Wave Piezoelectric Transducers Research Articles

Metasubstrate-based SH guided wave piezoelectric transducer for unidirectional beam deflection without time delay

Focusing the energy of a guided wave along a desired direction is of great significance in structural health monitoring (SHM), since it can improve the sensitivity to defects and reduce the complexity of signal interpretations. The phased array method is the most common approach for beam deflection, which requires a high uniformity of each element and the support of expensive and complex electronics. Moreover, the mirrored wave beam cannot be avoided for a linear array, resulting in the difficulty of distinguishing the defects at the symmetric positions. In this work, a metasubstrate-based piezoelectric transducer (MSBPT) is developed for unidirectional SH0 wave (the fundamental shear horizontal wave) beam deflection under a single driving source. The proposed MSBPT is constituted by a metasubstrate and two columns of thickness-shear mode PZT wafers. The metasubstrate is designed to provide the required phase gradient for unidirectional beam deflection, so no extra time delay is needed during excitation and reception. An analytical model is proposed to guide the design of the transducer to obtain the desired wave beam. The beam deflection properties of the MSBPT are validated by finite element simulations and experiments. It is observed that the MSBPT can concentrate the energy of the generated SH0 mode only in one desired direction with a small divergence angle. Moreover, the MSBPT can also serve as a sensor that only receives the SH0 mode propagating from the deflection angle and filters out the wave energies from other incident angles. Due to the simple configuration, the proposed MSBPT will be helpful in the controllable generation and reception of SH0 mode in SHM.

Ultrasonic Thread Surface Inspection of Railroad Car Wheels and Locomotive Tires Using Rayleigh Waves

Ultrasonic surface inspection of cylindrical objects using the Rayleigh wave echo method has many advantages. Despite strict requirements for surface cleaning, the method is widely implemented in industries and transport, for example on the all-rolled wheel thread or tires of railroad rolling stock. Taking into account the ability of the Rayleigh wave transducer to receive waves with a side ('back') lobe of the ray diagram, the influence of possible contamination or excess of contact fluid on the input surface on the control results, it becomes particularly difficult to explain the position and amplitude of the signal. Using mathematical modeling and experimental studies, the ray diagrams of Rayleigh wave piezoelectric transducers with the 'back lobe' characteristic of piezoelectric transducers of this type were obtained. The efficiency of emission and (or) receiving depends on the parameters of control and apparatus; this effect is more evident if the width of the transducer piezoelement deviates (e.g., during fabrication) and the frequency varies within 20 %, allowed by GOST R 55724-2013. The acoustic path of the Rayleigh wave reflected from the flaw with a diameter of 7 mm on the wheel thread is experimentally constructed and tested on the model. It is shown that the amplitude of the echo-signal changes nonuniformly, the local maxima obtained experimentally reach 9 dB relative to the exponential relation characteristic of propagation over a boundless surface. The reason for such phenomena lies in multiple re-reflections of waves from the boundaries on the tread surface of railway wagon wheels and locomotive tires (rim chamfer, ridge to ridge transition) and their further interference.

A general structural design method, dynamics modeling and application study for built-in sandwich annular traveling wave piezoelectric transducers

This study first summarized a piezoelectric excitation source arrangement guideline for sandwich annular traveling wave transducers, which can excite ideal traveling waves for driving while ensuring consistent circumferential stiffness of the annular transducer. Consequently, a general structural design method was proposed for annular sandwich traveling wave piezoelectric transducers operating with out-of-plane traveling waves based on the above-mentioned arrangement guideline. Moreover, to study the vibration characteristics of the designed transducer based on the proposed method, an electromechanical coupled dynamics model was developed using the dynamic substructure method combined with the Lagrange equation. This model is universal and has guiding significance for the design and optimization of the same type of annular traveling wave piezoelectric transducers. Thereafter, the feasibility of the transducer structure design method and the correctness of the developed dynamics model were verified through appropriate vibration measurement experiments on the transducer prototype. The measured ideal mode shape and regular elliptical motion verified the feasibility of the transducer design method, and the comparison between the vibration measurement and theoretical model calculation results confirmed the correctness of the developed dynamic model. Finally, an application study of the proposed built-in sandwich traveling wave transducer was conducted to investigate its driving characteristics. The proposed transducer was used to drive a rotor to build a traveling wave piezoelectric motor. In addition, output performance experiments of the motor prototype were conducted. The experimental results indicated that under the driving conditions of 300 N pre-pressure, 500 Vpp voltage, and 19 kHz exciting frequency, the no-load speed of the motor was 19.04 RPM, its stall torque was 1.2 N·m, the maximum output power of the entire machine reached 0.6453 W, and its maximum output efficiency was 15.87%. Moreover, the normal operation of the rotating motor further verified the feasibility of the transducer structure design approach and the reliability of the drive application. The structural design method and the corresponding dynamic model proposed in this study are expected to have important guiding significance for the design and optimization of sandwich-type annular traveling wave transducers. Furthermore, the application investigation in this study provides a typical example for the traveling wave transducer.

Beam-focused SH wave transducer based on YSr3(PO4)3 piezoelectric crystal for high temperature structural health monitoring

Guided-wave-based structural health monitoring (SHM) technology is of great importance for real-time inspection of high-temperature structures. The fundamental shear horizontal (SH0) wave is believed to be an ideal wave mode for developing SHM systems due to its nondispersive characteristics. However, currently very limited SH0 wave transducers can be used for SHM of high-temperature structures due to the limitation of materials. Herein, a novel YSr3(PO4)3 (YSP) piezoelectric crystal in the space group I4¯3d was grown. Experiments show that the face-shear piezoelectric coefficient d14 (d14 = d25 = d36) is 9.7 pC/N and varies little from 25 to 800 °C. Then a beam-focused SH0 wave piezoelectric transducer is developed based on face-shear-mode YSP wafers. Both finite element simulations and experimental results indicate that the YSP-based transducer can excite pure SH0 wave and focus the wave energy along two opposite main directions. Especially, the obtained SH0 wave beam is highly concentrated with a small divergence angle of less than 30°, originating from the high working frequency range from 300 to 400 kHz. The excellent temperature stability of the as-grown YSP crystal makes the proposed SH0 wave piezoelectric transducer very suitable for SHM of high-temperature structures.

Monitoring of thermal stress in metal plates by using bonded shear horizontal wave piezoelectric transducers

Thermal stress is one of the major causes of failure of engineering structures and its measurement has attracted more attention in recent years. The ultrasonic wave method is very promising in stress measurement due to its non-destructive nature and easy manipulation. The traditional ultrasonic wave transducers require a coupling medium which would introduce large repeatability errors in travel time measurement and thus in the measured stress. In this work, a methodology based on bonded shear horizontal (SH) guided wave piezoelectric transducers was developed to monitor thermal stress in metal plates. The adhesive bonding between the transducer and the specimen ensures the repeatability in travel time measurements, and the strain gauges are also employed to monitor the wave path length. The dispersive equation of acoustoelastic SH wave propagating in an isotropic medium under the uniaxial stress is derived. Both the uniaxial tension test and thermal modulation test have been performed in aluminum and steel plates. The results show that the acoustoelastic constants of the SH0 wave are identical to that of the shear bulk wave as predicted by the acoustoelastic theory and the thermal stress measured from −60 °C to 100 °C by the proposed method has a very good repeatability (better than 2 MPa) in both the aluminum and steel plates. Considering the convenience and reliability of the bonded SH0 wave piezoelectric transducer, the proposed method is very promising for monitoring of thermal stress in engineering structures, such as rails, etc.

Damage detection in thick plate structures based on ultrasonic SH wave

Thick plate structures are widely used in large deep-sea scientific detection equipment, such as deep-sea human-occupied vehicles, but they are prone to local damage caused by fatigue due to the complex working environment load. To prevent structural failure due to damage accumulation, it is necessary to carry out structural health monitoring for thick plate structures. In this work, two half-ring thickness poled d15 mode omnidirectional shear horizontal (SH) wave piezoelectric transducers are used to form an exciter/receiver network, and the SH2 mode wave packet is selected as the damage location wave packet. Then, the reconstruction algorithm for the probabilistic inspection of damage and an algorithm based on the time of guided wave flight are applied for damage detection in a 30 mm-thick aluminium plate. Experimental results show that structural damage detection based on SH waves can effectively identify and locate thick plate structural damage and evaluate the damage depth.

A circumferential SH wave piezoelectric transducer array for damage detection in large-diameter pipes

Abstract Torsional or longitudinal wave-based inspection is an effective method for rapid defect screening in pipes, but it has difficulty to locate the circumferential position of a defect due to the axially symmetric properties of torsional and longitudinal modes. In order to overcome this problem, several flexural-wave-based focusing methods have been proposed while multiple mode propagation associated with dispersion has to be taken into account in them, which increases the complexity of identifying defects in practical applications. In this work, based on our recently developed bidirectional SH wave piezoelectric transducer (BSH-PT), a fundamental circumferential shear horizontal (CSH0) wave linear phased array was proposed for in-situ defect localization in large-diameter pipes. Firstly, the working principle of the BSH-PT array was presented and its performance on defects detection was assessed by simulations using the total focusing method (TFM). Then, experiments were conducted to examine the array’s performance. Results showed that the proposed BSH-PT array can detect multiple defects simultaneously at varied frequencies. Finally, a method for eliminating the mirrored artifact was proposed and then validated by both simulations and experiments on pipes. Considering the quasi-nondispersive character of CSH0 wave and the desirable performance of the BSH-PT on exciting and receiving pure CSH0 wave, the proposed CSH0 wave phased array may be useful in structural health monitoring (SHM) of large-diameter pipes such as pressure vessels and oil tanks.

Novel Traveling Wave Sandwich Piezoelectric Transducer with Single Phase Drive: Theoretical Modeling, Experimental Validation, and Application Investigation

Most of traditional traveling wave piezoelectric transducers are driven by two phase different excitation signals, leading to a complex control system and seriously limiting their applications in industry. To overcome these issues, a novel traveling wave sandwich piezoelectric transducer with a single-phase drive is proposed in this study. Traveling waves are produced in two driving rings of the transducer while the longitudinal vibration is excited in its sandwich composite beam, due to the coupling property of the combined structure. This results in the production of elliptical motions in the two driving rings to achieve the drive function. An analytical model is firstly developed using the transfer matrix method to analyze the dynamic behavior of the proposed transducer. Its vibration characteristics are measured and compared with computational results to validate the effectiveness of the proposed analytical model. Besides, the driving concept of the transducer is investigated by computing the motion trajectory of surface points of the driving ring and the quality of traveling wave of the driving ring. Additionally, application example investigations on the driving effect of the proposed transducer are carried out by constructing and assembling a tracked mobile system. Experimental results indicated that 1) the assembled tracked mobile system moved in the driving frequency of 19410 Hz corresponding to its maximum mean velocity through frequency sensitivity experiments; 2) motion characteristic and traction performance measurements of the system prototype presented its maximum mean velocity with 59 mm/s and its maximum stalling traction force with 1.65 N, at the excitation voltage of 500 VRMS. These experimental results demonstrate the feasibility of the proposed traveling wave sandwich piezoelectric transducer.

Theoretical modeling and experimental investigation of a V-shaped traveling wave piezoelectric transducer for ultrasonic cavitation Peening: Part A

Ultrasonic cavitation peening is a surface enhancement technology that can increase surface hardness without obviously introducing a high surface roughness of the treatment surfaces. However, the most commonly used traditional transducers for ultrasonic cavitation peening are operated in the longitudinal mode and generate a standing wave in the working gap between a sonotrode end and a treated surface, causing a small cavitation treated area and low efficiency for treating the axis components. To achieve a high efficiency on the surfaces of axis components, a new V-shaped traveling wave transducer is proposed in this paper. Two Langevin transducers are employed to couple on the ring sonotrode, generating two third order in-plane bending vibrations with a spatial phase difference of 90° in the ring. Therefore, same elliptical vibrations are generated in all points on inner surface of the ring, resulting in a uniform cavitation effect around the axis component which is located in the center of the ring sonotrode. It is the first time to propose that cavitation bubbles are generated utilizing traveling waves. An analytical model is created utilizing the transfer matrix method for the vibrating system to describe the dynamic behavior and predict the acoustic field of the working area in the ring sonotrode. To validate the developed transfer matrix model, experimental investigations were conducted to measure the vibration characteristics of the proposed transducer and compared to the transfer matrix model calculation results. For further proving the feasibility of the cavitation peening, sonochemiluminescence was carried out to evaluate the distribution of cavitation bubbles in the working area. The results show that calculation results have a good agreement with the experimental results and a higher voltage and smaller working gap were beneficial for the ultrasonic cavitation treatment utilizing this novel transducer.

Theoretical modeling and experimental investigation of a V-Shaped traveling wave piezoelectric transducer for ultrasonic cavitation Peening: Part B

In industrial applications, the highly stressed axial components are easily damaged by cyclic loads if their surfaces are not enhanced, since cracks and stress concentrations occur on the cylindrical components after turning. By the peening processes, the residual tensile stress can be removed and the surface hardness can be increased, thereby increasing the tool life. Compared to other peening processes, ultrasonic cavitation peening is a potential method to achieve surface processing of metal specimens and attracted much attention, since fewer polluting effluents are produced, with the exception of metal oxide powder that can be easily collected and recycled. Additionally, this method is cheap to perform and there are no thermal effects. In this project, we proposed a V-shaped traveling wave piezoelectric vibrating system for the ultrasonic cavitation peening to achieve an even performance on a cylindrical surface upon single treatment. The transfer matrix modeling and the cavitation field have already been investigated. In the investigations of the ultrasonic cavitation field, only the radial vibration in the inner surface of the ring sonotrode was considered and studied. However, the effects of the tangential vibration on the cavitation intensity and distribution as well as the mechanical properties of the workpiece surfaces are important to evaluate the performances of the proposed V-shaped transducer. In this paper, the numerical simulation of fluid–structure interaction (FSI) was investigated by first using the finite element method, considering liquid pressure, liquid flow velocity and vapor volume fraction. Then, the liquid flow velocity was measured using the color tracer method to validate the FSI results. Finally, the treatments for cylindrical workpieces using ultrasonic cavitation peening were conducted, and the performances of the treatments were evaluated by means of micro-hardness and surface roughness. The results show that the roughness of the cylindrical workpiece surfaces did not increase significantly, but micro hardness increased by as high as 54%.

Evaluation of Defect Detection in Aluminium, CFRP and Epoxy Resin Plates Using Non-contact Air-Coupled Ultrasonic Waves

In the field of aerospace materials and structures, water coupled ultrasonic testing was not desirable due to changes in properties and contamination reason. Therefore, in the present study non-contact air coupled ultrasonic transducers were proposed as an alternative examination for such applications. This present study describes our practical knowledge of using piezoelectric air-coupled transducers for visualization and defect identification of aluminium, CFRP and epoxy-resin specimens. Further, the effectiveness of the system is verified by experiments conducted in the inspection of artificial defects and 2D images was reconstructed through scanning of specific specimen surface. Results show that, all the created defects were identified successfully using C-scan technique. The measured defect values were compared with experimental defect values and shows good agreement. The 1 MHz piezoelectric transducer was the best for all the three specimens which is used in the present study because of the material structure being evaluated. The bulk wave piezoelectric transducer was identified as extensive pulse attenuation by the structure of the aluminium, CFRP and epoxy resin.

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.

Linear Phase Design of Piezoelectric Transducers for Acousto-Optic Dispersion Delay Lines Using Differential Evolution for Matching Circuit Optimization.

A numerical method for providing linear phase response of bulk acoustic wave (BAW) piezoelectric transducers (PTs) of acousto-optic devices is developed. Our approach is based on the analytic approximation of the phase response of the PT with the matching electrical circuit and optimization with a differential evolution genetic algorithm. Simulations and experiments were performed for two typical BAW excitation schemes: direct and reflective. Variance of the group delay in the specified frequency range was reduced by an order of magnitude for reflective excitation of BAWs.

A high-resolution structural health monitoring system based on SH wave piezoelectric transducers phased array

Guided wave based structural health monitoring (SHM) has been regarded as an effective tool to detect the early damage in large structures and thus avoid possible catastrophic failure. In recent years, Lamb wave phased array SHM technology had been intensively investigated while the inherent multi-mode and dispersive characteristic of Lamb waves limits its further applications. In comparison, the fundamental shear horizontal (SH0) wave is non-dispersive with uncoupled displacements and thus more promising for defect detection. In this work, we proposed an SH0 wave linear phased array SHM system based on our recently proposed omni-directional SH wave piezoelectric transducer (OSH-PT). Firstly, the working principle of the phased array system was presented and the total focusing method (TFM) was employed for imaging. Then the SH0 wave mode generated by the OSH-PT was confirmed in a defect-free plate. Finally, experiments were carried out to examine the performances of this SHM system. Results showed that the proposed system can detect a through-thickness hole as small as 2 mm in diameter with the location error only about 6.3 mm. Moreover, the proposed phased array system can also detect multi-defects. Due to its low working frequency and thus low attenuation, the proposed phased array system is capable of monitoring large structures. This work will lay the foundations of SH wave based phased array SHM.

Comparison of Time-Domain and Frequency-Domain Contact Resonant Ultrasound Spectroscopy

In this work, two contact resonant ultrasound spectroscopy arrangements, one measuring in the frequency- and the other in the time-domain, are compared. Both set-ups use the same shear wave piezoelectric transducers for excitation and detection. Frequency-scan and acquisition with a lock-in amplifier is used for the frequency-domain measurement. As for the time-domain measurement, narrow square-pulse excitation and oscilloscope signal acquisition are employed. Output-to-input ultrasonic amplitude spectra are obtained with both arrangements for two samples of brass and polymethyl methacrylate. Fits of the more distinctive peaks of the spectra to Lorentzians yields limits of agreement between time- and frequency-domain measurements of the order of 5 × 10–2 kHz for most of the peaks considered. For brass, the full widths at half maximum of the frequency-domain peaks are of the order of 69 to 85% smaller than those of the corresponding time-domain peaks. For polymethyl methacrylate the differences in full widths at half maximum between corresponding frequency- and time-domain peaks fall into the range of error.

Development of an Omni-Directional Shear Horizontal Wave Transducer Based on a Radially Poled Piezoelectric Ring

For the inspection of large plate-like structures, the omni-directional guided wave transducer-based system has been regarded as an effective tool since only a few transducers are required to cover the entire inspection area without blind zones. In comparison with Lamb waves, the shear horizontal (SH) wave is more promising because its fundamental mode is non-dispersive. In this work, we proposed an omni-directional SH wave piezoelectric transducer (OSH-PT) based on a radially poled $$\hbox {d}_{24}$$ -mode PZT ring. Both the finite element simulations and experiments were carried out to demonstrate its performance in generating and receiving $$\hbox {SH}_{0}$$ wave. Results showed that the radially poled OSH-PT could generate single-mode $$\hbox {SH}_{0}$$ wave and receive $$\hbox {SH}_{0}$$ wave only over a wide frequency range from 70 to 200 kHz. The obtained signal-to-noise ratio can reach up to 26 dB in generation and 24 dB in reception. The omni-directivity of this OSH-PT is also very good with the deviation only about 6% in both generation and reception of $$\hbox {SH}_{0}$$ wave. Considering its easy fabrication, low cost and superior performances, this proposed OSH-PT may promote the applications of $$\hbox {SH}_{0}$$ wave-based inspection in structural health monitoring and nondestructive testing.

A practical omni-directional SH wave transducer for structural health monitoring based on two thickness-poled piezoelectric half-rings

Structural health monitoring (SHM) has become more and more important in modern industries as it can monitor the safety of structures during the full service life and prevent possible losses of life and economics. Shear horizontal wave in plate-like structures is very useful for long distance inspection since its fundamental mode (SH0) is totally non-dispersive. However, all the currently available SH wave transducers are not suitable for practical SHM because of their complicated structures. In this work, we firstly investigated via finite element (FEM) simulations the performances of thickness-poled d15 PZT ring based omni-directional SH wave piezoelectric transducers (OSH-PT) consisting of different number of elements. Results show that the two-half-ring based OSH-PT can have perfect omni-directivity and acceptable performances in excitation/reception of SH0 waves. Then, experimental testing on a 21 mm outer-diameter (OD), 9 mm inner-diameter (ID) two-half-ring OSH-PT shows that it exhibits acceptable but not desirable performances in both excitation and reception of SH0 wave. Finally, size optimization was conducted on the two-half-ring based OSH-PT using FEM simulations and results showed that its performances can be fairly enhanced by reducing the outer diameter of the half-ring. Testing results on a 12 mm-OD, 6 mm-ID OSH-PT show that the SH0-to-Lamb waves ratio in the case of self-excitation and self-reception can be over 20 dB from 115 kHz to 250 kHz, which is good enough for practical applications. The proposed two-half-ring OSH-PT is expected to be widely used in SH0 wave based SHM due to its simple structure, easy fabrication/assembling, low cost and good performances.

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.

A variable-frequency structural health monitoring system based on omnidirectional shear horizontal wave piezoelectric transducers

Structural health monitoring (SHM) is of great importance for engineering structures as it may detect the early degradation and thus avoid life and financial loss. Guided wave based inspection is very useful in SHM due to its capability for long distance and wide range monitoring. The fundamental shear horizontal (SH0) wave based method should be most promising since SH0 is the unique non-dispersive wave mode in plate-like structures. In this work, a sparse array SHM system based on omnidirectional SH wave piezoelectric transducers (OSH-PT) was proposed and the multi data fusion method was used for defect inspection in a 2 mm thick aluminum plate. Firstly, the performances of three types OSH-PTs was comprehensively compared and the thickness-poled d15 mode OSH-PT used in this work was demonstrated obviously superior to the other two. Then, the signal processing method and imaging algorithm for this SHM system was presented. Finally, experiments were carried out to examine the performance of the proposed SHM system in defect localization and imaging. Results indicated that this SHM system can locate a through hole as small as 0.12λ (4 mm) in diameter (where λ is the wavelength corresponding to the central operation frequency) under frequencies from 90 to 150 kHz. It can also locate multiple defects accurately based on the baseline subtraction method. Obviously, this SHM system can detect larger areas with sparse sensors because of the adopted single mode, non-dispersive and low frequency SH0 wave which can propagate long distance with small attenuation. Considering its good performances, simple data processing and sparse array, this SH0 wave-based SHM system is expected to greatly promote the applications of guided wave inspection.

A uniform-sensitivity omnidirectional shear-horizontal (SH) wave transducer based on a thickness poled, thickness-shear (d15) piezoelectric ring

The fundamental shear horizontal (SH0) wave in plates is of great importance in the field of nondestructive testing (NDT) and structural health monitoring (SHM) since it is the unique non-dispersive guided wave mode. For practical applications, a phased array system based on omnidirectional SH0 wave transducers is most useful as it can cover a wide range of a plate. However, so far very few omnidirectional SH wave transducers have been developed. In this work, we proposed an omnidirectional SH wave piezoelectric transducer (OSH-PT) based on a thickness poled piezoelectric ring. The ring is equally divided into twelve sectors and the electric field is circumferentially applied, resulting in a new thickness-shear (d15) mode. Finite element analysis shows that the proposed OSH-PT can excite single-mode SH0 wave and receive the SH0 wave only. Experiments were then conducted to examine the performance of the proposed OSH-PT. Results indicated that it can generate and receive single-mode SH0 wave in a wide frequency range with nearly uniform sensitivities along all directions. Considering its quite simple configuration, compact size and low cost, the proposed OSH-PT is expected to greatly promote the applications of SH waves in the field of NDT and SHM.