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TECHNICAL PAPERS

Frequency Range Selection for Impedance-Based Structural Health Monitoring

[+] Author and Article Information
Daniel M. Peairs

Center for Intelligent Material Systems and Structures, Virginia Polytechnic Institute and State University, 310 Durham Hall, Mail Code 0261, Blacksburg, Virginia 24061-0261dpeairs@vt.edu

Pablo A. Tarazaga, Daniel J. Inman

Center for Intelligent Material Systems and Structures, Virginia Polytechnic Institute and State University, 310 Durham Hall, Mail Code 0261, Blacksburg, Virginia 24061-0261

J. Vib. Acoust 129(6), 701-709 (Jul 26, 2007) (9 pages) doi:10.1115/1.2775506 History: Received April 03, 2006; Revised July 26, 2007

Impedance-based structural health monitoring uses collocated piezoelectric transducers to locally excite a structure at high frequencies. The response of the structure is measured by the same transducer. Changes in this response indicate damage. Frequency range selection for monitoring with impedance-based structural health monitoring has, in the past, been done by trial and error methods or has been selected after analysis by engineers familiar with the method. This study aims to determine if, in future applications, it is possible to automatically select preferred frequency ranges based on sensor characteristics, perhaps even before installing the system. In addition, the paper demonstrates a method for determining preferable frequency ranges for monitoring. The study examines the analysis of the measurement change through a damage metric and relates the results of the analysis to characteristics of the measurement. Specifically, outlier detection concepts were used to statistically evaluate the damage detection ability of the transducers at various frequency ranges. The variation in undamaged measurements is compared to the amount of change in the measurement upon various levels of damage. Testing was performed with both solid piezoceramic transducers and macrofiber composite piezoelectric devices of different sizes bonded to aluminum and fiber reinforced composite structures. The results indicate that characteristics of the structure, not the sensor alone, determine the optimal monitoring frequency ranges.

Copyright © 2007 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

(a) Small PZT and (b) large PZT (attached to the same end, opposite side of beam)

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Figure 2

Impedance response of unbonded PZTs

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Figure 3

Mean of small PZT base line damage metric including ±1 standard deviation interval for each frequency band of the base line measurements

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Figure 4

Mean of large PZT base line damage metric including ±1 standard deviation interval for each frequency band of the base line measurements

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Figure 5

(a) Base line and unbonded impedance of large PZT. (b) Base line and unbonded impedances of small PZT. (c) Z statistic for small and large PZTs for added mass. (d) Ratio of Z statistics for added mass.

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Figure 6

Average base line including ±1 standard deviation interval for each frequency band of the base line measurements for the small PZT with increased mass variability

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Figure 7

(a) Base line and unbonded impedance of the large PZT. (b) Base line and unbonded impedance of the small PZT. (c) Z statistic for the small and large PZT for quarter width cut. (d) Ratio of Z statistics for the quarter-width cut and half-width cut, with the result from the previous test shown for reference.

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Figure 8

(a) Suspended boom configuration and HP 4194A impedance analyzer used for testing. (b) Close-up pictures of large MFC (109×73.7mm2) used as collocated sensor/actuators. (c) Close-up pictures of small MFC (31.8×25.4mm2) used as collocated sensor/actuators.

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Figure 9

Boom schematic showing regions of induced damage, indicated by rectangles

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Figure 10

Sample base line impedance measurements (real part) for (a) large and (b) small MFCs on composite boom. The unbonded responses have been included for reference.

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Figure 11

(a) Base line measurement of large MFC (shown for reference). (b) Average base line damage metric for large MFC.

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Figure 12

(a) Base line measurement of small MFC (shown for reference). (b) Average base line damage metric for small MFC.

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Figure 13

Damage metric of large MFC at (a) 128–136kHz and (b) 192–200Hz for increasing levels of damage at the midpoint (region 1) of the boom. Each bar represents one measurement of the impedance response at that frequency range.

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Figure 14

Test statistics of each frequency range for the large MFC with damage induced at the midpoint of the boom (region 1) for increasing hole size. (a) Mean base line impedance measurement of the large MFC (for reference). (b) Test statistics for damage at the midpoint (region 1) with all frequency ranges. (c) Test statistics for damage at the midpoint (region 1) without frequency ranges with false positives.

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Figure 15

Test statistics of each frequency range without a false positive indication for the small MFC at each damage location for increasing hole size. (a) Mean base line impedance measurement. (b) Test statistics for damage close to MFCs (region 2). (c) Test statistics for damage at the midpoint of the beam (region 1). (d) Test statistics for damage at the far end of the beam (region 3). Test statistic values over 50 have been truncated for.improved visualization.

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Figure 16

Test statistics of each frequency range without a false positive indication for the large MFC at each damage location for increasing hole size. (a) Mean base line impedance measurement. (b) Test statistics for damage close to MFCs (region 2). (c) Test statistics for damage at the midpoint of the beam (region 1). (d) Test statistics for damage at the far end of the beam (region 3). Test statistic values over 50 have been truncated for improved visualization.

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