Fatigue damage in copper polycrystals subjected to ultrahigh-cycle fatigue below the PSB threshold

In High-Cycle Fatigue (HCF) of materials like copper, the most common failure modes originate from cyclic strain localization in persistent slip bands (PSBs). The latter form only when the loading amplitudes exceed the PSB thresholds. In contrast to this well-known HCF behaviour, it is shown in the present study that fatigue damage (cyclic strain localization, surface roughening, stage I crack initiation) develops even at loading amplitudes well below the PSB threshold in copper subjected to UltraHigh-Cycle Fatigue (UHCF). These findings are attributed to the accumulation of very small cyclic slip irreversibilities over very large numbers of cycles (>1010).