Emmanuel Moulin

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.

Experimental study of the A0 and S0 Lamb waves interaction with symmetrical notches

The aim of this work is to study the fundamental Lamb modes interaction with defects in isotropic plates. For these experimental investigations, symmetrical notches with various depths milled in aluminum plates are considered. Moreover, the incident Lamb wave of a specific mode is generated by means of two identical thin piezoceramic transducers placed at the opposite sides of the plate. The waves scattered by the notch are recorded with conventional transducers located on the plate surface in front and behind the defect. The selection of the A(0) or the S(0) modes is obtained by exciting the transducers with anti-phased or in-phased signals, respectively. Furthermore, a calibration process is investigated to correct errors caused by the presence of the receiver between the emitters and the defects. The power reflection and transmission coefficients are then obtained and the power balance is verified. Finally, these measurements are compared successfully with those obtained by a numerical method using the finite-element modeling described in a previous work.

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.

Applicability of acoustic noise correlation for structural health monitoring in nondiffuse field conditions

Ambient noise correlation offers an elegant way of passively caracterize a medium. The principle is that in diffuse field condition, Green’s function between two sensors can be retrieved from the cross-correlated received signals. Here, the technique is applied in conditions where convergence toward Green’s function is not ensured. Since the obtained information is sensible to the medium state, it is still exploitable for damage detection provided a baseline version is available for comparison. This requires the identification of the acoustic source characteristics at the instant of measurement, which could be achieved using the signal measured at a “reference” sensor.

Pseudo-3D modeling of a surface-bonded Lamb wave source

In this paper, a simple technique allowing the prediction of the Lamb wave field excited in an isotropic plate by a transducer of finite dimensions is presented. The basic idea is to separate the problem into two uncoupled parts. First, the Lamb wave excitation problem, in the plane defined by the thickness and width directions of the transducer, is treated. Then the source diffraction effects in the plane of the plate can be quantitatively estimated. The subsequent formulation thus offers a simple way of developing three-dimensional (3D) solutions from a two-dimensional (2D) modeling. Compared to a heavy full-3D numerical modeling, this technique appears to be a very satisfying alternative.

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.

Transient modeling of Lamb waves generated in viscoelastic materials by surface bonded piezoelectric transducers

In order to help optimize health monitoring systems based on Lamb wave propagation, a hybrid finite element–normal mode expansion modeling technique has been developed. This technique allows one to predict the Lamb waves generated in a structure as a function of the transducer parameters (size, location, material, etc.). The present paper shows how this hybrid modeling, initially valid for nonlossy materials and harmonic excitation, can be extended to lossy (viscoelastic) materials and transient excitation. For this purpose, simplifying assumptions have been proposed and validity conditions have been established and verified. Moreover, numerical results concerning Lamb wave generation have been successfully compared to experimental ones. As a result, the method has proved to be pertinent and practically unrestrictive.

Detection and localization of a defect in a reverberant plate using acoustic field correlation

Greens function can be retrieved from cross-correlation of a diffuse field generated by noise sources. Today, this important result is the fundamental of several passive imaging techniques. The aim of this paper is to establish the suitability of these methods to detect and locate a defect in a reverberant elastic plate. The relations between the noise cross-correlation function over a few number of noise sources and the imaginary part of the Greens function are derived and numerically validated. Then we show through numerical experiments that this technique is exploitable for defect detection and localization in a differential mode, despite a non-perfect estimation of the Greens functions. Finally, a filtering technique based on the singular value decomposition is shown to improve the detection.

Lamb wave assessment of fatigue damage in aluminum plates

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.