Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Nowadays, welded structures are widely employed in engineering design. Generally, these structures have internal stresses (residual stresses) generated by the thermal cycle to which these parts are submitted during welding process. In many cases, these stresses are not minimized by means of thermal treatment. Thus, one must take into account the residual stresses in the design of welded components. However, computation of residual stress field is not an easy task and, besides, it is important to evaluate the quality of welded components. Therefore, by using the advantage of the fact that the stress state influences the mechanical behavior of components and structures (stress stiffening effect), it was proposed to evaluate the viability of a methodology to control the quality of welded components, by means of dynamic responses. This way, it was verified experimentally the sensitivity of vibrating natural frequencies to welding residual stress in different structures. It was confirmed that thin structures are more sensitive to the stress stiffening effect. To evaluate this effect numerically, a procedure to simulate welding was validated by using experimental data from literature. This procedure was used to obtain the residual stress field. This stress field is included in a modal analysis to verify the modifications of natural frequencies. Numerical results were compared to experimental results obtained here, showing a good agreement. It was also proposed to use the electromechanic impedance technique to evaluate the stress stiffening effect. An analysis of the results shows the possibility of use the stress stiffening effect to implement a methodology for quality control of welded components. / Atualmente, estruturas soldadas encontram-se amplamente aplicadas em projetos de engenharia. Normalmente, estas estruturas apresentam tensões internas (residuais) devidas ao ciclo térmico ao qual são submetidas durante a soldagem. Em muitos casos, estas tensões não são minimizadas por meio de tratamento térmico, havendo assim a necessidade de inclusão destas em projeto. Entretanto, a determinação do campo de tensões residuais não é tarefa fácil e, no caso de uma produção em série, é importante avaliar a qualidade do componente confeccionado. Sendo assim, aproveitando o bem conhecido fato de o comportamento mecânico de componentes e estruturas sofrer influência do estado de tensão (enrijecimento por tensão), propõe-se avaliar a viabilidade de uma metodologia de controle de qualidade de peças soldadas por meio de respostas dinâmicas. Desta forma foi verificada experimentalmente a sensibilidade das freqüências naturais de vibração ao estado de tensões residuais de soldagem em diferentes estruturas. Confirmou-se que estruturas esbeltas são mais sensíveis ao enrijecimento por tensões residuais de soldagem. Com a finalidade de avaliar este efeito numericamente, um procedimento para simulação da soldagem foi validado utilizando-se dados experimentais da literatura. Na seqüência, utilizou-se tal procedimento para, após a determinação das tensões residuais de soldagem, realizar uma análise modal e avaliar as alterações nas freqüências naturais de vibração (e conseqüentemente o efeito do enrijecimento por tensão). Os resultados numéricos foram confrontados com os resultados experimentais obtidos neste trabalho, apresentando uma boa concordância. Finalizando o trabalho, propõe-se utilizar a técnica da impedância eletromecânica para avaliar o enrijecimento por tensão. Uma avaliação dos resultados mostra ser possível utilizar o enrijecimento por tensão para implementar uma metodologia para controle de qualidade de componentes soldados. / Doutor em Engenharia Mecânica

Basic resonator structures for frequency-selective capacitive vibration sensors which exploit their mechanical resonance peak for spectral selectivity are developed and analyzed. As an important capability, the stuctures offer voltage-controlled frequency tuning realized by electrostatic principles. Direct electrostatic tuning based on displacement-dependent electrostatic forces as well as tuning by stress-stiffening based on constant electrostatic forces are discussed. The sensor structures are designed for fabrication using common silicon surface microtechnologies. Experimental tests of both mentioned tuning principles are carried out at structures fabricated using a surface technology known as SCREAM. A considerable part of the work focusses on nonlinear oscillations of the mechanical resonator at large amplitudes arising in resonance. Dimensioning rules for minimum nonlinear disturbance are derived from a detailed analysis of the Duffing oscillator. Various capacitive principles for signal detection and electrostatic frequency tuning are evaluated in terms of nonlinearity. A novel type of specially shaped electrode systems offering linear properties within an amplitude range of 10 micrometers, so-called curved comb capacitors, is developed for fabrication in SCREAM technology. / In der vorliegenden Arbeit werden Grundstrukturen frequenzselektiver kapazitiver Vibrationssensoren entwickelt und analysiert, deren spektrale Selektionswirkung durch Ausnutzung ihrer mechanischen Resonanzüberhöhung entsteht. Wesentliches Merkmal der Strukturen ist ihre spannungsgesteuerte Abstimmbarkeit, die auf elektrostatischen Prinzipien beruht. Es werden die direkte elektrostatische Frequenzabstimmung, basierend auf positionsabhängigen Feldkräften, sowie das Prinzip des Stress-Stiffening, basierend auf einer konstanten elektrostatischen Kraft, diskutiert. Die Entwicklung konzentriert sich auf Sensorstrukturen, die in klassischen Oberflächentechnologien gefertigt werden können. Experimentelle Tests der beiden genannten Abstimmprinzipien werden anhand von Strukturen in oberflächennaher Silizium-bulk-Mikromechanik (SCREAM) durchgeführt. Schwerpunkt der Arbeit sind nichtlineare Schwingungen der mechanischen Resonatorkomponente bei großen Amplituden, die durch die Resonanzüberhöhung entstehen. Für den Sonderfall des Duffing-Schwingers werden Dimensionierungsregeln zur Minimierung der Nichtlinearitäten entwickelt. Kapazitive Prinzipien zur Detektion bzw. elektrostatischen Abstimmung werden hinsichtlich ihrer Linearität geprüft. Es werden neuartige Elektrodensysteme für die SCREAM-Technologie, sogenannte Kurvenkammsysteme entwickelt, die bei Schwingamplituden bis zu 10 Mikrometer linear arbeiten.

Basic resonator structures for frequency-selective capacitive vibration sensors which exploit their mechanical resonance peak for spectral selectivity are developed and analyzed. As an important capability, the stuctures offer voltage-controlled frequency tuning realized by electrostatic principles. Direct electrostatic tuning based on displacement-dependent electrostatic forces as well as tuning by stress-stiffening based on constant electrostatic forces are discussed. The sensor structures are designed for fabrication using common silicon surface microtechnologies. Experimental tests of both mentioned tuning principles are carried out at structures fabricated using a surface technology known as SCREAM. A considerable part of the work focusses on nonlinear oscillations of the mechanical resonator at large amplitudes arising in resonance. Dimensioning rules for minimum nonlinear disturbance are derived from a detailed analysis of the Duffing oscillator. Various capacitive principles for signal detection and electrostatic frequency tuning are evaluated in terms of nonlinearity. A novel type of specially shaped electrode systems offering linear properties within an amplitude range of 10 micrometers, so-called curved comb capacitors, is developed for fabrication in SCREAM technology. / In der vorliegenden Arbeit werden Grundstrukturen frequenzselektiver kapazitiver Vibrationssensoren entwickelt und analysiert, deren spektrale Selektionswirkung durch Ausnutzung ihrer mechanischen Resonanzüberhöhung entsteht. Wesentliches Merkmal der Strukturen ist ihre spannungsgesteuerte Abstimmbarkeit, die auf elektrostatischen Prinzipien beruht. Es werden die direkte elektrostatische Frequenzabstimmung, basierend auf positionsabhängigen Feldkräften, sowie das Prinzip des Stress-Stiffening, basierend auf einer konstanten elektrostatischen Kraft, diskutiert. Die Entwicklung konzentriert sich auf Sensorstrukturen, die in klassischen Oberflächentechnologien gefertigt werden können. Experimentelle Tests der beiden genannten Abstimmprinzipien werden anhand von Strukturen in oberflächennaher Silizium-bulk-Mikromechanik (SCREAM) durchgeführt. Schwerpunkt der Arbeit sind nichtlineare Schwingungen der mechanischen Resonatorkomponente bei großen Amplituden, die durch die Resonanzüberhöhung entstehen. Für den Sonderfall des Duffing-Schwingers werden Dimensionierungsregeln zur Minimierung der Nichtlinearitäten entwickelt. Kapazitive Prinzipien zur Detektion bzw. elektrostatischen Abstimmung werden hinsichtlich ihrer Linearität geprüft. Es werden neuartige Elektrodensysteme für die SCREAM-Technologie, sogenannte Kurvenkammsysteme entwickelt, die bei Schwingamplituden bis zu 10 Mikrometer linear arbeiten.