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Fretting--small amplitude back-and-forth slip of mating parts--accelerates the initiation and early stages of growth of fatigue cracks. This is a common problem in aircraft structural joints, where fretting may lead to fatigue cracking at faying surfaces and also within fastener holes. Present knowledge concerning the mechanisms of fretting fatigue is reviewed, with particular attention to the following factors: stresses at and near the contacting surfaces; location of crack initiation sites with respect to regions of highest surface stress and the slip-nonslip boundary; contributions of surface damage to crack initiation; propagation of microcracks initiated by fretting; the effects of mean stress; and fretting fatigue under variable amplitude loading. In the experimental program, a specially constructed apparatus was used to simulate conditions at the faying surface in a structural joint.