AFM observations of grain boundary step in low-cycle fatigue in copper

The push-pull low cycle fatigue test were performed under the strain-controlled condition and the step height formed at grain boundary, which is precursors of crack initiation, was measured in annealed copper using AFM. Then the step growth behaviors in cyclic loading process were compared with the one obtained from fatigue test performed under stress-controlled condition. In the early fatigue process, grain boundary step was not formed under strain-controlled condition. In the early fatigue process, grain boundary step was not formed under strain-controlled condition, whereas steps with 20-50nm in height were generated under stress- controlled condition. The critical step height at grain boundary at crack initiation Hc was quite large under strain controlled condition as compared with the one under stress controlled condition.