Mechanical fatigue of thin copper foil
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The electrodeposited and the rolled 12 to 35 µm 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 (Δε/2) and fatigue life (Nf) is established in the high Δε/2 (low Nf) and the low Δε/2 (high Nf) regimes. The Nf, Δε/2, and C-M slopes (c,b) are utilized to calculate the cyclic strain hardening (n′) and fatigue ductility (Df) parameters. It is shown that for a given foil thickness, an universal relationship exists between Df and the strength (σ) normalized fatigue life (Nf/σ). 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 (and the associated dislocation configurations generally observed for the bulk samples) has been found.
Key wordsCopper foil mechanical fatigue Coffin-Manson relationship electrodeposited
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