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TECHNICAL PAPERS

Lamb Wave-Mode Tuning of Piezoelectric Wafer Active Sensors for Structural Health Monitoring

[+] Author and Article Information
Giola B. Santoni

Department of Mechanical Engineering,  University of South Carolina, Columbia, SC 29208bottai@engr.sc.edu

Lingyu Yu

Department of Mechanical Engineering,  University of South Carolina, Columbia, SC 29208Yu3@engr.sc.edu

Buli Xu

Department of Mechanical Engineering,  University of South Carolina, Columbia, SC 29208Xub@engr.sc.edu

Victor Giurgiutiu

Department of Mechanical Engineering,  University of South Carolina, Columbia, SC 29208victorg@sc.edu

J. Vib. Acoust 129(6), 752-762 (Feb 19, 2007) (11 pages) doi:10.1115/1.2748469 History: Received May 20, 2006; Revised February 19, 2007

Abstract

An analytical and experimental investigation of the Lamb wave-mode tuning with piezoelectric wafer active sensors (PWASs) is presented. The analytical investigation assumes a PWAS transducer bonded to the upper surface of an isotropic flat plate. Shear lag transfer of tractions and strains is assumed, and an analytical solution using the spacewise Fourier transform is reviewed, closed-form solutions are presented for the case of ideal bonding (i.e., load transfer mechanism localized at the PWAS boundary). The analytical solutions are used to derive Lamb wave-mode tuning curves, which indicate that frequencies exist at which the A0 mode or the S0 mode can be either suppressed or enhanced. Extensive experimental tests that verify these tuning curves are reported. The concept of “effective PWAS dimension” is introduced to account for the discrepancies between the ideal bonding hypothesis and the actual shear-lag load transfer mechanism. The paper further shows that the capability to excite only one desired Lamb wave mode is critical for practical structural health monitoring (SHM) applications such as PWAS phased array technique (e.g., the embedded ultrasonics structural radar (EUSR)) and the time reversal process (TRP). In PWAS phased array EUSR applications, the basic assumption of the presence of a single low-dispersion Lamb wave mode (S0) is invoked since several Lamb wave modes traveling at different speeds would disturb the damage imaging results. Examples are given of correctly tuned EUSR images versus detuned cases, which illustrate the paramount importance of Lamb wave-mode tuning for the success of the EUSR method. In the TRP study, an input wave packet is reconstructed at a transmission PWAS when the signal recorded at the receiving PWAS is reversed in the time domain and transmitted back to the original PWAS. Ideally, TRP could be used for damage detection without a prior baseline. However, the application of TRP to Lamb waves SHM is impended by the dispersive and multimodal nature of the Lamb waves. The presence of more then one mode usually produces additional wave packets on both sides of the original wave packet due to the coupling of the Lamb wave modes. The PWAS Lamb wave tuning technique described in this paper is used to resolve the side packets problem. Several tuning cases are illustrated. It is found that the $30kHz$ tuning of the A0 Lamb wave mode with a 16-count smoothed tone burst leads to the complete elimination of the side wave packets. However, the elimination was less perfect for the $290kHz$ tuning of the S0 mode due to the frequency sidebands present in the tone-burst wave packet.

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Figures

Figure 9

PWAS phased array interrogation: (a) illustration of a PWAS phased array to be directed to a certain direction and (b) beamforming of a PWAS array using eight PWAS aligned along a straight line

Figure 12

Frequency tuning for EUSR phased array application: (a) pitch-catch voltage measurement, (b) pulse echo signal at 210kHz tuning frequency, and (c) pulse echo signal at 300kHz tuning frequency

Figure 13

EUSR inspection using frequency tuning: (a) EUSR mapped image at 300kHz tuning frequency, (b) EUSR image at 210kHz, and (c) EUSR image at 450kHz

Figure 14

Lamb wave time-reversal procedure block diagram

Figure 15

Predicted Lamb wave response of a 1mm aluminum plate under PWAS excitation: normalized strain response for a 7mm round PWAS (6.4mm equivalent length)

Figure 1

Layer interaction between the PWAS and the structure modeling (3)

Figure 2

Variation of shear-lag transfer mechanism with bond thickness tb for a APC-850 PWAS (Ea=63GPa, ta=0.2mm, la=7mm, d31=−175mm∕kV) attached to a thin-wall aluminum structure (E=70GPa and t=1mm) through a bond layer of Gb=2GPa (normalized position covers a half-PWAS length from center outward)

Figure 3

Load on a plate due to the PWAS actuation: (a) symmetric and (b) asntisymmetric (3)

Figure 4

Predicted Lamb wave response of a 1.6mm aluminum plate under a 7mm PWAS excitation: strain response

Figure 5

Aluminum plate 2024-T3 1.07mm with square, rectangular, and round PWASs

Figure 6

Tuning: Aluminum 2024-T3, 1.07mm thickness, 7mm square PWAS: (a) experimental data and (b) prediction with Eq. 3

Figure 7

Aluminum 2024-T3, 3.15mm thickness, 7mm square PWAS: (a) experimental data and (b) prediction (Eq. 3)

Figure 8

Tuning on plate 2024-T3, 1.07mm thick, rectangular PWAS (P1-P2): (a) experimental data and (b) prediction with Eq. 3

Figure 10

Frequency tuning for crack detection in a 1mm thick aluminum plate specimen: (a) strain-frequency plot and (b) group velocity-frequency plot

Figure 11

PWAS guided wave phased array application: (a) laboratory specimen layout and (b) experiment equipment

Figure 16

Untuned time reversal: reconstructed input using 16-count tone burst with 500kHz carrier frequency. Strong residual signals due to multimode Lamb waves are present.

Figure 17

Time reversal with S0 Lamb wave-mode tuning: reconstructed input using 16-count tone burst with 290kHz carrier frequency. Weak residual wave packets due to residual A0 mode component are still present due to the side band frequencies present in the tone burst.

Figure 18

Time reversal with A0 Lamb mode tuning: reconstructed input using 16-count tone burst with 30kHz carrier frequency; no residual wave packets are present

Figure 19

Reconstructed wave and residual wave in terms of their maximum amplitudes using 16-count tone burst over wide frequency range (10–1100kHz)

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