Long-life multiaxial fatigue of SG cast iron

Long life fatigue tests have been performed on cast spheroidal graphite iron using uniaxial tension, torsion and equi-biaxial loading. Data are compared to a Goodman-type fatigue limit criterion previously developed for this material which successfully correlates data for a variety of stress states: uniaxial tension with and without mean stresses, fully reversed torsion, cyclic torsion with mean shear stress, and cyclic torsion with static normal stresses. The criterion includes a multiaxial correction physically based on the growth of small cracks from notches and small defects. It is able to account for the detrimental effect of the negative second principal stress in torsion but the expected benefit of a positive second principal stress in biaxial tension was not observed in experiments. The biaxial stress state greatly increased the observed tortuosity of fatigue cracking and it is hypothesised that this produced a lower than expected fatigue limit as cracks were able to avoid natural non-propagation boundaries.