Combined high–low cycle fatigue behavior and failure mechanism of EH36 steel welded joints

To investigate the effect of combined high–low cycle fatigue on the fatigue life of EH36 offshore engineering steel welded joints, constant-amplitude and combined high–low cycle fatigue tests were conducted on 20-mm thick double-sided full-penetration butt joints under various stress ratios and mean stresses. The results showed that under constant-amplitude loading, the fatigue life increased with a decrease in the mean stress at a constant stress ratio and increased with a decrease in the stress range at a constant mean stress. In the fixed stress range, the fatigue life increased as the mean stress decreased. Under combined high–low cycle loading, the fatigue strength decreased with an increase in the stress ratio or mean stress, and the extent of fatigue life reduction due to combined loading was more pronounced at higher high-cycle stress ratios. Digital image correlation analysis revealed that the fatigue life reduction under combined loading was associated with changes in plastic deformation at the crack tip following low-cycle loading. A key mechanism was the attenuation of the plasticity-induced crack closure effect at the crack tip after combined high–low cycle loading. A decrease in the high–low cycle ratio increased the proportion of low-cycle events, further suppressing the crack closure effect and leading to a greater fatigue life reduction. Macroscopically, the fracture surfaces under combined loading exhibited higher roughness with tear ridges near the weld toe due to the high stress concentrations, fatigue arrest lines in the propagation zone from cyclic loading, and deep secondary cracks on the fracture surface.