Influence of casting defects on the fatigue limit of nodular cast iron

The high cycle fatigue behaviour of a nodular cast iron has been investigated under tension loading. Casting defects are at the origin of crack initiation for all samples tested. Critical defects (shrinkage) are located either at the surface or within the bulk. The role of the size and position of natural defect on the fatigue limit has been established. Results show that, for a given size, internal defects are less damaging than surface located ones. SEM observations revealed the existence of non-propagating surface cracks below the fatigue limit. By means of a marking technique, the 3D shape of these cracks initiated around the defects was fully described. In an attempt to simulate the evolution of the fatigue limit with defect size, some existing approaches are compared to experimental data; it is shown that 2D analysis of defects (crack or notch) is conservative.