A simplified extension of the Crack Analogue model for fretting fatigue with varying normal load

We suggest here a very crude extension to include the case of varying normal load, where we don't attempt to solve the contact mechanics problem rigorously, only providing an estimate of the equivalent Stress Intensity Factor from the instantaneous load and contact area. Even this results in a problem which has no equivalent in standard fatigue, since it corresponds to a “crack” that is oscillating in size at each cycle. This defines the cycles of stress in the actual material points in the fretted area, but the uncertainties in the approximations suggest that only some limit cases should be approaches this way: for example, when the peaks of tangential load are obtained at about a “mean contact area” (out-of-phase loading) then the original CLNA model could be used.