Stress–strain response and fatigue life of copper single crystals cyclically loaded with a positive mean stress

Copper single crystals oriented for single slip were cyclically loaded under load control with a non-zero tensile mean stress. No plateau was found in the cyclic stress–strain plot, and no persistent slip bands were found in the dislocation structure. The dislocation structure consists of cells arranged into slabs lying along the primary slip plane. The S/N curve is shifted towards shorter lives with respect to the S/N curve for symmetrical stress controlled tests. Microcracks initiate — as in the case of symmetrical cycling — at coarse slip markings with a sharp notch-peak topography. This leads to the conclusion that cyclic slip localisation always plays the crucial role in the initiation of fatigue microcracks, but the modes of this slip localisation are different for different loading conditions. In the case studied, the localisation is due to relative sliding of the slabs of cells.