MICROSTRUCTURAL DEVELOPMENT DURING FATIGUE OF COPPER FOILS 20-100-MU-M THICK

Fatigue tests were conducted on metallic copper thin sheets of 100 mu m and 20 mu m in the traction-traction mode. Adequate specimens, suita ble grips and a high frequency fatigue apparatus were designed and con structed. The development of dislocation structure during fatigue test s was shown by transmission electron microscopy. Upon cycling we obser ved the formation of an equiaxed cellular microstructure which gradual ly, owing to the stress concentration, transforms to a duplex microstr ucture consisting of elongated grain regions and cellular regions. Fin ally the elongated regions transform into persistent slip bands. Crack initiation occurs at stress concentration sites at the specimen surfa ces but do not appear to be the essential cause of fatigue failure. Fo r samples with grain sizes comparable with the sample thickness, thinn er specimens show higher cycle numbers at the same stress amplitude.