A Study Of Fatigue Properties Of AZ21 Mg Alloy Under Complicated Loading Conditions And Corrosive Environment

The mechanical and fatigue properties of magnesium alloys differ widely in different loading directions due to their hexagonal close-packed crystral structure. As widely used for engineering structural components, Mg alloys are normally subjected to complicated multiaxial loads, and generally suffer multiaxial fatigue failure. Moreover, corrosive environment also degrades the fatigue properties of Mg alloys, and reduces their service life. This work aims to reveal the effects of load paths and corrosive environment on the mechanical and fatigue properties of Mg alloys by conducting a series of multiaxial fatigue tests under stress control. On this basis, appropriate multiaxial fatigue life criteron were selected to reasonablely predict fatigue life under complicated loading condtions and corrosive environment. The multiaxial loading – fatigue life curve is of great reference and guide meaning to the pratical application of Mg alloys.In this study, the cyclic mechanical properties of AZ21 Mg alloy were investigated under uniaxial tension-compression, torsion, and multiaxial loading paths in air and corrosive environment, respectively, with the following major findings: 1) the dominant deformation mechanisms vary under different loading paths: under uniaxial tensioncompression path, tensile twining occurs during compression, resulting in the lower yield strength in compression than in tension; under torsion path, the shear stress-strain hysteresis loops remain symmetrical, indicating slip as the dominant mechanism for inelastic shear deformation; under non-proportional multiaxial loading path, the axis of principal stress is alwaying changing, making twining-detwining the main deformation mechanism; 2) under proportional and non-proportional multiaxial loading paths, the material shows siginicant cyclic hardening with the cyclic strain amplitude droping drastically during the early loading, and reaching a saturated state gradually, and the saturated cyclic strain amplitude shows little dependence on the loading path; 3) fatigue life under multiaxial loading paths are much shorter than that under uniaxial tensioncompression and torsion paths; with the same equivalent stress amplitude, the area of stress-strain hysteresis loops under round path is the largest, corresponding to the maximium energy dissipation, leading to the shortest fatigue life; 4) under corrosive environment, the area of stress-strain hysteresis loops is larger than in air, leading to a higher fatigue damage rate, and thus a shorter fatigue life.Basquin equation and Findley-Park-Nelson(FPN) equation were used to predict fatigue life under different environments and loading paths, and Basquin equation was found to yield better prediction than FPN equation in general. However, both equations are not able to distinguish the effect of loading path where the loading amplitudes are identical; by introducing a non-proportionality factor into Basquin equation, reasonable life predictions were achieved for the round and diamond paths with the same loading amplitude.