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Residual stress and surface roughness in fretting fatigue

Fretting involves contact between surfaces undergoing small cyclic relative tangential motion. The resultant wear and initiation of fatigue cracks are strongly influenced by the nature of the surface. Shot-peening is an established surface treatment which produces both surface roughening and the development of compressive stresses in the surface. If the material is susceptible to work hardening it also produces surface hardening, since local plastic deformation occurs. Experiments have been devised to separate these three effects in a study on the behaviour of an Al-4Cu-1Mg age-hardening alloy. The results on the fretting - fatigue behaviour of the alloy show that the residual compressive stress has the greatest effect on retarding the propagation of fatigue cracks initiated by fretting. The present investigation is designed to assess the effect of the roughening produced by different levels of shot-peening on the fatigue and fretting fatigue of an age-hardened aluminium alloy after removal of the residual compressive surface stress. On removal of the stress by heat treatment, the surface damage resulting from shot-peening is more damaging in plain fatigue than the increase in roughness would suggest, due to the presence of incipient cracks. These are less damaging in fretting fatigue because they are nearly parallel to the surface and are closed up by pressure from the fretting pad.