CYCLIC STRESS-STRAIN RESPONSE AND MICROSTRUCTURE UNDER VARIABLE AMPLITUDE LOADING

In order to study the cyclic stress-strain behaviour under variable am plitude loading in terms of dislocation glide character and microstruc ture, various metals and alloys were cyclically loaded in single-step tests and in an incremental step test (IST). The cyclic stress-strain curves (CSS curves) which correspond to the different loading conditio ns were compared and the shapes of the hysteresis loops were analysed with respect to Masing or non-Masing behaviour. As expected, cyclic lo ading with constant amplitude leads to the formation of a characterist ic dislocation arrangement that depends on the amplitude applied. The amplitude-related microstructure corresponds to the observation that M asing behaviour was not found in single-step tests. However, in the IS T a state of saturation, or at least approximate saturation, is attain ed after some or many loading blocks. In the case of single-phase poly crystalline materials, wavy slip behaviour was found to be a prerequis ite for the fulfilment of Masing behaviour. Then, the CSS curve, obtai ned by means of IST, intersects that of the single-step tests at an in termediate amplitude. Planar-slip materials show very similar CSS curv es in spite of considerable differences in the dislocation arrangement s for different testing modes. The behaviour of particle-hardened allo ys depends very strongly on the relative effectiveness of the precipit ates as obstacles for dislocation motion and can be understood analogo usly to single-phase materials on the basis of the resulting dislocati on glide character.