Dissertations / Theses on the topic 'Fractographic analyzes'

A indústria aeronáutica tem buscado melhorias no conceito de integridade estrutural, a partir do desenvolvimento de projetos mais otimizados com a produção de aeronaves mais leves e seguras. Tais projetos garantem aeronaves operacionalmente mais viáveis e redução no consumo de combustível, beneficiando o fabricante, os operadores e a população em geral, pois aplica o conceito de aeronave verde. Estruturas aeronáuticas em operação recebem solicitações de cargas típicas de vôo, como pressurização, sustentação, que por sua vez, geram esforços dos mais variados tipos e suas combinações, como tração, compressão, fadiga, torção, flexão, flambagem, cargas aeroelásticas, vibrações e flutter, associados a ambientes críticos. O conhecimento sobre o comportamento do material estrutural perante ambientes corrosivo ou de baixa temperatura, é importante para avaliação da vida útil das aeronaves. Neste contexto, o objetivo deste trabalho foi avaliar o comportamento da liga AA2050-T84 sob ponto de vista de corrosão-fadiga e tenacidade a fratura em criogenia e, correlacionar com a microestrutura e fractografia através de ensaios mecânicos e de tenacidade a fratura em temperatura ambiente e criogênica. As condições de ensaios foram determinadas próximas a de serviço a que o material será inserido sendo, para criogenia -54 ºC e para os ensaios de propagação de trinca por fadiga em atmosfera ambiente e névoa salina com concentração de solução de 3,5% e 5% NaCl, frequência de 15Hz e razões de carga R = 0,1 e 0,5. Os resultados em criogenia mostraram que há melhora nas propriedades trativas e estabilidade na tenacidade, pois o material retém dutilidade com o decréscimo da temperatura. Os ensaios de propagação de trinca por fadiga foram executados com ΔK decrescente e os parâmetros para geração de névoa salina foram determinados experimentalmente. As curvas de crescimento de trinca e determinação do fator limite foram comparadas quanto a razão de carga e atmosfera de ensaio. Observou-se que para os ensaios de propagação de trinca por fadiga a presença da névoa salina acelera a taxa de propagação e resultam em um fator limite menor.
The aviation industry has sought improvements with the concept of structural integrity, from the development of more optimized designs with production aircraft with low weight and secure. These projects ensure an aircraft more operationally doable and reduced fuel consumption, benefiting the manufacturer, operators and the people, applying the concept of green aircraft. Aerospace structures in operation receive requests loads typical of flight, how pressurization support and lift that produce several types and combinations of requests, such as tensile, compression, fatigue, torsion, bending, buckling , aeroelastic loads, vibration and flutter associated with critical environments. Knowledge about the behavior of the structural material into corrosive environments or cryogenic temperature, it is important for evaluation to the lifetime of the aircraft. Accordingly , the aim of this study was to estimate the behavior of AA2050 - T84 alloy, about corrosion-fatigue and fracture toughness at cryogenic situations and correlate with microstructure and fractography, by mechanical tests and fracture toughness at room temperature and cryogenic. The test conditions has been determined near of the service that the material will be inserted, for cryogenic temperature -54 ºC and testing of fatigue crack propagation in air and salt spray with concentration of 3.5% and 5% NaCl, frequency 15Hz and load ratios R = 0.1 and 0.5. The cryogenic results showed that there improvement in tensile properties and stability in toughness due the restraint of ductility with decreasing temperature. Tests of fatigue crack propagation were carried out with ΔK decreasing and the parameters to obtain salt spray were determined experimentally. The crack growth curves and determination of the threshold were compared the according to load ratio and test atmosphere. In the fatigue crack propagation it was observed that presence of salt spray accelerates the rate of propagation and result in a lower threshold.

This thesis looks into the examination of polymer composite wreckage from the perspective of the aircraft accident investigator. It develops an understanding of the process of wreckage examination as well as identifying the potential for visual and macroscopic interpretation of polymer composite aircraft wreckage. The in-field examination of aircraft wreckage, and subsequent interpretations of material failures, can be a significant part of an aircraft accident investigation. As the use of composite materials in aircraft construction increases, the understanding of how macroscopic failure characteristics of composite materials may aid the field investigator is becoming of increasing importance. The first phase of this research project was to explore how investigation practitioners conduct wreckage examinations. Four accident investigation case studies were examined. The analysis of the case studies provided a framework of the wreckage examination process. Subsequently, a literature survey was conducted to establish the current level of knowledge on the visual and macroscopic interpretation of polymer composite failures. Relevant literature was identified and a compendium of visual and macroscopic characteristics was created. Two full-scale polymer composite wing structures were loaded statically, in an upward bending direction, until each wing structure fractured and separated. The wing structures were subsequently examined for the existence of failure characteristics. The examination revealed that whilst characteristics were present, the fragmentation of the structure destroyed valuable evidence. A hypothetical accident scenario utilising the fractured wing structures was developed, which UK government accident investigators subsequently investigated. This provided refinement to the investigative framework and suggested further guidance on the interpretation of polymer composite failures by accident investigators.

Contrairement aux aciers inoxydables austenitiques conventionnels fecrni, nous montrons que l'acier inoxydable austenitique fe18cr18mn0. 9n est fragile a basse temperature ou en presence d'hydrogene. Le present travail consiste a analyser la microfractographie de la rupture fragile obtenue afin d'en determiner les mecanismes. Les differents facies de rupture obtenus a basse temperature en resilience, en fatigue ou en traction montre une rupture fragile de nature variee: transgranulaire, intergranulaire et mixte. L'analyse microfractographique de la rupture transgranulaire basee sur l'observation stereographique des traces de glissement ou de macles ainsi que des figures d'attaque met en evidence une fissuration cristallographique predominante caracterisee par un vrai clivage sans riviere principalement sur des plans 111 et occasionnellement sur des plans 100 et 110. Frequemment, les plans 111 presentent des rivieres partant des joints de grain et se propageant selon une direction locale <110>. Une fissuration alternee sur un plan moyen 110, formee par alternance de deux microfacettes 111 a ete egalement observee. Les fissures amorcees sur ces plans se propagent selon une direction locale <100>. L'absence de seconde phase et la presence de traces de plasticite indiquent que la rupture fragile peut etre associee a une deformation plastique restreinte comme le montre les valeurs tres elevees des contraintes effectives. Cet acier montre egalement une sensibilite a la fragilisation par l'hydrogene. Il presente une rupture fragile de nature variee (trans-intergranulaire) dependant des conditions d'essai. Contrairement a celle observee a basse temperature, la fissuration transgranulaire par clivage ou discontinue est principalement identifiee sur des plans 100. Les fissures s'amorcent aux joints de grains et se propagent selon une direction locale <100>. La rupture fragile obtenue peut etre expliquee par un mecanisme de decohesion.

The main goal of this project was to compare the changes in statistical variation and Weibull characteristics of the strength of glass rods as modified by heat treatment with and without an ion exchange bath. Several sample groups of 30 sodium borosilicate glass rod specimens were heat treated at various temperatures in air and in a potassium nitrate salt bath to induce an ion exchange process. All samples were then tested to failure in 4-point bending to assess the resulting Modulus of Rupture (MOR). Statistical analysis techniques and Weibull analysis were used to study the variations which occur within and between strength distributions of each sample group. A smaller sampling of test groups was subjected to fractographic analysis to study the effect of ion exchange on fracture features. The data shows that the ion exchange process caused a statistically significant increase in the strength of the glass rods. Samples which were heat treated do not show any significant changes in average strength. The fractographic analysis suggests that no changes in fracture morphology occurred as a result of ion exchange process, and that the critical flaw size population was not significantly different.
Master of Science

This thesis deals with the low cycle fatigue nickel-based superalloy IN713LC with applied TBC barrier at high temperature. The theoretical part is divided into four sections. The first one focuses on description of fatigue damage. The second one provides the basic characteristics of nickel-based superalloys. The third section describes the use of the surface layers - diffusion layers and thermal barriers and the fourth section deals with the influence of these layers on fatigue properties. Experimental part is focused on the evaluation of low cycle fatigue tests and on the explanation of the mechanisms of initiation and propagation of fatigue cracks. For the experimental part, fatigue samples were prepared by vacuum precision investment casting. TBC barrier was applied by atmospheric plasma spraying and consists of two sublayers - the lower metallic bond coating type CoNiCrAlY and top ceramic coating type YSZ. Low cycle fatigue tests were conducted under strain control at controlled temperature of 900 ° C. Fractographical analysis of fracture surfaces was carried out by using light and electron microscopy. Effect of applied barrier to fatigue life was determined - the parameters of Manson-Coffin and Basquin curve. A cyclic stress-strain curve was also obtained. The curves softening / hardening and number of transit cycles were determined. The obtained parameters and values from fatigue tests were compared with available data from fatigue tests of superalloy IN713LC without the layer, as applied AlSi type diffusion layer, at high temperatures. The initiation site on the fracture surfaces was determined within the fractographic evaluation and the influence of the layer on the initiation and propagation of fatigue cracks was discussed. A helpful tool was the assessment of longitudinal sections using scanning electron microscopy.

Este trabalho versa sobre a inspeção fratográfica por Microscopia Eletrônica de Varredura (MEV) e a análise micro-química por Energia Dispersiva de Raios-X de superfícies de fratura de um compósito particulado de matriz metálica (CMM = liga AA356 + SiC) manufaturado por compofundição e submetido a diferentes classes de ensaios mecânicos, a saber, impacto Charpy, flexão quase-estática e fadiga fletiva. A fratografia por MEV em modo de imageamento por elétrons secundários se mostrou uma poderosa ferramenta na análise dos aspectos topográficos de CMM particulados fraturados, que se relaciona diretamente ao aporte de energia (ou ao nível de tensão desenvolvido) para a criação da superfície de fratura, independentemente do tipo de solicitação mecânica aplicada. Mecanismos de tenacificação em CMM, tais como descolamento, trincamento e arrancamento de partículas de SiC da matriz metálica AA356 foram prontamente identificados e documentados por esta modalidade MEV, e correlacionados ao desempenho mecânico dos materiais investigados. Aglomeração de partículas de SiC, assim como a presença de poros e/ou vazios se revelaram freqüentemente como variáveis determinantes do desempenho mecânico do CMM em ensaios dinâmicos, quase-estáticos e cíclicos. MEV em modo de imageamento por elétrons retro-espalhados se mostrou extremamente útil na identificação de partículas de SiC aflorando em superfícies de fratura do CMM quando o emprego de elétrons secundários não foi bem sucedido na tarefa. Micro-análise química por EDS permitiu o mapeamento dos elementos Fe e Cr, além de Mn, e possibilitou classificá-los como formadores de precipitados potencialmente fragilizantes do CMM. Esta técnica também assegurou a identificação de reticulados ricos em Si ao redor dos glóbulos de fase pró-eutética (alfa), os quais (reticulados) são altamente favorecedores de fraturas intergranulares.
This work focuses on the fractographic inspection by Scanning Electron Microscopy (SEM) and micro-chemical analysis by Energy Dispersive X-Ray Spectroscopy (EDS) of fractured surfaces of a compocast particulate metal matrix composite (MMC = AA356 alloy + SiC) subjected to different classes of mechanical testing, namely, Charpy impact, quasi-static flexure and flexural fatigue. SEM fractography in secondary electron imaging mode has shown to be a powerful tool in analyzing topographic aspects of fractured particulate MMC, which are straightforwardly related to the energy apportion (or the developed stress level) to the fracture surface creation, regardless the applied mechanical loading type. Toughening mechanisms for MMC, such as SiC particle debonding, cracking and pulling-out from the metallic matrix AA356 alloy were promptly identified and documented through this SEM modality, and correlated to the mechanical performance of investigated materials. SiC particle clusters as well as pore and/or void presence were very often discovered as the main controlling variables of mechanical performance of MMC during dynamic, quasi-static and cyclic testing. SEM fractography in backscatted electron imaging mode has shown to be extremely useful on the identification of SiC particles emerging from fractured surfaces of MMC when secondary electron mode did not succeed in this task. EDS micro-chemical analysis allowed to map Fe and Cr, besides Mn, and permitted to classify them as potentially MMC-embrittling precipitated forming elements. This technique also assured the identification of Si-rich reticulated structure around pro-eutectic phase globules, which (reticulated structure) highly favors intergranular fracture.

Notre environnement quotidien est constitué de multiples exemples d’éléments ou de structures susceptibles de se rompre par fatigue. En effet, si le secteur du transport aérien, terrestre et maritime, est à l’origine de nombreuses avancées dans le domaine de l’endommagement, les problèmes de fatigue concernent aujourd’hui des domaines d’activité très divers tels que la production d’énergie, le biomédical, l’automobile… D’une manière générale, on estime aujourd’hui qu’environ 80% des ruptures en service sont dues à une mauvaise prise en compte de l’endommagement par fatigue au stade de la conception et/ou du dimensionnement. En mécanique de la rupture, la « fractographie » est une science qui consiste à expertiser les débris d’une pièce rompue en vue de comprendre les mécanismes de l’endommagement, et ainsi définir l’histoire et les causes de la rupture. Une telle stratégie peut permettre d’améliorer par la suite la qualité du matériau pour mieux résister à la fatigue. Cette thèse est consacrée à l’analyse de ces débris que nous appelons surfaces de fracture, avec comme objectif la discrimination de trois étapes d’endommagement préconisées en mécanique de la rupture: l’étape d’amorçage de la fissure, l’étape de propagation stable et l’étape de la rupture finale du matériau. La rugosité de ces zones de fissuration est différente car la vitesse de fissuration change d’une zone à une autre. En effet, la fissure s’amorce à une vitesse, se propage à une autre vitesse et s’accélère pour mener le matériau à la rupture finale. Ces trois modes de comportement de la fissure sont confirmés par les graphes d’évolution de la fissure en fonction du nombre de cycles à la rupture, estimés à l’aide de la nouvelle stratégie de suivi de fissure développée durant cette thèse. Elle peut être adaptée à tout type de matériaux indépendamment de leur texture ou de leur conductivité électrique, avec une précision de l’ordre de 8 µm. Une campagne d’essais de fatigue a été conduite sur un ensemble d’éprouvettes d’un matériau élastomère destiné à la fabrication de pièces d’accouplements souples en industrie automobile. Les essais de fatigue ont été menés jusqu’à la rupture. Les surfaces de fracture ainsi obtenues sont numérisées à l’aide de deux techniques : l’interférométrie pour obtenir des cartes de hauteurs et la microscopie électronique à balayage pour obtenir des images microscopiques qui décrivent la texture de ces surfaces. La caractérisation de la rugosité de ces zones demande une classification très fine des irrégularités estampillées par la propagation de la fissure. D’où la nécessité d’adopter des méthodes de traitement de signal performantes pour quantifier d’une manière efficace ces structures singulières. Nous exploitons le paramètre de rugosité globale, appelé « exposant de Hurst », pour identifier l’axe de fissuration et par analyse fractale des profils de fracture (coupes transversales des cartes de hauteurs) nous montrons qu’il est possible de reconstruire le parcours de la fissure. L’analyse multifractale basée sur les maxima locaux de la transformée en ondelettes de ces profils a permis la discrimination des trois zones de fissuration à l’aide des spectres de singularités qui quantifient la force des singularités et leur distribution sur les trois zones d’intérêt
This thesis is devoted to analyse the fracture surfaces of a broken piece. The aim is to discriminate the three principles damage stages advocated in the field of fracture mechanics: fracture initiation stage, fracture propagation stage and finale rupture of the material. The behaviour of these fracture modes is confirmed by the graphs of the fatigue crack growth according to the number of fatigue cycles. These graphs are estimated using a new strategy developed for monitoring fatigue crack growth based on thermographic measures applicable to a wide range of materials regardless of their electrical conductivity and their surface texture. A campaign fatigue testing was conducted on a set of an elastomeric material samples, this material is used for the manufacture of parts of flexible couplings automotive industry. The fracture surfaces are digitized using two techniques: interferometry to obtain maps of heights and scanning electron microscopy to obtain microscopic images. The fracture phenomena being highly nonlinear and non-stationary; therefore, the classical roughness parameters measurement of fracture lines development is not adapted for their characterization. In this investigation, multifractal analysis based on the continuous Wavelet Transform Modulus Maxima method (WTMM) is proposed to give the discrimination of the profile lines development at three principal fracture stages. Indeed, the discrimination of these three fracture stages provides a powerful diagnostic tool to identify the fracture initiation site, and thus delineate the causes of the cracking of the material. We have used the global roughness parameter, called Hurst exponent, to identify the axis of cracking. The fractal analysis of the fracture profiles show that it is possible to reconstruct the crack path. It was established that multifractal analysis based WTMM describes reasonably well the scaling properties of local regularity of the fracture. It performs a fine discrimination of the three fracture zones using the singularities spectra which quantify the strength of singularities and their distribution

Este trabalho visa determinar a razão de carga cíclica R através da medida, na superficie de fratura, da relação entre a largura inter-estrias (s) e a altura da estria (H). É de conhecimento que o espaçamento (s) se correlaciona com a taxa de crescimento da trinca da/dN, enquanto que a curva da/dN x ΔK depende da razão de carga R. Para esta avaliação, corpos de prova do tipo compacto C(T) foram extraídos na orientação LT do centro de uma placa laminada da liga de alumínio SAE-AMS 7475 T7351 para realização dos ensaios de propagação de trinca por fadiga. Estes ensaios foram realizados de duas formas diferentes. A primeira, para obtenção de dados relativos à região II da curva da/dN x ΔK, sendo, neste caso, utilizadas quatro diferentes razões de carga (R=O,1; R=0,3; R=0,5; R=0,7). No outro caso, os ensaios de fadiga foram realizados sob ΔK constante e utilizados os mesmos quatro níveis de razão de carregamento R para determinação da curva da/dN x ΔK constante. Os níveis de ΔK aplicados foram tomados na região de Paris da curva da/dN x ΔK. A análise fratográfica por microscopia eletrônica foi conduzida nas superfícies dos corpos de prova do ensaio de fadiga com ΔK constante. Isto foi feito para determinar os espaçamentos inter-estria (s) e a altura H das estrias para cada nível de ΔK aplicado. Observou-se que a taxa de crescimento da trinca (da/dN) e o espaçamento inter-estrias (s) sofrem influência direta com a variação do fator de intensidade de tensão (ΔK), entretanto, observou-se, também, que a relação de proporcionalidade de 1:1 entre eles somente é válida para níveis de da/dN variando entre 0,1 a 1 μ/ciclo. Os valores da morfologia da estrias (H/s) sofrem influência significativa com o aumento da razão de carga R, entretanto, ficou evidenciado que o fator de intensidade de tensão máxima Kmáx. não tem influência nesta relação. Evidenciou-se que a largura da estria corresponde à taxa de crescimento da trinca (da/dN), e que a relação entre a altura e o espaçamento da estria (H/s) possui uma correspondência direta com a razão de carga R, que tem influência significativa sobre a propagação de trinca por fadiga.
This work aims at to method to determine the load ratio R through of measure, on the fractured of surfaces the relation between the width striation spacing (s) and the height of the striation (H). It is of knowledge that the spacing (s), if correlates with the fatigue crack growth rates da/dN, whereas the curve da/dN x ΔK depends on the load ratio R. For this evaluation compact tension C(T) specimens had been extracted in the L- T orientation of a laminate plate of the aluminum alloy SAE-AMS 7475 T7351.These test had been carried out of two different forms. The first one, for attainment of relative data to region II of the curve da/dN x ΔK, being, in this in case that used four different load ratios (R=O,1; R=0,3; R=0,5; R=0,7). In the other in case that, the fatigue crack growth rates had been carried out under ΔK constant and used same the four levels of load ratios R for determination of the curve of constant da/dN x ΔK. The applied levels of ΔK had been taken in the region II of the curve da/dN x ΔK. The fractographic analysis for electronic microscopy was lead on the surfaces of the specimens tested in fatigue with constant ΔK. This was made to determine the striation spacing (s) and height (H) of the striations for each constant level of ΔK. It was observed that the fatigue crack growth rates (da/dN) and the striation spacing (s) suffers direct influence with the variation of ΔK. Also, was observed that the relation of proportionality of 1:1 between them is only valid for levels of da/dN in the range of 0,1 to 1,0 mm/cycle. The values of the relationship of the striation dimensions (H/s) suffer significant influence with the increasing of load ratios R. However, were evidenced that the value of Kmax. does not have influence in this relationship. It was found that the width of the striation corresponds to the value of (da/dN) and that the relation between the height and the striation of spacing (H/s) had a direct correspondence with the load ratios R that has signifficant effect on the fatigue crack growth rates.

In the common design and certification process, a large number of experimental tests, going from small specimens to the full structure, are performed. In these tests, the environmental conditions, especially temperature and humidity, which the structural component will be submitted are considered. This multiplies the number of tests to be performed. With the aim of reducing the economic cost and the time needed for the design and the certification of components, numerical tools that allow a reduction of experimental tests and help in the analysis of results, are developed. In this framework, the present thesis tackles the topic of hygrothermal effects in composite materials in both fields: experimental tests and numerical tools. Specifically, the thesis focuses its attention on two analysis levels which are seldom present in the scientific literature: the optimization of structural components and the experimental characterization of the translaminar failure of the material, both considering environmental changes
En el proceso habitual de diseño y certificación de componentes aeronáuticos se realiza un elevado número de ensayos experimentales que van desde pequeñas probetas hasta la estructura final. La consideración de las condiciones ambientales, especialmente temperatura y humedad, a las que el componente se verá sometido, incrementará el número de ensayos a realizar. Para disminuir el elevado coste y el tiempo de diseño y certificación, se desarrollan herramientas numéricas que permiten sustituir parte de los ensayos experimentales y facilitan el análisis de resultados. En este ámbito, la presente tesis enfoca la problemática de la consideración de los efectos ambientales tanto en los ensayos experimentales como en el desarrollo de herramientas numéricas. Concretamente, se centra en dos niveles de análisis con escasa o nula presencia en la literatura científica sobre efectos higrotérmicos: optimización de elementos estructurales considerando cambios ambientales y la caracterización experimental del fallo translaminar del material con variaciones higrotérmicas

Capítulo 1. Objetivos: Medir e comparar o perfil de tensão residual da cerâmica de recobrimento aplicada sobre infraestruturas em zircônia tetragonal policristalina estabilizada por ítria (Y-TZP), em um compósito de alumina/zircônia (ZTA) e em uma alumina policristalina (AL). Material e Métodos: Os perfis de tensão residual de cada um dos materiais foi medido por meio do método \"hole-drilling\" em discos de 19 mm de diâmetro e 2,2 mm de espessura (0,7 mm de infraestrutura + 1,5 mm de porcelana de cobertura) .Resultados: Os espécimes de AL exibiram tensões de compressão que aumentaram numericamente com a profundidade, enquanto que as tensões compressivas mudaram para tensões de tração no interior das amostras de Y-TZP. As amostras de ZTA exibiram tensões de compressão na superfície, decrescendo com a profundidade até 0,6 mm, tornando-se compressivas novamente próximo à infraestrutura. Conclusão: O ZTA não apresentou tensões de tração em nenhum ponto ao longo da espessura da cerâmica de recobrimento. A AL apresentou um perfil de tensão residual mais favorável, gerado pelas tensões compressivas. A Y-TZP apresentou o perfil mais desfavorável, por apresentar tensões de tração próximo à infraestrutura. Capítulo 2. Objetivos: 1) Comparar os coeficientes de susceptibilidade ao crescimento subcrítico (n), de uma zircônia tetragonal policristalina estabilizada por ítria, obtidos por meio de dois ensaios de fadiga: cíclica e dinâmica. 2) Verificar o efeito da frequência usada no ensaio de fadiga cíclica no tempo de vida característico. Material e Métodos: Espécimes em forma de barras (25 mm x 4 mm x 3 mm) foram confeccionadas de acordo com as recomendações do fabricante. Os ensaios de fadiga dinâmica (n=70) e de fadiga cíclica (n=75) foram realizados por meio de resistência à flexão em quatro pontos para a obtenção dos parâmetros de crescimento de trincas subcrítico (SCG). O ensaio de fadiga cíclica também foi realizado em duas frequências (2 e 10 Hz), utilizando valores de tensão máxima entre 350 e 600 MPa. Todos os espécimes fraturados foram inspecionados em microscópio eletrônico de varredura para que fosse identificada a origem da fratura. Os dados de fadiga dinâmica foram analisados por meio das fórmulas apresentadas na ASTM C 1368-00. Os dados de fadiga cíclica foram analisados por meio da estatística de Weibull e por meio de análise tipo \"General Log Linear Model\". Resultados: A Y-TZP apresentou valores dos parâmetros de SCG obtidos no ensaio de fadiga dinâmica de ?fo = 667 MPa e n = 54. Os parâmetros de Weibull obtidos a partir dos resultados do mesmo ensaio foram de m = 7,9, ?0 = 968,9 MPa e ?5% = 767 MPa. Os parâmetros de Weibull obtidos no ensaio de fadiga cíclica foram estatisticamente semelhantes para as duas frequências utilizadas, o m* foi de 0,17 para 2Hz e de 0,21 10Hz; os tempos de vida característicos (?, em número de ciclos) foram de 1,93 x 106 e 40.768, respectivamente para 2 e 10 Hz. O valor de n obtido na fadiga cíclica foi de 48 e 40, nas frequências de 2 e 10 Hz, respectivamente. Não foi observado o efeito da frequência da aplicação da tensão, do nível de tensão, nem da interação dos dois no tempo de vida da Y-TZP, quando analisados pelo General Log Linear Model. Conclusão: Os coeficientes de susceptibilidade ao crescimento subcrítico (n) obtidos por meio de fadiga cíclica e dinâmica foram semelhantes para a Y-TZP testada. Os extensos intervalos de confiança obtidos nos ensaios de fadiga cíclica indicaram não haver efeito da frequência usado nesse método de fadiga nos tempos de vida característicos da Y-TZP testada.
Chapter 1. Objective: was: to measure and compare the residual stress profile in the veneering ceramic layered on three different polycrystalline ceramic frame-work materials: Y-TZP, alumina polycrystalline (AL) and zirconia toughened alumina (ZTA).Materials and Methods. The stress profile was measured with the hole-drilling method in bilayered disk samples of 19 mm diameter with a 0.7 mm thick Y-TZP, AL or ZTA framework and a 1.5 mm thick layer of the corresponding veneering ceramic. Results: The AL samples exhibited increasing compressive stresses with depth, while compressive stresses switching into interior tensile stresses were measured in Y-TZP samples. ZTA samples exhibited compressive stress at the ceramic surface, decreasing with depth up to 0.6 mm from the surface, and then becoming compressive again near the framework. Conclusions: Y-TZP samples exhibited a less favorable stress profile than those of AL and ZTA samples. Chapter 2. Objectives: 1) To compare the stress corrosion coefficient (n) of a Y-TZP, obtained by two fatigue tests: cyclic and dynamic and 2) Evaluate the effect of frequency used in the cyclic fatigue test in the characteristic lifetime. Methods: Bar shaped specimens (4 mm x 25 mm x 3 mm) were prepared according to the manufacturer\'s recommendations. The dynamic fatigue test (n = 70) and cyclic fatigue (n = 75) were performed by four-point bending to obtain the slow crack growth parameters (n) .The cyclic fatigue test was also conducted in two frequencies (2 and 10Hz), using stress levels between 350 and 600 MPa. All fractured specimens were inspected by SEM so that the origin of the fracture were identified. The dynamic fatigue data were analyzed using the formulas given in ASTM C 1368-00. Cyclic fatigue data were analyzed using Weibull statistics and by the Log General Linear Model. Results: The material showed values of parameters obtained in the SCG assay dynamic fatigue ?fo = 667 and n = 54. The Weibull parameters obtained from the same test results were m = 7.9, ?0 = 968, 9 and ?5% = 767. The Weibull parameters obtained by cyclic fatigue were statistically similar for the two frequencies used, the m* was 0.17 (2 Hz) and 0.21 (10 Hz); characteristic lifetimes (?) were 1.93 x 106 and 40768, respectively. The n value obtained by cyclic fatigue was 48 and 40 at frequencies of 2 and 10 Hz, respectively. There was no effect of the frequency, the stress level or the interaction of the two in the Y-TZP lifetime, when analyzed by General Log Linear Model. Conclusion: The n values obtained by cyclic and dynamic fatigue tests were similar to Y-TZP tested. The extensive confidence intervals obtained in cyclic fatigue tests showed no effect of frequency used this method in fatigue characteristic lifetimes of the Y-TZP tested.

La caractérisation de la rupture des grains via des techniques d'imagerie par tomographie est un sujet de recherche d'actualité. Les principaux verrous scientifiques et techniques sont associés à la complexité des algorithmes de traitement d'images permettant de suivre le mouvement des grains et leurs ruptures.Dans cette thèse, des approches numériques basées sur le traitement d'images sont proposées afin d'améliorer la compréhension des phénomènes de rupture et de broyage dans les matériaux granulaires dont le comportement est décrit par imagerie.Premièrement, en raison de l'interdépendance de la précision de la segmentation et de la résolution des techniques d'imagerie à l'échelle de la rupture des grains, nous commençons par un état de l'art sur les différentes techniques d’élimination des bruits dans les images de milieux granulaires. Ainsi, une analyse critique des méthodes existantes est établie en se référant aux différents matériaux d'application associés.Deuxièmement, et en vue de mieux capter les informations à partir des images sources, nous proposons une nouvelle approche qui consiste à passer d'une approche morphologique par bassin versant à la segmentation des géo matériaux à une approche hiérarchique. Aussi itérons-nous les moyens par lesquels la segmentation spécifique aux différents contextes d'application de l'imagerie peut être réalisée.Troisièmement, nous présentons un modèle original pour capter les ruptures des grains en se basant sur le traitement d'images statiques; sans considération de motion pour, enfin, présenter des modèles spatio-temporels suivant l'évolution de la casse dans les images de matériaux granulaires
Grain breakage in granular materials has been relatively, difficult to compute and characterise in tomography images. This is based on the perceived complexity of an algorithmic formulation for the characterisation of grains that move and break.In this thesis, we highlight computational approaches that augment the understanding of breakage and crushing phenomena in granular materials. Due to the inter-connectedness of segmentation accuracy and ability to compute for breakage, we start by examining noise removal techniques in granular materials. Noise removal techniques are analysed based on a set of materials to which they applied. Secondly, we deviate from a morphological watershed approach to segmentation of geomaterials, to a hierarchical approach that better captures apriori information from data sources. The ways by which context or image specific segmentation can be achieved is iterated. Thirdly, we present a model for capturing breakage in static images; without the consideration of motion. Finally, we present spatiotemporal models that track the evolution of breakage in images of granular materials

L'objectif de ce travail est de favoriser le developpement du verre trempe en tant que reel materiau de structure, dans le domaine du genie civil. Dans l'optique de realisation de poutres de longue portee ou de poutres profilees, faites a partir d'elements en verre plat, l'etude des zones de connexion se revele indispensable. Ce memoire presente l'etude detaillee du potentiel de la liaison par introduction d'un connecteur metallique dans un trou de la plaque de verre. Dans un premier temps, l'etat de contraintes residuelles, apres trempe thermique, est modelise par elements finis. Le modele de narayanaswamy est utilise pour decrire le comportement thermomecanique du verre. Les differents echanges de chaleur sont analyses. Le rayonnement thermique, complexe dans le cas d'un materiau semi-transparent, est pris en compte par le biais des emissivites surfacique et volumique. Les coefficients de convection forcee sont identifies de maniere experimentale grace a la realisation d'un appareillage specifique experimental. Les contraintes residuelles calculees sont ensuite verifiees par des mesures photoelastiques. Dans un deuxieme temps, l'etat de contraintes du au chargement du connecteur metallique est considere. Ce travail comporte une partie experimentale et une partie numerique, ces deux approches etant developpees en parallele puis confrontees. La modelisation par elements finis elaboree est validee en deux temps : par fractographie en comparant les valeurs des contraintes mesurees et calculees, et par photoelasticite en simulant en 3d les images photoelastiques. Une methode de superposition des deux etats de contraintes est proposee, et utilisee afin d'etablir un critere de rupture sur le verre trempe thermiquement. Ces resultats sont alors appliques au dimensionnement des trous, et une premiere methode de dimensionnement des zones de connexion est proposee. Cette methode garantit la perennite des assemblages. Enfin, des discussions concernant la rupture sont abordees. Le role endommageant de la bague est montre, la guerison par la trempe des defauts superficiels est prouvee et quantifiee, et finalement des elements pour une approche deterministe de la rupture sont avances.

The demand for flat glass is high and increasing significantly in the building industry as a direct result of architectural requirements for lightness, transparency and natural light. Current architectural trends require glass in curvilinear forms for smooth free-form façades. Two principal challenges arise from this: to cost-effectively produce the desired curvature and; to ensure its safe performance after exposure to ageing. The recent availability of high strength glass provides an opportunity to address the first challenge by developing cold bent glass. Cold bending involves the straining of relatively thin glass components, at ambient temperatures, and is a low energy and cost effective manner of creating curvilinear forms. However, cold bending is not yet widely established as a reliable method. The aim of this thesis is to develop the understanding of cold bent glass during the bending process and to evaluate its post-ageing performance. This thesis, firstly, investigates the mechanical response of monolithic glass plates during the cold bending process. The stability of cold bent glass is investigated experimentally by bending it in double curved anticlastic shapes. A parametric numerical analysis involves different boundary conditions, geometrical plate characteristics and bending parameters. The principal outcome is that a local instability, now termed cold bending distortion, occurs when certain displacement limits are exceeded and could degrade the optical quality of the glass. An evaluation procedure is also formulated to set limits and aid designers/manufacturers to predict the mechanical response and the optical quality of the glass. Cold bent glass is subjected to permanent bending stresses throughout its service life and therefore, its strength degradation after ageing needs to be quantified. Analytical, experimental and numerical investigations are undertaken in this thesis to identify the most effective method for estimating glass strength (evaluation of destructive tests, required number of specimens, statistical analysis methods and sub-critical crack growth). The limited availability of naturally aged toughened glass and the absence of a reliable ageing standard impede the evaluation of its aged performance. Therefore, a parametric experimental investigation of artificial ageing methods on glass is undertaken in this thesis. A procedure for the evaluation of the strength of aged glass is finally, formulated to allow the selection of artificial ageing parameters that correspond to a target level of erosion. The knowledge on artificial ageing and strength prediction acquired above is finally implemented on different types of glass to determine their strength after ageing and assess their safe use in cold bending / load bearing applications. The investigation showed that fully toughened glass has a superior performance to chemically toughened or annealed glass. Overall, the research presented in this thesis demonstrates that high quality cold bent toughened glass can be created when certain applied displacement limits are respected. These can be used as a safe, cost-effective and energy efficient replacement to the more conventional hot bent glass. However, cold bending / load bearing applications in which the stressed glass surface is exposed to ageing, require glass with a relatively high case depth such as fully toughened or bi-tempered glass.

Este trabalho discutiu o estudo da inferência de carregamento e a reconstrução fratográfica da taxa de propagação da trinca por fadiga em histórias de vôos simulados do tipo Twist, mini-Twist, Falstaff e mini-Falstaff. O material estudado foi uma liga de alumínio de grau aeronáutico SAE-AMS 7475 T7351 na qual foram obtidas as propriedades mecânicas a partir dos ensaios de tração e dureza, análise química, metalográfica. Foram realizados ensaios de amplitude constante de carregamento em corpos de prova com geometria tipo C(T) extraídos de um bloco e ensaios de amplitude variável em corpos de prova com geometria tipo M(T) extraídos de uma placa. Corpos de prova do tipo C(T) foram extraídos da orientação L-T e T-L, à temperatura ambiente com diferentes espessuras para a realização de ensaios de propagação de trinca por fadiga (da/dN vs. ΔK) com distintos valores de razão de carregamento,R (0,1;0,3; 0,5; 0,7 e 0,8) de acordo com a norma ASTM-E647 (1993). Foram realizados dois tipos de ensaios de fadiga com amplitude constante de carregamento. Primeiro, o ensaio com ΔP constante para obtenção dos dados dos estágios II e III das curvas da/dN vs. ΔK para distintos valores de R e, posteriormente, o ensaio com ΔK constante e R crescente para ser utilizado no estudo da inferência de carregamento de amplitude constante a partir da relação entre a morfologia da estria, H/s e a razão de carregamento R. Adicionalmente foram realizados ensaios de tenacidade à fratura, nas direções L-T e T-L, conforme a norma ASTME1820 (1999). As taxas de propagação macroscópica e microscópica foram comparadas e boas correlações foram obtidas para um intervalo entre 0,1 a 1,0 μm/ciclo, mesmo com distintos R. Através da técnica proposta por Berkovitz (1995), o espectro de carregamento foi estimado para distintos valores de R, através da técnica de parametrização do fator K e os resultados foram comparados ao espectro de carregamento real imposto pela máquina de ensaio. Após os ensaios de amplitude constante de carregamento foram realizados ensaios de amplitude variável com a aplicação em corpos de prova M(T) de espectros de vôos simulados padronizados pela NLR do tipo Twist e Falstaff e seus vôos reduzidos, mini-Twist e mini- Falstaff, em uma máquina servohidráulica de ensaios dinâmicos. A partir destes ensaios notou-se que suprimir os vôos com os carregamentos menos severos influencia muito a vida da propagação da trinca de fadiga, quando vôos simulados do tipo Twist e mini-Twist, fato este não observado para o Falstaff e mini-Falstaff. Foi possível observar também o efeito da aceleração e desaceleração da trinca por fadiga observando os gráficos de d2a/dvôo vs. a obtidos dos ensaios de amplitude variável. Foi realizado um estudo da superfície de fratura do CP05 por meio da técnica fractográfica do reconhecimento de blocos marcadores sendo possível mostrar uma satisfatória reconstituição da taxa de propagação da trinca por fadiga de vôo simulado do tipo Falstaff por meio de medidas das marcações na superfície de fratura dos vôos mais severos, realizadas em imagens obtidas por MEV.
The loading inference and the fractographic reconstruction of fatigue crack propagation rate in simulated flight stories, as Twist, mini-Twist, Falstaff and mini-Falstaff were investigated in this work. The studied material was an aluminum alloy SAE-AMS 7475 T7351. Tensile and hardness tests were carried out in order to determine mechanical properties, as well as chemical and microstructure analysis of the material. C(T) specimens were extracted from a block of material for constant amplitude load testing and M(T) specimens were extracted from a plate and were tested in variable amplitude loading. The curves (da/dN vs. ΔK) for constant-amplitude loading were obtained from fatigue crack propagation tests performed in L-T and T-L directions in C(T) specimens for R (0.1; 0.3; 0.5; 0.7 and 0.8), at room temperature, according to ASTM-E647 (1993). Two different constant loading fatigue tests were performed. At constant ΔP and distinct values of R for obtaining data of stages II and III of da/dN - ΔK curves; and at constant ΔK and increasing the R ratio to be used in the inference of constant amplitude loading in the relation of H/s striation morphology (obtained from fractographic images) and the loading ratio, R. Additionally, fracture toughness tests were executed in T-L and L-T directions, according to ASTM-E1820 (1999). The macroscopic and microscopic fatigue crack propagations were compared and good correlation was obtained for the range 0.1-1.0 μm/cycle, for all R values. By using the K factor parametrization method, Berkovitz (1995), the loading spectrum was estimated for the distinct R-values and the results were compared to the actual spectrum imposed by the test machine. Variable amplitude tests were performed in M(T) specimens by applying simulating flight spectra normalized by NLR, as Twist, mini-Twist, Falstaff and mini-Falstaff, in a servohydraulic test machine MTS, in which a Flextest GT controller was coupled. The suppression of less severe loading flights showed a great influence in fatigue crack propagation life, in simulated flight tests Twist and mini-Twist, but not in Falstaff and mini-Falstaff. The acceleration and slowing down of fatigue crack were observed in the d2a/dflight - a curve obtained from variable loading tests. The fractured surfaces of some samples were examined by the fractographic technique of blocking marks recognition. A good reconstitution of fatigue crack propagation rate of a simulated flight like Falstaff was obtained by the measurements of marks in the fracture surfaces of the more severe flights, in MEV images.

Lo scopo della presente ricerca è valutare l’idoneità dell’acciaio da utensili innovativo K890, prodotto da Böhler con metallurgia delle polveri, ad essere impiegato per la realizzazione di componenti motore di moto da corsa, quali alberi a camme e a gomiti, in sostituzione dell’acciaio da nitrurazione GKHW, prodotto da Aubert&Duval con processo ESR (Electro Slag Remelting). Lo studio è stato condotto a valle di una precedente sperimentazione condotta su provini non intagliati ed ha previsto l'esecuzione di prove di trazione e prove di fatica a flessione rotante con intaglio oltre a prove di durezza ed analisi microstrutturali e frattografiche. Sono stati testati due lotti di acciaio K890, aventi durezza di 752 e 821 HV rispettivamente. Durante i test di fatica il lotto in K890 a durezza maggiore ha mostrato un limite di fatica con intaglio superiore al GKHW nitrurato. I risultati ottenuti sui due lotti di K890 sono stati impiegati per valutare l’applicabilità di relazioni empiriche per la stima del coefficiente di concentrazione a fatica delle tensioni ad acciai ultra-alto resistenziali. E’ emerso che le relazioni più tradizionali sovrastimano l’effetto dell’intaglio per questa tipologia di acciaio. A conclusione dello studio, considerando l'esito delle stesse prove condotte sull'acciaio GKHW nitrurato attualmente in uso, si può affermare che l'acciaio K890 è un possibile candidato per la produzione di alberi in ambito racing. La disamina dei risultati sperimentali è preceduta da tre capitoli incentrati su un inquadramento teorico, derivanti da ricerche bibliografiche: il primo riporta un’introduzione al funzionamento ed alle caratteristiche degli alberi a camme e a gomiti; il secondo è relativo alle proprietà degli acciai da utensili tradizionali ed innovativi; infine il terzo riguarda una trattazione sul danneggiamento da fatica, con particolare riguardo all’effetto dell’intaglio ed alle caratteristiche del fenomeno negli acciai ultra-alto resistenziali.

碩士
國立臺灣大學
土木工程學研究所
89
The feature of material fracture depends on its characteristic and stress condition. We can use these features to understand material fracture mechanism, stress condition and stress history. For this reason, fractography and fracture mechanics are useful tools for us to understand material fracture mechanism. We use the viewpoint of fractography and fracture mechanics in our study to distinguish rock fracture feature and then we can control the joint condition in engineering. First, we use the concept of fractography and fracture mechanics to understand the fracture condition of uniaxial compressive strength test and Brazil test. We can explain the meanings of strength by these tests and find that the specimens with dead zone can represent higher strength. Second, we do CSTBD (Cracked Straight-Through Brazilian Disk) test, uniaxial compressive test by oblique crack specimen and penetrative test. These tests can help us to clear the occurrence of fracture feature. Besides, we use fracture energy to quantify the roughness on fracture surface. Finally, according to our study, the direction of crack front may protrude the fracture origin. Therefore, in order to explain the fracture feature efficiently, we have to observe more than one fracture feature in the field.

Adhesively bonded joints have been increasingly applied in primary aerostructures, mainly due to their numerous advantages over mechanically fastened joints. More specifically, bonded Tjoints made from carbon fibre reinforced plastics (CFRPs) are commonly found as stiffening members on wing paneis and fuselage sections. Despite the well-known advantages of composites, aluminium stiffeners can yet be considered a safer and more conservative solution, since composite T-joints face delamination problems that typically initiate at the central noodle of the stiffener - the part where the three arms of the stiffener come together - and propagate through the adhesive bondtine. [...]
As juntas adesivas têm sido cada vez mais utilizadas em estruturas aeronáuticas primárias, principalmente devido às suas numerosas vantagens em relação a juntas rebitadas ou aparafusadas. Particularmente, juntas em "T" compósitas de matriz polimérica e reforçadas com fibras de carbono (CFRP) são frequentemente encontradas como membros enrijecedores (stifteners) em painéis de asas e seções de fuselagem. Apesar das vantagens bem conhecidas dos materiais compósitos, as juntas em "T" em alumínio ainda podem ser consideradas uma solução mais segura e conservadora, visto que as juntas em "T" compósitas enfrentam problemas de delaminação que normalmente se iniciam no preenchimento central do enrijecedor - a parte onde a alma e o banzo se juntam - e propagam-se ao longo da conexão adesiva. [...]

Laser cladding is a more recent approach to repair of aviation components within a damage tolerant framework, with its ability to restore not simply the geometric shape but the static and fatigue strength as well. This research analysed the fatigue performance of Ti-6Al-4V that has undergone a laser clad repair, comparing baseline specimens with laser clad repaired, and repaired and heat treated specimens. First an understanding of the microstructure was achieved by use of BSE imagery of the substrate, clad repaired region and post heat treated regions. The substrate of the material was identified with large grains which compared to a repaired clad region with a much finer grain structure that did not change with heat treatment. Next, performance of the specimens under tensile fatigue loading was conducted, with the clad specimens experiencing unexpectedly high fatigue performance when compared to baseline samples; the post heat treated specimen lasting significantly longer than all other specimens. It is theorised that the clad may have contributed to an increase in fatigue resilience due to its fine microstructure, when compared to the softer, more coarse substrate. The heat treatment is likely to have relaxed any residual stresses in the specimens leading to a reduction in any potential undesirable stresses, without impacting the microstructure. Residual stress analysis using EDD was unproductive due to the unexpected coarse microstructure and did not provide meaningful results. Fractography using the marker-band technique was explored with some success, proving a feesable method for measuring fatigue crack growth through a specimen post failure. Unfortunately fatigue crack growth throughout the entire fatigue life was not possible due to the tortuous fracture surface and potentially due to the fine micro-structure of the clad, resulting in interrupted marker-band formation. Future research shall expand on this work with a greater focus on residual stress analysis and its impact on fatigue.