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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]

• The tensile and shear elastic moduli of the plates can be estimated using the S 0 and A 0 modes. • The phase velocity of the S 0 mode depends only on the equivalent tensile elastic modulus and in-plane Poisson's ratio, whereas the phase velocity of the A 0 mode depends almost only on the equivalent shear elastic modulus. • The in-plane Poisson's ratio of cross-ply laminates changes sufficiently small to neglect its effect on the phase velocities of the S 0 mode. This paper proposes a simple and effective Lamb wave-based measurement method for the elastic moduli of cross-ply composite plates, considered equivalent to a single-layer orthotropic plate with nine independent elastic constants. The effects of theses constants on the phase velocities of the fundamental S 0 and A 0 modes are investigated. The phase velocity of the S 0 mode depends only on the equivalent tensile elastic modulus and in-plane Poisson's ratio in the low-dispersive frequency-thickness product regime, whereas the phase velocity of the A 0 mode depends almost only on the equivalent shear elastic modulus. Moreover, the in-plane Poisson's ratio of cross-ply laminates changes sufficiently small to neglect its effect on the phase velocities of the S 0 mode. Therefore, the tensile and shear elastic moduli of the plates can be estimated using the S 0 and A 0 modes, and the mapping relations between the phase velocities of these modes and the elastic moduli are established. The proposed approach has been numerically and experimentally tested on cross-ply carbon fiber reinforced laminates, and validated by theoretical values and conventional tensile tests. [ABSTRACT FROM AUTHOR]

The time reversal method has been widely used to detect damage in composite plates. Based on the time reversibility of Lamb waves, the waveform of the original input signal will be reconstructed after the time reversal process. However, the time reversal operator of Lamb waves in composite plates is a function of frequency. Thus the shape of the reconstructed signal will not be identical to that of the original input signal. To alleviate the effects of the time reversal operator, an improved time reversal method (ITRM) is proposed. In this method, the response signal in the forward path and the reconstructed signal in the time reversed path are both modulated by a weight vector which is obtained as the product of a window function and the reciprocal of amplitude dispersion. The window function varies with the excitation signal adaptively, and its shape is also determined by a threshold. An experimental example is then introduced to illustrate the effectiveness of the ITRM. The results show that the time reversibility can be greatly enhanced and the impact damage in the composite plate can be exactly located by the ITRM. [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]

Adhesively bonded joints between metallic plates and composite laminates are frequently used in various fields including aviation, automobile industry, and marine manufacturing. The edge of adhesive layer is exposed to pollutants in the air, which implies that the joints are inclined to suffer debonding defects along the adhesive bond line. To evaluate the contours of the edge disbond, a method based on propagation characteristic analysis and Lamb wave excitation recovery is proposed. Firstly, the dispersion curves of Lamb waves in bonded area and single-layer plate are obtained via the Semi-Analytical Finite Element (SAFE). Subsequently, the effect of disbond on propagation characteristics of Lamb waves is analyzed and discussed numerically. On this basis, the excitation recovery method is proposed and applied to the received waves. The value of envelope at the initial position is extracted as the index during backward propagation for quantitative evaluation of disband size for every transmitter-receiver propagation path. Finally, the disbond contours are sketched by synthesizing the information of all paths with a hypothesis that disbond exists at the edge of the adhesive layer. The proposed method is applied to a lap joint specimen composed of a quasi-isotropic CFRP laminate and an aluminum plate experimentally, and the result demonstrates the effectiveness of proposed method for the accurate evaluation of disbond contours. [ABSTRACT FROM AUTHOR]

As a good information carrier, solitary waves have broad application prospects in the field of non-destructive testing. In this paper, we studied the coupling effect between highly nonlinear solitary waves in the one-dimensional particle chain and composite plate with matrix cracking. We based on the tensor internal variable damage model and the residual stiffness model, obtained the couple differential equation of the one-dimensional particle chain and the composite plate with matrix cracking. The fourth-order Runge-Kutta method is used to solve the differential equations, and the change law of the particle velocity, internal force, and displacement curve is obtained. This paper analyzed the influence of the damage amount and the geometric parameters on the reflected solitary waves of composite plates with different material properties. It is found that the reflected waves are sensitive to the thickness of the plate, the average distance between cracks, and the thickness of the damage layer. The research results of this paper have enriched and perfected the theory of nondestructive testing of solitary waves in the field of composite materials and provided broad application prospects for the practical application of one-dimensional particle chains. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the investigation of the Lamb wave propagation in particle reinforced composites excited by piezoelectric patch actuators. A three-dimensional finite element method (FEM) modeling approach is set up to perform parameter studies in order to better understand how the Lamb wave propagation in particle reinforced composite plates is affected by change of central frequency of excitation signal, volume fraction of particles, size of particles and stiffness to density ratio of particles. Furthermore, the influence of different arrangements is investigated. Finally, the results of simplified models using material data obtained from numerical homogenization are compared to the results of models with heterogeneous build-up. The results show that the Lamb wave propagation properties are mainly affected by the volume fraction and ratio of stiffness to density of particles, whereas the particle size does not affect the Lamb wave propagation in the considered range. As the contribution of the stiffer material increases, the group velocity and the wave length also increase while the energy transmission reduces. Simplified models based on homogenization technique enabled a tremendous drop in computational costs and show reasonable agreement in terms of group velocity and wave length. [Copyright &y& Elsevier]

The problem of dispersion of guided waves propagating in stratified composite plates containing layers with alternating physical properties is analyzed by using combination of sextic formalism Cauchy, exponential fundamental matrices and Thomson – Haskell method for constructing transfer matrices. The developed method is applicable to analyzing stratified media containing homogeneous anisotropic periodic layers with arbitrary elastic anisotropy. It is revealed that the dispersion portraits of guided waves in plates containing even (asymmetric layout) or odd (symmetric layout) number of alternating layers are significantly different, remaining different even at large number of alternating layers. It is also found that the observed discrepancy between even and odd sets should in principle, be persistent for any composite plates with odd and even numbers of alternating layers. The latter circumstance prevents application of the existing methods of homogenization for obtaining dispersion portraits of guided waves in the stratified media with alternating layers. [ABSTRACT FROM AUTHOR]

Lamb wave is a promising means for active structural health monitoring and nondestructive evaluation of laminated composite plates. The inevitable reflections from structural geometric boundaries and other interferences limits the imaging performance of many existing imaging methods. The image generated through the well-known delay-and-sum method holds a wide mainlobe width and high level of sidelobes. Although the sparse reconstruction method is free from those deficiencies, it is sensitive to the regularization parameter. To overcome those limitations, a Lamb wave imaging method to locate anomalies or damage in laminated composite plates is proposed. The scattering signals are sparsely decomposed one by one with a prior weights penalized on the undetermined sparse coefficients, and the minimum variance distortionless response algorithm is adopted to process those sparse coefficients so as to generate the image. Experimental results on a quasi-isotropic laminated composite plate demonstrate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper presents a baseline-free damage detection method which leverages the multipath reflected Lamb waves for structural prognosis. From the viewpoint of ray tracing, the reflection of a propagating wave at a free edge is considered as a virtual transducer sitting at the mirrored location of the actual one, injecting excitation into the structure, or recording Lamb waves traveling from the source. These extra echoes arise waves that inspect the structure from different directions, which is equivalent to adding sensing paths into the sensor network. On this basis, the reciprocity breaking effects of each sensing path is quantitatively evaluated by measuring the waveform differences of the associated wave packets in the two response signals that recorded before and after the transmitter and receiver exchange their roles. Note that the indirect sensing paths related to edge reflections consist of several segments. The probabilistic imaging method is modified to accommodate both direct and indirect paths. The proposed multipath Lamb wave imaging algorithm is validated using an experiment conducted on a composite plate with simulated damages. The results demonstrate its availability and effectiveness, i.e., damage detection and localization is achieved using a single transmitter–receiver pair. [ABSTRACT FROM AUTHOR]

This study aims to use the passive and active acoustic-based health monitoring methods for impact damage assessment of composite structures. To this aim, a Carbon Fiber Reinforced Polymer (CFRP) composite plate was fabricated and subjected to a simulated low-velocity impact by performing repeated quasi-static indentation tests where a loading-unloading-reloading test profile with 5 repetitions was adopted. Two Acoustic Emission (AE) broadband sensors and a network of eight piezoelectric (PZT) sensors were attached on the composite plate surface. AE (passive method) was employed during the loading and reloading phases of the indentation tests to online monitor the critical damage occurrence and also specify the damage type while scanning of the plate with Lamb waves (active method) was done to localize the damage when the structure was unloaded. Felicity Ratio (FR) index which was calculated based on the AE data could accurately detect that critical damage occurred during the 5th loading-unloading-reloading stage when the structural integrity dropped to 60% of its initial stage. Furthermore, Lamb wave signals of central frequency 150 kHz localized the impact damage with error of 0.89 cm (3.6% error respect to the shortest dimension of the scanned area). [ABSTRACT FROM AUTHOR]