Estimation of Notch Sensitivity and Size Effect on Fatigue Resistance

Abstract

This paper addresses the problem of high cycle fatigue resistance associated to notches and the role of short crack propagation in the fatigue notch sensitivity quantified by the notch factor kf. An integrated fracture mechanics approach is proposed to estimate the fatigue notch sensitivity, by including the effect of both blunt and sharp notches.Whether fatigue strength at a given life is controlled by crack initiation (very blunt notches, kf = kt), by microstructuraly short cracks (blunt notches, kf <kt), or by mechanically short crack propagation (sharp notches, kf <<kt), depends on the stress concentration kt, the notch length D and the material threshold to crack initiation ΔσeR, to short crack propagation ΔKth and to long crack propagation ΔKthR.The approach includes the prediction of the fatigue crack propagation threshold for short cracks, previously developed to analyze the short crack behavior in metallic materials with or without blunt notches, and is integrated adding the influence of sharp notches and accounting for the controlling parameters. It estimates the fatigue resistance of the component by comparing the threshold for fatigue crack propagation as a function of crack length, ΔKth, with the applied ΔK for the given configuration. Estimations for results reported in published bibliography are presented.The proposed fracture mechanics approach allows accounting for the effects of notch acuity, notch size and intrinsic material fatigue properties on fatigue notch sensitivity. It opens the door to a new simple method for predicting fatigue notch sensitivity and fatigue strength of components with geometric concentrators by using parameters that can be easily measured or estimated, without the necessity of any fitting parameter.