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J.T.P de Castro et alii, Frattura ed Integrità Strutturale, 25 (2013) 79-86; DOI: 10.3221/IGF-ESIS.25.12 79 Special Issue: Characterization of Crack Tip Stress Field Is notch sensitivity a stress analysis problem? Jaime Tupiassú Pinho de Castro, Marco Antonio Meggiolaro Mechanical Engineering Department, Pontifical Catholic University of Rio de Janeiro (PUC-Rio), Brazil A BSTRACT . Semi–empirical notch sensitivity factors q have been widely used to properly account for notch effects in fatigue design for a long time. However, the intrinsically empirical nature of this old concept can be avoided by modeling it using sound mechanical concepts that properly consider the influence of notch tip stress gradients on the growth behavior of mechanically short cracks. Moreover, this model requires only well- established mechanical properties, as it has no need for data-fitting or similar ill-defined empirical parameters. In this way, the q value can now be calculated considering the characteristics of the notch geometry and of the loading, as well as the basic mechanical properties of the material, such as its fatigue limit and crack propagation threshold, if the problem is fatigue, or its equivalent resistances to crack initiation and to crack propagation under corrosion conditions, if the problem is environmentally assisted or stress corrosion cracking. Predictions based on this purely mechanical model have been validated by proper tests both in the fatigue and in the SCC cases, indicating that notch sensitivity can indeed be treated as a stress analysis problem. K EYWORDS . Notch sensitivity; Stress gradient effects; Non-propagating short cracks. I NTRODUCTION emi-empirical notch sensitivity factors 0  q  1 have been long used to relate linear elastic (LE) stress concentration factors (SCF) K t =  max /  n , to their corresponding fatigue SCF K f = 1 + q  ( K t – 1) = S L (R)/S Lntc (R) , where  max and  min are the maximum and minimum LE stress at the notch root caused by  n , the nominal stress that would act at that point if the notch did not affect the stress field around the notch, whereas S L (R) and S Lntc (R) are the fatigue limits measured on standard (smooth and polished) and on notched test specimens (TS), respectively, at a given R =  min /  max ratio. Despite their empirical nature, such K f values are still widely used to quantify notch effects on the fatigue strength of structural components [1]. On the other hand, it is well known that the notch sensitivity in fatigue can be associated with the relatively fast generation of tiny non-propagating cracks at notch roots when S L (R)/K t <  n < S L (R)/K f , see Fig. 1 [2]. Based on this behavior, a model has been proposed to calculate the q values from the fatigue behavior of short cracks emanating from notch tips, using only relatively simple but sound mechanical principles, which do not require heuristic arguments, neither any arbitrary data-fitting parameter [3-5]. In particular, this model does not require a critical distance concept, a good idea which originally assumed that the notch sensitivity should be dependent on some (yet never discovered) microstructural parameter [6-10]. In fact, to operationalize the critical distance concept it needs in practice to be fitted to the measured material properties, not to its microstructural parameters. The alternative explanation proposed here may be appealing for those who tackle such problems from a more mechanical viewpoint. Such an approach allowed it to explain notch effects not only on fatigue, but also on environmentally assisted cracking problems. Indeed, it is claimed here that notch sensitivity exists on such problems as well, and that it can be described using the same techniques successfully applied to model the fatigue problem. In fact, this can be done by just considering the proper resistances to crack initiation and to crack propagation in the given material/environment pair, both standard well-defined mechanical properties. Data is already available to support the proposed model predictions both in fatigue and SCC conditions. S