Formation process of shear bands and protrusions in ultrafine grained copper under cyclic stresses

Ultrafine grained copper processed by equal channel angular pressing was fatigued at two constant stress amplitudes: σa = 240 and 120 MPa (corresponding fatigue lives were Nf ≑ 2 × 105, and 4 × 106 cycles, respectively). Significant differences in the morphological features of post-fatigued surfaces between the high and low cyclic stress amplitudes were observed. To clarify the formation process of the surface damage, the morphological changes in the surface damage caused by the cyclic stresses were monitored successively by an optical microscope. Putting the optical microscopy and scanning electron microscopy observations of the post-fatigued surfaces together, the formation process of surface damage was discussed.