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Le travail présenté ici a pour but d’évaluer l’influence de surcharges incidentelles sur le comportement en fatigue de tôles en acier soudées par points et destinées à une application automobile. Une campagne d’essais expérimentale sur éprouvettes a permis d’établir les courbes effort – durée de vie de trois nuances d’acier soudées par point et soumis à des sollicitations de traction-cisaillement soit à amplitude constante (courbe de référence), soit avec des sollicitations incidentelles survenant à raison d’un cycle de surcharge pour 100 cycles appliqués. Les éprouvettes utilisées sont constituées de deux rectangles de tôle assemblés par un point soudé réalisé par un soudage par résistance. Deux rapports de surcharge, 1,4 et 2,3, sont testés et analysés. L’ensemble des cycles de tous les chargements présente le rapport de charge de 0,1 (traction ondulée). Pour deux nuances d’acier, les surcharges périodiques sont bénéfiques à la tenue en fatigue, ils permettent d’augmenter sensiblement la durée de vie. Pour la troisième nuance d’acier en revanche, l’effet enregistré est un affaissement très marqué des propriétés de fatigue. Deux types de ruine par fatigue sont observés, l’un par fissuration des tôles au droit du congé de raccordement du noyau fondu du point soudé sur la tôle, du côté de la zone en traction. L’autre se traduit par un cisaillement du noyau fondu dans le plan de sa section droite minimale et ne se produit qu’à haut niveau d’effort appliqué. Ces calculs par Eléments finis ont mis en évidence des états de contraintes multiaxiaux, principalement de traction et de cisaillement dans les zones expérimentales d’amorçage et de fissuration des éprouvettes. Il est observé de façon très claire une prépondérance de la contrainte de traction là où s’amorce la fissure de fatigue conduisant à la ruine de l’éprouvette par fissuration complète d’une des deux tôles, et une prépondérance tout aussi évidente de la contrainte de cisaillement dans la section minimale du noyau fondu du point soudé où se produit dans certains cas la rupture du point soudé par cisaillement. Les calculs effectués corroborent donc très bien les observations expérimentales. L’utilisation de nouvelles lois d’endommagement basées sur un cumul non linéaire du dommage, dont une qui a été proposée au laboratoire, conduit à une prévision des durées de vie en bon accord avec l’expérimentation alors que la loi de Miner demeure pour sa part très éloignée des résultats expérimentaux. / The objective of this work is to evaluate the influence of incidental overloads on the fatigue behavior of steel plates welded by points and aimed for automobile application. An experimental test campaign on specimens had allowed the establishment of effort curves – life cycle of three types of steel welded by points and subjected to tensile-shear efforts under constant amplitude (reference curve), and with incidental overloads happening once (overloaded cycle) for 100 applied cycles. Specimens used in the test are made of two rectangle plates joined by a welded point and done by resistance welding. Two overloads ratio, 1,4 and 2,3 were tested and analyzed. The overall overloading cycle presents the ratio of 0,1 (undulated tensile effort). For two out of three steel types, the periodicals overloading are benefit in term of fatigue resisting, their life duration has been slightly improved. The third steel type has in contrary showed observable deterioration of fatigue properties.Two types of ruin by fatigue have been observed. The first one shows cracking of plates on the right axle journal fillet of the melted nucleus of the welded point on the plate, in the tensile zone side. The second is showed by a shearing of the melted nucleus in the plan of its minimal right “section” and this happens at a high level of applied effort. Calculation done using Finite Elements method has evidently showed states of multiaxial stresses essentially tensile stress and shearing stress in the experimental areas of starting and cracking of specimens. It is clearly observed a mainly dominant tensile stress where the fatigue cracking starts leading to the ruin of the specimen by the complete cracking of one of the two plates, and also evidently a mainly dominant shearing stress in the minimal section of the melted nucleus of welded point, where in some case the rupture by shearing of the welded point is observed. The calculations done are then in total agreement with the experimental observations. The use of new damage laws, which one has been proposed in the laboratory based on a nonlinear cumulating of damage, leads to a forecasting of life duration in accordance with experimentation while the Miner law results stays far away from the experimental results.

Οι περιοχές συγκέντρωσης τάσης ή/και παραμόρφωσης στα σύνθετα υλικά, όπως και στα συμβατικά -μεταλλικά- υλικά, αποτελούν σημαντικά στοιχεία στο σχεδιασμό τμημάτων δομών και ολόκληρων κατασκευών. Ειδικότερα η μέθοδος σχεδιασμού με ανοχή στη βλάβη, η οποία έχει επικρατήσει, καθιστά την κατανόηση και μελέτη της συμπεριφοράς των περιοχών αυτών εκ των ων ουκ άνευ. Στο παρόν, γίνεται μια προσπάθεια για την καλύτερη κατανόηση της συμπεριφοράς των σύνθετων υλικών, όταν εμπεριέχουν περιοχές συγκέντρωσης τάσης. / Stress concentration or strain magnification locations on composite materials, similar to conventional-metallic materials, are a very important case in designing structural elements or structures in general. Damage tolerance design, particularly, which has become a dominant design method, makes the study and understanding of the behaviour of these locations necessary. In the present, an effort is presented for the better understanding of the composite materials behaviour in the presence of stress concentration locations.

This doctoral thesis is concerned with fatigue life of welded structures. Several topics related to fatigue of welded structures are treated such as; weld defects and their influence on fatigue performance of welded structures, fatigue life prediction using LEFM (Linear Elastic Fracture Mechanics), fatigue testing, welding simulation, residual stress prediction and measurement and their influence on fatigue life. The work that is reported in this doctoral thesis is part results of the Nordic R&D project QFAB (Quality and Cost of Fabricated Advanced Welded Structures) and the Swedish R&D project LOST (Light Optimized Welded Structures). One of the main objectives is to compare different welding processes for the fatigue performance, weld quality and gain understanding of the weld defects, their appearance in different welding processes and their effect on fatigue life. Another main objective is to study welding residual stresses and their effect on fatigue. The design rules are in some cases conservative and especially on the weld root sides the knowledge about the residual stress field may improve the life prediction. The aim is to develop simplified procedures for analysis of residual stresses, their relaxation and influence on fatigue life. Fatigue testing of Hybrid Nd: YAG laser/MAG and MAG welded (tandem arc solid wire, flux cored wire, tandem flux cored wire) non-load carrying cruciform joints was carried out. Four batches were produced, tested and the results were compared. The local weld geometry of the cruciform welded joints was measured and analyzed. Residual stress measurement was carried out close to the toe region using X-ray diffraction. Weld defects, in most cases cold laps, in the cracked specimens were measured. Further fatigue testing, weld defect assessment and residual stress and local weld geometry measurements were carried out on joints welded with flux cored and metal cored arc wires. Two-and three dimensional LEFM crack growth analysis were carried out in order to predict the influence of weld defects, local weld geometry and residual stresses. Residual stresses in multi-pass welded tube-to-plates were studied for two different tubular joint configurations; a three-pass single-U weld groove for maximum weld penetration and a two-pass fillet (no groove) welded tube-to-plates for minimum weld penetration. Torsion fatigue tests were performed in order to study crack propagation from the weld root. Mode III propagation from the lower and upper weld toe on the same tubular joints was also studied. Some tubes were stress relieved (PWHT) and some were fatigue tested with internal static pressure. A three dimensional finite element welding simulation of the multi-pass welded tubular joint was carried out. The calculated temperatures in the transient thermal analysis were compared with measured temperatures. The FE predicted residual stresses in the as-welded conditions were verified with hole drilling strain gage measurements. The residual stresses were used as internal stresses in the finite element model for the torsion fatigue simulation in order to study the cycle by cycle relaxation of the residual stresses in constant amplitude torsion loading. A two dimensional finite element welding simulation procedure was developed in order to predict welding residual stress. The predicted residual stresses were used together with a developed 2D LEFM subroutine to predict the fatigue life, crack path and the effect of residual stresses on weld root defects. The developed simulation subroutines were validated with results found in the literature. Residual stresses measurement, two-and three dimensional welding simulations were carried out in fillet welded joints in order to study the three dimensional effects of the welding process, boundary conditions and modelling technique on the formation of residual stresses. / QC 20100706

The presented work aims to optimize the performance of piezoresistive cantilevers in cases where the output signal originates either from a static deflection of the cantilever or from the dynamic (resonance) characteristic of the beam. Based on a new stress concentration technique, which utilizes silicon beams and wires embedded in the cantilever, the force sensitivity of the cantilever is increased up to 8 fold with only about a 15% decrease in the cantilever stiffness. Moreover, the developed stress-concentrating cantilevers show almost the same resonance characteristic as conventional cantilevers. The focus of the second part of the present work is to provide guidelines for designing rectangular silicon cantilever beams to achieve maximum quality factors for the fundamental and higher flexural resonance at atmospheric pressure. The applied methodology is based on experimental data acquisition of resonance characteristics of silicon cantilevers, combined with modification of analytical damping models to match the measurement data. To this end, rectangular silicon cantilever beams with thicknesses of 5, 7, 8, 11 and 17 um and lengths and widths ranging from 70 to 1050 um and 80 to 230 um, respectively, have been fabricated and tested. To better describe the experimental data, modified models for air damping have been developed. Moreover, to better understand the damping mechanisms in a resonant cantilever system, analytical models have been developed to describe the cantilever effective mass in any flexural resonance mode. To be able to extract reliable Q-factor data for low signal-to-noise ratios, a new iterative curve fitting technique is developed and implemented. To address the challenge of frequency drift in (mass-sensitive) resonant sensors, and especially cantilever-based devices, the last part of the research deals with a novel compensation technique to cancel the unwanted environmental effects (e.g., temperature and humidity). This technique is based on exploring the resonance frequency difference of two flexural modes. Experimental data show improvements in temperature and humidity coefficients of frequency from -19.5 to 0.2 ppm/˚C and from 0.7 to -0.03 ppm/%RH, respectively. The last part of the work also aims on techniques to enhance or suppress the flexural vibration amplitude in desired overtones.

[Truncated abstract] This thesis presents three different methods to investigate flexural wave propagation and scattering, power flow and transmission efficiencies, and dynamic stress concentration and fatigue failures in structural dynamics. The first method is based on the acoustical wave propagator (AWP) technique, which is the main part described in this thesis. Through the numerical implementation of the AWP, the complete information of the vibrating structure can be obtained including displacement, velocity, acceleration, bending moments, strain and stresses. The AWP technique has been applied to systems consisting of a one-dimensional stepped beam, a two-dimensional thin plate, a thin plate with a sharp change of section, a heterogeneous plate with multiple cylindrical patches, and a Mindlin?s plate with a reinforced rib. For this Mindlin?s plate structure, through the comparison of the results obtained by Mindlin?s thick plate theory and Kirchhoff?s classical thin plate theory, the difference of theoretical predicted results is investigated. As part of these investigations, reflection and transmission coefficients, power flow and transmission efficiencies in a onedimensional stepped beam, and power flow in a two-dimensional circular plate structure, are studied. In particular, this technique has been successfully extended to investigate wave propagation and scattering, and dynamic stress concentration at discontinuities. Potential applications are fatigue failure prediction and damage detection in complex structures. The second method is based on experimental techniques to investigate the structural response under impact loads, which consist of the waveform measuring technique in the time domain by using the WAVEVIEW software, and steady-state measurements by using the Polytec Laser Scanning Vibrometer (PLSV) in the frequency domain. The waveform measuring technique is introduced to obtain the waveform at different locations in the time domain. These experimental results can be used to verify the validity of predicted results obtained by the AWP technique. Furthermore, distributions of dynamic strain and stress in both near-field (close to discontinuities) and far-field regions are investigated for the study of the effects of the discontinuities on reflection and transmission coefficients in a one-dimensional stepped beam structure. Experimental results in the time domain can be easily transferred into those in the frequency domain by the fast Fourier transformation, and compared with those obtained by other researchers. This PLSV technique provides an accurate and efficient tool to investigate mode shape and power flow in some coupled structures, such as a ribbed plate. Through the finite differencing technique, autospectral and spatial of dynamic strain can be obtained. The third method considered uses the travelling wave solution method to solve reflection and transmission coefficients in a one-dimensional stepped beam structure in the time domain. In particular, analytical exact solutions of reflection and transmission coefficients under the given initial-value problem are derived. These analytical solutions together with experimental results can be used to compare with those obtained by the AWP technique.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Neste trabalho, apresenta-se o estudo de um conversor CC-CC isolado com comutação suave e saída em tensão, destinado a aplicações com ampla faixa de tensão de entrada, com o nome em inglês Hybrid T-type Zero-Voltage-Switching Pulse-Width-Modulated DC-DC converter (HT2-ZVS-PWM). O conversor estudado possui seis interruptores, sendo quatro deles submetidos ao nível máximo da tensão de entrada e os outros dois, que constituem a célula tipo T, submetidos a metade desta tensão. São propostas modulações de três níveis e quatro níveis aplicadas no comando dos interruptores, as quais possibilitam comutação suave do tipo ZVS para os mesmos, no modo de condução contínua. O trabalho é estruturado em análise teórica para os modos de condução contínua e descontínua, análise de esforços nos elementos do conversor, análise da comutação dos interruptores, análise dinâmica, metodologias de projeto e resultados experimentais, em que se analisam resultados para uma tensão mínima e uma tensão máxima de entrada. Utiliza-se 1,2 kW, 240 V – 480 V de entrada, 300 V de saída e frequência de comutação de 50kHz para a comprovação das análises que serão realizadas. Rendimentos são obtidos a partir de ensaios experimentais com o protótipo para operação com ponto fixo e ampla faixa de tensão de entrada. Para ponto fixo o rendimento foi de 97,38% em potência nominal e para ampla faixa foram de 96,35% (240 V) e 95,72% (480 V) em 60% e 70% da potencia nominal, respectivamente. / This work presents the analysis of an isolated DC-DC converter with soft switching and capacitive output filter, for applications with a wide input voltage range, called Hybrid T-type Zero-Voltage-Switching Pulse-Width-Modulated DC-DC converter (HT2-ZVSPWM). The converter studied contains six switches, four of which are submitted to the entire input voltage value and the other two, which constitute the Type T cell, are subjected to half of this voltage. The proposed three-level and four-level modulations are applied to generate the switching signals, which ensures ZVS operation for all converter switches in the continuous conduction mode. The work is structured in theoretical analysis for both continuous and discontinuous conduction modes, stress analysis, dynamic analysis, design methodology and experimental results in which results are analyzed for a minimum voltage and a maximum input voltage. Theoretical analysis, design example and experimental data for 1.2 kW, 240 – 480 Vdc input, 300 Vdc output, and 50 kHz switching frequency laboratory prototype are analyzed. Measured efficiency are obtained from experimental trials with the prototype for fixed point operation and wide input voltage range. For fixed point the efficiency was of 97.38% at full load and for wide range were of 96.35% (240 V) and 95.72% (480 V) in 60% and 70% of the nominal power, respectively.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Neste trabalho, apresenta-se o estudo de um conversor CC-CC isolado com comutação suave e saída em tensão, destinado a aplicações com ampla faixa de tensão de entrada, com o nome em inglês Hybrid T-type Zero-Voltage-Switching Pulse-Width-Modulated DC-DC converter (HT2-ZVS-PWM). O conversor estudado possui seis interruptores, sendo quatro deles submetidos ao nível máximo da tensão de entrada e os outros dois, que constituem a célula tipo T, submetidos a metade desta tensão. São propostas modulações de três níveis e quatro níveis aplicadas no comando dos interruptores, as quais possibilitam comutação suave do tipo ZVS para os mesmos, no modo de condução contínua. O trabalho é estruturado em análise teórica para os modos de condução contínua e descontínua, análise de esforços nos elementos do conversor, análise da comutação dos interruptores, análise dinâmica, metodologias de projeto e resultados experimentais, em que se analisam resultados para uma tensão mínima e uma tensão máxima de entrada. Utiliza-se 1,2 kW, 240 V – 480 V de entrada, 300 V de saída e frequência de comutação de 50kHz para a comprovação das análises que serão realizadas. Rendimentos são obtidos a partir de ensaios experimentais com o protótipo para operação com ponto fixo e ampla faixa de tensão de entrada. Para ponto fixo o rendimento foi de 97,38% em potência nominal e para ampla faixa foram de 96,35% (240 V) e 95,72% (480 V) em 60% e 70% da potencia nominal, respectivamente. / This work presents the analysis of an isolated DC-DC converter with soft switching and capacitive output filter, for applications with a wide input voltage range, called Hybrid T-type Zero-Voltage-Switching Pulse-Width-Modulated DC-DC converter (HT2-ZVSPWM). The converter studied contains six switches, four of which are submitted to the entire input voltage value and the other two, which constitute the Type T cell, are subjected to half of this voltage. The proposed three-level and four-level modulations are applied to generate the switching signals, which ensures ZVS operation for all converter switches in the continuous conduction mode. The work is structured in theoretical analysis for both continuous and discontinuous conduction modes, stress analysis, dynamic analysis, design methodology and experimental results in which results are analyzed for a minimum voltage and a maximum input voltage. Theoretical analysis, design example and experimental data for 1.2 kW, 240 – 480 Vdc input, 300 Vdc output, and 50 kHz switching frequency laboratory prototype are analyzed. Measured efficiency are obtained from experimental trials with the prototype for fixed point operation and wide input voltage range. For fixed point the efficiency was of 97.38% at full load and for wide range were of 96.35% (240 V) and 95.72% (480 V) in 60% and 70% of the nominal power, respectively.

Orientador: Itamar Ferreira / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-09T20:08:21Z (GMT). No. of bitstreams: 1 CamargoJunior_JoseAntoniode_M.pdf: 5956974 bytes, checksum: 439c5177de65f09b9855f932ec564537 (MD5) Previous issue date: 2007 / Resumo: Atualmente, praticamente todo o processo de desenvolvimento de componentes mecânicos que giram em alta rotação e que têm grande responsabilidade estrutural, principalmente quando fabricados em materiais de natureza frágil, como é o caso do ferro fundido, passa necessariamente por ensaios de centrifugação, que são realizados visando à determinação da rotação de ruptura. Além disso, os projetos atuais utilizam componentes com configurações geométricas que envolvem furos e outros tipos de concentradores de tensão. Em função disso, torna-se importante o conhecimento da influência de certos tipos de concentradores de tensão nesses componentes mecânicos. Este trabalho tem por objetivos determinar e analisar curvas de fator de concentração de tensão experimentais, por meio de ensaios de centrifugação, e numéricas, por meio de elementos finitos, em corpos anulares de espessura constante, submetidos à rotação e fabricados em ferro fundido cinzento perlítico GG25. Foram executados ensaios de centrifugação de cinco diferentes geometrias de corpos anulares: (a) sem concentrador de tensão; (b) com três concentradores de tensão do tipo semi-circular no raio interno; (c) com três concentradores de tensão do tipo semi-circular no raio externo; (d) com três furos passantes localizados nas proximidades do raio interno; (e) com três furos passantes localizados nas proximidades do raio externo. As tensões radial e tangencial foram determinadas por meio das equações clássicas para rotação de corpos anulares, sendo que a tensão equivalente foi determinada por meio do critério de von Mises, sendo esta comparada com o limite de resistência à tração do material determinada por meio de ensaios de tração. Foi também realizada uma análise simplificada por elementos finitos visando à determinação da tensão equivalente, também pelo critério de von Mises, para as cinco geometrias estudadas. Observou-se que cada geometria de descontinuidade geométrica apresenta um fator de concentração de tensão diferente a qual reduz a rotação de ruptura, quando comparada com o corpo anular sem concentrador de tensão. O corpo anular com concentrador de tensão do tipo semi-circular no raio interno possui os maiores níveis de fator de concentração de tensão médio, sendo este da ordem de 48% maior do que o sem concentrador. Por outro lado, o que apresentou menor nível de concentração de tensão foi o corpo anular com furos localizados nas proximidades do raio externo, sendo este da ordem de 5% maior do que o sem concentrador. Essas curvas poderão orientar o desenvolvimento de projetos de componentes mecânicos com relação aos efeitos dos concentradores de tensão analisados neste trabalho / Abstract: Currently, practically all the process of development of mechanical components working in high rotation and that they have great structural responsibility, mainly when manufactured in materials of brittle nature, as it is the case of the cast iron, passes necessarily for burst tests conducted in order to determine the burst speed. Moreover, the current projects use components with geometric configurations that involve holes and other types of stress raisers. As a function of this, the knowledge of the influence of certain types of stress raisers in these mechanical components becomes important. The objectives of this work is to determine and to analyze two types of curves of stress concentration factors, experimental, by means of the burst tests, and numerical, by means of finite element method, in annular specimens of constant thickness, submitted to rotation, and manufactured in perlitic gray iron GG25. Burst tests of five different shapes of annular specimens had been executed: (a) without stress raiser; (b) with three stress raisers of half-circular shape located in the internal radius; (c) with three stress raisers of halfcircular shape located in the external radius; (d) with three stress raisers of passing holes located near the internal radius; (e) with three stress raisers of passing holes located near the external radius. The radial and tangential stresses had been determined by means of the classic equations for rotation of annular bodies, the equivalent stress was determined by means of the von Mises criterion, and this was compared with the tensile strength of the material determined from tensile tests. Also a simplified analysis of finite element method was conducted in order to determine the equivalent stress, also by using von Mises criterion, for the five studied geometry. It was observed that each shape of geometry discontinuity presents a different stress concentration factor which reduces the rupture rotation, when compared with the annular specimen without stress raiser. The annular specimen with stress raiser of the half-circular type in the internal radius posses the biggest level of average stress concentration factor, about 48% greater of that without stress raiser. On the other hand, the annular specimen with three holes near the external radius presented minor level of stress concentration, about 5% greater of that without stress raiser. These curves will be able to guide the development of projects of mechanical components with regard to the effect of the analyzed stress concentrators in this work / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Wind power, especially offshore, is considered to be one of the most promising sources of ‘clean’ energy towards meeting the EU targets for 2020 and 2050. However, its popularity has always fluctuated with the price of fossil fuels since nowadays wind electricity production cannot compete with nuclear or coal electricity production. Support structures are thought to be one of the main drivers for reducing costs in order to make the wind industry more economically efficient. Foundations and towers should be fit for purpose, extending their effective service life but avoiding costs of oversizing. An exhaustive review of the background and state of the art of the Fatigue-Life assessment approaches has been carried out, combining analysis of the gathered experimental data and the development of Finite Element models based on contemporary 3D solid models with diverse Regression Analyses, in order to identify their weakness and evaluate their accuracy. This research shows that the guides and practices currently employed in the design and during the operation of the offshore wind turbine support structures are obsolete and not useful for optimisation, which generally leads to conservationism and an unnecessary increase in costs. The basis for a comprehensive update of the Girth Weld and Tubular Joint S-N curves and the Stress Concentration Factors of Tubular Joints has been set out. Furthermore, a reliable methodology for deriving the Stress Intensity Factor at the deepest point of a semi-elliptical surface saddle crack in a tubular welded T-joint has been proposed.