Investigation on the stress state along the crack front in the fatigue crack growth rate test for thick high strength steel plates

Complex environmental loads would unavoidably lead to the fatigue damage in marine structures, and the case worsens in high strength steel thick-welded structures which have typical tri-axial stress and the feature of brittleness.To study the stress state along the crack front, a standard fatigue crack growth rate test for high strength steel Q370QE with a thickness of 36 mm is conducted, and the corresponding material parameters and the threshold of stress intensity factor are obtained.Next, the abnormal behavior of a crack front during crack growth and the stress state along crack front during crack growth are analyzed with the finite element method.Accordingly, the varying tendency of crack front shape could be qualitatively defined.Finally, local constraint factors are introduced in order to quantitatively describe the stress state along the crack front with the growth of fatigue crack based on plasticity analysis, which provides proper explanation to why the crack front changes from slant to flat with the crack growing.The calculated stress intensity factor could only describe the stress state in the center area, but distortion is inevitable on the surface, while the local constraint factor can fully describe the stress state along the crack tip zone.