Microstructure-Sensitive Fatigue Modeling for Magnesium Materials and Components
Team members: PI, Marcos Lugo
Strain-life fatigue tests of selected magnesium alloys (AZ31, AM30, AZ91, and AM60) were conducted to investigate the fatigue behavior of magnesium (Mg) alloys used as automotive components. Structure-property analyses were conducted to determine relations between microstructural features and fatigue life. In addition, scanning electron microscope (SEM) experiments were conducted to determine fatigue crack incubation and microstructurally/physically small fatigue crack stages. Experiments were also performed to determine the fatigue performance of several different types of joints.
Fatigue performance of Mg alloys was characterized, and a structure-property modeling framework for fatigue life using the Multistage Fatigue (MSF) modeling approach was developed. We used a microstructure-sensitive fatigue model which decomposes total fatigue lifetimes into crack incubation, as well as microstructurally small crack (MSC), physically small crack (PSC), and long crack growth, to correlate the differences in fatigue behavior of Mg alloys. The utilized model is capable of capturing competing structure-property relations, including grain size, inclusion size, and texture, and their consequential impact on fatigue lifetimes. While McDowell et al.  originally developed the model for a cast A356 aluminum (Al) alloy, it has been modified to extend its application to wrought materials [2,3]. The total fatigue life NTotal is given by equation (1). N_Total=N_Inc+N_(MSC/PSC)+N_LC, (1) where NTotal is the total fatigue life. NInc is the number of cycles to incubate a crack at a micronotch formed by an inclusion, which can be a relatively large constituent particle, a large pore, or a cluster of each or both. The incubated crack extends from the inclusion into the matrix and propagates through a region of the micronotch root influence. NMSC/PSC is the number of cycles required for propagation of a microstructurally small/physically small crack. Finally, NLC is the number of cycles required for long crack (LC) propagation to final failure, which depends on the amplitude of loading and the corresponding extent of microplasticity ahead of the crack tip. This multistage framework was evaluated for the prediction of fatigue damage in Mg alloys and welded joints. The MSF will also be implemented into commercial finite element analysis (FEA) codes and validated by solving specific problems concerning the mechanical response and reliability/safety aspects of Mg alloys used in automotive applications.
1. Cyclic Behavior of an AZ31 Sheet Magnesium Alloy
2. Fatigue Damage and Microstructure Properties in an AM30 Extruded Magnesium Alloy
3. Multiscale Fatigue Modeling of AZ91 and AM60 Magnesium Alloys
4. Fatigue of Friction Stir Spot Welding in Magnesium Alloys