A modelagem adequada da propagação de trincas por fadiga em fuselagens pressurizadas é complexa devido a uma série de fatores, tais como: o efeito geometricamente não-linear conhecido como bulging, o revestimento das fuselagens ser bastante delgado e a presença de reforçadores que alteram a distribuição de tensão na fuselagem. Uma metodologia para simular computacionalmente a propagação de trincas por fadiga em estruturas de casca pressurizadas baseada no método dos elementos finitos e em conceitos da Mecânica do Fraturamento Linear Elástica é apresentada. Como resultado dessa simulação, obtêm-se a trajetória de propagação da trinca e o número de ciclos para ocorrer a propagação. Um estudo paramétrico é realizado para avaliar a influência de diversos parâmetros, tais como: espessura do revestimento, nível de pressurização, comprimento de trinca, área da seção transversal e distância entre reforçadores, entre outros, no fator de bulging em estruturas de cascas pressurizadas com ou sem reforçadores. A metodologia apresentada é validada por meio da simulação de propagação de trinca em uma fuselagem de um Boeing 737, sendo que os resultados obtidos nessa simulação são comparados com resultados experimentais e com os obtidos por POTYONDY (1993). A influência do tamanho do incremento de trinca e do número de nós por elemento (4 ou 9 nós) nos resultados obtidos é investigada. / The modeling of fatigue crack growth in pressurized fuselages is very complex due to many factors, such as the nonlinear stiffening effect referred to as bulging, the fuselage skin being very thin and the presence of stiffeners which modifies the stress distribution on the fuselage. A methodology for the computational modeling of the fatigue crack growth in pressurized shell structures, based on the finite element method and concepts of Linear Elastic Fracture Mechanics, is presented. As a result of this numerical simulation, the crack trajectory and the fatigue propagation life are obtained. A parametric study is performed to investigate the influence of several parameters, such as skin thickness, pressurization level, crack length, structural area of the stiffeners, distance between stiffeners, on the bulging effect for unstiffened and stiffened pressurized shells. The methodology is validated by means of a fatigue crack propagation simulation in a Boeing 737 airplane fuselage. The results of this simulation are compared with experimental results and those obtained by POTYONDY (1993). Additionally, the influence of the crack increment and the number of nodes per element (8 or 9 nodes) on the simulation results are investigated.

O Método dos Elementos Finitos Generalizados (MEFG) tem se mostrado uma ferramenta bastante eficaz para a obtenção de soluções dos problemas da Mecânica da Fratura. O MEFG/MEFX permite que trincas estejam imersas na malha de elementos, o que contribui para a redução do custo computacional. Particularmente, ao se tratar da modelagem do processo de propagação, a combinação do método geométrico denominado level-set com o MEFG/MEFX traz a vantagem de permitir descrever mais claramente o posicionamento e o caminho de propagação da trinca. A previsão das soluções dos problemas de fratura obtidas com o MEFG/MEFX o indicam como ferramenta importante para a simulação de processos que envolvem carregamento repetido e previsão de resposta em fadiga. Nesse contexto, a vida em fadiga é um dos principais fatores determinantes na análise de integridade estrutural. Esta pesquisa tem por objetivo avaliar, mediante simulações numéricas, a representatividade e as dificuldades da aplicação do MEFG/MEFX, combinado ao método level-set, para a estimativa de vida em fadiga de elementos estruturais aeronáuticos. Consideram-se abordagens estática e cinemática, de um modo geral e em particular para as análises em fadiga. A abordagem estática requer precisão na determinação dos fatores de intensidade de tensão. A abordagem cinemática inclui a propagação da trinca. Os exemplos considerados consistem em problemas planos e tridimensionais. As ferramentas empregadas são códigos computacionais para o MEFG/MEFX: MXFEM e ABAQUS. Conclui-se que o MEFG/MEFX pode se constituir em instrumento de grande interesse para a indústria aeronáutica, ao permitir análises de integridade estrutural que viabilizam a definição de um plano de inspeção personalizado para os operadores. Além disso, o uso do MEFG/MEFX no campo de definição e projeto de reparos estruturais também é algo promissor, dadas suas características e vantagens demonstradas neste trabalho. / The Generalized Finite Element Method (GFEM) has proved to be a very effective tool for obtaining solutions to the problems of Fracture Mechanics. The GFEM/XFEM allows cracks to be immersed in a finite the element mesh, which contributes to the reduction of the computational cost. Particularly, regarding modeling the propagation process, the combination of the geometric method called \'level-set\' with the GFEM/XFEM has the advantage of being able to describe more clearly the positioning and propagation path of the crack. The prediction of solutions of the fracture problems obtained with the GFEM/XFEM indicate it as an important tool for the simulation of processes involving repeated loading and prediction of fatigue response. In this context, life in fatigue is one of the main determining factors in the structural integrity analysis. The aim of this research was to evaluate the representativeness and difficulties of the application of the GFEM/XFEM, combined with the \'level-set\' method, to estimate fatigue life of aeronautical structural elements using numerical simulations. Static and kinematic approaches were considered, and in particular for fatigue analysis. The static approach requires precision in the determination of the stress intensity factors. The kinematic approach includes the propagation of the crack. The considered examples consist of plane and three-dimensional problems. The tools used are computational codes for the GFEM/XFEM: MXFEM and ABAQUS. As a conclusion, the GFEM/XFEM can be an instrument of great interest for the aeronautical industry, allowing structural integrity analyzes that allow the definition of a personalized inspection plan for the operators. In addition, the use of GFEM/XFEM in the field of definition and design of structural repairs is also promising, given its characteristics and advantages demonstrated in this work.

[pt] Uma metodologia eficiente e segura é proposta para prever a propagação de trincas de fadiga sob carregamento complexo em estruturas bidimensionais com geometria genérica. Primeiro, o caminho da trinca (em geral curvo) e os fatores de intensidade de tensão KI(a) e KII(a) ao longo do comprimento da trinca a são calculados num programa de elementos finitos especialmente desenvolvido para este fim, o Quebra2D. Estes cálculos são feitos usando pequenos incrementos especificáveis no tamanho da trinca e técnicas de remalhamento automatizadas. Os valores de KI(a) são usados como dados de entrada num programa de previsão de vida à fadiga, o ViDa. Esse programa foi desenvolvido para prever a iniciação e a propagação de trincas 1D e 2D sob carregamento complexo por todos os métodos clássicos, incluindo SN, eN e IIW (estruturas soldadas) para a iniciação da trinca, e o método da/dN para a propagação. Em particular, o módulo que propaga a trinca aceita qualquer expressão de KI(a) e qualquer regra da/dN, e usa o método DKrms ou CCC (crescimento ciclo-a-ciclo) para prever a propagação de trincas uni e bidimensionais sob carregamento complexo. A análise numérica proposta foi verificada através de vários experimentos representativos, cuja metodologia experimental é discutida em detalhes. / [en] A reliable and cost effective two-phase methodology is proposed to predict fatigue crack propagation in generic two-dimensional structural components under complex loading. First, the fatigue crack path and its stress intensity factor are calculated in a specialized finite- element software, using small crack increments. Numerical methods are used to calculate the crack propagation path, based on the computation of the crack incremental direction, and the stress-intensity factors KI, from the finite element response. Then, an analytical expression is adjusted to the calculated KI(a) values, where a is the length along the crack path. This KI(a) expression is used as an input to a powerful general purpose fatigue design software based on the local approach, developed to predict both initiation and propagation fatigue lives under complex loading by all classical design methods, including the SN, the eN and the IIW (for welded structures) to deal with crack initiation, and the da/dN to treat propagation problems. In particular, its crack propagation module accepts any KI expression and any da/dN rule, using the DKrms or the cycle-by-cycle propagation methods to deal with one and twodimensional crack propagation under complex loading. If requested, this latter method may include overload-induced crack retardation effects. This two-phase methodology is experimentally validated by fatigue tests on compact tension and bending single edge notch specimens, modified with holes positioned to attract or to deflect the cracks.

Welding, as one of the most effective joining methods for metals, has been extensively applied in engineering usage for a long time. When cracks occur in the vicinity of weldments, weld repairs are frequently considered for crack repair to extend service life. In order to evaluate to what extent the weld repair has improved the fatigue life of a cracked welded structure, it is necessary to be able to determine the residual life of the cracked welded joint, as well as the life of the weld repaired joint. Both these assessments require that the fatigue crack growth data be available. The determination of crack propagation rates of welded and weld repaired structures is thus of paramount importance to implement a damage tolerant approach to structural life extension. However, since most studies on welded joints so far have concentrated on fatigue life evaluation, at the present time only limited information is available on crack propagation rates in welded joints, and virtually none on fatigue behaviour and crack propagation in weld repaired joints. This thesis has focused on examination of fatigue and crack propagation behaviour in as welded and weld repaired aluminium alloy 5083, a weldable marine grade alloy extensively used in construction of high speed ferries and aerospace structures. Crack growth rates were measured during constant amplitude fatigue testing on unwelded, as-welded and weld repaired specimens of 5083-H321 aluminium alloy. A 3-D finite element analysis was conducted to determine the stress intensity factors for different lengths of crack taking into account the three-dimensional nature of the weld profile. The effects of crack closure due to weld residual stresses were evaluated by taking measurements of the crack opening displacements and utilised to determine the effective stress intensity factors for each condition. Metallurgical examinations and fractography of the fracture surface were conducted using an optical microscope and SEM. It was found that crack growth rates in welded plates are of the same order of magnitude as those of parent material when effective stress intensity factors were applied. However weld repaired plates exhibit higher crack growth rates compared to those of unwelded and once-only welded plates.

The objective of this work was to use LEFM in order to assess the detrimental influence of surrounding chloride-containing environments for stainless steels, hardened steel as well as for a cast aluminium alloy. An additional aim was also to use LEFM to assess the influence of porosity on the fatigue properties for different commercial cast aluminium alloys and manufacturing methods. The environmental influence on fatigue performance was mainly evaluated from fatigue crack growth measurements using compact tension (CT) specimens. In addition, fatigue performance in the high cycle regime was studied using spot welded specimens and smooth specimens. Corrosion fatigue tests for stainless steels were performed in different chloride-containing aqueous solutions and compared to the behaviour in air. Variables, which have been investigated, included temperature, redox potential and fatigue test frequency. The environmental influence on fatigue performance has also been compared to localised corrosion properties. Fatigue crack propagation rates were found to be higher in 3% NaCl than in air for all stainless steels investigated. The highest alloyed austenitic steel, 654SMO, showed the least influence of the environment. For duplex stainless steels the environment enhanced fatigue crack propagation rate to a higher degree than for austenitic stainless steels. This is explained by a material-dependent corrosion fatigue mechanism. In the high cycle regime, fatigue properties for spot welded stainless steels specimens were found to be decreased between 30%-40% due to the presence of 3% NaCl. For the hardened steel 100CrMnMo8 a fracture mechanics approach was employed for prediction of corrosion fatigue properties. In this model corrosion pit growth rate and the threshold stress intensity factor for fatigue crack propagation are needed as input parameters. For the high pressure die cast aluminium alloy the environmental influence of fatigue initiation through pre-exposure of smooth specimens was studied. Depending on environment used for pre-exposure, fatigue strength was found to be reduced by up to 50 % compared to the fatigue strength in air. Fatigue strength reduction was clearly associated to corrosion pits in the aluminium material. A fracture mechanics model was further successfully used to predict the environmental influence. The influence of porosity on the fatigue strength for the cast aluminium alloys tested has been described by a Kitagawa diagram. In design, the Kitagawa diagram can be used to predict the largest allowable pore size if the load situation in the component is known. The size of the porosity could either be evaluated directly from x-ray images or from metallographic prepared cross-sections using a method of extreme value analysis / QC 20100907

No-flow underfill materials that cure during the solder reflow process is a relatively new technology. Although there are several advantages in terms of cost, time and processing ease, there are several reliability challenges associated with no-flow underfills. When micron-sized filler particles are introduced in no-flow underfills to enhance the solder bump reliability, such filler particles could prevent the solder bumps making reliable electrical contacts with the substrate pads during solder reflow, and therefore, the assembly yield would be adversely affected. The use of nano-sized filler particles can potentially improve assembly yield while offering the advantages associated with filled underfill materials. The objective of this thesis is to study the thermo-mechanical reliability of nano-filled epoxy underfills (NFU) through experiments and theoretical modeling. In this work, the thermo-mechanical properties of NFUs with 20-nm filler particles have been measured. An innovative residual stress test method has been developed to measure the interfacial fracture toughness. Using the developed residual stress method and the single-leg bending test, the mode-mixity-dependent fracture toughness for NFU-SiN interface has been determined. In addition to such monotonic interfacial fracture characterization, the interface crack propagation under thermo-mechanical fatigue loading has been experimentally characterized, and a model for fatigue interface crack propagation has been developed. A test vehicle comprising of several flip chips was assembled using the NFU material and the reliability of the flip-chip assemblies was assessed under thermal shock cycles between -40oC and 125oC. The NFU-SiN interfacial delamination propagation and the solder bump reliability were monitored. In parallel, numerical models were developed to study the interfacial delamination propagation in the flip chip assembly using conventional interfacial fracture mechanics as well as cohesive zone modeling. Predictions for interfacial delamination propagation using the two approaches have been compared. Based on the theoretical models and the experimental data, guidelines for design of NFUs against interfacial delamination have been developed.

In the present study, the applicability of the hybrid plasma arc welding, in which a keyhole is responsible of deep penetration and a filler wire electrode supplies a high deposition rate, was examined. The microstructural evolutions in grade 2205 duplex stainless steel plates joined by keyhole and melt-in techniques were investigated. The specimens obtained from welded plates having thickness of 8 mm were examined via optical and scanning electron microscopy. Metallographic investigations were supported by X-ray diffraction and energy dispersed spectra analyses by characterizing the phases formed after welding. Impact toughness properties, hardness profiles, and crack propagation behavior of welding zones were quantitatively and qualitatively compared for mechanical characterization. Fracture characteristics were determined via scanning electron microscopy examinations. It was observed that single-pass HPA weldment seemed to be free of secondary austenite precipitation in acicular form, which is inevitable in multi-pass conventional arc welding methods. Besides &delta / -ferrite was successfully kept under 70%, which is presented as a limit to not to deteriorate the mechanical properties of DSS. High linear welding speed and high power density supplied by HPAW presented narrower weld metal and heat affected zone with not only lower hardness but also higher impact toughness energies. Synergic effect of the keyhole formed by a plasma arc and the metal transfer supplied by gas metal arc gave reasonable dilution in the weld metal. Furthermore, fatigue crack growth tests revealed that crack propagation rates in HPAW joints were comparable to GMAW joints.

Better understanding and control of crack growth direction during hydraulic fracturing are essential for enhancing productivity of geothermal and petroleum reservoirs. Structural analysis of fracture propagation and impact on fluid flow is a challenging issue because of the complexity of rock properties and physical aspects of rock failure and fracture growth. Realistic interpretation of the complex interactions between rock deformation, fluid flow, heat transfer, and fracture propagation induced by fluid injection is important for fracture network design. In this work, numerical models are developed to simulate rock failure and hydraulic fracture propagation. The influences of rock deformation, fluid flow, and heat transfer on fracturing processes are studied using a coupled thermo-hydro-mechanical (THM) analysis. The models are used to simulate microscopic and macroscopic fracture behaviors of laboratory-scale uniaxial and triaxial experiments on rock using an elastic/brittle damage model considering a stochastic heterogeneity distribution. The constitutive modeling by the energy release rate-based damage evolution allows characterizing brittle rock failure and strength degradation. This approach is then used to simulate the sequential process of heterogeneous rock failures from the initiation of microcracks to the growth of macrocracks. The hydraulic fracturing path, especially for fractures emanating from inclined wellbores and closed natural fractures, often involves mixed mode fracture propagation. Especially, when the fracture is inclined in a 3D stress field, the propagation cannot be modeled using 2D fracture models. Hence, 2D/3D mixed-modes fracture growth from an initially embedded circular crack is studied using the damage mechanics approach implemented in a finite element method. As a practical problem, hydraulic fracturing stimulation often involves fluid pressure change caused by injected fracturing fluid, fluid leakoff, and fracture propagation with brittle rock behavior and stress heterogeneities. In this dissertation, hydraulic fracture propagation is simulated using a coupled fluid flow/diffusion and rock deformation analysis. Later THM analysis is also carried out. The hydraulic forces in extended fractures are solved using a lubrication equation. Using a new moving-boundary element partition methodology (EPM), fracture propagation through heterogeneous media is predicted simply and efficiently. The method allows coupling fluid flow and rock deformation, and fracture propagation using the lubrication equation to solve for the fluid pressure through newly propagating crack paths. Using the proposed model, the 2D/3D hydraulic fracturing simulations are performed to investigate the role of material and rock heterogeneity. Furthermore, in geothermal and petroleum reservoir design, engineers can take advantage of thermal fracturing that occurs when heat transfers between injected flow and the rock matrix to create reservoir permeability. These thermal stresses are calculated using coupled THM analysis and their influence on crack propagation during reservoir stimulation are investigated using damage mechanics and thermal loading algorithms for newly fractured surfaces.

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