基于弯折频率变换的Lamb波高分辨率损伤成像

Lamb波在结构健康监测中受到广泛关注,但其在传播过程中存在着多模和频散特性,不利于损伤定位和高分辨率成像。弯折频率变换（Warped Frequency Transform, WFT）通过构建合理的弯折映射可实现对频率轴的弯折。基于Lamb波群速度频散曲线设计弯折频率变换,则可用于Lamb波信号的处理。本文从直接补偿角度出发,利用WFT对传感信号进行频散抑制。提出了基于WFT的高分辨率损伤成像方法,利用有限元软件ABAQUS进行了带损伤铝板中Lamb波传播的仿真。仿真结果表明WFT能有效压缩频散的波包,通过本文所提成像方法可实现高分辨率损伤成像。     

Guided wave expansion in warped curvelet frames

Lamb wave testing for structural health monitoring (SHM) often relies on analysis of wavefields recorded through scanning laser Doppler vibrometers (SLDVs) or ultrasonic scanners. Damage detection and characterization with these techniques requires isolation of defect-induced reflections in the wavefield from the injected wave packet and from scattering events associated with structural features such as boundaries, rivets, joints, etc. This is a challenging task when dealing with complex structures and multimodal, dispersive propagation regimes, whereby various wave contributions in both the time/space and the frequency/wavenumber domain overlap. A new mathematical tool named warped curvelet frames (WCFs) is proposed to effectively decompose the recorded wavefields. The presented technique results from the combination of two operators, i.e., the curvelet transform (CT) and the warped frequency transform (WFT). The CT provides an optimally sparse representation of nondispersive wave propagators. Combining the CT with the WFT allows for a flexible analysis of multimodal wave propagation in dispersive media. Exploiting the spatial and temporal localization of curvelets, as well as the spectro-temporal adaptation of the analysis frame to the characteristics of each propagating mode, provided by frequency warping, a convenient decomposition of guided waves is achieved and relevant contributions can be effectively isolated. The proposed approach is validated through dedicated simulations and further tested experimentally to demonstrate the effectiveness of the method in separating guided wave modes corresponding to acoustic events in close spatial proximity.     

海洋涡旋地震成像的时域有限差分仿真

传统的水声建模方法只考虑水中声波的折射影响,忽略了水层间的声反射,因此不能对反射波地震勘探的海水分层结构成像结果进行分析。文章首先建立了基于时域有限差分的低频宽带脉冲信号传输预报模型,该模型考虑海水分层结构的声反射影响,通过数值求解波动方程计算低频宽带脉冲信号近距离传播过程中的时域波形;其次将某次海上多道地震测量的水文条件作为输入环境参数,计算了近海面地震勘探接收阵所接收的气枪低频脉冲信号时域波形;最后按照反射波地震勘探处理流程,对接收阵信号在共反射点进行相干累加和地震成像,所得到的海水分层结构成像结果与海上多道测量成像结果符合较好。仿真结果验证了水层声反射对海水分层结构反射波地震成像的影响。     

Simulation of Acoustic Emission in Planar Carbon Fiber Reinforced Plastic Specimens

The simulation of acoustic emission waveforms resulting from failure during mechanical loading of carbon fiber reinforced plastic structures is investigated using a finite element simulation approach. For this investigation we focus on the dominant failure mechanisms in fiber reinforced structures consisting of matrix cracking, fiber breakage and fiber-matrix interface failure. To simulate the failure process accurately, we present a new acoustic emission source model that is based on the microscopic source geometry and micromechanical properties of fiber and resin. We demonstrate that based on this microscopic source model these failure mechanisms result in excitation of macroscopic plate waves. The propagation of these plate waves is described using a macroscopic three-dimensional model geometry which includes contributions of reflections from the specimen boundaries. We further present a model of the acoustic emission sensors used in experiments to simulate the influence of aperture effects. To enhance the understanding of correlation between macroscopically detectable acoustic emission signals and microscopic failure mechanisms we simulate the response to different source excitation times, crack surface displacements and displacement directions. The results obtained show good agreement with fundamental assumptions about the crack process reported by various other authors. The simulated acoustic emission signals obtained are compared to experimentally measured waveforms during four-point bending experiments of carbon fiber reinforced plastic structures. The simulated signals of fiber-breakage, matrix-cracking and fiber-matrix interface failure show systematic agreement with the respective experimental signals.     

A simplified broadband coupling approach applied to chemically robust sol-gel, planar integrated optical waveguides

A new generation waveguide spectrometer with broadband coupling capabilities has been developed. As opposed to previous devices, this attenuated total reflection (ATR) spectrometer is much simpler in design, is more chemically robust, and transmits light down to at least 400 nm. The attenuated total reflection element consists of a single-mode, planar integrated optical waveguide fabricated by dip-coating a approximately 300 nm thick, sol-gel composite layer on a glass substrate. A commercially available prism is used as the incoupler with an integral holographic diffraction grating acting as the dispersive outcoupling element. The transmission of narrow band-pass filters was used to compare the response of the waveguide spectrometer to that of a conventional transmission instrument. Spectral resolution was assessed by measuring the fwhm of various laser lines, which were found to range from 0.5 to 1.3 nm. The measured limits of detection for the waveguide spectrometer from 400 to 600 nm are 8.0 and 10.1 milliabsorbance units for TE and TM polarizations, respectively. Finally, to demonstrate the application of this technology to a molecular film confined to a solid-liquid interface, visible ATR spectra of an adsorbed submonolayer of horse heart cytochrome c were acquired. A procedure to correct the waveguide spectra for the wavelength dependence in ATR path length is described.     

Lamb wave tomography and its application in pipe erosion/corrosion monitoring

Ultrasonic Lamb wave techniques are widely used in a number of NDE applications. To excite Lamb waves, mode conversion of bulk waves or photoacoustic excitation often are used. Both of these approaches suffer from the need for liquid couplant or ablation of materials to reach a good signal-to-noise ratio. In this paper, we propose a novel technique that utilizes point source excitation and detection of Lamb waves through dry, elastic contacts to monitor velocity changes. A pair of pin transducers is used to excite and detect theA ₀ mode Lamb wave in the pipe wall, and the wave velocity is obtained by time-of-flight measurement. Any change in the pipe wall thickness can be detected by the change in the Lamb wave velocity due to the dispersive nature of theA ₀ mode. We demonstrate the power of this approach in ultrasonic pipe erosion/corrosion monitoring and its potential application in aircraft skin defect imaging. We present results of measurements of plate thickness and erosion/corrosion in a section of pipe that was removed from service, as well as imaging of defects in an aluminum thin plate.     

A Comparison of Q-Switched and Intensity Modulated Laser Pulses for Ultrasonic NDT

The purpose of this work was to investigate the potential of intensity modulated laser pulses for ultrasonic signal enhancement in NDT using a numerical model. Q-switched lasers produce broadband ultrasound which is susceptible to measurement noise. Intensity modulation can alleviate this problem by focussing the ultrasonic energy into a band centered around the modulation frequency. Ultrasonic waveforms and their DFTs are considered here in both aluminum and steel for a wide range of observation angles, for Q-switched laser pulses and for a 1 MHz intensity modulated laser pulse. These waveforms are useful to NDT researchers becauses they indicate what kinds of signals can be expected from unflawed materials. It is found that P waves are not as easily enhanced by modulation as are SV or Rayleigh surface waves. The time window which should be used to measure ultrasonic signals produced by intensity modulated laser pulses is determined.     

Photoacoustic tomography using a Mach-Zehnder interferometer as an acoustic line detector

A three-dimensional photoacoustic imaging method is presented that uses a Mach-Zehnder interferometer for measurement of acoustic waves generated in an object by irradiation with short laser pulses. The signals acquired with the interferometer correspond to line integrals over the acoustic wave field. An algorithm for reconstruction of a three-dimensional image from such signals measured at multiple positions around the object is shown that is a combination of a frequency-domain technique and the inverse Radon transform. From images of a small source scanning across the interferometer beam it is estimated that the spatial resolution of the imaging system is in the range of 100 to about 300 mum, depending on the interferometer beam width and the size of the aperture formed by the scan length divided by the source-detector distance. By taking an image of a phantom it could be shown that the imaging system in its present configuration is capable of producing three-dimensional images of objects with an overall size in the range of several millimeters to centimeters. Strategies are proposed how the technique can be scaled for imaging of smaller objects with higher resolution.     

Broadband generation of ultrasonic guided waves using piezoceramics and sub-band decomposition

Classically, damage detection or dispersion curve determination using piezoceramic-generated guided waves has been based on analysis of propagation properties of multiple narrowband excitation signals. However, dispersion and multimodal propagation impair the determination of propagation properties. More recently, it has been proposed to consider broadband excitations for both damage imaging and group velocity estimation. Among existing transducer technologies, although laser excitation is prone to practical limitations in terms of dimensions and generated amplitudes, it allows generation of noncontact, point-like broadband displacement. Thus, broadband generation of guided waves using piezoceramics can be envisioned. However, direct impulse response measurements are limited by the generated amplitude, leading to low SNR measurements. For this purpose, chirp excitations have been proposed using variable-frequency bursts, leading to phase and amplitude variations with respect to the frequency, such that this approach is not suitable for precise estimation of time of flight (ToF) or modal amplitude. In this paper, a sub-band decomposition technique that allows high-SNR measurements of impulse response in a given frequency range is proposed. Broadband excitation is decomposed over a given number of frequency sub-bands, generated by a piezoceramic element and measurement is performed using a laser Doppler vibrometer (LDV) or a piezoceramic sensor. Application to experimental estimation of group velocity and damage detection in pitchcatch configuration is proposed. It is shown that the proposed method allows damage estimation without a priori knowledge of the damage size, whereas narrowband techniques can fail at specific wavelengths.     

Time-reversed Lamb waves

Lamb waves are extensively involved in plate structure inspection because of their guided nature. However, their dispersive nature often limits their use in flaw detection. In this paper we show that the use of a time-reversal mirror (TRM) allows to automatically compensate for the dispersive nature of Lamb waves. Experiments showing the spatial and temporal behavior of time-reversed Lamb waves, demonstrate the ability of TRMs to self-focus and to recompress dispersive pulses. This is demonstrated in a set of experiments in which a broadband ultrasonic laser source is used to simulate a point Lamb wave source and an optical interferometer is used to map the time reversed elastic field. We also show that TRM may work in pulse echo mode and allows to detect and to focus along large 2-D plates on any flaws located in the inspected area.     

Two-dimensional photoacoustic imaging by use of Fourier-transform image reconstruction and a detector with an anisotropic response

Theoretical and experimental aspects of two-dimensional (2D) biomedical photoacoustic imaging have been investigated. A 2D Fourier-transform-based reconstruction algorithm that is significantly faster and produces fewer artifacts than simple radial backprojection methods is described. The image-reconstruction time for a 208 x 482 pixel image is approximately 1 s. For the practical implementation of 2D photoacoustic imaging, a rectangular detector geometry was used to obtain an anisotropic detection sensitivity in order to reject out-of-plane signals, thereby permitting a tomographic image slice to be reconstructed. This approach was investigated by the numerical modeling of the broadband directional response of a rectangular detector and imaging of various spatially calibrated absorbing targets immersed in a turbid phantom. The experimental setup was based on a Q-switched Nd:YAG excitation laser source and a mechanically line-scanned Fabry-Perot polymer-film ultrasound sensor. For a 800 microm x 200 microm rectangular detector, the reconstructed image slice thickness was 0.8 mm up to a vertical distance of z = 3.5 mm from the detector, increasing thereafter to 2 mm at z = 10 mm. Horizontal and vertical spatial resolutions within the reconstructed slice were approximately 200 and 60 microm, respectively.     

Wideband chaotic microwave signal generation in a ring system with a nonlinear delay line on coupled ferromagnetic films

Various regimes of chaotic signal generation have been studied in an self-oscillating ring system with a wideband nonlinear delay line based on coupled ferromagnetic films, in which magnetostatic backward volume waves (MSBVWs) are excited. It is shown for the first time that the simultaneous excitation of two waves in the coupled films leads to the appearance of a falling part in the dynamic characteristic of the MSBVW delay line and the generation of a broadband chaotic signal with a continuous spectrum. Under certain conditions, the ring exhibits a regime of intermittency, in which broadband chaotic signals with both continuous and discrete spectra are generated.     

Three-dimensional ultrasonic reflection and attenuation imaging

A three-dimensional, volumetric, data acquisition and reconstruction system for ultrasonic imaging is described. The method entails storing the digitized waveforms from each element of a multielement annular array, with bidirectional mechanical scanning. The system was implemented through digitization of the RF signals from each of the rings of an annular-array transducer, storage of the signals in computer memory, and offline image reconstruction. Initial reflection and reflex transmission images acquired with this system are presented. The results of different image reconstruction algorithms are demonstrated.     

Integrated planar terahertz resonators for femtomolar sensitivity label-free detection of DNA hybridization

A promising label-free approach for the analysis of genetic material by means of detecting the hybridization of polynucleotides with electromagnetic waves at terahertz (THz) frequencies is presented. Using an integrated waveguide approach, incorporating resonant THz structures as sample carriers and transducers for the analysis of the DNA molecules, we achieve a sensitivity down to femtomolar levels. The approach is demonstrated with time-domain ultrafast techniques based on femtosecond laser pulses for generating and electro-optically detecting broadband THz signals, although the principle can certainly be transferred to other THz technologies.     

Measurement of Broadband Temperature-Dependent Ultrasonic Attenuation and Dispersion Using Photoacoustics

The broadband ultrasonic characterization of biological fluids and tissues is important for the continued development and application of high-resolution ultrasound imaging modalities. Here, a photoacoustic technique for the transmission measurement of temperature-dependent ultrasonic attenuation and dispersion is described. The system uses a photoacoustic plane wave source constructed from a polymethylmethacrylate substrate with a thin optically absorbent layer. Broadband ultrasonic waves are generated by illuminating the absorbent layer with nanosecond pulses of laser light. The transmitted ultrasound waves are detected by a planar 7-μm high-finesse Fabry-Perot interferometer. Temperatureinduced thickness changes in the Fabry-Perot interferometer are tracked to monitor the sample temperature and maintain the sensor sensitivity. The measured ?6-dB bandwidth for the combined source and sensor is 1 to 35 MHz, with an attenuation corrected signal level at 100 MHz of ?10 dB. The system is demonstrated through temperature-dependent ultrasound measurements in castor oil and olive oil. Power law attenuation parameters are extracted by fitting the experimental attenuation data to a frequency power law while simultaneously fitting the dispersion data to the corresponding Kramers-Kr?nig relation. The extracted parameters are compared with other calibration measurements previously reported in the literature.     

基于波导不变量的双阵元被动探测方法研究

利用一种水声信道统计不变特征(波导不变量)实现被动探测。采用双阵元,分别对每一个阵元接收的宽带噪声信号进行LOFAR分析,对LOFAR图利用Hough变换等图像处理方法进行参数估计,得到波导不变量和曲线(文中讨论抛物线)参数估计值,进而运用几何关系估计出运动目标的航向角、速度和距离信息。利用Matlab仿真验证了该方法的正确性。     

Mid-infrared supercontinuum generation in tapered As2S3 chalcogenide planar waveguide

Abstract

We numerically demonstrate mid-infrared supercontinuum generation in a non-uniformly tapered chalcogenide planar waveguide. This planar rib waveguide of As₂S₃ glass on MgF₂ is 2 cm long with increasing etch depth longitudinally to manage the total dispersion. This waveguide has zero dispersion at two wavelengths. The dispersion profile varies along the propagation distance, leading to continuous modification of the phase-matching condition for dispersive wave emission and enhancement of energy transfer efficiency between solitons and dispersive waves. Numerical simulations are conducted for secant input pulses at a wavelength of 1.55 μm with a width of 50 fs and peak power of 2 kW. Results show this proposed scheme significantly broadens the generated continuum, extending from ~1 to ~7 μm.     

Frequency division transmission imaging and synthetic aperture reconstruction

In synthetic transmit aperture imaging only a few transducer elements are used in every transmission, which limits the signal-to-noise ratio (SNR). The penetration depth can be increased by using all transmitters in every transmission. In this paper, a method for exciting all transmitters in every transmission and separating them at the receiver is proposed. The coding is done by designing narrow-band linearly frequency modulated signals, which are approximately disjointed in the frequency domain and assigning one waveform to each transmitter. By designing a filterbank consisting of the matched filters corresponding to the excitation waveforms, the different transmitters can be decoded at the receiver. The matched filter of a specific waveform will allow information only from this waveform to pass through, thereby separating it from the other waveforms. This means that all transmitters can be used in every transmission, and the information from the different transmitters can be separated instantaneously. Compared to traditional synthetic transmit aperture (STA) imaging, in which the different transmitters are excited sequentially, more energy is transmitted in every transmission, and a better signal-to-noise-ratio is attained. The method has been tested in simulation, in which the resolution and contrast was compared to a standard synthetic transmit aperture system with a single sinusoid excitation. The resolution and contrast was comparable for the two systems. The method also has been tested using the experimental ultrasound scanner RASMUS. The resolution was evaluated using a string phantom. The method was compared to a conventional STA using both sinusoidal excitation and linear frequency modulated (FM) signals as excitation. The system using the FM signals and the frequency division approach yielded the same performance concerning both axial (of approximately equal to 3 wavelengths) and lateral resolution (of approximately equal to 4.5 wavelengths). A SNR measurement showed an increase in SNR of 6.5 dB compared to the system using the conventional STA method and FM signal excitation.     

Non-contact measurement of the mechanical properties of materials using an all-optical technique

Describes an optically-based measurement mechanism which realizes a totally noncontact assessment of the most important mechanical properties of structural materials - namely effective stiffness and Poisson ratio. These parameters are sensitive indicators of material integrity. The technique uses laser generated broadband ultrasound as a probe and interferometric optical detection as the detector again exploiting the broadband capability of optics in both space and time. Both detection and excitation systems are most conveniently realized in practical systems through optical fiber linkages. Observing the coupled waveforms between source and detector as a function of source: detector separation after a space : time Fourier transform yields a set of dispersion curves for the ultrasonic (typically Lamb wave) transfer function of the sample. This, in turn, can be inverted using curve fitting routines to obtain effective values of modulus and stiffness. An initial assessment of this inversion process is presented and demonstrates that the effective modulus can be extracted with a confidence level of better than a few percent with slightly larger errors in the Poisson ratio.     

Generation of narrowband antisymmetric lamb waves using a formed laser source in the ablative regime

A formed laser source, using a four-element lenticular array, is used in the ablative regime to generate select, narrowband, acoustic plate waves. The arrangement of the array produces acoustical signals that have frequencies compatible with the response of the broadband capacitive air-coupled transducer used in this study. A simplified concept is presented to explain the effect of a line array source on the frequency content of acoustic waves. The analytical model for a point pulse surface displacement is derived from the point load solution to Lamb's problem. The point pulse displacement elements of a line array source are summed mathematically, taking into account all applicable propagation modes and dispersion of plate waves. The model considers only the out-of-plane displacement of the antisymmetric plate modes to represent the detection capability of the broadband receiver. The distribution function of the laser beam energy profile is modified to depict the actual energy distribution that illuminates the surface of the plate. Filtering functions are made compatible with the sensitivity of the broadband receiver so as to retain only the detected frequencies in the model. The theoretical model showed good agreement with experimental results.