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TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

Subsurface Stress Fields in Face-Centered-Cubic Single-Crystal Anisotropic Contacts

[+] Author and Article Information
Nagaraj K. Arakere1

Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611-6300nagaraj@ufl.edu

Erik Knudsen

Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL 32611-6300

Gregory R. Swanson

 NASA Marshall Space Center Flight Center, ED22, Structural Mechanics Group, Huntsville, ALgreg.swanson@nasa.gov

Gregory Duke

 JE Sverdrup, Huntsville, ALgreg.duke@msfc.nasa.gov

Gilda Ham-Battista

 ERC, Inc., Huntsville, ALbattista@msfc.nasa.gov

1

To whom correspondence should be addressed.

J. Eng. Gas Turbines Power 128(4), 879-888 (Nov 03, 2005) (10 pages) doi:10.1115/1.2180276 History: Received August 02, 2004; Revised November 03, 2005

Single-crystal superalloy turbine blades used in high-pressure turbomachinery are subject to conditions of high temperature, triaxial steady and alternating stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent high-cycle fatigue (HCF) failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes and noncrystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single-crystal turbine blades. This paper presents analytical and numerical techniques for evaluating two- and three-dimensional (3D) subsurface stress fields in anisotropic contacts. The subsurface stress results are required for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single-crystal nickel-base superalloy turbine blades. An analytical procedure is presented for evaluating the subsurface stresses in the elastic half-space, based on the adaptation of a stress function method outlined by Lekhnitskii (1963, Theory of Elasticity of an Anisotropic Elastic Body, Holden-Day, Inc., San Francisco, pp. 1–40). Numerical results are presented for cylindrical and spherical anisotropic contacts, using finite element analysis. Effects of crystal orientation on stress response and fatigue life are examined. Obtaining accurate subsurface stress results for anisotropic single-crystal contact problems require extremely refined 3D finite element grids, especially in the edge of contact region. Obtaining resolved shear stresses on the principal slip planes also involves considerable postprocessing work. For these reasons, it is very advantageous to develop analytical solution schemes for subsurface stresses, whenever possible.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Damper contact locations on the turbine blade

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Figure 2

Crystallographic crack initiation at the damper contact location shown in Fig. 1(12)

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Figure 3

Anisotropic elastic half-space under generalized plane deformation subjected to normal and tangential traction forces

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Figure 4

Three-Dimensional FE model of the elastic anisotropic half space (28)

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Figure 5

Stress (σy) contours using analytical solution and finite element (ANSYS ) solution (28)

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Figure 6

Three-dimensional FE model of a cylindrical anisotropic contact and close-up view of the meshed contact region (28)

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Figure 7

Comparison of FEA contact and analytical subsurface stresses σx and σy, as a function of depth, for crystallographic orientation defined by case C (Δ=−15deg, γ=0deg, θ=0deg) (28)

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Figure 8

Contour plots of RSS τ1, τ3, and τ11, for cases B and C under the contact region (28)

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Figure 9

Three-dimensional FE model of the spherical isotropic indenter on a single-crystal orthotropic substrate

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Figure 10

Comparison of subsurface stresses between the full FEA contact solution and simulated contact, for the orthotropic spherical contact

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Figure 11

Weighted percentage difference in normal contact pressure for the orthotropic and isotropic spherical contact, as compared to case A

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Figure 12

Fatigue damage parameter, Δτmax versus cycles to failure (8)

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Figure 13

A critical subsurface point near the leading edge, for a cylindrical single-crystal contact of width 2a

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Figure 14

Variation of Δτ at the critical point shown in Fig. 1 as a function of secondary crystallographic orientation θ (primary orientation=case A)

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