Fatigue fracture of Mg-Al-Zn alloys

Abstract

Rotary-bending stress fatigue tests were carried out on Mg, AZ31 and AZ61 alloys. Mg, AZ31 and AZ61 have fatigue strengths in this order. They have ratios of bending stress to tensile strength at 10⁷ cycles about 0.15, 0.32 and 0.29 respectively. The relationship between crack propagation rate and stress intensity factor is expressed as: da/dN=C(K_max)^m, where m and C are constants. The value of m is 4 for Mg and AZ31 and 3 for AZ61. The crack propagation rate of AZ31 is some one-tenth as much as that of Mg. The crack in AZ31 runs zigzag in the early stage and propagates nearly normal to the stress direction accompanying a distinct plastic zone. The fractograph of AZ31 changes from cleavage-like fracture mode to the fracture mode by striations with cracks propagated.