An NDT technique for composite structures using visualized Lamb-wave propagation

This study proposes a non-destructive testing (NDT) technique that visualizes the propagation of ultrasonic waves in solids, including composite laminates. This technique provides a moving diagram of traveling waves through the use of a pulsed laser that scans a test piece. A non-contact scan by the pulsed laser for ultrasound generation with reception at a fixed point enables us to inspect an arbitrarily shaped object and also facilitates easy operation of the measurement system. We applied the proposed technique to the inspection of CFRP laminates and successfully visualized the wave scattering due to impact-induced delamination as well as the propagation of the S₀ and A₀ Lamb modes. We also addressed disbonding detection in a composite skin-stringer structure and concluded that the reliable detection of damage and the advantages of the proposed technique are applicable to inspections of composite structures.     

Development of smart composite structures with small-diameter fiber Bragg grating sensors for damage detection: Quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing

The authors and Hitachi Cable, Ltd. have recently developed small-diameter optical fiber and its fiber Bragg grating (FBG) sensor for embedment inside a lamina of composite laminates without strength reduction. The outside diameters of the cladding and polyimide coating are 40 and 52 μm, respectively. First, a brief summary is presented for applications of small-diameter FBG sensors to damage monitoring in composite structures. Then, we propose a new damage detection system for quantitative evaluation of delamination length in CFRP laminates using Lamb wave sensing. In this system, a piezo-ceramic actuator generates Lamb waves in a CFRP laminate. After the waves propagate in the laminate, transmitted waves are received by an FBG sensor attached on or embedded in the laminate using a newly developed high-speed optical wavelength interrogation system. This system was applied to detect interlaminar delamination in CFRP cross-ply laminates. When the Lamb waves passed through the delamination, the amplitude decreased and a new wave mode appeared. These phenomena could be well simulated using a finite element analysis. From the changes in the amplitude ratio and the arrival time of the new mode depending on the delamination length, it was found that this system could evaluate the delamination length quantitatively. Furthermore, small-diameter FBG sensors were embedded in a double-lap type coupon specimen, and the debonding progress could be evaluated using the wavelet transform.     

Remote Optical Non-Destructive Inspection Technique for Metal and Composite Structures

This paper presents a laser based non-destructive inspection technique which is purely non-contact. An acoustic device is used to remotely excite the material under inspection while a scanning laser vibrometer is employed to measure the resulting vibrations. The test materials, mostly aluminum or composite aircraft structures, have been successfully inspected from a standoff distance of at least a meter. The NDI system clearly shows the defect locations overlaid on the images of the test objects for ease of the end user. Metal and composite delamination and subsurface corrosion in aluminum structures are among the defects that were detected.     

Variation in the group velocity of Lamb waves as a tool for the detection of delamination in GLARE aluminium plate-like structures

With the development of new technology and use of lightweight material such as composite laminates, new methods must be developed for in situ structural health monitoring of these materials. This paper introduces an approach for the detection of delamination present in GLARE aluminium specimens. The approach is based on the change in group velocity of Lamb waves with frequency–thickness product as the determinant parameter for the detection of delamination. Two methods are applied: a surface contact method, which utilises a wedge probe tuned to excite a single Lamb mode, and the embedded PZT method, which involves incorporating lead zirconate titanate (PZT) elements in the glass fibre reinforced resin matrix during the manufacture of the GLARE aluminium specimens. It was found that both methods enabled the detection of delaminations in the GLARE aluminium specimens, within certain limits, which are stated.     

Damage assessment in layered composites using spectral analysis and Lamb wave

Piezo-ceramic transducers of the surface mounted type are commonly used for structural health monitoring (SHM) techniques. But, there is a disadvantage to use piezo-ceramic transducers of the surface mounted type in Lamb wave application. Due to the symmetric and antisymmetric Lamb wave modes generated by the surface mounted piezo-ceramic transducers simultaneously, the received signals are very complex and it is difficult to extract damage information from the signals.

In this paper, the practical method for SHM was proposed using piezo-ceramic transducers of the surface mounted type and Lamb wave. In order to overcome the difficulties in the signal processing of the simultaneous modes, the symmetric and antisymmetric modes were separated by using the two sensors bonded on the opposite surfaces at the same point. Also, spectral analyses of the separated symmetric and antisymmetric Lamb waves showed that each mode propagated with different frequency characteristics in the exciting frequency range.

By making use of these findings, the changes of power spectrum density in characteristic frequency band of symmetric and antisymmetric modes are proportional to the delamination size in quasi-isotropic Gr/Ep laminates. Therefore, this paper presents the damage assessment technique to extract damage information from the complicated PZT signals that could not be interpreted in time domain.     

混凝土分层缺陷的超声检测方法研究

为解决混凝土结构中分层缺陷的在线非接触检测难题,论文提出了利用空气耦合(简称:空耦)超声导波定量检测混凝土结构中分层缺陷的新方法。首先研究了空耦超声导波在混凝土结构中的传播特性,理论分析和实验表明,利用空耦超声波以入射角8.7°入射厚度为50 mm的混凝土板时,可以激发以A0模态为主的导波。然后构建了空耦超声导波扫查实验系统,在混凝土结构单侧利用一对倾斜8.7°的空耦探头激励和接收导波信号,通过分析发现A0模态对分层缺陷敏感,且其幅度与扫查路径中的分层缺陷尺寸存在单调变化关系;在此基础上,对检测区域进行扫查,利用不同位置处的导波信号幅度实现分层缺陷的二维成像。实验结果表明,该方法不仅可以避免耦合剂对检测结果的影响,同时可实现对服役状态下混凝土结构中分层位置及尺寸的定量检测。     

Lamb wave-based quantitative identification of delamination in CF/EP composite structures using artificial neural algorithm

Delamination in composite structures plays a major role in lowering structural strength and stiffness, consequently downgrading system integrity and reliability. A Lamb wave-based quantitative identification technique for delamination in CF/EP composite structures was established. Propagation of Lamb waves in a series of composite laminates, individually bearing a delamination, was evaluated using dynamic FEM analyses. Taking advantage of wavelet transform and artificial neural algorithms, an Intelligent Signal Processing and Pattern Recognition (ISPPR) package was developed, by which the spectrographic characteristics of simulated Lamb wave signals in the time-frequency domain were extracted and digitised as Digital Damage Fingerprints (DDF), to construct a Damage Parameters Database (DPD). The DPD was then used offline to train a multi-layer feedforward artificial neural network (ANN) under supervision of an error-backpropagation (BP) algorithm. Assisted by an active online structural health monitoring (AO-SHM) system with an active piezoelectric actuator/sensor network, the proposed methodology was validated online by identifying actual delaminations in CF/EP (T650/F584) quasi-isotropic composite laminates.     

Monitoring of delamination onset and growth during Mode I and Mode II interlaminar fracture tests using guided waves

A delamination monitoring method was proposed to characterize Mode I and Mode II delamination onset in carbon fiber/epoxy (CF/EP) composite laminates through interrogation of guided waves activated and captured using piezoelectric actuators and sensors in a pitch–catch configuration. Mode I and Mode II interlaminar fracture tests were conducted using double cantilever beam (DCB) and end notch flexure (ENF) specimens to evaluate the proposed method. The changes in wave propagation velocity and wave magnitude (or attenuation), and the degree of waveform similarity between excitation and response signals, were calculated as delamination-sensitive wave parameters and plotted versus displacement recorded using a materials testing system. The kink points determined from wave parameter–displacement curves agreed well with the deviation from linearity (NL), visual observation (VIS) and maximum load (Max) points, which are often used in conventional methods for determining interlaminar fracture toughness. The propagation characteristics of the A₀ wave mode in a low frequency range were demonstrated to have high sensitivity to Mode I and in particular Mode II delamination onset in CF/EP composite laminates. It was concluded that the guided waves propagating in the DCB and ENF specimens were capable of determining Mode I and Mode II interlaminar fracture toughness, complementing current practices based on visual inspection or trivial interrogation using load–displacement curve alone.     

Assessment of debonding in sandwich CF/EP composite beams using A0 Lamb wave at low frequency

The aim of this study is to quantitatively assess debonding in sandwich CF/EP composite structures with a honeycomb core using acoustic waves activated and captured by surface-mounted PZT elements. For experimental investigation, debonding was introduced at different locations in sandwich CF/EP composite beams. The fundamental anti-symmetric A₀ Lamb mode was excited at a low frequency. The transmitted and reflected wave signals in both surface panels were captured by PZT elements after interacting with the debonding damage and specimen boundaries. Aided by finite element analysis (FEA), the differences in wave propagation characteristics in sandwich composite beams and composite laminate (e.g. skin panel only) were investigated. The debonding location was assessed using the time-of-flight (ToF) of damage-reflected waves, and the severity of debonding was evaluated using both the magnitude of the reflected wave signal and the delay in the ToF of Lamb wave signals. Good correlation between the experimental and FEA simulation results was observed. The results demonstrate the effectiveness of Lamb waves activated and captured by surface-mounted PZT elements on either surface of sandwich composite structures in assessing debonding.     

基于弹性导波的厚钢梁结构的损伤检测

弹性导波由于其对损伤的敏感性和长距离传播特性,成为近年来结构健康监测领域的一个研究热点.探讨了利用 PZT 换能器在较厚结构中进行基于弹性导波方法的损伤识别的可能性.首先参考 Lamb 波的频散曲线,设置了导波的激励信号参数如激励频率、激励波形周期数等;基于优化的激励波形和 PZT 换能器布局,在结构健康监测实验平台上对试件进行了检测.通过对实验结果的处理分析,计算出导波的群速度,并根据群速度和飞行时间(ToF)得到了损伤的位置信息.结果表明利用导波方法能够针对较厚结构进行损伤定位并能识别出不同大小的损伤.     

Structural Health Monitoring Using Lamb Wave Reflections and Total Focusing Method for Image Reconstruction

This investigation aimed to adapt the total focusing method (TFM) algorithm (originated from the synthetic aperture focusing technique in digital signal processing) to accommodate a circular array of piezoelectric sensors (PZT) and characterise defects using guided wave signals for the development of a structural health monitoring system. This research presents the initial results of a broader study focusing on the development of a structural health monitoring (SHM) guided wave system for advance carbon fibre reinforced plastic (CFRP) composite materials. The current material investigated was an isotropic (aluminium) square plate with 16 transducers operating successively as emitter or sensor in pitch and catch configuration enabling the collection of 240 signals per assessment. The Lamb wave signals collected were tuned on the symmetric fundamental mode with a wavelength of 17 mm, by setting the excitation frequency to 300 kHz. The initial condition for the imaging system, such as wave speed and transducer position, were determined with post processing of the baseline signals through a method involving the identification of the waves reflected from the free edge of the plate. The imaging algorithm was adapted to accommodate multiple transmitting transducers in random positions. A circular defect of 10 mm in diameter was drilled in the plate, which is similar to the delamination size introduced by a low velocity impact event in a composite plate. Images were obtained by applying the TFM to the baseline signals, Test 1 data (corresponding to the signals obtained after introduction of the defect) and to the data derived from the subtraction of the baseline to the Test 1 signals. The result shows that despite the damage diameter being 40 % smaller than the wavelength, the image (of the subtracted baseline data) demonstrated that the system can locate where the waves were reflected from the defect boundary. In other words, the contour of the damaged area was highlighted enabling its size and position to be determined.     

Measurement of Interfacial Fracture Toughness of Surface Coatings Using Pulsed-Laser-Induced Ultrasonic Waves

This research develops a new technique for the measurement of interfacial fracture toughness of films/surface coatings using laser-induced ultrasonic waves. Using pulsed laser ablation on the bottom substrate surface, strong stress waves are generated leading to interfacial fractures and coating delamination. Simultaneously, a laser ultrasonic interferometer is used to measure the normal (out-of-plane) displacement of the top surface coating in order to detect coating delamination in a non-destructive manner. We can thus determine the critical laser energy for delamination, yielding the critical stress (that is, the interfacial strength). Subsequently, to examine the interfacial fracture toughness, additional pulsed laser irradiation is applied to a pre-delaminated specimen to show that the delamination area expands. This type of interfacial crack growth can be visualized using laser ultrasonic scanning. Furthermore, the calculation of elastic wave propagation was carried out using a finite-difference time-domain method) in order to accurately estimate the interfacial stress field. In this calculation, the stress distribution around the initial delamination is calculated to obtain the stress intensity factor. Based on the experimental and computational results, interfacial fracture toughness can be quantitatively evaluated. Since this technique relies on a two-laser system in a non-contact approach, it may be useful for a quantitative evaluation of adhesion/bonding quality (including both interfacial fracture strength and toughness) in various environments.     

Imaging of damage in sandwich composite structures using a scanning laser source technique

The Scanning Laser Source (SLS) technique described in this paper enables non-contact high-resolution imaging of damage in sandwich composite structures. In the SLS technique, ultrasound is generated using a pulsed YAG:Nd laser that is scanned over the structure to be inspected. Both conventional piezo-electric transducers and laser interferometers can be used to monitor the ultrasound. Characteristic changes in the ultrasonic signal are observed as the laser source moves over the damaged part of the structure, and these changes can be related to underlying matrix cracking or fiber-breakage. The SLS technique has several significant advantages over conventional methods, including: (i) non-contact in situ measurements, (ii) remote placement of laser equipment using fiber optics, (iii) ability to inspect surfaces with complex geometry, (iv) high spatial resolution, and (v) sensitivity high enough to detect fiber-breakage in a single ply. Example applications to image artificial and impact-induced defects in composite structures are demonstrated.     

Mode-selective excitation and detection of ultrasonic guided waves for delamination detection in laminated aluminum plates

Selective modes of guided Lamb waves are generated in a laminated aluminum plate for damage detection using a broadband piezoelectric transducer structured with a rigid electrode. Appropriate excitation frequencies and modes for inspection are selected from theoretical and experimental dispersion curves. Dispersion curves are obtained experimentally by short time Fourier transform of the transient signals. Sensitivity of antisymmetric and symmetric modes for delamination detection are investigated. The antisymmetric mode is found to be more reliable for delamination detection. Unlike other studies, in which the attenuation of the propagating waves is related to the extent of the internal damage, in this investigation, the changes in the time-of-flight (TOF) of guided Lamb waves are related to the damage progression. The mode conversion phenomenon of Lamb waves during progressive delamination is investigated. Close matching between the theoretical and experimentally derived dispersion curves and TOF assures the reliability of the results presented here.     

光纤非本征法布里-玻罗传感器兰姆波检测灵敏度分析

基于兰姆波的结构工况检测技术在评估复合材料和金属结构的安全性和耐久性方面发挥着重要的作用。作为对传统的压电换能器（PZT）的一种很好的替代,光纤传感器在传感方面的应用正被广泛地挖掘出来,包括兰姆波检测。本文从理论上建立了超声兰姆波作用下光纤非本征法布里．玻罗（EFPI）传感器参数与其输出性能之间的关系。数值结果显示了传感器的性能与其相对于声源的方向角以及传感器的计量长度与超声波长的比值相关。所得出的结论对于EFPI传感器精确地探测兰姆波提供了理论依据。     

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.     

Air-Coupled Nondestructive Evaluation Dissected

This work develops a two-dimensional theoretical model to simulate the behavior of a fully non-contact air-coupled nondestructive evaluation system for a thin isotropic plate. The model is divided into transmission, guided wave propagation and reception phase. The validation of the complete model was carried out by modeling the same system by means of finite element method using a Multiphysics software. In addition, the dependency of the generated Lamb waves on different transmitter’s parameters and incidence angle is thoroughly investigated. The results of the acoustic pressure excited by the transducer, the out-of-plane velocity amplitudes for the generated first antisymmetric Lamb wave mode, and the radiated pressure from the plate caused by the leaky Lamb wavefield were all compared between the two models and a reasonable degree of similarity was found.     

Capacitive micromachined ultrasonic Lamb wave transducers using rectangular membranes

This paper details the theory, fabrication, and characterization of a new Lamb wave device. Built using capacitive micromachined ultrasonic transducers (CMUTs), the structure described uses rectangular membranes to excite and receive Lamb waves on a silicon substrate. An equivalent circuit model for the transducer is proposed that produces results, which match well with those observed by experiment. During the derivation of this model, emphasis is placed on the resistance presented to the transducer membranes by the Lamb wave modes. Finite element analysis performed in this effort shows that the dominant propagating mode in the device is the lowest order antisymmetric flexural wave (A/sub 0/). Furthermore, most of the power that couples into the Lamb wave is due to energy in the vibrating membrane that is transferred to the substrate through the supporting posts of the device. The manufacturing process of the structure, which relies solely on fundamental IC-fabrication techniques, is also discussed. The resulting device has an 18 /spl mu/m-thick substrate that is almost entirely made up of crystalline silicon and operates at a frequency of 2.1 MHz. The characterization of this device includes S-parameter and laser vibrometer measurements as well as delay-line transmission data. The insertion loss, as determined by both S-parameter and delay-line transmission measurements, is 20 dB at 2.1 MHz. When configured as a delay-line oscillator, the device functions well as a sensor with sensitivity to changes in the mass loading of its substrate.     

基于三维弹性理论的碳纤维板中Lamb波建模

基于主动Lamb波的结构健康监测是目前复合材料结构损伤监测技术研究的热点之一,了解Lamb波的传播特性对进行可靠的损伤监测非常重要.本文结合经典三维弹性理论与Lamb波的运动位移方程,对碳纤维复合材料板中传播的Lamb波传播特性进行了建模研究,在此基础上推导了碳纤维板的相速度频散曲线,并讨论了Lamb波传播方向与坐标轴之间的夹角及碳纤维铺层方向对频散曲线的影响,建模结果证明了这种建模方法的正确性.     

Health monitoring of aerospace composite structures – Active and passive approach

Impact damage is one of the major concerns in maintenance of aircraft structures built from composite materials. Damage detection in composite materials can be divided into active and passive approaches. The active approach is usually based on various non-destructive techniques utilizing actuators and/or receivers. In contrast passive approaches do not involve any actuators; receivers are used to “sense and/or hear” any perturbations caused by possible hidden damage. Often strain data are used to localize impacts and estimate their energy. The assumption is that damage occurs above well-defined energy of impacts. The paper illustrates one active and one passive method recently developed for impact damage detection. The first method, based on guided ultrasonic waves, utilises 3-D laser vibrometry and does not require any signal processing. Simple laser scans, revealing the change in Lamb wave response amplitudes, have been used to locate delamination and estimate its severity in a composite plate. In contrast, the second method does not require any sophisticated instrumentation but relies on advanced signal processing. An array of piezoceramic sensors has been to detect strain waves transmitted from an impact applied to the composite aircraft structure. The modified multilateration procedure with Genetic Algorithms has been used to locate impact position.