Life time and cyclic slip of copper in the VHCF regime

Studies were performed on the fatigue properties of polycrystalline copper at very low cyclic stress amplitudes and very high numbers of cycles. The experiments were carried out with the time-saving ultrasonic fatigue technique. Thus, experiments up to more than 1010 cycles could be performed within reasonable testing time. Main result is that below the PSB threshold, irreversible cyclic slip along crystallographic planes still occurs, as evidenced by changes in the surface slip line pattern down to strain and stress amplitudes that are as low as approximately only half of the PSB threshold values. These values are named SB (slip band) thresholds. No fatigue limit in the conventional sense could be detected below 1 × 1010 cycles. Thus, a threshold value at 1 × 1010 cycles is defined (named fatigue life threshold in this study) which is roughly 50% higher than the PSB threshold. The question whether the SBs are actually PSBs or whether continued cycling below the PSB threshold would ultimately lead to PSB formation and finally failure after extremely long testing has to be answered in future studies.