High Cycle Fatigue Property and Micro Crack Propagation Behavior in Extruded AZ31 Magnesium Alloys

High cycle fatigue tests in rotating bending loading were carried out in extruded AZ31 magnesium alloys with two kinds of grain size (about 30 (mu)m and about 50 (mu)m). Effects of microstructure and grain size on fatigue behaviors such as fatigue strength, crack initiation and propagation behaviors were discussed. As the results, there were no clear difference in the fatigue strength, the crack initiation and propagation behaviors in the both alloys. There showed a clear bending point in the relation between crack propagation rate da/dN and the stress intensity factor range (delta) K at about (delta) K=3 MPa(square root)m. The fatigue lives in the higher stress level were evaluated from the inclusion size and the relation of da/dN-(delta) K.