CYCLIC STRESS-STRAIN RESPONSE AND STRAIN LOCALIZATION EFFECTS UNDER STRESS-CONTROL CONDITIONS

Polycrystalline nickel was cyclically deformed under stress control. E xperiments with different maximum and mean stresses were performed, al ways applying the maximum stress in the first quarter cycle. During th e tests the cyclic mechanical behaviour was studied and the occurrence of strain localization was investigated by surface relief observation and by transmission electron microscopy of bulk foils. The cyclic str ess-strain response was shown to be affected characteristically by the mean and maximum stresses. On the surface, persistent slip markings w ere found not only for zero mean stress but also for wide range of non -vanishing mean stresses. Persistent slip bands with the typical ladde r-like dislocation structure appeared in the bulk material only at sym metrical or nearly symmetrical cycling.