Search

This work was devoted to the development of a health monitoring system assigned to aerospace applications. Those applications concerned the detection of damaging impacts and debonding between stiffeners and composite skins, since they are the major causes of in-service damage of aircraft structures. The chosen health monitoring system was first based on the excitation and reception of Lamb waves along the structure by using thin piezoelectric transducers (active mode) and secondly on a continuous monitoring taking the same transducers used as acoustic emission sensors (passive mode). The composite specimen used was consistent with aircraft wingbox in terms of structure and loading. Several impacts with increasing energy increments were applied on the composite specimen. In passive mode, the study showed the ability of using the acoustic signature of an impact to detect possible damage. Moreover, the damage emergence in the case of damaging impact was confirmed in active mode. Further measurements during fatigue testing were performed. The aim was to demonstrate the ability of the system to monitor disbond growth between the stiffener and the composite skin. The sensitivity of the health monitoring system to the disbond growth was further demonstrated.

With intent to ensure health monitoring of foam core sandwich structures by a system based on ultrasonic methods, the use of Lamb waves is investigated. The propagation mechanisms are analyzed and reveal leaky waves properties. Moreover, the wave sensitivity to damages induced by low-velocity impacts is tested and the feasibility to detect debonding and foam failure is evaluated.

Thin piezoelectric transducers attached to or embedded within composite structures could be used for in situ structural health monitoring. For plate-shaped structures, the useful ultrasonic vibration modes are Lamb waves. Preliminary testing has already demonstrated the suitability and practical feasibility of such integrated transducers, but better control of the generation of Lamb modes seems to be necessary. Therefore, an original modeling approach has been developed, which can be used to design and optimize these "sensitive materials." This modeling technique allows the determination of the amplitude of each Lamb mode excited in a composite plate with surface-bonded or bulk-embedded piezoelectric elements. The method consists of a coupling of the finite element method (FEM) and the normal modes expansion method. The limited finite element mesh of the transducer and its vicinity enables the computation of the mechanical field created by the transducer, which is then introduced as a forcing function into the normal modes equations. The adequacy and accuracy of this modeling method have been numerically and experimentally verified.

Owing to the dispersive nature and complexity of the Lamb waves generated in a composite plate, the measurement of the phase velocities by using classical methods is complicated. This paper describes a measurement method based upon the spectrum-analysis technique, which allows one to overcome these problems. The technique consists of using the fast Fourier transform to compute the spatial power-density spectrum. Additionally, weighted functions are used to increase the probability of detecting the various propagation modes. Experimental Lamb-wave dispersion curves of multilayered plates are successfully compared with the analytical ones. This technique is expected to be a useful way to design composite parts integrating ultrasonic transducers in the field of health monitoring. Indeed, Lamb waves and particularly their velocities are very sensitive to defects.

The aim of this paper is to show that Lamb waves may be effectively generated using piezoelectric transducers embedded inside a composite plate, for nondestructive evaluation and health monitoring applications. A cylindrical transducer embedded in a composite host plate is considered. The electrical impedance of the transducer alone in vacuum and then of the embedded transducer, which allows the identification of the resonance modes, have been obtained by the finite element method (FEM). Moreover, the displacement fields in the plate, which allow the identification of the types of Lamb waves, have been computed at the resonance frequencies. Comparison between the FEM results and the Lamb wave dispersion curves of the host material are in good agreement. Experimental results (electrical impedance, frequency response, and phase velocities) concerning a composite plate specimen containing the same piezoelectric transducer inside it are shown. A good agreement is generally obtained between numerical and exper...

The identification of Lamb modes is still the most difficult step in the process of damage detection. Therefore, the aim of this paper is to use a dual signal processing approach in order to better identify Lamb modes. This approach is based on the use of a relationship between the Short Time Fourier Transform (STFT) and the Two Dimensional Fourier Transform (2DFT). Indeed, one direct theoretical relationship between their amplitudes is given in the case of both monomode and multimode signals. This relationship is then numerically verified by a two dimensional finite element method. This system is suitable for defect detection and can be easily implemented for real application to structural health monitoring.

Since a few years, Lamb waves have been seriously considered as a potential technique for integrated damage assessment systems. Since they are easily propagated over long distances in plate-shaped structures and are able to interact with inner material defects, Lamb waves are seen indeed as a most promising technique. Real-time, continuous health monitoring applications require the development of thin piezoelectric Lamb wave transducers intimately coupled to the structure to be monitored.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

International audience; The use of integrated Lamb wave sources (piezoelectric transducers) is known as a possible way of performing integrated, on-line health monitoring. Either omnidirectional (circular) or quasi-unidirectional (bar-shaped) transducers can be used. However, both of them have their own drawbacks, which makes them not optimal. A much more satisfying solution could be the use of phase-delayed multi-element arrays to perform angular steering of the emitted Lamb wave beam. In this paper both the proper conditions and the limitations for the applicability and performance of Lamb wave beam steering using integrated piezoelectric arrays are established. Then experimental damage detection capabilities using this principle are demonstrated.

A wavelet technique was used in an active system for the damage detection of aerospace composites. The active system was based on the generation and reception of Lamb waves by embedded piezoceramic transducers. The wavelets were used to decompose the Lamb-wave response into wavelet coefficients. The decomposition performance was improved by utilizing more adapted wavelets, based on the recurrent waveforms of Lamb waves. The changes in the Lamb waves interacting with damage in the plate were successfully characterized by this wavelet technique, through the amplitude change of the wavelet coefficients. The wavelet technique also showed great sensitivity in detecting damage of small sizes. This technique was found to be straightforward for detection of impact damage and evaluation of the damage size.

Lamb waves are used in the nondestructive evaluation (NDE) of composite materials as they can propagate over long distances. The usual method of generation relying on a transducer that can not be reduced to a low thickness, the development of a thin transducer using a comb-like array of piezoelectric elements has been investigated. It requires a deep understanding of the dispersion relations in the material, in order to determine its geometrical parameters for a set of frequencies. Then, a prototype designed to work on surface is presented and Lamb wave generation using "in-phase" and "varying phase" methods is discussed.

International audience; With the aim of developing an integrated health monitoring system, small piezoelectric Lamb wave transducers can be implemented into the structures to be monitored. Optimization of these transducers requires precise modeling of their radiation characteristics. In previous studies, bar-shaped integrated transducers have been modeled using a two-dimensional (2D) plane strain formulation. Thus prediction of the Lamb waves generated has been allowed. In this paper, additional information is provided by quantifying the in-plane directivity of such transducers. Prediction of the directivity pattern, as a function of the transducer length, is allowed by using a simple approximate model. Experimental measurements performed using various transducer lengths are in very good agreement with these theoretical predictions

This work concerned the study and the development of a health monitoring system for aeronautical applications. This system was based on the generation and sensing of Lamb waves in composite structures by thin piezoelectric transducers that were surface bonded. In order to optimise such a system, a hybrid modelling technique was used to conceive transducers better adapted to Lamb waves generation. The technique was successfully applied in the selection of a Lamb-mode that was sensitive to damage.

This paper describes an investigation into the feasibility of using an integrated system based on Lamb waves in order to assess the integrity of riveted aluminium joints during cyclical loading. An Acoustic Emission system is used jointly with the Lamb wave analysis in order to discuss about damage development. Finally, it is demonstrated that both methods can work together and that the results obtained are in good keeping.

This work was devoted to the development of a health monitoring system assigned to aerospace applications. The application concerned the detection of low damaging impacts on composite structures due to the extreme sensitivity of this material to this kind of solicitation. The chosen health monitoring was based first on the excitation and reception of Lamb waves along the structure by using thin piezoelectric transducers (active monitoring) and second on a continuous monitoring with the same transducers (passive monitoring). Preliminary tests were performed on a composite plate and the sensitivity of the A0 mode to the damage was demonstrated. Concerning the passive monitoring, the study showed the ability to use the acoustic signature of the impact to detect a possible damage. Further tests were performed on a wingbox composite structure. This wingbox structure consisted of composite skins with variable thickness mounted onto a metallic substructure. Moreover, four stringers were bonded on these skins. The aim was to demonstrate the ability of the system to perform rapid inspections of complex structures. After the application of a serial of impacts at diffent locations of the composite wingbox, the feasibility of the health monitoring was demonstrated.

The performance of embedded piezoceramic transducers (PZTs) used as Lamb wave generators was investigated in this paper. The composite specimens with a PZT embedded in the mid-plane were subjected to tensile and compressive monotonic loading as well as tension–compression fatigue loading. Both the static and fatigue tests revealed the large working range of embedded PZTs, despite the presence of damage observed by microscopy. The Lamb wave response remained unchanged after a large number of fatigue cycles; around 50 000–100 000 cycles at strain levels of ± 0.20%. At larger numbers of cycles, the changes in terms of amplitude and frequency in the Lamb wave response were believed to be associated with increasing matrix cracking in the specimens.

This work concerned a technique for a health monitoring system based on the generation and sensing of Lamb waves in composite structures by thin surface-bonded piezoceramic transducers. The objective was to develop transducers that are adapted for the damage detection in orthotropic composites. The key problem with the investigated Lamb waves was to select a mode to be sensitive to the damage. A hybrid modeling technique was therefore used to conceive transducers that were adapted to achieve such a feature. This modeling technique enabled studying the influence of the transducer characteristics on the Lamb waves propagating in orthotropic plates. It was demonstrated that a Lamb mode could be generated dominantly to other modes by using a multi-element transducer. The effectiveness of this technique was successfully verified experimentally on composite plates. It was shown that the dominant Lamb mode, obtained by use of dual-element transducers, was an appropriate mode for successfully detecting a damage in composites.

For a few years, the concept of integrated health monitoring of aeronautic structures has become an important issue. A promising solution is the emission of ultrasonic Lamb waves using thin piezoelectric transducers integrated to the structure. Indeed Lamb waves are known to be able to propagate over long distance and to interact with inner structural defects. One important aspect when applying Lamb waves to health monitoring is the control and optimization of their generation. Therefore, a hybrid finite element–normal mode expansion modeling technique adapted to piezoelectric transducers has been recently developed. This technique allows the determination of the amplitude of each Lamb mode as a function of the transducer parameters. So far, it had been successfully applied to the case of harmonic excitation only. In this paper, adaptation of this hybrid technique to transient excitation is presented, which enables to simulate actual experimental conditions. Contrary to other techniques presented in the literature, the present technique allows to take into account the actual electromechanical behavior of the integrated transducer. Predicted results have been successfully compared to experimental measurements performed on different types of materials.

The concept of a built-in structural health monitoring system has attracted great interest as such systems may allow reduced maintenance cost and increased safety. One attractive technique to realize such a system in composites is to use embedded transducers to generate Lamb waves. The damage in the structure is detected by determining the change in the character of the Lamb waves as an effect of damage. In this paper, the performance of embedded piezoceramic transducers used as Lamb-wave generators was investigated. The composite specimens with a piezoceramic transducer embedded in the mid- plane were subjected to tensile and compressive static loading as well as fatigue loading. A surface-attached acoustic emission sensor further detected the Lamb waves. Measurements of the impedance were also performed during static loading to evaluate the electrical conduction of the piezoceramic transducer. During static loading, the embedded piezoceramic transducers functioned near to the final failure of the composite without significant changes in the generated Lamb waves, although the microscopic examination indicated damage. Debonding between the surfaces of the piezoceramic element and the interconnectors as well as failure in the piezoceramic element had occurred. In fatigue, for a stress ratio of -1, the piezoceramic transducers also showed a large working range. The amplitude and frequency of the Lamb waves, generated by the piezoceramic transducers, were not significantly changed after a large number of cycles, around 50,000 - 100,000 cycles at strain levels of ± 0.20%. The changes in terms of amplitude and frequency of the Lamb waves, occurring after a large number of cycles, were associated with increasing matrix cracks in the specimen.

As part of the research work on Smart Materials and Structures, the development of self-monitoring materials is an emerging issue. In the case of plate-shaped structures, Lamb waves can be used for their relevant properties: long-range propagation, sensitivity to internal flaws and whole-thickness interrogation. This concept requires the use of thin piezoelectric transducers integrated to the structure. Since it is of primary importance to be able to control the generated modes, a suitable modeling technique of this kind of system has been tested on different cases of practical interest. The model uses a coupled finite element-normal modes expansion approach, which allows one to consider either the case of bonded or embedded transducers. The results presented deal with examples of multi-element transducers integrated to composite materials. The influence of parameters such as the dimensions, positions and relative excitation delays of the transducers is studied.

In this paper an experimental analysis of Lamb waves interaction with riveted aluminum plates representative of aircraft splice joints submitted to fatigue tests is given. In this joint evaluation technique Lamb waves are excited and received outside the joint area using piezoelectric transducers bonded onto the plates. Detected damages are cracks in joint resulting from fatigue loading. These cracks lead to waveform transformations. This phenomenon is studied in this paper by considering the first derivative of the envelope of the time domain signal. The position of the first derivative curve maximum during cyclical loading gives information about crack development. It suggests, that monitoring the change in the delay of this maximum may provide a means of sizing the defects. By using X-rays, it was possible to measure the size of the cracks and compare it with delay evolution. Results are obtained for two types of fatigue sequences. It is experimentally shown that the relative delay measured is very sensitive to crack development. With the continued progress in the field of damage assessment techniques such as methods relying on Lamb waves, the safety of such structures can be ensured.

Specific characteristics of Lamb waves, and more particularly their ability to propagate over long distances, make them a promising technique for health monitoring of composite structures using piezoelectric transducers bonded on the surface or embedded inside the material. In order to evaluate the possibilities of the technique, a theoretical dispersion model for the Lamb waves propagation has been realized for a carbon-epoxy composite material, and the control parameters, incidence angle, emission mode and frequency, have been fixed. The technique has then been experimented in a transmission mode with contact conversion mode transducers. The propagation over long distance and the detection of delaminations have been put in evidence. Then, optimized transducers have been manufactured in order to instrument, first, monolithic composite structures using surface bonding, and second, sandwich structures with embedded elements in the bulk. Experimental results show that a large network could be envisaged. This technique presents serious interests compared to conventional ultrasonic techniques, testing at a point, or passive measurements of local stress and strain modifications due to the delamination.

Lamb waves are used in the non destructive evaluation of composite materials as they can propagate over long distances. The usual method of generation relying on a transducer that can not be reduced to a low thickness (oblic incidence transducer), the development of a thin transducer using a comb-like array of piezoelectric elements has been investigated. It requires a deep understanding of the dispersion relations in the material, in order to determine its geometrical parameters for a set of frequencies. Then, a prototype designed to work on surface is presented and Lamb waves generation is compared with classical oblic incidence sensors.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Conventional ultrasonic inspection of large structures is very time consuming due to the surface scanning necessity. Since they can propagate over long distances, Lamb waves are an attractive solution to this problem. Here is shown the feasibility of using Lamb wave generation using a transducer embedded inside the material and two methods are given to determine the presence of a defect along a propagation line. In a first step, the embedded transducer is used to generate and receive Lamb waves. Then, a method based on the spatial Fourier transform is used to measure their phase velocities. The experimental measurements are in good agreement with a numerical phase velocity computation. In a second step, the determination of the presence of a defect along a propagation line is shown. One method consists in measuring the attenuation of Lamb waves during the propagation. Another one is based on the Hilbert transform, which can be used to determine mode conversions due to the damage. In both cases, the results show the feasibility of determining the presence of a defect.

The use of miniaturized piezoelectric transducers integrated to a host composite material could enable the development of an on‐line ultrasound health monitoring system for composite structures. Preliminary testing has already demonstrated the suitability and practical feasibility of this kind of integrated system, but a better control of these features now seems to be necessary. And especially, since the exploited ultrasonic vibration modes for plate‐shaped structures are Lamb waves, the generation of desired Lamb modes should be mastered. Therefore, an original modeling approach has been developed, in order to help to design and optimize these ‘‘smart materials.’’ This modeling technique aims at determining the amplitude of each Lamb mode excited in a composite plate with surface‐bonded or bulk‐embedded piezoelectric elements. It consists of a coupling of the numerical finite‐element method (FEM) and the normal modes expansion method. Indeed, a limited finite‐element mesh of the transducer and its close vicinity is built. Then the numerically computed mechanical field is introduced as a forcing function into the normal modes’ equations. In addition, the adequacy and accuracy of this modeling method have been experimentally demonstrated by using composite specimens containing small integrated piezoelectric elements.

Since Lamb waves are very efficient in the nondestructive testing of composite plates, new transducers are developed nowadays. They may be movable, stuck onto the surface, or integrated into the plates (‘‘smart structures’’). In order to detect defaults with few transducers, a multi‐element linear array transducer has been developed to generate single‐mode Lamb waves able to propagate over long distances. Movable prototypes were designed in order to evaluate their ability to generate the appropriate waves. Then, composite plates have been equipped with the same kind of transducers consisting of several thin PZT plates. High response levels have been obtained using a unidirectional composite with propagation along the fiber direction. Results for cross‐ply laminates were inferior but still acceptable. The main result obtained during this experimental work was the understanding of the generation process for this kind of transducer. Unlike comb transducers, each element generates a separate wave and suitable time delays or spacing between the elements can improve the global response in accordance with linear rules. Furthermore, the delays are easily determined from the responses of each element. This behavior is very interesting as default detection is possible using the properties of the generation process.

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.

With the aim of designing nondestructive evaluation and health monitoring systems for composite parts of complex structures (e.g., aircrafts), Lamb waves generated using embedded miniaturized transducers seem to be a very promising technique. Therefore, a cylindrical transducer embedded in a composite host plate has been considered. The electrical impedance of the transducer alone in vacuum and then of the embedded transducer, which allows the identification of the resonance modes, have been obtained by the finite‐element method (FEM). Moreover, the displacement fields in the plate, which allow the identification of the type of Lamb waves generated, have been computed at the resonance frequencies. Comparison between the FEM results and the Lamb waves dispersion curves of the host material are in good agreement. Experimental results (electrical impedance, frequency response, and phase velocities) concerning a composite plate specimen containing the same piezoelectric transducer inside it will be shown. A g...