Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

Rotordynamic Crack Diagnosis: Distinguishing Crack Depth and Location

[+] Author and Article Information
Philip Varney

e-mail: pvarney3@gatech.edu

Itzhak Green

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

1Corresponding author.

Contributed by the Manufacturing Materials and Metallurgy Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 5, 2013; final manuscript received July 8, 2013; published online September 17, 2013. Editor: David Wisler.

J. Eng. Gas Turbines Power 135(11), 112101 (Sep 17, 2013) (8 pages) Paper No: GTP-13-1233; doi: 10.1115/1.4025039 History: Received July 05, 2013; Revised July 08, 2013

The goal of this work is to establish simple condition monitoring principles for diagnosing the depth and location of transverse fatigue cracks in a rotordynamic system. The success of an on-line crack diagnosis regimen hinges on the accuracy of the crack model, which should account for the crack's depth and location. Two gaping crack models are presented; the first emulates a finite-width notch typically manufactured for experimental purposes, while the second models a gaping fatigue crack. The rotordynamic model used herein is based upon an available overhung rotordynamic test rig that was originally constructed to monitor the dynamics of a mechanical face seal. Four degree-of-freedom, linear equations of motion for both crack models are presented and discussed. Free and forced response analyses are presented, emphasizing results applicable to condition monitoring and, particularly, to diagnosing the crack parameters. The results demonstrate that two identifiers are required to diagnose the crack parameters: the 2X resonance shaft speed and the magnitude of the angular 2X subharmonic resonance. First, a contour plot of the 2X resonance shaft speed versus crack depth and location is generated. The magnitude of the 2X resonance along the desired 2X frequency contour is then obtained, narrowing the possible pairs of crack location and depth to either one or two possibilities. Practical aspects of the suggested diagnostic procedure are discussed, as well as qualitative observations concerning crack detection.

Copyright © 2013 by ASME
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Fig. 1

Comparison of overhung rotordynamic systems: (a) undamaged overhung rotordynamic system; (b) overhung shaft with notch; (c) overhung shaft with gaping fatigue crack

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Fig. 2

Rotor degrees of freedom

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Fig. 3

Relationship between inertial and rotating reference frames

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Fig. 5

Cross section of shaft containing transverse crack

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Fig. 6

2X resonance shaft speed versus crack depth for a fixed-location crack

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Fig. 7

Free response contour plot: 2X resonance shaft speeds: (a) notch; (b) gaping fatigue crack

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Fig. 8

Forced response of a gaping fatigue crack: (a) forced response including imbalance and gravity, at n = 100 Hz; (b) forced response over a range of shaft speeds to demonstrate the 2X resonance shaft speed

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Fig. 9

Magnitude contours of 2X angular resonance (in radians) versus crack location and depth

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Fig. 10

Locus of 2X angular resonance magnitudes for a range of crack depth and location pairs: (a) 73 Hz 2X resonant contour; (b) 70 Hz 2X resonant contour

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Fig. 11

Loci of 2X angular resonance magnitude for many 2X resonance shaft speeds




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