Loading history effect on fatigue crack growth of extruded AZ31B magnesium alloy

An experimental investigation was conducted on the fatigue crack growth (FCG) behavior of an extruded AZ31B magnesium (Mg) alloy in ambient air. Compact tension (CT) specimens oriented in three different directions with respect to the extrusion direction were employed in the study. The influences of overloading and two-step high-low sequence loading on FCG were investigated in detail. Single tensile overloads with three overload ratios (1.5, 1.75, and 2.0) were applied during otherwise constant amplitude loading. A single overload retarded the crack growth rate in all the three specimen orientations. The crack growth rate decreased immediately to a minimum value right after the application of a tensile overload and increased quickly to a stable value expected at constant amplitude loading. No visible torn fracture was observed at the crack tip right after overloading but the crack tip was clearly blunted. Results from the two-step high-low sequence loading reveal that FCG retardation occurred at the beginning of the lower amplitude step when the maximum load was lowered in the second loading step, which is similar to the case of a single overload. Walker’s model can correlate well the crack growth experiments with different R-ratios. Wheeler’s model can reasonably predict the influence of overload and high-low loading sequence on the FCG in the AZ31B Mg alloy.