Assessment of Fatigue Reliability of Power Transmission Gearing for Aerospace Propulsion Applications

The fatigue strength in presence of inclusions is one of the main topics for the fatigue of high strength steels, especially the case hardened ones adopted for power transmission. In this paper we analyze the fatigue strength of carburized gears in presence of inclusions in order to develop the reliability assessment of a power transmission for aerospace propulsion. The high cycle fatigue properties of a high strength carburizing steel have been experimentally determined by conducting bending fatigue tests with specimens with artificial defects and a coherent model for the assessment of fatigue crack growth thresholds by taking into account of the residual stresses due to carburizing has been developed. In parallel, the distribution of non-metallic inclusions has been estimated by carrying out high cycle fatigue tests onto cylindrical smooth specimens and by inspection of mirror polished sections of the material. Eventually, by employing a simplified finite element model of a highly loaded gear wheel, the bending fatigue strength of gear teeth and the reliability of a turbo-propeller power transmission was determined by incorporating the fatigue crack growth thresholds, the measured residual stress profiles and the statistics of extreme non-metallic inclusions into a competing risks model applied to FE discretization.