Fatigue Behavior of AZ31 Extruded Magnesium Alloy in Laboratory Air

Rotating bending fatigue test was carried out using AZ31 extruded magnesium alloy in order to clarify the S-N curve, crack initiation and propagation behavior of the alloy. The mechanisms of both crack initiation and crack arrest at the grain boundary during the fatigue process were investigated in detail with a special attention to the microstructure of the material. The material is divided roughly into 2 kinds of banded texture, the phase A (white zone) and B (black zone). The phase B consists of the crystal grains (phase C). Near the endurance limit of the material, fatigue life is greatly affected by a slight difference in the stress amplitude. This feature in fatigue lives is responsible to the sharply curved S-N curve at the endurance limit, a general characteristic of the extruded magnesium alloy. The fatigue crack initiates at the upper and lower edges of the grain boundary of the phase C in the early stage of the fatigue life. At stress amplitude of 122.5MPa which is slightly larger than the fatigue limit, the crack propagates to the next B-phase without being blocked by the grain boundary of the phase C, however at 120MPa which is slightly less than the fatigue limit, the crack advance was blocked by the grain boundary. The sharp curved S-N curve is attributable to the crack arrest which is caused by the grain boundary of the phase C. By integrating the relationship between stress intensity factor and crack growth rate, the calculated S-N curve agrees well with the experimental result