Thesis (M. Phil.)--University of Hong Kong, 1996. / Includes bibliographical references (leaf 207-212).

Pesquisas sobre a tenacidade à fratura dos aços ferríticos mostram uma grande dispersão dos resultados quando avaliados na região de transição dúctil-fragil. Esta dispersão é fortemente influenciada pela temperatura de ensaio, dimensões e geometria dos corpos-de-prova e também pelo nível de restrições plástica na ponta da trinca. Tais fatores dificultam a previsão, a partir de valores experimentais, de valores de tenacidade à fratura de aços ferríticos para aplicações em casos práticos. Uma forma de contornar essa dificuldade é através do uso das curvas de referencia, descritas no Código ASME. Estas curvas representam envoltórias inferiores de tenacidade à fratura obtida a partir de ensaios envolvendo diversos tipos de aços. Portanto os valores de tenacidade estabelecidos por essas curvas são, em geral, muito conservadores. Então foi proposta pela ASTM uma nova metodologia, chamada curva Mestra, que trata estatisticamente o comportamento à fratura dos aços ferríticos na região de transição e caracteriza um material em particular, através de um parâmetro denominado Temperatura de Referencia (T0). Nesta pesquisa, avaliou-se a influencia da sensibilidade dos parâmetros: Kmin, m e M envolvidos no procedimento do calculo de um parâmetro denominado temperatura de referencia. (T0). A metodologia da curva mestra foi aplicada através dos dados (experimentos) do euro-teste , para as temperaturas: -91 ºC, -60°C, -40°C e 20°C . Em análises de sensibilidade aos parâmetros com os seguintes valores kmin = 0, 10, 30; m= (3), (3,5), (4,5), (5,0) e M = 40, 50, 60. Os resultados apresentados são compatíveis comprovando a eficiência da metodologia para avaliação da tenacidade à fratura dos aços ferríticos na região de transição. Portanto, nestas condições, podemos afirmar que o conceito da curva mestra foi, realmente validado. / The caracterization of the fracture behavior in the ductile-to-brittle transition for ferritic steels is a hard task because fracture toughness is a strong function of temperature in the transition, with the toughness values increasing rapidly over a relatively narrow temperature baud/range and therefore cannot be easily used to evaluate integrity in structural components because of the uncertainty in how to handle these teste results. To overcome this problem in practice, the ASME code has provided reference curves that give estimates of toughness versus temperature. These curves represent lower envelopes to a large set of fracture toughness experimental data for several heats of steel . Therefore, the fracture toughness values obtained from these curves are in general too conservative. In the last decade, a new methodology, formalized in the ASTM standard method E1921-97, has revolutionized the approach used to caracterize transition fracture behavior and has given a rationale for treating the several factors that influence the scatter in the results. Statistical models are used to define a transition curve, called Master Curve, and the transition temperature of a given material is stated by a parameter called reference temperature (T0). In this research, it was evaluated the sensitivity influence of the parameter: Kmin, m and M, involved in the to measurement procedure. The master curve methodology was applied over the data (experiments) of EuroTest for the following temperatures: - 91°C, -60°C, -40°C and -20°C. A parameters sensitivity analysis was made with following values: Kmin=0, 10, 30; m=(3), (3,5), (4,5), (5,0) and M=40, 50, 60. The results are compatible, which confirm the efficiency of the methodology for evaluating the toughness to ferritic steel fracture in the transition region. Therefore, in these conditions, we can state the master curve concept has indeed been validated.

The effect of in-situ or redistributed stress on solute transport in fractured rocks is one of the major concerns for many subsurface engineering problems. However, it remains poorly understood due to the difficulties in experiments and numerical modeling. The main aim of this thesis is to systematically investigate the influences of stress on solute transport in fractured rocks, at scales of single fractures and fracture networks, respectively. For a single fracture embedded in a porous rock matrix, a closed-form solution was derived for modeling the coupled stress-flow-transport processes without considering damage on the fracture surfaces. Afterwards, a retardation coefficient model was developed to consider the influences of damage of the fracture surfaces during shear processes on the solute sorption. Integrated with particle mechanics models, a numerical procedure was proposed to investigate the effects of gouge generation and microcrack development in the damaged zones of fracture on the solute retardation in single fractures. The results show that fracture aperture changes have a significant influence on the solute concentration distribution and residence time. Under compression, the decreasing matrix porosity can slightly increase the solute concentration. The shear process can increase the solute retardation coefficient by offering more sorption surfaces in the fracture due to gouge generation, microcracking and gouge crushing. To study the stress effects on solute transport in fracture systems, a hybrid approach combing the discrete element method for stress-flow simulations and a particle tracking algorithm for solute transport was developed for two-dimensional irregular discrete fracture network models. Advection, hydrodynamic dispersion and matrix diffusion in single fractures were considered. The particle migration paths were tracked first by following the flowing fluid (advection), and then the hydrodynamic dispersion and matrix diffusion were considered using statistic methods. The numerical results show an important impact of stress on the solute transport, by changing the solute residence time, distribution and travel paths. The equivalent dispersion coefficient is scale dependent in an asymptotic or exponential form without stress applied or under isotropic compression conditions. Matrix diffusion plays a dominant role in solute transport when the hydraulic gradient is small. Outstanding issues and main scientific achievements are also discussed. / QC 20111011

Pesquisas sobre a tenacidade à fratura dos aços ferríticos mostram uma grande dispersão dos resultados quando avaliados na região de transição dúctil-fragil. Esta dispersão é fortemente influenciada pela temperatura de ensaio, dimensões e geometria dos corpos-de-prova e também pelo nível de restrições plástica na ponta da trinca. Tais fatores dificultam a previsão, a partir de valores experimentais, de valores de tenacidade à fratura de aços ferríticos para aplicações em casos práticos. Uma forma de contornar essa dificuldade é através do uso das curvas de referencia, descritas no Código ASME. Estas curvas representam envoltórias inferiores de tenacidade à fratura obtida a partir de ensaios envolvendo diversos tipos de aços. Portanto os valores de tenacidade estabelecidos por essas curvas são, em geral, muito conservadores. Então foi proposta pela ASTM uma nova metodologia, chamada curva Mestra, que trata estatisticamente o comportamento à fratura dos aços ferríticos na região de transição e caracteriza um material em particular, através de um parâmetro denominado Temperatura de Referencia (T0). Nesta pesquisa, avaliou-se a influencia da sensibilidade dos parâmetros: Kmin, m e M envolvidos no procedimento do calculo de um parâmetro denominado temperatura de referencia. (T0). A metodologia da curva mestra foi aplicada através dos dados (experimentos) do euro-teste , para as temperaturas: -91 ºC, -60°C, -40°C e 20°C . Em análises de sensibilidade aos parâmetros com os seguintes valores kmin = 0, 10, 30; m= (3), (3,5), (4,5), (5,0) e M = 40, 50, 60. Os resultados apresentados são compatíveis comprovando a eficiência da metodologia para avaliação da tenacidade à fratura dos aços ferríticos na região de transição. Portanto, nestas condições, podemos afirmar que o conceito da curva mestra foi, realmente validado. / The caracterization of the fracture behavior in the ductile-to-brittle transition for ferritic steels is a hard task because fracture toughness is a strong function of temperature in the transition, with the toughness values increasing rapidly over a relatively narrow temperature baud/range and therefore cannot be easily used to evaluate integrity in structural components because of the uncertainty in how to handle these teste results. To overcome this problem in practice, the ASME code has provided reference curves that give estimates of toughness versus temperature. These curves represent lower envelopes to a large set of fracture toughness experimental data for several heats of steel . Therefore, the fracture toughness values obtained from these curves are in general too conservative. In the last decade, a new methodology, formalized in the ASTM standard method E1921-97, has revolutionized the approach used to caracterize transition fracture behavior and has given a rationale for treating the several factors that influence the scatter in the results. Statistical models are used to define a transition curve, called Master Curve, and the transition temperature of a given material is stated by a parameter called reference temperature (T0). In this research, it was evaluated the sensitivity influence of the parameter: Kmin, m and M, involved in the to measurement procedure. The master curve methodology was applied over the data (experiments) of EuroTest for the following temperatures: - 91°C, -60°C, -40°C and -20°C. A parameters sensitivity analysis was made with following values: Kmin=0, 10, 30; m=(3), (3,5), (4,5), (5,0) and M=40, 50, 60. The results are compatible, which confirm the efficiency of the methodology for evaluating the toughness to ferritic steel fracture in the transition region. Therefore, in these conditions, we can state the master curve concept has indeed been validated.

Among the plurality of numerical methods to simulate fracture and fatigue processes, the phase-field method gained a lot of attention during the past years. The strength of the method is located in its implicit ability to capture crack phenomena like crack initiation, branching, and merging without consulting further criteria. However, it lacks efficiency due to the fine finite element meshes required in order to accurately capture the high gradients of the phase-field as well as of the displacements across a crack by standard LAGRANGian shape functions. Adding enrichment functions (either gained from analytical considerations or obtained numerically) to the ansatz space, as in the XFEM/GFEM (extended/generalized finite element method), can improve the approximation quality significantly. Particularly functions like discontinuous functions, which otherwise can be approximated by LAGRANGian shape functions only on really fine meshes, are reproduced independently of the mesh orientation and size in the XFEM/GFEM. In the scope of this thesis, a novel framework called extended phase-field method (XPFM), based on a standard phase-field formulation for fracture processes in linear elastic fracture mechanics, is presented. The main idea is to introduce a transformed ansatz formulation for the approximation of the phase-field, which depends on second-order LAGRANGian shape functions. It can exactly reproduce, independently of the mesh, the exponential phase-field profile known from analytical considerations. From this transformed ansatz, enrichment functions are derived to improve the approximation quality of the displacements. In contrast to the XFEM, no additional tracking of the crack geometry is necessary because it is given directly by the phase-field. As a result, the XPFM can handle crack propagation simulations on meshes five to ten times coarser in each spatial direction than the original phase-field formulation without loss of accuracy. Thus, a remarkable reduction of the size of the equation systems to be solved is reached, leading to a considerable reduction of the numerical effort, especially for fatigue simulations. / In den vergangenen Jahren hat die Phasenfeldmethode zur Simulation von Riss- und Ermüdungsvorgängen viel Aufmerksamkeit erlangt. Der Vorteil der Methode besteht darin, dass Rissphänomene wie Rissinitiierung, -verzweigung und -vereinigung ohne die Berücksichtigung weiterer Kriterien implizit behandelt werden. Jedoch sind sehr feine Finite-Elemente-Netze notwendig, um die hohen Gradienten des Phasenfeldes und der Verschiebungen akkurat mit LAGRANGEschen Formfunktionen zu approximieren. Anreicherungsfunktionen (entweder basierend auf analytischen Lösungen des Problems oder auf numerischen Betrachtungen), wie sie in der XFEM/GFEM (erweiterte/generalisierte Finite-Elemente-Methode) eingesetzt werden, können die Approximationsqualität erheblich verbessern. Insbesondere Funktionen mit Diskontinuitäten, welche sonst nur auf äußerst feinen Netzen mit LAGRANGEschen Formfunktionen angenähert werden können, können mithilfe der XFEM/GFEM unabhängig von Elementgröße und -orientierung exakt reproduziert werden. Im Rahmen dieser Arbeit wird eine neuartige Methode, die erweitere Phasenfeldmethode (XPFM), vorgestellt. Sie basiert auf einem Standardphasenfeldmodell für Rissvorgänge in linear-elastischen Materialien. Die Grundidee der Methode besteht darin, einen transformierten Ansatz für das Phasenfeld zu konstruieren, welcher auf quadratischen LAGRANGE-Formfunktionen basiert, jedoch das aus der analytischen Lösung folgende exponentielle Phasenfeldprofil exakt reproduzieren kann. Darauf aufbauend werden Anreicherungsfunktionen für eine verbesserte Approximation der Verschiebungen abgeleitet. Im Gegensatz zur XFEM/GFEM wird kein gesonderter Algorithmus zur Verfolgung eines Risses benötigt, da die Rissgeometrie durch das Phasenfeld gegeben ist. Mit der XPFM können Rissprozesse auf fünf- bis zehnmal so groben Netzen (in jeder Raumrichtung) im Vergleich zum ursprünglichen Phasenfeldmodell ohne Verlust der Genauigkeit erfasst werden. Dadurch wird eine bemerkenswerte Reduktion der Größe der zu lösenden Gleichungssysteme und eine spürbare Reduktion des numerischen Aufwands, insbesondere bei der Simulation von Ermüdungsvorgängen, erreicht.