Surface roughness and crack initiation in fretting fatigue

The presence of fretting is often thought to accelerate both the initiation and the propagation phases of fatigue. The propagation phase can in principle be understood using a macroscopic stress analysis and LEFM concepts. Fretting fatigue crack initiation, however, invariably involves processes at a microscopic scale and it is questionable whether such processes can be properly described using 'bulk' parameters. In particular, it may not be appropriate to consider smooth contact between surfaces, and contact at the asperity scale may need to be analysed. In this paper we present a rough contact analysis which predicts the mean size and spacing of asperity contacts from information obtained from a profilometer trace. The use of this information in the modelling of fretting fatigue crack initiation is then discussed.