Effects of Humidity and Water Environment on Fatigue Crack Propagation in Magnesium Alloys

Abstract

This paper presents the effects of environment on fatigue crack propagation (FCP) in two magnesium alloys, rolled AZ31 and extruded AZ61. FCP tests have been performed using electro-hydraulic fatigue testing machine operating at a frequency of 1Hz at a stress ratio, R, of 0.05 in distilled water and in dry air. The dew point of dry air was −60°C. The obtained results were compared with those already established in laboratory air, and the effects of humidity and water environment were discussed on the basis of fractographic analysis of fracture surfaces. The effect of cyclic frequency was also studied. Both alloys exhibited basically the same FCP behaviour regardless of environment. FCP rates were nearly the same in laboratory air and in distilled water, while approximately an order of magnitude slower in dry air than in those environments. After allowing for crack closure, the effect of environment still existed, where FCP rates were the fastest in laboratory air, then in distilled water, in dry air in decreasing order. Fractographic analysis revealed that fracture surfaces were extensively brittle in laboratory air, while entirely covered with corrosion products in distilled water. It was believed, therefore, that different FCP mechanisms operated in laboratory air and in distilled water, possibly hydrogen embrittlement and anodic dissolution, respectively. The frequency dependence of FCP rate was not recognized in dry air and less remarkable in laboratory air over a wide range of frequencies of 0.01Hz to 10Hz, while FCP rates became faster with decreasing frequency in distilled water.