Journal Articles

Piezoelectric-wafer active sensors are small, inexpensive, noninvasive, elastic wave transmitters/receivers that can be easily affixed to a structure. As wide-band nonresonant devices, piezoelectric-wafer active sensors can selectively tune in various Lamb-wave modes traveling in a thin-wall structure. This paper presents results obtained using a linear piezoelectric-wafer phased array to in situ image crack growth during a simulated structural health monitoring test on a large 2024-T3 aluminum plate. During the test, in situ readings of the piezoelectric-wafer phased array were taken in a round-robin fashion while the testing machine was running. Additional hardware was incorporated to prefilter the received signals before digitization to obtain usable readings. The received signals were postprocessed with the embedded ultrasonic structural radar phased-array algorithm and a direct imaging of the crack in the test plate was obtained. The imaging results were compared with physical measurements of the crack size using a digital camera. Good consistency was observed. The results of this investigation could be used to predict the gradual growth of a crack during structural health monitoring.

An analytical and experimental investigation of the Lamb wave-mode tuning with piezo-electric 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 space-wise 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 as-sumption 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 30 kHz 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 290 kHz tuning of the S0 mode due to the frequency sidebands present in the tone-burst wave packet.

Lamb wave time reversal method is a new and tempting baseline-free damage detection technique for structural health monitoring. With this method, certain types of damage can be detected without baseline data. However, the application of this method using piezoelectric wafer active sensors (PWAS) is complicated by the existence of at least two Lamb wave modes at any given frequency, and by the dispersion nature of the Lamb wave modes existing in thin-wall structures. The theory of PWAS-related Lamb wave time reversal has not yet been fully studied. This paper addresses this problem by developing a theoretical model for the analysis of PWAS-related Lamb wave time reversal based on the exact solutions of the RayleighLamb wave equation. The theoretical model is first used to predict the existence of single-mode Lamb waves. Then the time reversal behavior of single-mode and two-mode Lamb waves is studied numerically. The advantages of singlemode tuning in the application of time reversal damage detection are highlighted. The validity of the proposed theoretical model is verified through experimental studies. In addition, a similarity metric for judging time invariance of Lamb wave time reversal is presented. It is shown that, under certain condition, the use of PWAS-tuned single-mode Lamb waves can greatly improve the effectiveness of the time-reversal damage detection procedure

The use of piezoelectric wafer active sensors (PWAS) phased arrays for Lamb wave damage detection in thin-wall structures is presented. The PWAS capability to tune into specific Lamb-wave modes (which is an enabling factor for our approach) is first reviewed. Then, a generic beamforming formulation that does not require the conventional parallel-ray approximation is developed for PWAS phased arrays in connection with the delay-and-sum beamforming principles. This generic formulation is applied to a 1-D linear PWAS phased array. Particularly, 1-D PWAS array beamforming reduces to the simplified parallel ray algorithm when the parallel ray approximation is invoked. The embedded ultrasonic structural radar (EUSR) algorithm is presented. A couple of simple experiments are used to show that the linear EUSR PWAS phased array system can successfully detect cracks in large aluminum thin plates. To improve the EUSR image quality, advanced signal processing is studied for possible integration into the EUSR system. The approaches include Hilbert transform for envelope detection, thresholding techniques for removing background noise, discrete wavelet transform for denoising, continuous wavelet transform for single frequency component extraction, and cross-correlation for time-of-flight detection. The optimization of linear PWAS arrays is studied next. First we consider the effect of several parameters affecting the phased-array beamforming: (1) number of elements M; (2) elementary spacing d; (3) steering angle φ0; (4) location of the target r. Second, we examine the so-called nonuniform PWAS arrays which are generated by assigning different excitation weights to each of the array elements. The design of two nonuniform linear PWAS arrays, the binomial array and the Dolph–Chebyshev array, is presented. Significant improvement of the EUSR image is observed when using these nonuniform arrays.

Ferroelectric Ba Ti O ₃ (BTO) thin films were deposited on NiO buffered polycrystalline Ni tapes by pulsed laser deposition. Microstructural studies by x-ray diffractometer and transmission electron microscopy reveal that the as-grown BTO films have the nanopillar structures with an average size of approximately 80 nm in diameter and the good interface structures with no interdiffusion or reaction. The dielectric and ferroelectric property measurements exhibit that the BTO films have a relatively large dielectric constant, a small dielectric loss, and an extremely large piezoelectric response with a symmetric hysteresis loop. These excellent properties indicate that the as-fabricated BTO films are promising for the development of the structural health monitoring systems.

Piezoelectric sensors have been shown to respond reproducibly to changes in tissue mechanical properties surrounding an implant over a 4-month period. The vibrational amplitude at a frequency corresponding to the radial resonance shows a statistically significant change over time. The initial period of inflammation is marked by a significant reduction in amplitude, which is indicative of an increase in viscous dissipation of the tissue. As collagen displaces the cellular response, the amplitude continues to decrease. Finally, as the tissue matures, the capsule becomes stiffer, and the viscous dissipation lessens. These results are consistent with qualitative assessments of explanted capsules.

Strain gauges encased in a monolithic block of silicone exhibited a greater degree of variability, yet show similar trends over time. The strain increases in the initial 4-week period and remains relatively steady over the following 4 weeks. Beyond 8 weeks, the gauges begin to extrude from the animal or suffer a loss of electrical continuity. Steps are being taken to improve the strain sensor longevity in the animals.

For some time, engineers and scientists have been using the nature as an inspiration for theirdesign of advanced robotics and mechanical systems. Today, nature has also become the in-spiration for an entire new class of material systems – theadaptronic structures[1]. Usingbiological analogies and nature’s ability to adapt a material’s structure, morphology, shape,and properties to accommodate its changing environment and its aging process, present dayengineers and scientists are designing material systems that can change their shape, con-trol their vibrations, monitor their own health, and perform many other biological-inspiredfunctions.

Über viele Jahre ließen sich Ingenieure und Naturwissenschaftler von der Natur anregen, umfortschrittliche Roboter und andere mechanische Systeme zu entwerfen. Heutzutage animiertdie Natur darüber hinaus zu einer ganz neuen Klasse von Materialsystemen:adaptronischeStrukturen. Durch Nutzung biologischer Analogien sind Ingenieure und Naturwissenschaftlernun in der Lage, Struktur, Gestalt, Form und Eigenschaften von Werkstoffen so zu designen,dass sie sich selbsttätig an Umweltbedingungen und Alterungsprozesse anpassen. Hierzuwerden Materialsysteme entwickelt, die ihre Form verändern und ihre dynamische Belastungkontrollieren können sowie ihren eigenen Zustand überwachen und viele andere, durch dieBiologie inspirierte Funktionen ausführen.

Structural health monitoring results obtained with the electro-mechanical (E/M) impedance technique and Lamb wave transmission methods during fatigue crack propagation of an Arcan specimen instrumented with piezoelectric wafer active sensors (PWAS) are presented. The specimen was subjected in mixed-mode fatigue loading and a crack was propagated in stages. At each stage, an image of the crack and the location of the crack tip were recorded and the PWAS readings were taken. Hence, the crack-growth in the specimen could be correlated with the PWAS readings. The E/M impedance signature was recorded in the 100 - 500 kHz frequency range. The Lamb-wave transmission method used the pitch-catch approach with a 3-count sine tone burst of 474 kHz transmitted and received between various PWAS pairs. Fatigue loading was applied to initiate and propagate the crack damage of controlled magnitude. As damage progressed, the E/M impedance signatures and the waveforms received by receivers were recorded at predetermined intervals and compared.