Fatigue crack propagation damaging micromechanisms in ductile cast irons

Ductile iron discovery in 1948 gave a new lease on life to the cast iron family. In fact, these cast irons are characterized both by a high castability and by high toughness values, combining cast irons and steel good properties. Ductile cast irons are also characterized by high fatigue crack propagation resistance, although this property is still not widely investigated.In the present work, three different ferritic–pearlitic ductile cast irons, characterized by different ferrite/pearlite volume fractions, and an austempered ductile cast iron were considered. Their fatigue crack propagation resistance was investigated in air by means of fatigue crack propagation tests according to ASTM E647 standard, considering three different stress ratios (R = Kmin/Kmax = 0.1; 0.5; 0.75). Crack paths were investigated by means of a crack path profile analysis performed with an optical microscope. Crack surfaces were extensively analysed by means of a scanning electron microscope both considering a traditional procedure and performing a quantitative analysis of 3D reconstructed surfaces, mainly focusing graphite nodules debonding.