Crack initiation mechanisms in copper polycrystals cycled under constant strain amplitudes and in step tests

Abstract

Crack nucleation mechanisms at plastic strain amplitudes of 10 −3 and 10 −4 are reported for polycrystalline pure copper. In specimens cycled at 10 −3, cell structures are formed and, as well known, cracks nucleate at the grain boundaries. At the lower amplitude the dislocation structures consist of persistent slip bands (PSBs) and loop patches. Nevertheless, while some cracks do nucleate in the PSBs, the grain boundaries are the preferred site, rather surprisingly, for both crack nucleation and crack propagation. Crack nucleation mechanisms for low-high and high-low step tests using these two amplitudes were also explored. In low-high tests, for a rather wide range of fatigue life fractions the subsequent cycles at the high amplitude tend to ignore the damage produced first at the low amplitude. The potency of the damage interaction is much greater in high-low tests for the conditions studied here.