Mean stress in long-life torsion fatigue

Simple fatigue tests have been performed on two common engineering materials, cast ductile iron and low carbon steel, using three stress states, fully reversed cyclic torsion, cyclic torsion with mean shear stress and cyclic torsion with static axial and hoop stresses. Tests were designed to illustrate the effect of different types of mean stress and discriminate between normal stress and hydrostatic stress as the most suitable mean stress correction term for high cycle fatigue analysis. Nucleation and early crack growth in the low carbon steel is along maximum shear planes while for cast iron, pre-existing flaws grow on maximum normal stress planes. Data illustrates that tensile normal stress acting on a shear plane significantly reduced fatigue life and is an appropriate input for fatigue analysis of ductile materials. Static normal stresses did not significantly affect the fatigue life for the cast iron because the net mean stress on the maximum normal stress plane was zero. Mean shear stress reduced the fatigue strength of the low carbon steel. For cast iron the effect of mean shear could be modelled by resolving the mean shear into a mean tensile stress on the principal stress plane. Suitable stress based damage models for both materials are presented.