Research Articles

New Finite Element Modeling Approach of a Propagating Shaft Crack

[+] Author and Article Information
Zbigniew Kulesza

Faculty of Mechanical Engineering,
Bialystok University of Technology,
15-351 Bialystok, Poland
e-mail: z.kulesza@pb.edu.pl

Jerzy T. Sawicki

Fellow ASME
Center for Rotating Machinery
Dynamics and Control (RoMaDyC),
Fenn College of Engineering,
Cleveland State University,
Cleveland, OH 44115-2214
e-mail: j.sawicki@csuohio.edu

Manuscript received March 12, 2012; final manuscript received September 26, 2012; accepted manuscript posted October 8, 2012; published online January 30, 2013. Assoc. Editor: Marc Geers.

J. Appl. Mech 80(2), 021025 (Jan 30, 2013) (17 pages) Paper No: JAM-12-1101; doi: 10.1115/1.4007787 History: Received March 12, 2012; Revised September 26, 2012; Accepted October 08, 2012

Transverse shaft cracks are one of the most dangerous malfunctions of the rotating machines, including turbo- and hydrogenerators, high-speed machine tool spindles, etc. The undetected crack may grow slowly and not disturb normal machine operation. However, if it extends to a critical depth, the immediate shaft fracture may completely damage the machine, resulting in a catastrophic accident. Therefore, in-depth knowledge of the crack propagation process is essential to ensure reliable and safe operation of rotating machinery. The article introduces a new model of the propagating shaft crack. The approach is based on the rigid finite element (RFE) method, which has previously proven its effectiveness in the dynamical analysis of numerous complicated machines and structures. The crack is modeled using several dozen spring-damping elements (SDEs), connecting the faces of the cracked section of the shaft. By controlling the exact behavior of individual SDEs, not only the breathing mechanism, but also the crack propagation process can be simply introduced. In order to accomplish this, the stress intensity factors (SIFs) along the crack edge are calculated using the novel approach based on the modified virtual crack closure technique (VCCT). Based on the SIF values, the crack propagation rate is calculated from the Paris law. If the number of load cycles is greater than the constantly updated threshold number, then the crack edge is shifted by a small increment. This way, starting from the first initially cracked SDE, the crack is extended little by little, continuously changing its shape. The approach is illustrated with numerical results, demonstrating the changes in the rotor vibration response and in the crack shape and also explaining some issues about the breathing mechanism due to the propagating shaft crack. The increasing amplitude of the 2X harmonic component is recognized as an evident propagating crack signature. The numerical results correspond well with the data reported in the literature. The RFE model of the rotor is validated by comparing the vibration responses obtained experimentally and numerically. A good agreement between these data confirms the correctness and accuracy of the proposed model. The suggested approach may be utilized for a more reliable dynamic analysis of the rotating shafts, having the potential to experience propagating transverse cracks.

Copyright © 2013 by ASME
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Fig. 1

Model of the rotor: (a) dimensions and the first discretization into 46 spring-damping elements; (b) the second discretization into 47 rigid finite elements; (c) local coordinate system ξ,η,ζ of the rotor; and local coordinate systems of individual RFEs (xr,1,xr,2,xr,3) and SDEs (yk,1,yk,2,yk,3)

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Fig. 4

Subsequent steps (a), (b), (c), (d) of crack propagation process

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Fig. 3

Model of the propagating crack: (a) the rotor at the location of the crack; (b) the two RFEs and several SDEs modeling the crack in the partially cracked shaft under external loads; (c) uncracked shaft cross-section at the crack location, (d) shaft cross-section at the crack location in the partially cracked shaft under external loads

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Fig. 5

Cracked shaft finite element: (a) acting forces and coordinate systems; (b) cracked cross-section (based on Ref. [7])

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Fig. 6

Modified virtual crack closure method (based on Ref. [37])

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Fig. 11

Time history of the horizontal vibration of the rotor with the propagating crack

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Fig. 12

Time history of the vertical vibration of the rotor with the propagating crack

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Fig. 7

Virtual crack closure method applied to the rigid finite element model of the crack

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Fig. 2

Typical da/dN versus ΔKcurve

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Fig. 16

Frequency vibration spectra of the rotating shaft with a propagating crack

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Fig. 8

Schematic diagram of the algorithm for the numerical model of the rotor with the breathing and propagating crack

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Fig. 20

Vertical vibration spectrum of the 40% cracked rotor: (a) simulation; (b) experiment

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Fig. 9

Campbell diagram of the uncracked rotor

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Fig. 10

The first four mode shapes of the uncracked rotor at 1620 rpm

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Fig. 13

Snapshots of the growing shaft crack shapes

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Fig. 14

Breathing of the propagating 25% deep crack within the 4436th revolution

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Fig. 15

Breathing of the propagating 40% deep crack within the 4595th revolution

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Fig. 17

Vertical vibration of the rotor: (a) with no crack; (b) with the 25% deep crack; (c) with the 40% deep crack, solid line—experiment, dotted line—simulation

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Fig. 18

Vertical vibration spectrum of the uncracked rotor: (a) simulation; (b) experiment

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Fig. 19

Vertical vibration spectrum of the 25% cracked rotor: (a) simulation; (b) experiment




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