Long-life multiaxial fatigue of a nodular graphite cast iron

Nodular graphite cast iron is one example of a material that fails in fatigue primarily by the initiation and growth of Mode I cracks. With this in mind, a tensile critical plane damage parameter would be expected to correlate life for different stress states. Long life fatigue tests have been performed on cast nodular graphite iron using uniaxial tension, torsion and equi-biaxial loading. Results are compared to a Goodman-type fatigue limit criterion previously developed for this material that 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, and is able to account for the detrimental effect of the negative second principal stress in torsion. The expected benefit of a positive second principal stress in biaxial tension was not observed in experiments. The equi-biaxial stress state greatly increased the observed tortuosity of fatigue cracking as compared to the uniaxial or torsion stress states. It is assumed that this produced a lower fatigue limit because the crack driving force was nearly equal in all directions thus allowing cracks to link up weaker regions of the complex cast iron microstructure.