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.