Constrained thickness-shear vibration-based piezoelectric transducers for generating unidirectional-propagation SH0 wave.

Excitation of a pure guided wave with a controllable wavefield is essential in structural health monitoring (SHM). For example, a unidirectional-propagation guided wave can significantly reduce the complexity of signal interpretations by avoiding unwanted reflections. However, few transducers are currently capable of exciting a pure unidirectional-propagation guided wave, which cannot satisfy the emerging demands from the field of SHM. In this work, the thickness-shear vibration characteristics of the piezoelectric PZT wafer bonded on a waveguide are investigated by theoretical modeling and numerical simulations. It is found that there is a phase difference between the electric-excitation signal applied on the PZT wafer and the mechanical response signal of the bottom surface of the viscoelastic adhesive layer that connects the PZT wafer and waveguide. Moreover, such a phase difference can be adjusted by changing the equivalent width of the PZT wafer. Based on this finding, two piezoelectric transducers with different shape configurations are proposed to excite the unidirectional-propagation SH0 wave (the fundamental shear horizontal wave). Finite element simulations and experiments are conducted to verify the performances of the two unidirectional transducers. Results show that the two transducers can excite a pure SH0 wave and enhance the wave energy along a single direction. No time delay is required to excite the proposed transducers. Due to their simple configurations, the developed unidirectional SH0 wave transducers will have great potential applications in SHM.

Excitation of unidirectional SH wave within a frequency range of 50 kHz by piezoelectric transducers without frequency-dependent time delay.

Unidirectional generation of a pure guided wave mode is of practical importance in structural health monitoring (SHM). Currently, unidirectional propagation guided waves can only be generated by using transducer array in a phased array form, which is limited within a very narrow frequency range. If a variable-wavelength unidirectional wave source is required, both the transducer spacing and time delay usually need to be changed with frequency, which is against the permanent arrangement demand of transducers in SHM. In this work, inspired by the unique features of bidirectional SH0 wave (the fundamental shear horizontal mode) piezoelectric face-shear and thickness-shear transducers, a new method was proposed for generating unidirectional SH0 wave. Two types of transducer configurations (double-side and single-side) were designed to unidirectionally generate SH0 wave without frequency-dependent time delay. Both finite element simulations and experiments were conducted to validate the unidirectional features of the proposed double-side and single-side transducer configurations. Results shows that the double-side transducer configuration is capable of generating unidirectional propagation SH0 wave at the frequency range from 100 kHz to 150 kHz, while the corresponding working frequency range for the single-side one is from 100 kHz to 140 kHz. The proposed method provides a cheap way for generation of the unidirectional SH0 wave within a certain frequency range without adjusting the relative transducer spacing and the time delay with frequency, so it will have great potential applications in SHM.

Shear horizontal wave transducers for structural health monitoring and nondestructive testing: A review.

Shear horizontal (SH) waves are of great importance in structural health monitoring (SHM) and nondestructive testing (NDT), since the lowest order SH wave in isotropic plates is non-dispersive. The SH waves in plates, circumferential SH waves and torsional waves in pipes have remarkable resemblances in dispersion characteristics and wave structures, so the latter two can also be called as SH waves in pipes. This paper reviews the state-of-the-art research on SH wave transducers for SHM and NDT. These transducers are grouped into the following categories: Lorentz-force-based electromagnetic acoustic transducers (EMATs), magnetostrictive EMATs, shear wave piezoelectric wedge transducers, thickness-shear piezoelectric transducers and face-shear piezoelectric transducers. The working principles, applications, merits and limitations of different kinds of SH wave transducers are summarized, with a focus on discussing the various configurations for exciting and receiving directional, omnidirectional SH waves in plates and torsional waves in pipes. This paper is expected to greatly promote the applications of SH waves in SHM, NDT and the related areas such as elastic metamaterials.

Circumferential SH Wave Piezoelectric Transducer System for Monitoring Corrosion-Like Defect in Large-Diameter Pipes.

The fundamental circumferential shear horizontal (CSH0) wave is of practical importance in monitoring corrosion defects in large-diameter pipes due to its virtually non-dispersive characteristics. However, so far, there have been limited CSH0 wave transducers which can be used to constitute a structural health monitoring (SHM) system for pipes. Moreover, the CSH0 wave's capability of sizing the corrosion-like defect has not yet been confirmed by experiments. In this work, firstly, the mechanism of exciting CSH waves was analyzed. A method based on our previously developed bidirectional SH wave piezoelectric transducers was then proposed to excite the pure CSH0 mode and first order circumferential shear horizontal (CSH1) mode. Both finite element simulations and experiments show that the bidirectional transducer is capable of exciting pure CSH0 mode traveling in both circumferential directions of a 1 - mm thick steel pipe from 100 to 300 kHz. Moreover, this transducer can also serve a sensor to detect CSH0 mode only by filtering circumferential Lamb waves over a wide frequency range from 100 to 450 kHz. After that, a method of sizing a rectangular notch defect by using CSH0 wave was proposed. Experiments on an 11 - mm thick steel pipe show that the depth and circumferential extent of a notch can be accurately determined by using the proposed method. Finally, experiments were performed to investigate the reflection and transmission characteristics of CSH0 and CSH1 waves from notches with different depths. It was found that transmission coefficients of CSH0 mode decrease with the increasing of notch depth, which indicates that it is possible to monitor the depth change of corrosion defects by using CSH0 wave.

Enhanced Formaldehyde Removal from Air Using Fully Biodegradable Chitosan Grafted ß-Cyclodextrin Adsorbent with Weak Chemical Interaction.

Formaldehyde (HCHO) is an important indoor air pollutant. Herein, a fully biodegradable adsorbent was synthesized by the crosslinking reaction of ß-cyclodextrin (ß-CD) and chitosan via glutaraldehyde (CGC). The as-prepared CGC showed large adsorption capacities for gaseous formaldehyde. To clarify the adsorption performance of the as-synthesized HCHO adsorbents, changing the adsorption parameters performed various continuous flow adsorption tests. It was found that the adsorption data agreed best with the Freundlich isotherm, and the HCHO adsorption kinetic data fitted well with the pseudo second order model. The breakthrough curves indicated that the HCHO adsorbing capacity of CGC was up to 15.5 mg/g, with the inlet HCHO concentration of 46.1 mg/m³, GHSV of 28 mL/min, and temperature of 20 °C. The regeneration and reusability of the adsorbent were evaluated and CGC was found to retain its adsorptive capacity after four cycles. The introduction of ß-CD was a key factor for the satisfied HCHO adsorption performance of CGC. A plausible HCHO adsorption mechanism by CGC with the consideration of the synergistic effects of Schiff base reaction and the hydrogen bonding interaction was proposed based on in situ DRIFTS studies. The present study suggests that CGC is a promising adsorbent for the indoor formaldehyde treatment.

A variable-frequency bidirectional shear horizontal (SH) wave transducer based on dual face-shear (d24) piezoelectric wafers.

Focusing the incident wave beam along a given direction is very useful in guided wave based structural health monitoring (SHM), as it will not only save input power but also simplify the interpretation of signals. Although the fundamental shear horizontal (SH0) wave is of practical importance in SHM due to its non-dispersive characteristics so far there have been very limited transducers which can control the radiation patterns of SH0 wave. In this work, a variable-frequency bidirectional SH0 wave piezoelectric transducer (BSH-PT) is proposed, which consists of two rectangular face-shear (d24) PZT wafers. The opposite face-shear deformation of the two PZT wafers under applied electric fields makes the BSH-PT capable of exciting SH0 wave along two opposite directions (0° and 180°). Both finite element simulations and experimental testings are conducted to examine the performance of the proposed BSH-PT. Results show that pure SH0 wave can be generated by this BSH-PT and its wave beam can be focused bi-directionally. Moreover, the bidirectional characteristics of the BSH-PT can be kept over a wide frequency range from 150 kHz to 250 kHz. As the circumferential SH0 (CSH0) wave in a thin hollow cylindrical structure is essentially equivalent to the SH0 wave in a plate, the proposed BSH-PT may also be very useful to develop a CSH0-wave-based SHM system for hollow cylindrical structures.

Generation and reception of shear horizontal waves using the synthetic face-shear mode of a thickness-poled piezoelectric wafer.

The guided wave based inspection technique has been playing an important role in modern industries due to its capability in rapid detection of large structures. Among all the wave modes in plate-like structures, the fundamental shear horizontal wave (SH0) is of great importance since it is the unique non-dispersive mode. However, the generation and reception of SH0 wave using piezoelectrics is always a challenge. In this work, we synthesized face-shear deformation mode in a thickness-poled piezoelectric wafer and successfully excited/received SH0 wave in a thin aluminum plate. Firstly, the frequency response of the proposed wafer was analyzed using the finite element method (FEM) to show that the face-shear deformation can be synthesized via applying anti-parallel electric fields on different parts of the wafer. Subsequently, time-transient FEM simulations were carried out to predict its capacity in generation/reception of SH0 wave. Finally, experiments were conducted to examine the performance of the proposed wafer on SH0 wave generation/reception. The obtained results indicate that the synthetic face-shear piezoelectric wafer can generate SH0 wave along two principal directions (0° and 90°) with the amplitudes symmetric along the 45° direction. The amplitude of the generated SH0 wave reached its maxima along the principal direction and decreased to nearly zero at 45° direction, which is in good agreement with the FEM results. Besides, the wafer can only receive SH0 wave in a wide range of frequency, i.e., it can act as an inherent wave filter. Due to its compact size and easy fabrication, the proposed wafer has a great potential in promoting the applications of SH0 wave in nondestructive testing and structural health monitoring.

A new omnidirectional shear horizontal wave transducer using face-shear (d24) piezoelectric ring array.

The non-dispersive fundamental shear horizontal (SH0) wave in plate-like structures is of practical importance in non-destructive testing (NDT) and structural health monitoring (SHM). Theoretically, an omnidirectional SH0 transducer phased array system can be used to inspect defects in a large plate in the similar manner to the phased array transducers used in medical B-scan ultrasonics. However, very few omnidirectional SH0 transducers have been proposed so far. In this work, an omnidirectional SH0 wave piezoelectric transducer (OSH-PT) was proposed, which consists of a ring array of twelve face-shear (d24) trapezoidal PZT elements. Each PZT element can produce face-shear deformation under applied voltage, resulting in circumferential shear deformation in the OSH-PT and omnidirectional SH0 waves in the hosting plate. Both finite element simulations and experiments were conducted to examine the performance of the proposed OSH-PT. Experimental testing shows that the OSH-PT exhibits good omnidirectional properties, no matter it is used as a SH0 wave transmitter or a SH0 wave receiver. This work may greatly promote the applications of SH0 waves in NDT and SHM.

Magnetic-field-induced ferroelectric polarization reversal in magnetoelectric composites revealed by piezoresponse force microscopy.

Controlling electric polarization (or magnetization) in multiferroic materials with external magnetic fields (or electric fields) is very important for fundamental physics and spintronic devices. Although there has been some progress on magnetic-field-induced polarization reversal in single-phase multiferroics, such behavior has so far never been realized in composites. Here we show that it is possible to reverse ferroelectric polarization using magnetic fields in a bilayer Terfenol-D/PMN-33%PT composite. We realized this by ferroelectric domain imaging using piezoresponse force microscopy (PFM) under applied magnetic field loading. The internal electric field caused by the magnetoelectric (ME) effect in the PMN-PT crystal is considered as the driving force for the 180° polarization switching, and its existence is verified by switching spectroscopy PFM testing under a series of external magnetic fields. A quantitative method is further suggested to estimate the local ME coefficient based on the switching spectroscopy PFM testing results.

Nanoscale structural and functional mapping of nacre by scanning probe microscopy techniques.

Nacre has received great attention due to its nanoscale hierarchical structure and extraordinary mechanical properties. Meanwhile, the nanoscale piezoelectric properties of nacre have also been investigated but the structure-function relationship has never been addressed. In this work, firstly we realized quantitative nanomechanical mapping of nacre of a green abalone using atomic force acoustic microscopy (AFAM). The modulus of the mineral tablets is determined to be ~80 GPa and that of the organic biopolymer no more than 23 GPa, and the organic-inorganic interface width is determined to be about 34 ± 9 nm. Then, we conducted both AFAM and piezoresponse force microscopy (PFM) mapping in the same scanning area to explore the correlations between the nanomechanical and piezoelectric properties. The PFM testing shows that the organic biopolymer exhibits a significantly stronger piezoresponse than the mineral tablets, and they permeate each other, which is very difficult to reproduce in artificial materials. Finally, the phase hysteresis loops and amplitude butterfly loops were also observed using switching spectroscopy PFM, implying that nacre may also be a bio-ferroelectric material. The obtained nanoscale structural and functional properties of nacre could be very helpful in understanding its deformation mechanism and designing biomimetic materials of extraordinary properties.