Dissertations / Theses on the topic 'Multiaxial'

An experimental study has been carried out to investigate the behaviour of rock under multiaxial compression and assess the influence of both the stress conditions and test configuration on the apparent characteristics of this behaviour. Over three hundred specimens of Springwell sandstone, of various forms, have been tested using different loading techniques and most encountered stress fields. Cubes and thick-walled hollow cylinders have been subjected to uniaxial, biaxial, triaxial and polyaxial compression, and solid cylinders have been subjected to standard uniaxial and triaxial compression. Extensive work has initially been put into designing and developing the testing facilities required. A new multiaxial hollow cylinder test apparatus has been devised using a Hoek triaxial cell and specially designed system for the application of internal pressure, major modifications have been made to an existing multiaxial cubical test apparatus, and appropriate testing arrangements and procedures have been developed. Prior to initiating the experimental programme, characterisation tests have been conducted to determine the fundamental properties of the rock, and non-destructive ultrasonic wave velocity tests have been utilised together with statistical methods to examine any inherent variations in the specimens used. A remarkable agreement has emerged between the rock static and dynamic Young's moduli and the results have confirmed that the Springwell sandstone can practically be regarded as linear elastic, homogeneous and isotropic. The concept of the multiple failure state triaxial test has been utilised and extended to conduct multiple failure state polyaxial tests. While the concept remains useful, strain results obtained beyond the first failure state are likely to be inconsistent. Results of cube tests have been found to be highly influenced by the boundary conditions. Although the use of PTFE sheets can reduce the effect of friction between the specimen and the loading platens, it may equally have a weakening effect on the test specimen. The theory of elasticity has been found adequate to calculate the stresses in the hollow cylinders but remains inexact when deviation from linear behaviour occurs prior to failure. When the outer and inner surfaces of the cylinder are not perfectly concentric, the effect on the test results has proved negligible. Hollow cylinders have been found to provide an alternative means for measuring the rock indirect tensile strength. Results of biaxial and polyaxial tests on both cubes and hollow cylinders have confirmed the marked influence of the intermediate principal stress on the apparent strength of rock. Comparison of results from multiaxial tests on cubes, hollow and solid cylinders have shown that the apparent strength, deformability and failure characteristics of the rock are remarkably influenced by the stress conditions imposed as well as the test configuration used. Available failure criteria have advantages and disadvantages, but none of them can explain the diversity of the results obtained. On the whole, the results appear to cast doubt on common conceptions of rock failure and ultimately pose the question of how realistic current testing techniques are in their prediction of the actual behaviour of rock.

The multiaxial deformation of magnesium alloys is important for developing reliable, robust models for both the forming of components and also analysis of in service performance of structures, for example, in the case of crash worthiness. This work presents a combination of unique biaxial experimental tests and biaxial crystal plasticity simulations using a visco-plastic self-consistent (VPSC) formulation conducted on AZ80 magnesium alloy in two different conditions - extruded and a more weakly textured as cast condition. The experiments were conducted on tubular samples which are loaded in axial tension or compression along the tube and with internal pressure to generate hoop stresses orthogonal to the axial direction. The results were analyzed in stress and strain space and also in terms of the evolution of crystallographic texture. In general, it was found that the VPSC simulations matched well with the experiments, particularly for the more weakly textured cast material. However, some differences were observed for cases where basal < a > slip and {10¯12} extension twinning were in close competition such as in the biaxial tension quadrant of the plastic potential. The evolution of texture measured experimentally and predicted from the VPSC simulations was qualitatively in good agreement. Finally, experiments and VPSC simulations were conducted in which samples of the extruded AZ80 material were subjected to a small uniaxial strain prior to biaxial loading in order to further explore the competition between basal slip and extension twinning.

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Mecânica, 2006.
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O objetivo deste trabalho é o estudo de critérios de resistência à fadiga multiaxial de metais em regime de alto número de ciclos. Os modelos apresentados por vários autores propõem, como medidas principais, a contribuição das tensões normais e das tensões cisalhantes para a degradação por fadiga do componente, além dos parâmetros do material. A questão que se coloca no contexto de solicitações multiaxiais é: qual é a melhor medida para caracterizar a amplitude de tensões cisalhantes e como incorporar o efeito das tensões normais? O estudo desenvolve então, uma análise destas questões relacionadas à modelagem de um critério de resistência à fadiga. Tensões normais trativas contribuem de forma maléfica para a degradação por fadiga por agirem no processo de abertura de microtrincas; quase a totalidade dos modelos de fadiga multiaxial considera a tensão hidrostática como medida das tensões normais atuantes na solicitação à fadiga. Sabe-se que esta é basicamente uma média das tensões normais e propõe-se aqui a substituição desta, pela máxima tensão principal. A aplicação da proposta a um conjunto grande de resultados experimentais disponíveis na literatura confirmou a hipótese de que a pior situação, que corresponde à existência de uma micro-trinca ortogonalmente orientada à máxima tensão principal, deve ser considerada e fornece uma previsão de resistência à fadiga mais conservativa e portanto, a favor da segurança. Quanto às tensões cisalhantes, primeiro apresentam-se as propostas de alguns autores, destacando-se dentre elas a abordagem do envelope elíptico e do envelope prismático. As duas aproximações fornecem as mesmas boas previsões de resistência à fadiga para dados experimentais de carregamentos senoidais com ciclos de mesma freqüência. Avança-se a análise para carregamentos mais gerais cujas trajetórias se distanciam da forma de um elipsóide e verifica-se de maneira inédita que, para uma ampla faixa de histórias de carregamento, as medidas de amplitude de tensões cisalhantes obtidas pelo máximo envelope prismático são equivalentes às medidas correspondentes obtidas pelo mínimo envelope elíptico. Tal verificação foi comprovada considerando-se trajetórias com ciclos senoidais assíncronos proporcionais e fora de fase, e ciclos não senoidais selecionadas a partir de resultados experimentais relativos a situações limites de resistência à fadiga. ________________________________________________________________________________________ ABSTRACT
The aim of this work is to evaluate multiaxial high cycle fatigue criteria for metals. The models presented by many authors propose that the the normal and shear stresses are the main variables controlling the fatigue damage, as well as the materials parameters. In the multiaxial context, the fundamental question to be answered is: which is the best measure to characterize the shear stress amplitude and how the well known effect of the mean normal stresses can be incorporated in the modeling process? This work carries out an analysis of such issues! Tensile stresses reduce the fatigue strength of metals as they keep the crack faces opened. Almost the totally of the multiaxial fatigue models available in the literature considers the hydrostatic stress as a measure for the normal stresses acting upon the fatigue solicitation. The hydrostatic stress is basically an average of the normal stresses acting in three orthogonal planes passing through a material point. Here we claim that the worst situation in terms of fatigue solicitation corresponds to the existence of a micro-crack orthogonally oriented to the maximum principal stress. Therefore, the maximum principal stress rather than the hydrostatic stress should be considered as an appropriate measure of the mean normal stress effect on the fatigue solicitation. To validate this hypothesis available experimental data published in the literature were selected and compared with the estimates provided by a modified version of the Prismatic Hull criterion developed by Mamiya and Araujo. Concerning the shear stresses, some models which consider the Minimum Circunscribing Ellipsoid or the Maximum Prismatic Hull of the deviatoric stress path as an appropriate measure for the shear stress amplitude are presented. The analysis carried out considering different materials subjected to a broad range of loading paths involving sinusoidal loadings with distinct frequencies and non-harmonic loadings revealed the shear stress amplitudes measured by the prismatic hull are equivalent to the ones measured by the elliptic hull.

Eine nachträgliche Verfestigung von nähgewirkten multiaxialen Gelegen führt zu einer verbesserten Ausnutzung der Verstärkungsfadeneigenschaften. Zu diesem Zweck wurden Lösungsansätze für eine entsprechende Verfestigungseinrichtung gesucht und bewertet. Drei Anlagenvarianten wurden durch die Kombination verschiedener Verfahren (Strahlungswärme, die Kombination aus Wärme und Druck sowie Walzenbeschichtung) und Bindemittel (Thermoplaste in verschiedenen Aufmachungen, Beschichtungsmassen) entworfen. Sie bieten auf die Einsatzzwecke Produktion, Laborbetrieb und geringster Aufwand abgestimmte Eigenschaften
The additional stabilization of open grid warp knits provides a better exploitation of the reinforcing yarns. To realize such an additional stabilization, various possible methods have been examined and assessed. Three different types of stabilization installations have been developed by combining the most promising technologies (infrared radiation, combination of heat and pressure, roll coater) and binding agents (thermoplastics, liquid agents). These installations offer special fea-tures for different needs: production, laboratory and least expense

Dieser Beitrag bietet einen Überblick über die in zwölf Jahren Forschungsarbeit am Institut für Textilmaschinen und Textile Hochleistungswerkstofftechnik (ITM) erzielten Ergebnisse auf dem Gebiet textiler Betonbewehrungen unter Einsatz der Nähwirktechnik. Standen zunächst die Weiterentwicklung der Nähwirk- bzw. Multiaxial-Kettenwirktechnik und die Integration zusätzlicher Prozessschritte im Mittelpunkt, so wurde dies mit der Einführung neuer Faserwerkstoffe durch die Beantwortung grundlegender Fragen zum Materialverhalten von Glas- und Carbonfasern unter verschiedensten Belastungsszenarien ergänzt. Aufbauend auf den gewonnen Erkenntnissen stehen heute Multiaxialgelege als Bewehrung für Beton zur Verfügung, die ein weites Anforderungsspektrum abdecken können, mit hoher Qualität und Produktivität herstellbar sind und damit den praktischen Einsatz des Textilbetons auf breiter Basis ermöglichen
This paper provides an overview on the results of textile concrete achieved in twelve years of research at the Institute of Textile Machinery and High Performance Material Technology (ITM) in the field of textile reinforcements for concrete based on the multiaxial stitch-bonding technology. During the early years the research focused on the development of the textile manufacturing process and the integration of additional functions in stitch-bonding machines. With the introduction of new fiber materials this was shifted towards the description of the material behavior of glass and carbon fibers under different load scenarios. Based on the results of this research, multiaxial multi-ply fabrics are available now as reinforcements for concrete, covering a broad range of applications. These fabrics can be produced with high quality and productivity and enable the practical usage of textile reinforced concrete

Despite real applications having complex triaxial loading, current physical test methods to predict component behaviour are mainly uniaxial. But previous work has indicated that there may be substantial differences between the rubber's uniaxial and biaxial behaviour and hence through incompressibility, its triaxial properties. In order to quantify these differences equipment was developed to assess the biaxial performance of selected rubber compounds using inflated circular diaphragms. Although allowing higher extensions than stretching a sheet in its own plane, such tests do not allow stress and strain to be measured directly, requiring careful marking of the sample, or calculation through simulation. On the grounds of perceived accuracy, the latter was chosen, requiring accurate, general, elastic constants to high extensions. In this thesis the development of this apparatus, along with the associated techniques is described, along with the development of a new elastic theory. The tests on this new apparatus indicated significant differences between the uniaxial and biaxial strength and fatigue of rubber. In a unimdal test natural rubber (NR) is much stronger than styrene butadiene rubber (SBR) below 35pphr of carbon black. In a biaxial test though the converse is true, although there is some evidence of crystallinity in NR during the biaxial test. Distinct differences were also found in fatigue between the two load cases. When plotted against extension ratio the biaxial life of SBR was found to increase, while the converse is true for NR. However if life is plotted against a function of strain energy, the biaxial life of both polymers increases for a given energy.

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
COORDENAÇÃO DE APERFEIÇOAMENTO DO PESSOAL DE ENSINO SUPERIOR
PROGRAMA DE SUPORTE À PÓS-GRADUAÇÃO DE INSTS. DE ENSINO
PROGRAMA DE SUPORTE À PÓS-GRADUAÇÃO DE INSTITUIÇÕES COMUNITÁRIAS DE ENSINO PARTICULARES
Eixos de virabrequim estão sujeitos a carregamentos multiaxiais quando em serviço. Por se tratar de um estado complexo de tensões, os modelos aplicados na determinação da vida em fadiga de tais componentes devem permitir, também, uma abordagem multiaxial, mais avançados do que aqueles adotados para carregamentos uniaxiais. O objetivo deste trabalho foi prever a falha em serviço de eixos virabrequins de unidades geradoras Diesel ou gás de plantas termoelétricas. Falhas recentemente ocorridas em eixos virabrequins no parque termoelétrico nacional justificam o presente estudo, para garantir eficiência e segurança nas operações industriais. Com base no método de elementos finitos, foram calculadas as tensões atuantes em um eixo virabrequim de unidade geradora a gás, com 10 mancais, duas bielas por mancal e fabricado com aço estrutural DIN 34CrNiMo6. Em sequência, adotando-se estas tensões atuantes calculadas, foram aplicados diversos critérios de fadiga para prever sua falha. Para tal, adotaram-se os modelos de Papadopoulos, Findley, Matake, McDiarmid, Carpinteri e Spagnoli, Liu e Mahadevan, Mises, Sines e Crossland, todos de fadiga de alto ciclo, baseados no plano crítico ou na tensão de von Mises. Propriedades de resistência à fadiga do material foram retiradas da literatura. Os resultados obtidos indicaram que o componente é seguro quando avaliado usando tais critérios.
Crankshaft axles are subject to multiaxial loading when in service. Because the resulting state of stresses is complex, models applied to determining the fatigue life of such components employ a multiaxial approach as well, more advanced than those adopted for uniaxial loads. The objective of this work is to predict the failure in service of crankshafts of diesel or gas generating units of thermoelectric plants. Crankshafts reported recent failures in the national thermoelectric power plant justifies the present study, to guarantee efficiency and safety in such industrial operations. Based on the finite element method, the resulting stresses on a DIN 34CrNiMo6 structural steel gas generating unit crankshaft, with 10 bearings, two connecting rods per bearing were calculated. Using these finite element calculated stresses, several fatigue criteria were applied to predict this cranckshaft structural failure. Models due to Papadopoulos, Findley, Matake, McDiarmid, Carpinteri and Spagnoli, Liu and Mahadevan, Mises, Sines and Crossland, all of high cycle fatigue based on the critical plane or von Mises strain, were adopted. Material fatigue properties used in the analyses were compiled from specidized literature. Obtained results indicated that the component considered is safe regarding fatigue loadings, as evaluated using such criteria.

In the introduction to a recent symposium on rolling contact fatigue, R.A.Smith stated that it was difficult to apply our greatly increased understanding of metal fatigue, to rolling contact fatigue, because of "the apparent lack of alternating tensile stresses to drive the cracks." He went on to say "alternating shear stresses are easily found, but the reproduction of continuous crack growth controlled by shear (Mode II in fracture mechanics terms), has proved to be near impossible." This project has demonstrated that under specific conditions this mode of growth does occur. The project began by studying rolling contact fatigue defects, in particular the 'squat' defect in railway lines, and the stress analyses that have been performed on them. It was concluded that the largest stress cycle experienced by the cracks must be a shear stress. It. series of tests were then performed that loaded a crack in pure shear, or a mixture of tension and shear, looking at the effects of using fully reversed shear loading, and the effects of applying tensile mean stresses to reduce the friction on the crack flanks. However these tests all produced less than one millimetre of mode II growth, before the cracks arrested or branched. The final series of tests however applied a tensile load cycle before each shear load cycle. This time coplanar growth was produced, that is the crack grew in the direction of the maximum shear stress. This type of load cycle is a simplification of the load cycle calculated by Bower and Johnson of Cambridge University, where the tensile load is produced by fluid trapped in the crack. Two crack growth rate formulae were produced that fitted the data, indicating that the growth rate was dependent on both the tensile and the shear parts of the cycle.

Eine nachträgliche Verfestigung von nähgewirkten multiaxialen Gelegen führt zu einer verbesserten Ausnutzung der Verstärkungsfadeneigenschaften. Zu diesem Zweck wurden Lösungsansätze für eine entsprechende Verfestigungseinrichtung gesucht und bewertet. Drei Anlagenvarianten wurden durch die Kombination verschiedener Verfahren (Strahlungswärme, die Kombination aus Wärme und Druck sowie Walzenbeschichtung) und Bindemittel (Thermoplaste in verschiedenen Aufmachungen, Beschichtungsmassen) entworfen. Sie bieten auf die Einsatzzwecke Produktion, Laborbetrieb und geringster Aufwand abgestimmte Eigenschaften.
The additional stabilization of open grid warp knits provides a better exploitation of the reinforcing yarns. To realize such an additional stabilization, various possible methods have been examined and assessed. Three different types of stabilization installations have been developed by combining the most promising technologies (infrared radiation, combination of heat and pressure, roll coater) and binding agents (thermoplastics, liquid agents). These installations offer special fea-tures for different needs: production, laboratory and least expense.

Static and dynamic behavior of the dry and saturated Ottawa sand are studied in the constitutive modeling laboratory, by using cubical multiaxial device with servo-controlled loading system and high speed automatic data acquisition system. In this investigation, the multiaxial cubical device with servo-controlled loading system and high speed automatic data acquisition system is used to apply independently the three-dimensional loading and measure corresponding strains and stress. The cubical multiaxial device is modified to include the servo-controlled pressure system and automatic data acquisition system. To understand the cyclic behavior and post cyclic behavior, comprehensive laboratory tests have been performed on the dry and saturated Ottawa sand, covering a number of relative densities and confinements.

Modélisation du comportement du béton sous des charges multiaxiales avec une loi incrémentale non linéaire. Simulation numérique d'essais uniaxiaux, biaxiaux et triaxiaux. Comparaison avec les résultats expérimentaux obtenus avec une presse biaxiale.

De nouvelles approches phénoménologiques sur la modélisation du comportement des AMFs sont nécessaires pour tenir en compte leur réponse complexe sous chargement multiaxial. L’effet de l’anisotropie induit une dépendance de leur comportement inélastique de la direction du chargement pour des cas superélastiques. La réorientation martensitique affecte drastiquement la réponse du matériau sous chargement non-proportionnel. La charge répétitive modifie aussi certaines propriétés du matériau. L’objectif de cette étude est de proposer un nouveau modèle constitutif thermodynamique robuste pour les AMFs, focalisé surtout sur des compositions NiTi équiatomiques pour capter la transformation martensitique anisotrope et la réorientation des variantes martensitiques. Une nouvelle approche mathématique est introduite pour permettre la prise en compte de l’anisotropie de contraintes et l’évolution des déformations inélastiques lors de la transformation directe, causée par les conditions de mise en forme de structures en AMFs. Cette méthode est évaluée en employant des courbes contraintes-déformations résultant de chargements proportionnels simulés par un modèle micromécanique. Un modèle phénoménologique considérant surtout la réorientation martensitique et mettant en évidence le fort couplage thermomécanique est développé. Il est implémenté dans une plate-forme numérique en C++, SMART+, et évalué en exécutant des simulations des expériences non-proportionnelles existantes. Des structures complexes sont également simulées en employant la Méthode des Élements Finis. La dernière partie de ce travail concerne l’étude expérimentale des effets du chargement cyclique sur l’évolution des déformations résiduelles et le seuil de transformation des alliages NiTi sous sollicitation uniaxiale et biaxiale
New phenomenological approaches in modeling the behavior of SMAs are needed to account for their complex response under multiaxial loading. The effect of anisotropy induces a dependence of their inelastic behavior to the direction of the loading for superelastic cases. Martensitic reorientation affects drastically material response under non-proportional loading. Repeated loading also alters certain material properties. The goal of this study is to propose a new robust thermodynamic constitutive model for SMAs with focus on equiatomic NiTi compositions to capture anisotropic martensitic transformation and reorientation of martensitic variants, always taking in mind the strong thermomechanical coupling. A new mathematical approach is introduced to account for the anisotropy of stresses and the evolution of inelastic strains during forward transformation caused by the forming conditions of SMA structures. This method is evaluated by utilizing stress-strain curves resulting from proportional loading simulated with a micromechanical model. A phenomenological thermodynamic model considering especially martensitic reorientation and exhibiting the strong thermomechanical coupling is developed. It is implemented on a numerical platform in C++, SMART, and evaluated by simulating existing non-proportional experiments. Complex structures are also simulated using Finite Element Analysis. The last part of this work concerns the experimental study of the effects of cyclic loading to the evolution of residual strain and transformation threshold of NiTi under uniaxial and biaxial testing

Registra mediante la aplicación del modelo de diagnóstico multiaxial de la CIE 10, además de la enfermedad biomédica motivo de la hospitalización, la presencia de patología mental, de limitaciones funcionales como consecuencia de la enfermedad y de factores sociales y circunstanciales que influyen en la salud, en el 30% o más de los pacientes hospitalizados en un Servicio de Medicina del Hospital Guillermo Almenara Irigoyen - EsSalud. El estudio es descriptivo, observacional y transversal realizado en una muestra de pacientes hospitalizados en el Servicio de Medicina Interna N°1, del Hospital Nacional Guillermo Almenara, en el período comprendido entre el 1° de agosto de 2014 y el 31 de octubre de 2014. Para la interpretación de las variables se usaron distribuciones de frecuencia absoluta y relativa (%). Resultados: el 51.5% de los pacientes seleccionados presentaban trastornos mentales, el 97% compromiso funcional como consecuencia de la enfermedad y el 87.9% factores sociales y circunstanciales que afectan el estado de salud. Se concluye que mediante la aplicación del modelo de diagnóstico multiaxial, ha sido posible registrar además de la patología biomédica, la presencia de patología mental, de limitaciones funcionales como consecuencia de la enfermedad y de factores sociales y circunstanciales que influyen en la salud, en más del 30% de los pacientes hospitalizados en un Servicio de Medicina del Hospital Guillermo Almenara.
Tesis

Les travaux de cette thèse s’intéressent au comportement mécanique d’une mousse destinée à l’absorption d’énergie dans une assise de siège pilote. Les méthodes de caractérisation habituelles proposent de solliciter le matériau suivant une seule direction. Cependant, cette caractérisation ne permet pas d’être représentatif des sollicitations lors de l’utilisation de l’assise, qui sont multiaxiales. Cette étude s’intéresse donc à la caractérisation du comportement multiaxial d’une mousse. L’approche originale utilisée est une séparation du comportement en deux contributions: changement de volume (pression-volume) et de forme (distorsion-cisaillement). Un premier moyen d’essais de compression hydrostatique a été développé afin de caractériser le changement de volume. Les résultats mettent en évidence une forte influence de la contribution en changement de volume, lors d’une sollicitation de compression uniaxiale. Un deuxième moyen d’essais a été développé permettant d’appliquer des sollicitations radiales suivant un angle cinématique ϑε, imposant une proportion de volume et de distorsion. Les résultats montrent une forte influence de l’angle cinématique sur les comportements des contributions de changement de volume et de forme. D’autres sollicitations impliquant de la compression et du cisaillement d’une manière séquentielle ont montré une influence du niveau de volume sur le comportement en changement de forme. Enfin, un modèle de simulation 2D par assemblage d’éléments finis 1D, montre une bonne représentation des différents comportements des contributions de changement de volume et de forme obtenus expérimentalement
This thesis is focused on the mechanical behavior of foam designed to absorb energy in an airplane pilot seat cushion. Usually, these materials are characterized using uniaxial compressive test. Nevertheless, this uniaxial characterization doesn’t represent the real in-use loading of cushion. To complete these data, this work focuses on multiaxial behavior characterization of foam. The analysis of behavior is realized by using a separation into two contributions linked to the volume (pressure-volume) and the shape (distortion-shear) change. A hydrostatic testing system was developed with the aim to characterize the volume change behavior. Results highlight a strong influence of the volume change behavior during an uniaxial compression solicitation. A second testing system was developed allowing to apply radial solicitations following a kinematic angle, which imposes a non-proportional variation of volume and distortion. A kinematic angle influence is observed on the volume and shape change behavior. Other solicitations composed of compression and shear applied in a sequential way, permit to observe a volume influence on the shape change behavior. Finally, a 2D simulation model composed of 1D element composition shows a good representation of the volume and shape changes behavior obtained from experimentation

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.
Includes bibliographical references (p. 93-100).
This thesis presents the design, development, and construction of an instrument for biaxial mechanical testing of inhomogeneous elastic membranes. The instrument incorporates an arrangement of linear motion stages for applying arbitrary deformation profiles on the material under test, purpose-built two-axis force transducers for high-resolution measurement of applied loads, and a digital imaging system for full-field strain measurement. The components described herein provide the foundation for a sophisticated biaxial testing platform for determining the mechanical properties of anisotropic, inhomogeneous membrane materials.
by Ariel Marc Herrmann.
S.M.

Orientador: Auteliano Antunes dos Santos Júnior
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
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Resumo: O virabrequim é um dos principais componentes do motor de combustão interna e sua função é, em conjunto com a biela, transformar o movimento de translação do pistão em movimento de rotação. Como sofrem carregamentos cíclicos durante toda sua vida útil, o principal tipo de falha de um virabrequim é por fadiga. Porém, por estarem sujeitos a carregamentos complexos, tensões multiaxiais são geradas resultando no problema da fadiga multiaxial. Nos últimos anos, diversos métodos que tratam desse problema foram apresentados, porém, até hoje, nenhuma abordagem foi universalmente aceita. Este trabalho apresenta a avaliação da vida de dois virabrequins fornecidos pela empresa Thyssen Krupp Metalúrgica Campo Limpo Ltda quanto à falha por fadiga utilizando os critérios de fadiga multiaxial para elevado número de ciclos de Matake, McDiarmid e DangVan, baseados nos planos críticos, e os critérios de Sines, Crossland e Kakuno-kawada, baseados nos invariantes de tensão. Ao longo do trabalho, é descrita a cinemática e a dinâmica do movimento de um virabrequim bem como uma forma adequada de tratamento para a vibração torcional. Com base nos esforços, é feita a análise de tensões através do método de superposição de esforços unitários. Os resultados das tensões são utilizados como entradas em um algoritmo especialmente desenvolvido para os critérios de fadiga citados. As análises feitas mostram que os projetos analisados são seguros quando avaliados utilizando tais critérios
Abstract : The crankshaft is one of the most important components of the internal combustion engine and its function is, with the rods, transforming the linear movement of the piston into rotation. As it is under cyclic loading during all of his life, the main type of failure in these components is due to fatigue. However, as they are subject to complex loads, multiaxial stresses arise, which results in the problem of multiaxial fatigue. Recently, several approaches to this problem were proposed but until now, there is no universally accepted approach. This work evaluates the life of two designs of crankshafts provided by Thyssen Krupp Metalúrgica Campo Limpo Ltda with respect to the fatigue failure using the multiaxial high-cycle fatigue criteria of Matake, McDiarmid and Dang Van, based on critical planes, and Sines, Crossland and Kakuno-Kawada, which are based on the stress invariants. Throughout the work it is described the kinematics and the dynamics of the cranckshaft movement and an appropriate form of treatment for the torsional vibration. Based on the loads, a stress analysis is performed using the finite elements method and the superposition principle. The resulting stresses are used as inputs in a specially developed algorithm to evaluate the fatigue criteria cited. These analyses show that the designs are safe when they are evaluated using such criteria
Mestrado
Mecanica dos Sólidos e Projeto Mecanico
Mestre em Engenharia Mecânica

In spite of many multiaxial fatigue life prediction methods proposed over decades of research, no universally accepted approach yet exists. A multiaxial fatigue damage model developed for approximately proportional random amplitude loading is proposed in this study. A normal strain based analysis incorporating the multiaxial state of stress is conducted along a critical orientation assuming a constant strain ratio. The dominant deformation direction is chosen to be the critical orientation which is selected with the help of a principal strain histogram generated from the given multiaxial loading history. The uniaxial cyclic stress-strain curve is modified for the biaxial state of stress present along the critical orientation for the plane stress conditions. Modified versions of Morrow's and of Smith, Watson, and Topper's (SWT) mean-stress models are used to incorporate mean stresses. A maximum shear strain based analysis is, in addition, conducted to check for the shear dominant fatigue crack growth possibility along the critical direction. The most damaging maximum shear strain is chosen after analyzing the in-plane and the two out-of-plane shear strains.

The minimum of the two life values obtained from SWT model and the shear strain model is compared with the life estimated by the proposed model with the modified Morrow's mean stress model. The former is essentially the life predicted by Socie. The results of the proposed model, as reduced to the uniaxial case, are also compared with the experimental data obtained by conducting one-channel random amplitude loading history experiments.
Master of Science

Thanks to their lightness and versatility combined with excellent mechanical properties, composite materials underwent an increasing relevance in the last twenty years in many industrial fields, also for structural applications. This class of materials offer a wide range of advantages but it is still characterized by a quite high cost with respect to more traditional structural materials. One of the reasons is the lack of reliable design procedures, as well as of extensive experimental investigations in the literature providing clear and general information. This is particularly evident with reference to the behaviour of composite laminates and bonded joints under multiaxial fatigue loading, which often characterise in-service conditions. With the aim to acquire information on the multiaxial fatigue behaviour of unidirectional composites, a specimen configuration was first defined, suitable to characterise the matrix-dominated multiaxial fatigue behaviour of a composite lamina, which is of great importance for the damage evolution in laminates (chapter 2). Tubular specimens subjected to combined tension/torsion loading were identified as the best compromise between reliability of results, easiness of testing and possibility to obtain multiaxial stress states of interest. In the third chapter the results of an extensive experimental investigation on tubular specimens is presented. Tubes were fatigue tested with several values of the biaxiality (shear to transverse) ratio and load ratio (ratio between the minimum and the maximum fatigue loads). A strong influence of both parameters was found on the off-axis crack initiation and propagation phenomena, as well as on the damage mechanisms at the micro-scale. Uniaxial fatigue tests were then carried out on multidirectional flat laminates, designed to achieve local multiaxial stress states similar to those applied to the tubes by means of tension/torsion external loads. This activity, presented in the fourth chapter, revealed the equivalence between external (obtained by applying external loads in different directions) and internal (due to material anisotropy) multiaxial stress states. This represents a fundamental step for the extension of experimental results and predictive models to general loading conditions. The experimental activity on tubes provided information on the damage mechanisms at the micro-scale, responsible for fatigue failure of a unidirectional lamina. On the basis of these mechanisms a criterion for predicting crack initiation in a unidirectional lamina under multiaxial fatigue was developed by means of a multiscale approach (chapter 5). This criterion resulted in sound agreement with the new data on tubes, with data on flat unidirectional laminates from the literature and with crack initiation data on the off-axis layers of the laminates reported in chapter 4. A basic topic related to the analysis of multidirectional laminates is the stiffness degradation due to off-axis cracks in their plies. Dealing with such a topic, an analytical model was developed for predicting the stiffness of a laminate as a function of the crack density in its layers accounting for the interaction between cracks in different layers (chapter 6). In addition the model is capable of calculating the stress re-distribution due to the presence of cracks. This is fundamental for the development of a new procedure for predicting the fatigue crack density evolution in multidirectional laminates, presented in chapter 7. To this aim the experimental observations, the analytical models and criteria previously presented, combined with a statistical approach, have been used to predict the initiation and propagation of multiple cracks in a laminate. As a consequence, when this procedure is combined with the model presented in chapter 6, both the stiffness degradation and the stress re-distribution, useful for the estimation of the total fatigue life, can be predicted. When composite laminates are used as adherends in bonded joints the bonding surface represents a critical position for the onset of fatigue cracks. As a consequence an experimental investigation on the propagation of a bondline crack in composite bonded joints subjected to mixed mode I + II (opening + sliding) fatigue loading was carried out and presented in chapter 8. A criterion to predict the crack propagation rate under mixed mode loading was also developed, based on the damage mechanisms observed during the experimental campaign. Eventually, in Appendix A an analytical model for predicting the initiation of a fibre-matrix debond crack under biaxial static loads is presented. The model provides useful information on the influence of the main geometrical and interface parameters of the fibre-matrix interface strength.
Grazie alla loro leggerezza e versatilità combinate ad eccellenti proprietà meccaniche, i materiali compositi hanno acquisito un'importanza sempre maggiore negli ultimi vent'anni in molti settori industriali, anche per applicazioni strutturali. A fronte dei numerosi vantaggi offerti da questa classe di materiali vi è un costo che rimane ancora piuttosto elevato rispetto ai più tradizionali materiali da costruzione. Una delle ragioni è la mancanza di procedure di progettazione affidabili e riconosciute, nonché l'assenza, in letteratura, di estese caratterizzazioni sperimentali da cui acquisire informazioni di carattere generale. Ciò è particolarmente evidente in riferimento al comportamento a fatica multiassiale di lamine, laminati e giunzioni incollate in composito. Al fine di sopperire alla mancanza di informazioni sul comportamento a fatica multiassiale di lamine unidirezionali, nonché all'assenza di una procedura adeguata di test, è stata inizialmente definita una configurazione di provini adatta a caratterizzare la risposta matrix-dominated (particolarmente significativa per il danneggiamento a fatica di laminati) di materiali compositi unidirezionali (capitolo 2). Provini tubolari soggetti a carichi ciclici di trazione e torsione combinati sono stati identificati come il miglior compromesso tra affidabilità dei risultati, semplicità di testing e possibilità di ottenere condizioni multiassiali di interesse. Nel terzo capitolo sono riportati i risultati di un'estesa campagna sperimentale su tali provini tubolari in presenza di diversi rapporti di biassialità (tensione di taglio su tensione trasversale) e rapporti di ciclo (rapporto tra il minimo e il massimo carico di fatica). É stata riscontrata una notevole influenza di tali parametri sull'innesco e propagazione di cricche off-axis, nonché sui meccanismi di danneggiamento su scala microscopica. Sono poi stati testati a fatica uni-assiale dei laminai piani multi-direzionali progettati per avere condizioni di multiassialità locali simili a quelle ottenute sui provini tubolari tramite carichi esterni in diverse direzioni. L'attività, presentata al quarto capitolo, ha permesso di verificare l'equivalenza tra condizioni multiassiali ti tipo esterno (carichi in più direzioni) e interno (dovute all'anisotropia di lamine e laminati in composito). Ciò rappresenta uno step fondamentale per l'estensione di risultati sperimentali e modelli previsionali a condizioni di carico generiche. L'attività sperimentale sviluppata sui tubi ha fornito informazioni sui meccanismi di danneggiamento a livello microscopico che sono responsabili del cedimento a fatica della lamina unidirezionale. Sulla base di tali meccanismi è stato proposto un criterio per l'innesco di cricche a fatica multiassiale in lamine in composito basato su un approccio multiscala (capitolo 5). Il criterio è risultato in ottimo accordo con i nuovi dati sperimentali sui campioni tubolari, con dati disponibili in letteratura riguardanti lamine unidirezionali piane e con i dati ad innesco sugli strati off-axis dei laminati testati al capitolo 4. Parlando quindi di laminati multi-direzionali, un aspetto fondamentale è la diminuzione di rigidezza di questi ultimi dovuto all'innesco e propagazione di cricche multiple negli strati off-axis. A tal proposito è stato proposto un modello analitico in grado di legare la densità di cricche in ciascuno strato di un laminato alla diminuzione di rigidezza globale considerando anche l'interazione tra cricche presenti su strati diversi (capitolo 6). Tale modello fornisce anche le distribuzioni di tensione dovute alla presenza delle cricche stesse. Questo è un aspetto di fondamentale importanza per lo sviluppo di una procedura per prevedere l'evoluzione della densità di cricche in laminati multi-direzionali sollecitati a fatica, presentata al capitolo 7. A tale scopo le osservazioni sperimentali, i modelli analitici e i criteri sviluppati in precedenza, combinati ad un approccio di tipo statistico, vengono utilizzati per prevedere l'innesco e propagazione di cricche multiple in un laminato. Di conseguenza, combinata con il modello precedentemente illustrato, la procedura consente di prevedere sia la diminuzione di rigidezza di laminati sia la ridistribuzione delle tensioni per effetto del danneggiamento rappresentando quindi uno strumento utile anche alla stima della vita a fatica totale di un laminato. Quando i laminati in composito sono utilizzati come aderendi in giunzioni incollate, l'interfaccia di incollaggio rappresenta una zona particolarmente critica per l'innesco di cricche a fatica. Di conseguenza è stata analizzata sperimentalmente la propagazione di cricche in giunzioni incollate soggette a carichi ciclici di modo misto I + II (apertura + scorrimento). Ancora una volta i meccanismi osservati su scala microscopica sono stati utilizzati per la formulazione di un criterio damage-based per la previsione della velocità di propagazione di cricche in giunzioni incollate sollecitate in modo misto (capitolo 8). In fine, in Appendice A è presentato un modello analitico sviluppato per la previsione dell'innesco di una cricca di debonding tra fibra e matrice in condizioni di carico statico biassiale. Il modello è utile per trarre informazioni sull'influenza dei principali parametri geometrici e interfacciali sulla resistenza dell'interfaccia fibra-matrice.

A associação entre epilepsia e transtornos psicóticos é amplamente relatada na literatura. Características clínicas e do seu impacto, tais como suicídio, tentativas de suicídio, homicídio e alterações cognitivas são evidenciados em poucos estudos que associam epilepsia e psicose. Este estudo comparou o impacto clínico e funcionamento psicossocial dos pacientes com epilepsia e transtornos psicóticos, analisando ainda as diferenças entre pacientes com psicose interictal e pós-ictal. O estudo consiste de uma revisão de prontuário de todos os pacientes atendidos no PRONEPSI com epilepsia e transtornos mentais. Foram estudados 143 indivíduos, divididos em dois grupos: 82 pacientes com diagnóstico de epilepsia e transtornos psicóticos e 61 pacientes com epilepsia e outro transtorno mental. O grupo de pacientes psicóticos foi estudado comparando dados entre pacientes com psicose interictal (53) e pós-ictal (17). O grupo de pacientes com transtornos psicóticos apresentou menor escolaridade, mais história familiar de psicose, maior número de tentativas de homicídio, mais estados de mal epiléptico, mais internações psiquiátricas, história pregressa de insulto ao SNC e retardo mental. Além disso, a epilepsia foi considerada um fator causal importante para desenvolvimento da psicose. O grupo de pacientes com transtornos psicóticos também se diferenciou, revelando maior impacto na esfera cognitiva, vocacional e pessoal. Os pacientes com psicose interictal tiveram mais história familiar para psicose e a epilepsia foi considerada um fator causal importante para seu desenvolvimento. Pacientes com epilepsia e transtornos psicóticos apresentam um maior comprometimento clínico geral o que interfere diretamente no seu comprometimento funcional e na gravidade do impacto. Os pacientes com epilepsias mais graves e algum tipo de insulto ao SNC parecem ser mais vulneráveis ao desenvolvimento de transtornos psicóticos em comparação aos pacientes com formas menos graves de epilepsia
The association between epilepsy and psychotic disorders has been well documented in literature. Although this association is well-known, few studies regarding psychosis and epilepsy investigated the clinical characteristics of these patients and its impact on psychosocial function, suicide and suicide attempts, homicide attempts and cognitive deficits. The aim of this chart review was to compare the clinical impact and the psychosocial function between patients with epilepsy and psychotic disorders and patients with epilepsy and other psychiatric disorders. We also compared the clinical characteristics and psychosocial function between patients with postictal psychosis and interictal psychosis. We reviewed 143 charts, divided in two groups: 82 charts of patients with epilepsy and psychotic disorders and 61 charts of patients with epilepsy and other psychiatric disorders. In the group of patients with epilepsy and psychosis, 53 had a diagnosis of interictal psychosis and 17 of postictal psychosis. Patients with psychotic disorders had fewer years of education, more family history of psychotic disorders and higher number of homicide attempts, status epilepticus, psychiatric admissions and history of central nervous system insults. They also presented more impact on cognitive, vocational and interpersonal scales. Epilepsy was considered a major cause to the development of psychosis. Regarding the differences between patients with interictal and postictal psychosis, the only difference found was that the patients with interictal psychosis presented more family history of psychosis. Also, in both groups epilepsy was considered a major cause to the development of psychosis. Patients with psychotic disorders had a more severe clinical impairment in comparison with patients with other psychiatric disorders, which may have interfered in psychosocial functioning and severity of impact. Patients with central nervous system\'s insults and severe epilepsy may be likely more prone to psychosis\'s development than other patients with less severe forms of epilepsy

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
The evolution of techniques for the development of equipment capable of translating the results taken from a diverse and complex set of tasks carried out by the human body, has become an important item on the scientific communities agenda. Within the context of such equipment, the force plate has shown itself capable of being implemented for the task of measuring phenomena, which before could only be observed visually. This work has as its objective to describe and develop an extensometric, multiaxial force plate, for use as a biomedical assistance apparatus. Sensors for the detection of forces such as strain gauges were used and fixed in predetermined areas on the four transducers, which are an integral and fundamental part of the force plate apparatus. The cell design was completed using geometry capable of coupling with the minimum of interference the two different forces present within the structure. In this manner it is possible to quantify the forces in three orthogonal directions. The prototype developed can be utilized, for example as an evaluation tool for both gait and stability research. In parallel to the development of the force plate, equipment for carrying out the conditioning and digitalization of signals originating from the load cells was designed and constructed. The control of the acquisition processes as well as the processing of signals received from the force plate is carried out by the use of a specific applicative, which makes the quantitative values of the forces, strength and momentum available, allowing for the calculation of the parameters used for the respective biomedical evaluations, especially those dealing with the oscillation of the human body in an erect near static posture. Still further the applicative provides a confidence ellipse obtained from the stability evaluations carried out along with the coordinates of pressure center and standard stability indexes obtained instantaneously.
A evolução das técnicas para desenvolver equipamentos capazes de traduzir o resultado de diversas tarefas complexas exercidas pelo corpo humano, tornou-se evidente para a comunidade científica. Dentre esses equipamentos, a plataforma de força mostrou ser uma ferramenta capaz de ser empregada para a mensuração de fenômenos antes só observados. Este trabalho tem por objetivo descrever o projeto e o desenvolvimento de uma plataforma de força extensométrica, multiaxial, para uso em instrumentação biomédica. Sensores para detecção de esforços, do tipo strain gauge, foram utlizados e fixados em regiões previamente determinadas nos quatro transdutores que compõem o conjunto da plataforma. O projeto das células considera uma geometria capaz de desacoplar, com mínima interferência, dois esforços diferentes, presentes na mesma estrutura. Assim, pode-se quantizar esforços em três direções ortogonais. O protótipo desenvolvido pode ser utilizado como ferramenta para avaliações de marcha e estabilidade, por exemplo. Paralelamente ao desenvolvimento da plataforma, um equipamento para realizar o condicionamento e digitalização dos sinais provenientes das células de carga foi projetado e construído. O controle dos processos de aquisição e processamento dos sinais advindos da plataforma de força é realizado através de um aplicativo específico que disponibiliza os valores quantitativos dos esforços de força e momento, os quais permitem o cálculo de parâmetros relevantes às avaliações biomédicas, particularmente em avaliações a cerca da oscilação do corpo humano na postura ereta quasi-estática. O aplicativo fornece ainda as elipses de confiança obtidas a partir das avaliações de estabilidade realizadas, juntamente com as coordenadas do centro de pressão e índices de estabilidade padrão e instantâneos obtidos.
Mestre em Ciências

Literature concerning the development of mechanistic and empirical models of creep deformation and failure, under uniaxial and multiaxial stress systems are reviewed. The detailed metallurgy of the alloy is discussed and its specific creep behaviour in the experimental temperature range is reviewed from published experimental data, mostly obtained from the Alloy 800 Data Bank of the EEC Joint Research Centre at Petten.

Une géométrie d'éprouvette tubulaire de traction-torsion entaillée a été optimisée pour caractériser l'effet de l'état des contraintes (triaxialité et paramètre de Lode) sur la ductilité des métaux à faible taux de triaxialité. Des essais biaxiaux proportionnels ou non, accompagnés de mesures des champs cinématiques par stéréo-corrélation d'images ont été réalisés sur un acier 36NiCrMo16 revenu et un alliage d'aluminium 2024-T351. Les trajets de chargement à rupture (évolution au point critique de la déformation plastique équivalente, de la triaxialité des contraintes et du paramètre de Lode) ont été obtenus. L'évolution de la ductilité de l'alliage d'aluminium en fonction de la triaxialité obtenue en traction-torsion diffère notablement de celle obtenue par Bao et Wierbicki en 2004 qui suggérait une ductilité minimale en cisaillement, tandis que la technique de traction-torsion révèle une ductilité maximale en cisaillement. Un effet notable des trajets de chargement non monotones ou non proportionnels sur la ductilité a été mis en évidence au moyen d'essais séquentiels composés d'un pré-chargement en compression, torsion ou traction, suivi d'une séquence de chargement proportionnel en traction-torsion combinées. Des observations au MEB de coupes métallographiques à l'issue d'essais biaxiaux interrompus, ainsi qu'un suivi en temps réel de la déformation et de l'endommagement en surface, lors d'essais de torsion dans le MEB et un essai de fissuration couplé à un suivi 3D de l'endommagement par laminographie sous rayonnement synchrotron ont mis en évidence des phénomènes de localisation à de multiples échelles ainsi que la croissance de certaines cavités, même en cisaillement pur, qui contraste avec l'affaissement total prédit par les modèles de cellule élémentaire. Ceci semble lié à l'allongement axial significatif mesuré en torsion pure (effet Swift). La localisation par cisaillement apparaît comme le mécanisme prédominant de coalescence des cavités, ce qui justifie l'adoption d'un critère d'initiation de la rupture de type Hosford-Coulomb. Utilisé en conjonction avec un indicateur non-linéaire d'endommagement il permet de rendre compte des ductilités mesurées, y compris sous chargement potentiellement non-proportionnel
A stocky tubular tension-torsion specimen geometry was optimized to characterize the effect of the stress state (stress triaxiality and Lode angle parameter) on metals ductility, at low stress triaxialities. Biaxial tests (proportional and non-proportional) were performed on 36NiCrMo16 steel and 2024-T351 aluminum alloy. Strain fields were measured by stereo-correlation of digital images during the tests. Loading paths to fracture (evolution of the equivalent plastic strain, the stress triaxiality and the Lode angle parameter at the critical point) were determined. The evolution of aluminum ductility with respect to the stress triaxiality measured from tension-torsion tests differed substantially from that obtained by Bao and Wierzbicki in 2004. Indeed, the latter suggested a minimal ductility under shear, while the tension-torsion technique revealed a maximal ductility under shear. Non-proportional loading paths were shown to have an influence on ductility, by means of tests consisting in a pre-compression, pre-tension or pre-torsion, followed by a proportional loading sequence under combined tension-torsion. SEM observations of metallographic sections from biaxial interrupted tests, a real-time monitoring of the surface strain and damage during in-situ torsion tests in the SEM, and a crack propagation test coupled with in-situ X-ray synchrotron laminography brought evidences of localization phenomena at different scales, and of the growth of some cavities, even under pure shear, by contrast with the total collapse predicted by unit cell models. This growth may be due to the significant axial elongation measured under pure torsion (Swift effect). Shear localization was identified as the main coalescence mechanism, which justifies the choice of the Hosford-Coulomb fracture initiation criterion. Used in conjunction with a non-linear damage indicator, it accounts for the measured ductilities, even under possibly non-proportional loadings

Fatigue is considered to be one of the most frequent phenomena in the failure of many machine parts. Most of the prior studies on fatigue have been limited to uniaxial loading cases with a primary focus on constant amplitude cycles. A detailed exploration of multiaxial fatigue under constant and variable amplitude loading scenarios for a wide variety of aircraft engine alloys has been performed in this study, and a new methodology for the accurate prediction of fatigue damage is developed. A critical-plane based constant amplitude fatigue damage model has been developed in this study which is simple in comparison to prior models developed by other researchers and reduces the computational effort. The constant amplitude fatigue damage model is further used in the development of a multiaxial variable amplitude damage estimation method, with an emphasis on estimating the damage created by both low cycle fatigue (LCF) and high cycle fatigue (HCF) cycles. A significant increase in overall fatigue damage was observed in the tests with the introduction of HCF cycles in the mission histories. The damage due to the HCF cycles has been found to be much greater than predicted by linear damage accumulation theories, although the degree of interaction between the LCF and HCF cycles was found to be very dependent on the multiaxial load paths. In addition, the HCF cycles did not contribute significantly to the accumulation of damage until a certain amount of ?pre-damage? had been caused by the LCF cycles. Separate HCF damage computing approaches have been adopted in this study to accurately compute the damage produced by tensile and shear dominant HCF cycles, and a significant improvement in the accuracy of fatigue life prediction has been achieved using the new methodology.
General Electric (Aviation)
Airforce Office of Scientific Research

Characterization of stress-deformation behavior of concrete and rocks have been a subject of active research for a long time. Linear elastic, nonlinear (piecewise) linear elastic, elastic-plastic and endochronic models have been proposed and used by various investigators and the literature on the subjects is very wide. A review of various models together with their implementation is numerical (finite element) procedures is presented in Ref. (77). The primary objective of the present study is to develop a generalized constitutive model based on the theory of plasticity. Although such a model can be used for a wide range of materials, in this dissertation its applications to plain concrete and rocks are emphasised. One of the main objectives of this dissertation is to study constitutive behavior of concrete and soapstone under multiaxial load histories by using a truly triaxial or multiaxial testing device. The truly triaxial device is capable of applying a general three-dimensional state of stress. Samples can be tested along any three dimensional stress path. Therefore, constitutive behavior of concrete and soapstone can be studied under all possible states of stress. The conventional, octahedral, proportional loading and circular stress test series are conducted using the truly triaxial cubical device. For meaningful results, samples with consistent initial properties are essential. In order to produce samples with uniform initial properties such as density, equipment and procedures are developed to standardize the sample preparation process. The test data is used to determine the material constants associated with the proposed constitutive model. The model is then verified by back-predicting the stress-strain curves obtained from the laboratory.

Materiais metálicos com grãos ultrafinos obtidos por métodos de deformação plástica severa (SPD) possuem alta resistência mecânica. Porém, uma das principais limitações para o uso destes materiais é sua baixa ductilidade. De acordo com os estudos da literatura, o processamento cíclico em materiais metálicos pré-deformados plasticamente pode promover o amaciamento ou encruamento, de acordo com seu estado inicial e condições de processamento. Dessa forma, uma opção para o aumento da ductilidade destes materiais seria submetê-los a esforços cíclicos. Estudos sobre o comportamento de materiais com grãos ultrafinos sob fadiga de baixo ciclo mostram que para poucos ciclos de deformação, pode ocorrer o amaciamento após este carregamento. Neste trabalho se avalia o comportamento do alumínio comercialmente puro quando submetido ao processamento multiaxial cíclico após ser processado pelo método de SPD denominado Extrusão Angular em Canais Iguais (ECAP). Para tal, o material pré-deformado por ECAP, em um passe, foi submetido a ensaios de compressão multiaxiais cíclicas, com amplitude de deformação de 0,075. Em função da deformação heterogênea associada a compressão de metais, foi necessário o desenvolvimento de uma técnica especial para a realização desses ensaios. Os resultados mostram que a deformação multiaxial cíclica promove a redução na taxa de encruamento para o alumínio recozido e o amaciamento do material pré-deformado por um passe de ECAP, e que após o processamento cíclico o material recozido e processado por ECAP encrua com taxa maior que o material apenas recozido.

Práce se zabývá tvárným lomem, který je výsledkem víceosého kvazi-statického monotónního namáhání doprovázeného rozsáhlými plastickými deformacemi, přičemž pro degradaci materiálu je uvažován lokální přístup. Ve výpočtech o rozvoji poškození rozhodují použité mezní podmínky tvárného lomu. Tyto byly teoreticky studovány v úvodu práce a po výběru vhodné mezní podmínky byl stanoven postup kalibrace. Dále byl rozpracován plán měření a realizovány zkoušky při pokojové teplotě na slitině hliníku 2024-T351, zahrnující tah, krut a tlak, pro studium rozvoje poškození a věrohodnou kalibraci vybraného fenomenologického modelu tvárného porušování, vyjádřeného pomocí lomového přetvoření a závislého na hydrostatickém tlaku a deviátoru tenzoru napětí. Mezní podmínka tvárného lomu byla posléze svázána s podmínkou plasticity. Plasticita byla pro zkoumaný materiál uvažována ve tvaru zohledňujícím i stav třetího invariantu deviátoru tenzoru napětí. Celý navržený přístup, plně aplikovatelný na víceosé úlohy, byl implementován pomocí uživatelské rutiny do komerčního programu založeného na explicitní variantě metody konečných prvků. V závěru práce je předložena aplikace navrženého přístupu k modelování tvárného porušování v podobě verifikace na vybraných zkušebních testech, z níž plynou závěry a doporučení pro další práci.

Trois étapes sont mises en oeuvre ici : la méthode de localisation, la fonction de dommage matériau et le processus de détermination des chargements équivalents en fatigue (CEF). De nombreux essais de fatigue ont été réalisés (415 au total) pour étudier le comportement en fatigue sous chargement multiaxial relaxant et non relaxant sur des éprouvettes de caoutchouc naturel. La durée de vie et les caractéristiques des fissures sont analysées pour finalement introduire un critère de fatigue approprié basé sur le plan critique et permettant de rendre compte de l'effet de déformation moyenne. Ce critère est généralisé à travers une méthode originale de recherche du plan critique. Pour estimer la réponse mécanique locale (méthode de localisation), une méthode de couplage des axes adaptée à la nature non linéaire des structures élastomères est proposée. Elle est basée sur la décomposition multiplicative des tenseurs du gradient de la déformation. Ces deux étapes sont ensuite mises en oeuvre dans le cadre du processus de détermination du CEF. Pour cela, une méthode d'optimisation globale est ajoutée pour déterminer les chargements simplifiés, induisant partout localement le même endommagement en fatigue dans la structure étudiée que le RLD. Le temps de calcul de cette optimisation est réduit en ne considérant qu'un sous-ensemble de points matériels, les plus endommagés, pour la détermination du CEF. Enfin, la méthode a été testée sur une éprouvette afin de souligner ses capacités et de valider l'approche
This thesis introduces an Equivalent Fatigue Load (EFL) approach for the multiaxial fatigue design of elastomeric parts. As direct Finite Element Analysis (FEA) calculations of automotive in-service loads (Road Load Data (RLD)) are too expensive, the objective is to derive simplified load blocks as a realistic input for numerical damage calculations. Three streps are applied for this method: the localization method, the material damage function and the EFL determination process. Various fatigue tests have been conducted (415 samples) to study the fatigue behavior of this complex type of relaxing and non-relaxing multiaxial loading on natural rubber specimens. Lifetime and crack features are analyzed to eventually introduce an appropriate critical planebased fatigue measure and to establish a novel mean strain effect model. This criterion is generalized throughout an original critical plane search method. To estimate the local mechanical response (localization method), this thesis identifies an axes-coupling method that is fitted for the nonlinear nature of elastomeric structures. It is based on the multiplicative decomposition of the deformation gradient tensors. These two steps are then implemented in the framework of the EFLdetermination process. For this, a global optimization method is added to determine the simplified load blocks, causing locally the same fatigue behavior in the given structure. The computational costs of this optimization are reduced by only considering a subset of the most damaged material points for EFLdetermination. Finally, the method has been challenged on a specimen to outline its capabilities and to validate the approach

Orientador: Auteliano Antunes dos Santos Júnior
Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica
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Resumo: Este trabalho apresenta a comparação entre os diversos critérios de fadiga multiaxial de alto ciclo aplicados ao problema de contato roda-trilho. Para isto, é utilizado um modelo elastoplástico tridimensional de elementos finitos capaz de permitir o cálculo tanto das tensões geradas pelo rolamento de uma roda livre de tensões residuais de fabricação como de uma roda que contenha as tensões residuais provenientes do processo de tratamento térmico, obtidas através de uma simulação térmico-estrutural. Estes dois cenários são avaliados segundo critérios de fadiga multiaxial baseados tanto em planos críticos, como Dang Van, Matake e McDiarmid, quanto nos baseados em invariantes do tensor de tensões, como Sines, Crossland e Kakuno Kawada. Pela natureza da fadiga de contato, que não possui condição de vida infinita, é estimado para cada critério um número de ciclos para o aparecimento das trincas. Como resultado, observa-se que para alguns critérios como Dang Van, Sines e Kakuno-Kawada, a vida em fadiga é beneficiada pelo processo de tratamento térmico, enquanto para os demais, as tensões residuais de fabricação provocam uma redução no número de ciclos para o aparecimento das trincas
Abstract: This work presents a comparison of different high cycle multiaxial fatigue criteria, applied to wheel- rail contact. For this, it is used a three-dimensional elastoplastic finite element model able to calculate stresses generated by the rolling of a wheel free of residual stresses from manufacturing process and also by a wheel containing the residual stresses from heat treatment process, generated by a thermal-structural simulation. These two scenarios are evaluated according to multiaxial fatigue criteria based on critical planes, as Dang Van, Matake and McDiarmid, and also based on the invariants of the stress tensor, as Sines, Crossland and Kakuno Kawada. Due to the nature of rolling contact fatigue, which has no condition of infinite life, a number of cycles for crack appearance are estimated for each criterion. It can be notice that for some criteria, such as Dang Van, Sines and Kakuno-Kawada, fatigue life is benefited by the heat treatment process, while for others the residual stresses from manufacturing promote a reduction in the number of cycles
Mestrado
Mecanica dos Sólidos e Projeto Mecanico
Mestre em Engenharia Mecânica

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Tecnologia, Departamento de Engenharia Mecânica, 2018.
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A presente dissertação avalia três métodos para cálculo da amplitude de tensão cisalhante equivalente utilizada em modelos de fadiga multiaxial baseados na abordagem do plano crítico. Avaliou-se o Método da Mínima Circunferência Circunscrita (MCC), o Método da Caixa (MRH) e o Método do Momento de Inércia (MOI). Com os resultados desses métodos foram aplicados três critérios de fadiga multiaxial: o critério de Susmel e Lazzarin (2002), o critério de Findley (1956) e o critério de Matake (1977). O objetivo foi a avaliação de quais métodos aplicados aos critérios resultam em melhores estimativas de resistência à fadiga multiaxial. Para a validação da análise, foram utilizados dados disponíveis na literatura para espécimes fabricados em ligas de aço submetidos a carregamentos combinados de flexão e torção em iminência de falha por fadiga. Um Índice de Erro (IE), que fornece uma medida da distância entre a previsão dos modelos e os dados experimentais foi utilizado como medida da acurácia nesta análise. Conclui-se que, em um contexto geral, para os dados sob carregamentos síncronos em fase ou fora de fase, os melhores resultados foram obtidos pelo Método MRH aplicado conjuntamente com o modelo de Findley (1956) e, para os dados sob assíncronos, as melhores estimativas foram obtidas quando o MRH foi utilizado juntamente com o modelo de Susmel e Lazzarin (2002).
This thesis evaluate three different methods that calculate the equivalent shear stress amplitude using Multiaxial Fatigue models based on the Critical Plane approach. It was evaluated the Minimum Circumscribed Circle (MCC), the Maximum Rectangular Hull (MRH) and the Moment of Inertia (MOI). With the result of these methods on hands, it was applied three multiaxial fatigue criteria: Susmel and Lazzarin (2002), the Findley (1956) criteria and the Matake (1977) criteria. The aim of this was to evaluate which of these methods applied in these criteria result in the closest estimated of multiaxial fatigue resistance. In order to validate the analysis, it was used available data base to different types of steel loaded to combined torsion and bending at the limit of fatigue failure. In this way, a percentual error index was used to measure how distant the prediction of the criteria was to the empirical result. Therefore, it was showed that the best results were obtained by using the MRH method when applied to the Findley criteria for synchronous loadings and, for asynchronous loadings, the same method using the Susmel and Lazzarin (2002).

A study was conducted to test the therapeutic effects of assessment feedback on rapport-building and self-enhancement variables (self-verification, self-discovery, self-esteem), as well as symptomatology. Assessment feedback was provided in the form of interpretive information based on the results of the Millon Clinical Multiaxial Inventory- III (MCMI-III). Participants (N = 89) were randomly assigned to three groups: a Feedback group, a Reflective-Counseling group, and a No-Feedback group. The Feedback group was provided with assessment feedback, the Reflective-Counseling group was asked to comment on the meaning of the taking the MCMI-III, the No- Feedback group received general information about the MCMI-III. Results revealed that assessment feedback, when provided in the form of interpretive interpretation positively affects rapport-building and self-enhancement variables (self-verification and self-discovery). No significant results were found in terms of self-esteem or symptom decrease as a function of feedback. However, a significant decrease in symptoms across groups was found. Results indicate that assessment feedback in the form of interpretive information can be used as a starting point in therapy. Implications of the findings are discussed with respect to theory and clinical practice.

The majority of failures of mechanical components are caused by fatigue. Unlike many conventional engineering components, implants in the body are subjected to complex multidirectional loading patterns, thus fatigue not only occurs under axial, fully or partly reversed loading, but also under torsional loading. The fatigue behaviours of self-reinforced poly lactide composite (PLA) of unidirectional PLA fibres in PLA matrix containing tricalcium phosphates (TCP), (PLA-PLA-TCP) produced via pre-pregging technique has been investigated. Quasi-static test results indicated that PLA-PLA-TCP is stronger in tension than in compression and torsion and is significantly influence by moulding temperature. Uniaxial fatigue testing at 37° C in saline solution established S-N (Wöhler) curves for both axial and torsional loading for two moulding temperatures (140°C and 150°C). Compression loading showed significant effect on the axial fatigue behaviour. Biaxial fatigue results showed that the addition of torsion to axial loading significantly reduced the fatigue life. Out-of-phase loading was less detrimental to the fatigue life than in-phase. Fatigue development was evaluated by reduction in secant modulus and increase in energy absorbed. The threshold number of cycles at which damage starts to accumulate in the composite was found to be load ratio and direction dependent. The effects of the degradation process in saline solution on the fatigue behaviour of the composite were also studied at 25% of the ultimate tensile and shear stresses. The specimens were immersed for 8, 12, 16 and 20 weeks periods before testing. The results indicated that even though immersion in saline solution reduced the static and cyclic properties of the composite, it still had good strength retention and comparable to the cortical bone at the end of 20 weeks of degradation period. Microscopy examination on the fracture surface indicated that in uniaxial tension-compression fatigue, the failure was dominated by compression and failed via microbuckling mechanism. Biaxial fatigue failure was dominated by shear mechanisms with evidence of interfacial failure and fatigue striations.

Les élastomères présentent une diversité d'utilisation et des caractéristiques mécaniques spécifiques (grandes déformations, comportement dissipatif, ...) qui en font une famille de matériaux très utilisés dans l'industrie. Lors de leur fonctionnement, les pièces réelles subissent des sollicitations complexes. Comprendre les phénomènes induits par la fatigue multiaxiale constitue ainsi un enjeu important dans la phase de conception industrielle. Le matériau utilisé au cours de cette étude est un polychloroprène (CR), fourni par la société Hutchinson et présent dans les poulies découpleuses. Celui-ci possède une réponse dissipative en grandes déformations. Son comportement est modélisé à partir de lois de comportements viscohyperélastiques suivant deux approches : une méthode analytique impliquant un calcul simple en un point d'un cylindre et l'autre utilisant un calcul éléments finis implémenté dans ANSYS. Une campagne expérimentale en fatigue multiaxiale est alors réalisée, en traction-torsion afin de tester l'énergie dissipée comme critère de fatigue multiaxial. Celui-ci présente des résultats intéressants. Des diagramme de Haigh ont été établis afin de mettre en évidence le phénomène de cristallisation. Des analyses post-mortem ont été menés avec un microscope électronique à balayage et expose des spécificités morphologiques liées à la sollicitation vue par le matériau.