Une meilleure compréhension des relations entre la structure complexe de l'os cortical et ses propriétés mécaniques est nécessaire à l'évaluation de la qualité osseuse. Les méthodes conventionnelles ex vivo de mesure de l'élasticité à l'échelle du millimètre ont des limitations liées à l'anisotropie du tissu, à son inhomogénéité et à la petite taille des échantillons. Au contraire, la spectroscopie par résonance ultrasonore (RUS) est bien adaptée à la mesure de petits échantillons anisotropes. Cette méthode estime l'élasticité à partir des fréquences de résonance de l'échantillon, et l'amortissement à partir de la largeur des pics de résonance. Son application à l'os était considérée difficile, du fait de l'amortissement important des modes de vibration, qui induit un recouvrement des pics de résonance et complique la mesure des fréquences. Pour surmonter cette difficulté, des adaptions de la méthode - dans la mesure, le traitement du signal et l'estimation des propriétés du matériau - ont été proposées. Elles ont été validées sur de l'os cortical et sur des échantillons de polymère et de matériau composite imitant l'os. La précision de la méthode a été démontrée, ainsi que sa capacité à mesurer tous les termes du tenseur d'élasticité à partir d'un seul échantillon. De plus, une nouvelle formulation Bayésienne de l'inversion apporte une solution automatique à un problème qui nécessitait une stratégie fastidieuse d'essai-erreur ou de complexes modifications du dispositif expérimental. Finalement, l'application à une grande collection d'échantillons de tibias humains démontre que la méthode RUS pourrait être utilisée en routine pour la mesure de la viscoélasticité de l'os. / Deep understanding of the structure-function relationships of cortical bone in the context of bone quality assessment is still missing. Currently available methods to measure millimeter-scale elasticity ex vivo have limitations arising from theanisotropy of the tissue, its heterogeneity, and the small size of the specimens.Resonant ultrasound spectroscopy is particularly suitable for the measurement of small anisotropic specimens. This method estimates elasticity from the free resonant frequencies of a specimen, and damping from the width of the resonant peaks. Its application to cortical bone was considered challenging because of the high damping of the vibrations modes, which causes overlapping of the resonant peaks and prevents a direct measurement of the resonant frequencies. To overcome the difficulty,adaptations of all the steps of RUS – measurement, signal processing and inverse estimation of the material properties – have been introduced. Validation of each step of the procedure has been achieved by application to several test samples, including a cortical bone specimen and bone-mimicking composite and polymer specimens.RUS was shown to be precise and accurate, with the advantage of providing the complete stiffness tensor from the measurement of a single specimen. Additionally, an original Bayesian formulation of the inversion provides an automated solution toa problem that was previously solved by tedious trial-and-error procedures or complex additions to the basic experimental setup. Finally, the application to a large collection of human tibiae specimens demonstrates that RUS can be considered a routine method to characterize the viscoelasticity of bone.

Wooden objects have been widely used in the history of humanity and play an important role in our cultural heritage. The preservation of such objects is of great importance and can be a challenging task. This thesis investigates the static and time-dependent mechanical behaviour of archaeological oak wood from the Vasa warship. Characterisation of mechanical properties is necessary for the formulation of a numerical model to design an improved support structure. The ship was impregnated with polyethylene glycol (PEG) for dimensional stabilisation. All elastic engineering constants of the Vasa oak have been identified and compared with those of recent oak by means of the static and dynamic testing. The experiments were done on samples with cubic geometry, which allowed obtaining all elastic constants from a single sample. The usage of cubic samples with orthotropic mechanical properties during compressive experiments was validated with finite-element simulations. The Young's moduli of the Vasa oak in all orthotropic directions were smaller than those for the recent oak. The shear moduli of Vasa oak was determined and verified with the resonant ultrasound spectroscopy. The time-dependent mechanical behaviour of the Vasa oak has been studied. Creep studies were performed in uniaxial compression on the cubic samples in all orthotropic directions. The samples loaded in the longitudinal direction were subjected to different stress levels. A stress level below 15% of the yield stress in the longitudinal direction did not result in non-linear creep with increasing creep rates within the time frame of the tests. The results of the studies in radial and tangential directions showed that creep was dominated by the effect of annual fluctuations in relative humidity and temperature. The weight changes based on annual fluctuations of relative humidity were measured for Vasa oak and recent oak. The Vasa oak showed higher variations due to an increased hygroscopicity which is the result of the impregnation with PEG. In conceiving a full-scale finite-element model of Vasa ship, not only the stress-strain relations of the material but also those of the structural joints are needed. Since the in-situ measurement of joints is not an option, a replica of a section of the ship hull was built and tested mechanically. The load-induced displacements were measured using 3D laser scanning which proved to have advantages to conventional point displacement measurements. The mechanical characteristics of the Vasa oak and joint information presented in this work can be used as input for a finite-element model of the Vasa ship for simulation of static and time-dependent behaviour on a larger scale. / Stötta Vasa

De nos jours, l’étude biomécanique des structures osseuses représentent un enjeu pour différents domaines: accidentologie, prise en charge des pathologies osseuses, confort des personnes âgées, conception de prothèses innovantes, etc. Le but de cette thèse est de fournir des valeurs de référence représentatives de la qualité de l'os enfant en caractérisant des propriétés mécaniques et morphométriques du tissu osseux en croissance à différentes échelles. Les propriétés mécaniques ont été mesurées à 2 échelles différentes – mésoscopique et microscopique – selon 2 modalités expérimentales – spectroscopie à résonance ultrasonore et microindentation. Un autre volet de cette thèse concerne le développement d’une procédure d’analyse morphométrique adaptée au tissu osseux pour le traitement d’images obtenues par micro-tomographie (RX). Mieux connaître le tissu osseux juvénile est indispensable pour développer des modèles dédiés et ainsi mieux comprendre les mécanismes pathologiques caractéristiques de l'os en croissance (fracture en bois vert) pour améliorer le diagnostic et adapter les choix thérapeutiques pour les jeunes patients. / Nowadays, the biomechanical study of bone structures is a challenge for different fields: accidentology, management of bone pathologies, comfort for the elderly, design of innovative prostheses, etc. The aim of this thesis is to provide reference values representative of the quality of child bone by characterizing the mechanical and morphometric properties of growing bone tissue at different scales. Mechanical properties were measured at 2 different scales - mesoscopic and microscopic - in 2 experimental modalities – resonant ultrasound spectroscopy and microindentation. Another part of this thesis concerns the development of a morphometric analysis procedure adapted to bone tissue for the treatment of images obtained by micro-tomography (RX). A better knowledge of juvenile bone tissue is essential to develop dedicated models and thus better understand the pathological mechanisms characteristic of growing bone (greenstick fracture) to improve diagnosis and adapt therapeutic choices for young patients.

Non-destructive testing (NDT) techniques are critical to ensure integrity and safety of engineered structures. Structural health monitoring (SHM) is considered as the next step in the field enabling continuous monitoring of structures. The first part of the thesis concerns NDT and SHM using guided waves in plates, or Lamb waves, to perform imaging of plate structures. The imaging is performed using a fixed active array setup covering a larger area of a plate. Current methods are based on conventional beamforming techniques that do not efficiently exploit the available data from the small arrays used for the purpose. In this thesis an adaptive signal processing approach based on the minimum variance distortionless response (MVDR) method is proposed to mitigate issues related to guided waves, such as dispersion and the presence of multiple propagating modes. Other benefits of the method include a significant increase in resolution. Simulation and experimental results show that the method outperforms current standard processing techniques. The second part of the thesis addresses transducer design issues for resonant ultrasound inspections. Resonant ultrasound methods utilize the shape and frequency of the object's natural modes of vibration to detect anomalies. The method considered in the thesis uses transducers that are acoustically coupled to the inspected structures. Changes in the transducer's electrical impedance are used to detect defects. The sensitivity that can be expected from such a setup is shown to highly depend on the transducer resonance frequency, as well as the working frequency of the instrument. Through simulations and a theoretical argumentation, optimal conditions to achieve high sensitivity are given.