Uniaxial cyclic deformation and fatigue behavior of AM50 magnesium alloy sheet metals under symmetric and asymmetric loadings

• Large plastic strain after exceeding CYS leads to a gap in the stress–life curve.
• Hysteresis shape at stress-controlled tests is strongly affected by the stress ratio.
• Twin bands are responsible for a comparatively large scatter in stain–life curves.
• Fatigue model with weighted elastic strain energy density shows best correlation.

The mechanical behavior of twin roll cast AM50 magnesium sheet metals was investigated via quasi-static and cyclic tests. Nine different stress or strain ratios were evaluated to cover the most technical relevant load range. Furthermore, the dependence of the mechanical behavior on loading direction was examined with the transverse direction showing the least tensile elongation and fatigue resistance. Due to the strong basal texture, twinning dominates the plastic deformation in compression of samples parallel to the sheet plane. Especially in stress-controlled cyclic tests, twinning leads to large compressive deformations. In quasi-static and strain-controlled cyclic tests, twin bands were observed and were found to be responsible for local plastic deformation. Special focus is given on the shape of hysteresis loops as well as the cyclic behavior of stress, strain, and energy density to track twinning and detwinning effects. Different possible fatigue parameters are compared concerning their evolution as a function of the fatigue life. An energy based fatigue model with weighted elastic strain energy density shows the best correlation between the lifetime prediction and the experiments.