Cyclic strain hardening in polycrystalline copper

Polycrystals of pure copper were cyclically deformed, at room temperature under symmetric tension-compression fatigue at constant total strain amplitude control with an approximate constant plastic strain rate of 10−4. The relationship between the saturation stress amplitude and strain amplitude over a range of plastic strain from 2 × 10−7 to 10−2 reveals three regions of cyclic hardening. A quasi-plateau, where the stresses show a slow constant increase, was observed in the intermediate region extending in the plastic strain range γpl, of 1.5 × 10−5 to 7.5 × 10−4. In this region, persistent slip bands (PSBs) which consist of “ladder” structures, similar to the case of single crystals, were found in the bulk of the fatigued polycrystals. The fatigue limit was found to be Δσs/2 = 73 MPa which corresponds to the plastic strain Δϵpl/2 = 1.5 × 10−5 where PSBs do not form.