Approach to Lamb Wave Lateral Crack Quantification in Elastic Plate Based on Reflection and Transmission Coefficients Surfaces

Ultrasonic guided wave is promising in structure integrity management, but there is still much to learn about it because of its complex mechanism. Defect sizing or quantification is very important for structure inspection using ultrasonic guided waves, and this inverse problem is also very difficult to solve. The work presented in this article aims to resolve the lateral crack quantification problem in the inspection of thin elastic plate using Lamb wave. First, two-dimensional (2D) boundary element method (BEM) simulation was used to study the interaction of one chosen pure mode Lamb wave with lateral crack defect, and this could be defined as the forward problem. Then reflection and transmission coefficients surfaces were built from the results of the numerical simulation. Finally, an approach for lateral crack quantification based on these reflection and transmission coefficients surfaces was proposed. The quantification examples show that the presented approach gives acceptable accuracy. Some limitations of this approach are also discussed.     

Lamb wave scattering analysis for reflector characterization

The potential use of guided waves for defect characterization is studied. The influence of defect shape and size on transmitted and reflected fields is considered. Using the hybrid boundary element technique, the reflection and transmission coefficients for selected guided wave modes are numerically calculated and compared to experimental data. Selecting the aspect ratio as a shape parameter for various defects, the transmission and reflection coefficients are measured for certain guided wave modes input to the defect. The influence of defect size is then studied by monitoring the transmission and reflection coefficients for defects of various shapes and depths. The studies presented indicate that defect characterization is possible if a proper mode selection criteria can be established. The suitable features related to transmission and reflection coefficient data can also be used for algorithm development and implementation purposes of defect characterization.     

Noncontact Air-Coupled Guided Wave Ultrasonics for Detection of Thinning Defects in Aluminum Plates

Ultrasonic guided waves are gaining increasing attention for the inspection of platelike and rodlike structures. At the same time, inspection methods that do not require contact with the test piece are being developed for advanced applications. This paper capitalizes on recent advances in the areas of guided wave ultrasonics and noncontact ultrasonics to demonstrate a superior method for the nondestructive detection of thinning defects simulating hidden corrosion in thin aluminum plates. The proposed approach uses micromachined gas (air)-coupled capacitive transducers for the noncontact generation and detection of guided plate waves. Interesting features in the dispersive behavior of selected guided modes are used for the detection of plate thinning. It is shown that mode cutoff measurements provide a qualitative detection of thinning defects, while frequency shift measurements allow quantification of thinning depth. Measurement of the mode group velocity can be also used to quantify thinning depth. Similarly, thinning length can be determined by mode time-of-flight measurements.     

Noncontact Air-Coupled Guided Wave Ultrasonics for Detection of Thinning Defects in Aluminum Plates

Abstract

Ultrasonic guided waves are gaining increasing attention for the inspection of platelike and rodlike structures. At the same time, inspection methods that do not require contact with the test piece are being developed for advanced applications. This paper capitalizes on recent advances in the areas of guided wave ultrasonics and noncontact ultrasonics to demonstrate a superior method for the nondestructive detection of thinning defects simulating hidden corrosion in thin aluminum plates.

The proposed approach uses micromachined gas (air)-coupled capacitive transducers for the noncontact generation and detection of guided plate waves. Interesting features in the dispersive behavior of selected guided modes are used for the detection of plate thinning. It is shown that mode cutoff measurements provide a qualitative detection of thinning defects, while frequency shift measurements allow quantification of thinning depth. Measurement of the mode group velocity can be also used to quantify thinning depth. Similarly, thinning length can be determined by mode time-of-flight measurements.     

The Reflection of Guided Waves from Multiple Flaws in Pipes

Inspection of pipes and pipelines for corrosion is vital for their safe and cost effective operation. Ultrasonic guided wave inspection is a relatively new technique used for this. Recent research has advanced the technique so that it may be possible to characterise individual flaws, making guided waves a potentially viable alternative to direct examination where internal in-line inspection (ILI) is not feasible. However, corrosion often occurs in clusters and the presence of one flaw could affect the signals received from subsequent flaws. Therefore, the issue of multiple flaws must be dealt with if flaws are to be adequately characterised in the field. In order to address this, a semi-analytical modelling methodology has first been developed for the reflection and transmission of guided waves from a single flaw and then, this has been combined with an analytical model for wave propagation to extend it to simulate the reflections from two or more flaws. The semi-analytical model for two flaws has been validated by comparison with experimental data and verified against three-dimensional finite element simulations.     

Guided wave nuances for ultrasonic nondestructive evaluation

Recent developments in guided wave generation, reception, and mode control show that increased penetration power and sensitivity are possible. A tone burst function generator and appropriate signal processing are generally used. Variable angle beam and comb-type transducers are the key to this effort. Problems in tubing, piping, hidden corrosion detection in aging aircraft, adhesive and diffusion bonding, and ice detection are discussed. Additionally, sample configurations, inspection objectives, and logic are being developed for such sample problems as defect detection and analysis in lap splice joints, tear straps, cracks in a second layer, hidden corrosion in multiple layers, cracks from rivet holes, transverse cracking in a beam, and cracks in landing gear assembly. Theoretical and experimental aspects of guided wave analysis include phase velocity, group velocity, and attenuation dispersion curves; boundary element model analysis for reflection and transmission factor analysis; use of wave structure for defect detection sensitivity; source influence on the phase velocity spectrum, and the use of angle beam and comb transducer technology. Probe design and modeling considerations are being explored. Utilization of in-plane and out-of-plane displacement patterns on the surface and longitudinal power distribution across the structural cross-section are considered for improved sensitivity, penetration power, and resolution in nondestructive evaluation. Methods of controlling the phase velocity spectrum for mode and frequency selection are available. Such features as group velocity change, mode cut-off measurements, mode conversion, amplitude ratios of transmission, and reflection factors of specific mode and frequency as input will be introduced for their ability to be used in flaw and material characterization analysis.     

Boundary element analysis of guided waves in a bar with an arbitrary cross-section

A boundary element method (BEM) is presented for analyzing the dispersion relation of guided waves in a bar with an arbitrary cross-section. A boundary integral equation for a harmonic motion in time and space is derived with respect to the boundary of the cross-section of the bar. By means of a collocation method, a homogeneous matrix equation which depends on the frequency and the wave number of the guided wave is obtained. The dispersion relation of guided waves is then obtained by finding nontrivial solutions of the matrix equation. The Newton's method is used to find the solution of the dispersion relation mode by mode for both propagating waves and nonpropagating waves. Numerical results are shown for a square bar, rectangular bars, and an L-shaped bar. Dispersive properties of guided waves in the bars are discussed in comparison with the results for Lamb modes in a 2D plate.     

Self-calibrating ultrasonic technique for crack depth measurement

A configuration of transducers together with a self-calibrating measurement technique is proposed to investigate the reflection and transmission of surface waves by a surface-breaking or near surface defect. By means of this technique, the ratio of the reflection and transmission coefficients (R/T and/orT/R) can be obtained in a reliable and accurate manner. The reflection and transmission of surface waves for oblique incidence on a surface breaking crack is investigated in detail. Information onT/R for the latter case can be used to determine the depth of the crack. The experimental measurements ofT/R show excellent agreement with theoretical results.     

Ultrasonic guided wave scattering in a plate overlap

Guided wave scattering in a plate overlap is investigated by numerical calculations and experimental measurements of transmission and reflection factors from the overlap region. In the numerical study, a hybrid boundary element-finite element method is used to calculate the guided wave scattered field from the overlap region. Transmission and reflection factors are calculated for incident A0 and S0 Lamb and n0 shear horizontal waves, including higher modes generated through mode conversion phenomena. In addition, parametric studies of transmission and reflection factors in this problem are performed numerically over various incident modes, frequencies, and overlap lengths. For verification and comparison with numerical results, experiments were conducted to measure the transmission and reflection factors for incident Lamb and shear horizontal waves in steel plates with two different overlap areas. The experimental results agree well with the numerical calculations. The numerical and experimental results show that it is highly feasible to carry out efficient Lamb wave nondestructive evaluation (NDE) in overlapped plates and in multilayer structures with various lap joints by selecting various modes and tuning frequency.     

Measurement of interfacial fracture parameters using coherent gradient sensing (CGS)

An optical interferometry called coherent gradient sensing (CGS) has been extended for mapping interface crack tip fields and for evaluating fracture parameters. The optical technique is a double grating shearing interferometer with an on line spatial filtering arrangement. The method offers real time full field measurements and can be used both in transmission made and reflection mode. The interferometer measures small angular deflections of light rays which can be further related to in plane gradients of _x + _y in transmission through elasto-optic relations. Direct interfacial crack tip measurements in a high stiffness mismatch PMMA-aluminium bimaterial system are performed. A variety of crack tip mode mixities are studied using asymmetric four point bend specimens subjected to different far field mechanical loads. The. complex stress intensity factors and the associated phase angles are measured from CGS patterns using an asymptotic expansion field. The measurements are compared with finite element results.     

Evaluation of general corrosion on pipes using the guided wave technique

The torsional mode of a guided wave, T(0,?1), is capable of detecting features and defects in pipelines, especially in the cases of coated and buried pipes. However, it is hard to find or locate general corrosion, since the signals of shallow corrosion easily merge into the noise signal. The situation worsens when an inspector sets the array ring of the guided wave transducer directly on a general corrosion area of the pipe and serious energy attenuation of the detecting signal occurs. The goal of this study is to investigate the effects of the above-mentioned general corrosion on guided wave tests, as determined by the finite element method, experiments, and site inspection cases. The results show that the deeper the corrosion depth or the higher the operating frequency, the larger the attenuation rate of the guided wave; also, the higher the operating frequency, the fewer the coherent signals caused by the general corrosion. In the case of detecting localized severe corrosion inside a section of general corrosion on a pipe, the results show that a higher operating frequency reduces the coherent noise of general corrosion, and gives an obvious reflection signal of the local severe corrosion that leads to a good measurement being obtained.     

Characterization of steel-concrete interface bonding conditions using attenuation characteristics of guided waves

This paper presents an air-coupled ultrasonic nondestructive evaluation approach based on the attenuation characteristics of guided waves to characterize interface bonding conditions of steel-clad concrete structures. Analytical guided wave modal solutions for various interface bonding conditions are obtained using the global matrix technique. The analytical results indicate that attenuation behavior of the fundamental symmetric (S0) guided wave mode is sensitive to interface bonding condition. A signal processing scheme is proposed to extract a dimensionless damage index, the normalized S0 mode magnitude, which reflects the attenuation behavior of the S0 mode. A series of numerical simulations are performed to verify the utility of the parameter to characterize interface bond condition. The feasibility of the testing approach is then established by experiments on steel-clad concrete specimens with different interface bonding conditions. The numerical simulation and experimental results demonstrate that interface bonding conditions of steel-clad concrete structures can be quantitatively evaluated using the proposed approach.     

振动声激发超声导波评估皮质骨厚度的研究

利用振动声激发超声导波来评价长骨是一种较新的思路。基于超声轴向传播方法,首先通过三维有限元法对导波在长骨中的传播进行建模仿真,再根据接收波形求解0A导波模式的相速度,最后依据理论相速度频散曲线得出皮质骨厚度。对不同厚度(2~6 mm)的骨板仿真的结果表明,皮质骨厚度估计的平均误差仅为2.61%。所提出的方法能够有效评估皮质骨厚度,对发展超声导波反演长骨的新技术具有一定意义,同时为临床超声导波长骨诊断的研究提供了新的思路。     

板状粘接结构中对称和反对称纵波与界面的相互作用

为了激发粘接结构中的导波或界面波,通常需要将声波从两半无限介质同相位或反相位地同时入射多层系统。针对此问题,基于矩阵方法,推导了界面处于理想连接的情况下,对称或反对称纵波从上下半无限空间入射时,三层板状粘接结构中纵、横波的反射与透射系数表达式。分析了入射角度、粘接层厚度以及基体材料等对声波反射(或透射)特性的影响。结果表明,对称或反对称纵波垂直入射时不发生波型转换。粘接结构中声波的反射(或透射)特性与入射角度、频率以及粘接层厚度等参数密切相关。在相同的粘接层厚度(或频率)范围内,随着声波频率(或粘接层厚度)的增加,谐振频率曲线向低频漂移。该方法可作为粘接结构中体波或导波传播特性研究的重要理论基础。     

A roller device to scan for surface-breaking cracks and to determine crack depth by a self-calibrating ultrasonic technique

A roller device using a square or a rectangular configuration of four roller transducers is used to scan a surface for surface-breaking cracks, by monitoring the reflection and transmission of surface waves. A self-calibrating ultrasonic technique is presented to determine the reflection and transmission coefficients or their ratio. For two configurations, namely, when one or both front rollers have crossed the surface trace of the crack, expressions have been obtained for the reflection and transmission coefficients in terms of measured voltages only, independent of parameters defining the coupling of the transducers to the specimen and the attenuation of surface waves propagating over the specimen. For a two dimensional configuration of a crack normal to the surface, the experimental results for the magnitude of the transmission coefficient show good agreement with theoretical results. For that case, the crack depth can be determined from the measured data.     

A finite element analysis of the time-delay periodic ring arrays for guided wave generation and reception in hollow cylinders

A new guided wave transducer model, time-delay periodic ring arrays (TDPRAs), is proposed and investigated in this paper for guided cylindrical wave generation and reception in hollow cylinders with application interests focusing on non-destructive testing (NDT) of piping/tubing. A finite element simulation has been performed for axisymmetric guided-mode excitation and reception with TDPRAs. By arranging a proper configuration of the time-delay profile and the electric-connection pattern of a ring array, unidirectional excitation and reception of guided waves can be achieved. The numerical results are obtained for the first three axisymmetrical modes and are compared with respect to generation efficiency and mode selectivity. Parametric influences on the performance of TDPRAs are discussed, combining a 2-D phase velocity-frequency spectrum approach with the mode dispersion and displacement structure analyses. The identification of converted modes in guided cylindrical wave reflections with a flexible TDPRA receiver has also been studied through sample notch reflection.     

基于空间拓展法的管道高阶螺旋导波波包追踪方法研究

螺旋导波是管道短距离高精度层析成像技术采用的主要导波模态,可以有效地提高成像分辨率,弥补目前管道导波长距离大范围检测精度低的缺点,在管道腐蚀检测中具有重要意义。然而由于导波的频散及螺旋导波的多路径传播特性,换能器接收到的检测信号波包众多,且经常发生重叠。为了便于分析这些波包的来源,需要有效的方法来计算各阶螺旋传播路径的长度及波包到达时间,进行波包追踪。推导了基于空间拓展法的管道高阶螺旋导波传播路径的计算模型,并针对各向均匀辐射的柱面波前兰姆波形成的管道螺旋导波,进行了数值计算和实验验证,证明了模型的正确性及其在波包追踪中的有效性。对利用管道螺旋导波进行检测及层析成像具有理论指导作用。     

CIRCUMFERENTIAL SHEAR HORIZONTAL WAVE AXIAL-CRACK SIZING IN PIPES

Circumferential shear horizontal (SH) waves are used for the inspection and sizing of axial cracks in pipelines. Experiments on two sample pipes having notches with different depths and lengths were carried out utilizing magnetostrictive sensor (MsS) technology for generating circumferential shear horizontal guided waves. A simplified two-dimensional model for crack sizing in pipes was studied through wave-reflection amplitude coefficients. The wave-reflection amplitudes are affected by both defect depths and lengths. To estimate the defect depth, which is critical, length compensation was taken for defects shorter than the beam width. An axial scan was carried out for the defect length estimations and the length compensation. Based on this axial scan, an approximate two-dimensional theory has been developed that has improved the defect depth estimation greatly. Two-dimensional boundary element modeling analysis and normal mode expansion technology are used to study defect sizing theoretically in a pipe-like structure by calculating reflection coefficients. The theoretical results agree with the experiments quite favorably.     

Detection of Axial Cracks in Pipes Using Focused Guided Waves

The implementation of synthetic guided wave focusing to locate axially aligned defects in pipes has been investigated. Results from both finite element computer models and experiments on real pipes are presented and the data show good agreement. Focusing is necessary to improve the reflection coefficient from axially aligned defects, as the signals are very weak. The Common Source Method (CSM) of synthetic focusing has been applied which makes it possible to apply focusing via post processing to previously collected data. The dependence of reflection coefficient on crack length was measured for both through thickness and part depth axially aligned defects, at a range of frequencies, using the torsional guided wave family. The results show that the reflection coefficient is approximately doubled when focusing is employed, compared to the sensitivity for unfocused fundamental torsional waves. However the sensitivity is still very low, so in practise this approach could only be used to find severe defects.