O objetivo deste trabalho é analisar materiais piezelétricos compósitos com conectividade 1-3 e 2-2 para aplicações em transdutores de ultra-som na faixa de MHz utilizando modelos matemáticos e verificações experimentais. O estudo de um material piezelétrico compósito pode ser feito através de seus três principais tipos de modos de vibração: modo planar, modo de espessura e modo lateral. Neste trabalho, é utilizado o método dos elementos finitos para modelar os modos planares, de espessura e laterais de um compósito, e modelos analíticos para modelar o modo de espessura e o modo lateral. A modelagem do modo de espessura de um transdutor de ultra-som é feita a partir de um modelo analítico unidimensional. A modelagem unidimensional de um transdutor de ultra-som é feita através do cálculo das propriedades efetivas do material piezelétrico compósito. Essas propriedades são utilizadas no modelo da matriz distribuída para prever a impedância elétrica de um compósito e a resposta impulsiva de um transdutor de ultra-som. Com o objetivo de validar os modelos, foram construídos um material piezelétrico compósito com conectividade 1-3 e outro com conectividade 2-2 através da técnica “dice-and-fill", utilizando cerâmica de PZT-5A e resina epóxi. O compósito com conectividade 1-3 foi utilizado na construção de um transdutor de ultra-som. Os resultados teóricos da impedância elétrica e da resposta impulsiva são comparados com os obtidos experimentalmente. A impedância elétrica experimental é obtida através de um analisador de impedâncias, enquanto que a resposta impulsiva experimental do eco do transdutor é medida acoplando o protótipo do transdutor a um tarugo de acrílico. Devido à periodicidade do compósito foi feito um estudo teórico da propagação de ondas mecânicas em meios periódicos, mostrando que existem determinadas faixas de freqüências que não se propagam no material. Foi verificado que esta periodicidade é responsável pela diminuição das amplitudes dos modos radiais de um material piezelétrico compósito quando comparados com os modos radiais de um disco de cerâmica piezelétrica. Também foram feitos ensaios em tanque de imersão para determinar as propriedades mecânicas de amostras de epóxi e amostras de tungstênio e epóxi em função da fração de volume de tungstênio na amostra. / The objective of this work is to analyze piezoelectric composite materials with 1-3 and 2-2 connectivity for applications in ultrasonic transducers in the megahertz frequency range. The analysis is done through mathematical models and experimental validation. The analysis of piezoelectric composite materials can be done through the study of its three main vibrational modes: planar mode, thickness mode, and the lateral mode. In this work, it is used the Finite Element Method to model the planar, thickness and the lateral modes of the composite, and it is used analytical models to model the thickness and the lateral modes. The modeling of the thickness mode of an ultrasonic transducer is obtained through an unidimensional analytical model. The unidimensional modeling of the transducer is done by calculating the effective properties of the piezoelectric composite material. The effective properties are used in a distributed matrix model to calculate the electrical impedance of the composite and the impulse response of an ultrasonic transducer. To validate the models, a 1-3 and a 2-2 piezoelectric composite were built using the “dice-and-fill" technique. These composite were constructed using a piezoelectric ceramic of PZT-5A and epoxy. The piezoelectric composite with 1-3 connectivity was used in the fabrication of an ultrasonic transducer. The theoretical results of the electrical impedance and the impulse response are compared with the experimental results. The experimental electrical impedance is measured by using an impedance analyzer, and the experimental impulse response is measured by coupling the ultrasonic transducer prototype to an acrylic block. Due to the periodicity of the composite, it was analyzed the behaviour of mechanical waves in periodic media, showing that there are frequency ranges that the waves cannot propagate. It was verified that the periodicity is responsible for the suppression of the radial modes in a piezoelectric composite when compared with the radial modes of a disk of piezoelectric ceramic. It is also conducted measurements in a water filled tank to determine the mechanical properties of samples of epoxy, and Tungsten/epoxy composites as a function of the volume fraction of Tungsten.

by Hon-chor Lee. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaf 56). / by Hon-chor Lee. / Introduction --- p.1 / Series expansion for the conductivity of a linear random resistor network / Chapter 1. --- Introduction --- p.4 / Chapter 2. --- Comparison of the EMA with symbolic simulations in 2D --- p.5 / Chapter 3. --- Comparison of the EMA with Bergman and Kantor's findings --- p.6 / Chapter 4. --- Conclusion --- p.7 / Current moments of linear random resistor network / Chapter 1. --- Introduction --- p.10 / Chapter 2. --- Review of the definition of current moment --- p.10 / Chapter 3. --- Tremblay et. al.'s findings and symbolic simulation of current moments --- p.11 / Chapter 4. --- Conclusion --- p.13 / Effective medium theory for strongly nonlinear composites: comparison with numerical simulations / Chapter 1. --- Introduction --- p.15 / Chapter 2. --- Variational principles --- p.16 / Chapter 3. --- Formalism of EMA --- p.17 / Chapter 4. --- Comparison with numerical simulations --- p.19 / Chapter 5. --- Discussion --- p.21 / Percolative conduction in two-component strongly nonlinear composites / Chapter 1. --- Introduction --- p.25 / Chapter 2. --- Spherical inclusions --- p.25 / Chapter 3. --- Effective medium approximation in the vicinity of the percolation threshold --- p.27 / Chapter 4. --- Acknowledgment --- p.29 / Percolation Effects in Two Component Strongly Nonlinear Composites: Universal Scaling Behaviours / Chapter 1. --- Introduction --- p.31 / Chapter 2. --- General Scaling Relations for Two-Component Composites --- p.33 / Chapter 3. --- Estimate of Critical Exponents --- p.35 / Chapter 4. --- Numerical Simulations --- p.38 / Chapter 5. --- Discussions and Conclusions --- p.40 / Chapter 6. --- Acknowledgment --- p.40 / Improved effective medium theory for strongly nonlinear composites / Chapter 1. --- Introduction --- p.46 / Chapter 2. --- Formalism of Improved EMA --- p.47 / Chapter 3. --- Comparisons with numerical simulations and HS bound --- p.50 / Chapter 4. --- Discussions --- p.51 / Conclusion --- p.54

This work describes a novel technique to prepare ultra-thin composite Pd-porous metal membranes for H2 separation. This novel technique consists of the gradual smoothing of the Porous Metal (PM) support's surface with several layers of pre-activated alumina particles of different sizes. The deposition of coarse, fine and ultra-fine alumina particles resulted in the narrowing of the PM' surface pore size distribution. The excellent surface smoothness achieved after the grading of the PM 's surface support allowed for the preparation of gas tight Pd layers as thin as 5.6?m. The Pd layers were extremely uniform due to the presence of the grade layer and strongly attached to the support. Composite Pd membranes prepared on graded supports showed H2 permeance as high as 50 m3/(m2 h bar0.5) at 500ºC and ideal selectivities (H2/He) as high as 27000. Moreover, the H2 permeance and ideal selectivity were stable over 1100 hours at 500ºC in H2 atmosphere. Composite Pd-Cu membranes showed H2 permeance as high as 30 m3/(m2 h bar0.5) at 450ºC and ideal selectivities (H2/He) as high as 900. The H2 permeance and ideal selectivity of Pd-Cu membranes were stable over 500 hours at 450ºC in H2 atmosphere. The outstanding long-term H2 permeance and ideal selectivity stability of all composite Pd and Pd-Cu membranes represented a breakthrough in composite Pd membrane synthesis. The thermal stresses arising from the mismatch in the coefficient of thermal expansion between the Pd film and the support were determined by means of x-ray diffraction. The results indicated that the release of stresses began to occur at temperatures close to 400ºC. Also, the release of stresses took place with a visible sintering of Pd clusters within the thin Pd film. The stresses due to the absorption of H2 were also studied and modeled. It was estimated that the maximum compressive stress under which these composite Pd membranes were characterized was equal to 260 MPa.

Traditional probability and statistics methodologies recommended by ISO and NIST were applied to standardize measurement uncertainty analysis on calorimetry bench scale apparatuses. The analysis was conducted for each component instrument (direct measurement) and each related physics quantity measured indirectly. There were many sources contributing to the ultimate uncertainty, however, initially, we dealt with the intrinsic uncertainty of each measuring instrument and the uncertainty from calibration. All other sources of uncertainty, i.e., drift, data acquisition, data reduction (round off, truncation, and curve smoothing) and personal operation were assumed to be negligible. Results were expressed as an interval having 95% confidence that the ¡°true¡± value would fall within. A Monte Carlo Simulation technique with sampling size of 10000 was conducted to model the experiments. It showed that at least 95% of the modeled experiment results were inside the estimate interval. The consistency validated our analysis method. An important characteristic of composite material systems is the ability to ¡°custom design¡± the system to meet performance criteria such as cost, durability, strength and / or reaction to fire. To determine whether a new system is an improvement over previous ones and can meet required performance criteria, sufficiently accurate and precise instruments are needed to measure the system¡¯s material properties in bench scale testing. Commonly used bench scale apparatuses are the cone calorimeter (Cone) and the FMGR fire propagation apparatus (FPA). For this thesis, thermally ¡°thin¡± and ¡°thick¡± specimens of a natural composite, red oak, were tested in the Cone in an air environment and in the FPA in a nitrogen environment. Cone test data of two FRP composite systems from the previous work of Alston are also considered. The material reaction to fire properties were estimated considering both ignition and pyrolysis measurements made via the Cone and FPA. Investigation of the ultimate uncertainty of these material fire properties based on the intrinsic uncertainty of the component instruments (e.g. load cell) as well as the uncertainty introduced via use of a current ignition and pyrolysis model are considered.

"This study presents a general approach for an inverse problem to locate a sudden structural damage in a plate. The sudden damage is modeled as an impulse load and response data are collected at various sensor locations. In this simulation study the response data were generated by the commercial finite element code ANSYS for three square plates: one is an isotropic plate and made of aluminum and the others are two different composite plates made of graphite-epoxy. All plates are simply supported along all their edges. The responses of these plates to both narrow band and wide band loading were analyzed by a wavelet transform. The wavelet coefficient maps for each type of signal was utilized to estimate the shortest path arrival times of flexural waves resulted from the damage by locating the wavelet coefficient peak values of the response data. Using the dispersion relations of wave propagation based on the Mindlin’s plate theory, a set of nonlinear equations were derived to solve this inverse problem and the location of the applied load, which models a structural damage, was determined. The estimated locations for all different types of plates have shown an excellent agreement with the actual location of the impact loads applied. "

Orientador: Edson Cocchieri Botelho / Coorientador: Humberto Araújo Machado / Banca: Michelle Leali Costa / Banca: José Atílio Fritz Fidel Rocco / Resumo: Materiais utilizados em componentes de veículos espaciais, como em tubeiras ou superfícies expostas à reentrada atmosférica, são sujeitos a ambientes termicamente agressivos. Este trabalho apresenta estudos envolvendo o desempenho de materiais compósitos utilizados em sistemas de proteção térmica, a partir da exposição a jatos de plasma, onde os fluxos de calor são comparáveis aos da reentrada atmosférica de componentes de veículos espaciais. Amostras de compósitos ablativos de carbono/fenólica foram ensaiadas no túnel de plasma do Laboratório de Plasmas e Processos do ITA (Instituto Tecnológico de Aeronáutica), por meio de uma tocha de plasma alimentada por uma fonte de energia elétrica de corrente contínua de 50kW. Os parâmetros de operação do túnel de plasma foram otimizados para reproduzirem as condições próximas do ponto crítico de reentrada das cargas úteis dos veículos espaciais desenvolvidos pelo IAE (Instituto de Aeronáutica e Espaço). As amostras em estudo foram desenvolvidas e fabricadas no Brasil, a partir de materiais de especial interesse do IAE. Para comparação, foi também ensaiado outro material com propriedades já bem estabelecidas como o teflon, sob as mesmas condições ablativas. Foram determinadas as perdas de massa e as taxas de perda de massa específicas das amostras, as temperaturas radiométricas superficiais e termométricas internas, em função do tempo de exposição ao fluxo térmico. Foi realizada também a avaliação da evolução das interfaces por compara... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Materials used in space vehicles components, such as nozzles or surfaces exposed to atmospheric reentry, are subjected to thermally aggressive environments. This work presents studies involving the performance of composite materials used in thermal protection systems, through the exposure to plasma jets, where the heat fluxes are comparable to atmospheric reentry of space vehicle components. Samples of ablative carbon/phenolic composites were tested in the plasma tunnel of ITA's (Aeronautics Institute of Technology) Plasma and Process Laboratory, by a plasma torch with a 50kW DC power source. The plasma tunnel operating parameters were optimized to reproduce the conditions close to the critical re-entry point of the space vehicles payloads developed by the IAE (Aeronautics and Space Institute). The samples in study were developed and manufactured in Brazil, from materials of special interest to IAE. For comparison, another material with well established properties such as teflon was also tested under the same ablative conditions. The mass loss and the specific mass loss rates of the samples, the surface radiometric and internal thermometric temperatures, as a function of the exposure time to the thermal flow, were determined. The evolution of the interfaces was also performed by comparison between simulation and the sample after the test. The results allowed to estimate the properties of the ablative behavior of the materials tested and to validate the theoretical model used ... (Complete abstract click electronic access below) / Mestre

Bio-composite hierarchical materials have attracted the interest of the academic community operating in the field of bio-inspired materials for their outstanding mechanical properties achieved via lightweight structural designs. Antler and mantis shrimp's cuticle are extreme examples of materials naturally optimised to resist impacts and bear dynamic loading. Firstly, a class of finite-element fibril models was developed to explain the origin of heterogeneous fibrillar deformation and hysteresis from the nanostructure of antler. Results were compared to synchrotron X-ray data and demonstrated that the key structural motif enabling a match to experimental data is an axially staggered arrangement of stiff mineralised collagen fibrils coupled with weak, damageable interfibrillar interfaces. Secondly, the cuticle of the crustacean Odontodactylus scyllarus, known as peacock mantis shrimp, was investigated. At the nanoscale it consists of mineralised chitin fibres and calcified protein matrix, which form plywood layers at the microscale. Lamination theory was used to calculate fibrillar deformation and reorientation and, in addition, an analytical formulation was used to decouple in-plane fibre reorientation from diffraction intensity changes induced by 3D lamellae tilting. This animal also attracted my attention for using its hammer-like appendages to attack and destroy the shells of prey with a sequence of two strikes. Inspired by this double impact strategy, I performed a set of parametric finite-element simulations of single, double and triple mechanical hits, to compute the damage energy of the target. My results reveal that the crustacean attack strategy has the most damaging effect among the double impact cases, and lead me to hypothesise, that optimal damaging dynamics exists, depending on the sequence of consecutive impacts and on their time separation values. These new insights may provide useful indications for the design of bio-inspired materials for high load-bearing applications.

Orientador: Edson Cocchieri Botelho / Coorientadora: Michelle Leali Costa / Banca: José Maria Fernandes Marlet / Banca: Evandro Luis Nohara / Banca: Geraldo Mauricio Candido / Banca: Maria Odila Hilario Cioffi / Resumo: As matrizes poliméricas atualmente disponíveis para compósitos avançados têm evoluído progressivamente como alternativa para a indústria aeronáutica sendo que nos últimos anos diversas matrizes termoplásticas especiais e de alto desempenho mecânico vem sendo estudadas para utilização na fabricação de compósitos avançados. Dentre as matrizes termoplásticas mais utilizadas na área aeronáutica destaca-se a poli(éter-imida) (PEI) por apresentar baixo custo, boas propriedades mecânicas e ser de fácil manuseio. Por outro lado, um dos principais problemas da utilização de compósitos termoplásticos em aplicações estruturais consiste em sua união efetiva para a integração de componentes. Entre os diversos métodos disponíveis destinados à soldagem, a utilização do processo de soldagem por resistência elétrica vem sendo considerado como um dos mais promissores para união de compósitos. O presente trabalho tem como objetivo a otimização do processo de soldagem por resistência elétrica utilizando compósitos de matriz constituída do polímero termoplástico PEI reforçado por fibras contínuas de vidro, carbono e por um sistema híbrido constituído de fibras de carbono e de vidro, com foco em aplicações aeronáuticas. Para o desenvolvimento deste trabalho, foi utilizado um planejamento experimental para a determinação e otimização dos melhores parâmetros sendo aqui considerados tempo, corrente elétrica e pressão na soldagem de laminados em estudo. Além disso, foram avaliados 4 tipos de elementos resistivos (malhas metálicas de aço inox de 100, 200 e 300 mesh e fibra de carbono com bronze). Os melhores parâmetros de otimização foram determinados a partir de ensaios de Lap Shear. Baseando-se nos resultados encontrados quanto à otimização destes sistemas, foi concluído que para o compósito PEI/ fibra de vidro os melhores valores ...( Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Thermoplastic composites reinforced with continuous fibers are being studied in a wide range of applications, such as commercial use for their low cost and for engineering with higher associated value. Currently, available polymer matrices for advanced composites have been progressively developed as an alternative for the aeronautical industry and, in the latest years, several special and high performance thermoplastic matrices have been studied for their use in manufacturing advanced composites. Among the thermoplastic matrixes, the poly (ether imide) (PEI) should be highlighted due mainly to its low cost, good mechanical properties, and for being easy to be handled. Moreover, a major problem concerning the use of thermoplastic polymer composites for structural applications is their effectiveness in the integration of structural components. Among the methods available for welding these composites is the use of the electrical resistance welding process which has been considered as the most promising for joining composites. The present work aims to optimize the welding process using electrical resistance matrix composites, consisting of thermoplastic polymer PEI reinforced by glass fiber, carbon fiber and a hybrid system consisting of carbon fiber and glass fiber for aeronautical applications. For the development this work, an experimental design was carried out to determine optimal parameters of time, electrical current and pressure on the welding of the laminates used in this study. Also it was evaluate 4 types of resistive elements (metal mesh stainless steel 100, 200 and 300 mesh, and carbon fiber with bronze). The best optimization parameters were confirmed from the Lap Shear tests. From the main results of these optimization, it was concluded that, for PEI / glass fiber laminates, the best acquired experimental values of current, ... (Complete abstract click electronic access below) / Doutor

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / by Avraham Benatar. / B.S.

Modification in the resin-rich region between plies, also known as the interlaminar region, was investigated to increase the toughness of laminate composites structures. To achieve suitable modifications, the complexities of the physical and chemical processes during the resin curing procedure must be studied. This includes analyses of the interactions among the co-dependent microstructure, process parameters, and material responses. This dissertation seeks to investigate these interactions via a series of experimental and numerical analyses of the geometric- and temperature-based effects on locally interleaving toughening methods and further interlaminar synergistic toughening without interleaf. Two major weaknesses in composite materials are the brittle resin-rich interlaminar region which forms between the fiber plies after resin infusion, and the ply dropoff region which introduces stress concentration under loads. To address these weaknesses and increase the delamination resistance of the composite specimens, a dual bonding process was explored to alleviate the dropoff effect and toughen the interlaminar region. Hot melt bonding was investigated by applying clamping pressure to ductile thermoplastic interleaf and fiber fabric at an elevated temperature, while diffusion bonding between thermoplastic interleaf and thermoset resin is performed during the resin infusion. This method increased the fracture energy level and thus delamination resistance in the interlaminar region because of deep interleaf penetration into fiber bundles which helped confining crack propagation in the toughened area. The diffusion and precipitation between thermosets and thermoplastics also improved the delamination resistance by forming a semi-interpenetration networks. This phenomenon was investigated in concoctions of low-concentration polystyrene additive modified epoxy system, which facilitates diffusion and precipitation without increasing the viscosity of the system. Additionally, chemical reaction induced phase separation, concentration of polystyrene, and various curing temperatures are used to evaluate their effects on diffusion and precipitation. These effects were directly investigated by performing attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The diffusivity and curing kinetics experiments are performed to quantify the diffusivity coefficient of epoxy, hardener and thermoplastics, as well as the reaction rate constant of curing epoxy at various temperatures. Finally, mechanical testing and fracture surface imaging were used to quantify the improvements and characterize the toughening mechanism. Further improvement on delamination resistance was studied through the synergistic effect of combining different modification methods without the interleaf. Polysulfone molecules are end-capped with epoxide groups. Fiber surface is functionalized with amino groups to generate micro-mechanical interlocks. The interaction between two individual modifications chemically links the modified semi-interpenetration networks to the improved interfacial strength between fiber and epoxy to. The impact of the additive on the crosslinking density was examined through glass transition temperatures, and the chemical modification was characterized by Raman spectroscopy. Mode I and II fracture tests were performed to quantify the improvement of delamination resistance under pure opening and shear loads. The mechanism of synergistic effect was explained based on the fracture surface morphology and the interactions between the modification methods.