Fatigue limits prediction of surface treated materials

The fatigue limit prediction methodology for cyclical loading of surface treated material has been developed in this paper by material investigation. The change in surface layer properties caused by surface treatments was evaluated by compressive residual stress and strain hardening.A model based on the Crossland's criterion has been developed. The X-ray diffraction method of measuring is used. We adopted the rate between X-ray widths measurement of base material and treated one to quantify the strain hardening. The theoretical model gives a better approach of the mechanism of fatigue behaviour of surface treated material. It was demonstrated that the increasing effect of strain hardening on fatigue strength is more important than that of compressive residual stress.To validate the model, the proposed fatigue limit criterion is formulated in two cases of surface treatment methods: shot peening and nitriding. Comparison of predictions of this criterion against experimental fatigue limit data showed good agreement. The fatigue limit criterion predicts that the crack origin is located into the region under the surface hardened layer. This goes in concordance with experimental observations. An important property of this model is that it can be used to optimise the surface treatment parameters and to choose the suitable material. The model is an approach for predicting the effect of compressive residual stress and strain hardening on high cycle fatigue behaviour.