Fatigue Damage and Microstructure Properties in an AM30 Extruded Magnesium Alloy
In this section the investigation of an AM30 extruded Mg alloy is reported. A strain controlled fatigue test program was conducted to quantify and evaluate the mechanisms of fatigue and fatigue life. A multistage fatigue (MSF) model was developed to predict the fatigue behavior. The MSF model comprises three scales of cyclic damage; crack incubation; microstructurally small crack (MSC), physically small crack (PSC), and long crack growth. The MSF model will predict crack development in a component.
The material employed in this research was an extruded AM30 Mg alloy provided in the form of an automotive crash rail profile with a thickness of approximately 2.5 mm. Samples for this study were taken in the extrusion direction (ED) and in the transverse direction (ETD) as shown in Figure 9.
Cyclic Deformation Response
Figure 12 shows the stress-strain hysteresis loops for the ED at strain amplitudes of 0.6, 0.4, 0.3, and 0.2 percent. An asymmetric pattern in the cyclic deformation is observed for the first cycle for all the strain amplitudes. This asymmetry is more pronounced for the first cycle, and at higher strain levels. The asymmetry from tensile to compressive is larger as the strain amplitude increases. The cyclic stress-strain asymmetry observed in AM30 Mg alloy is due to twinning activation during unloading in compression, and detwinning during loading in tension [3, 17, 18]. In addition, the mid-life hysteresis loops remained relatively asymmetric at higher strain amplitudes, while nearly symmetric patterns are seen at lower strain amplitudes.
The evolution of stress amplitude as a function of the number of cycles for the ED at the strain amplitudes from 0.2% to 0.6% is shown in Figure 14. Stress amplitude increased as the number of cycles increased. The strain amplitudes ranging from 0.3% to 0.6% showed a greater hardening effect than the others strain amplitude levels.
Fractured surfaces of the AM30 Mg alloy that were observed showed typical indications of fatigue damage. River marks flowing outward from a single location at the surface were observed on all specimens. Figure 16 shows a typical fracture surface with a particle-initiated crack. In addition, the SEM images revealed evidence of twinning on the fatigue fracture surfaces similar to that which has been reported in the literature [19, 20].
Multistage Fatigue Modeling
Figure 17 shows the correlations of the MSF model to experimental data for both ED and ETD. It is important to note that the basic material constants were identical regardless of the orientation of cyclic loading, but the microstructural features and mechanical properties were different. As shown in Figure 17, the MSF model appears to correlate to the general trend of the fatigue life in both directions (ED and ETD).