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Gas Turbines: Structures and Dynamics

Crack Detection in a Rotor Dynamic System by Vibration Monitoring—Part II: Extended Analysis and Experimental Results

[+] Author and Article Information
Philip Varney1

Woodruff School of Mechanical Engineering,  Georgia Institute of Technology, Atlanta, GA, 30332pvarney3@gatech.edu

Itzhak Green

Woodruff School of Mechanical Engineering,  Georgia Institute of Technology, Atlanta, GA, 30332itzhak.green@me.gatech.edu

1

Corresponding author.

J. Eng. Gas Turbines Power 134(11), 112501 (Sep 21, 2012) (10 pages) doi:10.1115/1.4007275 History: Received May 16, 2012; Revised July 28, 2012; Published September 20, 2012; Online September 21, 2012

An increase in the power-to-weight ratio demand on rotordynamic systems causes increased susceptibility to transverse fatigue cracking of the shaft. The ability to detect cracks at an early stage of progression is imperative for minimizing off-line repair time and cost. The vibration monitoring system initially proposed in Part I is employed herein, using the 2X harmonic response component of the rotor tilt as a signature indicating a transverse shaft crack. In addition, the analytic work presented in Part I is expanded to include a new notch crack model to better approximate experimental results. To effectively capture the 2X response, the crack model must include the local nature of the crack, the depth of the crack, and the stiffness asymmetry inducing the gravity-forced 2X harmonic response. The transfer matrix technique is well suited to incorporate these crack attributes due to its modular nature. Two transfer matrix models are proposed to predict the 2X harmonic response. The first model applies local crack flexibility coefficients determined using the strain energy release rate, while the second incorporates the crack as a rectangular notch to emulate a manufactured crack used in the experiments. Analytic results are compared to experimental measurement of the rotor tilt gleaned from an overhung rotor test rig originally designed to monitor seal face dynamics. The test rig is discussed, and experimental angular response orbits and 2X harmonic amplitudes of the rotor tilt are provided for shafts containing manufactured cracks of depths between 0% and 40%. Feasibility of simultaneous multiple-fault detection of transverse shaft cracks and seal face contact is discussed.

Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Cross section of shaft containing transverse crack

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Figure 2

Overhung rotor system with transverse shaft crack

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Figure 3

Cross section of shaft containing transverse notch crack

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Figure 4

Dimensionless crack flexibility

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Figure 5

Local crack flexibility model: magnitude of 2X tilt response

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Figure 6

Local crack flexibility model: magnitude of 2X tilt response versus shaft speed and crack depth

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Figure 7

Local crack flexibility model: magnitude and frequency of 2X tilt resonance

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Figure 8

Notch area moments of inertia

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Figure 9

Notch crack model: magnitude of 2X tilt response

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Figure 10

Notch crack model: 2X tilt response versus shaft speed and crack depth

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Figure 11

Notch crack model: magnitude and frequency of 2X harmonic tilt resonance

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Figure 12

Test rig cross section

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Figure 13

Monitoring system block diagram

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Figure 14

Adapted probe configuration

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Figure 15

Low speed range of experimental 2X response

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Figure 16

High speed range of experimental 2X response

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Figure 17

Experimental 2X content of tilt orbit: 40 Hz

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Figure 18

Experimental 2X content of tilt orbit: 71 Hz

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Figure 19

Experimental total tilt orbits near respective 2X resonance locations for cracks between 0% and 30% depth

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Figure 20

Experimental total tilt orbit near 2X resonance for 40% crack depth

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