Analysis of fatigue crack initiation in cycled austempered ductile cast irons

Interrupted total strain controlled tests have been performed on cylindrical specimens of austempered ductile cast iron (ADI) with the aim to study fatigue crack initiation and provide the fine numerical simulations at temperatures 23 degrees C and -45 degrees C. The systematic observation of the same places on surface specimen by SEM was done in interrupted low cycle fatigue test. Fatigue cracks initiated preferably from persistent slip markings at the interface of graphite nodule and the ausferrite matrix and places without visible graphite nodule. To prove this assumption the numerical simulations were performed in order to study this situation in more details. The three-dimensional (2D) model of the particle in the matrix was created in order to estimate the stress concentration factor caused by the particle. The loading of the model was composed of two steps - the cooling and then applied the tension stress. These steps are identical to the real testing conditions. To study the influence of the particle position near the surface the two-dimensional (2D) numerical analysis was performed where the position of the particle changed its position within the matrix. The highest sensitivity to the cracking of the matrix represented the position of the particle near by the surface of the specimen. The numerical analyses presented here helped to better understanding of the experimentally observed processes in the fatigued ADI. (C) 2010 Published by Elsevier Ltd.