Fatigue of copper single crystals at low constant plastic strain amplitudes

Abstract

Single crystals of copper with single slip orientation were cycled at constant plastic strain amplitudes until fatigue failure. The strain amplitudes were selected to vary the volumetric fraction of persistentslip bands in the gauge of the specimen. The cyclic response has been measured by monitoring the changes in hysteresis loop shape, energy absorption and peak, average and saturation stresses. The hysteresis loop shape is quantitatively expressed through a parameter which turns out to be particularly sensitive to changes in deformation characteristics. The results show that the plastic strain amplitude affects the initial hardening rate, the cumulative strain value at which the maximum peak stress is reached, the shape variation of the hysteresis loop, the energy input per unit amount of deformation and the fatigue life in general as measured by the number of cycles to failure or by the energy absorption per cubic millimetre to failure. The fatigue results are explained by assuming that the crack propagation rates in Stage 1 are dependent on the plastic strain amplitude even when only the volumetric fraction of persistent slip bands is varied in the gauge of the specimen.