«Effect of loading mode on the cyclic response and the associated substructure of polycrystalline copper in the high-cycle regime»

Studies on the influence of loading mode on the cyclic response of small-grained polycrystalline copper and the associated dislocation structures have been carried out in the high-cycle regime. It is found that the saturation behaviour under constant load control, for two sets of specimens, with and without initial ramp-loading, exhibits strong differences in the “intermediate” range of stress amplitudes, i.e., between 70 and 98 MPa. Within this range the ramp-loading mode promotes a gradual substructure evolution which leads to localization of slip in primary systems and formation of persistent slip bands (PSBs), whereas conventional loading leads to the formation of elongated cells and multiple sets of wall structures (e.g., labyrinth structure), both intimately associated with multiple slip conditions. At low stress amplitudes observed differences in the plastic strain amplitudes obtained at saturation, as an effect of different loading modes, are relatively small and related to equally small differences in the uniformity and homogeneity, from grain to grain, of the dislocation structures associated with that stage. At high stress amplitudes equiaxed cell structures are promoted under both loading modes, the deformation is homogenized, and the cyclic response as a function of loading mode shows no differences.