Microstructure-based multistage fatigue modeling of a cast AE44 magnesium alloy

The multistage fatigue model developed by McDowell et al. was modified to study the fatigue life of a magnesium alloy AE44 for automobile applications. The fractographic examination indicated three distinct stages of fatigue damage in the high cycle fatigue loading regime: crack incubation, microstructurally small crack growth, and long crack growth. Cracks incubated almost exclusively at the cast pores that were near the free surface, located near sharp geometry changes of the test specimen, or at extremely large pores inside the specimens. Microstructurally small cracks grew in the eutectic region along the weak boundaries of the grains and dendrites or at very closely packed microstructural discontinuities. Long cracks were observed to grow in a transgranular fashion. Specimens fabricated from as-cast bars and extracted from a cast engine cradle were tested at room temperature and an elevated temperature typically required for automotive powertrain applications. A large variation of fatigue life in the high cycle fatigue region was observed in specimens from both conditions due to the sensitivity from microstructural discontinuities. The microstructure-based multistage fatigue model was generalized for the AE44 magnesium alloy to capture the network of porosity and temperature dependence. The modified multistage fatigue model was also used to estimate the upper and lower bounds of the strain-life curves based on the extreme microstructural discontinuities.