Defect tolerant design of an helicopter tail rotor shaft

New design recommendations are going to be adopted for the fatigue design and assessment of helicopters structures and components. The new issue of these documents is the analysis of fatigue in terms of presence of a defect tolerance with a given size. The assessment of such a defect cannot be carried out on the basis of LEFM and a new approach based on the concepts of short cracks is needed. The fatigue strength of a component containing defects or inhomogeneities subjected to axial or bending loads can be predicted on the basis of the concepts of Murakami and co-authors. They showed that at the fatigue limit small non-propagating cracks are always present at the edge of small notches and defects. This means that the fatigue limit has to be calculated as the cyclic threshold stress at which the cracks emanating from the defects do not propagate. In this paper we address the analysis of a defect tolerant design methodology based on these concepts for the tail rotor shaft of an helicopter. The activity has firstly consisted in obtaining the DeltaKth for small cracks using fatigue limit tests on micronotched specimens under axial loading at different stress ratios. The data have then been used for a fatigue assessment of the component under load spectrum.