Crack initiation mechanism of extruded AZ31 magnesium alloy in the very high cycle fatigue regime

Ultrasonic fatigue testing as well as conventional fatigue testing has been conducted on commercial extruded AZ31 magnesium alloy. The S-N curve for this alloy appears to have a continuous decreasing trend in the very high cycle regime. Fatigue strength at 109 cycles is 88.7 ± 4.1 MPa. The ratio of endurance limit at one billion cycles to the tensile strength (σ−1/σb) is 0.301. Fatigue failure mainly originated from the specimen surface or near surface, where deformation twins were found after cyclic loading. Fatigue cracks were observed along twin bands. Based on cyclic deformation irreversibility caused by twinning, fatigue damage mechanisms are proposed in terms of surface/near surface crack initiation.