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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Ultrasons diffus pour la caractérisation d'une fissure dans le béton. : approche linéaire et non linéaire. / Diffuse ultrasound for the characterization of a crack in concrete. : linear and nonlinear approach.

Quiviger, Audrey 16 November 2012 (has links)
Les différents processus de dégradation des structures de génie civil induisent une micro, puis macro- fissuration du béton. Celle-ci génère alors une réduction des propriétés mécaniques de l'ouvrage et, à terme, sa perte d'étanchéité. Il est donc nécessaire de fournir des informations quant à la présence et à la taille de fissures pour procéder aux réparations nécessaires et conserver l'intégrité de l'ouvrage. Dans un premier temps, le contrôle du béton ainsi que la morphologie de la fissure réelle sont présentés. La notion de contacts entre ses lèvres est introduite pour définir la problématique de sa caractérisation. La bibliographie montre que les méthodes acoustiques standards ne sont pas adaptées à la caractérisation d'une fissure dans le béton. Deux pistes sont alors identifiées : les ultrasons diffus et l'acoustique non linéaire. Nous présentons dans un second temps la caractérisation de la fissure par analyse du transport de l'énergie suivant une équation de diffusion. Les paramètres associés (diffusivité et dissipation) sont déterminés expérimentalement sur des éprouvettes fissurées sur différentes profondeurs. Nous introduisons et définissons le temps d'arrivée du maximum de l'énergie (ATME) qui s'avère être le paramètre le plus sensible à la partie ouverte d'une fissure. Son évolution au regard des incertitudes de mesure ne permet toutefois pas de caractériser totalement la partie fermée. Une simulation numérique en différences finies est réalisée. Elle met en évidence le rôle des contacts au sein de la partie fermée de la fissure et confirme les observations expérimentales. / The various processes of deterioration of the building structures lead to a micro and macro-cracking of the concrete. Consequently, the mechanical properties of the structure are reduced and, eventually, the building is no longer airtight. It is therefore necessary to supply information regarding the presence and size of cracks to carry out the necessary repairs and keep the integrity of the structure. First, the control of concrete as well as the morphology of the actual crack are presented. The notion of contacts between its lips is introduced to define the problem of its characterization. The bibliography shows that the standard acoustic methods are not adapted for characterizing of a crack in concrete. Two tracks are then identified: the diffuse ultrasound and the nonlinear acoustics. Subsequently, we present the characterization of the crack by analyzing the transport of the energy with a diffusion equation. The associated parameters (diffusivity and dissipation) are experimentally determined on test tubes cracked at different depths. We introduce and define the arrival time of the maximum energy (ATME), which turns out to be the most sensitive parameter to the open part of a crack. Its evolution with regard to the measurement uncertainties does not allow a full characterization of the closed part. A digital modeling in finite differences is performed. It highlights the role of the contacts within the closed part of the crack and confirms the experimental observations. Then, we present the nonlinear acoustics and the associated methods applied to concrete.
2

Finite element simulation of crack depth measurements in concrete using diffuse ultrasound

Seher, Matthias Eugen 24 August 2011 (has links)
Surface-breaking cracks pose a serious threat to the service life of concrete structures and health monitoring is presently conducted by a visual inspection method, yielding a potential risk to safety. Diffuse ultrasonic techniques have shown their potential as an ultrasonic technique for measuring crack depth in concrete and are currently under development. In this research, the finite element method (FEM) is employed to model the ultrasound diffusion in a concrete specimen. The objectives are to use the commercial finite element (FE) tool Ansys to develop the finite element model of a concrete specimen and verify the applicability of the model by comparing with an analytic solution and experiment data. Further, various crack types are analyzed with the FE model in order to gain physical insight into the interpretation of experimental measurements. The results of this research suggest that a preliminary knowledge of the cracking process is required to correctly interpret the measured impulse responses for an unknown crack geometry, as the impulse response expresses the response of the shortest path through a system of cracks between source and receiver. Moreover, the impulse response can carry some ambiguity, as certain crack types are not uniquely determined.

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