The objective of this thesis is to develop a nondestructive evaluation method that could accurately locate and size damage in structures. The method is to be based on pre-damage and post-damage strain energies of beam and column elements. The method should apply to 1-D as well as 2-D and 3-D structures with single or multiple damage locations. To achieve the objectives listed above, the following four tasks are addressed: (1) the development of the theoretical foundations of the nondestructive evaluation theory; (2) the validation of the accuracy of the theory using exact structural deformational data generated from the static analysis of F. E. models in SAP2000; (3) the validation of the practical feasibility of the theory using approximated structural deformational data generated from the modal analyses of F.E. models in SAP2000; and (4) the application of the methodology to an existing structure. The numerical simulations of damage indicate that the proposed NDE method can clearly locate damage in the structures and provide an accurate quantitative value of damage severities, even when only a few lower frequencies and mode shapes are known. The field data analysis results indicate that the developed NDE method can locate damage and provide conservative values for damage severity estimations.

The response of beams resting on viscoelastically damped foundation under moving SDoF oscillators is scrutinized through a novel state-space formulation, in which a number of internal variables is introduced with the aim of representing the frequency-dependent behaviour of the viscoelastic foundation. A suitable single-step scheme is provided for the numerical integration of the equations of motion, and the Dimensional Analysis is applied in order to define the dimensionless combinations of the design parameters that rule the responses of beam and moving oscillator. The effects of boundary conditions, span length and number of modes of the beam, along with those of the mechanical properties of oscillator and foundation, are investigated in a new dimensionless form, and some interesting trends are highlighted. The inaccuracy associated with the use of effective values of stiffness and damping for the viscoelastic foundation, as usual in the present state-of-practice, is also quantified.

Carbon fiber-reinforced silicon carbide matrix (C/SiC) composite is a ceramic matrixcomposite (CMC) that has considerable promise for use in high-temperature structuralapplications. In this thesis, systematic numerical studies including the prediction of elasticand thermal properties, analysis and optimization of stresses and simulation ofhigh-temperature oxidations are presented for the investigation of C/SiC composites.A strain energy method is firstly proposed for the prediction of the effective elastic constantsand coefficients of thermal expansion (CTEs) of 3D orthotropic composite materials. Thismethod derives the effective elastic tensors and CTEs by analyzing the relationship betweenthe strain energy of the microstructure and that of the homogenized equivalent model underspecific thermo-elastic boundary conditions. Different kinds of composites are tested tovalidate the model.Geometrical configurations of the representative volume cell (RVC) of 2-D woven and 3-Dbraided C/SiC composites are analyzed in details. The finite element models of 2-D wovenand 3-D braided C/SiC composites are then established and combined with the stain energymethod to evaluate the effective elastic constants and CTEs of these composites. Numericalresults obtained by the proposed model are then compared with the results measuredexperimentally.A global/local analysis strategy is developed for the determination of the detailed stresses inthe 2-D woven C/SiC composite structures. On the basis of the finite element analysis, theprocedure is carried out sequentially from the homogenized composite structure of themacro-scale (global model) to the parameterized detailed fiber tow model of the micro-scale(local model). The bridge between two scales is realized by mapping the global analysisresult as the boundary conditions of the local tow model. The stress results by global/localmethod are finally compared to those by conventional finite element analyses.Optimal design for minimizing thermal residual stress (TRS) in 1-D unidirectional C/SiCcomposites is studied. The finite element models of RVC of 1-D unidirectional C/SiCIIcomposites with multi-layer interfaces are generated and finite element analysis is realized todetermine the TRS distributions. An optimization scheme which combines a modifiedParticle Swarm Optimization (PSO) algorithm and the finite element analysis is used toreduce the TRS in the C/SiC composites by controlling the multi-layer interfaces thicknesses.A numerical model is finally developed to study the microstructure oxidation process and thedegradation of elastic properties of 2-D woven C/SiC composites exposed to air oxidizingenvironments at intermediate temperature (T<900°C). The oxidized RVC microstructure ismodeled based on the oxidation kinetics analysis. The strain energy method is then combinedwith the finite element model of oxidized RVC to predict the elastic properties of composites.The environmental parameters, i.e., temperature and pressure are studied to show theirinfluences upon the oxidation behavior of C/SiC composites.

The effects of a composite overwrapped gun barrel with viscoelastic damping layers are investigated. Interlaminar stresses and constrained layer damping effects are described. The Modal Strain Energy method is developed for measuring the extent to which the barrel is damped. The equations of motion used in the finite element analysis are derived. The transient solution process is outlined. Decisions for selected parameters are discussed. The results of the finite element analyses are presented using the program written in FORTRAN. The static solution is solved with a constant internal pressure resulting in a calculated loss factor from the Modal Strain Energy Method. The transient solution is solved using the Newmark-Beta method and a variable internal pressure. The analyses conclude that strategically placed viscoelastic layers dissipate strain energy more effectively than a thick single viscoelastic layer. The optimal angle for maximizing the coefficient of mutual influence in a composite cylinder is not necessarily the optimal angle when viscoelastic layers are introduced between layers.

Les matériaux composites à matrice de carbure de silicium renforcée par des fibres decarbone (C/SiC) sont des composites à matrice céramique (CMC), très prometteurs pour desapplications à haute température, comme le secteur aéronautique. Dans cette thèse, sontmenées des études particulières concernant les propriétés de ces matériaux : prédiction despropriétés mécanique (élastiques), analyses thermiques (optimisation des contraintesthermiques), simulation de l’oxydation à haute température.Une méthode basée sur l’énergie de déformation est proposée pour la prédiction desconstantes élastiques et des coefficients de dilatation thermiques de matériaux compositesorthotropes 3D. Dans cette méthode, les constantes élastiques et les coefficients de dilatationthermique sont obtenus en analysant la relation entre l'énergie de déformation de lamicrostructure et celle du modèle homogénéisé équivalent sous certaines conditions auxlimites thermiques et élastiques. Différents types de matériaux composites sont testés pourvalider le modèle.Différentes configurations géométriques du volume élémentaire représentatif des compositesC/SiC (2D tissés et 3D tressés) sont analysées en détail. Pour ce faire, la méthode énergétiquea été couplée à une analyse éléments finis. Des modèles EF des composites C/SiC ont étédéveloppés et liés à cette méthode énergétique pour évaluer les constantes élastiques et lescoefficients de dilatation thermique. Pour valider la modélisation proposée, les résultatsnumériques sont ensuite comparés à des résultats expérimentaux.Pour poursuivre cette analyse, une nouvelle stratégie d'analyse « globale/locale »(multi-échelle) est développée pour la détermination détaillée des contraintes dans lesstructures composites 2D tissés C/SiC. Sur la base de l'analyse par éléments finis, laprocédure effectue un passage de la structure composite homogénéisée (Echelle macro :modèle global) au modèle détaillé de la fibre (Echelle micro : modèle local). Ce passage entreles deux échelles est réalisé à partir des résultats de l'analyse globale et des conditions auxlimites du modèle local. Les contraintes obtenues via cette approche sont ensuite comparées àcelles obtenues à l’aide d’une analyse EF classique.IVLa prise des contraintes résiduelles thermiques (contraintes d’origine thermique dans lesfibres et la matrice) joue un rôle majeur dans le comportement des composites à matricescéramiques. Leurs valeurs influencent fortement la contrainte de microfissuration de lamatrice. Dans cette thèse, on cherche donc à minimiser cette contrainte résiduelle thermique(TRS) par une méthode d’optimisation de type métaheuristique: Particle Swarm Optimization(PSO), Optimisation par essaims particulaires.Des modèles éléments finis du volume élémentaire représentatif de composites 1-Dunidirectionnels C/SiC avec des interfaces multi-couches sont générés et une analyse paréléments finis est réalisée afin de déterminer les contraintes résiduelles thermiques. Unschéma d'optimisation couple l'algorithme PSO avec la MEF pour réduire les contraintesrésiduelles thermiques dans les composites C/SiC en optimisant les épaisseurs des interfacesmulti-couches.Un modèle numérique est développé pour étudier le processus d'oxydation de microstructureet la dégradation des propriétés élastiques de composites 2-D tissés C/SiC oxydant àtempérature intermédiaire (T<900°C). La microstructure du volume élémentaire représentatifde composite oxydé est modélisée sur la base de la cinétique d'oxydation. La méthode del'énergie de déformation est ensuite appliquée au modèle éléments finis de la microstructureoxydé pour prédire les propriétés élastiques des composites. Les paramètres d'environnement,à savoir, la température et la pression, sont étudiées pour voir leurs influences sur lecomportement d'oxydation de composites C/SiC. / Carbon fiber-reinforced silicon carbide matrix (C/SiC) composite is a ceramic matrixcomposite (CMC) that has considerable promise for use in high-temperature structuralapplications. In this thesis, systematic numerical studies including the prediction of elasticand thermal properties, analysis and optimization of stresses and simulation ofhigh-temperature oxidations are presented for the investigation of C/SiC composites.A strain energy method is firstly proposed for the prediction of the effective elastic constantsand coefficients of thermal expansion (CTEs) of 3D orthotropic composite materials. Thismethod derives the effective elastic tensors and CTEs by analyzing the relationship betweenthe strain energy of the microstructure and that of the homogenized equivalent model underspecific thermo-elastic boundary conditions. Different kinds of composites are tested tovalidate the model.Geometrical configurations of the representative volume cell (RVC) of 2-D woven and 3-Dbraided C/SiC composites are analyzed in details. The finite element models of 2-D wovenand 3-D braided C/SiC composites are then established and combined with the stain energymethod to evaluate the effective elastic constants and CTEs of these composites. Numericalresults obtained by the proposed model are then compared with the results measuredexperimentally.A global/local analysis strategy is developed for the determination of the detailed stresses inthe 2-D woven C/SiC composite structures. On the basis of the finite element analysis, theprocedure is carried out sequentially from the homogenized composite structure of themacro-scale (global model) to the parameterized detailed fiber tow model of the micro-scale(local model). The bridge between two scales is realized by mapping the global analysisresult as the boundary conditions of the local tow model. The stress results by global/localmethod are finally compared to those by conventional finite element analyses.Optimal design for minimizing thermal residual stress (TRS) in 1-D unidirectional C/SiCcomposites is studied. The finite element models of RVC of 1-D unidirectional C/SiCIIcomposites with multi-layer interfaces are generated and finite element analysis is realized todetermine the TRS distributions. An optimization scheme which combines a modifiedParticle Swarm Optimization (PSO) algorithm and the finite element analysis is used toreduce the TRS in the C/SiC composites by controlling the multi-layer interfaces thicknesses.A numerical model is finally developed to study the microstructure oxidation process and thedegradation of elastic properties of 2-D woven C/SiC composites exposed to air oxidizingenvironments at intermediate temperature (T<900°C). The oxidized RVC microstructure ismodeled based on the oxidation kinetics analysis. The strain energy method is then combinedwith the finite element model of oxidized RVC to predict the elastic properties of composites.The environmental parameters, i.e., temperature and pressure are studied to show theirinfluences upon the oxidation behavior of C/SiC composites.