Christophe Delebarre

Damage assessment in composites by Lamb waves and wavelet coefficients

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.

Fatigue Crack Monitoring of Riveted Aluminium Strap Joints by Lamb Wave Analysis and Acoustic Emission Measurement Techniques

Abstract Statistics show that fatigue crack development comes first and foremost as a damage source in aerospace metallic structures. Currently, widespread methods are available to inspect these structures, but they are quite time-consuming, costly and require the structural system to be idle. Next, attempts to develop damage detection integrated systems are paramount for the safety and cost of such structures. 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. In this experimental analysis, Lamb waves are excited and received outside the joint area using piezoelectric transducers coupled onto the plates. The detected damage is cracks in joint resulting from fatigue loading. The collected signals on the piezoelectric transducers are analysed using Hilbert transform and time–frequency analysis. It is shown that the final interpretation of Lamb wave analysis may provide a means of sizing the defects and following the crack development. In addition to that, an acoustic emission system is used jointly with the Lamb wave analysis in order to discuss results and damage development. Finally, it is demonstrated that both methods can work together and the results obtained are in good agreement.

Design of optimal configuration for generating A0 Lamb mode in a composite plate using piezoceramic transducers

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.

Piezoelectric transducer embedded in a composite plate: Application to Lamb wave generation

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...

Radome health monitoring with Lamb waves: experimental approach

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.

Actuation performance of embedded piezoceramic transducer in mechanically loaded composites

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.

Modeling of integrated Lamb waves generation systems using a coupled finite element–normal modes expansion method

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.

The propagation of Lamb waves in multilayered plates: phase-velocity measurement

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.

Lamb wave beam-steering for integrated health monitoring applications

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.

ULTRASONIC CHARACTERIZATION OF HETEROGENEOUS MATERIALS USING A STOCHASTIC APPROACH

The ultrasonic nondestructive characterization of heterogeneous media is difficult due to their inherent inhomogeneity. Using high‐frequency ultrasonic waves, one gets very high spatial resolutions, and the characterization of heterogeneous materials appears then feasible. However, the echographic signal processing involves a new approach, different from the classical deterministic one. Due to the random nature of the scatterers distribution, a stochastic model allows the determination of the structural parameters, like the spacings between heterogeneities (mean value and probability density function). An identification process is used to define the most convenient values for the mathematical parameters involved in the probabilistic scatterers distribution law and enables one to deduce the local concentration of scatterers. The theoretical models have been confirmed through measurements on synthetic inhomogeneous media and on two different kinds of composite materials: Metal matrix composites and carbon e...