The electrodeposited and the rolled 12 to 35 mu m thick copper foils are su
bjected to the bending/unbending strain-controlled flex fatigue over a wide
range of strain amplitudes. The fatigue life is associated with an increas
e in electrical resistance of the specimen beyond a preassigned threshold.
For each foil type, in the rolled or as-deposited as well as in the (recrys
tallization-like) annealed conditions, the inverse Coffin-Manson (C-M) rela
tionship between strain amplitude (Delta epsilon/2) and fatigue life (N-f)
is established in the high Delta epsilon/2 (low N-f) and the low Delta epsi
lon/2 (high N-f) regimes. The N-f, Delta epsilon/2, and C-M slopes (c,b) ar
e utilized to calculate the cyclic strain hardening (n') and fatigue ductil
ity (D-f) parameters. It is shown that for a given foil thickness, an unive
rsal relationship exists between D-f and the strength (sigma) normalized fa
tigue life (N-f/sigma). The propagation of fatigue crack through the foil t
hickness and across the sample width is related to the unique fine grain st
ructure for each foil type: pancaked grains for the rolled foil and equiaxe
d grains for the electrodeposited foil. The fatal failure corresponds to co
nvergence of the through-thickness and the across-the-width fatigue cracks.
The variations in (i) electrical resistance, (ii) mid-thickness microhardn
ess and grain structure and (iii) dislocation configurations with fatigue a
re monitored. Except for a small but significant fatigue induced softening
(or hardening), no convincing evidence of strain localization land the asso
ciated dislocation configurations generally observed for the bulk samples)
has been found.