Micromechanical modelling of fatigue in nodular cast iron

Due to its many beneficial properties, nodular cast iron is of great importance for especially the automotive industry. The microstructure is clearly non-homogenous, with a characteristic blend of weak graphite nodules and a steel-like matrix. First, it is shown that the bulk properties of this composite can be represented well using straightforward micromechanical models. Second, many experiments have shown a close relationship between nodule size, shape and distribution,and fatigue properties. The nodules act as short crack initiators due to cyclic plasticity under fatigue loading. The crack properties are analysed using finite element models with patterns of holes, or voids, representing the graphite nodules. A closed form expression is derived for the stress intensity factor (K) of a crack emanating from a nodule in a nodule pattern. Assuming the existence of a short crack fatigue threshold, fatigue limit simulations are performed with variations in crack size, nodule size and spacing. On the basis of the simulations, the fatigue failure probability can be expressed on the classic Weibull form. The fatigue results are primarily qualitative, but are considered capable of capturing some important trends, since the qualitative agreement with experiments is good.