Structural concrete is the most widely used material in the construction of bridges, pave-ments, runways, dams and other infrastructures which are subjected to uctuating loads during its service period. Concrete contains internal aws in the form of micro-cracks as an inherent property. When subjected to fatigue loading, distributed micro-cracks are formed at the sites of pre-existing aws, which subsequently, localize to form a major crack and propagates. The crack growth curve of a structural component when subjected to fatigue loading depicts a sigmoidal pattern. This curve is divided into three distinct regions namely sub-threshold crack propagation (short crack), stable crack propagation (long crack) and unstable crack propagation depending on the crack propagation rate. Most of the fatigue life is spent in the sub-critical stage (small crack) before the for-mation of long cracks. Hence, from the view of estimating the fatigue life, the crack initiation and early crack propagation (short crack stage) phase are the most important and correct concepts need to be developed. Hence, in this work, the behavior of propa-gation and non-propagationof short cracks in concrete when subjected to fatigue loading is addressed. Small non-propagating cracks are usually found at notch roots when the nominal stress range is below certain limits that depend on the notch sensitivity. Analysis is performed on geometrically similar three-point bend beams of three di erent sizes and subjected to fatigue loading in order to determine the important factors that a ect the notch sensitivity and to determine the minimum stress range required for the initiation and propagation of short cracks. A criterion for crack initiation and propagation is proposed based on linear elastic fracture mechanics. Using this criterion, the maximum length of non-propagating crack that can be formed from fatigue loading alone and the minimum stress range required to propagate a crack without arrest are computed. It is observed that the notch sensitivity increases with increase in beam size, decrease in notch-tip aspect ratio and increase in the fatigue limit of the material. Since the probability of formation of a non-propagating crack at a notch tip decreases with increase in notch sensitivity, and since it is desirable not to have a non-propagating crack in experimental investigations, it is essential to design a specimen with higher notch sensitivity. A crack spends a considerable amount of time in the short crack regime. The short cracks are found to propagate at higher rates than the long cracks at the same nominal stress intensity factor which is known as the short crack anomaly. It is important to consider this anomaly in the prediction of the residual life of damaged concrete structures. Hence, in the present work, an analytical model is developed using the principles of dimensional analysis and self-similarity in order to estimate the rate of short crack growth in concrete. The important parameters such as load range, threshold value of stress intensity factor range, modulus of elasticity, tensile strength, fracture energy, stress ratio, crack size and the maximum aggregate size are considered in the development of the short crack growth model. The model is calibrated and validated using the experimental results that are available in the literature. A probabilistic analysis is carried out to determine the sensitivity of each of the di erent parameters that has been considered on the crack growth rate using the coe cient of variation method. It is found that the crack length is the most sensitive parameter to short crack growth rate followed by the load range. A term called `characteristic fatigue life of short crack' is de ned as the number of fatigue cycles that can be applied such that not more than ve percent of the short cracks is expected to proceed to the long crack regime. Furthermore, the fatigue life of a crack spent in the short crack regime is determined through a reliability based study using the Monte Carlo technique. It is found that the smaller sized specimens have larger fatigue life in the short crack regime than the larger specimens.

abstract: Pipeline infrastructure forms a vital aspect of the United States economy and standard of living. A majority of the current pipeline systems were installed in the early 1900’s and often lack a reliable database reporting the mechanical properties, and information about manufacturing and installation, thereby raising a concern for their safety and integrity. Testing for the aging pipe strength and toughness estimation without interrupting the transmission and operations thus becomes important. The state-of-the-art techniques tend to focus on the single modality deterministic estimation of pipe strength and do not account for inhomogeneity and uncertainties, many others appear to rely on destructive means. These gaps provide an impetus for novel methods to better characterize the pipe material properties. The focus of this study is the design of a Bayesian Network information fusion model for the prediction of accurate probabilistic pipe strength and consequently the maximum allowable operating pressure. A multimodal diagnosis is performed by assessing the mechanical property variation within the pipe in terms of material property measurements, such as microstructure, composition, hardness and other mechanical properties through experimental analysis, which are then integrated with the Bayesian network model that uses a Markov chain Monte Carlo (MCMC) algorithm. Prototype testing is carried out for model verification, validation and demonstration and data training of the model is employed to obtain a more accurate measure of the probabilistic pipe strength. With a view of providing a holistic measure of material performance in service, the fatigue properties of the pipe steel are investigated. The variation in the fatigue crack growth rate (da/dN) along the direction of the pipe wall thickness is studied in relation to the microstructure and the material constants for the crack growth have been reported. A combination of imaging and composition analysis is incorporated to study the fracture surface of the fatigue specimen. Finally, some well-known statistical inference models are employed for prediction of manufacturing process parameters for steel pipelines. The adaptability of the small datasets for the accuracy of the prediction outcomes is discussed and the models are compared for their performance. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

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Orientador: Luis Rogério de Oliveira Hein / Banca: Marcelo dos Santos Pereira / Banca: Miguel Justino Ribeiro Barboza / Resumo: No presente trabalho foi feita a analise quantitativa das superfícies de fratura obtidas através da propagação de trinca em fadiga. As trincas em fadiga são o modo de falha que ocorre por uma carga cíclica e repetida aplicada a um corpo de prova, o que irá causar a nucleação, propagação da trinca até a sua ruptura. Utilizou-se a ferramente da dimensão fractal para analisarmos correlações com a propagação de trincas. Os materiais que foram ensaiados são: liga de alumínio AA7475 e aço 300M, ambos materiais de uso aeronáutico. Foram realizados ensaios de propagação de trinca em fadiga em corpos de prova do tipo C(T) para a obtenção da superfície de fratura para análise. Posteriormente as superfícies foram analisadas em microscopio óptico e em microscópio eletrônico de varredura. Foi realizada a medição da dimensão monofractal (DF), quando a superfície é descrita por um único valor, também os valores de dimensão textural (DT), quando temos macroescalas e a dimensão estrutural (DS), quando temos microescalas. Com os resultados obtidos podemos analisar que o DS é quem melhor representa o comportamento das superfícies de fratura, já que este indica os micromecanismos de fratura presentes. Os valores de DS são menos dispersos na identificação das regiões de fadiga (pré-trinca, propagação estável e propagação instável da trinca),possibilitando a comparação com as imagens obtidas de cada etapa do mecanismo de fratura. Os valores de DT são melhores representados pelas reconstruções em microscópio óptico, pois estas permitem descrição mais precisa da topografia do material, considerando que o DT é característico em macroescalas. Já os valores de DS são melhores descritos em análises de microscopia eletrônica de varredura, já que esta apresenta melhor definição para a observação... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: In this paper we make quantitative analysis of fracture surfaces obtained through from fatigue crack propagation. The fatigue cracks are in the mode of failure that occurs for a repetead cycling loading and applied to a specimen, which will cause the nucleation, crack propagation until rupture. We will use the tool of fractal dimension to analyze correlations with the crack propagation. The materials that were tasted are: AA7475 aluminum alloy and 300M steel, both materials for aeronautical use. Tested were performed in fatigue crack propagation inspecimens of type C(T) to obtain the fracture surface for analysis. Subsequently the surfaces were examines in optical microscope and scanning electron microscope. Was performed to measure the monofractal dimension (DF), when the surface is described by a single value, so the textural dimension values (DT), and when we involved macroescale and finally structural dimension (DS) when was presente microscale. With the results we can analyze the DS is who represents the best value for the fractal fracture surface, as this indicates the micromechanics of fracture surfaces. The DS values are less dispersed in the identification of regions of fatigue (pre-crack propagation stable and unstable crack propagation), allowing comparison with the images obtained from each step of the mechanism of fracture. The values of DT are better represented when we do analysis in an optical microscope, because it a achieves the best results of the topography of the material, whereas the DT is characteristic macroscale. The values of DS are best described in the analysis of scanning electron microscopy, since it show better the micromechanics of fracture. So we can say the the DS would be the best representative of the fractal dimension to the analysis of micromechanics of fracture surfaces to their good representation at the microscale / Mestre

Orientador: Valdir Alves Guimarães / Banca: Aelcio Zangrandi / Banca: Marcos Venicius Soares Pereira / Resumo: Atualmente a crescente necessidade de aumento da carga por eixo e do número de passageiros transportados pelos trens, fez com que as ferrovias em todo o mundo iniciassem pesquisas voltadas para o desenvolvimento de novos materiais e processamentos térmicos. O aumento da confiabilidade dos sistemas ferroviários é uma exigência do mercado, o que é definido como sendo a garantia de que não ocorra falhas nos componentes no período entre as inspeções de rotina, ou seja, que um defeito possa ser detectado e reparado antes que atinja um tamanho crítico que leve a ruptura do componente. Por se tratar de um equipamento fundamental nos trens, os eixos ferroviários são alvos de grande parte das recentes pesquisas na área de fadiga, desenvolvimento de novos materiais e segurança dos intervalos de inspeção, uma vez que sua falha, certamente ocasionará um descarrilamento do vagão ou locomotiva. O comportamento em fadiga dos materiais empregados na fabricação de eixos ferroviários vem sendo estudado através de ensaios de propagação de trinca por fadiga, a fim de se obter dados experimentais para definição de intervalos de inspeção. Neste trabalho, foram estudados os aços SAE-1050, comumente utilizado na fabricação de eixos ferroviários, e o SAE- 4130, que recentemente vem sendo empregado para este fim. Estes materiais foram processados por dois diferentes tipos de tratamentos térmicos, sendo um composto por normalização e alívio de tensões, e o outro por normalização, têmpera e revenimento. O comportamento mecânico dos aços foi avaliado através de ensaios de tração, charpy em diversas temperaturas e tenacidade à fratura ( IC K ). O comportamento em fadiga, por sua vez, foi avaliado através de ensaios de propagação de trinca por fadiga, realizados em corpos de prova tração compacto (CT). Para a determinação da influência da razão... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Currently the growing need of increase in axle load and numbers of passengers transported by the trains, prompted the railroads all over the world began researches about the development of new materials and heat treatments. The increase of the reliability for railway systems is a requirement of the market, and can be defined as the warranty that doesn't occur failures in the components between the inspection intervals, in other words, that a defect can be detected and repaired before it reaches a critical size to take the rupture of the component. Axles are one of the most important components in railway vehicle, thus are present in the great part of the recent researches into fatigue, new materials and safe inspection intervals, since its flaw, certainly will cause a derailment of the wagon or locomotive. The fatigue behavior of the employed materials in the production of railway axles has been studied with aid of fatigue crack growth tests, in order to obtain experimental data for definition of inspection intervals. The present work, studied the steels SAE-1050, commonly used in the production of railway axles, and SAE-4130, that recently has been used for this application. These materials were submitted to different heat treatments: normalizing and stress relief, and normalizing, quenching and tempering. The mechanical properties of the steels were evaluated through tensile test, impact test in different temperatures and fracture toughness test. The fatigue behavior was evaluated through of fatigue crack growth tests in compact tension specimens (CT). The influence of the load ratio " R ", on fatigue crack growth was evaluated using the ratio of 0,1 and 0,3. The results showed that the steel SAE-4130, normalized, quenched and tempered, achieved the best mechanical properties and the smallest fatigue crack growth rate, being therefore, the best option as... (Complete abstract click electronic acccess below) / Mestre

<p>Damaged components and a lack of spare components are issues which are currently affecting military aircraft capability. Laser Cladding is an additive manufacturing technique which shows promise in repairing damaged aviation components. However, there are considerable certification requirements for critical components which stand to gain the most benefits from laser clad repair methodologies. These requirements involve establishing crack growth rate data for the laser clad material to gain confidence in the reliability of the repair's performance on in-service aircraft. This research seeks to understand the fatigue behavior of Ti-6Al-4V that has undergone a simulated laser clad repair, with unrepaired specimens also tested to allow for comparison. </p>

In this work, understanding the microstructural effects of monotonic and cyclic failure of wrought 7075-T651 and cast A356 aluminum alloys were examined. In particular, the structure-property relations were quantified for the plasticity/damage model and two fatigue crack models. Several types of experiments were employed to adapt an internal state variable plasticity and damage model to the wrought alloy. The damage model was originally developed for cast alloys and thus, the model was modified to account for void nucleation, growth, and coalescence for a wrought alloy. In addition, fatigue experiments were employed to determine structure-property relations for the cast alloy. Based on microstructural analysis of the fracture surfaces, modifications to the microstructurally-based MultiStage fatigue model were implemented. Additionally, experimental fatigue crack results were used to calibrate FASTRAN, a fatigue life prediction code, to small fatigue-crack-growth behavior. Lastly, a set of experiments were employed to explore the damage history effect associated with cast and wrought alloys and to provide motivation for monotonic and fatigue modeling efforts.

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The fatigue of materials and structures is a subject that has been under investigation for almost 160 years; yet reliable fatigue life predictions are still more of an empirical art than a science. The traditional safe-life approach to fatigue design is based upon the total time to failure of a virtually defect free component. This approach is heavily reliant on the use of safety factors and empirical equations, and therefore much scatter in the fatigue life predictions is normally observed. Furthermore, the safe-life approach is unsuitable for many important applications such as aircraft, pressure vessels, welded structures, and microelectronic devices. In these applications the existence of initial defects is practically unavoidable and the time of propagation from an initial defect to final failure is comparable with the total life of the component. In the early 1970’s, the aircraft industry pioneered a new approach for the analysis of fatigue crack growth, known as damage tolerant design. This approach utilises fracture mechanics principles to consider the propagation of fatigue cracks from an initial crack length until final fracture, or a critical crack length, is reached. Since the first implementation of damage tolerant design, much research and development has been undertaken. In particular, theoretical and experimental fracture mechanics techniques have been utilised for the investigation of a wide variety of fatigue crack growth phenomena. One such example is the retardation and acceleration in crack growth rate caused by spike overloads or underloads. It is generally accepted, however, that the current level of understanding of fatigue crack growth phenomena and the adequacy of fatigue life prediction techniques are still far from satisfactory. This thesis theoretically investigates various non-linear individual and interaction phenomena associated with fatigue crack growth. Specifically, the effect of plate thickness on crack growth under constant amplitude loading, crack growth retardation due to an overload cycle, and small crack growth from sharp notches are considered. A new semianalytical method is developed for the investigations, which utilises the distributed dislocation technique and the well-known concept of plasticity-induced crack closure. The effects of plate thickness are included through the use of first-order plate theory and a fundamental solution for an edge dislocation in plate of arbitrary thickness. Numerical results are obtained via the application of Gauss-Chebyshev quadrature and an iterative procedure. The developed methods are verified against previously published theoretical and experimental data. The elastic out-of-plane stress and displacement fields are first investigated using the developed method and are found to be in very good agreement with past experimental results and finite element simulations. Crack tip plasticity is then introduced by way of a strip-yield model. The effects of thickness on the crack tip plasticity zone and plasticity-induced crack closure are studied for both small and large-scale yielding conditions. It is shown that, in general, an increase in plate thickness will lead to a reduction in the extent of the plasticity and associated crack closure, and therefore an increase in the crack growth rates. This observation is in agreement with many findings of past experimental and theoretical studies. An incremental crack growth scheme is implemented into the developed method to allow for the investigation of variable amplitude loading and small fatigue crack growth. The case of a single tensile overload is first investigated for a range of overload ratios and plate thicknesses. This situation is of practical importance as an overload cycle can significantly increase the service life of a cracked component by temporarily retarding the crack growth. Next to be studied is growth of physically small cracks from sharp notches. Fatigue cracks typically initiate from stress concentrations, such as notches, and can grow at rates higher than as predicted for a long established crack. This can lead to non-conservative estimates for the total fatigue life of a structural component. For both the overload and small crack cases, the present theoretical predictions correlate well with past experimental results for a range of materials. Furthermore, trends observed in the experiments match those of the predictions and can be readily explained through use of crack closure arguments. This thesis is presented in the form of a collection of published or submitted journal articles that are the result of research by the author. These nine articles have been chosen to best demonstrate the development and application of the new theoretical techniques. Additional background information and an introduction into the chosen field of research are provided in order to establish the context and significance of this work. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1349588 / Thesis (Ph.D.) - University of Adelaide, School of Mechanical Engineering, 2008