Generation and detection of higher-order mode clusters of guided waves (HOMC-GW) using meander-coil EMATs

This paper reports on a new means of generating higher-order mode clusters of guided waves (HOMC-GW) using a meander-coil (MC) electromagnetic acoustic transducer (EMAT) in plates at frequencies significantly higher than the lower-order plate modes. These wave modes are considerably less dispersive and they occur at much higher frequency-thickness (f x d) products. Our studies cover the f x d range of 13 to 20 MHz·mm. Experimental measurements were carried out on Al plate samples of different thicknesses using three different EMAT coil periods. To understand the generation and propagation characteristics of HOMC-GW with EMATs, several simulations were carried out using 2-D finite element models at different f x d products. These simulations captured all features observed in the experiments. The time-frequency smoothed pseudo Wigner-Ville distribution (SPWVD) was used to analyze the HOMC-GW modes. Defect detection measurements using HOMC-GW generated using EMATs were made on Al plates with machined defects.     

Effect of liftoff on accuracy of phase velocity measurements made with electromagnetic-acoustic transducers

Electromagnetic-acoustic transducers (EMATs) work on transduction principles which allow them to operate with a clearance (liftoff) between them and a conducting specimen. They have the potential for on-line ultrasonic measurements of rapidly moving materials.Liftoff causes changes in the effective inductance and resistance of the EMAT. Consequently, it causes a phase shift in the output voltage of a receiving EMAT. This can cause errors when the EMAT is used for velocity measurements.In this paper, we develop a model for the effect of liftoff. The model gives good agreement with measured liftoff-induced arrival time changes. The model can be extended to the case of an EMAT used with a voltage stepup transformer.The maximum signal is obtained for EMATs operating at resonance; however, the maximum sensitivity to liftoff also occurs then. Thus, a tradeoff must be made between optimum signal and suppressing liftoff artifacts. Our model, and experimental results, can be used to make these tradeoffs.Contribution of the National Institute of Standards and Technology; not subject to copyright.NIST Guest Researcher; on leave from ADA, Haifa, Israel.     

Quantitative modeling of the transduction of electromagnetic acoustic transducers operating on ferromagnetic media

The noncontact nature of electromagnetic acoustic transducers (EMATs) offers a series of advantages over traditional piezoelectric transducers, but these features are counter-balanced by their relatively low signal-to-noise ratio and their strong dependence on material properties such as electric conductivity, magnetic permeability, and magnetostriction. The implication is that full exploitation of EMATs needs detailed modeling of their operation. A finite element model, accounting for the main transduction mechanisms, has been developed to allow the optimization of the transducers. Magnetostriction is included and described through an analogy with piezoelectricity. The model is used to predict the performance of a simple EMAT: a single current-carrying wire, parallel to a bias magnetic field generating shear horizontal waves in a nickel plate close to it. The results are validated against experiments. The model is able to successfully predict the wave amplitude dependence on significant parameters: the static bias field, the driving current amplitude, and the excitation frequency. The comparison does not employ any arbitrary adjustable parameter; for the first time an absolute validation of a magnetostrictive EMAT model has been achieved. The results are satisfactory: the discrepancy between the numerical predictions and the measured values of wave amplitude per unit current is less than 20% over a 200 kHz frequency range. The study has also shown that magnetostrictive EMAT sensitivity is not only a function of the magnetostrictive properties, because the magnetic permeability also plays a significant role in the transduction mechanism, partly counterbalancing the magnetostrictive effects.     

Half-space radiation by EMATs

A Green's function calculation of the far-field radiation patterns of EMATs is presented. The approach is based on (a) closed form expressions for the eddy current and static magnetic field distributions, established by the EMATs, which react to produce the driving Lorentz forces, and (b) a Green's function derived from the steepest descent approximation to the far-field response of an arbitrary surface point force on a half space. Numerical results are presented, illustrating the radiation patterns of the three common EMAT designs. Included are vertically polarized shear waves as radiated by both meander coil and periodic magnet EMATs and horizontally polarized shear waves as radiated by the latter.     

Minimizing influence of multi-modes and dispersion of electromagnetic ultrasonic lamb waves

Electromagnetic ultrasonic (EMU) Lamb waves excited by electromagnetic acoustic transducers (EMATs) possess many advantages in NDT. However, their characteristic multi-modes and dispersion are disadvantageous for inspection and restrict further improvements in their real applications. By deducing the excitation equation of EMU Lamb waves, the primary design parameters of EMATs and the characteristic equation of Lamb waves are combined, and excitation curves based on the excitation equation are plotted to aid the design of EMATs. The excitation characteristic of EMU Lamb waves on different thickness of plates is analyzed according to the excitation curves. The influence of multi-modes of EMU Lamb waves is minimized by choosing reasonable operating points and operating zones to excite a single-mode Lamb wave or multi-mode Lamb waves with identical or approximate propagation velocities. The influence of dispersion is minimized by searching corresponding points whose slope of group velocity tends to zero. The validity of the proposed method is verified by experiments.     

电磁超声兰姆波换能器多目标优化设计

针对电磁超声兰姆波换能器激发的兰姆波存在多模式、频散现象和信号较弱的问题,结合铝合金板材检测背景,提出一种基于＂双交点法＂、＂零斜率准则＂和正交试验设计相结合的电磁超声兰姆波换能器多目标优化设计方法。其中,＂双交点法＂可有效削弱兰姆波多模式现象的影响,＂零斜率准则＂能够有效降低兰姆波的频散现象,而正交试验设计方法可有效提高电磁超声兰姆波信号的幅值。依据所提优化设计方法,对一个在铝板检测中常用的电磁超声兰姆波换能器的9个主要参数进行了多目标优化设计。实验表明,优化后,兰姆波信号中的多模式、频散现象得到显著抑制,而且信号幅值得到明显提升,有效改善了电磁超声兰姆波换能器的工程实用性。     

On-line measurement of steel sheetr-value using magnetostrictive-type EMAT

An ultrasonic on-line system to measurer values in cold rolled steel sheets has been developed with electromagnetic acoustic transducers (EMATs). These EMATs are composed of meanderline coils and electromagnets and operate with the magnetostrictive mechanism. The EMAT instrument measures the propagation times of the fundamental symmetrical Lamb (S _o ) wave at a low frequency and relates them to ther values through a calibrated regression curve. Preliminary tests indicate that the on-line monitoring ofr is quite feasible with a standard deviation of 0.07 for various low carbon steel sheets; the thicknesses range from 0.5 mm to 2.5 mm. The measuring time is 20 msec per data. The liftoff is allowed to 5 mm with 2 mm thick sheet. Ther evaluation is independent of the line speed up to 325 m/min. These promising results promote installation of ultrasonicr-value measurement systems in steel production lines.     

Numerical simulations of an electromagnetic acoustictransducer-receiver system for NDT applications

The development of a multistage numerical model of an electromagnetic acoustic transducer (EMAT), with particular emphasis on an EMAT receiver, is presented. The model includes five separate modeling states: static magnetic field simulation of an electromagnet; pulsed eddy current distribution of a generic meander-line coil suspended over a conducting specimen; Lorentz force distribution due to the interaction of the static magnetic field with the eddy current distributions; acoustic wave generation and propagation based on the dynamic Lorentz forces; and acoustic wave detection by an EMAT receiver. In particular, it is shown how the transient particle displacement fields are converted into an induced voltage response as part of the EMAT receiver system. Numerical simulations show that the voltage response is dependent on the wire spacing of the receiver coil     

SH电磁超声换能器优化设计及粘接强度测量

电磁超声换能器(Electromagnetic Acoustic Transducer,EMAT)使用时无需耦合剂，可以便捷地应用于严苛工况下结构的超声检测。但由于EMAT复杂的多物理场换能机理，通常存在换能效率低，接收信号信噪比小等缺点。针对上述问题，文章开展了线圈宽度、线圈间距及磁致伸缩贴片等参数对EMAT换能效率的影响研究，优化设计了SH电磁超声换能器。实验结果表明，一定厚度的磁致伸缩贴片对EMAT的换能效率有较明显的提高。基于SH0导波对界面变化的敏感性，采用优化设计的EMAT激发SH0导波，对固化温度等因素引起的多层铝板弱粘接结构件进行粘接性能的检测，实验测得的粘接强度与结构拉伸强度的变化趋势一致，表明优化后的EMAT可以适用于粘接结构状态的检测。     

The interaction of Lamb waves with defects

The interaction of individual Lamb waves with a variety of defects simulated by notches is investigated using finite-element analysis, and the results are checked experimentally. Excellent agreement is obtained. It is shown that a 2-D Fourier transform method may be used to quantify Lamb wave interactions with defects. The sensitivity of individual Lamb waves to particular notches is dependent on the frequency-thickness product, the mode type and order, and the geometry of the notch. The sensitivity of the Lamb modes a(1), alpha(0), and s(0) to simulated defects in different frequency-thickness regions is predicted as a function of the defect depth to plate thickness ratio and the results indicate that Lamb waves may be used to find notches when the wavelength to notch depth ratio is on the order of 40. Transmission ratios of Lamb waves across defects are highly frequency dependent.     

基于COMSOL连铸坯壳测厚横波EMAT优化

为了研究电磁超声传感器(Electromagnetic Acoustic Transducer,EMAT)横波测量连铸坯壳厚度的机理及横波在连铸坯中的传播情况,选取坯壳厚度为10~50 mm的Q235小方坯为研究对象,利用有限元软件COMSOL建立脉冲电磁铁和螺旋线圈的电磁超声模型,分析在不同EMAT结构参数下,连铸坯中电磁场、力场、声场的分布规律。研究结果表明:脉冲电磁铁和螺旋线圈组成的EMAT能够在连铸坯壳集肤层激发出超声波横波。脉冲电磁铁空心螺线管线圈匝数、内半径、线圈导线半径对换能效率的影响依次减小,且当脉冲电磁铁内半径尺寸大于螺旋线圈尺寸时,产生横波的效率最高。坯壳厚度越小,螺旋线圈最优激励频率越大,测量精度越高,信号衰减越快。因此,坯壳厚度为10~50 mm的Q235小方坯选择1.1 MHz为最佳激励频率。     

A novel time-domain method of analysis of pulsed sine wave signals

Sine wave packs are used in the nondestructive evaluation of materials, most commonly in the form of ultrasonic waves. An example of such methods is the use of electromagnetic acoustic transducers (EMATs) in the evaluation of metallic structures. Reflected EMAT signals are often highly polluted by noise. Elimination of noise and extraction of peak amplitude are important signal processing tasks associated with the analysis of EMAT signals. This paper presents a method of noise elimination and information extraction for pulsed sinusoids. The functionality of the proposed method is exemplified through noise reduction and peak detection of EMAT signals. The proposed method offers a simple and robust technique of signal analysis which is suitable for real-time industrial applications since it requires a relatively low level of computational resources.     

电磁超声单向表面波对铝板微小缺陷的检测

电磁超声表面波被广泛用来检测表面或近表面缺陷。双向表面波电磁超声换能器(ElectromagneticAcoustic Transducer, EMAT)会在两侧同时产生能量较低且均衡的超声波,而微小缺陷(缺陷深度远小于表面波波长)的反射信号非常微弱,易被噪声淹没,根据回波信号,难以识别和定位缺陷。为此基于惠更斯叠加原理设计了单向表面波EMAT,对其声场进行了有限元分析;研究了增强侧表面波遇到不同缺陷的响应特性,得出缺陷深度、角度与反射波幅值的关系;并对含不同微小缺陷的铝板进行了实验研究。仿真和实验结果表明,所提方法提高了表面波检测微小缺陷的灵敏度,并实现了缺陷位置及深度的量化。     

Optimization design of a Lamb wave device for density sensing of nonviscous liquid

A Lamb wave device composed of a piezoelectric plate loaded with a nonviscous liquid layer is presented. The relation between the Lamb wave phase velocity and the liquid density can be used for liquid density sensing. In this paper, utilizing the partial wave theory, the concept of effective permittivity is introduced to analyze the Lamb wave's excitation and the phase velocity calculation under a certain liquid density. The interface between the Lamb wave device and the liquid layer is metallized to eliminate the influence of liquid electrical properties when sensing liquid density. Based on the theory model, the phase difference measurement method is adopted to study the device's sensitivity to liquid density. In order to achieve high sensitivity to liquid density with sufficient excitation efficiency of Lamb wave, the optimal parameters of the Lamb wave device including plate thickness and cut orientation are obtained by numerical calculation. The experimental results are found to be in agreement with the theoretical simulations, verifying the validity of the theory model and the practicability of the optimization design.     

Omni-directional guided wave transducer arrays for the rapid inspection of large areas of plate structures

Omni-directional guided wave array transducers contain a circular pattern of elements that individually behave as omni-directional point transmitters or receivers. The data set acquired from such an array contains time-domain signals from each permutation of transmitter and receiver. A phased addition algorithm is developed that allows an omni-directional, B-scan image of the surrounding plate to be synthesized from any geometry of array. Numerically simulated data from a single reflector is used to test the performance of the algorithm. The results from an array containing a fully populated circular area of elements (Type I array) are found to be good, but those from an array containing a single ring of elements (Type II array) contain many large side-lobes. An enhancement to the basic phased addition algorithm is presented that uses deconvolution to suppress these side-lobes. The deconvolution algorithm enables a Type II array to equal the performance of a Type I array of the same overall diameter. The effect of diameter on angular resolution is investigated. Experimental data obtained from a guided wave array containing electromagnetic acoustic transducers (EMAT) elements for exciting and detecting the S/sub 0/ Lamb wave mode in a 5-mm thick aluminium plate are processed with both algorithms and the results are discussed.     

Omnidirectional guided wave inspection of large metallic plate structures using an EMAT array

The design of an electromagnetic acoustic transducer (EMAT) array device for the inspection of large areas of metallic plate-like structures using the S0 guided wave mode is described. The reasons for using the S0 mode are discussed and it is shown how the choice of mode determines the nature of the EMAT array elements. A novel array construction technique is shown to be necessary whereby the EMAT coils for adjacent elements are overlapped in order to achieve the required element density. Results are presented that illustrate the operation of the device on steel and aluminum plate specimens in the thickness range from 5 to 10 mm. An area of at least 10 m2 can be inspected from a single location. Spurious signals in the results are caused both by the unwanted A0 mode and by S0 sidelobes, the latter occurring at the same radial distance from the array as the genuine S0 signal from a reflector, but in the wrong direction. The signal-to-coherent noise performance of the complete system is determined by the amplitude ratio of the largest genuine S0 signal to the largest spurious signal. This is typically around 30 dB. The sensitivity of the device to artificial defects and genuine corrosion patches is demonstrated and the limitations of its operation are discussed. The feasibility of using the device with the S1 guided wave mode to inspect a 20 mm thick plate is also demonstrated.     

Apparent texture symmetry deviations in aluminum sheet

An ultrasonic texture measurement system for sheet metal is being developed using rotating electromagnetic acoustic transducers (EMATs). We report on further investigations of deviation from theoretically predicted symmetries in the elastic constants (as measured ultrasonically using the aforementioned system) of cold-rolled aluminum sheet reported in earlier publications. A study of the effects of annealing and deliberate deformation (both elastic and plastic) are used to develop an explanation of the nature and likely origin of this asymmetry. These deviations from symmetry cannot be detected by ultrasonic Lamb wave measurement in three directions alone. Texture asymmetry is relevant to the sheet metal manufacturer as it affects formability and may indicate processing problems. Results indicate that the asymmetry in the measured ultrasonic Lamb wave velocities on either side of the rolling direction is due to a stress effect rather than crystallographic     

基于超声导波检测的角钢型材缺陷特性分析

输电线路铁塔长期暴露在大气环境之中受到腐蚀,会造成角钢型材不同位置出现壁厚减小的情况,严重时可能导致铁塔断裂倒塌。为此,对角钢型材内壁锈蚀情况的检测技术进行探究。首先,对超声兰姆波的传播机理进行分析,确定角钢型材超声兰姆波的检测模态;然后,基于有限元理论对角钢型材进行三维数值建模,通过数值模拟探究了导波在钢材中的传播特性,分析了不同激励信号频率下角钢型材端面回波的波形特征;最后,通过对不同位置、不同深度腐蚀缺陷的角钢型材进行数值模拟,得到不同缺陷状态下超声兰姆波在角钢型材中的传播特性。研究结果表明,所提检测方法能够有效检测出输电线路铁塔的缺陷,为实际应用提供参考依据。     

Dispersion relations and modes of wave propagation in inclusion-reinforced composite plates

This paper is intended to examine the effect of inclusion shapes, inclusion contents, inclusion elastic constants, and plate thickness on the dispersion relations and modes of wave propagation in inclusion-reinforced composite plates. The shape of inclusion is modeled as spheroid that enables the composite reinforcement geometrical configurations ranging from sphere to short and continuous fiber. Mori–Tanaka mean-field theory is used to predict the effective elastic moduli of the composite plate explicitly. The effective elastic moduli are able to elucidate the effect of inclusion’s shape, stiffness, and volume fraction on the composite’s anisotropic elastic behavior. The resulting moduli are then used to determine the dispersion relations and the modal patterns of Lamb waves using the dynamic stiffness matrix method. The types (symmetric or antisymmetric) of Lamb waves in an isotropic plate can be classified according to the wave motions are symmetrical or antisymmetric about the midplane of the plate. Classifying the wave type in an anisotropic plate is not as simple as that in an isotropic plate, and has not received proper attention in the literature. The wave types and orders are identified by analyzing the dispersion curves and inspecting the calculated modal patterns, and the results indicate that the Lamb waves in an orthotropic composite plate can also be classified as either symmetric or antisymmetric waves. It is also found that the inclusion contents, aspect ratios and plate thickness affect propagation velocities, higher-order mode cutoff frequencies, and modal patterns. Propagation speed is generally increased with the aspect ratio, e.g., using longer fibers generally results in a higher propagation speed.     

Laser generation of Lamb waves for defect detection: experimental methods and finite element modeling

The propagation of Lamb waves generated by a pulsed laser beam in an aluminum sheet is modeled using finite element analysis, and the interaction with defects is studied and compared to experimental results. The ultrasonic Lamb waves are detected by an electromagnetic acoustic transducer (EMAT). The frequency content of the received wave is shown to be enhanced when the generation point is situated directly over the defect in both the modeled and experimental cases. Time-frequency analysis using a Wigner transform has enabled individual modes to be identified.