Effects of surface morphology and residual stress on fatigue strength of A6061-T6 alloy and fatigue strength estimation

• Fatigue strength of A6061-T6 alloy decreases with forming large surface dents. • Size of surface dents and waviness parameter correlate with fatigue strength. • Fatigue strength of peened alloy is not correlated with stress concentration factor. • Achievable maximum fatigue strength increases with reducing surface dent size. • Fatigue strength of A6061-T6 alloy with dents and residual stress can be estimated. The influence of surface morphology on the fatigue strength of A6061-T6 alloy was investigated using polishing, peening and T6 heat treatment. Peening is typically performed after T6 treatment to introduce compressive residual stress and work hardening, thereby increasing the fatigue strength. In this study, this sequence was intentionally reversed, and polishing or peening was performed before T6 treatment. The subsequent T6 process strengthened the specimens through precipitation hardening while removing the residual stress and work hardening introduced by the prior treatments. This procedure produced samples with equivalent residual stress and hardness but different surface morphologies. Fatigue strength decreased as surface dent size increased. Although fatigue strength did not correlate with the stress concentration factor at fatigue crack initiation sites, it correlated with dent size as reflected by surface waviness parameters. Moreover, the combined effects of surface morphology and residual stress on the fatigue strength of the alloy were examined by constructing fatigue limit diagrams. The maximum fatigue strength achieved by introducing compressive residual stress increased as the dent size decreased. This trend was attributed to the formation of large dents, which decreased the effects of compressive residual stress and increased stress concentration. Thus, the fatigue strength of the A6061-T6 alloy under the combined effects of surface dents and residual stress was estimated from fatigue limit diagrams, as validated by a well-known √ area method that considering the size of peening dents.