Air-coupled Ultrasonic Transducers Research Articles

Lamb wave based ultrasonic imaging of interface delamination in a composite T-joint

In this article, the complete characterization of the interface delamination in a composite T-joint using ultrasonic guided Lamb waves is discussed. Correlation between the numerically simulated and experimentally obtained B-scans was established. In the B-scan images, different Zones were identified and could be utilized for differentiating the healthy and delaminated T-joints. The length and the width of delamination were computed from traversing the transducers in two orthogonal directions (i.e. called the B-scans, and the D-scans). For the first time, backward propagating mode (called “Turning Lamb Modes”) that are generated at corner of the T-joint was observed and verfied. Using a single linear line scan of the “Turning Lamb Modes”, both the width and length of interface delamination could be computed. In the present study, the experiments were carried out employing air-coupled ultrasonic transducers.

Modelling of attenuation of Lamb waves using Rayleigh damping: Numerical and experimental studies

In this study, the Lamb mode attenuation constants were derived in terms of the attenuation coefficient, group velocity and central frequency of excitation of the Lamb mode, using the Rayleigh damping model. Attenuation of Lamb waves, both fundamental symmetric and anti-symmetric modes, propagating through viscoelastic media (cross-ply glass/epoxy laminate) was modelled using the Finite Element Method. Numerically simulated attenuation of Lamb waves using Lamb mode attenuation constants was found to be in good agreement with the assumed attenuation. Experiments were performed on a quasi-isotropic laminate, employing air-coupled ultrasonic transducers, to measure the attenuation coefficient. Lamb mode attenuation constants, computed using the attenuation coefficient, were used to model the attenuation of the Lamb mode in quasi-isotropic laminates. Numerically simulated amplitude variation was found to be in good agreement with that computed from experiments.

Investigation of Acoustic Field Calculation and Measurement Method for Air-coupled Ultrasonic Transducer

The field characteristic of an air-coupled planar ultrasonic transducer was investigated using the spatial impulse response of transducer transmitting aperture,which is based on the assumption of linear sound theory.The impulse response of arbitrary point in field space is calculation by the summation of spherical wave from all of the small area on the transmitting aperture.Attenuation is one of important factor impacting acoustical field distribution for the air-coupled ultrasonic transducer,and which increases with the frequency,a frequency-dependent attenuation function is considered in the field calculation of frequency domain,and then the pressure distribution is obtained by convolution integral of the impulse response with excitation impulse.Compared field measurement experiments are implemented using a multiple-axis scanning system and a miniature aperture detector.The results of theoretic analysis and that of experiment are in a good agreement.Finally,the effects of the receiving aperture size are also considered in sound field measurements,the results indicate that because of the low efficiency and interface loss in air couple,a compromise between resolution and signal to noise ratio(SNR) should be taken into account to choose the receiving aperture size.And a smaller aperture could improve the measuring resolution on condition that the receiving amplification coefficient and the SNR are valid.

J042012 集束型空気超音波センサーによる円周Lamb波の効率的励起メカニズムと減肉計測への応用([J04201]超音波計測・解析法の新展開(1))

Efficient transduction of a circumferential (C-) Lamb wave by a pair of noncontact air-coupled ultrasonic transducers (NAUT) was presented. A line focus type (LFT-) of the NAUT was employed for the efficient transduction. Characteristics of the LFT-NAUT were theoretically and experimentally shown in comparison to those of the conventional plane type (CPT-) NAUT. The efficiency of the C-Lamb wave generated and detected by the LFT-NAUT took about 20 times higher than that by the CPT-NAUT. As for an application of the C-Lamb wave generated by the LFT-NAUT, a novel method of an accurate estimation of pipe wall thickness was introduced and evaluated with a theoretical model. It was confirmed that the averaged error between the experiments and the theoretical model was at less than 10 μm.

J042011 空中伝播超音波センサとリアルタイム雑音低減手法を用いた非接触型AE計測システムの開発([J04201]超音波計測・解析法の新展開(1))

Acoustic emission (AE) technique is widely used for health monitoring of machines. However, it is difficult to monitor structural components that has rough surface or that is under operating. In order to monitor these components, non-contact AE monitoring system with noise reduction function was developed. The developed system was composed of an air-coupled ultrasonic transducer and a digitizer with a real-time noise reduction function. Two types of digital signal processing for improving signal to noise ratio were embedded in the system. One is a spectral subtraction process for reduction of continuous noise. The other is E-filter process which filters out pulse-like noise. Artificial AE signals were monitored to compare the performances between conventional AE system and developed one. The S/N ratio of the AE signal was improved by 12.3 dB by the developed system.

Non-Contact Determination of Nugget Size of Spot-Welding Using Focused Air-Coupled Ultrasound

Since the size and shape of a nugget of a spot weld are closely related to the bonding strength of the weld, their quantitative evaluations are very important to improve the reliability of the welded structures. In this work, a focused air-coupled ultrasound technique has been used for non-contact measurements of spot welds of steel plates. One-dimensional scanning in transmission configuration using a pair of focused air-coupled ultrasonic transducers operating with tone burst waves at 0.3 MHz is performed to obtain the amplitude profile of the transmitted ultrasound through the nugget area of a spot weld. A simple method for determining the nugget size from the amplitude profile is proposed. The validity of the method is verified using a calibration sample which imitates a spot weld. The proposed method has then been applied to the evaluations of spot welds fabricated with different electric currents.

Cellular polypropylene polymer foam as air-coupled ultrasonic transducer materials

Cellular polypropylene polymer foams, also known as ferroelectrets, are compelling candidates for air-coupled ultrasonic transducer materials because of their excellent acoustic impedance match to air and because they have a piezoelectric d(33) coefficient superior to that of PVDF. This study investigates the performance of ferroelectret transducers in the generation and reception of ultrasonic waves in air. As previous studies have noted, the piezoelectric coupling coefficients of these foams depend on the number, size, and distribution of charged voids in the microstructure. The present work studies the influence of these parameters both theoretically and experimentally. First, a three-dimensional model is employed to explain the variation of piezoelectric coupling coefficients, elastic stiffness, and dielectric permittivity as a function of void fraction based on void-scale physics and void geometry. Laser Doppler vibrometer (LDV) measurements of the effective d(33) coefficient of a specially fabricated prototype transmitting transducer are then shown which clearly indicate that the charged voids in the ferroelectret material are randomly distributed in the plane of the foam. The frequency-dependent dynamic d(33) coefficient is then reported from 50 to 500 kHz for different excitation voltages and shown to be largely insensitive to drive voltage. Lastly, two ferroelectret transducers are operated in transmit-receive mode and the received signal is shown to accurately represent the corresponding signal generated by the transmitting transducer as measured using LDV.

초음파 음향임피던스 변환기를 이용한 저압 저진공 측정기술 연구

초음파 변환기의 기체 음향임피던스 변화를 이용해서 압력진공 측정기술을 연구하였다. 상용화된 한 쌍의 500 kHz 공중 초음파 변환기(air-coupled ultrasonic transducer)를 진공용기에 넣고 초음파 진폭을 측정하였다. 그 결과 6.66 kPa에서 202.65 kPa (약 2기압)까지 4회 반복 측정값으로부터 계산된 표준편차는 0.0093에서부터 0.3325 사이에 있었으며, 표준편차를 압력으로 나누어서 백분율로 나타낸 값은 133.32 kPa에서 0.018%이었고 202.65 kPa에서는 0.164%이었다. Pressure vacuum measurement technique using acoustic impedance change of ultrasonic transducers was studied. The sensor has been setup using two air-coupled ultrasonic transducers, one as a transmitter and the other as a receiver, and put it into vacuum chamber and measured pressure versus ultrasonic amplitude. The result confirms that the standard deviations of four repeat measurements were from 0.0093 to 0.3325 at pressure 6.66 kPa to 202.65 kPa(about two atmosphere), and the relative percents were 0.018% and 0.164% at pressure 133.32 kPa and 202.65 kPa, respectively.

J0402-2-5 空気超音波センサーで励起検出した円周Lamb波による円筒の非接触減肉計測([J0402-2]超音波計測・解析法の新展開(2))

J0402-2-5 空気超音波センサーで励起検出した円周Lamb波による円筒の非接触減肉計測([J0402-2]超音波計測・解析法の新展開(2))

Airborne ultrasonic phased arrays using ferroelectrets: a new fabrication approach

In this work, a novel procedure that considerably simplifies the fabrication process of ferroelectret-based multielement array transducers is proposed and evaluated. Also, the potential of ferroelectrets being used as active material for air-coupled ultrasonic transducer design is demonstrated. The new construction method of multi-element transducers introduces 2 distinctive improvements. First, active ferroelectret material is not discretized into elements, and second, the need of structuring upper and/or lower electrodes in advance of the permanent polarization of the film is removed. The aperture discretization and the mechanical connection are achieved in one step using a through-thickness conductive tape. To validate the procedure, 2 linear array prototypes of 32 elements, with a pitch of 3.43 mm and a wide usable frequency range from 30 to 300 kHz, were built and evaluated using a commercial phased-array system. A low crosstalk among elements, below -30 dB, was measured by interferometry. Likewise, a homogeneous response of the array elements, with a maximum deviation of +/-1.8 dB, was obtained. Acoustic beam steering measurements were accomplished at different deflection angles using a calibrated microphone. The ultrasonic beam parameters, namely, lateral resolution, side lobe level, grating lobes, and focus depth, were congruent with theory. Acoustic images of a single reflector were obtained using one of the array elements as the receiver. Resulting images are also in accordance with numerical simulation, demonstrating the feasibility of using these arrays in pulse-echo mode. The proposed procedure simplifies the manufacturing of multidimensional arrays with arbitrary shape elements and not uniformly distributed. Furthermore, this concept can be extended to nonflat arrays as long as the transducer substrate conforms to a developable surface.

Assessment of porosity of mortar using ultrasonic Rayleigh waves

The relationship between Rayleigh wave velocity at ultrasonic frequencies and porosity is investigated in dry and fully saturated mortar. Porosity is varied by changing the water/cement ratio. Rayleigh wave (RW) pulse velocity is measured accurately using air-coupled ultrasonic transducers as transmitter and receiver. The experimental results are compared with two single-phase models. Due to relatively weak correlation between RW velocity and porosity, measured data and the models are compared using statistical hypothesis tests. Although the parameters of the model are not accurately predicted, these tests conclude on a reasonable agreement between measured data and the models. This result can be useful to develop simplified models of wave propagation in concrete, or to estimate porosity in concrete cover from ultrasonic measurements.

B201 空気超音波センサーによる円周Lamb波の特徴を用いた配管の減肉計測(検査・モニタリング-1,OS6 保全・設備診断技術)

B201 空気超音波センサーによる円周Lamb波の特徴を用いた配管の減肉計測(検査・モニタリング-1,OS6 保全・設備診断技術)

Progress of Air-coupled Ultrasonic Non-destructive Testing Technology

Air-coupled ultrasonic non-destructive testing,possessing the characters of complete non-contact and non-destruction,can be used in some situations in which traditional ultrasonic testing is hardly applicable.The development of air-coupled ultrasonic non-destructive testing is summarized,and the main technical difficulties that restrict its progress are analyzed.Two methods to improve the efficiency of air-coupled ultrasonic transducers and corresponding research works are presented.Main detecting methods and corresponding application fields of air-coupled ultrasonic testing are introduced.It is concluded that air-coupled ultrasonic testing technique will be applied more widely in the future,however,the high efficiency transducer is a prerequisite for the successful application of the technology,and is also a focus of present research in this technical field.

Experimental Study on Intermediate Layer Made of (0–3) Composite Materials for Use in Air-Coupled Ultrasonic Transducers

The (0–3) composite materials for the intermediate layer of a MHz-range air-coupled ultrasonic transducer were studied experimentally. The optimal value of the characteristic acoustic impedance for this intermediate layer was calculated using a transmission line model. The (0–3) composite materials were formed from a mixture of silicone rubber and hollow thermoplastic microspheres at various weight ratios, and their acoustic properties were measured, and compared with those of a Reuss model. The measured values of the acoustic properties of the composite materials agreed well with the theoretical estimation. The transmission and reception of an air-coupled ultrasonic waves of 1 MHz using a piezoceramic transducer fitted with the intermediate layer was demonstrated with a pulse–echo system, and the sensitivity was improved by 20 dB by using the optimal composite material.

Air-Coupled Ultrasonic Transducers Based on Cellular Polypropylene Ferroelectret Films

Analysis of the dielectric and electromechanical properties of the cellular polypropylene ferroelectret films (EMFIT) evidences their high potential for the application in the air-coupled ultrasonic transducers. Simple prototype transducers, based on the EMFIT films, were developed and their high sensitivity was proved in the air-coupled ultrasonic experiment. Amplitude and delay time scanned images of the polyethylene step wedge with holes, obtained in both pulse-echo and transmission modes, demonstrate that non-contact ultrasonic imaging with EMFIT transducers is possible.

Study on the high power air-coupled ultrasonic compound transducer

The high power air-coupled compound ultrasonic transducer in flexural vibration is studied. The transducer consists of a sandwich longitudinal piezoelectric transducer and a circular thin plate in flexural vibration. The resonance frequency equation and the equivalent circuit of a circular radiator with clamped boundary condition are derived. The resonance frequency equation and the equivalent circuit of the compound transducer are also obtained. The radiated acoustic field of the circular thin plate radiator is analyzed and the directional pattern is calculated. It can be seen that when the vibrational order of the circular thin plate in flexural vibration is increased, the radiated acoustic field becomes complex.

Spherically focused capacitive-film, air-coupled ultrasonic transducer

A spherically focused (no mirrors) capacitive-film, air-coupled ultrasonic transducer, constructed using a spherically deformed backplate and metalized polymer film, has been designed, fabricated, and its performance characterized. A 1cm diameter device has a center frequency of 805kHz and a 6dB bandwidth of 760kHz. Comparisons of field strength in the focal zone with theoretical calculations for a spherically focused piston show that the device achieves diffraction-limited focusing. The nominal focal point of 25mm lies within 0.01mm of the calculated value for this device.

空中超音波送波器の音響整合層のための粒子分散型複合材料の設計とその音響的特性

The acoustic properties of (0-3) composite materials for acoustic matching layer of MHz-range air-coupled ultrasonic transducer are studied experimentally. The composite materials are formed from a mixture of silicone rubber and hollow thermoplastic microspheres. The acoustic properties of the composite materials are measured and compared with Reuss model, which is theoretical model for elastic properties of composite materials. The measured values of velocity and characteristic acoustic impedance of the composite materials agreed well with the theoretical estimation up to 10% weight fractions of hollow thermoplastic microspheres. The optimal value of characteristic acoustic impedance for acoustic matching layer of air-coupled ultrasonic transducer was calculated using transmission line model. The acoustic matching layer having the optimal value was obtained from the composite material, for which the weight percentage of hollow thermoplastic microspheres is 5%. Transmission of air-coupled ultrasonic wave of 1 MHz using piezoceramic transducer with the acoustic matching layer is demonstrated, and the sensitivity of transmission was improved by 10 dB using the optimal composite material.

P3-47 Detection of Lamb waves in metal plate using air-coupled ultrasonic transducers in MHz range

P3-47 Detection of Lamb waves in metal plate using air-coupled ultrasonic transducers in MHz range

Submicrometer-Order Displacement Measurements Using an Air-Coupled Ultrasonic Transducer at Frequencies of 40 and 400 kHz

Noncontact air-coupled ultrasonic systems for measuring micrometer- or submicrometer-order displacements are newly constructed at frequencies of 40 and 400 kHz. The phase detection accuracy of the displacement measurements under several thousandths of ultrasonic wavelengths λ is markedly improved by the introduction of phase differences between the reference waves and the transmitting wave signal. To verify our system, the displacement of the reflector, which is translated by a high-precision mechanical stage, is measured using a single piezoelectric ceramic transducer in the reflection mode. Resolutions of displacement of better than 1 µm or 0.1 µm (λ/8500) using ultrasonic waves of 40 or 400 kHz in air, respectively, are achieved.