Embedded Ultrasonics Structural Radar Research Articles

Shear lag solution for tuning ultrasonic piezoelectric wafer active sensors with applications to Lamb wave array imaging

An analytical investigation of the interaction between piezoelectric wafer active sensor (PWAS), guided Lamb waves, and host structure is presented in this paper, supported with application examples. 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 space-wise Fourier transform is reviewed, closed-form solutions are presented for the case of both ideal bonding (i.e., load transfer mechanism localized at the PWAS boundary) and not ideal bonding (i.e., load transfer mechanism localized close 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. The paper further shows that the capability to excite only one desired Lamb wave mode is critical for practical structural health monitoring applications such as PWAS phased array technique (e.g., the embedded ultrasonics structural radar, EUSR) and the sparse array imaging. Extensive experimental tests that verify the tuning mechanism and prediction curves are reported. Examples of correctly tuned EUSR images vs. detuned cases illustrate the paramount importance of Lamb wave mode tuning for the success of PWAS based damage detection.

In-Situ Imaging of Crack Growth with Piezoelectric-Wafer Active Sensors

∗Piezoelectric-wafer active sensors (PWAS) are small, inexpensive, non-invasive, elastic wave transmitters/receivers that can be easily affixed to a structure. PWAS are wide-band nonresonant devices that can selectively tune in various Lamb wave modes traveling in a thin-wall structure. PWAS can be wired into sensor arrays and connected to data concentrators and wireless communicators. They have the potential to bring about a revolution in structural health monitoring, damage detection, and non-destructive evaluation just as significant as ultrasonic inspection did 50 years ago. However, its development is not yet complete, and a number of issues have still to be resolved. This paper will present results obtained in using a PWAS phased array to in-situ image crack growth during a simulated structural health monitoring test. The fatigue crack growth experiment was set up on a 1000 mm by 1000 mm 2024-T3 1-mm thick aluminum plate. An initial pre-crack of 30 mm was grown under fatigue loading to a final length of 60 mm over a total of 58 kilocycles run at 10 Hz frequency in an MTS-810 testing machine. During the test, insitu readings of the PWAS phased array were taken while the testing machine was running. The PWAS were excited with a 10-Vpp smoothed 3-count tone burst of 372 kHz, which excited the symmetric S0 Lamb-wave mode. To minimize the equipment requirement to just a single channel, the excitation of the PWAS array was performed in a round robin fashion. Simultaneously, all the PWAS were in term considered as receivers. In this way, an N x N matrix of signals was collected and stored. In the beginning, the received signals were unusable due to the high 10 Hz noise superposed by the testing process. Additional hardware was constructed to pre-filter the received signals prior to digitization. Consequently, usable signals with clear crack reflections were successfully collected. The received signals were post processed with the embedded ultrasonics structural radar (EUSR) algorithm to obtain a direct imaging of the crack in the test plate (similar to the C-scan from ultrasonic NDE, only that was obtained from a single location using a sweeping beam of focused Lamb waves. The imaging results were correlated with physical measurements of the crack size using penetrating liquid and a digital camera. Remarkable consistency was obtained. Subsequently, the results were used to predict further crack growth and make a prognosis of component failure using the structural health monitoring results.

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

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.

Embedded-ultrasonics Structural Radar for In Situ Structural Health Monitoring of Thin-wall Structures

Embedded-ultrasonics structural radar (EUSR) is a new concept and methodology for in situ nondestructive evaluation (NDE) and structural health monitoring (SHM) of thin-wall structures. EUSR consists of: (a) an array of piezoelectric wafer active sensors (PWAS) embedded into the structure; and (b) electronic modules for signal transmission/reception, processing, and interpretation. EUSR utilizes guided elastic waves (Lamb waves) generated omnidirectionally into the thin-wall structure by surface-mounted permanently attached PWAS. The paper starts with the general concepts of the EUSR algorithm: transmission beamforming, reception beamforming, and time-of-fight (TOF) determination. Next, details of the Lamb wave generation with PWAS, verification of group-velocity dispersion curves, identification of optimal excitation frequency, and confirmation of wave front omnidirectionality are discussed. In the third part of the paper, the actual implementation of the EUSR method in a proof-of-concept demonstration is presented. The construction of the PWAS-phased array is described, and detection of cracks located broadside and offside of the PWAS array is illustrated. The method is shown to be easy to use through a visually interactive LabView™ interface. Very good detection accuracy is observed. The proof-of-concept experiments presented in this paper were illustrated on metallic structures; however, the EUSR concept may also work on composite and hybrid structures, although the range of detection may be reduced by the medium attenuation.