Stress-controlled fatigue behaviour of micro-sized polycrystalline copper wires

Micro-sized polycrystalline copper wires with diameters of 30 μm and 50 μm and mean grain sizes of 12 μm and 20.5 μm respectively were cyclically loaded under stress control over a life range of 1 × 102–4.7 × 105 cycles using a recently developed fibre tensile loading frame at frequencies of 2–9.9 Hz. This frame fits into a scanning electron microscope for in situ deformation and fatigue studies. The fatigue life of the thicker wires, compared to that of the thinner ones, was found to be longer at higher applied stress amplitude range, but shorter at lower applied stress amplitude range. The measurements of the elongation versus the number of cycles revealed three distinct regimes with different slopes, corresponding to different microstructural changes. The total elongation of the wire during the cyclic loading with a low applied stress amplitude was analysed with respect to microstructural changes observed in situ in a scanning electron microscope.