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Research Papers: Gas Turbines: Structures and Dynamics

Probabilistic Fretting Fatigue Assessment of Aircraft Engine Disks

[+] Author and Article Information
Michael P. Enright, Kwai S. Chan, Jonathan P. Moody

 Southwest Research Institute, San Antonio, TX 78238

Patrick J. Golden

Air Force Research Laboratory, Wright-Patterson AFB, OH 45433

Ramesh Chandra, Alan C. Pentz

 NAVAIR, Patuxent River, MD 20670

J. Eng. Gas Turbines Power 132(7), 072502 (Apr 14, 2010) (9 pages) doi:10.1115/1.4000130 History: Received April 09, 2009; Revised May 19, 2009; Published April 14, 2010; Online April 14, 2010

Fretting fatigue is a random process that continues to be a major source of damage associated with the failure of aircraft gas turbine engine components. Fretting fatigue is dominated by the fatigue crack growth phase and is strongly dependent on the magnitude of the stress values in the contact region. These stress values often have the most influence on small cracks where traditional long-crack fracture mechanics may not apply. A number of random variables can be used to model the uncertainty associated with the fatigue crack growth process. However, these variables can often be reduced to a few primary random variables related to the size and location of the initial crack, variability associated with applied stress and crack growth life models, and uncertainty in the quality and frequency of nondeterministic inspections. In this paper, an approach is presented for estimating the risk reduction associated with the nondestructive inspection of aircraft engine components subjected to fretting fatigue. Contact stress values in the blade attachment region are estimated using a fine mesh finite element model coupled with a singular integral equation solver and combined with bulk stress values to obtain the total stress gradient at the edge of contact. This stress gradient is applied to the crack growth life prediction of a mode I fretting fatigue crack. A probabilistic model of the fretting process is formulated and calibrated using failure data from an existing engine fleet. The resulting calibrated model is used to quantify the influence of inspection on the probability of fracture of an actual military engine disk under real life loading conditions. The results can be applied to quantitative risk predictions of gas turbine engine components subjected to fretting fatigue.

Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Illustration of the relationship among contact forces P and Q in the disk/blade interface of a typical gas turbine engine

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Figure 2

For the probabilistic calibration, the upper confidence bound of predicted crack area is set equal to the smallest observed crack area

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Figure 3

The probabilistic model is calibrated to the failure data by adjusting the predicted probability of cracking to match the occurrence rate observed in the actual disk population

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Figure 4

Due to the complex geometry of the fan blade, higher order tetrahedral elements (C3D10) were used for the associated finite element mesh

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Figure 6

To quantify the variation in contact forces along the length of the dovetail slot, the finite element model was sliced into 12 sections that could then be further analyzed independently

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Figure 7

Pressure and shear tractions from finite element model results for slice number 1 and load increment 1

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Figure 8

Contact force history for one slice of the finite element model: 1. 110% maximum speed, 2. 50%, 3. 110%, and 4. 72%

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Figure 9

Bulk stresses were obtained from the finite element model along a path perpendicular to the edge of contact for a single slice and load increment

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Figure 10

Typical fan speed profile based on the composite mission associated with actual engine usage histories

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Figure 11

Representative P and Q history for the typical fan speed profile shown in Fig. 1

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Figure 12

Computed total stress values for high pressure slice 1 at mission load steps 1 and 2

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Figure 13

Computed total stress values for high pressure slices 1, 6, and 12 at mission load steps 1 and 2

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Figure 14

Maximum delta stress values for high pressure slices

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Figure 15

Computed stress ranges for high pressure slice 1 corresponding to load steps 1 and 2 compared with the bulk stress range and the threshold stress ranges for a large-crack growth threshold ΔKth of 2 MPa(m)1/2

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Figure 16

The stress scatter coefficient of variation (COV) was adjusted so that the upper confidence bound was equal to the smallest observed crack size

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Figure 17

POD plots for representative eddy current and FPI inspection methods

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Figure 18

Influence of eddy current inspection on the normalized probability of fracture for selected inspection intervals

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Figure 19

Influence of FPI inspection on the normalized probability of fracture for selected inspection intervals

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Figure 20

The crack propagation plane associated with fretting fatigue often is not perpendicular to the surface of contact

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Figure 21

Principal stress values associated with a representative load step: (a) maximum principal stress contours and (b) enlarged view at the edge of contact

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Figure 22

Influence of crack path orientation on slice 12 delta stress gradient

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Figure 23

Influence of crack path orientation on the normalized probability of fracture of selected slices

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Figure 24

The crack propagation shape is dominated by the steep fretting stress gradient at the surface: (a) simulated crack propagation sequence and (b) typical failed surface

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Figure 5

A refined mesh of first order hexahedral elements (C3D8) was used for the dovetail portion of the fan blade and the disk

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