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Detection of Air-Voids in Foam-filled Sandwich Panels using Air-Coupled Lamb Waves
... In recent years, the topic of guided waves has become more and more popular, for example, to perform integral measurements in structures like metal sheets or pipes and also sandwich structures (Feng et al. 2023;Yaacoubi et al. 2019). Recent works use aircoupled lamb waves for fault detection and visualization in the addressed sandwich structures (Haugwitz et al. 2023) at 40 kHz. In the lower frequency range up to a few kHz, that contains information about the first system eigenfrequencies, other excitation principles can be of interest. ...
Sandwich panels in civil engineering have the advantage of a high flexural strength and thermal insulation while providing a very low density. They are made of two steel sheets and a core of polyurethane or polyisocyanurate foam. Due to the manufacturing process, the panels occasionally have defects, that are not visible from the outside. These may cause visible damages after installation in building facades. This paper presents a workflow for the detection and localization of the defects exploiting their influence on the response to a dynamic excitation. It uses an experimentally validated finite element model, feature engineering, and a neural network. The workflow initiates with the development and validation of a mechanical finite element model using experimental data. The model serves as the foundation for a simulation campaign, examining both defect-free and models with defects. The simulation process incorporates variations in material parameters based on experimentally obtained standard deviations, ensuring a robust representation of real-world uncertainties. The resultant simulation dataset is subjected to a feature engineering process encompassing time and frequency domain features. This multi-domain feature extraction enhances the dataset’s informativeness. The feature dataset is employed to train a neural network for two primary use cases: Firstly, to differentiate between faulty and fault-free models, providing an accurate fault detection mechanism. And secondly, to localize faults within the model. The effectiveness of this approach is demonstrated using various testing datasets. The proposed methodology represents a combination of a synthetically created database from a validated finite element model, multi-domain feature engineering, and neural network capabilities. It offers a methodology for fault detection and localization in sandwich panels. Using a dataset of several hundred samples, a detection rate of 94.8 % and a localization rate of 81.3 % are achieved in this work. In practical use, this approach can lead to significant reduction of customer complaints, and savings in replacement costs and hence carbon footprint.
... In addition, we are using air-coupled ultrasonic testing because the integration into production lines is important. This work extends our preliminary conference paper [31] by adding the 200-kHz setup for comparison and providing theoretical background on how the defect detection works. ...
Sandwich panels, composed of two steel faces and a rigid foam core, are an inexpensive and lightweight option for construction industry. However, voids can form in the foam core during the manufacturing process. This paper uses ultrasonic testing to detect such voids in the foam core of sandwich panels, buried a few millimeters below the surface. The testing setup employs both air-coupled and non-contact ultrasonic testing. Different frequencies are investigated for their influence on the detection capabilities. Two air-coupled experimental setups are constructed, one at 40 kHz and the other one at 200 kHz. Artificial defects are carved into the sandwich panel at different depths. The results are compared to a simulation. We found that detecting buried voids in these sandwich panels is feasible. The 40-kHz setup has a larger penetration depth of 14 mm, while the 200-kHz setup has a smaller penetration depth of 2.5 mm. The 200-kHz setup shows a better contrast, i.e. the amplitude at the defect increases by 27% compared to 6% with the 40-kHz setup. These methods enable air-coupled, non-contact ultrasonic testing of buried defects in sandwich panels. They have the potential to be integrated into production lines, contributing to improved material efficiency and quality control for these sandwich panels.
Aluminum/rigid polyurethane foam composite plates (ARCPs) are widely used for thermal insulation. The interface debonding generated during manufacturing degrades the thermal insulation performance of an ARCP. In this study, the debonding of an ARCP, a composite plate with a porous and damped layer of rigid polyurethane foam (RPUF), was detected using A0 mode Lamb wave electromagnetic acoustic transducers (EMATs). The low energy transmission coefficient at the interface caused by the large acoustic impedance difference between aluminum and RPUF made the detection difficult. Based on these structural characteristics, an A0 mode Lamb wave with large out-of-plane displacement was used to detect the debonding. EMATs are preferred for generating A0 mode Lamb waves due to their advantages of being noncontact, not requiring a coupling agent, and providing convenient detection. A finite element simulation model considering the damping of the RPUF layer, the damping of the PU film at the interface, and the bonding stiffness of the interface was established. The simulation results indicated that the Lamb wave energy in the aluminum plate transmits into the RPUF layer in small amounts. However, the transmitted energy rapidly attenuated and was not reflected into the aluminum plate, as the RPUF layer was thick and highly damped. Therefore, energy attenuation was evident and could be used to characterize the debonding. An approximately linear relationship between the amplitude of the received signals and the debonding length was obtained. Experiments were performed on an ARCP using EMATs, and the experimental results were in good agreement with the simulation results.
Light steel frame (LSF) building systems offer high structural resilience, lower costs due to fast prefabrication, and high ability to recycle and reuse. The main goal of this paper was to provide state-of-the-art main components for such systems with the intention to be implemented for use in nearly-zero energy buildings (NZEBs). A brief historical outline of the development of LSF systems was given, and the key parameters affecting the design and use of LSF systems were discussed. The influence of the individual components of the LSF system (steel studs, sheathing boards, and insulation materials) was then thoroughly discussed in light of relevant research on energy efficiency and other important properties (such as sound protection and fire resistance). Web of Science and Scopus databases were used for this purpose, using relevant key words: LSF, energy efficiency, sheathing boards, steel studs, insulation, etc. Several research gaps were identified that could be used for development and future research on new LSF systems. Finally, based on the analysis of each component, an innovative LSF composite wall panel was proposed which will be the subject of the authors’ future research. Conducted preliminary analysis showed low thermal transmittance of the system and indicates the path of its further research.
In this work we combine a multipath ultrasonic gas flow meter (UFM) with an ultrasonic air-coupled phased-array. This allows complementing the advantages of a multipath UFM, i.e. higher accuracy and more robustness to irregular flow, with the extended velocity measuring range due to sound drift compensation via a phased-array. We created a 3D-printed flow meter consisting of an 8×8 λ/2 phased-array for transmission and 14 individual receivers for seven upstream and seven downstream sound paths. Measurements were conducted in a test rig with a maximum gas flow rates of 8.3m3 s-1 (107ms-1). A differential pressure nozzle was used as reference sensor. Three configurations were compared: Parallel sound paths with a single transmitter; parallel sound paths with the phased-array as transmitter; and fan-shaped sound paths with the phased-array as transmitter. The signal-to-noise ratio (SNR) and deviation of measured flow were used as comparison criteria. In addition, we measured the optimum steering angles of the phased-array required to compensate the sound drift effect. Using the phased-array with the sound drift effect compensation enabled and disabled, the SNR increases by 10.6 dB and 4.95 dB, respectively, compared to the single transmitter setup at 83ms-1. Furthermore, the phased-array with compensation active, extends the velocity measuring range by 29%, from 83ms-1 to 107ms-1, while maintaining a similar standard deviation of the flow measured. Besides demonstrating that a phased-array in a gas flow meter significantly extends the measurement range, our setup qualifies as versatile research platform for designing future high-velocity gas flow meters.
Wind energy, with an exponential growth in the last years, is nowadays one of the most important renewable energy sources. Modern wind turbines are bigger and complex to produce more energy. This industry requires to reduce its operating and maintenance costs and to increase its reliability, safety, maintainability and availability. Condition monitoring systems are beginning to be employed for this purpose. They must be reliable and cost-effective to reduce the long periods of downtimes and high maintenance costs, and to avoid catastrophic scenarios caused by undetected failures. This paper presents a survey about the most important and updated condition monitoring techniques based on non-destructive testing and methods applied to wind turbine blades. In addition, it analyses the future trends and challenges of structural health monitoring systems in wind turbine blades.
In the rapidly expanding composite industry, novel inspection methods have been developed in recent years. Particularly promising for air-coupled testing are cellular polypropylene transducers which offer better impedance matching to air than piezoelectric transducers. Furthermore, broadband transmitters (laser-induced ultrasound and thermoacoustic emitters) and receivers (optical microphones) have opened a completely new chapter for advanced contact-free ultrasound inspection. X-ray dark-field radiography offers a different approach to detect porosity and microcracks, employing small angle X-ray scattering. These innovative ultrasonic and radiographic alternatives were evaluated in comparison with well-established inspection techniques. We applied thirteen different non-destructive methods to inspect the same specimen (a carbon fiber-reinforced polymer laminate with induced impact damage): air-coupled ultrasound testing (using piezoelectric transducers, broadband optical microphones, cellular polypropylene transducers, and a thermoacoustic emitter), laser-induced ultrasound testing, ultrasonic immersion testing, phased array ultrasonic testing, optically excited lock-in thermography, and X-ray radiography (projectional absorption and dark-field, tomosynthesis, and micro-computed tomography). The inspection methods were qualitatively characterized by comparing the scan results. The conclusions are advantageous for a decision on the optimal method for certain testing constraints.
Composite materials/structures are advancing in product efficiency, cost-effectiveness and the development of superior specific properties. There are increasing demands in their applications to load-carrying structures in aerospace, wind turbines, transportation, and medical equipment, etc. Thus robust and reliable non-destructive testing (NDT) of composites is essential to reduce safety concerns and maintenance costs. There have been various NDT methods built upon different principles for quality assurance during the whole lifecycle of a composite product. This paper reviews the most established NDT techniques for detection and evaluation of defects/damage evolution in composites. These include acoustic emission, ultrasonic testing, infrared thermography, terahertz testing, shearography, digital image correlation, as well as X-ray and neutron imaging. For each NDT technique, we cover a brief historical background, principles, standard practices, equipment and facilities used for composite research. We also compare and discuss their benefits and limitations, and further summarise their capabilities and applications to composite structures. Each NDT technique has its own potential and rarely achieves a full-scale diagnosis of structural integrity. Future development of NDT techniques for composites will be directed towards intelligent and automated inspection systems with high accuracy and efficient data processing capabilities.
Composite materials are increasingly used in the aerospace industry. To fully realise the weight saving potential along with superior mechanical properties that composites offer in safety critical applications, reliable Non-Destructive Testing (NDT) methods are required to prevent cata-strophic failures.This paper will review the state of the art in the field and point to highlight the success and challenges that different NDT methods are faced to evaluate the integrity of critical aerospace composites.The focus will be on advanced certificated NDT methods for damage detec-tion and characterization in composite laminates for use in the aircraft primary and secondary structures.
Composite materials are widely used in the industry, and the interest of this material is growing rapidly, due to its light weight, strength and various other desired mechanical properties. However, composite materials are prone to production defects and other defects originated during exploitation, which may jeopardize the safety of such a structure. Thus, non-destructive evaluation methods that are material-independent and suitable for a wide range of defects identification are needed. In this paper, a technique for damage characterization in composite plates is proposed. In the presented non-destructive testing method, guided waves are excited by a piezoelectric transducer, attached to tested specimens, and measured by a scanning laser Doppler vibrometer in a dense grid of points. By means of signal processing, irregularities in wavefield images caused by any material defects are extracted and used for damage characterization. The effectiveness of the proposed technique is validated on four different composite panels: Carbon fiber-reinforced polymer, glass fiber-reinforced polymer, composite reinforced by randomly-oriented short glass fibers and aluminum-honeycomb core sandwich composite. Obtained results confirm its versatility and efficacy in damage characterization in various types of composite plates.
In this work we model by finite element method (FEM) the Lamb waves’ propagation and their interactions with symmetric and asymmetric delamination in sandwich skin. The simulations were carried out using ABAQUS CAE by exciting the fundamental A 0 Lamb mode in the frequency 300 kHz. The delamination was then estimated by analysing the signal picked up at two sensors using two technics: Two-Dimensional Fast Fourier Transform (2D-FFT) to identify the propagating and converted modes, and wavelet transform (WT) to measure the arrival times. The results showed that the mode A 0 is sensible to symmetric and asymmetric delamination. Besides, based on signal changes with the delamination edges, a localization method is proposed to estimate the position and the length of the delamination. In the last section an experimental FEM verification is provided to validate the proposed method.
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. © 2018, Springer Science+Business Media, LLC, part of Springer Nature.
This article describes an experimental procedure for improving the detectability of small defects in composite
laminates based on modifications to the total focusing method (TFM) of processing ultrasonic array data to form an image. The TFM is modified to include the directional dependence of ultrasonic velocity. The maximum aperture angle is limited and a Gaussian frequency-domain filter is applied prior to processing. The parameters of maximum aperture angle, filter centre frequency and filter bandwidth are optimized.
Lightweight steel constructions made of sandwich panels are an economical solution for wall and roof claddings, especially in industrial construction. The panels consist of two thin sheets of steel and a core with thermal insulating properties. In the mounted state and at high temperatures on the outer face of the panel, damage to the components may occur due to blister formation. In a current research project, the influence of defects in sandwich panels with a PIR‐foam core is examined. This article presents the results of experimental tests regarding the influence of various parameters, such as face temperature, production defects and design of the longitudinal joint on blistering in sandwich wall panels with a PIR‐core. The aim is to identify the necessary conditions for the occurrence of blisters. Using an optical strain measuring method (Digital Image Correlation), the temperature‐induced deformations and strains in the covering nose of sandwich wall panels with hidden fastening are measured and will be analyzed with regard to blistering. First results reveal a significant dependence on the geometry and design of the covering nose.
Waveguides allow grating lobe free beamforming for air-coupled ultrasonic phased-arrays by reducing the effective inter-element spacing to half wavelength. Since the sound waves propagate through the waveguide ducts, additional time delays are introduced. In this work, we present analytical, numerical, and experimental methods to estimate these time delays. Afterwards, two different waveguides are compared. The first one consists of equal-length ducts, requiring a time-consuming assembly process of the ultrasonic phased-array. In contrast, the second waveguide consists of Bézier-shaped ducts of unequal lengths but a planar input port allowing fast assembly. The analytical model is based on the geometric lengths of the waveguide ducts. The numerical model relies on a transient finite element analysis. All simulations are validated in an anechoic chamber using a calibrated microphone. The analytical (7.6% deviation) and numerical (3.2% deviation) propagation time models are in good agreement with the measurements. By using the analyzed propagation times for the compensation of the unequal waveguide duct lengths, we restored the beamforming capability without significant sound pressure level (SPL) loss. This work shows the possibility of reduced transducer assembly time for waveguided air-coupled phased-arrays without a reduced SPL.
The study reports on the characterization of interfacial disbonds in an elastic-viscoelastic (steel-rubber) adhesively bonded bilayer structure where the elastic material (steel) has a significantly higher impedance than the viscoelastic material (rubber). The study uses the A0 mode guided wave at a low frequency of 100 kHz, which was generated and detected using non-contact air-coupled ultrasound. The A0 mode could be generated and detected from both steel and rubber sides, despite the high viscoelasticity of the rubber layer. However, the A0 mode guided wave propagation is sustained only in the steel layer and not in the rubber layer. The mode travelling along the steel layer leaks into the rubber layer and hence can be detected from the rubber side in the bonded region. When the disbond is below either the transmitter or the receiver, a better sensitivity in the disbond detection was achieved from the rubber side rather than the steel side. Using the experimental and 2D frequency-domain finite element simulation results, a linear correlation between the extent of disbond and the amplitude, time-of-flight (ToF) difference of the A0 mode arrival in the disbond region on the steel side was also established. The results show the potential to characterize interfacial disbonds regardless of the position with respect to the transmitter or receiver in elastic-viscoelastic multilayered structures using air-coupled guided wave inspection from single-side access.
We present an air-coupled ultrasonic imaging system based on a 40 kHz 8×8 phased-array for three-dimensional real-time localization of multiple objects in the far-field. By attaching a waveguide to the array, the effective inter-element spacing is reduced to half wavelength. This enables grating lobe free transmit and receive beamforming with a uniform rectangular array of efficient low-cost transducers. The system further includes custom transceiver electronics, an FPGA Systemon- Chip and a PC for GPU accelerated frequency domain signal processing, consisting of matched filtering, conventional beamforming and envelope extraction using Nvidia CUDA and OpenGL for visualization. The uniform rectangular layout allows utilizing multiple transmit and receive methods, known from medical imaging applications. Thus, the system is dynamically adaptable to maximize the frame rate or detection range. One implemented method demonstrates the real-time capability by transmitting a hemispherical pulse with a single transducer to irradiate the surroundings simultaneously, whereas all transducers are used for echo reception. The imaging properties, such as axial and lateral resolution, field of view and range of view, are characterized in an anechoic chamber. The object localization is validated for a horizontal and vertical field of view of ±80° and a range of view of 0.5m to 3m with 29 frames per second. Using the same system, a comparison between the hemispherical pulse method and the dynamic transmit beamforming method, which transmits multiple sequential beamformed pulses for long-range localization, is provided.
Sandwich composites with a polyurethane foam (PUF) core have become a versatile set of materials with applications ranging from basic construction elements to advanced engineering materials. This diverse range of applications has provided these sandwich materials with a distinctive position in today's engineering world. This review focuses on the broad research that has been carried out on PUF core‐based sandwich composites over the years—especially in the last decade. Thorough details have been presented focusing on basic PUF core sandwich structures, advanced PUF core sandwich composites with different kinds of reinforcements and complex structures such as hybrid sandwich panels. All the research presented here has been categorized carefully, such as the types of materials used in various studies, the types of reinforcements and testing techniques used, including mechanical, electrical, dynamic, thermal and acoustic methods etc. Comparisons have also been made based on similar materials, similar testing procedures and similar application areas. Research areas have been highlighted by giving a deep insight into the most promising research, manufacturing techniques and improvement procedures. Major consumption areas and application sectors for PUF core sandwich composites have also been discussed in this review. Finally, conclusions have been made based on the results found from a literature investigation.
Elastic guided wave based evaluation is useful to detect delamination in composites. This paper reports a detailed account of guided wave interaction with delamination in laminated composites modeled using time domain spectral finite element (TSFE) method. Wave scattering due to different delamination positions is studied. Wave field near the delamination in different layer-wise positions is investigated, which is useful to further characterize and correlate the far field wave packet carrying the parametric signature of delamination. Off-axis delamination in composite laminate creates an asymmetry in the elastodynamic stress transfer. It leads to wave mode conversions. Sensitivity of the delamination to the wavelengths is studied. The outcome of this study indicates potential possibilities to use frequency-wavelength information to discriminate various sizes of delamination. Energy of the scattered waves and dissipation/conversion of the wave energy due to the defects depend on the resonance characteristics of the sub-laminates. Wave scattering effect due to length-wise multiple delaminations and edge delamination is also analyzed. The resonance patterns in the signals are analyzed with reference to the defect quantification problem.
The objective of this work is to assess to which extent the interaction of antisymmetric ultrasonic guided waves with impact damage can be captured with an experimental model consisting of a single artificial delamination in composite structures. The structures of interest are composed of unidirectional prepreg carbon fiber-reinforced polymer (CFRP) with a quasi-isotropic layup. The artificial delamination is introduced into the laminate using two circular Teflon tapes during manufacturing and the realistic damage is simulated by impacting the samples at two energy levels. Two colocalized rectangular piezoceramics are used to generate an antisymmetric mode and noncontact measurement is performed using a three-dimensional (3D) laser Doppler vibrometer (3D-LDV) to extract the required information for evaluation of the reflection, transmission, as well as the scattering behavior of the antisymmetric mode. The corresponding coefficients as a function of frequency, incident angle, and type of damage are extracted. It is found that the amplitude of the coefficients and directivity patterns of scattered waves are barely affected by incident angle but significantly by the impact energy. In light of the results, design guidelines are proposed for efficient guided wave inspection of composite structures submitted to impacts.
Sandwich panels are being used increasingly as the cladding of buildings like factories, warehouses, cold stores and retail sheds. This is because they are light in weight, thermally efficient, aesthetically attractive and can be easily handled and erected. However, to date, an authoritative book on the subject was lacking. This new reference work aims to fill that gap. The designer, specifier and manufacturer of sandwich panels all require a great deal of information on a wide range of subjects. This book was written by a group of European experts under the editorship of a UK specialist in lightweight construction. It provides guidance on: materials used in manufacture thermal efficiency and air- and water-tightness acoustic performance performance in fire durability special problems of sandwich panels in cold stores and chill rooms architectural and aesthetic considerations structural design at the ultimate and serviceability limit states additional structural considerations including fastenings, the effect of openings and the use of sandwich panels as load-bearing walls test procedures The book concludes with some numerical design examples and is highly illustrated throughout.
Polyurethane sandwich structures are very useful in construction because of their excellent thermal insulating properties. Even though the chemicophysical processes occurring inside the production line crucially affect the quality of the final product, up to now, they are not monitored at all. Thus, the aim of this paper is to present a new fundamental approach for non-invasively monitoring these processes online by using air-coupled, low-frequency ultrasonic plate waves ("Lamb waves"). Hereby, a foaming cell incorporating the ultrasonic unit was constructed on a laboratory scale. Then, sandwich structures were fabricated with polyurethane consisting of various ratios of the educts di-isocyanate and polyol. Air, instead of liquid was used as the coupling medium to transport the ultrasonic waves that were subsequently monitored. For comparison, tensile strength and microscopic images were analyzed. Most importantly, various wave amplitudes and phases were recorded depending on the actual chemical properties of the polyurethane and its physical state. Consequently, this technique permits the direct and non-invasive tracking of the foaming process thereby avoiding expensive panel replacements.
Ultrasonic Lamb waves have been investigated extensively for damage detection in advanced composite materials. They are particularly suitable for proving thin plate structures of large area, where the Lamb wave approach offers a considerable saving in time over through-the-thickness inspection. However the potential complexity of the propagation can introduce significant difficulties to the technique. We present a review of work conducted at The University of Strathclyde in collaboration with several European partners into the feasibility of Lamb wave inspection. Specifically we will address issues of Lamb wave generation, propagation, defect interaction and detection.
Because of the complex nature of structural characteristics, damage detection in honeycomb sandwich structures inherently carries many challenges. In the study, effects of debonding on leaky guided wave propagation in honeycomb sandwich structures are first studied by using the finite element method. A surface-bonded piezoelectric wafer actuator/sensor network is used for elastic-guided wave propagation and reception in the composite. Experimental testing is then conducted to verify the numerical simulation and for damage detection. A damage localization image from each actuator-sensor pair is constructed by conducting probability analysis of differential features of transmitted guided waves in the structure with and without debonding. An image of the whole structure is then reconstructed by superimposing the image from each actuator-sensor pair. The quality of the final image is improved by using the image fusion technique. To detect multiple debondings in the honeycomb sandwich structure, a multilevel sensor network strategy is suggested for eliminating pseudoimages due to image coupling of multiple debondings. The final structure image demonstrates that the proposed method can provide quantitative information about location and size of debonding in the honeycomb sandwich structure.
Ultrasonic Lamb waves have been investigated extensively for damage detection in advanced composite materials. They are particularly suitable for proving thin plate structures of large area, where the Lamb wave approach offers a considerable saving in time over through-the-thickness inspection. However the potential complexity of the propagation can introduce significant difficulties to the technique. We present a review of work conducted at The University of Strathclyde in collaboration with several European partners into the feasibility of Lamb wave inspection. Specifically we will address issues of Lamb wave generation, propagation, defect interaction and detection.
The penetration depth into a medium of sagittally polarized Rayleigh waves propagating on a planar stress-free surface of an isotropic medium, has been considered. Using values of the ratio cR/ct (where cR denotes the velocity of the Rayleigh wave, and ct, the velocity of the transverse sound wave) calculated from the dispersion relation for the Rayleigh surface wave, the penetration depths (kβL)-1 and (kβt)-1 defined by Maradudin, have been calculated as functions of the ratio CR/CL, (where cL denotes the speed of the longitudinal acoustic wave), the ratio cR/ct and also of the Poisson's ratio σ. The amplitudes of the components U1 and U3 of the displacement field for sagittally polarized Rayleigh waves have been calculated as function of the distance x3 from the surface, for various values of the Poisson's ratio σ.
The guided Lamb wave is widely acknowledged as one of the most encouraging tools for quantitative identification of damage in composite structures, and relevant research has been conducted intensively since the 1980s. The main aim of this paper is to provide a comprehensive review on the state of the art of Lamb wave-based damage identification approaches for composite structures, addressing the advances and achievements in these techniques in the past decades. Major emphasis is placed on the unique characteristics and mechanisms of Lamb waves in laminated composites; approaches in wave mode selection, generation and collection; modelling and numerical simulation techniques; signal processing and identification algorithms; and sensor network technology for practical utility. Representative case studies are also briefly described in terms of various experimental validations and applications.