Prediction of the fatigue life in a textured AZ31 Mg alloy as a function of orientation using fatigue indicator parameters

Abstract

Computational homogenization was used to simulate the mechanical response of a textured AZ31Mg alloy subjected to fully-reversed cyclic deformation. The behavior of each grain in the polycrystal followed a phenomenological crystal plasticity model that considered different deformation mechanisms, namely pyramidal, basal, and prismatic slip, as well as twinning and detwinning. It also accounted for both kinematic and isotropic hardening effects. The crystal plasticity model's parameters were adjusted to replicate the cyclic stress-strain curves along three different orientations (rolling direction, normal direction and at 45° between the rolling and normal directions) at different cyclic strain semi-amplitudes. Three different fatigue indicator parameters were calculated from the simulations, based on the accumulated plastic shear strain in one stabilized fatigue cycle from basal slip and pyramidal slip, and from the shear strain accommodated by twinning/de-twinning during one fatigue cycle. The predictions of the fatigue life based on the fatigue indicator parameters were compared with the experimental results for the three orientations. It was found that the most accurate prediction was obtained by using the fatigue indicator parameter associated with twinning/de-twinning. Thus, it is concluded that fatigue cracks nucleated at twin boundaries control the fatigue life of textured Mg alloys.