Localisation de source en milieu réverbérant par Retournement Temporel

thèse co-encadrée par Julien de Rosny, Stéfan Catheline et Didier CassereauIn this thesis, the Time Reversal Process is applied in the acoustic domain, first to focus a few seconds-long random noise in a reverberant room, and second to localize impacts given at the surface of thin plates. Concerning the first application, 2D numerical simulations as well as 3D experiments show a spatial focusing of the signals. The signal to noise ratio is proved theoretically and experimentally to depend only on the number elements of the Time Reversal Mirror. The robustness of this focusing is studied while the medium reciprocity is broken between the two Time Reversal steps.Moreover, a technique is developed to localize impacts given at the surface of reverberant plates. It is based on the pattern matching of acoustic signatures. The link between this method and Time Reversal, and the study of the waves excited by such impacts allow to predict the resolution and the signal to noise ratio of the technique. The interaction of the predominant wave (the A0 Lamb wave) with the plate's borders is studied. Besides, a finite differences simulation of A0 mode propagation in thin plates is developed. At last, the sensibility of the localization technique to temperature variations is studied in many materials, numerically, theoretically and experimentally. When temperature changes, the impulse responses are simply stretched, which can easily be compensated for.Cette thèse concerne l'étude du Retournement Temporel (RT) en milieu réverbérant, appliqué d'une part à la focalisation d'ondes acoustiques audibles longues de plusieurs secondes à caractère aléatoire dans une chambre réverbérante, et d'autre par à la localisation d'impacts à la surface de plaques minces réverbérantes. Concernant cette première application, une focalisation spatiale des signaux est observée tant numériquement qu'expérimentalement. Il est montré à la fois théoriquement et expérimentalement que le contraste ne dépend que du nombre d'éléments du Miroir à Retournement Temporel. Enfin, la robustesse de la focalisation lorsque la réciprocité du milieu est brisée entre les deux étapes du RT est étudiée. D'autre part, afin de localiser des impacts à la surface de plaques réverbérantes, une technique reposant sur la comparaison des signatures acoustiques par corrélation est développée. L'analogie avec le RT, ainsi qu'une étude de la nature des ondes excitées par un impact à la surface de plaques, permettent de comprendre les capacités de cette techniques en terme de résolution et contraste. L'interaction de l'onde majoritaire (onde de Lamb A0) avec les bords de la plaque est également étudiée. En outre, un code de simulation numérique de la propagation de ce mode par différences finies dans l'approximation faible produit fréquence par épaisseur est développé. Enfin, l'influence des variations de température sur la technique de localisation par corrélation est étudiée numériquement, théoriquement et expérimentalement, dans plusieurs matériaux. Un changement de température entraîne une simple dilatation des réponses impulsionnelles, et est aisément compensé

Localisation de source en milieu réverbérant par retournement temporel

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Evaluation of metallic bonded plates with nonlinear ultrasound and comparison with destructive testing

International audienceIn the last decades, the use of structural adhesion has increased. It is indeed able to replace traditional bonded techniques such as rivets or bolts, and reduces the global weight of the structure, which is of great interest in the aeronautic industry. In addition, it allows the assembly of mixed or composite materials with better stress repartition. However, to be used for structural joining and critical application, reliable non-destructive testing techniques are compulsory for evident safety reasons, be it after fabrication or during the whole life of the structure. While linear ultrasound is known to detect easily decohesion or voids in a structure, its use for bond strength inspection is less straightforward. Constraining hypothesis have to be made to retrieve the bond strength [1]. However, in some cases, the effect of such bond degradation on ultrasound is the same as the effect of geometrical fluctuations of the structure (eg thickness of various layers)[2]. Another approach relies on the nonlinear signature of a bond defect inspected by high amplitude ultrasound [3], generated with a chaotic cavity transducer [4]. In the present paper, this method is applied to the inspection of various metallic bonded plates (titanium or aluminum) with several bond defects. The defects were introduced by degradation of the surface preparation process (introduction of PTFE spray, or finger prints). Combined with the pulse inversion technique, high amplitude plane waves were sent in the structure, leading to harmonic components observed in the defect region. Mechanical destructive tests were performed and display a good agreement with nondestructive tests

Nondestructive evaluation of adhesive joints by using nonlinear ultrasonics

International audienceAdhesive bonding technology has now gained much attention in many industries as a very versatile assembling technique. However, to be used for structural joining and critical application, high reliability is needed. Thus, efficient non-destructive control strategy should be proposed to evaluate the nominal bonding quality but also possible progressive in-service degradations. Promising results have been presented in the literature using ultrasound-based methods. While linear ultrasound is efficient to detect decohesion or voids in a structure, it is barely sensitive to bond strength. In this work, we present a method to generate high amplitude plane wave, which may produce nonlinear phenomenon that are able to reveal kissing bonds or other types of adhesion defects. In this purpose a method based on a chaotic cavity transducer has been developed to generate high energy plane wave. Then the method is evaluated on metallic bonds with bonding defects. Combined with the pulse inversion technique, nonlinear phenomenon can be measured in the form of harmonic generation in the defect zone. We use this method to image a defect created by spraying PTFE on one adherent prior to bonding

Nonlinear ultrasound for nondestructive evaluation of adhesive joints

International audienceAdhesive bonding technology has gained much attention in many industries, specifically aeronautic. However, to be used for structural joining and critical application, high reliability is needed. Thus, efficient non-destructive control strategy should be proposed to evaluate the nominal bonding quality but also possible progressive in-service degradations. On-going effort has been made to develop methods for cohesive defects detection and adhesion quality assessment. In this paper, we propose a method based on the measurements of nonlinear features of weak bond defects, without any hypotheses of known elements on the adhesive bond (e.g. thickness of the substrates or the adhesive). It is applied to nondestructive testing of metallic substrates joined with thin adhesive (100μm)

Evaluation of metallic bonded plates with nonlinear ultrasound and comparison with destructive testing

In the last decades, the use of structural adhesives has increased. Indeed, they are able to replace advantageously traditional assembly techniques such as riveting or bolting, and generally tend to reduce the global weight of the structure and/or improve the strength of the structure thanks to more homogeneous stress distribution. In addition, it allows the assembly of composite materials or materials having different nature, which is of great interest in the aeronautic industry. However, to be used for structural joining and critical application, reliable non-destructive testing techniques are compulsory for evident safety reasons, be it after fabrication or during the whole life of the structure. While linear ultrasounds have demonstrated their capability to detect easily decohesion or voids in a structure, their use for bond strength inspection is less straightforward. Another approach relies on the analysis of nonlinear signature of a bond defect inspected by high amplitude ultrasound. In the present paper, a Chaotic Cavity Transducer is used to inspect various metallic bonded plates (titanium or aluminum) with several bond defects. The defects were introduced by depositing localized surface pollution (introduction of PTFE spray, release agent, or fingerprints) before the adhesive is deposited on top of the surface. Combined with the pulse inversion technique, high amplitude plane waves were sent in the structure, leading to harmonic components observed in the defect region. Mechanical destructive tests were performed and display a good agreement with nondestructive tests

Detection and imaging of BVIDs in composite plates

International audienceComposite materials are increasingly used in the aerospace industry. However, they may be subjected to Barely Visible Impact Damages (BVIDs). BVIDs can occur during maintenance phases or during flight and are almost impossible to detect with the naked eye. Although very small, these defects greatly weaken the structure and present a significant safety risk if they are not detected early enough. Currently, very high frequency non-destructive testing methods can be used for BVIDs localization but involve immobilizing the aircraft. In addition, some parts are difficult to access. This explains the growing interest for Structural Health Monitoring or SHM in the industry. Indeed, the objective is to integrate the sensors throughout thelife of the aircraft and to carry out control in near real time. This makes possible to increase the frequency of inspections while reducing costs.Most aeronautical parts have plate-like geometries, and are therefore commonly inspected using ultrasound guided waves. Guided waves have the advantage of propagating over long distances, but are rapidly attenuated at high frequencies in composite materials. Linear methods require a sufficiently smallwavelength relative to the defect size and are sensitive to high-frequency heterogeneities. Furthermore, the microcracks created by BVIDs are partially closed and therefore difficult to detect with linear methods, even at high frequencies. Hence, we propose to use low frequency guided waves non-linear methods. Inorder to precisely locate defects, it is necessary to take the dispersive nature of guided waves into account. A Beam Forming algorithm with dispersion compensation in very anisotropic media is presented. In a first step, the algorithm has been validated numerically and experimentally in a linear imaging version for both A0 and S0, respectively from data simulated with the CIVA software and with measurements on a CFRP composite plate, in the frequency range 10kHz-90kHz. Then this algorithm will be adapted to the non-linear case with a pump-probe type setup, and evaluated on impact defects (BVID created by compressed air impactor)

Nondestructive evaluation of adhesive joints by using nonlinear ultrasonic guided waves

International audienceFor a couple of decades, the use of bonded structures has been increasing in various industries. However, the non-destructive testing of such bonds is still challenging. While linear ultrasound is efficient to detect decohesion or voids in a structure, it is barely sensitive to bond strength. In this communication we present a method to generate high amplitude guided waves (GW) able to give birth to nonlinear effects after interaction with a defect at the bonded interface. In this purpose a new device has been developed, it acts as a two-dimensional virtual array and can be used to generate pure Lamb modes in a plate. Numerical simulation is employed to select the most appropriate guided mode-frequency combination. First the method for the generation of guided waves is presented, as well as measurements of its performances in terms of modal selectivity and amplitudes. Then the method is applied and evaluated on metallic bonds with bonding defects

Multichannel Multiple Signal Classification for dispersion curves extraction of ultrasonic guided waves

Multichannel acquisition of ultrasonic guided waves can be used to extract dispersion curves, from the time-position domain $(t-x)$ to the frequency-wavenumber $(f-k)$, or frequency-velocity domain $(f-c)$. Accurate measurements are needed in order to be able to precisely characterize the specimen, by improving the extraction of low amplitude modes and enhance resolution. The proposed method is based on the MUltiple SIgnal Classification algorithm combined with a multi-emitter and multi-receiver acquisition. In this work, this method is applied on experimental data to extract dispersive information from multilayered bonded specimens.Quality assurance concepts for adhesive bonding of aircraft composite structures by advanced ND