Waves In Composite Plates Research Articles

Experiments and modelling of ultrasonic waves in composite plates under varying temperature

Guided wave (GW) structural health monitoring (SHM) systems offer an attractive solution as an in-situ quasi real-time assessment of structural damage, but their sensitivity and efficiency may be impaired under varied environmental and operational conditions. Thus, virtual tests, such as that based on the Finite Element (FE) method, represent a valid approach for simulating and investigating SHM systems, enabling a substantial reduction in experimental campaigns. In this work, GW propagation characteristics in a carbon fibre-reinforced composite plate are studied under a varying temperature condition, representative of the aeronautics application. At first, GW SHM system was physically tested at room temperature (20°C), and the results were used to calibrate and assess the proposed FE modelling approaches, characterised by different element types and mesh sizes. A temperature independent averaged time compensation factor is proposed to mitigate the numerical data dependency on excitation frequency and propagation angle. Two temperature variations (from 20°C to -50°C, and 20°C to 65°C) were experimentally and numerically considered to investigate the effect of varying temperature on the GW. For all test cases, the compensated numerical data was compared to the experimental results, and discussed in terms of dispersion curves, focusing on the zero-order symmetric, S0, and antisymmetric, A0, modes. Results show that both 2D and 3D FE approaches can accurately predict the changes in GW due to varying temperature, with the group velocity of the A0 mode being less sensitive to temperature variations.

Ultrasonic inspection for ice accretion assessment: effects on direct wave propagation in composite media

From aeroplane wings to overhead power lines, through giant blades of wind turbines, a build up of ice can cause problems ranging from impaired performance all the way to catastrophic failure. Therefore, it is of the utmost importance to control or prevent ice formation, especially on the critical areas of a structure. However, de-icing and anti-icing countermeasures can result energetically expensive and hazardous to the environment. In addition, excessive use of such of them will reduce the life of the ice protection system and introduce fatigue to the controlled structures. Thus, in order to manage properly the available resources, it is desirable to have an ice detection system that can both detect ice formation and monitor the thickness of ice on critical surfaces. This would allow acting when it is necessary. Ultrasonic guided-wave-based technique has been proved reliable for ice detection, but approaches to establish ice accretion over time have not been provided yet. The paper investigates the interaction of ultrasonic direct waves in composite plate with ice layers and the effects of ice thickening on the ultrasound propagation. Moreover, the use of metrics is discussed as indicators of ice growth.

The Detection of Impact Damage to the Edges of CFRP Plates Using Extensional Ultrasonic Edge Waves

Extensional edge waves propagate along the edges of plates, with low attenuation in the propagation direction and amplitude decreasing rapidly (within one or two wavelengths) in the direction perpendicular to the plate edge. This makes them an ideal candidate for inspecting the edges of plate-like structures. Here, finite-element models and experiments are used to investigate the propagation and scattering of extensional edge waves in composite plates and application to damage detection is demonstrated. Piezoceramic transducers attached to the edge of a 4-mm-thick carbon-fibre-reinforced polymer (CFRP) plate were used to excite 140-kHz edge waves and damage detection demonstrated using two experimental configurations: pitch-catch and two-transducer pulse-echo. Damage due to edge-on impacts of 5 J and 10 J were detected in both configurations. A mass-addition damage model was used to investigate the effect of damage location. Detection was specific to damage within 33 mm (1.5 wavelengths) of the plate edge with propagation unaffected by features beyond that distance. The time of arrival of reflected signals in pulse-echo mode was accurately predicted using the edge-wave group velocity indicating that this configuration can be used for locating damage on edges. The localisation of edge waves means that they can specifically detect damage at the edges of structures, and attenuation measurements indicate that their useful propagation distance is large (5.5 m).

Experiments and Modelling of Ultrasonic Waves in Composite Plates Under Varying Temperature

Experiments and Modelling of Ultrasonic Waves in Composite Plates Under Varying Temperature

Temperature Compensation Method of Lamb waves on Composite Plate using First Arrival Wave

There are two basic methods of temperature compensation on isotropic plates, Optimal Baseline Selection(OBS) and Baseline Signal Stretch(BSS). The OBS method compensates the temperature using multiple baseline signals, while the BSS method using one baseline signals and stretch factor. However, in composite materials, the velocity of lamb waves differs from one direction to another. In this paper, a temperature compensation algorithm is proposed using the First Arrival Wavepacket(FAW) factor considering the propagation direction of lamb waves in composite plates. The proposed algorithm consists of two parts. First, Fast Fourier Transform (FFT) was used to filter the response signal in the space domain. Second, the signal was compensated by using the FAW factor for each separated signal. And the accuracy of the algorithm was improved by considering the group velocity and phase velocity of the numerical value of the lamb waves. The numerical value is calculated by using the stress-strain relation and the equation of motion for particles in the composite. Finally, the error of the signals was calculated using the Maximum Error of the Subtracted Signal(MESS). Also, the temperature compensation method using the proposed algorithm was verified by comparing the existing OBS/BSS method. Also, the compensated signal was compared with the experimental signal.

Characteristics of In-Plane Waves in Composite Plates

The high frequency dynamic excitations generate both in-plane as well as bending waves in structures. In aerospace applications, many of these structures are made of composite materials. There are two types of in-plane motions, longitudinal and in-plane shear. Although these motions are uncoupled in isotropic materials, composite structures show coupled behaviour. The works reported on in-plane waves in composite structures assume that two in-plane motions are uncoupled as in isotropic plates. In this work, characteristics of the in-plane waves in composite laminated plates are investigated. Expressions for wavenumber, phase speed and group speed are derived. It is seen that in composite plates the two in-plane waves are coupled in longitudinal and shear propagations and are non-dispersive. The phase speeds of in-plane waves in composite plates can be much different from those determined using the expressions for isotropic plates where the waves are uncoupled. To validate the expressions derived the phase speeds of in-plane waves in a typical composite panel are determined experimentally. It is seen that the experimentally obtained phase speeds match well with the theoretical results.

Modeling Analysis of Guided Ultrasonic Waves in Composite Plates with Impact Damage

This paper investigates the propagation of guided ultrasonic waves and the interaction with impact damage in composite plates using a full three-dimensional Finite Element analysis. Impact damage in the composite plate was modeled as rectangular- and T-shaped delaminations. In order to provide guidelines for extending the modeling of realistic multimode impact damage, the impact damage was modeled as a combination of the delamination and reduced materials properties. The information obtained from these methods was compared to the experimental results around the damage area for a validation. There was a reasonable similarity between the experimental and FE results. The FE simulations can effectively model the scattering characteristics of the A0 mode wave propagation in anisotropic composite plates. This suggests that the simplified and easy-to-implement FE model could be used to represent the complex impact damage in composite plates. This could be useful for the improvement of the FE modeling and performance of guided wave methods for the in-situ NDE of large composite structures.

An improved time reversal method for diagnostics of composite plates using Lamb waves

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.

Propagation and scattering of guided waves in composite plates with defects

Failure in aerospace composites owing to low-velocity impact raises a significant maintenance concern because it can lead to invisible damage. For the aerospace industry, such defects pose a potential danger to the structural integrity of aircraft. This in turn jeopardises passenger safety and incurs high repair costs. Hence, it is important to efficiently monitor composite structures during the service life. In this study, the potential of low-frequency guided ultrasonic waves for health monitoring in laminated composite plates is investigated. This study focuses on the use of the first antisymmetric guided wave mode (A0). The first part of this study is to investigate the propagation of the A0 mode in three different undamaged composite plates experimentally. The dispersive and anisotropic behaviour are in agreement with the results of finite element simulations and semi-analytical analysis. The final part of this study presents the scattering of guided waves at the impact damage using a non-contact laser interferometer. Significant scattering activities were observed and the impact damage size can be estimated to be about 10 × 25 mm. In conclusion, these results demonstrate the potential of guided ultrasonic waves for the inspection of aerospace composite structures.

Propagation characteristics of laser-generated surface acoustic waves in composite plate with gradient changes of near-surface viscous moduli

Taking into account the gradient change of the near-surface viscous property, we develop a finite element model of laser-generated surface acoustic wave in composite plate. The propagation characteristics of the surface acoustic wave in the composite plate are studied in detail, and the influences of the near-surface viscous modulus, thickness, and Lam constant on the attenuation characteristics of the surface acoustic wave are discussed. In addition, the propagation characteristics of the surface acoustic wave are verified by the theoretical calculations of the dispersion and attenuation curves. The results show that the near-surface viscous modulus and thickness are related to the attenuations of the surface shear wave and the Rayleigh wave, but have no influence on the propagation velocity. Furthermore, the imaginary part of the Lam constant has great influence on the attenuations of the surface shear wave and the Rayleigh wave, whereas the imaginary part of has no effect on the attenuation characteristics of the two waves, which indicates that the attenuation of the surface acoustic wave is related to the near-surface shear viscous modulus. The study gives theoretical basis for evaluating the near-surface mechanical properties of the composite plates by the laser ultrasound technique. Besides, it provides a feasible way to study the surface micro-cracks on the composite plates.

Application of Artificial Neural Networks and Probability Ellipse methods for damage detection using Lamb waves

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 6mm has been investigated for aluminium panel while three damage entities (through-thickness hole with size equal to 2.5mm, 4.5mm and 9mm) 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 2.5mm up to 9mm. 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.

The propagation of coupled Lamb waves in multilayered arbitrary anisotropic composite laminates

Based on linear three-dimensional elasticity theory, the wave equations of coupled Lamb waves in multilayered arbitrary anisotropic composite laminates are derived using a Legendre orthogonal polynomial approach. The elastodynamic solution for the propagation of coupled Lamb waves in composite plates is also presented to determine the characteristics of coupled Lamb waves. To verify the applicability and validity of the method, two cases of bi-layered plates formed with isotropic components and anisotropic components, respectively, are primarily manipulated for comparison with earlier known results. Next, the dispersion curves, displacements and stress distributions of Lamb waves in multilayered anisotropic laminates are calculated. The effects of coupling and fiber orientation on the characteristics of the Lamb waves are illustrated. The potential usefulness of the fundamental modes of the coupled Lamb waves is discussed in detail.

Band structures of Lamb waves in one-dimensional piezoelectric composite plates: polarizations and boundary conditions

Theoretical studies are presented for the band structures of Lamb waves in a one-dimensional phononic crystal plate consisting of piezoelectric ceramics placed periodically in an epoxy substrate. The band structures for different polarizations and electric boundary conditions are calculated for the composite plates in this paper. It is found that the first band gap is always broadened by polarizing piezoelectric ceramics, and the band gap widths with short circuit boundary condition are always larger than that with open circuit situation for the same polarization. Employing harmonic frequency analysis, the numerical results show that the Lamb wave modes corresponding to certain frequencies in the composite plates can be modulated by the different electric boundary conditions. The researches show that it is possible to control the Lamb waves in the composite plates in the engineering according to need by choosing suitable polarizations and electric boundary conditions.

Quantitative Accessibility of Delamination in Composite Using Lamb Wave by Experiments and FEA

Delamination in composite structures plays a major role in lowering structural strength and stiffness, consequently downgrading system integrity and reliability. The aim of this study was to show that Lamb waves may be effectively generated using piezoelectric actuators embedded within a composite plate for use in health monitoring applications. A Lamb wave-based quantitative identification technique for delamination in VARTM (vacuum assisted resin transfer molding) composite structures was established. The propagation of Lamb waves in composite plates was evaluated using dynamic FEM analysis, then an experiment was carried out to verify the validity of the analytical study. The study assessed damage in the plates by fusing information from multiple sensing paths of the embedded network. To eliminate interference, a wavelet transform technique was applied to purify the acquired Lamb wave signals. The results showed that satisfactory detection of defects could be achieved with the proposed method.

Built-in transducer design for Lamb wave generation in composite structures

Conventional ultrasonic inspection of large composite structures is very time consuming due to the necessity of surface scanning. Because they can propagate over long distances, Lamb waves are an attractive solution to this problem. Two methods are available for the generation of Lamb waves in composite plates. The first method, and the most useful one, consists of using bonded transducers. The second solution is to use a built-in transducer. In both cases, the transducers must be designed to generate efficiently the signal and choose the mode of propagation. In the following, a method to design a built-in transducer will be discussed. First, the case of a unidirectional composite plate is considered. An analytical method is developed, then the results are compared with those obtained using finite element analysis. In the case of a square element transducer embedded in the composite plate, the computed results are compared with experimental data. Then, the method is extended to the case of a multilayered plate. An analysis is performed to determine the optimal design of a built-in multi-element transducer.

Floquet analysis of guided waves propagating in periodically layered composites

In this paper, we present an analysis and numerical calculations on the propagation of elastic guided waves in composite plates composed of periodic layers. The guided waves are constructed by superposing obliquely traveling Floquet partial plane wave solutions in such a way that they satisfy a standing wave resonance condition through the thickness of the periodic composite. We demonstrate that transverse resonance can be established only in the Floquet passbands of the periodic composite. The predicted phenomena observed is consistent with our previous experimental observations in the Lamb wave case for an aluminum/aramid-epoxy composite (Shull et al., 1994, J. Acoust. Soc. Am. 95, 99–108).

Propagation of impact-induced stress waves in composite plates

A theoretical investigation is carried out of the response of a laminated plate to a surface impact. The plate consists of four layers of a uni-directional fiber composite in a symmetric cross-ply configuration. The composite material is modeled as a transversely isotropic, homogeneous, elastic continuum. The four layers are assumed to be of finite depth, infinite lateral extent, and perfectly bonded to each other. The surface impact has been modeled by three different conditions, namely, a delta function impulse, a square pulse, and a half-sine pulse. The variation with depth of the normal component of stress is examined as a step toward gaining insight into the through thickness response of the plate.

Measurements of backscattered leaky Lamb waves in composite plates: 38154 Nagy, P.B.; Jungman, A.; Adler, L. Materials Evaluation, Vol. 46, No. 1, pp. 97–100 (Jan. 1988)

Measurements of backscattered leaky Lamb waves in composite plates: 38154 Nagy, P.B.; Jungman, A.; Adler, L. Materials Evaluation, Vol. 46, No. 1, pp. 97–100 (Jan. 1988)

Impact and Wave Propagation in Composite Plates

An analysis is presented of the stresses induced in a composite plate by impact loadings. The plate is represented by a refined theory which includes the effect of transverse shear, normal, and quadratic effects in the kinematic assumptions, and correctly predicts the dispersive effects of axial waves in composite plates. The method of characteristics is used to investigate the effect of longitudinal impact in a semi-infinite plate as predicted by the refined model. Results obtained by the refined theory are compared with corresponding results given by a less accurate six-mode theory and shown to be significantly different under certain impact loadings.

An analysis of waves in a two-layer composite plate and its application to surface wave propagation experiments on roads

In this paper a derivation is given of the theoretical dispersion curves relating phase velocity and wavelength of waves in a two-layer composite plate in which stresses and displacements are continuous across the interface. Computations are carried out for different ratios of shear moduli and thickness of the two media in the composite plate using a computer programme which is capable of deriving the real roots of the dispersion equation of waves in layered plates or a layered half-space with up to thirty layers. It is shown that part of the dispersion curve of flexural waves in the composite plate can be derived to an accuracy of 5 % in the wavelength by adding the wavelengths of the antisymmetric or flexural modes of the individual plates at each velocity. This approximation applies over a fairly wide range of shear modulus ratios (4:1 to 1: 2) and for a ratio of thickness of 2:7. The computations of the dispersion curves in free composite plates are shown to give an acceptable approximation to many features of the experimental flexural wave branch of the dispersion curves obtained on roads which have two upper layers of considerably higher shear moduli than the underlying media. Thus, the computational analysis provides a basis for obtaining the thicknesses and elastic properties of the upper layers of many types of road construction from the experimental dispersion curves.