Impact of variable loading conditions on fretting wear

Fretting is considered as a specific type of reciprocating sliding. It is defined as a small displacement amplitude oscillatory motion between two solids in contact, usually induced by vibrations. Depending on the loading conditions (displacement amplitudes, normal loading), fretting causes damage by surface fatigue and wear induced by debris formation. To prevent such damage, numerous hard coatings have been developed which improve the wear resistance of contacts. However, one difficulty is to estimate how long it will be before the coating wears through. Studies have been conducted to analyze the effect of displacement amplitude, normal force or ambient atmosphere, but usually under constant loading conditions. Such a situation is far from real operating components, where elements are subjected to variable loadings implying variable displacement amplitudes. To predict the durability of a coating under variable fretting displacements, wear depth is quantified as a function of the maximum accumulated dissipated energy density by derivation from a global energy wear approach. This model is compared to TiC vs. alumina fretting experiments. Very good correlation is observed between the prediction and the wear depth, independently of the applied variable amplitude sequences. An equivalent “Miner-Energy” wear model is introduced which permits the durability of the coating to be estimated.