A frequency-dependent high-cycle fatigue damage model and its application in crack nucleation of marine riser from corrosion pits

Based on the continuum damage mechanics theory, a frequency-dependent high-cycle fatigue damage evolution model was established. According to the fatigue results of API X80 steel, the frequency-dependent damage evolution parameters were regressed. Using the available riser mechanical model, the user subroutine of high-cycle fatigue damage evolution was compiled, and the accelerating effect of corrosion pits on fatigue crack initiation of the marine riser was analyzed. The results show that a single corrosion pit at the surface can reduce crack nucleation life by more than 80%. In a double corrosion pits' case, the stress interference between pits significantly affects the crack nucleation. Circumferential double pits can produce a fatigue crack earlier than the single pit, but the effect of axial double pits is just the opposite. When the spacing reaches a certain distance, the stress interference disappears, and the crack nucleation life approaches that of a single pit case.