No description available.

From its discovery, crack closure was recognized as a key aspect in understanding the fatigue crack growth process. Considering the condition of plane stress, a vast amount of research has been conducted experimentally, analytically, and numerically to understand the complex process of fatigue crack growth and crack closure. Nonzero crack opening stress values are routinely observed, and it seems that there is a general agreement regarding the incidence of the phenomenon under plane stress. However, investigations regarding crack closure under plane strain conditions are less abundant. Moreover, the existence of crack closure under the plane strain state of the stress has been questioned. The importance of accurate measurements of closure to predict adequately fatigue crack growth rates should not be underestimated. Models employed to predict fatigue crack growth rates rely on plasticity-induced crack closure concepts, and the validity of plasticity-induced crack closure depends on crack closure measurements. Crack closure measurements can be performed with Elber’s Method, the ASTM standard(Compliance offset method), or it may be done alternatively by the compliance ratio (CR) or the adjusted compliance ratio method (ACR). In this research, a small scale yielding two-parameter modified boundary layer analysis is performed to study the occurrence of plasticity-induced fatigue crack closure under constant amplitude loading and plane strain conditions. A wide range of T-stresses and KI levels are considered in the finite element analysis with the purpose of exploring the behavior of the crack opening stress. Crack closure was observed for some values of T-stress. Other values of T-stress resulted in an absence of closure under steady state conditions. In addition, an elastic-plastic finite element model was used to simulate a growing fatigue crack with WARP3D software. The computed displacements were used to determine the effective stress intensity factor range ΔKeff with the ASTM standard compliance offset approach, the (CR) method, and the (ACR) method. Finally, measurement location effects on ACR and the ability of ACR method to remove residuals stresses were investigated.

No description available.

No / This paper presents the details of a methodology for predicting the thermoelastic properties degradation in general symmetric laminates with uniform ply cracks in some or all of the 90° layers. First, a stress transfer method is derived by using the concept of state space equation. The laminate can be subjected to any combination of in-plane biaxial and shear loading, and the uniform thermal loading is also taken into account. The method takes into account all independent material constants and guarantees continuous fields of all interlaminar stresses across interfaces between material layers. By this method, a laminate may be composed of an arbitrary number of monoclinic layers and each layer may have different material property and thickness. Second, the concept of the effective thermoelastic properties of a cracked laminate is introduced. Based on the numerical solutions of specially designed loading cases, the effective thermoelastic constants of a cracked laminate can be obtained. Finally, the applications of the methodology are shown by numerical examples and compared with numerical results from other models and experiment data in the literature. It is found that the theory provides good predictions of the thermoelastic properties degradation in general symmetric laminates.

O polietileno de média densidade (PEMD) é um polímero termoplástico parcialmente cristalino, cujo uso tem crescido bastante em aplicações de engenharia, como em tubos plásticos para sistemas de distribuição de água e de gás. Sob carga constante, entretanto, este material pode eventualmente apresentar fratura por fluência, por meio de um mecanismo de crescimento lento de trincas, provocando acentuadas perdas por vazamento nos sistemas. Os métodos atuais empregados pela indústria para estimar a durabilidade dos tubos são caros, demorados, pouco práticos e imprecisos. Busca-se o desenvolvimento de técnicas mais eficientes, sendo que os métodos da mecânica da fratura são promissores no sentido de descrever a etapa de propagação da trinca. Ensaios de mecânica da fratura, entretanto, requerem a introdução de pré-trincas nos corpos de prova. Sabe-se que as técnicas artificiais de introdução de pré-trincas causam algum tipo de dano na matriz polimérica, que pode ou não afetar os resultados dos ensaios, dependendo dos eventos que ocorrerem após o carregamento inicial. A principal propriedade requerida de um método de introdução de pré-trinca, portanto, é reprodutibilidade. Neste estudo foram aplicados três importantes métodos de mecânica da fratura ao PEMD e investigados os efeitos de diferentes técnicas de introdução de pré-trincas sobre os resultados dos ensaios e sobre as estruturas de deformação na matriz do material na ponta da trinca. Os ensaios de tenacidade à fratura no estado plano de deformação - KIc - foram realizados em condições criogênicas, em vista do comportamento dúctil do material à temperatura ambiente, sendo que a estratégia mostrou-se satisfatória. Os resultados dos ensaios de Integral-J sugerem que o método pode não ser aplicável ao PEMD, devido ao peculiar mecanismo de fratura do material. O método do trabalho essencial de fratura - EWF - se aplica bem ao PEMD. Os resultados dos ensaios de KIc e EWF demonstraram que diferentes técnicas de introdução de pré-trincas provocam diferentes estruturas de deformação no material na ponta da trinca e afetam os resultados de ensaios de fratura no PEMD. Os resultados de EWF demonstraram ainda que o processamento também afeta as propriedades de fratura do material. É proposta uma nova técnica para a introdução de pré-trincas, com características mais naturais, a qual requer estudos complementares para seu aperfeiçoamento. / Medium density polyethylene (MDPE) is a semicrystalline thermoplastic polymer that has been increasingly used in engineering applications, as plastic pipes for water and gas distribution systems. Under constant load, however, this material may occasionally present creep failure, by means of a mechanism of slow crack growth, leading to leakage losses in the systems. Current methods used by industry to estimate durability of pipes are expensive, time consuming, non practical and inaccurate. The development of more efficient methods is a common target and fracture mechanics methods are promising in describing the crack propagation stage. Fracture mechanics testing methods, however, require the introduction of pre-cracks into the specimens. It is known that artificial methods of introducing pre-cracks produce some damage on the polymeric matrix, which may or not affect the results of tests, depending on the events that occur after the initial loading. Main propriety required of a pre-crack introducing method, therefore, is reproducibility. In this study three important fracture mechanics methods were applied to MDPE and the effects of different methods of pre-cracking over test results and over deformation structures of material matrix at the crack tip were investigated. Plane-strain fracture toughness - KIc tests were performed under cryogenic conditions, in view of the ductile behavior of material at room temperature and this strategy was well succeed. Results of Integral-J tests suggest that this method may not be applicable to MDPE, due to the peculiar fracture mechanism of the material. The essential work of fracture method EWF is well suited to study MDPE. The results of KIc and EWF tests showed that different pre-cracking methods cause different deformation structures in the material at the crack tip and affect the fracture tests with MDPE. EWF results showed also that the processing affect the fracture properties of materials too. It is proposed a new method for introducing pre-cracks, with more natural characteristics that requires complementary studies for its improving.

O polietileno de média densidade (PEMD) é um polímero termoplástico parcialmente cristalino, cujo uso tem crescido bastante em aplicações de engenharia, como em tubos plásticos para sistemas de distribuição de água e de gás. Sob carga constante, entretanto, este material pode eventualmente apresentar fratura por fluência, por meio de um mecanismo de crescimento lento de trincas, provocando acentuadas perdas por vazamento nos sistemas. Os métodos atuais empregados pela indústria para estimar a durabilidade dos tubos são caros, demorados, pouco práticos e imprecisos. Busca-se o desenvolvimento de técnicas mais eficientes, sendo que os métodos da mecânica da fratura são promissores no sentido de descrever a etapa de propagação da trinca. Ensaios de mecânica da fratura, entretanto, requerem a introdução de pré-trincas nos corpos de prova. Sabe-se que as técnicas artificiais de introdução de pré-trincas causam algum tipo de dano na matriz polimérica, que pode ou não afetar os resultados dos ensaios, dependendo dos eventos que ocorrerem após o carregamento inicial. A principal propriedade requerida de um método de introdução de pré-trinca, portanto, é reprodutibilidade. Neste estudo foram aplicados três importantes métodos de mecânica da fratura ao PEMD e investigados os efeitos de diferentes técnicas de introdução de pré-trincas sobre os resultados dos ensaios e sobre as estruturas de deformação na matriz do material na ponta da trinca. Os ensaios de tenacidade à fratura no estado plano de deformação - KIc - foram realizados em condições criogênicas, em vista do comportamento dúctil do material à temperatura ambiente, sendo que a estratégia mostrou-se satisfatória. Os resultados dos ensaios de Integral-J sugerem que o método pode não ser aplicável ao PEMD, devido ao peculiar mecanismo de fratura do material. O método do trabalho essencial de fratura - EWF - se aplica bem ao PEMD. Os resultados dos ensaios de KIc e EWF demonstraram que diferentes técnicas de introdução de pré-trincas provocam diferentes estruturas de deformação no material na ponta da trinca e afetam os resultados de ensaios de fratura no PEMD. Os resultados de EWF demonstraram ainda que o processamento também afeta as propriedades de fratura do material. É proposta uma nova técnica para a introdução de pré-trincas, com características mais naturais, a qual requer estudos complementares para seu aperfeiçoamento. / Medium density polyethylene (MDPE) is a semicrystalline thermoplastic polymer that has been increasingly used in engineering applications, as plastic pipes for water and gas distribution systems. Under constant load, however, this material may occasionally present creep failure, by means of a mechanism of slow crack growth, leading to leakage losses in the systems. Current methods used by industry to estimate durability of pipes are expensive, time consuming, non practical and inaccurate. The development of more efficient methods is a common target and fracture mechanics methods are promising in describing the crack propagation stage. Fracture mechanics testing methods, however, require the introduction of pre-cracks into the specimens. It is known that artificial methods of introducing pre-cracks produce some damage on the polymeric matrix, which may or not affect the results of tests, depending on the events that occur after the initial loading. Main propriety required of a pre-crack introducing method, therefore, is reproducibility. In this study three important fracture mechanics methods were applied to MDPE and the effects of different methods of pre-cracking over test results and over deformation structures of material matrix at the crack tip were investigated. Plane-strain fracture toughness - KIc tests were performed under cryogenic conditions, in view of the ductile behavior of material at room temperature and this strategy was well succeed. Results of Integral-J tests suggest that this method may not be applicable to MDPE, due to the peculiar fracture mechanism of the material. The essential work of fracture method EWF is well suited to study MDPE. The results of KIc and EWF tests showed that different pre-cracking methods cause different deformation structures in the material at the crack tip and affect the fracture tests with MDPE. EWF results showed also that the processing affect the fracture properties of materials too. It is proposed a new method for introducing pre-cracks, with more natural characteristics that requires complementary studies for its improving.

Un mur de soutènement en pierre sèche (MSPS) est un ouvrage vernaculaire construit par empilement de blocs rocheux sans liant. Ce type de mur a fortement sculpté les campagnes françaises mais a aussi été utilisé par le passé dans des ouvrages de soutènement que ce soit routier ou ferroviaire. Pourtant, l’utilisation de cette technologie a disparu au cours du 20e siècle au profit de solutions industrialisées comme le béton armé. Face à ce patrimoine vieillissant, il est très difficile de proposer des protocoles de réparation alors que la réglementation associée à ces ouvrages est inexistante. Pour les mêmes raisons, alors que cette technologie répond parfaitement aux questions soulevées par le développement durable, elle est rarement envisagée ou retenue par les prescripteurs. La rupture des MSPSs neufs peut apparaître à cause de : (1) une poussée excessive du remblai soutenu par le mur, (2) un effort concentré excessif présent sur la surface du remblai et proche de la tête du mur. Ces deux causes induisent deux types de rupture très différents, respectivement : (1) rupture en déformation plane, (2) rupture par apparition d’un ventre. Trois expériences récentes à l’échelle 1 des MSPSs chargés par : (1) une poussée hydrostatique, (2) un remblai, (3) un remblai avec une force concentrée sur la surface de ce dernier, ont permis d’identifier certains phénomènes mis en jeu dans ces ruptures. Ce travail de thèse contribuera alors au développement de nouveaux outils scientifiques mais aussi à valider des outils existants pour dimensionner les MSPSs. Les expériences réalisées à l’échelle 1 serviront alors de cadre à ce travail pour valider les outils développés. Nous avons choisi d’utiliser une approche aux éléments discrets (MED) où chaque bloc de pierre du mur est modélisé individuellement, respectant ainsi la nature-Même de l’ouvrage réel. Dans un premier temps, une rupture de type déformation plane est modélisée par une approche discrète pure et une approche discrète-Continue. Les atouts et inconvénients de chacune des méthodes sont mis en avant. Une validation quantitative des modèles numériques est faite en comparant la hauteur critique de chargement avec les expériences à l’échelle 1. Ensuite, on étudie la sensibilité de plusieurs paramètres géométriques et mécaniques sur la hauteur critique de chargement, tout comme l’influence de la cohésion du remblai. Enfin, une première modélisation 3D de type qualitative sur la rupture des MSPS par un chargement concentré en surface du remblai est présentée. Les problèmes de modélisation sont soulevés et l’influence de la forme des blocs dans le processus de rupture est étudiée. / A dry-Stone retaining wall (DSRW) is a vernacular structure constructed by stacking the stone blocks without using any binder. In the past, this wall has been extensively used, shaping the French countryside area, built either for road or railroad retaining wall. However, the use of this technology has disappeared during 20th century, due to the emergence of more industrialized materials such as reinforced concrete. Confronted by these ageing heritage structures, it is very difficult to propose a proper reparation procedure since the building codes associated with this structure are nonexistent. For the same reason, though this technology perfectly answers the questions raised by the concept of sustainable development, in practice this structure is rarely considered or used by the engineering advisors. The failure of the newly built DSRWs can be attributed to the following reasons: (1) an excessive pressure of backfill retained by the wall, (2) a presence of an excessive concentrated load on the backfill surface, close to the top part of the wall. These two types of loading lead to two very different types of failure, respectively: (1) plane strain failure, (2) bulging failure. Three recent experimental campaigns of DSRWs loaded with: (1) hydrostatic pressure, (2) backfill, (3) backfill with a concentrated on its surface, provided a better understanding of phenomena involved in these failures. The work presented in this PhD thesis contributes to the development of new scientific tools capable to help design the DSRWs, as well as validating existing tools. The recent full-Scale experimental campaign will serve as a basis to the validation of the developed numerical tools. We have chosen to use a discrete element method (DEM) where each stone block of the wall is modeled individually, complying with the nature of real DSRW. In the first place, the plane strain failure is modeled by a purely discrete approach and a discrete-Continuum approach. The advantage and inconvenient of each method will be presented in advance. A quantitative validation of numerical models is provided by comparing the critical height of loading with results derived from the full-Scale experimental campaign. Thereafter, a 3D qualitative model of DSRWs loaded with a concentrated load on the backfill surface is presented. The modeling problem is noted and the influence of the block form in regards of the related failure mechanism is studied.

In the first part of this thesis, the FE program MSC.Marc is applied for coupled thermomechanical simulations of wire-rod rolling. In order to predict material behaviour of an AISI 302 stainless steel at high strain rates generated during wire-rod rolling, a material model based on dislocation density is applied. Then, the evolution of temperature, strain rate and flow stress is predicted in the first four rolling passes of a wire block. In the second part of the thesis, an alternative approach to simulation of shape rolling is evaluated. The approach is applied in order to save the computational time in cases where many shape-rolling passes are to be simulated. The approach is a combination of the slab method and a 2D FEM with a generalized plane-strain formulation. A number of various isothermal shape-rolling passes are simulated applying the simplified approach. The simulations are carried out using an in-house 2D FE code implemented in Matlab. The results are compared to fully 3D FE analyses. The comparison shows that the simplified approach can predict roll forces and roll torques with a fair accuracy, but the predicted area reductions are a bit underestimated. The reasons for the deviations between the simplified approach and the 3D FEM are discussed. / QC 20101123

No description available.

The Two-Parameter-Fracture-Criterion (TPFC) was validated using an elastic-plastic two-dimensional (2D) finite-element code, ZIP2D, with the plane-strain-core concept. Fracture simulations were performed on three crack configurations: (1) middle-crack-tension, M(T), (2) single-edge-crack-tension, SE(T), and (3) single-edge crack-bend, SE(B), specimens. They were made of 2014-T6 (TL) aluminum alloy. Fracture test data from Thomas Orange work (NASA) were only available on M(T) specimens (one-half width, w = 1.5 to 6 in.) and they were all tested at cryogenic (-320oF) temperature. All crack configurations were analysed over a very wide range of widths (w = 0.75 to 24 in.) and crack-length-to-width ratios ranged from 0.2 to 0.8. The TPFC was shown to fit the simulated fracture data fairly well (within 6.5%) for all crack configurations for net-section stresses less than the material proportional limit. For M(T) specimens, a simple approximation was shown to work well for net-section stresses greater than the proportional limit. Further study is needed for net-section stresses greater than the proportional limit for the SE(T) and SE(B) specimens.