On fatigue lives of diecast and extruded Mg alloys

In this study, fatigue tests were carried out on both diecast and extruded Mg alloys to study their distributions of fatigue lives under constant stress amplitudes. During the fatigue process of the diecast Mg alloy, cracks initiated from the casting defects inside of the specimen, and then propagated prior to final failure of the specimen. While in the extruded Mg alloy, cracks initiated from the inclusions located on the specimen surface. With assuming the above defects as the initial cracks, the initial maximum stress intensity factors Kimax were evaluated. There are common relations between the initial maximum stress intensity factors Kimax and fatigue lives Nf, regardless of the stress amplitudes for the both Mg alloys at the constant R ratio of −1. The lower Kimax, the longer Nf becomes. Integrating the fatigue crack propagation law from the initial maximum stress intensity factor Kimax to the fatigue fracture toughness Kfc, the relations Kimax vs. Nf can be successfully evaluated. Distributions of fatigue lives at the constant stress amplitudes can be represented by the Weibull distributions. Dispersion in the fatigue lives becomes larger at the lower stress amplitude as compared with those at the higher stress amplitudes. This trend is observed commonly for both diecast and extruded Mg alloys.