Complex environmental loads unavoidably lead to fatigue damage in marine structures, and it becomes worse in high tensile steel thick-welded structure which has typical tri-axial stress state and the feature of brittleness. In this paper, a standard fatigue crack growth rate test for high-tensile steel Q370QE with thickness of 36 mm was carried out, and some material parameters and the threshold of stress intensity factor were obtained. Then, the abnormal behavior of a crack front during crack growth was analyzed by finite element method. Accordingly, the varying tendency of crack front shape could be qualitatively defined, and the plastic zoon at both flat and slope crack fronts for different crack depth was demonstrated based on plasticity analysis. Finally, local constraint factor was introduced to quantitatively describe the stress state along the crack front with the growth of fatigue crack, which provided proper explanation to why the crack front changes from slope to straight with the crack growing.

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