Effect of contact pressure and stress ratio on the fretting fatigue behaviour of Ti-6Al-4V

Fretting fatigue behaviour of Ti-6Al-4V was studied at two different contact pressures (150 MPa and 300 MPa) and two different stress ratios (0.1 and 0.7). The variation of frictional force and tangential force coefficient (TFC) with the number of fretting cycles was studied to understand the fretting fatigue behaviour of the alloy. As evidenced by the variation of frictional force with time as well as frictional force versus cyclic load hysteresis type plots, gross sliding was present at lower contact pressures and lower stress ratios, leading to higher TFC values and surface roughness. Oxide particles were detected in the slip region and in the initiated fretting cracks indicating fretting debris. While the effect of increasing contact pressure in increasing the fretting fatigue life was clearly seen at higher stress ratio, it was not observed at lower stress ratio. At both contact pressures, a significant increase in life with an increase in stress ratio was noticed. The contact problem was analysed using the existing numerical tools to obtain the contact stresses. From these stresses, fretting fatigue lives were estimated as a sum of initiation lives estimated from multi-axial fatigue parameters and propagation lives estimated from fracture mechanics approach. A good agreement while using Findley parameter indicates that the adopted procedure can effectively account for the effects of contact pressure as well as stress ratio.