Influence of nodule count on fatigue properties of ferritic thin wall ductile iron

The present work focuses on the study of the influence of nodule count on the fatigue resistance of ductile iron. Fatigue tests were carried out on specimens taken from thin wall ductile iron plates of 2 and 4 mm thickness and standard Y blocks of 12.7 mm thickness, showing nodule counts ranging between 1800 and 300 nod mm(-2). All samples were ferritised before testing to obtain a homogeneous ferritic matrix. The results showed a large dispersion of fatigue strength values. Nevertheless, careful examination of the fracture surfaces showed the presence of very small casting defects on many test samples. When only the results measured on sound test samples were accounted for, a significant increase in fatigue strength was found as the nodule count increases. Casting defects, particularly microshrinkage, revealed by scanning electron microscopy on the fracture surfaces, were responsible for the premature failure, due to shortening of the crack initiation stage. The fatigue lives measured experimentally were compared with estimations based on the fracture mechanics theory.