Analysis of fatigue crack growth rate of welded joint after immersion corrosion

Wind turbines operating in marine environments inevitably face fatigue loads from ocean currents, wind, waves, as well as corrosion from the complex marine environment, which often makes the welded structures on their towers vulnerable to corrosion fatigue failure, therefore, studying the corrosion fatigue phenomena in welded joints and exploring the underlying mechanisms is of great significance. S355 high strength low alloy structural steel is selected as the test material to carry out the pre-corrosion fatigue crack growth test of S355 steel welded joint in 5%NaCl environment. The test result shows that the fatigue crack growth rate of the test specimens after 48 hours of pre-corrosion is not significantly increased, while the fatigue crack growth rate of the test specimens pre-corrosion for 336 hours is significantly increased. Scanning electron microscope is used to characterize the pre-corrosion fatigue test specimens. Micro-cracks and branching cracks are observed around the crack growth paths, indicating that the metal is embrittlement. On the pre-corrosion fatigue section, the change of crack growth mode is observed, the surface metal shows brittleness fracture and the inner metal retaines fatigue toughness fracture. The results of microstructure characterization show that the pre-corrosion embrittles the surface of metal, which result in acceleration of the fatigue crack growth rate of the whole test specimen.