Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2007. / Committee Chair: Itzhak Green; Committee Member; Jeffrey Streator; Committee Member: Richard Neu. Part of the SMARTech Electronic Thesis and Dissertation Collection.

As coberturas constituem um elemento essencial para a sobrevivÃncia humana. As transformaÃÃes mais portantes das cobertas relacionaram-se, por um lado, com a prÃ-fabricaÃÃo do material a ser utilizado, que veio permitir maiores garantias de Ãxito em qualquer aplicaÃÃo, e por outro lado, com o aparecimento de novos materiais na construÃÃo, como o vidro laminado, peÃas de madeira, os materiais plÃsticos e principalmente o alumÃnio e o aÃo. Neste contexto os telhados de metal sÃo bem avaliados por vÃrios motivos, sÃo resistentes, durÃveis, vencem grandes vÃos, resistem à corrosÃo e sÃo mais leves em relaÃÃo Ãs cerÃmicas, gerando economia nos custos da estrutura e facilidade no manuseio, transporte e montagem. A problemÃtica dos ventos de alta velocidade, como furacÃes e tempestades, muitas vezes causam danos graves Ãs telhas metÃlicas. Os danos causados pelo vento mostraram que a fadiga promove uma trinca no telhado em torno dos furos do prendedor ocasionando um despendimento das telhas em sequÃncia. Nesse trabalho desenvolve-se uma mÃquina para estudos de simulaÃÃo de carregamento cÃclico em telhas metÃlicas simulando a forÃa do vento. Utilizando a tÃcnica da CorrelaÃÃo Digital de Imagem (CDI) desenvolveu-se uma metodologia que auxiliou em termos quantitativos e qualitativos a avaliaÃÃo da integridade da telha. / Shelter is an essential element for human survival. The most important changes in shelter have been in regard, on the one hand, of pre-fabrication of the material to be used, which better guarantees success at any application, and, on the other hand, of the rise of new building materials, such as laminated glass, wooden parts, plastic materials, and mainly aluminum and steel. In this context, metal roofs are praised for several reasons. They are sturdy and durable, span across wide areas, stand corrosion, and are lighter compared to ceramic tiles, which leads to savings in structure costs and ease of handling, transport, and assembly. The issue of high- speed winds, such as hurricanes and storms, often causes great damage to metal tiling. Wind damages show that the low-cycle wear fissures the roofing around the fastening holes, causing the tiles to become loose. This research developed a machine to study the simulation of cyclic loads in metal tiles by simulating wind force. By using the Digital Image Correlation (DIC) technique, a methodology was developed that will aid in assessing tile integrity both quantitatively and qualitatively.

O trabalho trata da formulação, implementação e aplicação de um modelo para análise não-linear de estruturas reticuladas planas em concreto armado, baseado no conceito simplificado de localização dos processos físicos irreversíveis (dano, plasticidade e histerese) em zonas previamente definidas denominadas rótulas que se localizam nas extremidades de um elemento finito de barra. O modelo proposto tem por objetivo reproduzir os deslocamentos residuais relacionados tanto ao comportamento plástico do concreto danificado quanto ao escoamento do aço, além de possibilitar a reprodução de laços de histerese observados experimentalmente em diagramas momento-rotação, ou equivalentemente força-deslocamento, quando a história de carregamento incluir ciclos de descarregamento e recarregamento. A formulação termodinâmica é desenvolvida fazendo-se referência as variáveis de tensão e deformação generalizadas em razão de sua aplicação no âmbito da teoria clássica de flexão de barras. Neste modelo a não-linearidade do elemento estrutural é totalmente decorrente da dissipação de energia localizada nas rótulas e é representada por três variáveis escalares a elas associadas: a variável adimensional de dano, a rotação plástica e a rotação por escorregamento, que representa o fenômeno físico do escorregamento entre as faces das microfissuras no concreto (encaixe e desencaixe dos agregados). Na fase de aplicações as respostas numéricas obtidas com a utilização do modelo proposto foram avaliadas mediante o confronto com diversos resultados experimentais disponíveis na literatura para estruturas de concreto armado sob solicitações estáticas e dinâmicas. Os resultados obtidos evidenciam o bom funcionamento do modelo no caso das solicitações estáticas, tendo sido obtidos resultados limitadamente satisfatórios no caso das solicitações dinâmicas. / This work deals with the formulation, implementation and application of a model for nonlinear analysis of reinforced concrete planed frame structures, based on the simplified concept of localization of the irreversible physical processes (damage, plasticity and hysteresis) in zones previously defined called hinges which are located in the extremities of a bar finite element. The proposed model has as objective to reproduce the residual displacements related to the plastic behavior of the damaged concrete as well as to the steel yielding, moreover making possible the reproduction of hysteresis loops experimentally observed in moment-rotation diagrams, or equivalently force-displacement, when the loading history includes unloading and reloading cycles. The thermodynamic formulation is developed referring to generalized stress and deformation variables due to their application in the scope of the classic theory of bars bending. In this model the nonlinearity of the structural element is totally due to the energy dissipation in the hinges and is represented by three scalar variables associated to them: the non-dimensional damage variable, the plastic rotation and the sliding rotation, which represents the physical phenomenon of sliding between the microcracks faces in the concrete (aggregate interlocking). In the applications phase, the numerical solutions obtained with the use of the proposed model had been evaluated by confrontation with various experimental results available in the literature of reinforced concrete structures under static and dynamic solicitations. The obtained results evidence the good performance of the model in the case of static solicitations, having been obtained limited satisfactory results in the case of dynamic solicitations.

Este trabalho trata da formulação de leis constitutivas para meios elásticos, que uma vez danificados passam a apresentar diferentes comportamentos em tração e em compressão e certo grau de anisotropia. Inicialmente é revista uma formulação para meios elásticos anisótropos e bimodulares, sendo a mesma, então, estendida para incorporar os casos de meios elásticos com anisotropia e bimodularidade induzidas pelo dano. Seguindo os conceitos da mecânica do dano contínuo e a extensão mencionada da formulação, propõe-se um modelo constitutivo para o concreto assumindo por hipótese fundamental, a equivalência de energia entre meio danificado real e meio contínuo equivalente. Tal hipótese garante a simetria do tensor constitutivo e a sua consistência termodinâmica. De acordo com o modelo proposto, o material é considerado como um meio elástico inicialmente isótropo que passa a apresentar anisotropia induzida pela evolução do dano. Além disso, a danificação pode também induzir uma resposta bimodular no material, isto é, respostas elásticas diferentes para estados de tensão de tração ou de compressão predominantes. Nesse sentido, dois tensores de dano governando as rigidezes em regimes predominantes de tração ou de compressão são introduzidos. Sugere-se então, um critério afim de caracterizar os estados dominantes. As deformações permanentes induzidas pelo dano são, de uma forma geral, desconsideradas. No entanto, propõe-se uma versão unidimensional do modelo que permite a sua consideração. Por outro lado, os critérios para a ativação inicial dos processos de danificação e de sua posterior evolução são escritos em termos de densidade de energia de deformação. Os parâmetros do modelo podem ser identificados mediante experimentos, onde estados de tensão uniaxial e biaxial são induzidos. Também propõem-se leis de evolução de dano com base nos resultados experimentais. A boa coerência do modelo é ilustrada comparando-se uma série de respostas experimentais e numéricas no concreto relativas a estados de tensão uni, bi e triaxiais. Por fim, o modelo é empregado em análises unidimensionais e planas de vigas e pórtico em concreto armado com o objetivo de mostrar a sua potencialidade. / This thesis deals with the formulation of constitutive laws for elastic media that start to present different behaviours in tension and compression and some anisotropy degree when damaged. Initially a formulation for bimodular and anisotropic elastic media is reviewed; then it is extended to take into account the bimodularity and anisotropy induced by damage. Following the concepts of the continuum damage mechanics and the formulation extension, a constitutive model is proposed here by exploring the fundamental hypothesis of energy equivalence between real and continuous medium. Such hypothesis guarantees symmetry to the constitutive tensor and its thermodynamic consistency. According to the proposed modeling, the material is assumed as an initial elastic isotropic medium presenting anisotropy induced by damage evolution. Moreover, damage can also induce a bimodular response in the material, i.e., distinct elastic responses whether traction or compression stress states prevail. To take into account the bimodularity conveniently, two damage tensors governing the rigidity in traction or compression regimes are introduced. A criterion is proposed in order to characterize the dominant states. In general, the permanent strains induced by damage are disregarded. However, a one-dimensional version of the model that allows considering permanent strains is proposed. On the other hand, damage criteria indicating the initial and further evolution of damage are expressed in terms of strain energy densities. The model parameters can be identified from experiments where one-axial and two-axial stress states are induced. Damage evolution laws are also proposed on the basis of experimental results. The good performance of the model is illustrated by comparing a variety of numerical and experimental responses, from one to three-axial stress states. Finally, the model is used in one-dimensional and plane analysis of reinforced concrete beams and frame in order to show its potentiality.

No trabalho estudam-se alguns aspectos relativos à formulação teórica e à simulação numérica de modelos constitutivos para o concreto fundamentados na Mecânica do Dano Contínuo, incluindo-se os chamados métodos simplificados de análise estrutural. Inicialmente apresenta-se uma discussão sobre deformações permanentes e anisotropia induzidas pela evolução do dano. A resposta unilateral do concreto submetido a solicitações com inversão de sinal, também é comentada. Cada um dos fenômenos é ilustrado por respostas observadas experimentalmente. O modelo de dano proposto por Mazars para o concreto submetido a carregamento proporcionalmente crescente é então analisado. Em seguida, apresenta-se uma extensão do modelo considerando-se o aspecto unilateral no comportamento do concreto. Na sequência, analisa-se o modelo constitutivo proposto por La Borderie em seus aspectos de formulação e resposta numérica. O modelo é mais completo permitindo levar em conta todos os fenômenos discutidos anteriormente. Um outro aspecto considerado no trabalho é relativo à aplicação dos modelos estudados à análise de estruturas aporticadas. Em termos de discretização destacam-se a técnica de divisão dos elementos estruturais em estratos e os chamados modelos simplificados. Neste último caso, o modelo de Flórez-López é analisado e a simplificação consiste na definição prévia, sobre a estrutura discretizada através de elementos de viga e de coluna, de zonas de localização da plastificação e do dano; no limite com a evolução do processo de carregamento, aquelas zonas passam a se constituir em rótulas. Por último uma generalização do modelo anterior proposta por Álvares é estudada. Os resultados numéricos fornecidos pelos modelos são confrontados com os experimentais de vigas em concreto armado (biapoiadas e com diferentes taxas de armadura) e de um pórtico em concreto armado. / In this work some aspects related to the theoretical formulation and numerical simulation of constitutive models for the concrete based on the Continuum Damage Mechanics are studied, including simplified methods of structural analysis. Initially a discussion about permanent strains and anisotropy induced by the evolution of the damage is presented. The unilateral response of the concrete submitted to loadings with signal inversion is commented as well. Each one of the phenomena is illustrated by experimental tests. The damage model proposed by Mazars for the concrete under proportional increasing load is then analysed. Afterwards, it is shown an extension of the model considering the unilateral aspect ofthe concrete behaviour. In the sequence the constitutive model proposed by La Borderie is presented in its aspects of formulation and numerical response. This model is more complete, allowing to take in to account all the phenomena discussed previously. Another aspect considered in the work is related to the application of the models studied to the analysis of framed structures. Regarding the discretization, the technique of division of the structural elements in layers and the so called simplified models are enhanced. In this last case, the model proposed by Flórez-López is analysed and the simplification consists on previous definition of yielding and damage zones over the assembly of beam and colunm elements, which compose the discretized structure; in the limit evolution of the loading process, these zones become hinges. Finally, a generalization of the previous model proposed by Álvares is studied. The numerical results supplied by the models are compared with the experimental results of reinforced concrete beams (simply supported and with different reinforced rates) and of a reinforced concrete frame.

The high specific stiffness and strength of composites makes it advantageous for load carrying structures in the automotive industry. By successfully be able to numerically simulate the crush behaviour of composites, structure with high specific energy absorption can be implemented in the automotive industry. The purpose of this thesis is to verify the predictive capabilities of a crush model developed at SICOMP. Initially currently available material models are investigated. Puck’s criterion is deeper studied. An improvement of the criterion is suggested and the model is updated to be able to output fracture angles in Abaqus.The material model developed by SICOMP is a three-dimensional physically based damage model where failure initiation is estimated with proven failure criteria and damage growth is combined with friction to account for the right energy absorption. The crush damage model has been implemented in Abaqus/Explicit as a VUMAT subroutine. Numerical predictions are compared with experimental results. Specimens with different fibre layups and crash triggers are tested.

Blunt carotid artery injury (BCAI), resulting primarily from automobile accidents, is a major contributor to the high mortality and morbidity rates associated with carotid artery dissection. More work is needed to characterize carotid artery injury mechanisms, quantify stages of damage, and elucidate failure modalities as a result of this type of injury. The present study examines the structure and mechanics of the carotid artery in the circumferential and axial directions by employing uniaxial tensile testing, high speed videography, interruption testing, scanning electron microscopy (SEM), histological analysis, real-time environmental SEM assessment, and atomic force microscopy (AFM). Results are as follows: (i) the carotid artery exhibits anisotropic, viscoelastic behavior; (ii) intimal failure precedes ultimate tissue failure, and the layers in order of increasing strength are intima, adventitia, and media; (iii) tissue damage accumulates as strain level increases, and failure occurs as a result of void nucleation, void growth, and void coalescence.

A three-dimensional finite element code to analyze coupled thermomechanical deformations of composites has been developed. It incorporates geometric nonlinearities, delamination between adjoining layers, and damage due to fiber breakage, fiber/matrix debonding, and matrix cracking. The three damage modes are modeled using the theory of internal variables and the delamination by postulating a failure envelope in terms of the transverse stresses; the damage degrades elastic moduli. The delamination of adjoining layers is simulated by the nodal release technique. Coupled nonlinear partial differential equations governing deformations of a composite, and the pertinent initial and boundary conditions are first reduced to coupled ordinary differential equations (ODEs) by the Galerkin method. These are integrated with respect to time with the Livermore solver for ODEs. After each time step, the damage in an element is computed, and material properties modified. The code has been used to analyze several static and transient problems; computed results have been found to compare well with the corresponding test results. The effect of various factors such as the fiber orientation, ply stacking sequence, and laminate thickness on composite's resistance to shock loads induced by underwater explosions has been delineated. / Ph. D.

This work discusses the use of Positive Position Feedback (PPF) for Active Vibration Control as part of an Active Damage Control System (ADCS). Vibration control increases the fatigue life of a structure and decreases the in-plane stresses that can cause delamination in a composite. PPF is a collocated direct-output feedback control method that increases the effective damping in a structure. A simply-supported beam was used as the testbed which used strain gages as the sensing element and piezoelectric ceramics as the actuator. Initial investigations into sampled-data systems using PPF are presented. The issues addressed are: stability of the sampled system, the effects of the sampling rate on the system, and degradation from predicted analog performance. A digital design procedure for the tuning filters in the Z-plane is suggested if the sampling rate to be used is known. If the sampling rate varies significantly, to avoid redesigning the filters for each new sampling rate, they should be designed in the continuous-time and transformed to the Z-plane. The Tustin transformation was found to adequately map the poles and zeros of the compensator to the Z-plane for digital control. Experimental implementation of PPF on a simply-supported beam resulted in vibration suppression of three modes with a S180 controller. The beam was subjected to both a single-frequency harmonic disturbance and a broadband harmonic disturbance. One, two, and three-mode controllers were designed with disturbance suppression up to 15dB achieved. / Master of Science

Simulations using the Finite Element Method (FEM) play an increasingly important role in the design process of joints and fasteners in the aerospace industry. In order to utilize the potential of such adhesive bonding, there is an increasing need for effective and accurate computational methods. The geometry and the nature of an adhesive joint are, however, not so simple to describe effectively using standard FEM-codes. To overcome this difficulty, special FEM-elements can be developed that provide a material surface treatment of the adhesive and the joined parts. In order to create a model that reflects the above features, one may introduce proper scalings on the geometry and on the material properties in terms of a perturbation parameter. Within the framework of three-dimensional elasticity, together with an asymptotic expansion method, a material surface model is obtained through a systematic procedure. In such a derivation, no a priori assumptions for the displacements or stress fields are needed. The final result is a variational equation posed over a single reference surface which forms the basis of a structural element for the compound joint. Through the usage of continuum damage mechanics and the framework of a generalized standard material, the linear elastic model is extended to include an elastic-plastic material model with damage for the adhesive. The model is FE-discretized and an important implication is that the (quasi-static) propagation of the local failure zone in the adhesive layer can be simulated. The failure load is obtained as a computational result and consequently no postulated failure criterion is needed. The derived FE-method opens up the possibility to efficiently model and analyze the mechanical behavior of large bonded structures. / At the time the thesis was defended paper I. was in fact two manuscripts, which later were combined to give the published article.