High-cycle fatigue crack paths in specimens having different stress concentration features

This paper summarises an attempt to study the high-cycle fatigue cracking behaviour in specimens of low carbon steel weakened by U-notches. The specimens were tested under uniaxial fatigue loading with a load ratio equal to 0.1, and the considered Kt values, calculated with respect to the gross area, ranged from 3.8 up to about 25. The generated crack paths were quite irregular showing a propagation occurring in alternate trans- and intra-crystalline mode: in many cases, this made difficult to unambiguously measure orientation and length of Stage 1 planes. In spite of these experimental difficulties, the observed material cracking behaviour seemed to suggest that a Stage 1-like process could always be assumed to be representative of the crack initiation phenomenon, and this held true independently of the notch sharpness. In light of the fact that, at a mesoscopic level, crack initiations never occurred on material planes parallel to the notch bisector, we attempted to investigate whether it was possible to use a critical plane approach to estimate high-cycle fatigue damage in notched components under uniaxial fatigue loading. In more detail, the generated results have initially been re-analysed by using the Modified Wo\ua8 hler Curve Method re-interpreted in terms of the Theory of Critical Distances [Susmel L. A unifying approach to estimate the high-cycle fatigue strength of notched components subjected to both uniaxial and multiaxial cyclic loadings. Fatigue Fract Eng Mater Struct 2004;27:391\u2013411]. The accuracy in predicting the high-cycle fatigue behaviour of the considered multiaxial fatigue method was then compared to the accuracy of two other uniaxial approaches: the classical one by Smith and Miller [Smith RA, Miller KJ. Prediction of fatigue regimes in notched components. Int J Mech Sci 1978;20:201\u2013206] and the one recently proposed by Atzori and co-workers [Atzori B, Lazzarin P, Meneghetti G. A unified treatment of the mode I fatigue limit of components containing notches or defects. Int J Fract 2005;133:61\u201387] and based on the use of some classic LEFM concepts. In particular, this comparison was performed considering virtual specimens having the same geometries as the ones investigated in the present study, but assuming that they were made of materials having mechanical properties known from the literature. This exercise allowed us to see that the high-cycle fatigue damage in notched specimens under uniaxial fatigue loading can satisfactorily be predicted not only using Mode I-crack based methods, but also using multiaxial fatigue criteria modelling the crack initiation phenomenon