Mechanical fatigue of thin copper foil

The electrodeposited and the rolled 12 to 35 mu m thick copper foils are subjected to the bending/unbending strain-controlled flex fatigue over a wide range of strain amplitudes. The fatigue life is associated with an increase 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 (recrystallization-like) annealed conditions, the inverse Coffin-Manson (C-M) relationship 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 epsilon/2 (high N-f) regimes. The N-f, Delta epsilon/2, and C-M slopes (c,b) are utilized to calculate the cyclic strain hardening (n') and fatigue ductility (D-f) parameters. It is shown that for a given foil thickness, an universal relationship exists between D-f and the strength (sigma) normalized fatigue life (N-f/sigma). The propagation of fatigue crack through the foil thickness and across the sample width is related to the unique fine grain structure for each foil type: pancaked grains for the rolled foil and equiaxed grains for the electrodeposited foil. The fatal failure corresponds to convergence of the through-thickness and the across-the-width fatigue cracks. The variations in (i) electrical resistance, (ii) mid-thickness microhardness and grain structure and (iii) dislocation configurations with fatigue are monitored. Except for a small but significant fatigue induced softening (or hardening), no convincing evidence of strain localization land the associated dislocation configurations generally observed for the bulk samples) has been found.