The Relaxation of Residual Stresses During Fatigue

Microplastic deformation during fatigue of aluminum alloys permits relaxation of residual surface stresses at peak external stress amplitudes that are appreciably less than the yield strength. A review of the literature reveals that this process has not been adequately modeled. We propose and evaluate a model of relaxation which is based upon consideration of the nature of the microplastic deformation process. Residual surface stresses measured during fatigue of an Al 2219-T851 alloy are predicted from the cyclic stress amplitude, the initial magnitude of the residual stress and the ambient humidity during fatigue. It is shown that the rate of relaxation is greatest for those surface preparation processes which produce the steepest residual stress gradient normal to the surface. The rapid relaxation of surface residual stresses in aluminum alloys is an important effect that must be quantitatively understood before adequate prediction of the effect of such stresses on fatigue life can be made.