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Structural Health Monitoring of Rotordynamic Systems

Mani, Girindra N.
http://rave.ohiolink.edu/etdc/view?acc_num=akron1144522032

Abstract Details

2006, Doctor of Philosophy, University of Akron, Mechanical Engineering.
Structural health monitoring (SHM) deals with monitoring and detecting damage of a structure. Vibration-based SHM methods examine changes in the vibration characteristics of the structure to identify and quantify any possible damage. This thesis focuses on two aspects of vibration-based SHM of rotordynamic systems. To detect damage at earlier stages, the first demand of a successful vibration-based SHM method is to measure appropriate damage sensitive vibration signal, either passively, or by active interrogation of a rotating structure. Another important aspect is the signal processing stage where the vibration data is prepared for feature extraction to finally make decision about the health of the structure. We present a framework for active SHM techniques that addresses these two aspects, and utilizes resonant behavior (when two or more frequencies satisfy a rational relationship) of a rotordynamic system. Specifically, we attempt to develop external forcing strategies that create damage sensitive measures from the vibration response that are then analyzed using advanced technologies such as wavelet analysis. We consider a rotating cracked shaft with a specified, time-dependent forcing applied at the mid-shaft location. The crack is assumed to be “breathing” (continuously opens and closes as the shaft rotates) and produces a time-dependent stiffness in the governing equations. We begin with a Jeffcott rotor (a simply supported, massless shaft carrying a rigid disk in the middle) subject to harmonic external forcing. The equations of motion are linearized about the static equilibrium position and then analyzed using the method of multiple scales to identify resonant operating conditions. In particular, a combination resonance is identified in which the response is sensitive to the magnitude of the accumulating damage, serving as a novel strategy for health monitoring in the system. Furthermore, this proposed damage detection technique does not require the shaft to operate at a specific speed and can therefore be used as an “on-line” health-monitoring strategy under normal loads and steady-state operating conditions. This strategy is verified against numerical simulations of the original equations of motion and even in the presence of noise the response is shown to remain sensitive to the magnitude of the damage. The analytical predictions for the response of the damaged system are shown to compare favorably with the numerical results obtained from spectral analysis. Furthermore, we examine the application of various non-harmonic periodic external forcing inputs that satisfy the combination resonance condition. The response to some non-harmonic excitations is found to be more sensitive to the presence of the damage compared to the harmonic excitation with an equal force amplitude. Next, the application of the continuous wavelet transform (CWT) to the numerically simulated signals is discussed. In particular, CWT coefficients near an appropriate “scale” (related to frequency) are monitored for both resonant and non-resonant conditions. The results compare favorably with the analytical predictions and those obtained from the spectral analysis. We also offer an approximate method to better analyze the noise influence on the damaged responses. Furthermore, we investigate the intermittent application of external forcing which can facilitate the “on-line” applicability of the proposed technique. We then extend our study to a uniform and elastic rotating cracked shaft modeled as an Euler-Bernoulli beam. Once again, the equations of motion are linearized about the static equilibrium position and Galerkin's method is applied to the equations of motion with assumed mode displacement functions. This leads to a set of time-dependent bilinear equations which are then analyzed using the method of multiple scales to identify the resonant operating conditions. As in the case of the Jeffcott rotor, combination resonance conditions are identified in which the response is sensitive to the magnitude of the accumulating damage. Spectral and wavelet analyses are repeated as before and results are comparable to the theoretical predictions. Finally, we would like to mention that experiments are being conducted (at Virginia Tech. under the supervision of Dr. Mary Kasarda) as a verification of this work. The preliminary experimental results [74] demonstrate that a viable SHM approach, as outlined in this thesis, can be achieved for improved identification of the shaft damage.
Donald Quinn (Advisor)
¿¿¿¿¿¿¿; shaft; ¿¿¿¿¿¿¿2; critical frequency

Recommended Citations

Citations

  • Mani, G. N. (2006). Structural Health Monitoring of Rotordynamic Systems [Doctoral dissertation, University of Akron]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=akron1144522032

    APA Style (7th edition)

  • Mani, Girindra. Structural Health Monitoring of Rotordynamic Systems. 2006. University of Akron, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=akron1144522032.

    MLA Style (8th edition)

  • Mani, Girindra. "Structural Health Monitoring of Rotordynamic Systems." Doctoral dissertation, University of Akron, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=akron1144522032

    Chicago Manual of Style (17th edition)

akron1144522032
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© 2006, all rights reserved.
This open access ETD is published by University of Akron and OhioLINK.