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 unde r 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 arra nged into slabs lying along the primary slip plane. The SIN curve is shifte d towards shorter lives with respect to the SIN curve for symmetrical stres s controlled tests. Microcracks initiate - as in the case of symmetrical cy cling - at coarse slip markings with a sharp notch-peak topography. This le ads to the conclusion that cyclic slip localisation always plays the crucia l role in the initiation of fatigue microcracks, but the modes of this slip localisation are different for different loading conditions. In the case s tudied, the localisation is due to relative sliding of the slabs of cells. (C) 1999 Elsevier Science S.A. All rights reserved.