The high temperature fatigue crack growth behaviour of the newly developed, metastable titanium-based alloy, TIMETAL 21S, was investigated in an inert and an oxidizing environment. The investigation adopted a two pronged approached, namely, to initially establish the pure microstructural behaviour under oxidizing and inert environments at various elevated temperatures, and consequently, to establish the environmental effects on the fatigue crack growth behaviour in the various environments at high temperature. The effect of the oxidizing environment on the metastable alloy and on the mechanical and chemical events occurring at the fatigue crack were studied by using optical and scanning electron microscopy, including ED X analysis, x-ray diffraction, and Auger Electron Spectroscopy (AES) . For the microstructural investigation, the TIMETAL 21S samples were exposed for 5 hours to a pure argon and argon + 20% O2 environment at 300°C to 750°C in increments of 50°C. The results showed that in the oxidizing environment a more homogeneous nucleation of the alpha phase had occurred at higher temperatures and that the oxide Ti02, in addition to the alpha case, had predominantly formed on the exposed surfaces. AES analysis showed that dissolution of the oxygen into the alloy occurred even at low temperatures. An LEFM approach was used to investigate fatigue crack growth rate (FCGR) of C(T) specimens at 375°C, 450°C, 550°C and 620°C in the argon and argon + 20% oxygen environment. The crack growth rates were monitored using load-line compliance and the beachmarking method - a method by which beach marks were impressed on the fracture surface to track the progressing crack. The results showed that the crack growth rates were lower in the oxidizing environment and was influenced by a synergistic effect of the temperature, stress intensity at the crack tip and the environment. In addition to the phenomena of crack tip shielding (a process whereby the effective crack tip driving force experienced at the crack tip was locally reduced), other mechanisms such as slip character modification and secondary cracking ahead of the crack tip, leading to crack tip blunting and branching, had to be incorporated to fully explain the crack growth behaviour. The tests conducted in the inert environment effectively excluded the effect of oxygen on the crack growth behaviour and substantiated that various mechanisms ultimately determined the FCGR in TIMETAL 21S at elevated temperatures. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 1995.

No description available.

Thesis (Ph. D.)--Technische Universiteit Delft, 1997. / Cover title. Summary in Dutch. Includes bibliographical references.

Thesis (Ph. D.)--Technische Universiteit Delft, 1997. / Cover title. Summary in Dutch. Includes bibliographical references.

No description available.

Dynamic loading of polycrystalline metallic materials can result in brittle or ductile fracture depending on the loading rates, geometry, and material type. At high strain rates, mechanical energy due to plastic deformation may lead to significant temperature rise and shear localization due to thermal softening. These shear bands reduce the stress-bearing capacity of the material and act as a precursor to ductile fracture (e.g. cracks that develop rapidly on top of a shear band). Reliable models are needed to predict the response of metals subject to dynamic loads. Understanding the heat transfer physics in thermo-mechanical problems when cracks are developed is of great importance. In particular, capturing the interplay between heat conduction and crack propagation is still an open research field. To accurately capture the heat transfer physics across crack surfaces, damage models degrading thermal-conductivity are necessary. In this thesis, a novel set of isotropic thermal-conductivity degradation functions is derived based on a micro-mechanics void extension model of Laplace's equation. The key idea is to employ an analytical homogenization process to find the effective thermal-conductivity of an equivalent sphere with an expanding spherical void. The closed-form solution is obtained by minimization of the flux differences at the outer surfaces of the two problems, which can be achieved using the analytical solution of Laplace's equations, so-called spherical-harmonics. Additionally, a new anisotropic approach is proposed in which thermal-conductivity, which depends on the phase-field gradient, is degraded solely across the crack. We show that this approach improves the near-field approximation of temperature and heat flux compared with isotropic degradation when taking the discontinuous crack solutions as reference. To demonstrate the viability of the proposed (isotropic and anisotropic) approaches, a unified model, which accounts for the simultaneous formation of shear bands and cracks, is used as a numerical tool. In this model, the phase-field method is used to model crack initiation and propagation and is coupled to a temperature-dependent visco-plastic model that captures shear bands. Benchmark problems are presented to show the necessity of the anisotropic thermal-conductivity approach using physics-based degradation functions in dynamic fracture problems. On the other hand, the computational burden in dynamic fracture problems with localized solution features is highly demanding. Iterative methods used for their analysis often require special treatment to be more efficient. Specifically, the nonlinear thermomechanical problems we study in this thesis lead to strain localizations, such as shear bands and/or cracks, and iterative solvers may have difficult time converging. To address this issue, we develop a novel updating domain decomposition preconditioner for parallel solution of dynamic fracture problems. The domain decomposition method is based on the Additive Schwarz Method (ASM). The key idea is to decompose the computational domain into two subdomains, a localized subdomain that includes all localized features of the solution and a healthy subdomain for the remaining part of the domain. In this way, one can apply different solvers in each subdomain, i.e. focus more effort in the localized subdomain. In this work, an LU solver is applied in both subdomains, however, while the localized subdomain is solved exactly at every nonlinear iteration, the healthy subdomain LU operator is reused and only selectively updated. Hence, significant CPU time savings associated with the setup of the preconditioner can be achieved. In particular, we propose a strategy for updating the preconditioner in the healthy subdomain. The strategy is based on an idealized performance-based optimization procedure that takes into account machine on-the-fly execution time. Three dynamic fracture problems corresponding to different failure modes are investigated. Excellent performance of the proposed updating preconditioner is reported in serial and parallel simulations.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING. / Vita. / Includes bibliographical references. / by Hyo-Chol Sin. / Ph.D.

Many experiments demonstrate that the effects of hydrogen solutes decrease macroscopic fracture stresses and strains in ductile materials. Hydrogen-related failures have occurred in nearly all industries involving hydrogen-producing environments. The financial losses incurred from those failures reaches millions if not billions of dollars annually. With the ever-urgent needs for alternative energy sources, there is a strong push for a hydrogen economy from government and private sectors. Safe storage and transportation of hydrogen increases the momentum for studying hydrogen-related failures, especially in ductile materials. To quantify ductile material damage with the effects of hydrogen embrittlement, it is necessary to add hydrogen effects into the void nucleation, void growth, and void coalescence equations. In this research, hydrogen-enhanced void nucleation is our focus, with hydrogen-enhanced void growth and void coalescence t be studied in the future. Molecular Dynamic (MD) and Monte Carlo (MC) simulations with Embedded Atom Method (EAM) potentials were performed to study how hydrogen affects dislocation nucleation, dislocation structure formation and nanovoid nucleation at nickel grain boundaries. The results were inserted into the continuum void nucleation model by Horstemeyer and Gokhale, and the relationships between stress triaxiality-driven void nucleation, grain boundary hydrogen concentrations and local grain geometries were extracted. MD and MC simulations with EAM potentials were also performed to study how hydrogen interstitials affect the dislocation nucleation, dislocation structure formation and subsequent anovoid nucleation of single crystal nickel in different hydrogen-charging conditions. Evolutions of dislocation structures of nickel single crystal with different hydrogen concentrations were compared. The effects of nanovoid nucleation stress and strain at different hydrogen concentrations were quantified. The results were also inserted into the Horstemeyer and Gokhale model and the relationship between stress triaxiality-driven void nucleation and hydrogen concentration caused by stress gradient, which showed similar trends as the grain boundary studies. From nanoscale studies and existing experimental observations, a continuum void nucleation model with hydrogen effects was proposed and used in a continuum damage model based upon Bammann and coworkers. The damage model was implemented into user material code in FEA code ABAQUS. Finite element analyses were performed and the results were compared to the experimental data by Kwon and Asaro.

Fatigue-crack propagation and residual static-strength data on PH15-7Mo (TH 1050) stainless steel are presented in this thesis. In addition, the capability of McEvily and Illg's crack-growth analysis and Kuhn and Figge's residual strength analysis to correlate the test data has been investigated. Axial-load fatigue-crack propagation (at R = 0 and -1) and residual static-strength tests were conducted, on 2-inch-wide sheet specimens made of PH15-7Mo (TH 1050) stainless steel. Analysis of the data showed that as individual analysis methods both analyses satisfactorily correlated the majority of the test data. However, the material constants derived in the two analyses differed significantly. This difference was attributed to the different amounts of work-hardening which occurs in the material prior to failure in the two cases. The effects of the different stress ratios on fatigue-crack growth were studied. In addition, the capability of the residual-strength analysis to predict the effects of changing buckling restraint in the vicinity of the crack.and of changing specimen width were investigated. / Master of Science

Em uma abordagem realística de estruturas e componentes mecânicos admite-se a existência de trincas. A presença destas, geralmente, está associada ao fenômeno da fadiga. Existem diversos modelos matemáticos que descrevem a propagação de uma trinca. De forma geral, os modelos de propagação de trinca são classificados pelo tipo de carregamento, que pode ter amplitude de tensão constante (CATC) ou amplitude de tensão variável (CATV). Neste trabalho foi utilizado o modelo do tipo CATC proposto por Collipriest. Para muitas aplicações de engenharia, até um certo momento, não se faz necessário uma grande acurácia nas previsões do comportamento das estatísticas, a cerca da evolução de uma trinca, mas uma previsão confiável, dentro de certos limites, desse comportamento. Este trabalho apresenta resultados teóricos, que consistem em obter cotas, inferiores e superiores, que “envelopam” os estimadores dos momentos estatísticos de primeira e de segunda ordem da função “tamanho de trinca” baseadas no método “Fast Crack Bounds”. Essas cotas são polinômios, definidos na variável número de ciclos, que consideram as incertezas nos parâmetros que descrevem os modelos de propagação de trinca. O método de simulação de Monte Carlo foi utilizado para obter as realizações da função tamanho de trinca a partir de um conjunto de 10.000 amostras randômicas dos parâmetros característicos do modelo de Collipriest. Essas realizações foram utilizadas para obter os estimadores dos momentos estatísticos do tamanho de trinca. A eficiência das cotas para os estimadores dos momentos estatísticos do tamanho de trinca é avaliada através de funções “desvio relativo” entre as cotas e as soluções numéricas aproximadas do problema de valor inicial (PVI) que descreve o modelo de Collipriest. Em geral, a solução dos PVI que descrevem os modelos de propagação de trincas é obtida através do uso de métodos numéricos, como o método de Runge-Kutta de quarta ordem explícito (RK4). Neste trabalho foi utilizado o software MATLAB para obter as soluções do PVI que descreve o modelo de Collipriest, avaliar o tempo computacional da metodologia proposta, além dos desvios das cotas em relação às soluções aproximadas, confirmando sua eficiência. / In a realistic approach of structures and mechanical components, cracks are admitted. Their presence is usually associated with the fatigue phenomenon. There are several mathematical models that describe the propagation of a crack. In general, the crack propagation models are classified by the type of load, which can have constant stress amplitude (CSA) or variable stress amplitude (VSA). In this work, the CSA type model proposed by Collipriest was used. For many engineering applications, until a certain point, it is not necessary to have great accuracy in predictions of the behavior of statistics, about the evolution of a crack, but a reliable prediction, within certain limits, of this behavior. This work presents theoretical results, which consist of obtaining lower and upper bounds that "envelop" the estimators of the first and second order statistical moments of the crack size function based on the Fast Crack Bounds method. These bounds are polynomials defined in the variable “number of cycles” that consider the Metais - Fadiga uncertainties in the parameters that describe the crack propagation models. The efficiency of the bounds for the statistical moments of crack size is evaluated through the deviation between the bounds and the approximate numerical solutions of the initial value problems (IVP) that describes the Collipriest model. In general, the solution of the IVPs describing crack propagation models is obtained through the use of numerical methods, such as the explicit fourth order Runge-Kutta method (RK4). In this work, the MATLAB software was used to obtain the solutions of the IVP that describes the Collipriest model, to evaluate the computational time of the proposed methodology, besides the deviations of the bounds in relation to the approximated solutions, confirming its efficiency.