Stress Ratio Effect on Fatigue Crack Initiation Mechanism of Magnesium Alloy AZ31

Fatigue tests were conducted under several stress ratios, including negative maximum stress to elucidate the fatigue crack initiation mechanism of a magnesium alloy, AZ31. The specimen surface near the crack initiation site was analyzed by EBSD. On the basis of the results of EBSD analysis, it is concluded for an alternating cyclic stress condition (fully reversed cyclic stress) that fatigue cracks formed from grains where both the grain size and Schmid factor of the basal slip system are large, and that the crack initiation mechanism is based on irreversible slipping and unrelated to twinning. Under compression-compression fatigue test ( R =10), cracks were formed along boundary of grains with large Schmid factor and misfit of both side grain are large. At the tip of the initiated crack, twin bands were observed.