Evaluating deformation behavior of a TBC-System during thermal gradient mechanical fatigue by means of high energy X-ray diffraction

Applications of TBC-systems involve complex thermal mechanical loading pattern including transient thermal gradients across the coated system, which result in multiaxial stresses and stress gradients affecting the damage behavior. In an ongoing research, starting more than 10 years ago, the authors developed laboratory test facilities for evaluating the damage behavior of TBC-systems for gas turbine blades in aeroengines under realistic thermal mechanical loading conditions [1]. Fatigue tests involving thermal gradients have been conducted and damage behavior in dependence of load pattern and pre heat treatment has been intensively investigated on TBC-systems comprising a partially yttria stabilized zirconia (YSZ) topcoat and a MCrAlY bond coat both applied by electron physical vapor deposition (EB-PVD) onto nickel based super alloys serving as substrate [2]. Numerical analyses by means of FE-calculations did provide hypotheses explaining the observed damage behavior [3], but even though the results are plausible they did depend on reasonable assumptions on materials properties since reliable data on the properties of the thin coating layers are still lacking, especially for high temperatures. High energy X-ray diffraction can provide the requested information since it is possible to achieve information on the local deformation processes in each layer with high spatial resolution, and short acquisition times allow for in situ investigation of time dependent deformation processes. A new test facility based on concepts after [1] for cyclic thermal loading of tubular specimens and applying a controlled thermal gradient across the coated specimen’s wall has been developed for implementation into an electro-mechanical test machine at the advanced photon source (APS) at Argonne National Laboratory. A precision positioning rig allows for exact µm-positioning of the entire test machine with respect to the focused X-ray beam, and X-ray diffraction patterns were taken using a 2D detector, giving accurate 360° lattice parameter data [4]. Tests have been performed with varying thermal and mechanical load schemata intending to determine material properties from the respective strain response. The beam energy was 65 keV, and throughout all experiments the beam scanned through the coating layers with a window and step size of 30 µm. Strain data were acquired in plane parallel to the specimen’s length axis and out of plane. Results of the strain data evaluation will be presented and discussed. Exemplary results are: - Elastic properties of the YSZ showed a gradient across the coating thickness reflecting the microstructure gradient of the YSZ resulting from the EB-PVD process. - The YSZ strain was – below the deposition temperature - in plane compressive and out of plane tensile, which is a consequence of (i) the higher thermal expansion coefficient of YSZ with respect to the substrate and (ii) the cylindrical specimen geometry with the YSZ at the outer surface. [1] M. Bartsch, G. Marci, K. Mull, C. Sick, Adv. Eng. Mater. (1999), 1(2), 127–9 [2] M. Bartsch, B. Baufeld, S. Dalkilic, L. Chernova, M. Heinzelmann, Int. J. Fatigue (2008) 30, 211–8 [3] M. T. Hernandez, A. M. Karlsson, M. Bartsch, Surf. Coat. Technol. (2009) 203, 3549–58 [4] S.F. Siddiqui, K. Knipe, A. Manero, C. Meid, J. Wischek, J. Okasinski, J. Almer, A.M. Karlsson, M. Bartsch, S. Raghavan, Review of Scientific Instruments (2013) 84, 08390

Ein miniaturisierter Kriechversuch zur Kalibrierung eines effizienten Lebensdauermodells für Hochdruckturbinenschaufeln

Jet engines of airplanes are designed such that in some components damage occurs and accumulates in service without being critical up to a certain level of damage. Since maintenance, repair, and component exchange are very cost-intensive, it is necessary to predict efficiently the component lifetime with high accuracy. A former developed lifetime model, based on interpolated results of aerodynamic and structural mechanics simulations, uses material parameters estimated from literature values of standard creep experiments. For improved accuracy, an experimental procedure is developed for the characterization of the short-time creep behavior, which is relevant for the operation of turbine blades of jet engines. To consider microstructural influences resulting from the manufacturing of thin-walled single crystal turbine blades, small-scale specimens from used turbine blades are extracted and tested in short- and medium-time creep experiments. Based on experimental results and literature values, a creep model, which describes the fracture behavior for a wide range of creep loads, is calibrated and is now used for the lifetime prediction of turbine blades under real loading conditions.Flugzeugtriebwerke sind so ausgelegt, dass in einigen Komponenten während des Betriebs Schädigungen auftreten und akkumulieren können, ohne dass diese kritisch werden. Wegen der hohen Kosten von Wartung, Reparatur und Austausch ist es notwendig, die Lebensdauer der Komponenten möglichst effizient und genau vorherzusagen. Ein vorab entwickeltes Lebensdauermodell, welches auf interpolierten Ergebnissen von aerodynamischen und strukturmechanischen Simulationen basiert, nutzt in der Literatur verfügbare Daten aus Standard-Kriechversuchen. Um die Genauigkeit der Lebensdauervorhersage zu verbessern, wird eine Versuchstechnik zur experimentellen Charakterisierung des betriebsrelevanten Kurzzeit-Kriechverhaltens entwickelt. Mikrostrukturelle Einflüsse aus der Herstellung der dünnwandigen einkristallinen Turbinenschaufeln werden berücksichtigt, indem die Proben für die Kriechversuche aus gebrauchten Turbinenschaufeln extrahiert werden. Mit den Daten von Kriechversuchen mit kurzen bzw. mittleren Standzeiten wird nun das Lebensdauermodell kalibriert, so dass für einen weiten Bereich von Kriechlasten das Bruchverhalten von Turbinenschaufeln unter Betriebsbedingungen erfasst wird

Miniaturization of low cycle fatigue‐testing of single crystal superalloys at high temperature for uncoated and coated specimens

A newly developed miniature specimen and respective fixture for high temperature low cycle fatigue testing of nickel based single crystal superalloys is presented. Miniaturization allows the preparation of test specimens in all main crystallographic orientations of the cubic nickel crystal using laboratory sized material samples and enables excellent utilization of the costly material. The specimen geometry is optimized by means of parameter studies employing numerical calculations such that for the main crystallographic orientations the stress concentration at the fillet between gauge length and specimen head is minimized, and failure is likely to occur within the gauge length. The designed fixture allows easy specimen mounting and provides sufficient support for applying an extensometer for strain measurement. Protective metallic coatings against oxidation can be applied on the specimen by plasma spraying for studying the effect of coatings on the fatigue lifetime. The functionality of the specimen geometry and fixture design for low cycle fatigue testing is demonstrated for temperatures up to 950 °C