Air-coupled Ultrasonic Transducers Research Articles

Acoustic and rheological characterization of hollow glass microsphere composite for development of optimized air-coupled ultrasonic matching layer

An air-coupled acoustic matching layer, based on hollow glass microsphere composite, was developed for air-coupled ultrasonic transducers. The effects of the matrix, powder series, and volume fraction were systematically analyzed by acoustic and rheological characterization. The optimized hollow glass microsphere-epoxy composite showed a superior air-coupling performance, with a characteristics acoustic impedance of 1.08 MRayl and attenuation coefficient of 0.05 dB/mm, respectively. To validate the developed composite, an air-coupled ultrasonic transducer was fabricated with piezoelectric ceramic vibrator, and its feasibility for application in a gas flowmeter was demonstrated. The developed composite exhibits a suitable air-coupling performance for a high-accuracy ultrasonic gas flowmeter.

Fabrication and Modeling of Matching System for Air-Coupled Transducer.

The tremendous acoustic impedance difference between the piezoelectric composite and air prevents the ultrasonic transition, resulting in low amplitude for the received signal for the composite defect detection using an air-coupled transducer. The matching system, which includes the matching layers and bonding layers attached to the piezoelectric composite, can reduce the acoustic impedance difference and benefit the acoustic transition. In this paper, the fabrication method and modeling for the matching layers are proposed to optimize the transducer performance. The effects of bonding layer material on the transducer performance are also discussed. Experiments were conducted for modeling validation. The proposed model can predict the matching layer acoustic properties with an error of less than 11%. The bonding layer using the same material as the first matching layer can help to increase the sensitivity by about 33% compared to the traditional epoxy bonding. The optimized air-coupled ultrasonic transducer, based on the results of this study, has a 1283 mV amplitude in the air, which is 56% higher than commercially available transducers, and can identify the defects in two typical non-metallic composite materials easily.

Tangential bond stiffness evaluation of adhesive lap joints by spectral interference of the low-frequency A0 lamb wave

Transmission spectra of the lowest-order antisymmetric (A0) Lamb wave for adhesively bonded single lap joints are experimentally investigated in a frequency range below 0.4 MHz. Based on the theoretical results of the wave interference, a tangential bond stiffness evaluation method is proposed for the lap joints. Aluminum alloy plates bonded with epoxy adhesive on different conditions were prepared as bonded specimens. An air-coupled ultrasonic transducer was used to generate the A0 mode in the specimens, and the signals of the transmitted waves across the joints were measured with a pin-type piezoelectric transducer. Spectral analysis was performed for the measured waveforms, and the transmission coefficient was calculated for each specimen as a function of frequency. The experimental results showed that the transmission coefficient has local maxima and minima at multiple frequencies, called peak and notch frequencies, respectively. The sets of the peak and notch frequencies were different among the bonded specimens. Based on the peak and notch frequencies, the tangential bond stiffness was estimated for each bonded specimen. As a result, the bond stiffnesses obtained for the specimens with the same nominal bond thickness and adherend pretreatment were almost equal even if the bond lengths were different. Furthermore, the evaluation results showed that pretreatments such as sanding and contamination of adherend surfaces affect the bond stiffness due to the change in the interfacial stiffness.

Numerical and experimental analysis of focused air-coupled ultrasonic transducer based on cross-linked polypropylene film

Air-coupled ultrasonic transducers based on novel piezoelectrets with extremely small acoustic impedance have some distinctive advantages including no coupling agent, flexibility, and eco-friendliness, compared to conventional ultrasonic transducers. Such advantages make them unique in the applications of nondestructive testing. In this study, a focused air-coupled ultrasonic transducer featuring a sphere-shaped transduction surface is designed by introducing flexible irradiated cross-linked polypropylene (IXPP) films. The characteristic of focusing is evaluated by numerical and experimental analysis. The results show that, by bending the IXPP film into a sphere, the ultrasonic energy can be concentrated to a certain small area to achieve better sensitivity and higher resolution. Furthermore, the IXPP focusing transducer has an obvious focusing effect when the opening radius D of the transducer is increased. Meanwhile, with the reducing radius R of the spherical surface and increasing frequency f of excitation, the value of the deviation coefficient α can be made smaller so that the transducer designed in this way can obtain a better focusing effect.

Experimental and Numerical Study of Hydrodynamic and Heat Transfer Characteristics of Falling Film Over Metal Foam Layered Horizontal Tube

AbstractA novel nonintrusive technique based on an air-coupled ultrasonic transducer was used to study the hydrodynamic behavior of falling film over metal foam layered horizontal tube. Copper foam having a porosity of 90.5%, brazed over a copper tube of 25.4 mm diameter was used in this study. Falling film thickness distribution in the circumferential direction and the dynamic characteristics of falling film were studied in the falling film Reynolds number range of 356–715, and at a tube spacing of 5 mm and 15 mm. The falling film characteristics over metal foam layered horizontal tubes were compared with that over a plain horizontal tube surface. Heat transfer studies of falling film over metal foam layered tube were studied in an evaporator of a multi-effect desalination system by experiment. It was observed that the falling film heat transfer coefficient was enhanced 2.7 times by the application of metal foam over the plain horizontal tube. The measurements obtained from hydrodynamic and heat transfer studies were compared with the predictions made by a computational model and were found to be in good agreement. Metal foam properties required for the computational model were obtained using a microcomputed tomography-based study.

Numerical Investigation of Excitation of Various Lamb Waves Modes in Thin Plastic Films

Ultrasonic-guided waves are widely used for the non-destructive testing and material characterization of plates and thin films. In the case of thin plastic polyvinyl chloride (PVC), films up to 3.2 MHz with only two Lamb wave modes, antisymmetrical A0 and symmetrical S0, may propagate. At frequencies lower that 240 kHz, the velocity of the A0 mode becomes slower than the ultrasonic velocity in air which makes excitation and reception of such mode complicated. For excitation of both modes, we propose instead a single air-coupled ultrasonic transducer to use linear air-coupled arrays, which can be electronically readjusted to optimally excite and receive the A0 and S0 guided wave modes. The objective of this article was the numerical investigation of feasibility to excite different types of ultrasonic-guided waves, such as S0 and A0 modes in thin plastic films with the same electronically readjusted linear phased array. Three-dimensional and two-dimensional simulations of A0 and S0 Lamb wave modes using a single ultrasonic transducer and a linear phased array were performed. The obtained results clearly demonstrate feasibility to excite efficiently different guided wave modes in thin plastic films with readjusted phased array.

Features of Laser-Vibrometric Nondestructive Testing of Polymer Composite Materials Using Air-Coupled Ultrasonic Transducers

Features of Laser-Vibrometric Nondestructive Testing of Polymer Composite Materials Using Air-Coupled Ultrasonic Transducers

Application of Air-Coupled Ultrasonic Nondestructive Testing in the Measurement of Elastic Modulus of Materials

It is difficult to measure elastic modulus simply and accurately in the testing of mechanical properties of materials. Combined with static tensile method, this paper presents a method for measuring elastic modulus of materials based on air-coupled ultrasonic nondestructive testing. Firstly, the 1–3 piezoelectric composite material and the matching material of low acoustic impedance are self-made, and 400 kHz air-coupled ultrasonic transducer is fabricated. Then, the performance of the transducer is tested, and the insertion loss and bandwidth of −6 dB are −33.5 dB and 23.4%, respectively. Compared with the traditional instrument for measuring elastic modulus, the measurement of elastic modulus of carbon steel rod material is realized in this paper, and the measured results are in agreement with the accepted value. In addition, from the angle of relative uncertainty, how to reduce the measurement error by improving the device is analyzed. It can be shown that the method has high linearity, high symmetry, and good stability and repeatability. This paper provides a new way for the selection and design of measuring instrument components.

Modification of Mechanical and Electromechanical Resonances of Cellular Ferroelectret Films Depending on the External Load.

Ferroelectret films are cellular polymers with electrically charged pores that exhibit piezoelectric response. Among other applications, ferroelectret films have been widely used as active elements in air-coupled ultrasonic transducers. More recently, they have also been tested in water immersion. They show a promising wide frequency band response, but a poor sensitivity produced by the disappearance of the electromechanical resonances. This paper studies in detail the modification of FE films response when put into water immersion, both the mechanical and the electromechanical responses (the latter in transmission and reception modes). The lack of electromechanical thickness resonances when the films are put into water is explained as the result of the different profile of the modification of the polarization vector along the film thickness imposed by the large mechanical load produced by the water. This different electromechanical response can also be the reason for the subtle modification of the mechanical thickness resonances that is also observed and analyzed.

FLAUT: A mutual sensitivity improvement through matched pipe, cavity and thin plate resonance

Electrostatic transducers promises a great potential in alternative to piezoelectric transducer based on certain advantages such as inherently wide bandwidth and good acoustic matching to air due to the membrane’s low acoustics impedance. There are two basic designs that are popular among electrostatic ultrasonic transducer developer – rigid backplate and micromachine backplate. This paper presents a methodology for improving the sensitivity of an air-coupled ultrasonic transducer by coupling the resonating thin plate, cavity and pipe in a single cell. The proposed device is termed Fluidically Amplified Ultrasonic Transducer (FLAUT) for an air-coupled application. Investigation of the concept of matched thin plate, cavity and pipe, of which the individual geometry is expected to mutually enhance one another. Analytical modelling is utilized to the matched thin plate, cavity and pipe. The analytical modelling identifies the required geometry for the FLAUT based on the matched operating resonant frequency of 25 kHz. At the end of the paper the prototype of FLAUT is presented where the device was fabricated using additive manufacturing process (3-D printing) which consist of a 50 µm Kapton thin film over a micro stereolithography designed backplate. Here aluminum is coated as the electrode utilizing the thermal evaporation process for both the Kapton film and the backplate. A laser interferometer is utilized to measure FLAUT thin plate displacement which indicates the device is running at 25 kHz fundamental mode. A 30 dB difference is also observed between the deformation velocity of the cavity active region and its surrounding.

Non-contact in-process ultrasonic screening of thin fusion welded joints

Joining of metal structures, or welding, plays a critical role in our modern world, where safety critical welds of thin sheet metals are used in pipework, nuclear cannisters and aircraft ducting among others. A potential failure in such welds could prove to be catastrophic, hence the need for thorough inspection and testing. With the ever-increasing automation of welding operations, manually deployed Non-destructive Evaluation (NDE) has become a major bottleneck in the supply chain. This paper introduces for the first time, non-contact gas-coupled ultrasonic sensors deployed in-process during weld deposition, for screening of weld penetration directly at the point of manufacture. 3 mm thick mild steel plates were butt-welded together using Gas Tungsten Arc Welding (GTAW) while non-contact air-coupled ultrasonic transducers, generating surface guided Lamb waves, performed inspection screening of the weld seam. Optimised Signal-to-Noise Ratio (SNR) was achieved through a customised acoustic matching layer, low-noise amplifiers and real-time signal processing. By performing multiple trials at varying levels of welding input power (0.59 kJ/mm to 1.03 kJ/mm), it was demonstrated that the amplitude of the through transmission A0 Lamb wave is correlated to the Weld Penetration Depth (WPD) and can be used for on-line weld quality screening. Advantages of the outlined method include higher production rates, reduced levels of scrap and higher production quality in regards to thin metal sheet welded components.

Design, Fabrication and Characterization of a Spherically Focused Capacitive Air-Coupled Ultrasonic Transducer

We report the development, fabrication, and testing of spherically focused capacitive air-coupled ultrasonic transducers, that need no mirror, zone plate, or similar external device. To achieve native focusing, we have employed a flexible copper/polyimide backplate that per- mits a conformal fit to a spherical fixture whose radius determines the focal length. A spheri- cally deformed aluminized Mylar foil, conforms to the spherical backplate, completing the transducer. Two devices have been fabricated and tested, one having a 1-cm diameter and 2.54-cm focus, and another with a 5-cm diameter and 5.1-cm focus. Both devices have frequency spectra centered at 840 kHz with -6 dB points at 310 and 1200 kHz. Using a quasi-point re- ceiver, the beam diameter is found to be 2.7 mm (1-cm device) and 1.32 mm (5-cm device) in the focal plane, when excited with a broadband signal. The monochromatic diffraction behavior of the 1-cm device agrees with model calculations based on a focused piston radiator to within the measurement resolution of about 2%. Features as small as a 250-micron wire have been successfully imaged using the 5-cm devices in a confocal geometry, demonstrating resolution at the diffraction limit for this device. Further examples of defect imaging in honeycomb com- posites and in wood are also presented.

The excitation and detection of Lamb waves in a droplet-loaded plate using air-coupled ultrasonic transducers

The signals in an air-coupled ultrasonic inspection system using Lamb waves would be affected by liquid droplets on the surface of test plates. To investigate the wave behaviour, a plate-droplet inspection system comprising a thin isotropic plate loaded with water droplets and a pitch–catch transducer configuration was modelled and studied experimentally. The Lamb waves were generated by a non-contact airborne transmitter. Their energy leaking into the adjacent air was collected by a receiver. To obtain a deep understanding of the effects of water droplets, a multiphysics model based on finite element method was established. The model facilitated the visualization of the wave fields scattered by the droplets on the surface of the plate and evaluated the received signals which were then verified by experimental measurements. This entirely modelling method could be a convenient way to interpret the inspection signals for test plate loaded with liquid droplets.

Ferroelectret Ultrasonic Transducers for Pulse-Echo Water Immersion

Ferroelectrets are thin and porous polymeric films with a cellular microstructure, high porosity, permanent polarization and piezoelectric response. They have been used for different applications, where one of the most interesting ones is for the fabrication of air-coupled ultrasonic transducers. More recently they have been tested as water immersion transducers, showing a promising wide bandwidth but limited sensitivity along with other technical problems. This paper investigates ultrasonic transducers for water immersion and pulse-echo operation based on ferroelectret films. Two different ferroelectret foams with different resonant frequencies, acoustic impedances and cellular structures were tried. Flat and spherically focused prototypes (radius of curvature of 22 and 35 mm) were produced and tested. Finally, different materials and methods were tried to provide a protective surface coating. Acoustic field measurements for the focused transducers confirm the possibility to efficiently focus the ultrasonic beam by the proposed fabrication method, with focal spot size of 1.86 mm at −6 dB. Results show that in spite of the reduced sensitivity (about −115 dB), some of the tried ferroelectret films provide a very wide band response (−6 dB band from 0.29 to 2.7 MHz) and short pulse duration (2–3 us) that can be of interest for different applications.

Low insertion loss air-coupled ultrasonic transducer with parallel laminated piezoelectric structure

This work presents a low insertion loss air-coupled ultrasonic transducer with a parallel laminated piezoelectric structure. The relevant parameters of this structure are theoretically analyzed. For overcoming the enormous acoustic impedance mismatch between the piezoelectric material and air, a 1–3 connectivity piezoelectric composite, and a stack of two low acoustic impedance matching layers are adopted. The experimental results show that the insertion loss of −17.5 dB is obtained under the pitch-catch mode with the length of the air column of 50 mm by selecting a suitable air-coupled ultrasonic transducer combination. The insertion loss improvement is 3.8 dB compared with a monolayer piezoelectric structure with an identical active area.

Test, modeling, and analysis of air-coupled ultrasonic transducer based on piezoelectret film

Piezoelectrets, a new type of electromechanical transducer material with low acoustic impedance and high piezoelectric d33 and g33 coefficients, are promising candidates for developing high-performance air-coupled ultrasonic transducers. In this work, we design polypropylene-piezoelectret-film based air-coupled transducers (ACTs) with two different structures and investigate the influence of boundary conditions of the film with experimental measurement, electromechanical equivalent circuit modeling (EECM), and finite element analysis (FEA) for both inverse and direct piezoelectric effects. The experimental data are in great agreement with the results of EECM and FEA. The resonant frequency of the ACT with a free-front-clamped-back (FFCB) film is about half of that of the device with a free-front-free-back (FFFB) film, the response peak of the FFCB device is higher than that of the FFFB device, and the −3dB bandwidth of the former is much narrower than that of the latter. The results show that the experimental measurements can be predicted by EECM and FEA, which can provide the basis for the development of high-performance air-coupled ultrasonic transducers based on piezoelectret films.

Effects of a nano-scale embossed surface on the acoustic emission of air-coupled piezoelectric ultrasonic transducers

An air-coupled piezoelectric ultrasonic transducer (PUT) was fabricated to assess the effects of a nano-scale embossed Al diaphragm on the acoustic emission characteristics. A 310-μm-thick multilayer piezoelectric film and a 300-μm-thick Al plate were used for the PUT. It was found that the flexural modulus of the Al plate was decreased slightly from 3.73 GPa to 3.41 GPa by the effect of the embossed surface morphology, having an array of Al bumps with dimensions of 800 nm × 800 nm and a height of 320 nm. The decreased flexural modulus had a strong effect on the decrement of the acoustic resonant frequency and on the enhancement of the sound power radiation for the PUT. In comparison to a sound pressure level (SPL) of 117.8 dB at 30 cm away from a flat PUT at 41.4 kHz, the embossed PUT could result in a significantly enhanced SPL of 121.7 dB at the secondary resonant frequency of 40.5 kHz for the implementation of a smaller transducer. This result can motivate an efficient design parameter for better performance of a PUT through modifications of the nm-scale embossed surface.

Multi-Channel Signal-Generator ASIC for Acoustic Holograms.

A complementary metal-oxide-semiconductor (CMOS) application-specific integrated circuit (ASIC) has been developed to generate arbitrary, dynamic phase patterns for acoustic hologram applications. An experimental prototype has been fabricated to demonstrate phase shaping. It comprises a cascadable 1 ×9 array of identical, independently controlled signal generators implemented in a 0.35- [Formula: see text] minimum-feature-size process. It can individually control the phase of a square wave on each of the nine output pads. The footprint of the integrated circuit is [Formula: see text]. A 128-MHz clock frequency is used to produce outputs at 8 MHz with a phase resolution of 16 levels (4 bits) per channel. A 6 ×6 air-coupled matrix array ultrasonic transducer was built and driven by four ASICs, with the help of commercial buffer amplifiers, for the application demonstration. Acoustic pressure mapping and particle manipulation were performed. In addition, a 2 ×2 array piezoelectric micromachined ultrasonic transducer (PMUT) was connected and driven by four output channels of a single ASIC, demonstrating the flexibility of the ASIC to work with different transducers and the potential for direct integration of CMOS and PMUTs.

Falling film thickness measurement using air-coupled ultrasonic transducer

In this work, a new, non-intrusive, increased sensitivity technique based on air-coupled ultrasonics was developed to measure the falling liquid film thickness. Air-coupled transducer of frequency 1 MHz was used to make static and dynamic measurements of water level/film thickness. The air-coupled method were compared with a metal-side contact ultrasonic approach as well as calculating water levels in the collection tank. The falling film thickness measurement over inclined plate was used in this study. Measurement of film thickness was taken at flow rates in the range of 0.34–0.9 lpm. Relatively good agreement was found between the readings, which validates the dynamic measurement capability of the new technique. The technique was applied to measure the horizontal tube falling film thickness for falling film Reynolds number ranging from 130 to 350, at different circumferential angles ranging from 40° to 130° and at tube spacing of 5 mm and 15 mm. Measurements were compared with correlations available in the literature and found to be in good agreement.

Investigating vibration characteristics of magnetostrictive transducers for air-coupled ultrasonic NDT of composites

In this study, high-power ultrasonics has been used for non-contact nondestructive testing of composites. Air-coupled magnetostrictive ultrasonic transducers provide reasonable amplitudes of horn oscillations even if this type of ultrasonic stimulation is close to the idle mode. Four types of ultrasonic horns have been designed and manufactured in order to choose an optimal horn configuration. For each horn, the dependence of vibration amplitude on ultrasonic unit power has been found to exhibit a good match between the theoretical and experimental results. The resonance frequencies and amplitudes of oscillatory velocity were experimentally evaluated by using the technique of scanning laser vibrometry and modeled by means of the Autodesk Inventor software. An air-coupled magnetostrictive transducer, in conjunction with an optimized horn, has been used in the evaluation of impact damage in a graphite epoxy composite and detection of water and excessive epoxy adhesive in an aviation honeycomb panel. It is expected that the technique of air-coupled ultrasonics may be useful for inspecting thin, fragile and hydrophilic materials.