Synthetic aperture focusing for defect reconstruction in anisotropic media

Ultrasonic inspection plays an important role in numerous industrial fields. One of the prominent tasks with respect to quantitative nondestructive evaluation is the determination of location, shape, size and orientation of defects. In this respect, the synthetic aperture focusing technique (SAFT) has been successfully applied to isotropic materials over the years. In anisotropic media, however, its application suffers from several phenomena, which are the direction dependence of the ultrasonic velocities, the beam skewing effect and the modified transducer radiation characteristics. In this article, a SAFT imaging algorithm is presented which fully accounts for the nature of wave radiation and propagation within anisotropic materials. For three-dimensional defect reconstruction, the spatial dependence of the ultrasonic group velocities as well as the radiation characteristics of the transducer are exploited--respective algorithms have been implemented for orthotropic material symmetry. Tests have been performed on unidirectional composite material.     

基于虚拟源技术的双层固体结构超声成像

本文提出虚拟源与合成孔径技术相结合的成像方法来实现对双层结构的内部缺陷成像。通过相控阵探头延时聚焦的方法在界面形成虚拟声源,并对有机玻璃/钢双层结构中的聚焦声场进行了仿真,结果显示聚焦声束穿过界面后形成扩散球面波,适合采用合成孔径聚焦技术成像。实验中采用128阵元探头对含有横穿孔缺陷的有机玻璃/钢样品进行测量和波形采集,应用自适应滤波方法抑制界面回波,最后对波形使用合成孔径聚焦成像,得到的图像比B扫描成像具有较高的信噪比和横向分辨率,并且分辨率不随深度变化。     

A Green's function method for surface acoustic waves in functionally graded materials

Acoustic wave propagation in anisotropic media with one-dimensional inhomogeneity is discussed. Using a Green's function approach, the wave equation with inhomogeneous variation of elastic property and mass density is transformed into an integral equation, which is then solved numerically. The method is applied to find the dispersion relation of surface acoustic waves for a medium with continuously or discontinuously varying elastic property and mass density profiles.     

Analytical methods for modeling of ultrasonic nondestructive testing of anisotropic media

Many modern structural materials exhibit anisotropic elastic behavior leading to complicated wave propagation phenomena. To ensure the reliability of ultrasonic nondestructive testing techniques, these material properties as well as the influence of microstructural inhomogeneities and the effects of interfaces on ultrasonic wave propagation have to be taken into account. In this respect, mathematical modeling provides an efficient method of assisting analysis. Two computationally efficient analytical approaches--a Gaussian beam and a point source superposition technique--are presented, which are well-suited for performing ultrasonic wave propagation and scattering simulations for anisotropic media. Results for homogeneous as well as inhomogeneous anisotropic media like composites and weld material are presented.     

基于模式分离的兰姆波逆时偏移成像

应用频率域逆时偏移方法实现各向同性和各向异性板中缺陷的兰姆波成像.由于缺陷引起的多模态散射信号会在重建图像中形成伪像,根据基本导波模式振动对称性的差别进行了模式分离预处理.基于多元阵列超声技术,开展了铝板和复合板内缺陷频率域逆时偏移超声成像方法的数值仿真研究.首先,建立有限元模型,采用环形传感器数值采集由缺陷引起的兰姆波散射信号,然后,将采集到的多模式散射信号进行模式分离处理,再将模式分离后的兰姆波散射信号经时间反转后并在相应的接收器处重新激励,在频域中运用格林函数反向传播兰姆波散射信号,获取监测区域的声场信息,与正向传播声场进行互相关,重建缺陷图像.首先对铝板中单缺陷以及复合材料板中相邻的两个相同缺陷进行数值仿真,对比有无模式分离处理的缺陷逆时偏移成像效果,体现出模式分离的重要性.在此基础上,采用逆时偏移方法对复合板材内位置邻近、深度不同的双缺陷进行识别.数值结果表明,模式分离预处理后的缺陷重建图像能够有效去除多模式干扰产生的伪像.文中提出的成像方法对各向同性板和各向异性板内缺陷的检测和成像具有很好的发展潜力,可以准确地探测多个缺陷的形状、尺寸和深度.     

Laser ultrasonic propagation imaging method in the frequency domain based on wavelet transformation

A wavelet-transformed ultrasonic propagation imaging method capable of ultrasonic propagation imaging in the frequency domain was developed and applied as a new structural damage or flaw visualization algorithm. Since the wavelet-transformed ultrasonic propagation imaging method has strong frequency selectivity, it can visualize the propagation of ultrasonic waves of a specific frequency (for example, to isolate ultrasonic mode of interest and a damage-related ultrasonic wave). The strong frequency selectivity of the wavelet-transformed ultrasonic propagation imaging method was demonstrated, isolating only the zeroth-order asymmetrical mode of the fundamental Lamb wave modes in an anisotropic carbon fiber-reinforced plastic plate with a thickness of 5 mm. The wavelet-transformed ultrasonic propagation imaging method can also convert a complex time domain multiple wavefield into a simple frequency domain single wavefield. This feature enables easy interpretation of the results, and facilitates the precise evaluation of the location and size of structural damage or flaws. We demonstrated this capability by detecting a disbond in a sandwich structure made of Al-alloy skins and a foam core. A disbond with a diameter of 20 mm, which is representative of a common manufacturing flaw, was successfully detected, localized, and evaluated. Since a method to determine the allowable maximum pulse repetition frequency depending on target materials and structures was found by investigating the residual wave caused from the previous laser impinging, our laser ultrasonic system can scan rapidly the target with an optimal pulse repetition rate. In addition, the proposed wavelet-transformed ultrasonic propagation imaging method can visualize damage or flaw without the need for reference data from the intact state of the structure. Hence, we propose the wavelet-transformed ultrasonic propagation imaging approach for automatic inspection of in-service engineering structures, or in-process quality inspection in manufacturing.     

Band gap behaviours of periodic magnetoelectroelastic composite structures with Kagome lattices

In this paper, the elastic wave propagation in periodic cylinder magnetoelectroelastic composite structures is studied using the plane wave expansion method. The band structure characteristics of magnetoelectroelastic rods embedded in polymer matrix and the reverse case are investigated taking the electric, magnetic and mechanical coupling effects into account. The generalised eigenvalue equation is derived to analyse the in-plane and out-of-plane modes, respectively. The numerical calculations for both the cases with Kagome lattices are performed. The relation between the gap widths and filling fractions are discussed in detail. The effects of the magnetoelectricity on the band structures and widths of band gaps are analysed. The band gap characteristics are illustrated further and the results will be helpful to design such kind of composite structures.     

The form-invariance of wave equations without requiring a priori relations between field variables

According to the principle of relativity,the equations describing the laws of physics should have the same forms in all admissible frames of reference,i.e.,form-invariance is an intrinsic property of correct wave equations.However,so far in the design of metamaterials by transformation methods,the form-invariance is always proved by using certain relations between field variables before and after coordinate transformation.The main contribution of this paper is to give general proofs of form-invariance of electromagnetic,sound and elastic wave equations in the global Cartesian coordinate system without using any assumption of the relation between field variables.The results show that electromagnetic wave equations and sound wave equations are intrinsically form-invariant,but traditional elastodynamic equations are not.As a by-product,one can naturally obtain new elastodynamic equations in the time domain that are locally accurate to describe the elastic wave propagation in inhomogeneous media.The validity of these new equations is demonstrated by some numerical simulations of a perfect elastic wave rotator and an approximate elastic wave cloak.These findings are important for solving inverse scattering problems in many fields such as seismology,nondestructive evaluation and metamaterials.     

各向异性渗流条件下弹性波的传播特征

研究了弹性波在非均匀裂纹孔隙介质中的传播特性,建立了各向异性喷射流模型.当弹性波通过裂纹孔隙介质时,由于波的扰动及裂纹和孔隙几何结构的不一致,导致在裂纹内部及裂纹与周边孔隙之间同时存在着流体压力梯度.此时的弹性波波动响应中包含着裂纹内连通性特征和背景孔隙渗透率信息.流体的动态流动过程使得介质的等效弹性参数为复数(非完全弹性),并且具有频率依赖性.当弹性波为低频和高频极限时,介质为完全弹性;当处于中间频段时,波有衰减和频率依赖.裂纹孔隙介质的各向异性连通性(渗透率)对应着各向异性特征频率(当渗流长度等于非均匀尺度时的弹性波频率),波的传播受到裂纹内连通性的影响.在一定频段内,随着裂纹厚度的增加,将出现第二峰值,峰值大小同时受到裂纹厚度和半径的影响.     

Laser ultrasonic anomalous wave propagation imaging method with adjacent wave subtraction: Algorithm

The anomalous wave propagation imaging (AWPI) method is proposed for the laser ultrasonic propagation imaging system using a Q-switched laser and a laser mirror scanner to highlight the anomalies in complex structures. The AWPI algorithm was developed based on the observation that the waves from two adjacent scanning points are very similar, and that the propagation direction of the incident wave is different from that of the anomalous wave caused by structural anomalies including damage. The structural anomaly is highlighted by suppressing the incident waves and exaggerating the anomalous wave through adjacent waves subtraction after arrival time and amplitude matching. The variable time window amplitude mapping (VTWAM) method was also developed, based on the difference in arrival time between the residual incident wave and anomalous waves. The VTWAM method enhances anomaly visualization and sizing, notably for composite damages, by mapping the amplitudes of the confining wave within the damage. Our results showed that the AWPI increased the signal-to-noise ratio of a back-surface hole damage in a steel plate by 13.76 dB, while in another inspection of a composite wing with two impact damages, the AWPI results enhanced by the VTWAM compared favorably with the results of the immersion ultrasonic C-scan. The AWPI and VTWAM adopt implicit spatial referencing wherein all necessary data can be obtained through a single-time scan, therefore circumvent the disadvantages of conventional temporal baseline referencing.     

A Wave Automaton for Wave Propagation in Inhomogeneous Anisotropic Media

This paper presents an extension built on a hexagonal grid of the wave automaton, which was introduced in past few years for describing wave propagation in inhomogeneous media. This new method is capable of computing wave propagation in 2D anisotropic media without the need for introducing interpolating schemes. After a comparison of isotropic single scattering with analytical results using Mie theory, the method is used to compute the field scattered by one anisotropic particle for various orientations of its principal axes. Scattering by a collection of anisotropic particles is also presented.     

Frequency modulation in the transmittivity of wave guides in elastic-wave band-gap materials

It is shown, for the first time, that the transmittivity of wave guides created as rectilinear defects in periodic elastic band-gap materials oscillates as a function of frequency. The results are obtained using the finite difference time domain method for elastic waves propagating in two-dimensional inhomogeneous media. The oscillations of the transmittivity are due to the richness of modes in the elastic systems and, mainly, due to the periodicity of the potential in the direction of the wave propagation. Results are presented for a periodic array of Pb and Ag cylinders inserted in an epoxy host, as well as for Hg cylinders in an Al host.     

Guided waves in anisotropic and quasi-isotropic aerospace composites: Three-dimensional simulation and experiment

Three-dimensional (3D) elastic wave simulations can be used to investigate and optimize nondestructive evaluation (NDE) and structural health monitoring (SHM) ultrasonic damage detection techniques for aerospace materials. 3D anisotropic elastodynamic finite integration technique (EFIT) has been implemented for ultrasonic waves in carbon fiber reinforced polymer (CFRP) composite laminates. This paper describes 3D EFIT simulations of guided wave propagation in undamaged and damaged anisotropic and quasi-isotropic composite plates. Comparisons are made between simulations of guided waves in undamaged anisotropic composite plates and both experimental laser Doppler vibrometer (LDV) wavefield data and dispersion curves. Time domain and wavenumber domain comparisons are described. Wave interaction with complex geometry delamination damage is then simulated to investigate how simulation tools incorporating realistic damage geometries can aid in the understanding of wave interaction with CFRP damage. In order to move beyond simplistic assumptions of damage geometry, volumetric delamination data acquired via X-ray microfocus computed tomography is directly incorporated into the simulation. Simulated guided wave interaction with the complex geometry delamination is compared to experimental LDV time domain data and 3D wave interaction with the volumetric damage is discussed.     

Propagation of nonlinear longitudinal waves in a solid with regard to the interaction between the strain field and the field of defects

A model of nonlinear longitudinal wave propagation in a solid with quadratic nonlinearity of an elastic continuum exposed to laser impulses is developed in view of the interaction between the strain field and the field of point defects. The influence of the generation and recombination of laser-induced defects on the propagation of an elastic strain wave is analyzed. The existence of a nonlinear elastic shock wave of low intensity is revealed in the system and its structure is studied. The estimations of the depth and velocity of the wave front are performed. The contributions due to the interaction of the strain field and the field of defects to both a linear elastic modulus and the dispersion parameters of a lattice are found.     

热粘弹波在变温非均匀合金熔体中的传播

机械波在金属凝固过程中传播的定量计算一直是一个难题,主要原因就是在这个过程中的熔体结构非常复杂.本研究考虑到熔体的变温、非均匀和粘弹性的特点,采用Kelvin粘弹性介质模型,建立了具有粘热损失特性的热粘弹性波动方程,通过隐式有限差分方法对波动方程进行求解,并以ZL203A合金熔体为研究对象,探究了热粘弹波在变温非均匀介质中的传播规律.结果表明:热粘弹波从合金熔体的低温区向高温区传播时,非均匀的温度场对波的传播有较大影响;相反,当波从合金熔体的高温区向低温区传播时,非均匀的温度场对波的传播几乎没有影响.热粘弹波在合金熔体中的衰减系数随频率的增大呈线性增大,而随温度的升高先增大后减小,在熔体的枝晶搭接温度附近达到最大值.     

Spatiotemporal mapping of surface acoustic waves in isotropic and anisotropic materials

We demonstrate a new method for a real-time imaging of surface acoustic waves at frequencies up to 1 GHz with picosecond temporal and micron spatial resolutions using an ultrafast optical pump and probe technique combined with a common path interferometer. Using samples with isotropic or anisotropic substrates coated with metallic thin films, we observe the propagation of Rayleigh-like modes and surface-skimming bulk modes as well as resolving surface phonon focusing effects. In addition we image surface acoustic wave propagation in a laterally inhomogeneous sample.     

电导率各向异性金属薄板感应式磁声图像的仿真

为了给金属薄板感应式磁声(MAT-MI)成像算法的研究提供数据源,提出一种电导率各向异性金属薄板表面MAT-MI图像的数值仿真方法。建立含缺陷的电导率各向异性金属薄板仿真模型,并将其置于静磁场中。将通入交变电流的折线线圈置于金属薄板上方,对金属薄板在静磁场和交变磁场共同作用下产生的感应涡流以及声源(即洛伦兹力)进行数值仿真,得到金属薄板表面波位移分布的灰度图像。仿真实验结果表明,根据表面波位移在缺陷处迅速衰减的特性,可从图像中准确地识别并定位金属薄板表面的缺陷。忽略金属材料的电导率各向异性会降低成像质量,进而导致对缺陷的误判。通过提高表面波位移信号的信噪比可改善成像质量。减小提高距离或增大激励电流频率,可提高系统对微小缺陷和不规则缺陷检测的分辨力。      

固/固型二维正方晶格声子晶体缺陷态研究*

固/固型声子晶体在抑制噪音和隔离振动等工程领域有着潜在的应用。利用有限元方法对存在缺陷的二维正方晶格金/环氧树脂声子晶体进行了研究,数值计算结果表明弹性波在点缺陷处局域,带隙中出现多条缺陷模,点缺陷数目的增加对声子晶体局域特性产生显著影响;弹性波沿着线缺陷传播形成波导,改变线缺陷结构可以改变弹性波传播方向和实现信号的分离。对声子晶体缺陷特性的研究可为声学滤波器和波导等器件的设计提供参考。