Fretting induced cracking is commonly observed in industrial components that are in contact and are subjected to small oscillatory movements between them. Fretting causes a considerable reduction in fatigue strength. In this paper recent knowledge on the short and long crack growth behaviour is applied to estimate crack propagation and fatigue life in fretting. The model is based on mode I stress intensity factors with a threshold modified for short cracks. The predicted results are compared with experiments and the influence of the contact pressure is examined. A good correlation between predictions and experimental results are obtained for crack growth rates as well as fatigue lives in terms of number of cycles to failure. It is seen that the increase of fatigue life observed for contact pressures above a certain level can be predicted by the crack growth model.
NOMENCLATUREa, ai = crack length, initial crack length a, = crack length correction da/dN = crack growth rate f= Q/P = ratio of contact friction and normal force P = contact normal force Q = contact friction force R = stress ratio s = pad span K,,,,,, = stress intensity factor caused by maximum stress of bulk load KQ,,,, = stress intensity factor caused by the maximum value of Q K p = stress intensity factor caused by the constant P &(a) = threshold stress intensity factor as a function of crack length AK,, = effective stress intensity range ALKth(m) = threshold stress intensity factors for long cracks Aoax = stress range of axial load Ao,, = smooth specimen fatigue limit
A new multiaxial fatigue testing facility is described. It can strain a thin-walled tubular specimen in three independently controlled loading modes by the use of torsion, axial load, and internal and external pressure. Any stress state having a principal stress ratio between equibiaxial (λ = +1) and torsion (λ = −1) with any orientation of the maximum principal stress and the specimen axis can be chosen, with a maximum principal stress of 700 MN/m2. The significance of specimen geometry is examined in relation to multiaxial fatigue testing and the design of a suitable specimen is discussed. Test data for a 1 Cr-Mo-V steel are presented to show the variation in multiaxial fatigue results for similar stress states obtained using different test systems and specimen geometries. The possibility of undertaking wider investigation of the effects of anisotrophy and cumulative damage is discussed.
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