History effects in metals during constant and variable amplitude testing I: Wavy dislocation glide behaviour

The influence of a mechanical prehistory on the cyclic deformation behaviour and the fatigue life was studied on polycrystalline copper as a material showing wavy dislocation slip character. The specimens were predeformed monotonically either by straining in tension up to different strain values or by swaging. Subsequent fatigue testing was performed at constant plastic strain in the high amplitude, low cycle fatigue region as well as at low amplitudes. Effects of cyclic predeformation were studied in multiple step tests with blockwise constant amplitudes and different loading sequences. Detailed transmission electron microscopy was used to characterize the dislocation arrangements formed under the various conditions. The results of the mechanical tests are compared with constant amplitude data of samples without predeformation in order to describe the degree of dependence on history as a function of testing mode. Differences are interpreted and discussed on the basis of observed changes in the dislocation arrangement.