Effect of Stress Ratio on Fatigue Crack Growth Behavior of Magnesium Alloy AZ61 under 3.5 Mass% NaCl Spray Environment*

Fatigue crack growth behavior of AZ61 magnesium alloy was investigated in low humidity (55% relative humidity) and 3.5% NaCl spray environments under various stress ratios. Fatigue crack growth rates became higher with increasing load ratio for both environments. However, the results of fatigue crack growth test under 3.5% NaCl spray environment showed lower fatigue crack propagation rate compared to those in low humidity environment for all stress ratios. The difference in crack growth rate was resulted from the different crack closure behavior. The crack growth curves arranged by ∆Keff merged to one curve regardless of stress ratio. In order to investigate the higher crack growth resistance in 3.5% NaCl spray environment, fracture surface observation was carried out using a scanning electron microscope. Fracture surface of the specimen tested in 3.5% NaCl environment was fully corroded. However, no corrosion was observed for the specimen tested in low humidity environment. The corrosion products on the fracture surface could induce higher crack closure point. This might lead to the higher crack growth resistance observed for the specimen tested in 3.5% NaCl spray environment.