Bestimmung des Verformungs- und Versagensverhaltens von stengelkristallinen Gasturbinenschaufeln aus IN 792 DS

The achievement of high efficiency in large land-based stationary gas turbines for electric power generation requires increased gas temperatures. Gas temperatures higher than 1300 °C can only be handled using air-cooled structures to keep the metal temperature below 1000 °C. Only columnar grained (directionally solidified, DS) or single crystal (SC) superalloys have the required creep strength and thermo-mechanical fatigue resistance. The superalloy IN 792 DS shows anisotropic propertiesespecially with respect to Young's modulus and creep rates. For the estimation of life time a constitutive equation has been developed which describes the deformation behaviour of the anisotropic material. Thermo-mechanical fatigue tests have been carried out with internally cooled model blades of IN 792 DS. Cyclic temperature variations were used to simulate the loading and unloading of real gas turbine blades. In addition the model blade was subjected to a constant load to simulate the centrifugal forces. The test specimen was heated by a specially designed high frequency induction coil. Analyses based on inelastic material behaviour are necessary to understand the stress redistribution in a blade. The anisotropy was modified with the Hill approximation in the three dimensional structure dependant materials law. The developed constitutive equations have been verified by thermo-mechanical tests on model blades and by the results of finite element calculations. For different thermal cycles the temperature distribution in the cross-section of the blade was calculated with finite element methods. Based on the calculated temperature distribution, the stress and strain behaviour was evaluated using the anisotropic materials law. A two dimensional finite element mesh net (generalized plain strain element) was used, which allowed calculation in all three dimension. For the applied thermal cycles a ratcheting effect was not observed. The comparison of the experiments with the finite element calculations, using the constitutive equations for the anisotropic material IN 792 DS, showed a good conformity up to the tertiary creep behaviour. The finite element calculations gave a good description of the deformation behaviour of internally cooled model blades. The regions in which the first cracks could be predicted. For the life time prediction of the model blades, modified temperatures and stresses were used