ABSTRACT
The main steps of the fatigue assessment procedure (Bertoglio et al. 2014) are essentially seven:
– the assessment of the operating profiles and of their percentage of occurrence during the whole life of the ship;
– the determination of the pressures acting on the blade for 60 different positions of the blade during one revolution, i.e. every 6 degrees. The pressures are discretized as a distribution of forces acting on the blade surface. The coordinates of the nodes on the back and on the face of the blade, where the forces have been applied, were selected as the closest to the points where the pressures were calculated;
– the evaluation of the stresses time histories acting on the pressure side of the blade, which is always subjected to tensile stresses, i.e. the worst stress condition for a crack. The stresses evaluated for the calculation of the safety coefficients are the von Mises stress and the first principal stress. The former are considered because the blade has a complex geometry and it is subjected to complex loading conditions, the latter are considered as usually happens in fatigue assessments because the first principal stress governs the fatigue crack propagation in way of existing flaws;
– the analysis of the stress time history in selected points of the blade and counting of the cycles,
1 INTRODUCTION
Propeller design procedures still largely rely on rather old fashioned approaches and high safety coefficients are consequently needed in order to consider uncertainties and dynamic factors.
