On the Load Multiaxiality Effect on the Cyclic Behaviour of Magnesium Alloys

While most fatigue-related studies on wrought magnesium alloys are under uniaxial push–pull loading condition, structural members are mostly under multiaxial stresses in real-life applications. This study addresses the effect of load multiaxiality on the cyclic behaviour of several wrought magnesium alloys: AZ31B, AM30, AZ80, and ZK60 under multiaxial tension/compression–torsion loading. In particular, the influence of the presence of shear on normal stress response and vice versa is studied. In addition, phase angle effects on the stress–strain response and fatigue life are discussed. Strain energy density (SED) is introduced as a suitable fatigue damage parameter to connect and compare uniaxial and multiaxial cases. It is shown that irrespective of loading direction and/or phase angle, SED closely correlates experimental results. Beyond strain of ~0.4–0.5%, the strain-controlled cyclic behaviour in uniaxial push–pull is dominated by twining/de-twinning, while in pure shear deformation is dominated by basal slip. The effect of each of these load directions on the other in a multiaxial loading is considered in two cases: at low axial strain amplitudes the interaction is mutual, and at high axial strain amplitudes axial strain dominates. It is believed that the re-orientation of basal planes due to twinning/de-twinning caused by axial strain favours basal slip in twinned grains resulting in better accommodation of shear strain. Further, three load phase angles of 0, 45, and 90 were considered. It is observed that the phase angle has minimal effect on life at low axial strain values; however, at higher axial strain amplitudes out-of-phase angle causes more damage. The re-orientation of matrix due to twinning and rotation of the principal axis due to phase angle shift increase the chance of different slip/twin systems to be activated resulting in lower lives.