The goal of this research was to extend the investigation into a method to represent and analyze the stress field around two parallel edge cracks in a finite body. The Westergaard-Schwarz method combined with the local collocation method was used to analyze different cases of two parallel edge cracks in a finite body. Using this method a determination of when two parallel edge cracks could be analyzed as isolated single edge cracks was determined Numerical experimentation was conducted using ABAQUS. It was used to obtain the coordinate and stress information required in the local collocation method. The numerical models were created by maintaining one crack at a fixed length while varying the length of the second crack as well as the separation distance of the two cracks. The results obtained through the local collocation method were compared with the finite element obtained J-Integrals to verify the accuracy of the results. The results obtained in the analysis showed that the major factor in determining when the second crack’s stress field has to be considered was the crack separation distance. It was found that a reduction in the second crack’s length did not have a significant effect on overall stress intensity factors of the fixed crack. A larger change in the opening mode stress intensity factor can be seen by varying the crack separation distance. As well as seeing a steady reduction in shear mode stress intensity factors as the crack separation was increased. The results showed that after a certain crack separation distance the two cracks could be analyzed separately without introducing significant error into the stress field calculations.

Compressive residual stresses induced by tensile overloads, compressive under loads, or by a cold-expansion process in specimens containing a circular hole and their influence on subsequent fatigue crack growth in aluminum alloys are studied. The finite element method is used to calculate residual stresses. The superposition method, which uses crack-tip stress intensity factors for cases involving remote loading and residual stresses, is used to calculate crack growth life for three kinds of tests from the literature: (1) fatigue of a circular hole specimen after an overload or under load, (2) single crack growing from a circular hole after a severe tensile overload, and (3) single crack growing from a circular hole after cold-working, reaming and notching. All specimens were subjected to subsequent constant amplitude loading. The superposition method worked fairly well for most cases, but tended to over predict fatigue life for small cracks and for cracks growing under residual stresses, which produce compressive (maximum and minimum) stress intensity factors.

This thesis details an experimental study on the determination of the fracture parameters for a crack located at the interface between two dissimilar materials using the method of photoelasticity. The interface is potential1y an inherent weak spot of any composite material, structure"or adhesively bonded joint. Accurate description of the state of stress at the crack tip is required for strength prediction. The concept of the complex stress intensity factor is used to characterise the elastic crack tip stress field for an interface crack. Complex stress intensity factors and their moduli have been measured experimental1y for standard bimaterial crack geometries using the wel1 established technique of photo elasticity. Bimaterial specimens comprising aluminium al10y and epoxy resin components were used. This creates a large material mismatch at the interface and al10ws data to be col1ected from the epoxy component of the specimen using transmission photoelasticity. An automated ful1 field photoelastic technique was developed to significantly reduce the data col1ection time. The technique comprises elements from the approaches of three wavelength and phase stepping photoelasticity and is a significant improvement on techniques previously available. Stress intensity factors were determined by fitting a theoretical stress field solution for the bimaterial crack to the experimental data. A computational routine automatical1y selects the region of best fit between the experimental data and the theoretical solution. This data is then used to determine the complex stress intensity factor and its modulus value. In order to provide a robust fit between the experimental data and the theoretical field solution a weighting function was incorporated into the routine. The measured bimaterial stress intensity factors are compared with those determined experimental1y for equivalent homogeneous specimens made from epoxy resin. The differences between the two are then discussed. The experimental results agree with the wel1 known concept that tension and shear effects are inherently coupled at the crack tip. However, the effects of changing the load angle with respect to the interface also demonstrate that some contrasts exist with known numerical solutions.

Este trabalho apresenta os principais fenômenos que regem a propagação de trincas em anéis de rolamentos, por meio da aplicação de conceitos da Mecânica da Fratura Elástica Linear e Fadiga. O estado complexo de tensões atuante sobre o anel do rolamento culmina em carregamento modo misto sobre as paredes da trinca. Para o problema de rolamento proposto, são considerados os modos de abertura e de cisalhamento puro no plano do defeito, sob regime cíclico. O fator de intensidade de tensões K e a integral J são parâmetros que estabelecem condições de fratura e são suficientes, para caracterizar a criticidade de trincas instaladas em anéis de rolamentos. A avaliação da criticidade de defeitos é baseada na aplicação de Critérios de Falha distintos, oriundos de literaturas especializadas. A partir dos parâmetros de fratura, K e J , e da avaliação da criticidade de defeitos é possível determinar a vida remanescente à fadiga, por meio de modelos que descrevem a taxa de propagação de trincas. Os resultados indicam que o modo misto de carregamento de trinca considerado na dissertação, é um dos parâmetros mais importantes a considera-se na nucleação e propagação de defeitos em rolamentos, e são responsáveis por uma redução significativa da vida à fadiga originalmente estipulada. / This work presents the main phenomenon that governs the crack propagation in bearing rings, through the application of Linear Elastic Fracture Mechanics and Fatigue concepts. The complex stress state acting on the bearing ring produces a mixed mode loading over the crack faces. The proposed bearing problem considers a tensile mode and pure shear mode in the plane of the defect, both under a cyclic regime. The stress intensity factor K and the J -integral parameters define the fracture conditions and suffice to characterize the severity of cracks developed in bearing rings. The procedure for defect assessment is based on the application of different failure criteria. Using parameters, K and J, coupled to the defect assessment procedures, it is possible to estimate the remaining fatigue life, through models that describe the crack propagation rate. The results indicate that the mixed mode loading considered in this work is one of the most important parameters in nucleation and propagation of bearing defects and are responsible for a significant reduction in the original estimation of fatigue life.

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Este trabalho apresenta os principais fenômenos que regem a propagação de trincas em anéis de rolamentos, por meio da aplicação de conceitos da Mecânica da Fratura Elástica Linear e Fadiga. O estado complexo de tensões atuante sobre o anel do rolamento culmina em carregamento modo misto sobre as paredes da trinca. Para o problema de rolamento proposto, são considerados os modos de abertura e de cisalhamento puro no plano do defeito, sob regime cíclico. O fator de intensidade de tensões K e a integral J são parâmetros que estabelecem condições de fratura e são suficientes, para caracterizar a criticidade de trincas instaladas em anéis de rolamentos. A avaliação da criticidade de defeitos é baseada na aplicação de Critérios de Falha distintos, oriundos de literaturas especializadas. A partir dos parâmetros de fratura, K e J , e da avaliação da criticidade de defeitos é possível determinar a vida remanescente à fadiga, por meio de modelos que descrevem a taxa de propagação de trincas. Os resultados indicam que o modo misto de carregamento de trinca considerado na dissertação, é um dos parâmetros mais importantes a considera-se na nucleação e propagação de defeitos em rolamentos, e são responsáveis por uma redução significativa da vida à fadiga originalmente estipulada. / This work presents the main phenomenon that governs the crack propagation in bearing rings, through the application of Linear Elastic Fracture Mechanics and Fatigue concepts. The complex stress state acting on the bearing ring produces a mixed mode loading over the crack faces. The proposed bearing problem considers a tensile mode and pure shear mode in the plane of the defect, both under a cyclic regime. The stress intensity factor K and the J -integral parameters define the fracture conditions and suffice to characterize the severity of cracks developed in bearing rings. The procedure for defect assessment is based on the application of different failure criteria. Using parameters, K and J, coupled to the defect assessment procedures, it is possible to estimate the remaining fatigue life, through models that describe the crack propagation rate. The results indicate that the mixed mode loading considered in this work is one of the most important parameters in nucleation and propagation of bearing defects and are responsible for a significant reduction in the original estimation of fatigue life.

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Ce travail de thèse s’inscrit dans le cadre des problèmes de fissuration par fatigue, détectéesnotamment dans des structures nucléaires et se situe dans la continuité de travaux déjà réalisésau laboratoire. L’objectif de cette étude est la réalisation d’un outil numérique de prédictiondu phénomène de fermeture induite par plasticité, au cours de la propagation d’une fissure defatigue dans une éprouvette CT, dans un acier inoxydable 304L, en prenant en comptel’influence simultanée de la forme des fronts et de la longueur de fissure. Celle-ci a d’abordété considérée par le biais de modèles numérique tridimensionnels sous ABAQUS, avec desgéométries pré-imposées des fronts de fissure. Les évolutions des facteurs d’intensité effectifs(FIC) locaux le long des fronts et au cours de la propagation ont été comparées. Ensuite, unoutil numérique, utilisant le code ABAQUS et le langage de programmation PYTHON a étédéveloppé pour la prédiction automatique de la forme de la fissure en fonction des donnéesd’entrée (géométrie, charge, conditions aux limites, définition du contact au cours de lapropagation, maillage), à partir d’un entaille droite de longueur égale à 0.1mm. La variationeffective du FIC ΔK l eff local a été supposée étant la force motrice de la propagation. Lesessais de fatigue ciblés réalisés ont permis une comparaison critique avec le numérique, enterme de formes finales du front de fissure dans les différentes conditions de chargementimposées. / This PhD thesis deals with the problems of fatigue cracking, particularly detected in nuclearstructures, and is a continuation of work already carried out in the laboratory. The objective ofthis study is to provide a numerical prediction tool of the phenomenon of plasticity-inducedcrack closure, during the propagation of a fatigue crack in a CT specimen and in a 304Lstainless steel, taking into account the simultaneous influences of the crack shape and cracklength. This has been first considered through three-dimensional numerical models withABAQUS, through pre-imposed crack fronts geometries. The local stress intensity factors(FIC) evolutions along the crack fronts and over the whole propagation have been compared.Then, a numerical tool, using the ABAQUS code and the programming language PYTHONhas been developed in order to automatically predict the crack shape evolution, depending ondifferent input data (geometry, loads, boundary conditions, contact definition duringpropagation, mesh), starting from a 0.1mm straight notch. The local effective evolution of theSIF ΔK l eff has been supposed to be the driving force for the whole propagation. Fatiguetargeted tests have been carried out in order to allow a critical comparison with the numericalresults, in terms of final crack front shapes under different imposed loading conditions.

No description available.

A life prediction method is developed for spot-welds subject to fatigue loading. Stress intensity factors are used with the Walker equation to develop two crack growth approaches to the problem. The predictions fit data for lap joint configurations well, but not so for peel joint geometries. / Master of Science