Two sequences of tension–fatigue and fatigue–tension tests were performed on Cu polycrystalline sheet, with a mean grain size of 32μm. For the angle between the 2 successive loading directions, 2 typical values (0 and 45°) were chosen. The effect of strain path change on subsequent initial work hardening rate and saturation stress in tension–fatigue, as well as the effect of strain path change on subsequent yield and flow behaviour in fatigue–tension were investigated. The strain rate for the tension tests was 5

x 10-3/s, while the fatigue tests were performed under constant plastic strain amplitude control with various amplitudes (εpl = 6 x 10-4, 1.5 x 10-3, 3 x 10-3). Slip morphology and dislocation microstructure were observed by optical and transmission electron microscopy after mechanical tests. Under these conditions, in the case of fatigue–tension, it was found that fatigue pre-straining influenced the subsequent yield and flow behaviour in tension. However, the subsequent mechanical behaviour of samples seemed to be affected only by the magnitude of strain path change (that is, the angle between the 2 successive loading directions), and not by the value of the plastic strain amplitude of the preceding fatigue tests. In the case of tension–fatigue, the strain amount of pre-loading in tension obviously affected the initial cyclic hardening rate, while it had almost no effect on the saturation stress of subsequent fatigue tests, irrespective of the value of the angle between the 2 successive loading directions. The occurrence of micro-bands in the saturation fatigue dislocation structures of samples pre-strained in tension implied that fatigue was a more effective loading mode than was tension, in causing intense glide on the activated slip systems.

Mechanical Behaviour and the Evolution of the Dislocation Structure of Copper Polycrystal Deformed under Fatigue–Tension and Tension–Fatigue Sequential Strain Paths. W.P.Jia, J.V.Fernandes: Materials Science and Engineering A, 2003, 348[1-2], 133-44