Estimation of Notch Sensitivity and Size Effect on Fatigue Resistance

AbstractThis 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

Assessing fatigue endurance limit of pitted specimens by means of an integrated fracture mechanics approach

From an operator/engineering perspective, the correct assessment of the severity of corrosion defects (e.g., pits) can have enormous economic, social and environmental benefits; therefore the development of a generally applicable and simple to apply procedure for fatigue assessment of key components is recognised as a valuable tool, seeking to reduce the current overly conservative procedures whilst maintaining structural integrity. The critical condition for a crack emanating from a pit (pit-to-crack transition) to start to propagate is analysed in this paper. The pitcrack configuration is re-characterized into that of a hemispherical crack of length equal to the pit depth, and this assumption is analysed by detailed 3D FEA. A propagation threshold approach is used to estimate the fatigue resistance from intrinsic material properties. The proposed approach is validated by comparison with experimental results available in the open literature.</jats:p

Estimation of the Plain High-Cycle Fatigue Propagation Resistance in Steels

In this work a method to estimate the high cyclic fatigue propagation life of steel specimens under constant loading is presented. This method is based in experimental evidence that the fatigue limit represents the threshold stress for the propagation of nucleated cracks, so that both the fatigue limit and the fatigue resistance depend on the effective resistance of the microstructural barriers that have to be overcome by the nucleated cracks. It is proposed also, that in those cases where the number of cycles that is necessary for the nucleation of the cracks can be neglected, the fatigue crack propagation life can be taken as an estimation of the total fatigue life. The high cycle fatigue propagation life of a structural steel of the type JIS 10C is estimated