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Research Papers: Gas Turbines: Structures and Dynamics

A Viscoplastic Modeling Approach for MCrAlY Protective Coatings for Gas Turbine Applications

[+] Author and Article Information
Roland Mücke

 Alstom, Brown Boveri Strasse 7, CH-5401 Baden, Switzerland

J. Eng. Gas Turbines Power 131(6), 062501 (Jul 14, 2009) (7 pages) doi:10.1115/1.3094032 History: Received October 15, 2008; Revised October 16, 2008; Published July 14, 2009

MCrAlY coatings are applied in industrial gas turbines and aircraft engines to protect surfaces of hot gas exposed components from oxidation and corrosion at elevated temperature. Apart from oxidation resistance, coatings have to withstand cracking caused by cyclic deformation since coating cracks might propagate into the substrate material and thus limit the lifetime of the parts. In this context, the prediction of the coating maximum stress and the strain range during cyclic loading is important for the lifetime analysis of coated components. Analyzing the state of stress in the coating requires the application of viscoplastic material models. A coupled full-scale cyclic analysis of substrate and coating, however, is very expensive because of the different flow characteristics of both materials. Therefore, this paper proposes an uncoupled modeling approach, which consists of a full-scale cyclic analysis of the component without coating and a fast postprocessing procedure based on a node-by-node integration of the coating constitutive model. This paper presents different aspects of the coating viscoplastic behavior and their computational modeling. The uncoupled analysis is explained in detail and a validation of the procedure is addressed. Finally, the application of the uncoupled modeling approach to a coated turbine blade exposed to a complex engine start-up and shut-down procedure is shown. Throughout the paper bold symbols denote tensors and vectors, e.g., σ stands for the stress tensor with the components σij. The superscripts (.)S and (.)C symbolize the substrate and the coating, respectively, e.g., εthS stands for the tensor of substrate thermal strain. Further symbols are explained in the text.

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Copyright © 2009 by American Institute of Physics
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Figures

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Figure 2

MCrAlY coating after service: (left) coating layer intact and aluminum resource exhausted; (right) coating TMF cracks not propagating into the substrate

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Figure 3

Definition of cycle types

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Figure 4

Measured stress temperature response of a MCrAlY coating subjected to an OP TMF cycle (7)

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Figure 5

Temperature dependence of yield strength and rupture strain of MCrAlY coatings (7)

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Figure 6

Type of material tests for parameter identification

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Figure 7

Simulation of coating and substrate subjected to TMF OP cycles with different loading rates

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Figure 8

Coated turbine component and mesh details

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Figure 9

Stress-strain results using the coupled modeling approach (substrate at position A and coating at position B) and the uncoupled approach (coating at position B)

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Figure 10

Absolute difference in mechanical strain between coupled and uncoupled modeling approaches

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Figure 11

History of substrate metal temperature and mechanical strain for different engine start-up and shut-down operation concepts (only one cycle is shown)

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Figure 12

Stress-strain response of the coating for different engine start-up and shut-down operation concepts

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Figure 1

Virgin substrate/MCrAlY-coating system

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