Uma das dificuldades do processo de retificação center-less é o estabelecimento de padrões confiáveis que determine com pequena margem de erro problemas no processo. Este trabalho visa desenvolver um sistema de monitoramento capaz de identificar falhas ocorridas durante a dressagem e o processo de retificação center-less acima dos centros de hastes de válvulas. Para tanto, inicialmente, foi feito um estudo teórico e prático acerca do processo de retificação center-less com o objetivo de conhecer seus elementos básicos, características, principais problemas e fatores que influenciam o processo. Em função da grande aplicação da emissão acústica em operações de retificação, foi desenvolvida uma nova metodologia, onde foi possível criar um mapa digital, baseado em informações acústicas detalhadas do processo, usando para isto uma constante de tempo muito pequena. Os resultados mostraram a potencialidade do sistema de mapeamento em detectar e diagnosticar problemas ocorridos durante processos de retificação e dressagem. Assim, pode-se atuar sobre o processo de tal forma a corrigir estas anomalias. / One of the difficulties of the center-less grinding process is to establish reliable patterns to determine, with a small error, problems in the process. This work aims to develop an monitoring system able to identify flaws happening during the dressing and the center-less grinding process above the center of values stems. Initially a theoretical and practical study was conduct on the center-less grinding process with the objective of discussing its basic elements, characteristics, main problems and factors that influence the process. Due to the great application of acoustic emission in grinding operations a new methodology was developed, where it was possible to create a digital map, based on detailed acoustic information on the process using for this a very short time constant. The results showed the potentiality of this mapping system to detect and to diagnose problems during grinding and dressing. Therefore, it is possible to correct anomalies when plunge center-less grinding process.

A viabilização do conceito de manufatura enxuta depende da redução da necessidade de intervenções do operador no processo. Na usinagem dos metais, uma das principais situações onde está a intervenção se faz necessária é a determinação da condição de fim de vida das ferramentas de corte, a qual é muitas vezes subjetiva e não pode ser prescrita com segurança. Sendo a emissão acústica (EA) um sinal gerado pelos principais fenômenos envolvidos na usinagem, um sistema de monitoramento de desgaste de ferramentas baseado neste tipo de sinal foi avaliado em operação de torneamento de eixo de saída de transmissão mecânica caracterizado por geometria complexa. Verificou-se que há uma relação entre a trajetória da ferramenta e a \"assinatura\" do processo (tensão RMS plotada contra o tempo). Além disso, há uma forte correlação destes mais a adesão de material da peça à ferramenta determinaram pequena diferença de amplitude do sinal entre os estágios iniciais e finais de vida das ferramentas. Além disso, a influência do sistema de fixação na EA mostrou-se diversas vezes maior que a influência do desgaste. Portanto, um sistema de monitoramento de desgaste baseado apenas em emissão acústica não seria adequado para a situação estudada. / The Lean Manufacturing concept depends on the reduction of the necessity of interventions in the process by the operators. ln metal machining, one situation where these interventions are necessary is the determination of tool life, which is subjective. Besides that, the toollife prediction is not reliable. Acoustic emission signals are generated by many activities involved in machining. Therefore an acoustic emission based tool wear monitoring system was evaluated in a turning operation of a mechanical transmission main shaft characterised by complex geometry. lt was found that there is a relationship between the tool track and the AE \"signature\" of the process (RMS voltage plotted versus time). Furthermore, there is a strong correlation between crater and flank tool wear and the AE signal, but the combination of them plus adhesion phenomena determine low amplitudes between initial and final stages of the tool life. Besides that, the influence of the fixture system was found to be several times higher than the influence of the wear. Therefore, a tool wear monitoring system based on acoustic emission only would not be adequate for this application.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science; and, (B.S.)--Massachusetts Institute of Technology, Dept. of Biology, 1978. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Includes bibliographical references. / B.S.

Resumo: Na indústria moderna, a precisão e o custo são fundamentais para a qualidade do produto e competitividade no mercado. Desenvolver sistemas mais velozes, dinâmicos e autoajustáveis aplicando técnicas de controle é um objetivo dos pesquisadores de todas as áreas da engenharia. A retificação de materiais é um sistema complexo que possui muitas variáveis relacionadas, principalmente por possuir múltiplas arestas de corte associado a altas velocidades e potências de retificação. Para aumentar o domínio sobre o processo, sinais de emissão acústica e de potência de acionamento do rebolo têm sido monitorados em várias pesquisas. Nesse trabalho foram realizadas experiências utilizando a retificação plana tangencial com o aço ABNT 1020 e rebolo de óxido de alumínio, onde foi determinado a ocorrência e o início da queima através da variação da profundidade de corte, tendo-se o cuidado de manter os outros parâmetros constantes, inclusive a superfície de corte do rebolo através da operação de dressagem. Os sinais de emissão acústica e de potência elétrica foram adquiridos durante uma passada do rebolo sobre a peça, numa taxa de 2.5 milhões de amostras por segundo, e pro cessados através de um ferramental matemático, onde foram propostos outros parâmetros, para a análise da ocorrência da queima. Observou-se também a detecção do início da queima, onde o parâmetro proposto VARPO obteve bons resultados. Paralelamente foi realizado um estudo sobre os microcontroladores, onde foram inseridos os cálculos de detecção da queima do processo de retificação num PIC18F452, mostrando a viabilidade de um monitoramento. / Abstract: In modern industry, the accuracy and cost are extremely important to the quality of product and market competition. The development of faster dynamic and robust systems by applying control techniques is the goal of researchers of all engineering areas. The grinding process is a complex system which contains many related variables mainly because its multiple cutting edges associated with high speed and power. In order to improve the knowledge on the process, acoustic emission and power signals have been monitored in several research investigations. In this work, surface grinding tests with ABNT 1020 steel and aluminum oxide grinding wheel were carried out in which the burning occurrence as well as the onset of it were studied. This was done by varying the depth of cut and maintaining the other grinding parameters constant, including the grinding wheel surface through the dressing operation. Acoustic emission and electrical power signals were acquired during a single grinding pass of the grinding wheel over the work-piece at 2.5 million of samples per second rate. The signals were digitally processed through many statistic algorithms among which new parameters for burn detection have been proposed. Also, the onset of burning occurrence was observed and remarkably detected for the proposed parameter dubbed VARPO. In addition, a study on microcontrollers was performed, and the implementation of the algorithms previously mentioned were carried out into the microcontroller PIC18F452, showing the monitoring feasibility. / Orientador: Paulo Roberto de Aguiar / Coorientador: Eduardo Carlos Bianchi / Banca: Paulo José Amaral Serni / Banca: Leonardo Roberto da Silva / Mestre

O presente trabalho tem como objetivo caracterizar e correlacionar formas de onda de sinais de emissÃo acÃstica, utilizando mÃtodos de anÃlise espectral e fractal, em corpos de prova previamente soldados com seus respectivos defeitos, avaliando assim a integridade estrutural do material aÃo carbono ASTM 516 Grau 60, utilizado em vasos de pressÃo, tubulaÃÃes, esferas de armazenamento, etc. Os sinais de emissÃo acÃstica foram gerados submetendo os corpos de prova a ensaios de traÃÃo. Para anÃlise dos sinais no domÃnio dos tempos, foram utilizados o mÃtodo de contagem de caixas, que fornece a dimensÃo fractal, e o de intervalo re-escalado de Hurst, e no domÃnio das freqÃÃncias o mÃtodo espectral de Fourier. AtravÃs das relaÃÃes previstas pelas leis de escala, concluiu-se que os expoentes calculados pelos mÃtodos espectral de Fourier e reescalado de Hurst, correlacionam-se entre si, bem como com a dimensÃo fractal obtida pelo mÃtodo de contagem de caixas. Mostrou-se tambÃm a existÃncia de duas regiÃes de escala distintas que caracterizam a mudanÃa do regime elÃstico para o regime plÃstico quando as amostras foram submetidas ao carregamento de traÃÃo. Verificou-se ainda que este comporta-mento foi caracterÃstico tanto para os materiais soldados como para o metal de base. / The present work aims to characterize and correlate waveforms of acoustic emission signals, using methods of spectral analysis and fractal in specimens previously soldiers with their defects, thus evaluating the structural integrity of the material carbon steel ASTM 516 Grade 60 used in pressure vessels, piping, storage spheres, etc.. The acoustic emission signals were generated by subjecting the specimens to tensile tests. For analysis of the signals in the time domain, we used the method of box counting, which provides the fractal dimension, and re-scaled range of Hurst, and in the frequency domain the Fourier spectral method. Through the relations laid down by law scale, it was concluded that the exponents calculated by Fourier transform and spectral methods reescalado Hurst, are correlated with each other as well as the fractal dimension obtained from a box counting. It was also shown existence of two regions of different scale characterizing the change of the elastic to the plastic regime when the samples were subjected to traction. It was further found that this behavior is characteristic for both the welded material as for the parent metal.

Refratários são materiais com microestrutura heterogênea constituída de uma fração grosseira, os agregados, e de uma fração mais fina, a matriz, em que ambas exercem papéis fundamentais nas propriedades dos refratários, sendo a resistência ao dano por choque térmico, uma das mais importantes. Para avaliar essa questão crítica dos refratários há necessidade de se conhecer bem seu comportamento à propagação de trinca, principalmente quando submetido a uma tensão. Porém, devido à complexidade da estrutura desses materiais, o comportamento das regiões à frente e atrás da ponta da trinca sempre foi muito discutido, só que essa discussão sempre fez uso de modelos e simulações computacionais, já que é prevista uma zona de processo, em que diferentes mecanismos podem absorver energia aumentando a resistência à propagação da trinca principal. Nesta tese foi proposto o estudo experimental do comportamento da propagação de trinca em refratários, visando entender os mecanismos de resistência à propagação de trinca e o caminho das trincas propagantes, utilizando o método da cunha para propagação estável da trinca, que é o mais adequado para essa classe de materiais. Para isso foram utilizados, dois refratários distintos: tijolo e concreto, ambos de alta alumina. No tijolo, para visualização do caminho da trinca propagante após o ensaio, o caminho da trinca foi infiltrado com cola instantânea para garantir a integridade da mesma, a fim que amostras pudessem ser preparadas para análise de imagens em microscópio eletrônico de varredura. Devido à dificuldade dessa preparação, e de só ser possível observar a trinca após a propagação, um microscópio digital passou a ser utilizado in loco ao ensaio. Esse estudo foi realizado com o concreto, sendo possível associar o comprimento da trinca com a curva carga-deslocamento. A fim de complementar o estudo do processo de fratura, a técnica de emissão acústica (EA) passou a ser utilizada nos ensaios de propagação de trinca, já que quando um material é submetido a uma carga e as trincas se desenvolvem, há liberação de energia de deformação do material, sendo possível capturar os dados de energia dos sinais gerados pela propagação. Sendo assim foi possível correlacionar resultados de energia de fratura, início e tamanho de trinca com as curvas carga-deslocamento, carga-tempo, e inclusive, com a contagem de sinais acumulada-tempo, que foi complementar na estimativa da zona de processo completa, ou seja, os fenômenos produzidos atingiram o estado estacionário. A região em que se encontra o final da zona de processo coincide com o fim do regime estacionário, que é onde a trinca atravessa o corpo de prova. Dessa forma, mostra-se com essa tese, que o corpo de prova utilizado para a propagação estável de trinca pelo método da cunha, nas dimensões atuais, são suficientes para o desenvolvimento de todos os mecanismos de resistência à propagação de trinca em refratários. / Refractories are materials with heterogeneous microstructure, consisting of a coarse fraction, aggregates, and a finer fraction, the matrix, in which both play key roles in the properties of the refractory, and the resistance to thermal shock damage, one of the most important. To examine this critical issue of the refractory is no need to be familiar with their behavior to crack propagation, especially when subjected to a stress. The behavior of the regions ahead of and behind the crack tip has been discussed exhaustively, because a process zone was envisaged in which different mechanisms could absorb energy, thus increasing the propagation resistance of the main crack. However, this discussion has always been based on the use of models and computer simulations. The thesis presented here proposes an experimental study of the behavior of crack propagation in refractories, aiming to understand the mechanisms of crack propagation resistance and the crack propagation path, using the wedge splitting method to achieve stable crack propagation. To this end, two different refractory materials were used: brick and concrete, both high alumina. Based on the stable crack propagation test by the wedge method, techniques were sought that would aid in the visualization of crack propagation. In brick, the crack path was infiltrated with instant glue and infiltrated samples were examined by scanning electron microscopy (SEM); however, in addition to proving laborious, the crack was only visible after its propagation. In the case of concrete, this study was performed in loco during the test, using a digital microscope in combination with the acoustic emission (AE) technique. AE is defined as the generation of stress waves stored energy is suddenly released from localized sources within a material subjected to external loads. By means of the fracture energy data and the AE signals, it was possible to observe the entire fracture process and to correlate the results of fracture energy and crack onset and size with the load-displacement and load-time curves, and even the count of signals accumulated over time. This information was complementary to estimate the complete process zone, i.e., the phenomena produced reached the steady state. This study demonstrated that the dimensions of the test specimen used for stable crack propagation by the wedge splitting method suffice for the development of all the mechanisms of crack propagation resistance in refractories.

Les phases MAX sont des carbures et / ou des nitrures ternaires avec un fort potentiel dans des applications diverses. Cette étude a porté sur deux phases MAX, Ti3SiC2 et Ti2AlC qui sont les plus connues dans ce groupe de matériaux. La première partie de ces travaux était dédiée à l’élaboration des poudres et des matériaux frittés. L’objectif était d’obtenir une variété de matériaux présentant différentes caractéristiques microstructurales, en termes de composition chimique et de taille de grains. Ainsi, des poudres commerciales et synthétisées par SHS ont été densifiées à l'aide de deux techniques de frittage sous charge, i.e. SPS et HP. La deuxième partie du travail a été consacrée à une meilleure compréhension de l’influence de la composition chimique et de la taille des grains sur le comportement thermomécanique des phases MAX. Des informations supplémentaires ont été fournies en couplant deux techniques expérimentales, la flexion quatre points et l’émission acoustique, et en les associant à des observations SEM post-mortems. L’approche expérimentale développée, basée sur la comparaison des réponses mécaniques des matériaux Ti3SiC2 (contenant la phase MAX et des phases secondaires) et de Ti2AlC (phases MAX uniquement), a permis d’approfondir la compréhension des mécanismes de déformation et d’endommagement induits. Il était également montré que les phases sécondaires et la taille de grains influence la manière dont les différents endommagements sont accumulées dans le matériaux. Les résultats d’EA sont fourni les informations supplémentaires sur les type d’endommagements rencontrées et leur chronologie qui résultent avec le comportement nonlinéaire de phases MAX. La dérnière partie de cette thèse a montré que le température de transition fragile-plastique est autour de 1200˚C et que la taille de grains l’abaisse. / MAX phases are ternary carbide and/or nitride with a great potential in various application. This study concerned two MAX phase compounds, namely Ti3SiC2 and Ti2AlC, which are on the most studied among all known MAX phases. The first part dealt with materials elaboration which includes both synthesis and sintering stages. The objective was to obtain a variety of materials with different microstructural features, i.e. phase composition and grain size. For this purpose, both commercial and SHS-derived powders were densified with two pressure-assisted sintering technique, i.e. SPS and HP. The second part of work was devoted to deeper understanding of the influence of phase composition and grain size to thermomechanical behavior of MAX phases. More information was provided when two experimental techniques, four-point bending and AE monitoring, were coupled together with post-mortem SEM observations. The developed experimental approach based on comparison of mechanical responses of Ti3SiC2 (MAX phase and secondary phases) and Ti2AlC (containing only MAX phases) compounds, is implemented to investigate the induced deformation and damage mechanisms. It was shown that higher quantity of MAX phases in the material improves flexural strength with corresponding more pronounce nonlinear behavior and high dissipated energy. When MAX phase grains are coarser the mechanical strength lowers but the nonlinear part becomes more significative, which results in higher values of dissipated energy. It was also shown that the secondary phases and the grain size impact the way in which the damage is accumulated within the sample. AE data provided more information on possible damage mechanisms and their chronology leading to hysteretic behavior of MAX phases. The last part has shown that the BPTT for Ti3SiC2 is ≈1200˚C and the grain size of MAX phases lowers this BPTT.

Prefabricated and pre-stressed reinforced concrete beams and girders are integral components of many highway structures, including those built by rapid construction techniques. Concerns exist regarding the development of cracks during curing, form removal, detensioning, transport, installation, and operation. Non-destructive, Acoustic Emission (AE) sensing techniques have the potential for detecting and locating cracking in prefabricated, pre-stressed concrete girders used as Prefabricated Bridge Elements and Systems (PBES) used in rapid construction practices as part of a Quality Assurance/Quality Control (QA/QC) program. AE sensing records transient elastic waves produced by the release of stored elastic energy resulting in plastic deformations (i.e., crack nucleation and growth) with an array of point sensors. The AE instrument system is relatively portable which can allow for it to be an option for both off-site fabrication QA/QC as well as on-site field QA/QC. This thesis presents a multi-stage research initiative on acoustic emission monitoring of prefabricated and pre-stressed reinforced concrete beams used in highway bridge construction during detensioning, craned removal from formwork and transport to bridge sites, along with supporting laboratory tests and numerical analysis. The specific objectives of this research were to: 1. Identify suitable instruments to monitor pre-stressed and/or post-tensioned concrete girders for cracking activity; 2. Design and develop a reusable instrumentation package; 3. Measure performance and condition of concrete girders during fabrication and transport; and 4. Identify test protocols and possible accept/fix/reject criteria for structural elements based on information from monitoring system. Presented are results from laboratory, full-scale girder fabrication, and transport monitoring, along with overall conclusions and recommendations for future research.

Current demands on lithium ion battery (LIB) technology include high capacity retention over a life time of many charge and discharge cycles. Maximizing battery longevity is still a major challenge partly due to electrode degradation as a function of repeated cycling. The intercalation of lithium ions into an active material causes the development of stress and strain in active electrode materials which can result in fracture and shifting that can in turn lead to capacity fade and eventual cell failure. The processes leading to active material degradation in cycling LIBs has been studied using a combination of acoustic emission (AE) and in situ X-ray diffraction (XRD) techniques. Safe, low cost custom electrochemical cells were designed and developed for use in battery AE and XRD experiments. These tools were used to monitor the time of material fracture through AE and link these events to lattice strain and phase composition as determined by XRD. Both anode and cathode materials were studied with an emphasis on graphite, silicon, and Li(Mn1.5Ni0.5)O4, and tin. A thermal analogy model for lithiation/delithiation induced fracture of spherical particles capable of predicting when AE should be detected in a cell containing a composite silicon electrode. The results of this work were used to develop an understanding of when and how active materials are degrading as well as to suggest methods of improving their performance and operational longevity.

No description available.