No description available.

Methods for comprehensive structural dynamic analysis generally employ input-output modal analysis with a mathematical model of structural vibration using excitation and response data. Recently operational modal analysis methods using only vibration response data have been developed. In this thesis, both input-output and operational modal analysis, in the presence of significant unmeasured excitations, is considered. This situation arises when a test structure cannot be effectively isolated from ambient excitations or where the operating environment imposes dynamically-important boundary conditions. The limitations of existing deterministic frequency-domain methods are assessed. A novel time-domain estimation algorithm, based on the estimation of a discrete-time autoregressive moving average with exogenous excitation (ARMAX) model, is proposed. It includes a stochastic component to explicitly account for unmeasured excitations and measurement noise. A criterion, based on the sign of modal damping, is incorporated to distinguish vibration modes from spurious modes due to unmeasured excitations and measurement noise, and to identify the most complete set of modal parameters from a group of estimated models. Numerical tests demonstrate that the proposed algorithm effectively identifies vibration modes even with significant unmeasured random and periodic excitations. Random noise is superimposed on response measurements in all tests. Simulated systems with low modal damping, closely spaced modes and high modal damping are considered independently. The accuracy of estimated modal parameters is good except for degreesof- freedom with a low response level but this could be overcome by appropriate placement of excitation and response measurement points. These observations are reflected in experimental tests that include unmeasured periodic excitations over 200% the level of measured excitations, unmeasured random excitations at 90% the level of measured excitations, and the superposition of periodic and random unmeasured excitations. Results indicate advantages of the proposed algorithm over a deterministic frequency domain algorithm. Piezoceramic plates are used for structural excitation in one experimental case and the limitations of distributed excitation for broadband analysis are observed and characterised in terms of actuator geometry and modal deformation. The ARMAX algorithm is extended for use with response measurements exclusively. Numerical and experimental tests demonstrate its performance using time series data and correlation functions calculated from response measurements.

Les traumatismes du rachis incluent des fractures osseuses, des lésions disco-ligamentaires et des blessures médullaires. Ils impliquent un fort coût pour le système de santé, ainsi qu’une prise en charge (durée d’hospitalisation, atteinte de l’intégrité physique) lourdement handicapante pour leurs victimes. Les séquelles peuvent aller d’une déformation du profil sagittal à la tétraplégie. Leurs causes principales sont les accidents véhicules moteurs, les chutes et les accidents sportifs. Il existe de multiples mécanismes d’apparition des fractures vertébrales (compression, flexion-extension, cisaillement, torsion) et des blessures médullaires (contusion, traction, cisaillement), ce qui rend leur étude complexe. Malgré l’important investissement mis dans la recherche à ce sujet, des lacunes persistent dans la compréhension de ces traumatismes, qui ralentissent la progression des dispositifs de protection du rachis, et le développement des techniques chirurgicales. Dans ce contexte, l’objectif principal de ce doctorat est d’améliorer la compréhension fondamentale des mécanismes de fractures vertébrales et des blessures de la moelle épinière. Une première partie du travail a visé à développer des connaissances fondamentales sur les propriétés géométriques et mécaniques des vertèbres et de la moelle épinière. Ce type de propriétés a été utilisé pour créer et calibrer un modèle par éléments finis du rachis thoracolombaire. Ce modèle a ensuite été utilisé pour étudier le lien entre chargements mécaniques et une classification clinique des patrons de fractures vertébrales, ainsi que l’effet de différents éléments d’influences sur la mécanique de la contusion médullaire. / Spine traumas include bony fractures, intervertebral disc and ligaments lesions, and spinal cord injuries. They imply a high cost for the health system, as well as a highly incapacitating surgical management (length of hospitalization, physical handicap), and result in aftereffects such as sagittal curve deformity or tetraplegia. Their principal causes are motor vehicle accidents, falls and sports accidents. Multiple mechanisms of occurrence exist for vertebral fractures (compression, flexion-extension, shear, torsion) and spinal cord injury (contusion, traction, shear), which makes them complex to study. Despite major investments in research on this topic, there is a lack of the understanding of some aspects regarding these traumas. This slows down the progress in protection devices and surgical strategies development. In this context, the main objective of this Ph.D. is to improve fundamental knowledge of vertebral fractures and spinal cord injuries mechanisms. A first part of this work has been to develop fundamental knowledge on geometrical and mechanical properties of the vertebrae and the spinal cord. This type of properties has been used to create and calibrate a finite element model of the thoracolumbar spine. This model was then used to study the link between mechanical loadings and a clinical classification of vertebral fracture patterns, as well as the effect of various parameters on spinal cord contusion mechanisms.

Ce travail porte sur l’optimisation sous contraintes de masse de structures de type cabine de camion. Il s’agit ici d’un problème dans lequel deux objectifs antagonistes doivent être conciliés. En effet, la réduction de la masse de la structure entraîne une dégradation de ses performances vibro-acoustiques. Il faut donc pouvoir déterminer quelles zones de la structure influent le moins sur ce comportement afin de diminuer la masse localement. C’est dans cet esprit que, dans un premier temps, différentes méthodes d’analyse de sensibilité sont présentées afin de déterminer leurs avantages et inconvénients dans le cas de leur utilisation pour identifier les variables à retenir dans le cas d’une procédure d’optimisation donnée. Ensuite une procédure d’optimisation adaptative en basses fréquences est présentée et appliquée à un cas test représentant schématiquement une cabine de camion et les résultats obtenus sont comparés à ceux issus d’un algorithme d’optimisation sous contraintes non-linéaires couramment utilisé. Une seconde méthode est également développée dans le cas des moyennes et hautes fréquences où les informations disponibles sur la structure sont seulement des quantités énergétiques. Ici encore les résultats obtenus sont comparés à ceux issus d’un algorithme d’optimisation sous contraintes non-linéaires. / This work deals with mass optimization procedures underdesign constraints of truck cabs. It consists in a problem where two conflicting objectives must be conciliated as the mass reduction would deteriorate the vibroacoustic behaviour. One has to determine the zones in which the mass could be locally reduced. First, this work presents several sensitivity analysis procedures and discusses theirs advantages and drawbacks. Then an adaptive optimization procedure is developed in the low frequency range. This procedure is applied on atest-case and the obtained results are compared with results issued from a commonlyused optimization algorithm. The procedure is then extended to medium and high frequency range where the only available quantities are energetic ones. The obtained results are also compared with those from a commonly-used optimization algorithm.

Vertebral fractures affect at least 12-20% of men and women over the age of 50, and the risk of fracture increases exponentially with age. Despite their high prevalence, the failure mechanisms leading to these fractures are not well understood. For example, clinical observations of fractured vertebra often note that one or both vertebral endplates have collapsed, but the precise involvement of the endplates in the initiation and progression of failure has not yet been defined. The mechanisms of failure may also relate to spatial variations in the density and microstructure of the porous trabecular bone within the vertebra as well as to the health of the adjacent intervertebral discs (IVDs) which transfer loads directly to the vertebral endplates. Delineating the contributions of these factors would shed light on the etiology of vertebral fractures and would aid in development of clinically feasible, patient-specific finite element (FE) models of the vertebra. These models are built from a patient's quantitative computed tomography (QCT) scan and have shown tremendous promise for accurate, patient-specific estimates of bone strength and fracture risk. Further validation studies are required to assess the impact of the choices of material properties and boundary conditions, as a prerequisite for broad implementation of these FE models in clinical care. The overall goal of this work was to define the failure processes involved in vertebral fractures and to evaluate the accuracy of patient-specific FE models in simulating these processes. Mechanical testing of human spine segments, in conjunction with micro-computed tomography, enabled the assessment of deformation at the vertebral endplate and deformation throughout the entire bone, as the vertebra was loaded to failure under both axial compression and anterior flexion. These data were compared against predictions of vertebral deformation obtained from QCT-based FE models. The impact of the choice of boundary conditions was specifically examined by comparing the accuracy of the FE predictions between models that simulated applied loads based on measured distributions of pressure within IVDs and models that used highly idealized boundary conditions. The results of these studies demonstrated that sudden and non-recoverable endplate deflection is a defining feature of biomechanical failure of the vertebra, for both compression and flexion loading. The locations of endplate collapse as vertebral failure progressed were associated with the porosity of the endplate and the microstructure of the underlying trabecular bone. FE analyses incorporating the experimentally observed endplate deflections as boundary conditions provided more accurate predictions of displacements throughout the rest of the vertebra when compared to FE models with highly idealized boundary conditions. Under anterior flexion, the use of boundary conditions informed by measurements of IVD pressure mitigated, but did not eliminate, the inaccuracy of the idealized boundary conditions. No further improvement in accuracy was found when using boundary conditions based on pressure measurements corresponding only to IVDs whose level of degeneration matched that observed in the IVDs adjacent to the vertebra being modeled. Overall, the accuracy of the FE predictions of vertebral deformation was only moderate, particularly near the locations of endplate collapse. The outcomes of this work indicate that the vertebral endplate is principally involved in vertebral fractures and that current methods for QCT-based FE models do not adequately capture this failure mechanism. These outcomes provide a biomechanical rationale for clinical diagnoses of vertebral fracture based on endplate collapse. These outcomes also emphasize that future studies of patient-specific FE models should incorporate physiologically relevant loading conditions and also material properties that more accurately represent the vertebral endplate in order to obtain higher fidelity predictions of vertebral failure.

Finite element modeling has become a powerful tool in orthopedic biomechanics, allowing simulations with complex geometries. Current fatigue behavior simulations are unable to accurately predict the cycles to failure, creep, and damage or modulus loss even when applied to a bending model. It is thought that the inhomogeneity of the models may be the source of the problem. It has also been suggested that the volume size of the element will affect the fatigue behavior. This is called a stressed volume effect. In this thesis non-homogeneous finite element models were used to examine the effects of "sizing factors" on damage laws in fatigue simulations. Non-homogeneous finite element models were created from micro computed tomography (CT) images of dumbbell shaped fatigue samples. An automatic voxel meshing technique was used which converted the CT data directly into mesh geometry and material properties. My results showed that including these sizing factors improved the accuracy of the fatigue simulations on the non-homogeneous models. Using the Nelder-Mead optimization routine, I optimized the sizing factors for a group of 5 models. When these optimized sizing factors were applied to other models they improved the accuracy of the simulations but not as much as for the original models, but they improved the results more than with no sizing factors at all. I found that in our fatigue simulations we could account for the effects of stressed volume and inhomogeneity by including sizing factors in the life and damaging laws. / Master of Science

Este trabalho apresenta uma metodologia capaz de unir a análise hidrodinâmica de um sistema oceânico com sua análise estrutural, assim como o pós-processamento acoplado dos resultados. Foram criadas rotinas e códigos para que a série temporal de forças das linhas de risers e amarração de uma plataforma pudessem tornar-se dados passíveis de entrada num pré- processador de elementos finitos. Com a aplicação destas no modelo, e sua conseqüente análise no domínio do tempo, foi criada uma interface para os resultados do solver, para que pudesse ser importados no pós-processador hidrodinâmico, e visualizados com os mesmos movimentos que os obtidos na resposta da análise hidrodinâmica. O TPNView, atual pós-processador do laboratório Tanque de Provas Numérico(TPN), foi quem recebeu por fim as rotinas e interfaces criadas a partir das idéias apresentadas nesta dissertação. Com isso é possível ver em uma única ferramenta de visualização tanto o comportamento hidrodinâmico quanto o estrutural de uma estrutura do sistema de uma só vez.. / This work presents a methodology developed to treat the hydrodynamic analysis of an offshore system conjointly with its structural analysis; the same methodology also allows for combined post-processing of data. Programming routines were created so as to enable the use of the time series of the forces present at the risers and mooring lines as input data for a finite element analysis solver software. Applying this forces in to the finite element model, and its subsequent analysis in time domain, it was possible to create an interface between the solver output, so that structural analysis could be imported into the hydrodynamic post-processor and visualised with the same movements obtained in the hydrodynamic analysis response. TPNView, the post-processor developed at the Tanque de Provas Numérico laboratory, was benefited from the programming routines and interfaces developed for this thesis. Using the aforedescribed visualisation tools, it became possible to monitor at once both the hydrodynamic and the structural behaviour of a system component.

Los estímulos mecánicos son uno de los factores que afectan a la diferenciación celular en el proceso de regeneración del tejido óseo, por lo tanto, en el desarrollo de andamios para ingeniería de tejidos, se pueden aplicar las cargas mecánicas con el fin de inducir la actividad de las células. Cuando se aplican cargas mecánicas, los estímulos mecánicos específicos transmitidos a las células a nivel microscópico pueden estudiarse mediante técnicas numéricas. El objetivo de esta tesis fue estudiar la mecanoregulación de la diferenciación de tejido en andamios de fosfato de calcio utilizando modelos de elementos finitos basados en micro tomografía axial computarizada.Dos muestras de materiales porosos basados en fosfato de calcio fueron utilizadas. Se desarrollaron mallas de elementos finitos congruentes, discretizando la fase sólida y los macro poros interconectados, con el fin de tener en cuenta la morfología irregular de los andamios.En primer lugar, se estudió la distribución de los estímulos mecánicos. La fase sólida y el fluido intersticial se simularon como material elástico lineal y como fluido Newtoniano, respectivamente. Se simuló una compresión del 0.5% en el sólido y un fluido con velocidades de entrada de 1, 10 y 100 µm/s en los poros. Se encontraron distribuciones de deformación similares en las paredes ambos materiales, con valores máximos de 1.6% en compresión y de 0.6% en tracción. En algunos poros, la velocidad del fluido aumentó a 100 y 1000 veces la velocidad de entrada. Este estudio mostró como estímulos mecánicos macroscópicos pueden causar distintos niveles de estímulos mecánicos microscópicos dentro los andamios, debido a la morfología.A continuación se realizó un estudio en el tiempo de la diferenciación de tejido en un andamio sometido a condiciones in vitro. La compresión y la perfusión se modelaron como en el estudio anterior. Se simularon una compresión del 0.5% y una velocidad de entrada de fluido constante de 10 µm/s o una presión de entrada de fluido constante de 3 Pa. La deformación cortante octaédrica y el esfuerzo cortante del fluido se utilizaron como estímulos mecano-regulatorios basándose en la teoría de Prendergast et al. (1997). Al aplicar velocidad constante, se predijeron fluctuaciones entre los estímulos equivalentes a la formación de tejido y a la muerte celular, debido al aumento en el esfuerzo cortante del fluido cuando el tejido comienza a llenar los poros. Sin embargo, al aplicar presión constante, se predijo estímulo equivalente a la diferenciación de tejido óseo en la mitad del volumen de los poros. Estos resultados sugieren que para permitir la diferenciación de tejido, la velocidad del fluido debe disminuirse cuando el tejido empieza a mineralizarse.Finalmente, se llevó acabo un estudio en el tiempo de la angiogénesis y de la diferenciación de tejido en un andamio bajo condiciones in vivo. La deformación cortante octaédrica y la velocidad relativa del fluido se utilizaron como estímulos mecano-regulatorios. Las fases sólida y porosa fueron tratadas como materiales poroelásticos. Se simuló la actividad individual de las células. Compresiones de 0.5 y 1% fueron simuladas. La mayoría de los vasos crecieron en los poros de la periferia del andamio y se bloquearon por las paredes. Se formaron redes capilares similares independientemente de la magnitud de deformación utilizada. Al aplicar 0.5% de compresión, estímulos correspondientes a la formación de hueso se predijeron en el 70% del volumen de los poros, sin embargo, sólo el 40% del volumen se llenó de osteoblastos debido a la falta de oxigeno. Este estudio mostró el efecto de la falta de vascularización en el centro del andamio en la diferenciación de tejido.Ese tipo de estudios, combinados con estudios in vitro, deberían contribuir a la comprensión del proceso de diferenciación de los tejidos dentro de los andamios y por lo tanto a la mejora de los métodos de diseño de andamios. / Mechanical stimuli are one of the factors that affect cell differentiation in the process of bone tissue regeneration; therefore, in the development of scaffolds for tissue engineering, mechanical loads can be applied in order to induce cell activity. The specific mechanical stimuli transmitted to cells at a microscopic level when mechanical loads are applied can be studied using numerical techniques. The objective of this thesis was to study the mechanoregulation of tissue differentiation within calcium phosphate scaffolds using micro computed tomographed based finite element models.Two samples of porous calcium phosphate based materials were used. Congruent finite element meshes, with the solid phase and the interconnected pores discretized, were developed in order to account for the scaffold irregular morphology.First, a study of the distribution of mechanical stimuli was performed. The solid phase and the fluid flow within the pores were modeled as linear elastic solid material and Newtonian fluid respectively. Compressive strains of 0.5% of total deformation applied to the solid and interstitial fluid flows with inlet velocities of 1, 10 and 100 µm/s applied to the pores were simulated. Similar strain distributions for both materials were found, with compressive and tensile strain maximal values of 1.6% and 0.6% respectively. For the fluid flow models, the fluid velocity in some of the scaffold pores increased to 100 and 1000 times the inlet velocity. This study showed how mechanical loads and fluid flow applied to the scaffolds caused different levels of mechanical stimuli within the samples according to the morphology of the materials.Next, a study of the mechanoregulation of tissue differentiation over time in a scaffold subjected to in vitro loads was performed. The solid phase and the fluid flow were modeled as in the study described above. Compressive strain of 0.5% and fluid flow with constant inlet velocity of 10 µm/s or constant inlet pressure of 3 Pa were applied. Octahedral shear strain and fluid shear stress were used as mechano-regulatory stimuli based on the theory of Prendergast et al. (1997). When a constant velocity was simulated, fluctuations between stimuli equivalent to tissue formation and cell death were predicted due to the increase in the fluid shear stress when tissue started to fill the pores. However, when constant pressure was applied, stimuli equivalent to bone formation were predicted in about half of the pore volume. These results suggest that in order to allow tissue differentiation within a scaffold, the fluid velocity should be decreased when tissue starts mineralizing.Finally, a study of the angiogenesis and the mechanoregulation of tissue differentiation over time in a scaffold subjected to in vivo conditions was performed. Octahedral shear strain and relative fluid velocity were used as mechano-regulatory stimuli. The solid and pore phases were treated as poroelastic materials. Individual cell activity was simulated within the pore domain. Compressive strains of 0.5 and 1% of total deformation were simulated. Most vessels grew in the pores at the periphery of the scaffolds and were blocked by the scaffold walls. Similar capillary networks were formed independently of the magnitude of the mechanical strain applied. When 0.5% of strain was applied, 70% of the pore volume was affected by mechano-regulatory stimuli corresponding to bone formation; however, because of the lack of oxygen, only 40% of the volume was filled with osteoblasts. This study showed the effect of the lack of vascularization in the center of the scaffold on the tissue differentiation.Such kind of studies, combined with in vitro studies, should contribute to the understanding of the process of tissue differentiation within the constructs and therefore to the improvement of scaffold design methods.

Considering the deficiencies frequently observed in the existing reinforced concrete buildings, detailed assessment and rehabilitation must be conducted to avoid significant life and value loss in seismic zones. In this sense, performance based evaluation methods suggested in the regulations and codes must be examined and revised through experimental and analytical research to provide safe and economical rehabilitation solutions. In this study, seismic behavior of three reinforced concrete frames built and tested in Middle East Technical University Structural Mechanics Laboratory is examined. The specimens are extracted from a typical interior frame of 3-story 3-bay reinforced concrete structure. One of the specimens has compliant design according to Turkish Earthquake Code (2007) and each of the other two specimens represents different types of deficiencies in terms of material strength and detailing. The test specimens were modeled using different modeling approaches and nonlinear dynamic analyses were conducted on the numerical models. Results of continuous pseudo-dynamic testing of three ground motions are presented and compared with the numerical simulations on models. Calibrated finite element models were used for evaluation of performance assessment procedure of Turkish Earthquake Code (2007) and further investigation on local deformation components in light of experimental findings and observations. Deformation sources of columns and joints were studied in terms of their interaction and contributions to the total drift. Estimated plastic hinge lengths of columns were compared with the experimental observations and the proposed expressions in the literature.

Este trabalho apresenta uma metodologia capaz de unir a análise hidrodinâmica de um sistema oceânico com sua análise estrutural, assim como o pós-processamento acoplado dos resultados. Foram criadas rotinas e códigos para que a série temporal de forças das linhas de risers e amarração de uma plataforma pudessem tornar-se dados passíveis de entrada num pré- processador de elementos finitos. Com a aplicação destas no modelo, e sua conseqüente análise no domínio do tempo, foi criada uma interface para os resultados do solver, para que pudesse ser importados no pós-processador hidrodinâmico, e visualizados com os mesmos movimentos que os obtidos na resposta da análise hidrodinâmica. O TPNView, atual pós-processador do laboratório Tanque de Provas Numérico(TPN), foi quem recebeu por fim as rotinas e interfaces criadas a partir das idéias apresentadas nesta dissertação. Com isso é possível ver em uma única ferramenta de visualização tanto o comportamento hidrodinâmico quanto o estrutural de uma estrutura do sistema de uma só vez.. / This work presents a methodology developed to treat the hydrodynamic analysis of an offshore system conjointly with its structural analysis; the same methodology also allows for combined post-processing of data. Programming routines were created so as to enable the use of the time series of the forces present at the risers and mooring lines as input data for a finite element analysis solver software. Applying this forces in to the finite element model, and its subsequent analysis in time domain, it was possible to create an interface between the solver output, so that structural analysis could be imported into the hydrodynamic post-processor and visualised with the same movements obtained in the hydrodynamic analysis response. TPNView, the post-processor developed at the Tanque de Provas Numérico laboratory, was benefited from the programming routines and interfaces developed for this thesis. Using the aforedescribed visualisation tools, it became possible to monitor at once both the hydrodynamic and the structural behaviour of a system component.