Low-cycle fatigue of a multi-layered aluminum sheet alloy

Low-cycle fatigue (LCF) in engineering structures is caused by a relatively low-frequency strain cycling or thermal cycling. Regardless of the fact that it is much more dangerous than high-cycle fatigue, it has not been studied enough. Coated aluminum alloys are widely used in aerospace and transportation industries, mostly because of their high toughness and strength-weight ratio and improved surface properties. The effect of one-, two- and three-layer coatings, including an inner electroless nickel layer and, additionally, electrodeposited nickel, gold and silver, on the LCF behavior of 1.6-mm-thick 6061-T6 Al alloy was studied in a strain-controlled purely bending mode at maximum plastic strain varied from 0.003 to 0.010. The lifetime of the coated alloy drastically decreases as compared to the substrate. Incipient cracks were revealed, first of all, in the electroless nickel layer and in the substrate close to its surface.