Lynda Chehami

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.

Experimental study of passive defect localization in plates using ambient noise

Passive listening methodology has been shown to be a practical and effective method for passive structural health monitoring. In this work, this approach is applied experimentally to monitor the occurrence of defects in thin aluminum plates. A correlation matrix is estimated from noise vibrations recorded on a transducer array. A defect is localized by applying a beamforming algorithm to the difference between the correlation matrices obtained with and without the defect. We successfully detect defects for different kinds of noise sources. Moreover, we show that this technique is robust to detect massive inclusions, holes, and cracks. With a vibrometer, we observe that the fidelity of the estimated transient responses strongly depends on the number of uncorrelated noise sources. Finally, we show that the defect is successfully localized even if the noise source distribution is not uniform, provided that it remains spatially stationary between the states with and without defect. A simple theoretical framework is proposed to interpret these results.

Reverberation of flexural waves scattered by a local heterogeneity in a plate

A statistical model is proposed to relate the scattering properties of a local heterogeneity in a plate to the statistical properties of scattered and reverberated flexural waves. The contribution of the heterogeneity is isolated through the computation of differential signals consisting of a subtraction of the signals recorded after and before introduction of the heterogeneity. The theoretical expression of the average reverberation envelope of these differential signals is obtained as a function of the scattering cross-section of the heterogeneity. Successful numerical and experimental validations in various cases of canonical heterogeneities with known scattering cross-sections are shown. These satisfying results offer a way to estimate the scattering cross-section of an unknown scatterer from the reverberated differential signals.

The transmission spectrum of sound through a phononic crystal subjected to liquid flow

The influence of liquid-flow up to 7 mm/s is examined on transmission spectra of phononic crystals, revealing a potential use for slow liquid-flow measurement techniques. It is known that transmission of ultrasound through a phononic crystal is determined by its periodicity and depends on the material characteristics of the crystals constituents. Here, the crystal consists of metal rods with the space in between filled with water. Previous studies have assumed still water in the crystal, and here, we consider flowing liquid. First, the crystal bandgaps are investigated in still water, and the results of transmission experiments are compared with theoretical band structures obtained with the finite element method. Then, changes in transmission spectra are investigated for different speeds of liquid flow. Two situations are investigated: a crystal is placed with a principal symmetry axis in the flow direction ( ΓX) and then at an angle ( ΓM). The good stability of the bandgap structure of the transmission spectrum for both directions is observed, which may be of importance for the application of phononic crystals as acoustic filters in an environment of flowing liquid. Minor transmission amplitude changes on the other hand reveal a possibility for slow liquid flow measurements.The influence of liquid-flow up to 7 mm/s is examined on transmission spectra of phononic crystals, revealing a potential use for slow liquid-flow measurement techniques. It is known that transmission of ultrasound through a phononic crystal is determined by its periodicity and depends on the material characteristics of the crystals constituents. Here, the crystal consists of metal rods with the space in between filled with water. Previous studies have assumed still water in the crystal, and here, we consider flowing liquid. First, the crystal bandgaps are investigated in still water, and the results of transmission experiments are compared with theoretical band structures obtained with the finite element method. Then, changes in transmission spectra are investigated for different speeds of liquid flow. Two situations are investigated: a crystal is placed with a principal symmetry axis in the flow direction ( ΓX) and then at an angle ( ΓM). The good stability of the bandgap structure of the transmission ...

Ultrasonic investigation of fatigue, tensile damage, and impact damage in woven glass-fiber fabric reinforced composites with comparison to X-ray tomography

This work was done on woven glass-fiber fabric reinforced composite samples. Those materials exhibit a complex anisotropic evolution of defects induced by several damage mechanisms. In order to non-destructively evaluate the damage accumulation within this material, a methodology based on the measurement of the complete stiffness tensor is considered. After validation of the detectability of increasing damage state with this method, a new damage indicator is proposed to thoroughly quantify it. Samples were damaged by tensile tests (quasi-static and fatigue) at increasing stress levels along and out-of fibers axis. Afterwards, drop-weight impact is performed to consider several damage situations. Finally, an X-ray tomography is conducted to identify the damage mechanisms as well as the evolution of the void volume fraction. It is shown that this evolution has the same tendency with the ultrasonic damage indicator.

Non destructive evaluation of adhesion quality in a bi-material structures

The difficulty of applying ultrasonic techniques to bi-material structures lies in the fact that different phenomena coexist, such as the non-linear effects caused by the adhesion properties between matrix-fibers and the diffraction effect caused by the periodicity of those structures. This work deals with the study of the combined effect of non-linear effects on a 3D bi-materials structure “polymer-titanium.” For this purpose, a Snapscan system is used which generates high amplitude pulses and receives signals composed of the fundamental mode and the higher harmonics. The transducers used are chosen according to the thickness and the periodicity scale of the sample. First, measurements in transmission are made where the transmitted ultrasonic waves are measured by changing the angle between the receiver and the sample. For each angle, a spectrogram is realized. The experimental observations on the spectrograms show that for small incident amplitudes we measure only the fundamental frequency. When the amp...

Propagation of ultrasonic waves in complex media: Investigation of coda wave technique and traditional polar and C-scan imaging

A discussion of nondestructive techniques is presented for the investigation of complex media, with a focus on composite samples. Traditionally, one applies ultrasonic C-scans, or polar scans, which are easy to implement and to interpret. However, in many realistic cases, it is important to use more sophisticated approaches as C-scans often do not reveal any useful information. Typically, the early part of received signals is used to extract information, whereas the later part is considered either as noise or as a useless coda wave as in musical acoustics. Nevertheless, it appears that the coda part carries useful information about the medium, and therefore, it is important to explore techniques to extract that information. In addition, it turns out that the coda is very sensitive to material properties and damage as those sound waves interact longer with the material than early arrival waves. First, earlier results will be shown which compare experimental polar scans with numerical simulations, then, for...

Non destructive evaluation of bi-material structures: a case study

Due to their specific elastic properties (high stiffness to mass ratio), regular microstructure materials (architectural materials), are widely used in industry. This work deals with nondestructive evaluation of a 3D bi-material structure made of an architectural ABS polymer embedded in an Epoxy resin. The difficulty of applying ultrasonic techniques to bi-material structures lies in the fact that different phenomena coexist, such as diffraction effects caused by the periodicity and the nonlinear effects caused by the adhesion properties between matrix and fibers. A combination of techniques have, therefore, been applied. For this purpose, a series of measurements in reflection mode were first employed to measure the characteristic Bragg spectrum of a healthy sample and after compressive loadings. The results show that the damage disturbs the periodicity of the structure and consequently alters the Bragg spectrum. Then the nonlinear effects, possibly due to adhesion, are investigated using cross-modulatio...

Passive defect detection in plate from nonlinear conversion of low-frequency vibrational noise

It is known that, under the assumption of diffuse noise, the cross-correlation of acoustic signals recorded at two points of a medium allows to passively estimate the impulse response between these points. This principle, associated with coherent array processing, has been successfully applied to defect detection and localization in reverberant plates subject either to distributed noise sources or friction noise. In order to extend the applicability of this principle even in the absence of an adequate ambient noise, we have introduced the concept of secondary noise sources based on the conversion of low-frequency modal vibrations into high-frequency noise by exploiting frictional contact nonlinearities. The device consists of a mass-spring resonator coupled to a flexible beam by a rough frictional interface. The extremity of the beam, attached to the surface of a plate, excites efficiently flexural waves in the plate in the ultrasound range when the primary resonator vibrates around its natural frequency....

Characterization of a local scatterer in a plate from reverberation of flexural waves

In media where acoustic waves are subject to multiple propagation paths (scattering or reverberation), recorded acoustic signals present a random aspect. Still, such signals carry some information about the medium, and ensemble averaging can give access to the estimation of a number of useful parameters. A well-known example, in room acoustics, is the estimation of absorption coefficients of walls from average decrease of the reverberation envelopes. In the work reported here, we have proposed a statistical model allowing to relate the scattering properties of a local heterogeneity (defect) to the average properties of reverberated acoustic signals in a solid plate. A theoretical expression of the averaged envelope of signals produced by scattered and reverberated flexural waves has been derived and both numerically and experimentally validated. A simple curve-fitting procedure applied to signals recorded on a few receivers then allows an experimental estimation of the scattering cross-section of the hete...