Mechanisms of fatigue crack growth in Austempered Ductile Iron

The aim of this work is to study the propagation of fatigue cracks in Austempered Ductile Iron (ADI). Threshold and Propagation fatigue regimes are analyzed. Nominal and Effective Threshold stress intensity factors were measured for five different ADI matrix microstructures, and related to its mechanical properties. It was found that the effect of the microstructure is minor, and can be attributed to the different crack closure contribution of each matrix microstructure. The effective threshold values measured are greater than those reported for steels. The crack–nodule interaction was modeled numerically. Based on boundary element modeling results, a crack propagation mechanism was proposed. This mechanism is in good agreement with the observed crack behavior during propagation, and can be used to explain the low propagation rates and high effective threshold values measured in ADI when compared with those of steels.