Lamb and SH waves generated and detected by air-coupled ultrasonic transducers in composite material plates

Electrostatic, air-coupled, ultrasonic transducers are used to generate and detect guided waves in anisotropic solid plates. Waves considered in this study are Lamb-type and SH-type, guided modes. If the plane of propagation coincides with a plane of symmetry of the material, then Lamb modes only are launched and detected by the transducers. If the plane of propagation does not coincide with a plane of symmetry of the material, then Lamb modes are still generated and detected, but guided, SH-like modes are, too. The variation of phase velocity with frequency is measured for several modes propagating in different directions along a glass–epoxy composite plate.A numerical model that takes into account the anisotropy of composite materials is developed to predict the dispersion curves (phase velocity, group velocity or wave-number versus frequency) and the displacement fields of plate waves, the plane of propagation being either a plane of symmetry or not.The experimental phase velocities are in good agreement with the predicted dispersion curves, thus showing that the forward problem concerning the propagation of plate waves in anisotropic, homogeneous, composite material plates is properly solved. The dispersion curves associated with the predicted displacement fields show that guided modes in composite plates have different behaviors depending on their direction of propagation.     

Monitoring of the moisture content in carbon-epoxy plates using Lamb waves

Non-destructive material health monitoring is of great importance for aerospace industries, in particular for tanks made of composite materials, the mechanical properties of which can be altered by hydrothermal environment. In this context, the sensitivity of Lamb waves to various levels of moisture in a carbon-epoxy plate has been investigated. The plate has been manufactured and submitted to different cycles of hydrothermal aging and drying. For various steps of these cycles, both changes in weight and ultrasonic data are measured. Low-order A₀, S₀, S₁ Lamb modes are generated and detected using air-coupled transducers, and signal processing allows both real and imaginary parts of the wave-numbers to be measured. The real parts of the wave-numbers of all the generated modes are shown to be not sensitive to the moisture level of the material. However the imaginary part of the wave-number, i.e. the attenuation, of the A₀ mode is very sensitive to the moisture content, and its changes follow those of the plate weight, thus making it a promising mode for NDT of composite materials used in humid environments.     

Modelling of Lamb waves in composite laminated plates excited by interdigital transducers

The technique of permanently attaching interdigital transducers (IDT) to either flat or curved structural surfaces to excite single Lamb wave mode has demonstrated great potential for quantitative non-destructive evaluation and smart materials design. In this paper, the acoustic wave field in a composite laminated plate excited by an IDT is investigated. On the basis of discrete layer theory and a multiple integral transform method, an analytical–numerical approach is developed to evaluate the surface velocity response of the plate due to the IDTs excitation. In this approach, the frequency spectrum and wave number spectrum of the output of IDT are obtained directly. The corresponding time domain results are calculated by applying a standard inverse fast Fourier transformation technique. Numerical examples are presented to validate the developed method and show the ability of mode selection and isolation. A new effective way of transfer function estimation and interpretation is presented by considering the input wave number spectrum in addition to the commonly used input frequency spectrum. The new approach enables the simple physical evaluation of the influences of IDT geometrical features such as electrode finger widths and overall dimension and excitation signal properties on the input–output characteristics of IDT. Finally, considering the convenience of Mindlin plate wave theory in numerical computations as well as theoretical analysis, the validity is examined of using this approximate theory to design IDT for the excitation of the first and second anti-symmetric Lamb modes.     

Ultrasonic inspection of long steel pipes using Lamb waves

An ultrasonic inspection technique using Lamb waves was evaluated to detect and determine the exact location of flaws present in long steel pipes. Since multiple modes of Lamb waves are generated due to their dispersive characteristics in the inspected pipes, the selection of a specific Lamb wave mode is very important for inspection of flaws. Experimental studies of flaw detectability with the use of each Lamb wave mode, namely, the A₀ S₀, A₁, and S₁ modes and their ultrasonic attenuation characteristics were conducted. The experimental results showed that the A₀ mode is the most effective for detection and exact determination of the location flaws. A lucite wedge containing a water column that generates the A₀ Lamb wave mode was developed and used in the present inspection study. It was found that the ultrasonic beam divergence after wrapping around the inspected pipe once interferes with exact determination of the location of flaws and that the maximum reflection signals are obtained when the transducer is offset axially from the straight line with the position of the flaw. The present study showed feasibilities of ultrasonic inspection with the use of Lamb waves for detection of flaws in insulated or inaccessible steel pipes.     

FE modeling of Lamb mode diffraction by defects in anisotropic viscoelastic plates

A Finite Element code is used in a stationary mode to compute, at a single frequency, the stress and displacement fields in plates made of anisotropic, viscoelastic materials. An appropriated spatial load at one boundary of the plate is applied to generate a guided mode. This step is included in a frequency loop for which the number of iterations (≈ usually less than 50) is defined by the frequency spectrum of a temporal excitation. Then the temporal response at any location in the plate can be reconstructed with inverse Fourier transform. The phase velocity, attenuation and nature of one or several propagating modes can be identified in the classical frequency/wave number representation. The first example concerns the interaction of the symmetric Lamb mode S₀ with an opening notch in a Perspex plate. The proportion of S₀ and converted antisymmetric A₀ mode, transmitted past the notch is evaluated by the FE method and validated by a comparison with measurements made on a real system using an air coupled transducer. The second example shows the effect of the viscoelasticity on the propagation of the symmetric Lamb mode S₀ in a cross-ply, carbon-epoxy material plate. Focus is made on some difficulties that the attenuation causes on the detection of a delamination between plies.     

Comparison of Lamb waves and pulse echo in detection of near-surface defects in laminate plates

One of the problems faced in ultrasonic nondestructive testing (NDT) of composite laminates is near-surface delaminations which may be due to impact damage. The normal incidence pulse echo technique has difficulty in resolving echoes from this type of defect since they often lie within the length of the transmitted ultrasonic pulse. Although a high frequency probe may be used, it has its limitations and could be problematic in composite materials in which ply reflections may interfere with defect reflections. The use of an acoustic delay, together with a high frequency probe, may improve the detection. In this paper, a different approach has been developed using the fundamental Lamb wave (S₀ mode) to detect delaminations in unidirectional fibre composite materials. It has been found that the Lamb wave amplitude decreases significantly over a delamination region. The decrease in amplitude is strongly dependent upon the depth of the delamination and is most sensitive to the delaminations near to the surface of the laminate. By scanning the transducer over the surface, it is possible to measure the size and depth of this kind of delamination. This technique is comparable to the delayed pulse echo technique and can be used to complement other techniques.     

Measurement of the effects of rough surfaces on Lamb waves propagation

This work deals with the sensitivity to the plate roughness of Lamb waves. An experimental study is performed involving an air-coupling transducer system. Signal processing allows us to extract the Lamb waves characteristics: phase velocity and attenuation. Plate surface topographies are obtained by means of an optical surface profiler. The acoustic characteristics and the surface topographies are finally linked.     

钽钨合金薄板的兰姆波检测

采用超声兰姆波检测方法,和研制的1．25P15×20型38°入射角,大晶片专用换能器,对1．6mm（厚）×80mm（宽）的TaW合金板材实施了内部质量检测。试验表明,该超声换能器不但可以稳定检出1．6mm厚TaW板材内Ф0．4mm左右的缺陷,而且孔径与兰姆波反射幅值呈现出良好的线性关系,满足了生产中的质量控制要求。     

兰姆波质点振动位移对检测灵敏度的影响

为了探索兰姆波检测中敏感模式的选择与质点振动位移之间的关系,采用单发单收方式,对金属薄板中分层缺陷进行了兰姆波无损检测.运用时频分析方法,对有无缺陷时的时城信号进行了研究.结果表明,兰姆波检测的敏感性与其在板中的质点振动位移存在对应关系.     

Leaky Lamb waves for the detection and sizing of defects in bonded aluminium lap joints

The possibility of using ultrasonic leaky Lamb waves in a pitch and catch configuration for detecting and sizing defects in bonded aluminium lap joints is presented in this paper. Based in Transfer Matrix method the dispersion curves and the propagation modes behaviour are obtained. First the experimental scattering effects of circular holes with different dimensions on the fundamental symmetric mode S₀ are analysed and compared with the results obtained by analytical model. Very good agreement with the analytical model used is found. After, using the same methodology, scattering effects produced by artificial defects located in the bond line of lap joints are measured and the results show that the previous model could be used when an estimation of defect dimensions is needed. Finally an empirical relation between an experimental parameter and the defect dimensions is established revealing a high linear correlation meaning. So, this parameter could be used for more accurate defect dimensions evaluation.     

混合谱二维傅里叶变换法识别兰姆波模式

在分析二维傅里叶变换原理的基础上,将快速傅里叶变换（FFT）和自回归（AR）参数估计相结合,采用混合谱二维傅里叶变换方法,对铝板中的兰姆波模式进行识别。试验结果表明,与二维快速傅里叶变换相比,混合谱估计方法能精确检测多模式超声兰姆波相速度,对兰姆波模式识别力大大提高。混合谱二维傅里叶变换方法在超声导波检测研究中具有广泛的应用前景。     

复合材料Lamb波检测中S0单模式信号的提取

Lamb波的频散与多模式现象以及复合材料的各向异性限制了复合材料的应用研究。文章以玻璃纤维增强复合材料层合板为研究对象,利用图形法进行Lamb波频散方程求解,由此绘制频散曲线并分析了频散现象,计算了A0,S0的传播速度。搭建了试验系统,基于HHT变换,提取了信号特征并获取了信号的传播速度。结合A0,S0的速度理论值,实现了对Lamb波S0单模式的提取,由此简化了lamb波的使用,使之可广泛应用于复合材料的损伤检测。     

缝类缺陷的Lamb波检测

在ST12冷轧板上用线切割加工出不同深度的狭缝来模拟钢板中的缝类缺陷,采用一发一收方式,对缺陷进行Lamb波检测。利用短时傅里叶变换对有无缺陷时采集的时域信号进行了时频分析,有效识别出板中激发的Lamb波模式;将时间中心和频率中心作为数字参量,探讨了缝类缺陷对Lamb波的影响。结果表明,Lamb波信号时间中心的变化与缝类缺陷深度有较好的对应关系。     

一种求解兰姆波特征方程的方法

给出了兰姆波的原始特征方程式,以相速度为自变量,求解兰姆波特征方程。通过Matlab软件对求解过程进行编程,绘制出兰姆波在钢板中传播的频散曲线,以此作为进行无损检测的重要依据。     

Non-contact estimation of thickness and elastic properties of metallic foils by laser-generated Lamb waves

Non-contact estimation of the thickness and elastic properties of metallic foils was attempted by quantitative analysis of velocity dispersion of laser-generated Lamb waves. Lamb waves were generated in stainless steel (AISI304) foils with a thickness of less than 40 μm by a Q-switched Nd:YAG laser. Both the zeroth order symmetric S₀ and anti-symmetric A₀ waves were monitored using a heterodyne-type laser interferometer. Dispersion of group velocity of the A₀ mode was obtained by the wavelet transformation, and was found to agree well with the numerical solution of the Rayleigh–Lamb equation. A modified method to estimate both the thickness and acoustic (or elastic) properties from the sheet wave velocity and the group velocity dispersion of the A₀ mode was proposed. The modified method was found to provide a correct estimate for stainless steel foils thinner than 30 μm.     

一种优化激发电磁超声Lamb波的方法

超声Lamb波的频散特性和多模式特性导致回波信号复杂难于分辨。为了解决该问题,结合电磁超声换能器(EMAT)的主要设计参数和Lamb波的特征方程,推导电磁超声Lamb波的激发方程和激发曲线。通过选取电磁超声Lamb波在激发曲线上的工作点以削弱多模式和频散现象,进而提出了一种优化激发电磁超声Lamb波的方法。采用该方法分析了零模式Lamb波在工程应用中的有效性,并进行了试验验证。     

Lamb waves monitoring of the thermomechanical aging effects in ceramic coatings

Ceramic coatings are mainly used to protect underlying alloy structures, and cover a wide range of thermomechanical and electrochemical applications, especially in the energy management and related industries. The work described in this paper focuses on the use of the Acoustic Emission (AE) technique to produce a permanent Structural Health Monitoring (SHM) of various coating systems subject to thermal cycling conditions. SHM generally refers to those methods that provide the ability to detect and interpret adverse changes in a structure in order to improve their reliability. The AE consists of elastic wave that propagates in the material carrying the information of occurred and latent defects. Since the challenge in designing such a method is knowing what changes to look for and how to identify them, the emphasis in this particular paper is placed on the cumulative frequency analysis of such AE waves and their potential for determining damage to coatings prior to their final breakdown.     

Modal analysis of second-harmonic generation of generalized Lamb waves in layered planar structures

This research has developed an approach for studying second-harmonic generation of generalized Lamb waves (GLWs) in layered planar structures. On the basis of a modal analysis approach and second-order perturbation treatment, an effective theoretical model has been established. The material non-linearity can result in second-harmonic generation by primary GLWs propagating in layered structures, and the non-linearity is treated as a second-order perturbation of the elastic response of primary GLW propagation. There are second-order bulk and surface/interface driving sources in layered planar structures when primary GLWs propagate. These driving sources can be thought of as the forcing functions of a series of double frequency GLWs (DFGLWs) in terms of the approach of modal expansion analysis of guided waves. The total second-harmonic displacement fields consist of a series of DFGLWs in the stress-free layered structures. Thus, the complicated problem of second-harmonic generation of GLWs can be determined exactly within the second-order perturbation regime. Despite the strongly dispersive nature of primary GLWs in layered planar structures, it is found that the second harmonics of primary GLWs can grow with propagation distance along layered structures when the phase velocities of the DFGLWs are equal to those of the primary GLWs. Also, the formal solution of the second harmonics of the primary GLWs are presented. Numerical simulations were performed to understand the physical process of second-harmonic generation by primary GLW propagation in layered structures. The potential application of the present results to practical inspections of layered structures is discussed.     

金属薄板中缝类缺陷的兰姆波检测

在ST12冷轧板上用线切割加工出不同深度的狭缝来模拟钢板中缝类缺陷.采用一发一收和单发单收两种方式对缺陷进行兰姆波检测,利用短时傅里叶变换对有无缺陷时采集的时域信号进行了时频分析,有效识别出板中激发的兰姆波模式以及模式转换现象.进一步分析发现,试件中激发的主要兰姆波模式的能量分布中心与缺陷深度有较好的对应关系.