Showing papers by "Zheng Fan published in 2019"

Broadband ultrasonic focusing in water with an ultra-compact metasurface lens

Abstract: Focusing of ultrasonic waves in water plays an important role in various scenarios ranging from biomedical imaging to nondestructive testing. Acoustic metasurfaces have been largely explored for acoustic focusing, but they are generally narrowband and mainly implemented for airborne sound because of their structural complexity. Nevertheless, our previous development of metasurfaces provides a great opportunity to solve the challenges. Here, we present numerically and experimentally the broadband focusing of ultrasonic waves in water with a metasurface lens consisting of an array of deep-subwavelength sized and spaced slots. The slot widths of the metasurface are optimized based on microscopic coupled-wave theory. Due to the non-resonant arrangement, the focusing effect is demonstrated over a broad band of frequencies. The metasurface lens with simplicity and an ultra-compact size provides a feasible means for the design of thin and lightweight ultrasonic devices and is suitable for practical applications in biomedical and industrial fields.

Ultrasonic imaging of irregularly shaped notches based on elastic reverse time migration

Abstract: Ultrasonic techniques have been proved to be useful for detection and characterization of flaws in solid structures. However, it remains challenging to characterize flaws that do not have regular shapes. In this paper, an ultrasonic imaging technique based on elastic reverse time migration (ERTM) is developed for imaging notches with irregular shapes. In this method, the image is generated by cross-correlating the forward propagated wavefield from the source with the time-reversed backward propagated wavefield from the scatterer. Comparing to traditional ultrasonic imaging methods based on the travel time of ultrasonic signals, this method considers full waveforms which contain the information of mode conversions and multiple scattering, and therefore enables the possibility to image flaws with complex shapes. In this paper, the ERTM algorithm is applied to image a branched surface-breaking notch and an embedded stepped notch, showing excellent reconstruction results in both simulations and experiments.

Laser-Induced Full-Matrix Ultrasonic Imaging of Complex-Shaped Objects

Abstract: We present laser-induced full-matrix imaging of complex-shaped objects that combines the advantages of noncontact laser ultrasound inspection and high-contrast array imaging based on total focusing method (TFM). Full-matrix data were acquired by synthesizing a laser ultrasonic array through an alternate scanning of the generation and detection laser beams. Taking advantage of the simultaneous generation of all wave modes, surface Rayleigh waves were extracted for surface profiling, and longitudinal waves were utilized for imaging. To gain a better understanding, we analyzed the full-wave dynamics in generation, propagation, and scattering by using numerical simulations. TFM was adapted to accommodate complex surface and suppress interference from other waves, including surface waves and straightforward traveling bulk waves between emission and reception. Numerical and experimental studies on an aluminum sample with a sinusoidal surface and side-drilled holes were conducted, and results agreed well.

Fiber bragg grating based detection of part-thickness cracks in bent composite laminates using feature-guided waves

Abstract: Composite structures with bends are widely used in aerospace and industrial sectors. However health monitoring of such structures is challenging due to their complex topographical features. Recent literature shows that bends in composite laminates can confine and guide ultrasonic energy along their length, known as feature-guided waves (FGW). This article demonstrates a fiber Bragg grating (FBG) based technique using FGW modes for defect detection and identification in bent composite laminates. In addition, the effects of defect depth and excitation frequency on the FGW mode reflection coefficient are reported using 3D finite element simulations. Physical insight into the reflection behavior is discussed based on an analysis of mode interaction with part-thickness cracks.

Groove-structured meta-surface for patterned sub-diffraction sound focusing

Abstract: Confining acoustic fields in subwavelength volumes is of fundamental interest in wave-energy harvesting and high-resolution imaging. Phononic crystals have been shown to be capable of superfocusing but are highly limited by their very large dimensions. Acoustic metasurfaces can yield similar functionality with unit cells significantly smaller than the wavelength. However, they are studied mostly under effective medium theory and cannot manipulate evanescent waves directly to control near-field focusing. Here, we use a microscopic approach to study acoustic metasurfaces for subdiffraction focusing of reflected waves, which consist of an array of deep-subwavelength sized and spaced grooves. We further show that the focusing pattern can be tailored by the designer. To validate the effectiveness of our scheme, two representative metasurfaces are designed theoretically, proved numerically, and confirmed experimentally for subdiffraction sound focusing with different patterns. We hope that our approach can work as a general guideline to shape near-field signals in the broad field of acoustics.

New Type of Spectral Nonlinear Resonance Enhances Identification of Weak Signals.

Abstract: Some nonlinear systems possess innate capabilities of enhancing weak signal transmissions through a unique process called Stochastic Resonance (SR). However, existing SR mechanism suffers limited signal enhancement from inappropriate entraining signals. Here we propose a new and effective implementation, resulting in a new type of spectral resonance similar to SR but capable of achieving orders of magnitude higher signal enhancement than previously reported. By employing entraining frequency in the range of the weak signal, strong spectral resonances can be induced to facilitate nonlinear modulations and intermodulations, thereby strengthening the weak signal. The underlying physical mechanism governing the behavior of spectral resonances is examined, revealing the inherent advantages of the proposed spectral resonances over the existing implementation of SR. Wide range of parameters have been found for the optimal enhancement of any given weak signal and an analytical method is established to estimate these required parameters. A reliable algorithm is also developed for the identifications of weak signals using signal processing techniques. The present work can significantly improve existing SR performances and can have profound practical applications where SR is currently employed for its inherent technological advantages.

A cellular sound-absorbing metasurface with subwavelength thickness

Abstract: The sound absorbing metasurface efficient in the audible frequency range is proposed. The metasurface has a cellular structure, each cell having a hexagonal shape with a hollow tunnel inside. The wall of the hexagonal cell is sub-divided into six hollow chambers connected to the central tunnel via the six thinner channels of different diameters. The hollow chambers act as Helmholtz resonators providing six different resonant frequencies for each cell. The negative effective bulk modulus property of the metasurface allows full adsorption at the resonant frequencies. By carefully designing the size of the connecting channels we can manipulate the desired sound adsorption frequency range. The thickness of the metasurface is in the range of 0.03–0.1 wavelengths for the sound frequency range between 300 and 1000 Hz. Six absorption peaks are achieved for each unit cell providing broader range of absorption. The hexagonal shape of the unit cell allows full utilization of the metasurface volume by a standard honeycomb tessellation of the cells. By designing a metasurface containing individual cells with different inner channel diameters, the absorption peaks can be multiplied or overlapped and further broaden the frequency range with the absorption coefficient higher than the desired value.

A modular sound-attenuating metasurface with subwavelength thickness and hexagonal unit cells

Abstract: A sound attenuating subwavelength metamaterial efficient in the low audible frequency range is proposed. The metasurface unit cell has a hexagonal shape with an axial ventilation channel inside. The side walls of the hexagonal cell are sub-divided into six hollow chambers connected to the central air path via the six thinner channels of different diameters. The hollow chambers act as Helmholtz resonators providing six different resonant frequencies for each cell. The negative effective bulk modulus property of the metasurface allows near-full sound block at the resonant frequencies and partial sound adsorption inside the intermediate frequency bands. By carefully designing the size of the connecting side holes we can adjust the desired sound attenuation frequency range. The thickness of the metasurface is in the range of 0.05–0.15 wavelengths for the sound frequency range between 300 and 1000 Hz. The hexagonal shape of the unit cell allows full honeycomb tessellation of the metasurface volume. By designing a hybrid metasurface containing individual cells with different resonators, the absorption peaks can be multiplied or merged together thus further broadening the frequency range with the transmission coefficient lower than the desired threshold value.