An experimental study of the orientation effect on fatigue crack propagation in rolled AZ31B magnesium alloy

Mode I fatigue crack growth (FCG) experiments were performed using compact tension (CT) specimens made of rolled AZ31B magnesium alloy in ambient laboratory air. The testing specimens were made with respect to two material orientations: a crack surface perpendicular to the rolled direction (R-T) and a crack perpendicular to the thickness or normal direction (N-T). The constant amplitude load experiments were performed at three load (R) ratios (minimum load over maximum load in a loading cycle) of 0.1, 0.5, and 0.75, respectively. Material orientation was found to affect the early crack growth stage more than the later stable growth stage. For each R-ratio, the threshold stress intensity factor range for the R-T specimens was less than that for the N-T specimens. Three sub-stages of steady crack growth were observed following the threshold stage: a low Paris law slope for the first sub-stage, a second sub-stage with a very high slope, and an intermediate slope during the third sub-stage. The R-T specimens exhibited an overall typical Mode I cracking direction, with occasional local deviation from the horizontal crack path. The N-T specimens displayed a general Mode I cracking with irregular crack pathing into the specimen away from the observation surface. Transgranular cracking was the primary cracking mode for both specimen orientations. Slip induced cleavage dominated cracking in both orientations. Few residual twins were found in the plastic zone area surrounding the crack tip, and no evidence of twin boundary cracking was found.