Fatigue limit of new precipitation-hardened aluminium alloy with distinct fatigue crack propagation limit

The goal of this study was to confirm the presence of fatigue limit in a plain specimen of a new precipitation-hardened aluminium alloy. The alloy was fabricated by adding 0.53% excess Mg to the standard 6061-T6 aluminium alloy with a balanced Mg2Si composition. There were two challenges in this study: verification of the existence of fatigue limit in this new alloy and interpretation of the large scatter in specimen life at the low stress level amplitude. For the former, the existence of a fatigue limit was confirmed by a coaxing effect test. For the latter, the initiation and growth behaviors of a small fatigue crack at low stress amplitude were observed by plastic replica method not only with an optical microscope but also with a scanning electron microscope (SEM). The study results showed that the large scatter in specimen fatigue life at the low stress level amplitude was due to the anomalous initiation and growth behaviors of the small fatigue crack, which in turn was strongly influenced by local conditions. Finally, a conservative S–N curve of this new alloy evaluated by using a plain specimen could be obtained where a distinct knee point showed at cycles of about 106.

► A new alloy is fabricated by adding 0.53% Mg to the 6061-T6 aluminium alloy.
► The existence of a fatigue limit is confirmed by a coaxing effect test.
► The small fatigue crack behaviors is observed by plastic replica method.
► The large scatter of life is due to the anomalous behaviors of the small crack.
► A distinct knee point exists at about 106 in the conservative S–N curve.