Gross slip criteria in fretting

Abstract

One of the most important characteristics in fretting is the transition from mixed stick-slip conditions to gross slip, as given by the corresponding regime boundary in fretting maps. The regime boundary is determined by the critical values of frequency, normal force, tangential force amplitude and displacement amplitude. Three sets of criteria for incipient gross slip have been studied in the present investigation: fretting scar morphology, tangential force and displacement amplitude interrelations, and fretting energy dissipation. A fully computerized fretting tester has been used, in which the relative displacement between the specimens can be very accurately controlled and measured. Tangential force and relative displacement measurements can be made with resolutions of 2 nm and 5 mN respectively. Pure copper (UNS C11) specimens were studied. Post-test studies of fretting scars were performed by SEM. With some practice, it is possible to identify scar morphologies characteristic of the different regimes. The last two criteria were compared by plotting tangential force amplitude and energy dissipation together as functions of displacement amplitude in so-called FED (force energy displacement) diagrams. Some FED diagrams display force curves without a well-defined critical force amplitude, rendering the force criterion difficult to apply. The energy curves, however, always have a sharp bend, corresponding to a change in energy dissipation characteristics at the gross slip transition. The energy criterion occurs for a lower critical displacement amplitude than the force criterion (when applicable). A possible explanation of this observation is discussed.