Fatigue damage in fretting after crack formation is only initiated under very specific conditions of both amplitude and load. Experience shows, that for a given load the velocity difference between the core of the rubbing specimen and the friction pads is accommodated as amplitude increases through the three following mechanisms: 1. Elastic displacements in homogeneous rubbing solids (or first bodies) is a non-destructive process which will not alter specimen life. 2. Elastic displacements in cracked rubbing solids (or first bodies) can lead to fatigue failure. This mechanism is dangerous as it can lead to failure and thus to life reduction. 3. Shear in debris beds (third-bodies), formed by particles detached from first-bodies. In most instances, these beds protect the rubbing surfaces and prevent or at least retard crack formation and thus fatigue.
The significant drop in life observed experimentally in modified fatigue machines is noted only because the conditions which lead to Mechanism 2 above are often favored in this type of device. An original static fretting fatigue approach is proposed.