Advances in ultra-high cycle fatigue

The fatigue of metals has been extensively studied. However, most published research does not extend past around 10⁷ cycles. Because plots of the stress versus number of cycles to failure (S-N curves) of ferrous alloys and some other metals apparently reach a horizontal asymptote, it was assumed that specimens tested at stresses below the asymptote, called the fatigue limit, would have infinite lives. However, recent research has discovered fatigue failures at stresses below the fatigue limit and lives above 10⁷ cycles, termed ultra-high cycle fatigue (UHCF). This paper reviews published research in this area. The S-N curves found in this research are presented, illustrating the location of crack initiation in the UHCF region, the existence of primary and secondary plateaus, and the effects of test frequency, temperature, and environment on fatigue behavior in the UHCF region. This paper also reviews several mechanisms and models for UHCF, including the √area parameter model, the slip mechanism, hydrogen-embrittlement mechanism, and fatigue crack initiation at porosities and inhomogeneities.