Cyclic Behavior of an AZ31 Sheet Magnesium Alloy

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Introduction

Figure 1. Dimension of fatigue specimen.

The influence of the microstructural features due to the effect of in-plane constraint from rolling on the fatigue resistance of sheet-formed AZ31 was investigated. We employed a microstructure-sensitive fatigue model that comprises fatigue total fatigue lifetimes into crack incubation, microstructurally small crack (MSC) and physically small crack (PSC), and long crack growth [1, 2], to correlate the behavior of AZ31 Mg alloy to experimental results. The MSF model was used to capture competing structure-property relations, including grain size, inclusions size, and mechanical properties, and their accompanying impact on fatigue lifetimes.

Results

Fig. Figure 2. Etched microstructure image of the AZ31 Sheet magnesium alloy.

The material employed was a sheet of AZ31 Mg alloy. Dimensions of the fatigue specimens used in this research are shown in Figure 1. Fatigue tests were conducted on a servo-controlled electro-hydraulic system. These tests were performed under strain control mode, constant strain amplitude, and fully reversed. Tests were conducted at room temperature with a frequency of 5 Hz. A 50% drop of the peak stress load was used to determine failure of the sample.

Microstructure

The microstructural characteristics of the material were quantified to provide an understanding of the behavior under monotonic and cyclic loads. Figure 2 shows the grain morphology and size observed for a section parallel to the rolling direction, where elongated grains averaging 5.7 µm were observed. The intermetallic particles were found to have an average diameter of 3.7 µm and a nearest neighbor distance of 35.5 µm.


Fatigue Model Correlation

Figure 8 shows the correlations of the MSF models to the experimental strain-life data. The MSF model appears to correlate to the general trend of the fatigue life in the AZ31 sheet-formed alloy.
Figure 8. Multistage fatigue (MSF) model and strain-life results for an AZ31 sheet magnesium alloy


Conclusions

The cyclic behavior of AZ31 sheet-formed Mg alloy was investigated based on the variation in microstructural features and mechanical properties. A multistage fatigue (MSF) model was developed to capture competing structure-property and mechanical properties, and their consequential impact on fatigue lifetimes. In regard to the research conducted here, the following conclusions are made: • The strain-life curve for sheet-formed Mg alloy showed a similar pattern to the same for Mg extruded alloys. • The hysteresis lops of the AZ31 sheet Mg alloy showed almost a symmetric cyclic behavior for most of the strain amplitudes observed. In contrast with the behavior observed for an extruded alloy, which shows an asymmetric behavior. • A slight strain hardening effect was observed for all strain amplitudes. • Observations of the fracture surfaces under SEM revealed that fractured intermetallic particles initiated fatigue cracks. The crack initiation sites were found to occur more frequently at inclusions near the free surface of the specimens.

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