On the unusual R-ratio dependence of the high cycle fatigue performance of precipitate strengthened Al alloys

The high cycle fatigue (HCF) performance of precipitate strengthened Al alloys is important for their use in moving structures applications such as transportation. It was first shown in the 1950′s, and has been confirmed multiple times since, that sometimes the weaker underaged alloys can show longer high cycle fatigue lives under R = -1 loading than the stronger peak aged materials. However, under alternative R-ratio loadings (e.g. R = 0.1), the underaged alloys tend to show the same, or even worse, HCF performance as the peak aged alloys. This unusual R-ratio dependence is studied in this work using a combination of fatigue testing on smooth samples, optical profilometry of the evolution of surface roughness during cycling, and transmission electron microscopy. We demonstrate that the unusual R-ratio effect is really an effect of the maximum applied stress, and the critical role that this has in determining the strain localization that occurs during the first cycle of loading. This initial strain localization sets the path for subsequent plasticity accumulation which eventually leads to fatigue failure. The maximum applied stress plays a stronger role in determining the initial strain localization than either the applied stress amplitude or R-ratio and this is the critical factor determining the fatigue life of the precipitate strengthened Al alloys considered in this study.