Cyclic stress-strain response and strain localization of polycrystalline Cu tested under stress-control and different start-up procedures

The influence of the start-up procedure and ramp length (of 1 and 100 cycles) on the cyclic response under load controlled testing at room temperature was studied with specimens of polycrystalline Cu. Cyclic stress-strain curves were obtained for stress amplitudes between 30 and 110 MPa. An increase of the ramp length was found to cause a shift of the CSS curve towards smaller values of saturated plastic strain. This shift could be related to the amount of unidirectional strain which was found to depend on the ramp length and start-up procedure. The formation of slip bands was studied by SEM techniques. The magnitude of plastic strain corresponds to the density of PSBs which decreased considerably with increasing ramp length. Prior to the formation of PSBs tensile slip bands could be identified which were produced most profusely in the case of a ramp length of 1 cycle. The PSB nucleation was observed to occur at these tensile slip bands during further cycling, at shear stress amplitudes of larger than 18 MPa. This low nucleation shear stress can be explained on the basis of dislocation arrangements observed in TEM micrographs.