Biaxial low cycle fatigue under non-proportional loading of a magnesium-lithium alloy

The influence of the out-of-phase on the life duration is examined for a magnesium-lithium alloy subjected to biaxial low cycle fatigue in combined tension-compression and torsion. It has been found that the life duration depends on the angle of the phase shift phi imposed between the two perpendicular cyclic loadings. The maximum life duration is obtained in the case of the absence of such a phase shift (phi = 0 degrees); It has been shown that the deformation always remains plastic above a critical angle phi(c), i.e. no unloading of the sample can take place. The fatigue properties are examined employing von Mises equivalent stress and strain quantities. Results are also reported for the evolution of the density of deformation twinning;It is suggested that the loss of life duration is-due to the constantly changing loading direction in the case of phase shift, which hinders the stabilization of the plastic deformation in the constituent grains of the polycrystal. Copyright (C) 1996 Elsevier Science Ltd.