Even small surface deviations arising from manufacturing may lead to critical performance losses of mechanical components as e.g. blade shape deviations in aeroengines. Supposed that such deviations are often associated with complex geometries, the nominal parametrization of the part may be inadequate to correctly describe them. Therefore, this paper proposes to re-parametrize the surface of interest as a B-spline surface with a higher number of control points to allow accurate coverage of surface deviations. Further, it is demonstrated how such a B-spline surface may be fitted to surface measurements by B-spline morphing and how manufacturing uncertainties may be described statistically by a few manufacturing modes only. Such a small data characterization may be used as an excellent basis for performing robust design. The proposed procedures are validated by an application to the hub endwall of a high-pressure turbine blade.
Abstract. 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.
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