FATIGUE IN COPPER SINGLE CRYSTALS AND A NEW MODEL OF FATIGUE IN FACE-CENTERED CUBIC METALS

Abstract

The fatigue process has been studied in copper single crystals cycled in torsion at constant total shear strain. A discontinuity in the slope of the S–N curve of [Formula: see text] axially oriented specimens and a step discontinuity in the S–N curve of [Formula: see text] oriented specimens dividing the curves into a low-amplitude and high-amplitude region, together with metallographic changes, are interpreted as evidence that mechanisms of fatigue failure are dependent on amplitude. In the low-amplitude range the distributions of fatigue striations have been compared with the resolved shear stresses plotted as a function of azimuthal angle. This comparison shows that repeated cross-slip models do not account for the observed striation distribution but, instead, high stresses on both the primary slip system and another slip system in the primary plane are important. A new model of fatigue striation formation is proposed according to which the interaction of dislocations of at least two different Burgers vectors in the primary plane forms sequences of cells whose sides are twist boundaries parallel to the primary plane separated by a relatively dislocation-free interior. Slip is believed to concentrate between the twist boundaries giving rise to striations. The model is shown to be in good accord with available experimental evidence. At high strain amplitudes slip on intersecting planes will account for the change in macroscopic appearance and fracture behavior from those observed at low amplitudes.