Development of a linear array electromagnetic acoustic transducer for shear horizontal guided wave inspection

To meet the demands of highly integrated and miniaturized radio frequency front-end (RFFE) modules, multi-passband filters which support multi-channel compounding come to the foreground. In this work, we proposed a new design of a dual-passband surface acoustic wave (SAW) filter based on a 32°YX-LiNbO3 (LN)/SiO2/SiC multilayered structure. The filter is of a standalone ladder topology and comprises dual-mode resonators, in which the shear horizontal (SH) mode and high-order SH mode are simultaneously excited through electrode thickness modulation. The impact of electrode thickness on the performance of the dual-mode resonator was systematically investigated by the finite element method (FEM), and resonators were prepared and verified the simulation results. The electromechanical coupling coefficients (K2) of the SH modes are 15.1% and 17.0%, while the maximum Bode-Q (Qmax) values are 150 and 247, respectively, for the fabricated resonators with wavelengths of 1 μm and 1.1 μm. In terms of the high-order SH modes in these resonators, the K2 values are 9.8% and 8.4%, and Qmax values are 190 and 262, respectively. The fabricated dual-band filter shows the center frequencies (fc) of 3065 MHz and 4808 MHz as two bands, with 3-dB fractional bandwidths (FBW) of 5.1% and 5.9%, respectively. Such a dual-band SAW filter based on a conventional ladder topology is meaningful in terms of its compact layout and diminished area occupancy. This work provides a promising avenue to constitute a high-performance dual-passband SAW filter for sub-6 GHz RF application.

Detection of kissing bond type defects and evaluation of the bonding quality in metal/adhesive/composite structures by a wavenumber-frequency insensitive SH mode

A multi-mode P-matrix formalism accounting for energy conservation, causality and reciprocity is presented. The P-matrix of an electrode is represented as product of P-matrices for propagation, excitation and reflection. Reflection considers the interaction of forward and backward propagating waves of the same mode as well as different modes. Model parameters are determined by fitting to dispersion relation [1] and harmonic admittance, both obtained from finite element (FEM)computations. For a temperature compensated SAW (TCSAW)system a two-mode model of interacting Rayleigh and shear horizontal (SH)mode is determined. It is shown that the excitation of the SH mode is mainly determined by mode interaction, rather than direct excitation of the mode. A multi-mode model for leaky SAW (LSAW)systems is set up by considering the interaction of a single guided mode and a quasicontinuum of bulk acoustic wave (BAW)modes. In this way the surface bulk hybrid (SBH)mode and other characteristic features of LSAW devices automatically emerge in the simulation result.

An ultrasonic system using electromagnetic acoustic transducers has been developed at the Fraunhofer Institute for Non-Destructive Testing (IZFP), in order to test laser welds in tailored blanks. Shear horizontal modes are exploited to detect different types of defects in the laser welds, such as lack of penetration, lack of fusion, mismatch and cracks. In the present work, studies of the discontinuities of the laser weld that affect specularly reflected and non-specularly signals have been carried out on ferritic sheets. Geometric parameters of the laser weld, such as the fusion-zone width and the discontinuities at the weld root, were measured and compared with the amplitude of the reflected signals. Furthermore, X-ray radiographs and metallography were made to obtain additional information on the different defects, which affect the ultrasonic signal. From our results we conclude that under thickness and incomplete penetration at the root, together with the pores of the weld affect the reflected signal more than any other discontinuity.

The pure SH (shear horizontal) mode surface acoustic wave on rotated Y-cuts about the Z-axis of potassium niobate (KNbO/sub 3/) is theoretically analyzed. The results show that there exists a pure SH-mode surface wave, which has a velocity lower than the piezoelectrically inactive Rayleigh wave for most of the rotated cuts. The particle displacement of the SH-mode surface wave on Y-cut KNbO/sub 3/ has a depth profile different from that in 6 mm crystals. The electromechanical coupling factor K/sup 2/ ranges from a very large value of 0.53 to a relatively small value of 0.026, depending on substrate orientation. The collinear interaction between the SH-mode surface wave and optical waves in KNbO, is also theoretically analyzed. It is shown that both the photoelastic and electro-optic effects contribute constructively to the acousto-optic interaction and that the optical TE-to-TM mode conversion produced by the SH-mode surface wave is very efficient compared with that produced by the Rayleigh wave in Y-X LiNbO/sub 3/.

Electromagnetic acoustic transducers (EMATs) are well suited to generating and detecting a variety of different ultrasonic wavemodes, without the need for couplant, and they can be operated through some coatings. EMATs can be used to generate shear horizontal (SH) waves, which show promise for fast screening of wall thinning and other defects. However, commercial SH-wave EMATs are not suitable for robotic implementation on ferritic steel due to the large magnetic drag force from the magnets. This article describes the design and characterisation of miniaturised SH guided wave EMATs, which significantly reduce the magnetic drag and enable mounting onto a small crawler robot for sample scanning. The performance of the miniaturised EMATs is characterised and compared to a commercial EMAT. It is shown that signal to noise ratio is reduced, but remains within an acceptable range to use on steel. The bandwidth and directivity are increased, depending on the exact design used. Their ability to detect flat bottomed holes mimicking wall thinning is also tested.

Shear horizontal (SH) guided waves have been proved to be a viable method in the Non-Destructive Evaluation (NDE). Electromagnetic acoustic transducers (EMAT) can excite SH waves easily. By bonding the Fe-Co alloy to the test sample, the SH guided waves based on magnetostriction effect can be used to detect the flaw in nonferromagnetic material. The principle of exciting and receiving SH waves is introduced, and an experiment is carried out to validate the result.

Friction stir welding (FSW) is unique in terms of the mechanical properties and metallurgical structure of the joint. Like any other weld joints, FSW joints have some characteristic defects generated during the joining process. These defects lead to reduction in the overall quality of the weld, thereby weakening the joint over time. Thus, there is a need for an inspection technique to evaluate the weld quality in FSW joints. The fundamental SH0 mode offers a unique advantage because of its non-dispersive nature. SH waves are very difficult to generate through conventional ultrasonic generation methods, but electro-magnetic acoustic transducers (EMAT) have proven to be very efficient in this aspect. In this paper, we present an inspection technique involving shear horizontal (SH) ultrasonic waves that gives a qualitative view of FSW joint in aluminium plates. Studies were conducted on a 2.5 mm aluminium plate with a friction stir welded butt joint, with SH waves of wavelength 6.35 mm wavelength generated by EMAT probes. The configuration of the EMAT transducers was in such a way that the receiver measured the specular reflection from the weld line. The method was found to be useful in detecting defects in the weld-line. Results from EMAT inspection are compared with radiographic images.KeywordsFriction stir weldEMATShear horizontal mode

The phenomenon of the reduction in the propagation speed of an ultrasonic wave when it travels through a fatigue zone has been well studied in the literature. Additionally, it has been established that shear waves are more severely affected by the presence of such a zone, compared with longitudinal waves. Our study utilises these phenomena to develop a method able to characterise the fatigue state of steel pipes. Initially, the existing theory regarding the increased sensitivity of shear waves to the presence of fatigue is validated through measuring and comparing the change in propagation speed of both longitudinal and bulk shear waves on flat geometries, at different fatigue states. The comparison is achieved with the aid of ultrasonic speed C-scans of both longitudinal and shear waves, with the latter now being obtainable through our implementation of advances in Electromagnetic Acoustic Transducers (EMAT) technology. EMATs have not been traditionally used for producing C-scans, and their ability do to so with adequate repeatability is demonstrated here; we show that shear wave scanning with EMATs now provides a possibility for inspection of fatigue damage on the inner surface of pressure-containing components in the nuclear power industry. We find that the change in ultrasonic wave speed is amplified when shear waves are used, with the magnitude of this amplification agreeing well with the theory. Following the verification of the theory, the use of EMATs allowed us to tailor the shear wave scanning method to pipe geometries, where C-scans with conventional piezoelectric transducers would not have been possible, with the results successfully revealing the presence of fatigue zones.

Composite materials are widely used in aerospace industries due to their light weight, strength, and various other desired properties. However, they are susceptible to various defects occurring during the manufacturing process or in service. Typical defects include porosity, wrinkles, and delamination. Nondestructive means of detection of the defects at any stage are of great importance to ensure quality and safety of composite structures. A nonintrusive removable Lamb wave system and accompanied methodology that is not material dependent are presented in this paper to detect various types of typical defects in laminated composite plates, flat or curved. Through multidimensional data acquisition and processing, abnormality in waves caused by defects is captured and presented in inspection images. The methodologies are demonstrated in two cases: delamination in a curved plate and wrinkles in a flat plate. Overall, the results show that Lamb waves using the piezoelectric transducer and laser vibrometer system can be used for various types of defects inspection in flat or curved composite plates.

High-precision detection of crack is of great significance to ensure the safety of pressure equipment in service. Electromagnetic ultrasound has the advantages of no coupling agent, no surface pretreatment, and non-contact. However, several problems such as low energy conversion efficiency and poor sound field directivity limit the application in crack detection. This paper designs a multi-layer line-focusing electromagnetic acoustic bulk wave transducer (MLF-EMAT). A simulation model containing MLF-EMAT is established based on the Lorentz force and magnetostrictive force transduction mechanism. The influence of some transducer parameters (wire width, wire thickness, coil turns, lift-off distance) on the echo response of crack has been studied. The orthogonal experimental design method is used to optimize the controllable parameters of the transducer. When the width of the permanent magnet is equivalent to the width of the coil, the larger the number of coil layers, the larger the amplitude of the echo response of crack. An experimental platform for electromagnetic ultrasonic testing is established, and several line-focusing electromagnetic acoustic transducers with multiple parameters are prepared. The comparison results show that the optimized MLF-EMAT significantly improves the signal-to-noise ratio of echo response of crack. The amplitude of echo wave is increased by about 78% compared with that before optimization, and the detection accuracy of crack notch depth is up to 0.05 mm. The MLF-EMAT proposed in this study lays the foundation for the application of crack detection or monitoring of pressure vessels and pipes under complex working conditions.

A variety of signal processing algorithms have been proposed to detect and locate defects in plate-like structures. However, the signal-to-noise ratio in these algorithms is too small especially in the reflection wave from the boundary, which further degrades the accuracy of localization of defects. A novel method for localization of defects is proposed in this article, based on the direct wave and fuzzy c-means clustering algorithm. To verify its effectiveness, experiments using the parallel linear and circular array are conducted, respectively. The experimental results show that the proposed method not only accurately locates single defect but also locates double defects in plate-like structures, and by comparing with the current discrete elliptic imaging algorithm, its location error of single defect is reduced from 20–25 mm to 0–3 mm and double defects is also reduced from 60–90 mm to 0–3 mm.

Due to their low cost, hand-held convenience, wide selection of bandwidths, and ultrasound imaging capability, linear ultrasonic transducer arrays have been widely studied for photoacoustic computed tomography (PACT). As linear-array PACT suffers from a limited view, full-view imaging requires either the transducer or the object to be rotated. So far, both the central frequencies and bandwidth of linear transducer arrays applied in full-view PACT are low, limiting the spatial resolutions of the reconstructed images. Here, we present a multiview high-frequency PACT imaging system implemented with a commercial 40-MHz central frequency linear transducer array. By rotating the object through multiple angles with respect to the linear transducer array, we acquired full-view photoacoustic pressure measurements. Further, to quantify the unipolar initial pressures and overcome the limitations of the single-view Hilbert transformation, we developed a multiview Hilbert transformation method. The in-plane spatial resolution of this full-view linear-array PACT was quantified to be isotropically 60 μm within a 10×10 mm² field of view. The system was demonstrated by imaging both a leaf skeleton and a zebrafish in vivo.

Propagation characteristics of the zeroth-order symmetric (S0) mode shear-horizontal (SH) wave in a cell structure containing a thermotropic liquid crystal layer with chiral smectic-C (SmC*), smectic-A (SmA), chiral nematic (N*) and isotropic (Iso) phase states, are examined. The SH wave propagation is numerically analyzed with consideration of the directional dependence of the viscosity and elastic constant of the SmA liquid crystal. Temperature dependencies of acoustic phase delay and propagation loss of the SH wave are observed. The critical behavior of the acoustic phase delay and the propagation loss of the S0 mode SH wave in the vicinity of the N*-SmA phase transition temperature are explained by the existence of the elastic constant related to layer compression. The layer compression elasticity is predominant in the SmA phase, and is determined from the comparison of numerical analysis with experimental results.

An NDT guided wave technique for the identification of corrosion defects at support locations