An experimental study of fatigue crack propagation in extruded AZ31B magnesium alloy

Mode I fatigue crack growth (FCG) experiments were carried out using extruded AZ31B magnesium alloy in ambient air. The cracks in the experiments using compact tension (CT) specimens were oriented in three different directions with respect to the extrusion direction. With constant load amplitude, the experiments were conducted with three load ratios (minimum load over maximum load in a loading cycle): 0.1, 0.5 and 0.75. The experimental results reveal that the specimen orientation has a significant influence on the FCG rate and crack path. Under the same load ratio and identical stress intensity factor range, the FCG rate of the T–L specimen is the highest while the L–T specimen shows the lowest value (L = extrusion direction, R = radial direction of the extrusion bar, T = direction perpendicular to the L–R plane). For the L–T specimens, a plateau of constant crack growth rate occurs after the threshold stage as crack extends. Typical macroscopic Mode I crack is developed in the T–L and T–R specimens whereas a deviation from the Mode I crack path and branching occur in the L–T specimens. For all three specimen orientations, the FCG rate increases with increasing load ratio. The values of the threshold stress intensity factor range are approximately the same for the L–T and T–L specimens and they are not significantly influenced by the load ratio. For the T–R specimens, the load ratio has a great effect on the threshold intensity. Transgranular cracking is the major cracking mode. Slip induced cleavage dominates the transgranular cracking in the T–L and L–T specimens whereas both twin boundary cracking and slip induced cleavage exist in the T–R specimens.