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Transcranial ultrasound therapy has become a non-invasive method for treating neurological and psychiatric disorders, and studies have further demonstrated that multi-target transcranial ultrasound therapy is a better solution. At present, multi-target transcranial ultrasound therapy in small animals can only be achieved by the multi-transducer or phased array. However, multiple transducers may cause spatial interference, and the phased array system is complicated, expensive, and especially unsuitable for small animals. This study is the first to design and fabricate a miniature acoustic holography lens for multi-target transcranial ultrasound therapy in rats. The acoustic holographic lens, working at a frequency of 1.0 MHz, with a size of 10.08 mm × 10.08 mm and a pixel resolution of 0.72 mm, was designed, optimized, and fabricated. The dual-focus transcranial ultrasound generated based on the lens was measured; the full width at half maximum (FWHM) of the focal spots in y-direction were 2.15 and 2.27 mm, respectively, and in z-direction were 2.3 and 2.36 mm, respectively. The focal length was 5.4 mm, and the distance between the two focuses was 5.6 mm, close to the desired values of 5.4 mm and 6.0 mm, respectively. Finally, the multiple-target blood-brain barrier opening in rats' bilateral secondary visual cortex (mediolateral area, V2ML) was demonstrated using the transcranial ultrasound therapy system based on the lens. These results demonstrate the good performance of the multi-target transcranial ultrasound therapy system for small animals, including high spatial resolution, small size, and low cost.

The excellent performance of PMN–PT has received increasing attention and applications in high-frequency (>20 MHz) ultrasonic array transducers. However, due to its brittleness and low phase-transition temperature of PMN–PT, a decrease of piezoelectric properties in the fabrication of small-scale array elements is still an urgent problem and would substantially determine the performance of transducers. Here, in this study, ultrashort pulse laser technique is proposed to fabricate PMN–PT array transducers with outstanding performance. The effect of the laser parameters on domain structures has been studied systematically, and it was found that the ultrahigh piezoelectric properties of micro-sized elements still maintained after optimizing the laser parameters. A Fresnel array transducer with center frequency of 40.84 MHz and −6dB bandwidth as high as 122.28% was also designed and fabricated. This work demonstrates that it is feasible and promising to fabricate superior PMN–PT and other piezoelectric ultrasonic array transducers based on ultrashort pulse laser techniques.

A decrease of piezoelectric properties in the fabrication of ultra-small Pb(Mg1/3Nb2/3)–x%PbTiO3 (PMN–x%PT) for high-frequency (>20 MHz) ultrasonic array transducers remains an urgent problem. Here, PMN–31%PT with micron-sized kerfs and high piezoelectric performance was micromachined using a 355 nm laser. We studied the kerf profile as a function of laser parameters, revealing that micron-sized kerfs with designated profiles and fewer micro-cracks can be obtained by optimizing the laser parameters. The domain morphology of micromachined PMN–31%PT was thoroughly analyzed to validate the superior piezoelectric performance maintained near the kerfs. A high piezoresponse of the samples after micromachining was also successfully demonstrated by determining the effective piezoelectric coefficient (d33*~1200 pm/V). Our results are promising for fabricating superior PMN–31%PT and other piezoelectric high-frequency (>20 MHz) ultrasonic array transducers.

Defect inspection of braided carbon fiber reinforced polymer (CFRP) is very difficult due to its nonhomogeneity, anisotropy, and sensitivity to coupling agents. This paper presents a hybrid system for detecting microstructure and defects in braided CFRP that combines the advantages of laser-ultrasound and air-coupled ultrasonic testing . Through the finite element method, the ultrasonic field propagating on the braided CFRP was simulated during the laser-induced ultrasound, and the influences of laser parameters and surface braided structure were analyzed on the laser ultrasonic signal. This detection method based on air-coupled laser ultrasound can provide the near-surface microstructure characterization of the resin pockets and braided fiber bundles, and also can achieve both shallow and deep defect detection of CFRP sheets which is comparable to that of the contact-type high-frequency phased array with higher contrast and less distortion. These results indicate that it has the potential to develop a non-contact, high-resolution, and low-cost method for the detection and repairment of advanced composite materials.

Fiber Bragg gratings (FBGs) have being seen for a growing application in the field of structural health monitoring (SHM) because of their lightweight features. This letter introduces an adaptive SHM sensing system based on the ultrasonic guided wave (UGW) and FBG sensor technology. First, the capability of the proposed SHM sensing system used to detect UGW is demonstrated by the frequency detection of Rayleigh wave that is generated by the piezoelectric transducers with a function generator. Furthermore, a nanosecond pulse laser is utilized to generate Lamb wave on an aluminum plate, while the SHM sensing system is employed to detect the wave in a long distance. Finally, the SHM sensing system is used to locate the source of Rayleigh wave that is created by a pulsed laser. The experiment results show the proposed SHM sensing system is capable of localizing and monitoring the UGW by using four FBG cascaded on a loaded aluminum plate.

The excitation and propagation of an electromagnetic field is a very complex problem in Stratified conductive structures (SCS) because of their complicated physical properties. In single air-cored coil above multilayered conductive structures, the impedance of an eddy-current coil is calculated based on the electromagnetic field. This paper provides insight into eddy current models for stratified conductive structures using three types of analytical approaches, including Cheng’s matrix method, Truncated region eigenfunction expansion (TREE) method, and the innovative method based on the reflection and transmission theory of the electromagnetic waves. The analytical expressions of the three models are discussed and compared. Numerical simulation of an FEM model is performed, and the experimental results verify the developed models.

The anti-corrosive metal coatings on ferromagnetic tubes are the important components in power plants and need non-destructive evaluation (NDE) techniques to measure their thickness precisely. The objective of this research is to develop pulsed eddy current (PEC) technique for measuring the coating thickness accurately. PEC signals are investigated for the evaluation of coating thickness using a differential PEC probe, which consists of a driving coil and two pickup coils. The characteristics of PEC signals for coating thickness variations show that the related features, such as the time-to-peak and peak value, change linearly with thickness change. A new feature, called as time to the rising point, related to the propagation time of electromagnetic field in conductive metals is adopted for coating thickness measurement. These features can be used to inspect the coating thickness within the accuracy of 0.02 mm.

Rail health conditions are among the top concerns in the area of train safety. In this study, a fiber optic monitoring system is developed to achieve ultrasonic guided wave based rail crack detection. Although fiber Bragg grating (FBG) sensor is a wellknown suitable candidate for long-distance monitoring of rail, the sampling speed of commercially available optic spectrum analyzers limits their application to ultrasonic wave detection. A high-speed FBG interferometric interrogation module is developed, which constitutes the rail monitoring system in conjunction with an active wave generation module and a sensing network. To find appropriate excitation frequency and FBG dimension for ultrasonic guided wave generation and reception, dispersion analysis of rail, a waveguide with complex cross-section, is conducted to guide subsequent design of damage detection experiment. The system and the crack detection technique are then implemented on a long full-scale rail segment, by deploying PZT (lead zirconate titanate) actuator and FBG sensor in pitch-catch and pulse-echo configurations. Artificial cracks in different lengths are introduced to the rail. Frequency-domain analysis of the rail responses is used to identify the damageinduced discrimination after direct observation of time-domain signals. Power spectral density analysis of the purified signals, assisted by discrete wavelet filtering, leads to the graphic presentation of rail integrity.

This paper proposes an analytical approach that can describe the Pulsed Eddy Current Testing (PECT) of stratified conductive structures by a single air-coil used for detecting and sizing of metal loss due to interlayer corrosion. Specifically for a single air-cored coil above a multilayered conductive structure, the coil impendance response of the cylindrical eddy-current probe is calculated based on the truncated region eigenfunction expansion (TREE) method using the generalized matrix coefficients in series form. Analytical modeling of pulsed eddy current is developed using the Fourier series for multilayered conductive structures. A simulation study is carried out for demonstrating the capability of the proposed approach for sizing the metal loss in a two-layer specimen. Furthermore, the experiment is set up to confirm the simulating results of the analytical model for multilayered steels. Good agreement between the theoretical results and measured signals is observed.