Prediction of crack nucleation under partial slip fretting conditions

Fretting can be defined as small-amplitude oscillatory motions between two solid surfaces in contact. It has been known for a long time that such loading introduces cracking and/or wear in many structural parts. For very small amplitudes, the contact presents a composite structure where, during the loading cycle, some domains remain stuck whereas others endure sliding. Such a partial slip condition mainly favours crack nucleation and crack propagation. The loading path imposed through the contact requires precise identification of the sliding condition and the local friction coefficient operating through the sliding domain. An energy approach is here developed combining the online identification of the sliding domain and the determination of the local friction coefficient. A normalized fretting sliding chart is introduced and discussed, taking into account the impact of plasticity on the sliding condition. Applying analytical and finite element model computations, it has been shown that crack nucleation under stabilized partial slip can be predicted by transposing multiaxial fatigue criteria (Dang Van, Crossland, McDiarmid and Smith-Watson-Topper). Such a prediction nevertheless requires that the high stress gradient imposed by the contact loading is taken into account. The loading states, which are computed to determine the risk of crack nucleation, must be averaged over an elementary volume whose length scale can be compared with the microstructure.