stress and strain at crack tip, the crack growth rate caused by the anodic dissolution is obtained. Besides, this article presents a new model for CFCG rate, with the mechanism of both anodic dissolution and Hydrogen-induced cracking taken into consideration, and determines the parameters of the new model by fitting the test result. Then, a new model which takes the polarization potential into consideration is presented and the accuracy of the new model is verified in comparison with test results.Finally, the optimum cathodic protection for D36 steel is determined by the new model and it is of guiding significance to study the influence of amplitude of stress intensity factors, stress ratio and loading frequency in engineering practice utilizing this very new model.This article presents a new model for CFCG rate and it is of guiding significance to determine an optimum cathodic protection for D36 steel in seawater in engineering practice utilizing this very new model. |
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