Abstract
To discuss an effect of stress ratio on very high cycle fatigue properties of the extruded magnesium alloys, axial loading fatigue tests have been performed under three conditions of stress ratio, R, of 0, -1 and -1.5 in laboratory air at room temperature using hourglass shaped specimen of three kind of alloys. Specimens tested under R=-1 and -1.5 showed a step-wise S-N curve on which two knees appear while under the test of R=0 clear fatigue limit existed on the S-N diagram. From the detail observation of fracture surface, It is suggested that fatigue crack initiation mechanism changed from the twin-induced failure mode at high stress amplitude level to the slip-induced one at low stress amplitude level. This transition was occurred by the relation between the minimum stress during a cycle and the compressive yield stress at which deformation twin occurred. The deformation twin is formed during fatigue process when the minimum stress exceeds the yield stress in compression that is smaller than that in tension.
