Effect of triaxial stress on ductility and fracture morphology of ferritic spheroidal graphite cast iron

Four ferritic spheroidal graphite (SG) cast irons of different graphite nodule interparticle spacings were prepared for a tensile experiment. The results indicate a decreasing ductility with increasing nodule spacing. Transition from pure dimple to a mixed mode of dimple and cleavage occurs when the nodule spacing reaches a critical value. The data of the S.G. cast iron possessing the largest nodule spacing reveal that the ductility is increased by raising the superposed hydrostatic pressure. The mixed mode of dimple and cleavage fractures of this SG cast iron, which occurs under normal pressure, is replaced by pure dimple fracture under all the hydrostatic pressure conditions. These results are analyzed in this article in terms of the triaxiality ratio at the ferrite center region. The higher the local triaxiality ratio, the lower the ductility and the larger the chance for cleavage fracture to occur.