Effect of Water Environment on Fatigue Behaviors of Magnesium Alloys

Fatigue characteristics of two kinds of magnesium alloys ; AZ92A casting plate and AZ31M rolled plate, in air and in purified water have been investigated. Plane bending fatigue tests were carried out and fatigue crack initiation behaviors were observed successively. In the air, S-N curve for AZ92A was continuous curve type, however, the one for AZ31M was bi-line type with fatigue limit, because it has a non-propagating microcrack at the fatigue limit. In the water environment, the fatigue strength decreased for both alloys. The tendency of degradation was remarkable under long life region and the fatigue limits decreased by about 50% of that in the air. Crack growth tests under push-pull loading were carried out with center notched specimens. For AZ92A, crack growth rates were found to increase with water environment. However, for long crack their growth rates were lower than that in the air because of crack closure by wedge effect of a corrosion product. Such a effect of water environment on crack growth was not much notable for AZ31M.