| Summary: | Torsion and bending fatigue tests of magnesium alloy AZ31were carried out to investigate the initial damage under high cycle fatigue. The crystallographic orientation of crystals in which slip lines were occurred was analyzed by using electron back scatter diffraction (EBSD). The slip line occurrence angle was then calculated based on a crystallographic slip system with the maximum Schmid factor and compared with the experimental results to examine the active slip system. The slip direction from the specimen surface was also calculated for crystals with slip lines and the relationship between the calculated direction and Schmid factor was investigated under both loading conditions. Schmid factor of crystals with slip lines becomes the maximum in basal slip systems and the calculated slip line occurrence angle agreed well with that obtained from the experimental results. Therefore, the basal slip system in which Schmid factor becomes the maximum is active in both loading conditions. Although the slip direction from the specimen surface was small value in the crystals without slip lines, the crystals with slip lines had relatively large values. This means that the slip direction also affects the initial fatigue damage. Slip deformation was activated by the in-plane maximum shearing stress in torsion condition. On the other hand, both in-plane and out-of-plane maximum shearing stresses were responsible for the slip deformation in bending condition. Particularly, the fatigue crack was considered to be initiated by the out-of-plane maximum shearing in small number of stress cycles, since the crack-induced crystals have large values of the slip direction from the specimen surface.
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