Fatigue limit in 5056 aluminum alloy extrusion studied by rotating-bending tests

The fatigue properties of a 5056 aluminum alloy were investigated. The alloy is the most Mg-rich among commercial Al-Mg alloys and exhibits significant work hardening and strain aging. Rotation-bending tests with smooth specimen and holed specimen were performed at room temperature with stress ratio R=-1 and frequency of stress cycle F=55Hz. The experimental results show that a clear fatigue limit, as the one exhibited by steel, exists on the S-N curve of the smooth specimen. Surface observation with the replica method has revealed a wavy pattern around the second phase particle for N=10⁶-10⁷ but no nonpropagation crack was observed with smooth specimen. It is considered that the observed fatigue limit appeared as a result of marked work hardening and strain aging at the heavily deformed zone and fatigue cracks did not initiate from the zone up to the maximum number of stress cycle N=3.5×10⁷ employed in the present study. On the other hand, non-propagating crack was observed with holed specimen at the stress amplitude of σ_a=70MPa.