MIXED FRETTING REGIME

The mixed fretting regime (MFR) has been proven to be the most dangero us regime for crack nucleation and service failure. It can be identifi ed from the evolution of the ''tangential load-displacement'' loops, i .e. when there is a variation in the shape of the loops. Usually ellip tical (partial slip) and quasi-rectangular (gross slip) loops are enco untered in MFR. This regime has been obtained in fretting wear and fre tting fatigue tests on numerous metal- base alloys. In contrast with t he classic stick-slip condition in Mindlin's elastic theory, MFR depen ds strongly on the normal load, the imposed amplitude and material pro perties such as elongation. During fretting testing under MFR, the mat erial can fail due to overstraining or overstressing and the location of the maximum varies. For ductile materials, a competition exists bet ween the formation of the tribologically transformed structure, partic le detachment and the nucleation of fatigue cracks. In the case of bri ttle materials, MFR is generally avoided due to the rapid formation of debris and the establishment of a third body layer in which easier ve locity accommodation favours the gross slip condition. A mechanical an d material approach is used to analyse the specific behaviour of mater ials submitted to MFR. The study of fretting crack nucleation and prop agation is emphasized.