Comprehensive Multistage Fatigue Model of Magnesium Alloys

Environmental concerns and economic advantages have produced a demand for increased vehicle efficiency through lightweight structural materials in the automotive and aerospace industries. Magnesium is a desirable candidate for many applications with its high strength to weight ratio. Automotive structural components are repeatedly loaded and unloaded to support the shifting weight during use. To effectively implement magnesium alloys in these cyclic environments, an accurate fatigue model is necessary to predict cyclic failure. The MultiStage Fatigue model seeks to accomplish this by identifying the microstructural features and their effects during crack formation and propagation. Because this model is based on the alloy’s mechanical properties and microstructure, it can predict the fatigue life of many magnesium alloys and for a range of processing methods.

The purpose of this research is to characterize the constants for a single model capable of predicting the cyclic response of any magnesium alloy. Fifteen magnesium alloys have been selected for this analysis. These alloys include AZ, AM, NZ, ZM, and ZK type alloys.  Material processing methods include extruded, cast, plate, and sheet, with some extruded materials evaluated in multiple directions to represent the range of anisotropy in the material. Most fatigue data and microstructural information for this study was taken from literature. When microstructural data was not available, a typical value for the given processing method was used. When strain controlled fatigue data could not be acquired for a given material, the cyclic stress-strain relationship at mid-life was used to equate stress-life data to strain-life.

The MultiStage Fatigue model uses microstructural properties, such as grain size, particle size, and porosity, to predict crack formation and crack growth. Model parameters characterize the microstructural influences during each of the distinct crack regimes. The incubation regime is governed by plastic damage accumulating at critical inclusions. Once significant damage has accumulated, the small crack regime is calculated based on a cyclic crack tip displacement model. Finally, once the crack has reached significant length, the long crack regime operates according to the Paris law.

Results show that the model was able to successfully predict the wide range of magnesium alloys presented. In cast materials, pore size played a primary role, while grain size and particle size had a significant effect on fatigue life of wrought materials.  

This model can be applied to assist in the design process for geometry and material selection to accurately predict fatigue life. Additionally, when the microstructural heterogeneity throughout a component can be accounted for, this model can produce accurate results for the integrity of the component relative to the microstructure and load experienced in each region. Future work for this model should be expanded to include additively manufactured alloys and the respective defects that effect fatigue life. Fatigue dependence on operating temperature and corrosion should also be added to the model.