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Lamb waves (LWs) are widely used to achieve structural health monitoring of aeronautic composite structures. Composite materials are however anisotropic and LWs propagation characteristics depend on their propagation direction. Within one composite ply, when this direction coincides with the principal axis of the ply, they are not coupled with shear waves (SHWs) but become coupled with SHWs for any other direction. As composite materials are built up with layers at various orientations LWs and SHWs are coupled for some layers and uncoupled for some others when studying an arbitrary propagation direction. Transfer Matrix Method (TMM), Global Matrix Method (GMM), and Stiffness Matrix Method (SMM) are all methods allowing to predict guided waves (LWs and SHWs) behavior in composite materials. However those methods suffer from an incompatibility issue preventing them to manage cases when SHWs and LWs are coupled for some plies but uncoupled for some other plies. This issue is particularly frequent when dealing with metallic-composite plates or with composite plates made up with isotropic, orthotropic, and triclinic materials. In order to solve this incompatibility issue, a hybrid matrix strategy (HMS) is proposed here on the basis of SMM. The core idea of the HMS is to re-couple LWs and SHWs into hybrid guided waves when they are uncoupled in order to make them compatible with the general coupled waves cases. The numerical stability of HMS is proved theoretically and its effectiveness is validated through numerical investigations and using experimental data from the literature. The SMM-HMS framework can thus be considered as a state of the art benchmark approach for evaluating the performance of numerical methods dedicated to the computation of guided waves dispersion curves and can be confidently applied to any arbitrary composite material used in aeronautic and aerospace industries. • The coupling issue of Lamb and SH waves is studied via the Stiffness Matrix Method. • Hybrid Matrix Strategy is proposed to deal with the matrix incompatibility issue. • SMM-HMS is naturally extended to metallic-composite laminate case. • The numerical stability of SMM-HMS is theoretically proved. • The effectiveness of SMM-HMS is validated by numerical and experimental data. [ABSTRACT FROM AUTHOR]

Abstract This work reports a study aimed at quantifying damage severity in composite materials by an in-field inspection technique known as Acousto-Ultrasonics (AU) where Lamb waves are transmitted through a damage zone by exciting a pulse onto the composite surface and capturing the received waves after their interactions with the damage. It builds on a previous study that proposed as quantitative descriptors of the received signal a set of stress wave factors (SWFs) extracted from the power spectral density (PSD) distribution of the AU signal. The SWFs were previously shown to detect the spatial extent of impact damage, but the severity of the damage within the impact damage zone was not quantified. The present study takes a step in the direction to achieve that goal. Lamb waves are transmitted through a cross ply laminate in which transverse ply cracks of increasing density (number of cracks per unit axial length) are induced. The PSD distribution is first analyzed for the case of an undamaged laminate and then the SWFs are correlated with the crack density. The implications of the correlations obtained and the future prospects of quantifying the damage severity by such correlations are discussed. [ABSTRACT FROM AUTHOR]

In this work, a structural health monitoring system has been implemented with damage identification purpose on a winglet of a general aviation aircraft. Using a pitch-catch approach, guided waves (Lamb waves) have been measured by means of an array of piezoelectric sensors able to excite and record the dynamic response of the structure. The undamaged configuration has been used as reference state (baseline) for the structural health monitoring (SHM) purpose while the damaged configuration (current state) has been obtained by low velocity impact test. The effectiveness of the damage has been verified using a Non-Destructive Inspection (NDI) by means of a C-SCAN Olympus Omni SX. The calculation of the damage index obtained comparing the measured wave propagation data in a reference state and the current state is introduced as a determinant of structural damage. Its calculation in different paths associated with the Probability Ellipse (PE) method has been used to identify the position of the damage. Additional radar graphs have been developed for the measurement of the directionality of the Lamb waves and further considerations have been introduced in order to evaluate the sensitivity of the Lamb waves with the aerodynamic load. [ABSTRACT FROM AUTHOR]

In this paper, the attenuation of guided waves in damped multilayered arbitrary anisotropic composite laminates is investigated using a Legendre orthogonal polynomial approach. The laminate is made of the graphite–epoxy composite material with the layering sequence [0/ ϕ /0]. The validity of the polynomial approach is illustrated through a comparison with exact solution obtained from the Stiffness matrix method and a comparison with earlier known Lamb wave solution of pure elastic multilayered plate. The dispersion and attenuation of coupled Lamb waves with different layering sequences are investigated. The viscous effect on dispersion curves is shown. The combination of frequencies and modes with minimum attenuation in high and low frequencies are determined. These frequencies and modes are of great importance for NDT applications since they play a critical role in the selection of the optimal inspection frequencies. [ABSTRACT FROM AUTHOR]

On the basis of reverberation-ray theory, this article presents the development of a new mode tracing method which can be used to efficiently calculate dispersion curves of Lamb waves in composite laminates. The dispersion relations are built by reverberation-ray formulas in three-dimensional Cartesian coordinate, and the transcendental equations are numerically solved by the improved mode-tracing method. In contrast to conventional mode-tracing method, a slope control algorithm is adopted in the new approach to avoid the switch of tracer from one mode to another around the cross point of dispersion curves, which solves the problem of solution loss. Numerical verifications are conducted to assess the performance of the proposed method and the good agreement with the existing experiment results are demonstrated to show that the proposed method is accurate, efficient and robust for the dispersion analysis of Lamb waves in composite laminates with arbitrary lay-up. [ABSTRACT FROM AUTHOR]

In this paper an application of Artificial Neural Networks and Probability Ellipse methods for damage detection using Lamb waves is presented. The wave propagation data are used to determine the location and degree of damage in metallic and composite plates. The structural components are to be instrumented with array of actuators and sensors (PCB) able to excite and record its dynamic response. A damage index, calculated from the measured wave propagation data in a reference state (baseline) and the current state, is introduced as a determinant of structural damage. The index is a relative measure comparing the two states of the structure under the same ambient conditions. The damage index increase its intensity when the damage size increase too. Moreover, its calculation in different paths associated with the Artificial Neural Networks or Probability Ellipse methods allow to identify position and entity of the damage. Two panels made by aluminum and fabric composite material have been used for tests. A damage entities simulated as through-thickness hole with diameter equal to 6 mm has been investigated for aluminium panel while three damage entities (through-thickness hole with size equal to 2.5 mm, 4.5 mm and 9 mm) have been investigated for that in composite material. In order to get a training, needful for the application of the ANN method, finite elements analysis have been performed for the aluminum panel taking into account several damage paths. Simulations have been performed in order to characterize the propagation path of the Lamb waves in composite plate changing the damage entities from hole diameter equal to 2.5 mm up to 9 mm. Damage index for each damage size for both simulations and tests will be compared. The potential applications of these methods in developing health monitoring systems in defects-critical structures and in composite material structures are discussed. [ABSTRACT FROM AUTHOR]

This paper focuses on quantification of attenuation coefficient of fundamental anti-symmetric (A o ) Lamb wave when it propagates through hybrid structural composites with nanomaterials. Investigations were carried out on nanosilica and carbon nanofiber hybridized-glass and carbon fiber reinforced epoxy composites in comparison to their respective control composite laminates with no nanofillers. Composite specimens were fabricated through Resin Film Infusion (RFI), ensuring near zero void fractions. Effective and actual attenuation characteristics of A o mode were evaluated at three different frequencies – 100 kHz, 200 kHz and 500 kHz in hybrid glass and carbon based composites. It is observed that effect of the nanomaterials (nanofillers) on attenuation of A o mode is more significant at high frequencies. Furthermore, attenuation behavior of A o mode depends on the type of nanomaterial. It is also observed that A o mode undergoes more attenuation in glass fiber reinforced composites compared to their carbon fiber based counterparts, irrespective of the nanomaterials. [ABSTRACT FROM AUTHOR]

Lamb waves are useful for detecting changes in material properties, as their propagation characteristics are sensitive to changes in the medium through which they travel. Defects and damage in materials/structures can be detected by analyzing the difference between the phase and/or group velocity amplitude and mode changes of Lamb waves’ propagation in damaged versus un-damaged specimens. The propagation characteristics of Lamb waves are described in the form of dispersion curves, relating phase and/or group velocities to the product of frequency and thickness, with these curves being generated by solving the Lamb wave equations. In this article, the effect on the Lamb wave propagation/dispersion due to changes in the material properties such as E 11 , E 22 , G 12 , and density were analyzed for three different composite laminate types – unidirectional, cross-ply, and quasi-isotropic, all constructed out of 16-plies of unidirectional carbon-fiber epoxy prepregs. The analysis was performed by obtaining the characteristic Lamb waves’ phase velocity dispersion and slowness curves for each laminate type. This was done by reducing E 11 , E 22 , and G 12 by 5%, 10%, 15% and 30%, and density by 1%, 2%, 5% and 10% with the intent of representing manufacturing and in-service defects. Since the Lamb waves’ velocity in a composite laminate varies with the propagation direction, the laminates were analyzed at 0°, 20°, 45°, 70°, and 90° propagation angles, with all the aforementioned variations. A program developed in MATLAB was used to generate the dispersion curves for the fundamental anti-symmetric and symmetric Lamb wave. The information provided in this paper can be used to select the proper Lamb wave modes, excitation frequency, which in turn governs sensor selection, data acquisition hardware, and damage detection algorithm for structural health monitoring of composite structures. [ABSTRACT FROM AUTHOR]

Lamb wave is widely studied in the non-destructive testing of composite materials due to its wide detection range and high sensitivity to defects. In order to solve the problem of extracting the effective signal of damage in a strong noise environment to improve the accuracy of damage location determination. A damage location imaging method based on power spectral density and Lamb wave tomography is proposed in this paper. The damage location imaging of composite materials under strong noise environment is realized through simulation and experiment respectively. Firstly, the simulation analysis the propagation characteristics of Lamb waves in composite materials. The circular sensor array is evenly arranged on the composite material. Each sensor is used as an actuator to generate Lamb wave in turn clockwise. Other sensors are responsible for collecting signals. Strong noise is added to the collected signal to simulate the signal collected in the strong noise environment. Finally, the damage information is characterized by power spectral density to determine the damage factor, and by using the probability imaging algorithm, the damage location imaging is totally realized. The experimental results show that the maximum error of imaging positioning for single damage and multiple damage under strong noise environment is 5.10 mm and 8.04 mm, respectively. This method does not need to pretreatment the signal with strong noise, and can directly image the original signal. At the same time, the extraction process of complex reflected signals is avoided, and it has a great potential in the location and identification of composite materials damage in a strong noise environment. [ABSTRACT FROM AUTHOR]

In this study, a damage identification approach was developed for carbon fibre/epoxy composite laminates with localized internal delamination. Propagation of the Lamb wave in laminates and its interaction with the delamination were examined. The fundamental symmetric Lamb wave mode, S0, and the lowest order shear wave mode, S0′, were chosen to predict damage location. A real-time active diagnosis system was therefore established. This technique uses distributed piezoelectric transducers to generate and monitor the ultrasonic Lamb wave with narrowband frequency. The two-way switches were employed to minimize the number of transducers. A signal-processing scheme based on the time–frequency spectrographic analysis was utilised to extract useful diagnostic information. Also, an optimal identification method was applied on damage searching procedure to reduce errors and obtain the diagnostic results promptly. Experiments were conducted on [0/−45/45/90]s CF/EP laminates to verify this diagnosis system. The results obtained show that satisfactory detection accuracy could be achieved. [Copyright &y& Elsevier]

In this paper, the hybrid Delay-And-Sum (DAS) with Sparse Reconstruction (SR) method was further developed for damage location in composite plates. In composite materials, anisotropy leads to some challenges in using conventional damage location methods, which are developed for isotropic materials. In the hybrid DAS-SR method, the DAS and SR methods were combined as a complement of each other. To investigate the DAS-SR method for composite structures, the group velocity of the travelling wave for different directions was first measured experimentally via PZTs. The DAS and SR formulations were then modified to be compatible with the direction-dependent group velocities. The results show that this modification increases the accuracy of the DAS and SR methods for damage location in a composite plate. However, using group-velocity instead of phase-velocity in the standard SR method, causes some model mismatch and errors in the damage localization and this problem was solved by using the modified hybrid DAS-SR method. The experimental results demonstrated that the performance of the modified hybrid method for detection and location of barely visible impact damages as well as for multi-location artificial damages is better than that of the DAS and SR methods when separately used. [ABSTRACT FROM AUTHOR]

• Lamb wave propagation method can be used for the detection of matrix cracking. • The Lamb wave velocity and amplitude decreased with matrix crack density. • Artificial intelligence methods helped the classification of matrix cracking. • Employing the linear discriminant analysis enhanced the classification accuracy. The guided wave propagation and artificial intelligence (AI) approaches were used to propose an intelligent model for automatic detection and classification of the matrix cracking in composites. Glass/epoxy cross-ply laminated composites were fabricated and the matrix cracking with several densities was induced in 90° layers. A non-destructive testing procedure using the fundamental antisymmetric Lamb wave propagation was performed on the intact specimens and those with 0.05, 0.15, and 0.25 matrix cracking density. The velocity of propagated Lamb wave, wave amplitudes in four different distances from the actuator, and the ratio of these amplitudes to the first received wave amplitude, in three frequencies of 100, 200, and 330 kHz were extracted from the acquired signals. The extracted sensory features were used to train three types of supervised machine learning models for the crack density classification. The linear discriminant analysis (LDA) was performed for dimensional reduction to find a linear combination of features that can better discriminate the classes. Support vector machines (SVM), linear vector quantization (LVQ) neural network (NN), and multilayer perceptron (MLP) NN were used for the classification. It was shown that SVM accounted for the highest classification accuracy (91.7%) followed by LVQ NN (88.9%) and MLP NN (77.8%), respectively. [ABSTRACT FROM AUTHOR]

The purpose of the present study is to develop a nondestructive evaluation method and propose a model-independent viscoelastic characterization approach for the identification and classification of the matrix cracking in polymeric composites using the Lamb wave propagation method. To this end, cross-ply laminated composite specimens were fabricated and matrix cracking with different crack densities was induced in specimens using a tensile testing machine. The crack density was first measured by an optical microscope and compared with that of predicted by the Hashin model. Then, the fundamental anti-symmetric Lamb wave (A 0) at a frequency of 206 kHz was propagated in specimens using an ultrasound actuator and an ultrasound probe to obtain the characteristics of the induced waves including the displacement phase and the amplitude at different locations of the specimens along the propagation direction. Then, the corresponding inverse Lamb wave propagation problem was solved and the elastic and viscoelastic properties of specimens were estimated. The elastic modulus measured by the present method was in good agreement with the results obtained by an optical microscope and predicted by the Hashin model. The viscoelastic properties of specimens also showed good sensitivity to the severity of the induced matrix cracking. [ABSTRACT FROM AUTHOR]