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The amplitude and the phase information are both important for damage localization in Lamb wave–based structural health monitoring and non-destructive testing. Most previous studies in Lamb wave imaging are only based on either the amplitude or phase of the signal at the time of flight. In this study, a post-processing technique for Lamb wave phased array imaging is proposed for isotropic plates, which simultaneously utilizes both the amplitude and phase information within a small neighborhood centered at the time of flight. In the proposed imaging algorithm, a modified virtual time reversal is firstly implemented to compensate for dispersion and the amplitude decrease caused by wave diffusion. Then the waveform covariance between any two processed wave packets, which contains information of both amplitude and phase, is used as the indicator of the presence of damage. Finally, adaptive weights based on minimum variance are introduced to weight and sum the waveform covariance for damage imaging. Based on a uniform circular array, experimental results on an aluminum plate with three defects verify that the proposed algorithm is capable of localizing multiple defects, suppressing background noise and main lobe width.

In this paper, a contactless nonlinear acoustic method is developed for the evaluation of stress states in anisotropic metallic plates by a combination of electromagnetic acoustic resonance (EMAR) technique and higher harmonic generation. Electromagnetic transducers (EMATs) designed and applied for exciting and receiving ultrasonic signals can maintain the coupling condition consistently on the measure of higher harmonics generated. EMAR provides a sufficient magnitude of signals for higher harmonics generated. In addition, the conventional EMAR technique based on the measurement of shear-wave velocity and attenuation within a certain frequency range is also carried out in the specimens. The effect of stress on the higher harmonic generation is explored and discussed. It is found that nonlinear parameters measured by the nonlinear EMAR method change significantly versus the increase of external tension stress loadings, whereas the variations of linear acoustic parameters measured are negligible. In addition, the obtained results clearly indicate that the variation of the measured acoustic nonlinear parameters versus external stresses is direction-depended in anisotropic materials. The contactless nonlinear acoustic technique combines the feature of EMAR with the merit of higher harmonic generation, providing an effective means for stress evaluation in weakly anisotropic materials with improved reliability and sensitivity over linear ones.

It has long been proposed that cell competition functions to remove precancerous clones. A classical model is the removal of polarity-deficient clones such as thescribble(scrib) mutant clones inDrosophilaimaginal discs. The activation of Ras, Yki or Notch signaling robustly reverses thescribmutant clonal fate from elimination to tumorous growth. Using single-cell transcriptomics techniques to profile wing imaginal discs harboring thescribmutant clones in combination with different signals, we found that a critical converging point downstream of Ras, Yki and Notch signals is the upregulation of Upd2, which is necessary to promote tumorous growth. Unexpectedly, while Upd2 is not required for cell survivalper se, Upd2-deficient clones are efficiently wiped out from epithelia, indicating that Upd2 is a previously unrecognized cell competition factor.

The use of acoustic nonlinear response can be an effective method for the early-stage damage evaluation of materials. However, it is challenging to assess the thermal damage in highly attenuating materials, such as polymethyl methacrylates, by the measure of generated second harmonics with a double fundamental frequency of primary waves. Quasi-static component (QSC) generated by ultrasonic wave propagation in damaged structures, which combines the high sensitivity of acoustic nonlinearity and low attenuative effect, has a great potential for early-stage damage evaluation in highly attenuating materials. In this paper, an experimental approach for directly detecting the QSC pulse of an ultrasonic wave tone burst is proposed. While a high-frequency longitudinal wave transducer is used for exciting the primary ultrasonic wave tone burst, the co-propagating QSC pulse is captured using a low-frequency ultrasonic transducer. The phase-reversal technique is employed to enhance the signal-to-noise ratio of QSCs and to counteract the signals of primary longitudinal waves. Amplitude variations of QSCs under different thermal loadings are clearly illustrated in a repeatable manner, while those of linear ultrasonic features in the specimens are neglectable. Experimental results indicate that the measure of QSC generation of ultrasonic wave propagation can be a prospective alternative for evaluating thermal degradation of highly attenuating materials.

This study focuses on static component generation (SCG) and its measurement wherein a group velocity mismatch (GVM) exists between the primary guided wave and the generated static component (SC). The SCGs by primary S0, A0, and SH0 waves are investigated. It is confirmed that the SCs are S0 mode. The GVM causes the temporal waveforms of the SCs to tend to increase in width with propagation distance. A feasible method is proposed accordingly for measurement of SCG with GVM using only lead zirconic titanate based transducers, wherein the SCs generated by two counter-propagating primary waves are modulated and superposed on each other.

A microcrack localization method based on a static component (SC) induced by a primary A0 Lamb wave is proposed. Based on the bilinear stress-strain constitutive model, a two-dimensional finite element model is built to investigate the interaction between microcracks and Lamb waves. The A0 Lamb wave at low frequency is selected to be the primary Lamb wave, which is beneficial to microcracks localization. Based on the time of flight of the generated SC pulse, an indicator named normalized amplitude index is defined for finding the location and number of microcracks. Simulation results show that one or multiple microcracks can be effectively located.

Nonlinear Rayleigh waves are useful for material surface damage detection. Traditional non-destructive testing (NDT) techniques based on higher harmonics of Rayleigh waves are inevitably interfered with by the nonlinearity of measurement systems. To overcome this disadvantage, an approach based on one-way collinear mixing Rayleigh waves (MRWs) is proposed to evaluate the material surface properties. A wedge with two different incidence angles for the excitations of transverse and longitudinal waves is designed to generate one-way collinear MRWs. The one-way collinear MRWs can be generated on the whole surface of the material, and propagate along the propagation way under one excitation. Numerical simulations of one-way collinear MRWs under different surface properties, which are characterized using different third-order elastic constants, are studied on a semi-infinite aluminum. Experiments are also conducted on the aluminum specimen, whose surface is corroded by hydrochloric acid so as to change the material surface properties. Both numerical and experimental results show that the one-way collinear MRWs are generated effectively, and the defined relative acoustic nonlinearity parameter increases with the propagation distance as well as the severity of the material surface damages. The results verify the effectiveness of the proposed MRW-based method and prove that it would be a promising NDT technique for the accurate evaluation of material surface properties.

Low-intensity unfocused ultrasound (LIUU) is proposed for producing a temperature difference (TD) effect in biological tissues. A finite-element simulation model has been established to validate the method’s rationality, and its effectiveness in practical applications is further discussed through phantom experiments. Experimental results indicate that LIUU under suitable conditions can result in discrepancy of temperature increases in biological tissues with complex compositions. For photoacoustic imaging, the method leverages differences in acoustic absorption coefficients of biological tissues and extracts the TD as an imaging contrast, offering the potential to enhance tissue discrimination in conventional photoacoustic imaging with low light absorption coefficients.

Because the surface corrosion damage could susceptibly occur in metallic structures for the corrosive environment and fatigue loads, it essential to detect the corrosion timely and precisely before the loss of life and property caused by corrosion. Based on low-frequency Lamb wave technique, the low-frequency A0 Lamb wave, the low-frequency S0 Lamb wave and one-way S0-A0 Lamb mixing wave are experimentally investigated in the thin plates with the early stage damage caused by the surface corrosion, which were made by different times of hydrochloric corrosion. Firstly, it verified that the damage degree of the surface corrosion region increases with the number of corrosion times by the Scanning Electron Microscopy (SEM). Secondly, the linear and nonlinear effect caused by corrosion had been investigated, and the relationship between the characters of the different Lamb waves and the different corrosion damage levels were qualitatively discussed. Especially, the length and the location of the corrosion region were located successfully by one-way S0-A0 Lamb mixing wave. This paper explores experimentally the propagation mechanism of low frequency linear and nonlinear Lamb wave in thin plates with corrosion damage, which can provide an experimental basis for Lamb-wave detection techniques used in the evaluation and location of damages.

During spermatogenesis, multiple testis-specific histone variants are involved in the dynamic chromatin transitions. H2BFWT is a primate testis-specific H2B variant with hitherto unclear functions, and SNPs of H2BFWT are closely associated with male non-obstructive infertility. Here, we found that H2BFWT is preferentially localized in the sub-telomeric regions and the promoters of genes highly expressed in testis from differentiated spermatogonia to early spermatocytes. Cryo-EM structural analysis shows that H2BFWT nucleosomes are defined by weakened interactions between H2A-H2BFWT dimer and H4, and between histone octamer and DNA. Furthermore, one of its SNPs, H2BFWTH100R further destabilizes nucleosomes and increases the nucleosome unwrapping rate by interfering with the interaction with H4K91. Our results suggest that H2BFWT may be necessary for the regulation of spermatogenesis-related gene expression by decreasing transcriptional barriers, and that H2BFWTH100R overdrives its nucleosome-destabilizing effects which causes infertility.

Nickel and nickel-based composites are of vital importance in many fields, while temperature loading can greatly influence the strength and performance of the materials. Nondestructive evaluation and characterization of such thermal damage can be used to predict the failure of metallic structures, thermal barrier coatings and so on, especially in a non-contact way under certain strict circumstances, such as testing at high temperature or in radiative environment. Herein, a contactless ultrasonic technique employing electromagnetic acoustic transducers (EMATs) combined with the resonance ultrasound spectroscopy is applied to make up the low energy transition efficiency of EMATs and enhance the signal-to-noise ratio of ultrasonic testing signals. The method is adopted to assess the thermal damages of different levels in artificially heat loaded nickel plates. The damage sensitivity of third order harmonics generated from shear waves is discussed, along with linear ultrasonic features including wave velocity and attenuation. Experimental results show that the proposed nonlinear electromagnetic acoustic resonance (EMAR) technique can be used to evaluate the thermal damage in ferromagnetic material with improved reliability and sensitivity over linear ones.

Recent advances in single-cell RNA sequencing (scRNA-seq) techniques lead to an explosion of single-cell atlases from diverse biological contexts. The information of cell-cell signaling events, which underlie multicellular organism function, is embedded in these atlases. Here, we review current strategies of mining cell-cell signaling events from single-cell transcriptomics datasets and highlight examples where functions of predicted cell-cell signaling events from single-cell atlases are further pursued to yield new insights into biological processes.

This paper numerically and experimentally investigates the resonant behavior of one-way Lamb and SH (shear horizontal) mixing method in thin plates with quadratic nonlinearity. When the primary S

Using the nonlinear interaction effect between ultrasonic Lamb waves and microcracks to detect and locate microcracks has the advantages of fast detection speed and high sensitivity. In this paper, a method for microcrack localization based on cross-shaped sensor clusters in a plate is proposed by combining nonlinear ultrasonic Lamb wave technology and time difference of arrival (TDOA) technology. The antisymmetric (A0) mode at low frequency is chosen as the primary Lamb wave to simplify the complication of the dispersion and multi-mode properties of Lamb waves. The selected mode pair (A0-s0) weakens the influence of the cumulative growth effect of higher harmonics induced by the inherent material nonlinearity on the microcrack characteristic signals. Pulse inversion technique and cross correlation function are used to extract the TDOAs of the nonlinear characteristic signals including microcrack information. The cross-shaped sensor clusters approach proposed for the first time can achieve reliable and fast microcrack localization without being affected by the duration of the excitation signal, and a priori knowledge of group velocities of primary wave modes or generated harmonics. Experimental and numerical results validate the proposed method in isotropic and anisotropic plates. This paper provides a new idea for nonlinear ultrasonic nondestructive evaluation and structural health monitoring of microcracks in plates.

This work investigates the feasibility of using a nonlinear low-frequency Lamb wave approach for characterizing the interfacial property of a two-layered plate. Compared with the case of the exact phase-velocity matching, the approximate phase-velocity matching in the low-frequency region can still guarantee the cumulative second-harmonic generation (SHG) of primary Lamb wave propagation, which overcomes the drawbacks arising from the inherent dispersion and multimode features of Lamb wave propagation. For a given two-layered plate, the appropriate mode pair at low frequency consisting of primary Lamb wave and double-frequency Lamb wave (DFLW), which satisfies the approximate phase-velocity matching and nonzero energy flux, is selected to ensure that the amplitude of the generated second harmonic grows within the maximum cumulative distance (MCD). Meanwhile, the numerical analyses indicate that the variation of the SHG efficiency is maximized at the MCD during the interfacial degradation. Using the nonlinear ultrasonic measurement-based experimental setup, the time-domain signal of the second harmonic generated at different propagation distances is conveniently extracted, and then the relative nonlinear acoustic parameter curve consistent with the theoretical prediction is obtained. For examining the influence of interfacial property on the SHG effect of low-frequency Lamb wave propagation, the different annealing cycles of the thin adhesive layer (acrylics) are used to simulate minor changes in the interfacial property of the given two-layered plate. It is found that the relative nonlinear acoustic parameter at the MCD decreases monotonically and sensitively with the increment of annealing cycle number, which verifies the quantitative correlation between the SHG efficiency of low-frequency Lamb wave propagation and the degree of the interfacial degradation. The consistency between the numerical analysis and the experimental measurement shows the potential of using the SHG effect of low-frequency Lamb wave propagation to characterize a minor change in the interfacial properties of a layered composite plate.

The crucial ability to suppress low-frequency waterborne sound efficiently has far-reaching implications for underwater noise-control engineering. Here, we propose an ultrathin composite metasurface in deep subwavelength thickness based on pentamode metamaterials (PMs) backed by rubber-metal resonators (RMRs). As a demonstration, average absorptance $87.8\mathrm{%}$ at subkilohertz frequencies ranging from 760 to 920 Hz (wavelength $\ensuremath{\lambda}$ from 30.9 to 25.5 times of absorber thickness) is achieved. In the composite system, PM layer functions as an energy converter, which converts incident waterborne sound into mechanical vibrations, suggesting that sound waves could be absorbed only if the converted vibrations are eliminated by the backing RMRs. Beyond as a converter, PM itself also exhibits an additional stringlike resonant mode originating from the fixed-boundary constraints, which gives rise to intense vibrations of RMRs that facilitate energy dissipation. Further investigation demonstrates the unique property of robust high-efficiency absorption for extended frequency region and wide oblique incidence angles. The proposed methodology paves the way for a class of low-frequency underwater absorber design platforms, allowing devices for versatile applications to be envisioned.

Fatigue damage and micro-cracks growth have been proved to be closely related to the nonlinear effect of ultrasonic Lamb waves propagation in materials. In this paper, the relationship among fatigue life, length and width of micro-cracks, and the acoustic nonlinearity parameter was established. The variation of the acoustic nonlinearity parameter caused by fatigue damage was analyzed based on micro-cracks growth. Considering the multimodal nature of nonlinear Lamb waves, we selected the low-frequency mode pairs S0–s0 and S1–s2 to evaluate fatigue damage in 7075 aluminum alloy plates. Both FE simulations and experiments show that the mode pair S1–s2 is more suitable than S0–s0 for the assessment of fatigue damage. It was found that with increasing fatigue cycles, the relative acoustic nonlinearity parameter A2/A12 firstly increases because the micro-cracks length extends while the micro-cracks width does not change much. The relative acoustic nonlinearity parameter A2/A12 reaches its peak at about 60% of the fatigue life. Then the relative acoustic nonlinearity parameter A2/A12 decreases because of the rapid increase in the micro-cracks width. Our findings clarify the mountain-shape curve between the relative acoustic nonlinearity parameter A2/A12 and the fatigue cycles.

Acoustic vortices (AVs) based on artificial metamaterials have attracted intense research efforts for their potential use in subdiffraction imaging, particle manipulation, and acoustic communication. However, to date, major strategies to generate AVs have an intrinsic limitation of fundamentally depending on complex phase modulations, which require a higher number of multiple elements as the desired topological charge increases, let alone the elaborate dephasing between the individual microstructured elements in the switching process of topological charges. In this paper, we propose an annulus anisotropic structure coupled with a pair of eccentric asynchronous monopole sources that can realize multiple-order AV modes at different frequencies. The generated AVs originate from the interactions of two superimposed multipoles characterized with orthogonal emission patterns and $\ensuremath{\pi}/2$-phase differences. The AVs with topological charges from 1 to 5 are achieved simultaneously at dipole, quadrupole, hexapole, octupole, and decapole resonances, respectively. Numerical simulations and experimental results clearly confirm the feasibility and practical applicability of the proposed design strategy in realizing topologically diverse AVs. Our design, exempted from complex phase modulations, possesses flexible topological charge modes at the expense of frequency, which may foresee potential capabilities in multiplexing and demultiplexing of AVs.

Air-coupled ultrasonic testing (ACUT) has many advantages over other non-destructive testing (NDT) methods, among which the most outstanding one is completely no contamination of the sample. Therefore, it can be applied in various conditions where liquid coupling media is not suitable. With the development of ACUT, it is found that mismatch between air and solid is one of the biggest difficulties confronting researchers, and electromechanical film (EMFi) with low acoustic impedance can be a promising solution when used as vibration element in air-coupled ultrasonic transducers. In this contribution, one planar and two spherically focused EMFi-based transducers were designed, fabricated, and their properties were characterized through frequency response test as well as focal length test. Then imaging experiments of PMMA samples were performed to demonstrate the possibility and feasibility of EMFi transducers in non-contact testing.

In the ultrasonic detection using nonlinear Lamb wave, only the cumulative second-harmonic Lamb wave is useful for the nondestructive evaluation of plate-like structures. One of the conditions for cumulative second-harmonic Lamb wave generation is phase velocity matching between the primary and the secondary wave. However, in most cases, the phase velocities are approximate matching. In this paper, we theoretically analyzed and discussed the cumulative effect of second-harmonic Lamb wave. The theoretical results show that, when the phase velocities of the primary and the secondary Lamb waves are approximate matching, there is still a cumulative effect in the secondary wave within a specific distance, which is referred as accumulation distance. Based on the accumulation distance, an excitation window is proposed as the basis for mode selection and excitation condition determination of nonlinear Lamb wave. It is considered that, in the case of approximate phase velocity matching, the mode pair with larger excitation window could generate second-harmonic Lamb wave easier than the other mode pairs. Then, the excitation windows of different Lamb wave mode pairs are calculated and compared theoretically. In order to verify the validity of the excitation window as the basis for mode selection, the ranges of two mode pairs’ primary Lamb wave that could generate cumulative second-harmonic Lamb wave are measured and compared in a specific plate. In the experiment, the primary and the secondary Lamb waves around a longitudinal mode pair and a crossing mode pair are excited and measured using wedge piezoelectric transducers. In the measurement, the second-harmonic Lamb wave is generated by the primary wave in different frequency and phase velocity ranges, and then the ranges of the primary Lamb wave are confirmed in the phase velocity dispersion curves, using the time-frequency analysis. The experimental result shows that, the primary wave range of the longitudinal mode pair is larger than that of the crossing mode pair, which means that the selected longitudinal mode pair could generate second-harmonic Lamb wave easier than another one. The experimental result is in accordance with the comparing result of the excitation windows. The theoretical and experimental results show that, the excitation window could be an indicator for mode selection and excitation condition determination of cumulative second-harmonic Lamb wave generation, and it can be used in the field of ultrasonic detection using nonlinear Lamb wave.

Ultrasonic inspection is a widely used nondestructive testing approach in industrial fields for more accurate life prediction and efficient management strategies of critical structural components. However, it is quite challenging to detect a kind of micro-defect, whose size is much smaller than the ultrasonic wavelength but larger than the ultrasonic amplitude (namely, there is no appearance of non-classical acoustic nonlinearity). In this article, identification and imaging of a micro-defect of this size range with an improved resolution is conducted by the combination of the second-harmonic generation (SHG) of ultrasonic Lamb waves and the reconstruction algorithm for the probability inspection of damage. An intuitive model is first developed to explore the physical mechanism of a micro-defect-induced variation of generated second harmonic of a primary Lamb wave in a plate. Variations of amplitudes of second harmonics generated in propagation paths are used to construct the micro-defect image. A phase-reversal technique is employed to enhance the signal-to-noise ratio of the SHG. Comparisons between images constructed by linear and nonlinear acoustic features of Lamb wave propagation are presented. Results show that the image of the micro-defect with an improved resolution is successfully obtained by the proposed approach, while there is no visualized result obtained by the conventional linear ultrasonic one.

Micro-damages such as pores, closed delamination/debonding and fiber/matrix cracks in carbon fiber reinforced plastics (CFRP) are vital factors towards the performance of composite structures, which could collapse if defects are not detected in advance. Nonlinear ultrasonic technologies, especially ones involving guided waves, have drawn increasing attention for their better sensitivity to early damages than linear acoustic ones. The combination of nonlinear acoustics and guided waves technique can promisingly provide considerable accuracy and efficiency for damage assessment and materials characterization. Herein, numerical simulations in terms of finite element method are conducted to investigate the feasibility of micro-damage detection in multi-layered CFRP plates using the second harmonic generation (SHG) of asymmetric Lamb guided wave mode. Contact acoustic nonlinearity (CAN) is introduced into the constitutive model of micro-damages in composites, which leads to the distinct SHG compared with material nonlinearity. The results suggest that the generated second order harmonics due to CAN could be received and adopted for early damage evaluation without matching the phase of the primary waves.