CYCLIC STRAIN-HARDENING IN POLYCRYSTALLINE COPPER

Polycrystals of pure copper were cyclically deformed, at room temperat ure under symmetric tension-compression fatigue at constant total stra in amplitude control with an approximate constant plastic strain rate or 10(-4). The relationship between the saturation stress amplitude an d strain amplitude over a range of plastic strain from 2 x 10(-7) to 1 0(-2) reveals three regions of cyclic hardening. A quasi-plateau, wher e the stresses show a slow constant increase, was observed in the inte rmediate region extending in the plastic strain range gamma(pl), of 1. 5 X 10(-5) to 7.5 x 10(-4). In this region, persistent slip bands (PSB s) which consist of ''ladder'' structures, similar to the case of sing le crystals, were found in the bulk of the fatigued polycrystals. The fatigue limit was found to be DELTAsigma(s)/2 = 73 MPa which correspon ds to the plastic strain DELTAepsilon(pl)/2 = 1.5 x 10(-5) where PSBs do not form.