M. Judelewicz et al., MICROSTRUCTURAL DEVELOPMENT DURING FATIGUE OF COPPER FOILS 20-100-MU-M THICK, Materials science & engineering. A, Structural materials: properties, microstructure and processing, 186(1-2), 1994, pp. 135-142
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.