Microstructural development during fatigue of copper foils 20–100 μm thick

Fatigue tests were conducted on metallic copper thin sheets of 100 μm and 20 μm in the traction-traction mode. Adequate specimens, suitable grips and a high frequency fatigue apparatus were designed and constructed. The development of dislocation structure during fatigue tests was shown by transmission electron microscopy. Upon cycling we observed the formation of an equiaxed cellular microstructure which gradually, owing to the stress concentration, transforms to a duplex microstructure consisting of elongated grain regions and cellular regions. Finally the elongated regions transform into persistent slip bands. Crack initiation occurs at stress concentration sites at the specimen surfaces but do not appear to be the essential cause of fatigue failure. For samples with grain sizes comparable with the sample thickness, thinner specimens show higher cycle numbers at the same stress amplitude.