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BTU
The international standard ISO 6336-3 defines a calculation method for the tooth root capacity of standard gears with basic rack profile according to ISO 53. However, the results of this method may become too optimistic for non-standard gears with custom root fillets, large pressure angles or asymmetric teeth, since the effect of general mean stresses on the local tooth root strength are neglected. Therefore, a more generic calculation method is suggested, which considers the mean stress influence by utilizing the Haigh diagram. Both approaches are then used as basis of root fillet shape optimizations. A comparison of results demonstrates significant differences in the safety assessment and the resulting root geometry.
Currently, prediction of crack initiation by corrosion pits is only possible by assuming regular geometrical shapes, such as semi-spheres or semi-ellipsoids. Moreover, typical fatigue life diagrams associate the crack initiation life with geometrical features, such as pit depth or aspect ratio, often leading to unsatisfactory correlations due to high pit shape variability and data scatter. In the context of blade-disc fixation in aero engine turbines, this limitation translates into highly conservative life estimations. Therefore, a new crack initiation predictor is formulated based on experimental testing and numerical analysis of 28 artificial corrosion pits. A low-cycle fatigue test campaign is conducted using three-point bending test specimens to simulate maximum takeoff operation conditions of the aero engine and the associated loading of the blade root designed as firtree. An artificial pit is located at the critical point of each test specimen, respectively. The prediction criterion is based on finite element analysis and is formulated as the lowest plastic strain of a plastic region with a certain volume in the corrosion pit. This reference volume is varied until an optimum correlation with experimental crack initiation life is obtained. The criterion shows a superior correlation with crack initiation life compared to pure geometrical parameters such as pit depth.