Fatigue Crack Propagation in Magnesium Alloy AZ31 Rolled Plate

This paper describes the fatigue crack propagation (FCP) characteristics in magnesium alloy AZ31 rolled plate. FCP experiments have been performed at stress ratios, R, of 0.05 and 0.7 in laboratory air at ambient temperature using CT specimens with two different orientations of LT and T-L. The effects of orientation and stress ratio and fracture mechanism were discussed on the basis of crack closure measurement and fracture surface analysis. Regardless of orientation, the relationship between FCP rate and stress intensity factor at R = 0.05 consisted of two parts with different slops, which became much more remarkable when FCP rates were characterized in terms of the effective stress intensity factor. This was attributed to fracture mechanism transition because the fracture mechanisms operated were different above and below a stress intensity factor where the slops changed. After allowing for crack closure, a slight difference in FCP behaviour between both orientations and a clear stress ratio dependence of FCP rate were still seen, but these could be due to some problems in measuring crack closure, because fracture mechanism was not affected by orientation and stress ratio. The intrinsic FCP resistance of the present Mg alloy was found to be inferior to that of other materials such as aluminium alloys and pure titanium.