A Bidimensional Finite Element Study of Crack Propagation in Austempered Ductile Iron

Austempered ductile iron (ADI) is composed of an ausferritic matrix with graphite nodules and has a wide range of applications because of its high mechanical strength, fatigue resistance, and wear resistance compared to other cast irons. The amount and size of the nodules can be controlled by the chemical composition and austenitizing temperature. As the nodules have lower stiffness than the matrix and can act as stress concentrators, they influence crack propagation. However, the crack propagation mechanism in ADI is not yet fully understood. In this study, we describe a numerical investigation of crack propagation in ADIs subjected to cyclic loading. The numerical model used to calculate the stress intensity factors in the material under the given conditions is built with the aid of Abaqus commercial finite element code. The crack propagation routine, which is based on the Paris law, is implemented in Python. The results of the simulation show that the presence of a nodule generates a shear load on the crack tip. Consequently, even under uniaxial tensile loading, the presence of the nodule yields a non-zero stress intensity factor in mode II, resulting in a deviation in the crack propagation path. This is the primary factor responsible for changing the crack propagation direction towards the nodule. Modifying the parameters, for example, increasing the nodule size or decreasing the distance between the nodule and crack tip, can intensify this effect. In simulations comparing two different ADIs with the same graphite fraction area, the crack in the material with more nodules reaches another nodule in a shorter propagation time (or shorter number of cycles). This suggests that the high fatigue resistance observed in ADIs may be correlated with the number of nodules intercepted by a crack and the additional energy required to nucleate new cracks. In summary, these findings contribute to a better understanding of crack propagation in ADIs, provide insights into the relationship between the presence of nodules and the fatigue resistance of these materials, and support studies that associate the increased fatigue resistance with a higher number of graphite nodules. These results can also help justify the enhanced fatigue resistance of ADIs when compared to other cast irons.