GROSS SLIP CRITERIA IN FRETTING

One of the most important characteristics in fretting is the transitio n from mixed stick-slip conditions to gross slip, as given by the corr esponding regime boundary in fretting maps. The regime boundary is det ermined 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 betw een the specimens can be very accurately controlled and measured. Tang ential force and relative displacement measurements can be made with r esolutions of 2 nm and 5 mN respectively. Pure copper (UNS C11) specim ens were studied. Post-test studies of fretting scars were performed b y SEM. With some practice, it is possible to identify scar morphologie s 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 (for ce energy displacement) diagrams. Some FED diagrams display force curv es without a well-defined critical force amplitude, rendering the forc e criterion difficult to apply. The energy curves, however, always hav e a sharp bend, corresponding to a change in energy dissipation charac teristics at the gross slip transition. The energy criterion occurs fo r a lower critical displacement amplitude than the force criterion (wh en applicable). A possible explanation of this observation is discusse d.