Fatigue Life Evaluation Of Extruded Magnesium Alloy Under Multiaxial Loading

Magnesium alloy components were widely used in automobile and aircraft industries, due to their light weight, high specific strength, stiffness, damping capacity, machinability, and recyclability. Engineering components subjected cyclic loading inevitably and led to fatigue failure. Most studies on magnesium alloy were focus on uniaxial fatigue. Very limited work has been done of magnesium alloys under multiaxial loading. Studies on mulitiaxial fatigue of magnesium alloy have the engineering application and practical significance.In this study, multiaxial fatigue experiments were conducted on extruded AZ31B magnesium alloy. Five multiaxial fatigue criteria based on critical plane approach were employed to predict fatigue life. Estimated fatigue lives were compared between conventional fatigue parameters method and a three-parameter equation method. A new fatigue damage parameter was proposed based on extruded magnesium alloys fatigue crack behavior. The main work and conclusions are as follows:(1) Strain-controlled multiaxial fatigue experiments were conducted on extruded AZ31B magnesium alloy using thin-walled tubular specimens. Four loading paths, including fully reversed tension-compression, cyclic torsion,45oin-phase axial-torsion, and90oout-of-phase axial-torsion, were adopted in the fatigue experiments. It is observed that the strain-life curve displays a distinguishable kink under each loading path at the equivalent strain amplitude around0.3%to0.55%. The fatigue life under the proportional loading path is the highest when equivalent strain amplitudes higher than0.45%, and the fatigue life under the tension-compression loading path is the highest when equivalent strain amplitudes lower than0.45%,. For the same equivalent strain amplitude, fatigue life under nonproportional loading resulted in the shortest fatigue life.(2) Based on Fatemi-Socie model and Smith-Watson-Topper model, estimated fatigue lives were compared between conventional fatigue parameters method and a three-parameter equation method. Predicted fatigue life of three different structural metal materials including7075-T651aluminium alloy,16MnR steel and extruded AZ61A magnesium alloy were checked. It is demonstrated that the three-parameter equation gives predicted results more accurate than conventional fatigue parameters method.(3) Several fatigue models prediction which evaluated by both conventional fatigue parameters equation and a three-parameter equation on extruded AZ31B magnesium alloy. The Fatemi-Socie model and SWT model’s three-parameter equations give better results in factor-of-five boundaries. The KBM model’s three-parameter equation predicted more accurate in factor-of-two boundaries.(4) Five critical plane multiaxial fatigue criteria were employed to predict fatigue life. Most of the predicted results by Fatemi-Socie parameter are within factor-of-five boundaries. Predictions by SWT parameter did not agree well with the fatigue life.69%predicted results are within factor-of-five boundaries and60%results are dangerous. Modified Smith-Waston-Topper parameter was found to be able to predict fatigue lives reasonably well.95%predicted results are within factor-of-five boundaries. Jiang model give safe predictions and78.6%results within factor-of-five boundaries.(5) In addition, fatigue crack orientations were checked with each fatigue model’s critical plane. Fatemi-Socie model and KBM model could tell correct fatigue crack orientation under torsion and nonproportional loading; SWT model and modified SWT model predict the right orientation under tension-compression and torsion loading, respectively. Jiang model’s predictions on the fatigue crack direction for tension-compression, torsion and proportional loading path are consistent with observation.(6) A new fatigue damage parameter was proposed based on extiuded magnesium alloys fatigue crack behavior. It is convenient for engineering applications for has no material constants. It was demonstrated that the new model predicted fatigue life well for extruded AZ31B magnesium alloy and AZ61A magnesium alloy.