Observations of fretting fatigue micro-damage of Ti–6Al–4V

Small displacement contact fatigue below the threshold of sliding produces micro-slip near the edges of contact. The damage that results is known as fretting fatigue, and includes large surface stresses, wear in the regions of slip and crack nucleation, all of which contribute to drastic reductions in fatigue capability. Damage of this type that was generated using three independent test systems on four different contact geometries is compared. Characterization of the damage was conducted using scanning electron microscopy (SEM), semi-quantitative elemental analysis and metallographic cross-sectioning to identify damage micro-mechanisms. The evidence presented indicates the near simultaneous nucleation of multiple cracks, some of which will propagate if subjected to a sufficient bulk stress. While most cracks nucleated in regions of slip, cracks were observed outside of the slip regions under certain conditions. In regions of wear, micro-notches that can lead to the formation of fretting fatigue cracks were observed and appeared to have been the result of local plasticity induced by shot peening. Wear particles were observed in the mouths of cracks as small as 3 μm in depth. The implications of these observations on the development of life prediction models for fretting fatigue are discussed.