Investigation of non-Coulomb friction behaviour in reciprocating sliding

A commonly observed phenomenon where the friction force increases during the gross slip phase of individual fretting cycles is investigated with the aim of identifying the physical origins of this type of frictional behaviour. Measurements of sliding friction from linear and torsional fretting tests, using the aerospace nickel alloy Udimet 720, and subsequent analysis of the post-test worn surfaces were used to investigate the phenomenon. It was found that this friction variation is due to wear–scar interaction effects. These interactions were primarily found to occur at sites distributed throughout the nominal contact area via the interference of local interlocking peaks and troughs on the worn surfaces. Cross-correlation and auto-correlation analysis of the worn surfaces was used to identify, and to show the approximate size of, these local features. Many of the features were found to be similar in size to the applied fretting stroke, but on average, the features were somewhat larger. A simple one degree-of-freedom model of the interaction of an idealised surface peak with a corresponding surface groove was developed to show how these interactions produce the type of friction variation which is commonly observed during the sliding phase.

► The friction variation which is commonly observed during the gross-slip phase of fretting cycles is a result of wear-scar interactions.
► These wear-scar interactions were found to occur primarily at local ‘peaks and troughs’ distributed throughout the nominal contact region.
► 3D profilometry and cross/auto correlation methods were used to identify these local wear-scar features and determine their size.
► A simple model has been put forward to explain how interaction of wear-scar features can produce the friction variation which is commonly observed.
► A new test rig, designed to adapt in-line fretting test machines for torsional fretting has been designed, and shown to work effectively.