Plane bending fatigue tests were conducted under stress ratios of -1, -0.1, 0.1, and 0.5 at room temperature in a laboratory atmosphere to elucidate the fatigue crack initiation mechanism of an extruded AZ31 magnesium alloy. The fatigue life can be expressed as a unique function of the equivalent stress amplitude based on Smith-Watson-Topper theory independent of the stress ratio, and the dependence of the fatigue limit on the mean stress can be expressed by Morrow's equation, both of which were proposed for conventional metals without twinning. In addition, the specimen surface was observed by optical microscopy and scanning electron microscopy (SEM), and the surface near the crack initiation site was analyzed by electron backscatter diffraction (EBSD) analysis to discuss the fatigue crack initiation mechanism. On the basis of the results of EBSD analysis, it is concluded that the existence of large grain with large Schmid factor of the basal slip system is essential for crack initiation, and the crack initiation mechanism is based on irreversible slipping and unrelated to twinning under the alternating stress condition (R=-1).

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