F. Berto, Frattura ed Integrità Strutturale, 34 (2015) 169-179; DOI: 10.3221/IGF-ESIS.34.18 169 Focussed on Crack Paths Crack initiation at V-notch tip subjected to in-plane mixed mode loading: An application of the fictitious notch rounding concept F. Berto University of Padova, Italy berto@gest.unipd.it A BSTRACT . The fictitious notch rounding concept is applied for the first time to V-notches with root hole subjected to in-plane mixed mode loading. Out-of-bisector crack propagation is taken into account. The fictitious notch radius is determined as a function of the real notch radius the microstructural support length and the notch opening angle. Due to the complexity of the problem, a method based on the simple normal stress failure criterion has been used. It is combined with the maximum tangential stress criterion to determine the crack propagation angle. An analytical method based on Neuber's procedure has been developed. The method provides the values of the microstructural support factor as a function of the mode ratio and the notch opening angle. The support factor is considered to be independent of the microstructural support length. Finally, for comparison, the support factor is determined on a purely numerical basis by iterative analysis of FE models. K EYWORDS . Fictitious notch rounding; Mixed mode loading; V-notches with root hole; Microstructural support. I NTRODUCTION ll previous publications on the fictitious notch rounding (FNR) concept deal with the pure loading modes 1, 2 and 3. The difficulty of considering mixed mode loading conditions is due to the fact that the most critical direction, in which cracks might provisionally initiate and propagate, varies as a function of the mode ratio. This direction varies from the notch bisector line in the case of pure mode 1 loading, to a direction substantially out of the notch bisector line in the case of pure mode 2 loading. The present work extends the FNR concept to mixed mode 1 and 2 loading conditions for the first time and provides a solution to the problem as a function of the mode ratio. The FNR concept [1-3] refers to the fact that the theoretical maximum notch stress does not characterise the static strength or fatigue strength of pointed or sharply rounded notches. The notch stress averaged over a short radial distance at pointed notches or over a small distance normal to the notch edge at rounded notches (real notch radius ) is the key parameter of the method. The mentioned distance * is usually called ‘microstructural support length’. In the high-cycle fatigue regime, notch stress averaging should take a path which coincides with the point and direction of fatigue crack initiation and propagation. The basic idea of the FNR concept is to determine the fatigue-effective averaged notch stress directly (i.e. without actual notch stress averaging) by performing the notch stress analysis with a fictitiously enlarged notch radius f given by A

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