[pt] Aços ARBL são cada vez utilizados na construção de linhas de dutos (pipelines) devido aos benefícios de pressões de linha mais elevadas, redução de peso do tubo, e, principalmente, diminuição dos riscos de falhas estruturais. Entretanto, tais tubos, em serviço offshore, podem ser fragilizados pela presença de H2S no fluido transportado. Sendo assim, esta pesquisa estudou a influência do hidrogênio no comportamento à fratura do aço API 5L X65 por ensaios de tenacidade à fratura (CTOD e Integral J) no aço com e sem hidrogênio a – 30 Graus Celsius, o que possibilitou uma análise comparativa das duas condições. Pelos parâmetros CTOD e Integral J, se verificou ausência de redução da tenacidade à fratura do aço X65, uma vez que os valores dos parâmetros citados para a condição mais severa de hidrogenação foram similares àqueles encontrados na condição de referência. O efeito degradante provocado pelo hidrogênio foi associado a uma maior propagação de trinca durante os ensaios de tenacidade. Isto indica que o hidrogênio pode causar efeitos contraditórios no comportamento à fratura do material que estão relacionados ao tipo de investigação realizada para análise do comportamento mecânico do material (macroscópica ou microscópica), da microestrutura e às variáveis experimentais adotadas nos ensaios, tais como taxa de deformação, concentração de hidrogênio e nível de tensões. / [en] HSLA steels are increasingly used in the construction of pipelines due to its excellent mechanical properties, resulting in adjustments to higher line pressures, weight reduction of the tube, and mainly risk decrease of structural failure. However, such pipes, when in offshore operation, can be embrittled by the presence of H2S in the transported fluid. So that, this research aimed to study the influence of hydrogen on the fracture behavior of API 5L X65 steel with and without hydrogen at –30 Celsius Degree, which enabled a comparative analysis of the two conditions. Based on the CTOD and J Integral parameters, it was found that there was no reduction in the toughness of the X65 steel, since the CTOD and J related to the most severe hydrogenation conditions were similar to those found in the reference condition. The degrading effect caused by hydrogen was associated with a higher crack propagation during the toughness tests. This indicates that hydrogen can cause contradictory effects on the fracture behavior of the material. The discrepant effects are related to the type of research undertaken to analyze the mechanical behavior of the material (macroscopic or microscopic), microstructure and experimental variables adopted during the tests, such as strain rate, hydrogen concentration, stress levels.

No description available.

No description available.

No description available.

No description available.

The primary objective of this research is to develop and verify a methodology for modeling three dimensional discrete crack growth in concrete and reinforced concrete structures. Two main sources of damage, considered in this work, include the mechanical loading and the chemical interaction. The behavior of concrete is brittle in tension and becomes ductile behavior under compressive loading. At the same time, the chemical interaction triggers a progressive degradation of strength parameters. The main focus in this research is on numerical analysis of localized damage that is associated with formation of macrocracks. The specific form of chemical interaction examined here involves the alkali-silica reaction (ASR). The approach used in this work for describing the propagation of macrocraks is based on the volume averaging technique. This scheme represents a simplified form of strong discontinuity approach (SDA). It incorporates the notion of a ‘characteristic length’, which is defined as the ratio of area of the crack surface to the considered referential volume. It is demonstrated, based on an extensive numerical study, that this approach gives mesh-independent results which are consistent with the experimental evidence. The accuracy of the solutions is virtually the same as that based on SDA and/or the Extended Finite Element Method (XFEM), while the computational effort is significantly smaller. In order to describe the behavior of the fractured zone, a traction velocity discontinuity relation is formulated that is representative of different modes of damage propagation, including crack opening in tensile regime as well as shear band formation under compression. For tracing the discontinuity within domain, crack smoothening algorithm is employed to overcome any numerical instabilities that may occur close to ultimate load of the structure. The general methodology, as outlined above, has been enhanced by incorporating the chemoplasticity framework to describe the damage propagation in concrete affected by chemical interaction, i.e. continuing ASR. The latter is associated with progressive expansion of the silica gel that is coupled with degradation of strength properties. An implicit scheme has been developed, incorporating the return mapping algorithm, for the integration of the governing constitutive relations. The framework has been implemented in Abaqus software to examine the crack propagation pattern in structural elements subjected to continuing ASR. Another major topic addressed in this thesis is the ‘size effect’ phenomenon. The existing experimental studies, conducted primarily on various concrete structures, clearly show that the ultimate strength is strongly affected by the size of the structure. This phenomenon stems primarily from the effect of localized damage that accompanies the structural failure. The quantitative response depends on the geometry of the structure, type of loading and the material properties. The size effect has been investigated here for a number of notched and un-notched concrete beams, of different geometries, subjected to three-point bending. Both mechanical loading and the chemical interaction have been considered. The next topic considered in this study deals with analysis of localized fracture in 3D reinforced concrete structures. Here, a mesoscale approach is employed whereby the material is perceived as a composite medium comprising two constituents, i.e. concrete matrix and steel reinforcement. The response at the macroscale is obtained via a homogenization procedure that incorporates again the volume averaging. The latter incorporates a set of static and kinematic constraints that are representative of the response prior to the onset of fracture. After the formation of macrocracks, a traction-separation law within the fractured zone is modified by incorporating the Timoshenko beam theory in order to assess the stiffness characteristics in the presence of reinforcement. A number of numerical examples are given that examine the crack pattern formation and the associated fracture mechanism in concrete beams at different intensity of reinforcement. The final chapter of this thesis provides an illustrative example of the application of the proposed methodology to the analysis of a large scale structure. The focus here is on the assessment of structural damage in a hydraulic structure subjected to ASR continuing over of period of a few decades. The results, in term of the predicted extent of damage as well as the displacement history at some specific locations, are compared with in-situ monitoring. / Thesis / Doctor of Philosophy (PhD)

Crack propagation studies in nanocrystalline alpha-iron samples with grain sizes ranging from 6 to 12 nm are reported at temperatures ranging from 100K to 600K using atomistic simulations. For all grain sizes, a combination of intragranular and intergranular fracture is observed. Mechanisms such as grain boundary accommodation, grain boundary triple junction, grain nucleation and grain rotation are observed to dictate the plastic deformation energy release. Intergranular fracture is shown to proceed by the coalescence of nanovoids formed at the grain boundaries ahead of the crack. The simulations also show that at an atomistic scale the fracture resistance and plastic deformation energy release mechanisms increase with increasing temperature. Finally a softening of the material occurs with decreasing grain size. The elastic properties are found to decrease and the fracture resistance to increase with decreasing grain size. / Master of Science

This dissertation aims to contribute towards the understanding and modeling of tribological phenomena of contact, friction, and wear in viscoelastic materials with application in rubber compounds. Tribiological properties of rubber compounds are important for many applications such as tires, shoe heels and soles, wiper blades, artificial joints, O-ring seals, and so on. In all these applications, the objective is to maximize the friction coefficient to avoid slipping and reduce the wear rate to improve the life expectancy and performance of the products. The first topic in this study focuses on a novel multiscale contact theory proposed by Persson and explains the advantages of this theory over other classical contact theories. The shortcomings of this theory are also investigated, and three methods are proposed to improve Persson's original contact model by correcting the approximation of deformation in the contact area. The first method is based on the original Greenwood and Williamson (GW) contact theory, which neglects the effect of elastic coupling between asperities. The second method is based on an improved version of GW theory, which considers the elastic coupling effect of asperities in an approximate way. The third method is based on the distribution of local peaks of asperities, which is particularly suitable to determine the fraction of a skewed height profile involved in tribological processes. This method can be implemented within the framework of other proposed methods. Since the height profiles of rough surfaces studied in this dissertation are approximately normally distributed, the second correction method is applied to the original contact model to calculate the real contact area and friction coefficient. The second topic addresses the theoretical model of hysteresis friction in viscoelastic materials. The multiscale temperature rise of the rubber surface due to hysteresis friction is also modeled and the effect of flash temperature on the real contact area and friction coefficient is studied. Since the hysteresis friction is not the only mechanism involved in the rubber friction, a semi-empirical model is added to the hysteresis model to include the contribution of adhesion and other processes on the real contact area. Based on the improved multiscale contact theory, a pressure-dependent friction model is also developed for viscoelastic materials, which is in good agreement with experimental results. The third topic deals with the theory of stationary crack propagation in viscoelastic materials and the effect of crack tip flash temperature on the instability of crack propagation observed in some experimental results in the literature. Initially, a theoretical model is developed to calculate the tearing energy vs crack tip velocity in a Kelvin-Voigt rubber model. Besides, two coupled iterative algorithms are developed to calculate the temperature field around the crack tip in addition to the tearing energy as a function of crack tip velocity. In this model, the effect of crack tip flash temperature on the tearing energy is considered to update the relation between tearing energy vs crack tip velocity, which also affects the flash temperature. A theoretical model is also developed to calculate the contribution of the hysteresis effect to the tearing energy vs crack tip velocity using the dynamic modulus master curve of a rubber compound. Then, the low-frequency fatigue test results are compared with the theoretical predictions and used in the framework of powdery rubber wear theory to calculate the stationary rubber wear rate due to fatigue crack propagation. Moreover, a sliding friction and wear test set-up, with both indoor and outdoor testing capability, is developed to validate the theoretical models. The experimental results confirm that the theoretical model can successfully predict the friction coefficient when there is no trace of thermochemical degradation on the rubber surface. Investigating the wear mechanism of rubber samples on three different surfaces reveals that the contribution of fatigue wear rate is less important than other wear mechanisms such as abrasive wear due to sharp asperities or thermochemical degradation due to a significant rise of temperature on the contact area. Finally, the correlation between friction coefficient and wear rate on different surfaces is studied, and it is found that the relation between friction and wear rate strongly depends on the dominant wear mechanism, which is determined by the surface characteristics, sliding velocity, normal load, and contact flash temperature. / PHD / The objective of this dissertation is to understand and develop models for contact, friction, and wear in rubber-like materials. Friction and wear of rubber-like materials are important in many applications such as tires, shoe heels and soles, wiper blades, artificial joints, O-ring seals, and so on. In all these applications, it is desired to maximize the friction to avoid slipping and reduce the mass loss due to abrasion to improve the life expectancy of the products. The first topic in this dissertation focuses on a novel multiscale contact theory proposed by Persson and different approaches proposed in this work to improve this theory. Then, the real contact area is calculated using an improved version of the contact model. The second topic addresses the theoretical model of rubber friction due to hysteresis energy dissipation and the effect of frictional heating on the real contact area. Since the hysteresis friction is not the only mechanism involved in the rubber friction, a semi-empirical model is also used to include the contribution of adhesion and other processes on the real contact area. Based on the improved contact theory, a pressure-dependent friction model is also developed for rubber-like materials, which is in good agreement with the experimental results. The third topic deals with the theory of stationary crack propagation in rubberlike materials and the effect of crack tip temperature rise on the instability of crack propagation observed in some experimental results in the literature. The low-frequency fatigue test results are compared with the theoretical predictions, and the results are used in the framework of powdery rubber wear theory to calculate the rubber wear rate due to slow crack propagation. A sliding friction and wear test set-up is also developed to validate the theoretical models. The theoretical model of the friction coefficient is successfully validated by experimental results. Investigating the rubber wear on different surfaces reveals that the contribution of fatigue wear rate is less important than the other wear mechanisms. The correlation between friction coefficient and wear rate on different surfaces reveals that relation between friction and wear rate strongly depends on the dominant wear mechanism, which is determined by the surface characteristics, sliding velocity, normal load, and temperature rise on the contact surface.

<p>This dissertation studies the chemo-hygro-mechanical coupling involved in the process of crack propagation encountered both in natural and engineered context. Chemical processes are likely to affect the mechanical properties of geo-materials, resulting in possible weakening effect. The deformation and micro-cracking induced by material weakening in turn enhances the overall mass removal. In this study, several models within both elasticity and plasticity domain are developed for a better understanding of the enhanced crack propagation. A deformational plasticity model based on experimental observations is addressed. Rigid-plasticity models are applied to various boundary conditions. In the chemo-elasticity model, chemical dissolution is assumed to be a function of a comprehensive strain invariant. One-way coupling and two-way coupling models are discussed. In the two-way coupling model, volumetric strain coupling and deviatoric strain coupling are compared. A variety of loading modes are adopted to investigate the chemical enhancement of propagation of a single crack. The behavior of the material is either rigid-plastic, or elastic with the variable of mass removal enters the constitutive equation as a chemical strain. Comparison between the results from two models is presented and discussed.</p> / Dissertation

No presente trabalho foi realizado um estudo comparativo entre as ligas AA7050- T7451 e AA2050-T84, bem como entre as direções L-T e T-L para analisar o comportamento das ligas, quando submetidos à temperatura ambiente e criogênica. Para realizar a análise comparativa entre as ligas e entre as direções, foram utilizados corpos de prova tipo C(T) (Compacto Tension), pré-trincados em fadiga. A análise comportamental das ligas foram avaliadas através dos ensaios de tenacidade à fratura, KIC, Curva KR, fadiga, da/dN. A microestrutura foi caracterizada através da microscopia ótica, microscopia eletrônica de varredura e microscopia eletrônica de transmissão. Foi observado que na direção L-T há uma maior resistência à propagação da trinca em relação à direção T-L, no que diz respeito à Curva KR, tanto para temperatura ambiente quanto para temperatura criogênica. Quando comparou-se as duas ligas na direção L-T em temperatura ambiente, ambas apresentaram um fator de intensidade de tensão similar. Os resultados dos ensaios de fadiga mostraram que a direção T-L é mais sensível à razão de carga. Analisando os resultados entre as direções L-T e T-L, observou-se que as duas ligas apresentaram comportamento anisotrópico. / A comparative study was made of the AA7050-T7451 and AA2050-T84 alloys and of the L-T and T-L directions to analyze the behavior of the alloys when subjected to room and cryogenic temperatures. The comparative analyses of the alloys and directions were performed using fatigue-precracked compact tension (CT) test specimens. The behavior of the alloys was analyzed based on fracture toughness, KIC, KR curve, fatigue, and da/dN tests. Their microstructure was characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy. The KR curve indicated that resistance to crack propagation was higher in the L-T direction than in the T-L direction at both room and cryogenic temperatures. In a comparison of the two alloys in the L-T direction at room temperature, they were found to present a similar stress intensity factor. The results of the fatigue tests demonstrated that the T-L direction is more sensitive to the load ratio. An analysis of the results in the L-T and T-L directions indicated that the two alloys exhibited anisotropic behavior.