A threshold curve method was applied to study the fatigue behavior and fatigue blunt-notch sensitivity of austempered ductile iron (ADI) of grade 2 (ASTM 897M-90), a material with an excellent combination of high strength, ductility, toughness, fatigue strength, and wear resistance. The mechanical properties of ADI can be achieved by proper austempering treatments in relation to a unique microstructure consisting of acicular, carbide-free ferrite with carbon-enriched austenite. A mechanistic fatigue crack growth methodology that uses an integrated threshold curve for fatigue crack propagation was applied to analyze its fatigue limit and the influence of blunt notches. The existence and nature of non-propagating cracks and their influence on the definition of the fatigue limit was also analyzed.
Results show that the fatigue limit in ADI is controlled by a crack arrest. Ausferrite packet boundaries act as barriers for microstructurally-small cracks due to a requirement for stage I growth in the ausferrite matrix. The strength of the barrier depends on the relative orientations of the microstructure at the boundary. The fatigue limit of the ADI analyzed is influenced by the ''micronotch'' effect of the graphite nodules. However, it is possible to estimate a true ''matrix'' fatigue limit for the ausferrite microstructure, and the experimentally measured limit may be described as an ADI or component fatigue limit.