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.
Highlights
Corrosion of critical parts is one of the key cost drivers in aerospace industry
In the design of steel discs for high-pressure turbines (HPT), corrosion criticality is evaluated by estimating the number of cycles required to initiate a crack by a corrosion pit, and the number of cycles required for this crack to propagate and cause macroscopic fracture
Instead of developing an analytical model for crack growth based on geometric simplifications, the present paper aims at formulating a crack initiation predictor which considers the real pit geometry and individual load level
Summary
Corrosion of critical parts is one of the key cost drivers in aerospace industry. Corrosion problems typically appear in the mature fleet and are considered as most difficult and expensive due to the potential size of the affected fleet and variety of factors influencing life prediction and fleet management. Instead of developing an analytical model for crack growth based on geometric simplifications, the present paper aims at formulating a crack initiation predictor which considers the real pit geometry and individual load level. A crack initiation predictor is a criterion which exhibits a useful correlation with real crack initiation life To formulate this predictor, an experimental test campaign with 28 simplified test specimens representing the disc firtree is carried out. Each test specimen contains an artificially created corrosion pit (hereafter referred as artificial pit), which imitates the shape of corrosion pits found in used HPT discs (hereafter referred as ex-service pit) From this experimental test campaign, crack initiation lives NNII,ii of all artificial pits are obtained. The found correlation will be applied to estimate the crack initiation life of ex-service pits
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