An experimental investigation of fretting fatigue in Ti-6Al-4V: the role of contact conditions and microstructure

A systematic investigation of the fretting fatigue behavior of the titanium alloy Ti-6Al-4V in both the mill-annealed (MA) and the solution-treated an d overaged (STOA) conditions was carried out. A sphere-on-flat fretting fat igue device was used that facilitated real-time control and monitoring of a ll the relevant parameters such as the contact geometry, contact (normal an d tangential) loads, and bulk alternating stress. While different sets of e xperiments were conducted to examine the influence of the bulk stress, the tangential load, and the normal load, respectively, on fretting fatigue res ponse, the effect of microstructure on fretting fatigue was explored with e xperiments on the acicular, Widmanstatten, and martensitic microstructures as well. In experiments where the contact loads were maintained constant an d the bulk stress was varied, fretting reduced the fatigue strength of Ti-6 Al-4V. For this case, the "strength reduction factor" was higher for the ex periments with higher tangential loads. For cases where the bulk stress and the normal or the tangential loads were maintained constant, lower frettin g fatigue lives were obtained at larger tangential loads and at smaller nor mal loads. Of all the microstructures studied, preliminary results on the m artensitic structure suggest an enhanced fretting fatigue resistance, compa red to the basic STOA or the MA microstructure. Using the measured maximum static friction coefficient for Ti-6Al-4V, the experimentally observed cont act and stick-zone radii were found to exhibit good agreement with analytic al predictions. Furthermore, conditions for crack initiation were determine d through the application of the recently developed adhesion model for fret ting fatigue. The model predictions of weak adhesion and crack initiation w ere validated with experimental observations of stick-slip behavior and fre tting fatigue failures, respectively.