The IF steel was cycled at strain amplitude of £`max = 0.3% and the samples were cycled at 3000¡B6000 and 13000 for observation of surface crack, respectively. The microstructure of the surface and interior of IF steel were examined by the SEM under BEI/ECCI mode. At 3000 cycles, we can find that many persistent slip bands (PSBs) form in grains, but it¡¦s not easy to find cracks. The main microstructures in the interior are dislocation loop patches and dislocation walls. The microstructures at the surface are dislocation cells, and we find the dislocation cells arrange along their prefer orientation in some grains. At 6000 cycles, there are many cracks on the surface, but the depth of cracks may be less than 10£gm. The mainly microstructures in the interior and on the surface are almost the same as those at 3000 cycles, but very few dislocation cells can be observed in the interior. When the fatigue cycles increase to 13000 cycles, there are many long cracks on the surface. The microstructure of crack tips are misorientation cells and the size of misorientation cells are less than 2£gm. It means that the length of cracks will develop if the fatigue cycles increase. As the result of the observation of surface cracks longer than 50 um in samples at 6000 and 13000 cycles, the microstructure of crack tips consists of misorientation cells which imply a propagating crack.

Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-04-10T21:22:10Z No. of bitstreams: 1 GABRIEL DE CASTRO COÊLHO – DISSERTAÇÃO (PPGEM) 2018.pdf: 7666728 bytes, checksum: 7531b06d7808af28519bdc17c9bac069 (MD5) / Made available in DSpace on 2018-04-10T21:22:10Z (GMT). No. of bitstreams: 1 GABRIEL DE CASTRO COÊLHO – DISSERTAÇÃO (PPGEM) 2018.pdf: 7666728 bytes, checksum: 7531b06d7808af28519bdc17c9bac069 (MD5) Previous issue date: 2018-02-26 / Capes / Com o intuito de se ter uma melhor análise de engenharia (avaliação de vida útil remanescente) versus menor número de intervenções (parada de equipamento para manutenção) foram elaboradas normas técnicas de Fitness For Service (FFS – Adequação ao Serviço), que procuram explorar os limites de carregamentos, permitindo a utilização de estruturas e componentes danificados de forma mais eficiente, estendendo suas campanhas operacionais. Este trabalho teve como objetivo realizar os procedimentos de FFS das normas americana API-579/ASME FFS-1 (2016) e britânica BS 7910 (2013) por meio da análise de uma trinca superficial longitudinal localizada na superfície externa de um vaso de pressão cilíndrico, em cada nível de cada norma, tal que uma comparação entre as duas normas foi feita de modo a avaliar qual das duas normas é capaz de prover dimensões críticas maiores, que na prática proporcionariam maiores campanhas operacionais de estruturas e componentes danificados. Como ferramenta de análise, o MEF foi usado através do software ABAQUS®. Resultados indicaram em todas as comparações dos 3 (três) níveis de avaliação de cada norma que a BS 7910 (2013) oferece uma avaliação crítica de engenharia menos conservadora, já que tanto pelas soluções analíticas como pelos resultados via MEF, conseguiu-se por esta determinar tamanhos críticos maiores quando em comparação a norma americana. Conclui-se, portanto, que a BS 7910 (2013) desde a absorção da FITNET (2008) tornou-se um procedimento bem mais prático em termos de aplicação e menos conservador quando comparado a norma API 579/ASME FFS-1 (2016), para os casos estudados. / Intending on having a better engineering analysis (remaining life assessment) versus lower interventions (equipment stop for maintainance) Fitness For Service (FFS) standards were elaborated to explore the load limits, allowing the use of damaged structures and components in a more efficient way, extending their operational campains. This thesis implemented the FFS procedures of the American standard API 579/ASME FFS-1 (2016) and the British standard BS 7910 (2013) by the analysis of a surface longitudinal crack on the external surface of a cylindrical pressure vessel, in each level of assessment of each standard so that a comparison between each standard could be performed to evaluate which one is capable of providing higher critical dimensions that in real situation would mean higher operational time of dmamaged structures and components. FEM was used through the ABAQUS® software. Results indicated that in all comparisons of the 3 (three) assessment levels of each standard, BS 7910 (2013) provided a less conservative engineering critical assessment since, by both numerical and analytical solutions, this standard was able to determine higher critical sizes in comparison to the American standard. As a general concluding remark, BS 7910 (2013), since the absorption of the FITNET (2008) procedures, has become a much more practical and less conservative procedure when compared to the API 579/ASME FFS-1 (2016).

Surfaces subjected to rolling and sliding contacts maysuffer from contact fatigue. This thesisdeals with solidmechanic aspects of contact fatigue including the descriptionand verificationof explaining mechanisms. The new mechanism forsurface initiated contact fatigue is basedon tensile surfacestresses from local asperity contacts. It is also realised thatsub-surfaceinitiated contact fatigue is the result of tensileresidual stresses that emanate from plasticdeformation belowthe surface. These mechanisms clearly show that contact fatiguecracksfollow the same rules as ordinary fatigue cracks inhardened steel. The thesis contains four papers that treat a new testprocedure named Standing ContactFatigue (SCF). The results ofthe test procedure have played an important role inthedevelopment and verification of the mechanisms for surfaceand sub-surface contact fatigue. The first part of the research work was experimental. Inthis part the SCF test properties wasdecided, crack resultsconfirmed and crack detection methods developed. Herecomparativestudies were performed using some differentmaterials and mechanical properties. It wasverified that SCFcould detect differences in contact fatigue resistance. Next a finite element model of the SCF test was evaluatedthrough the general-purposeprogram MARC. The model includedgraded material properties that originate from heattreatment.The residual surface deformation and surface compliance wereverified againstexperimental results. Crack initiation wasinvestigated in two ways. Firstly, the principalstresses atcritical locations were computed and plotted in a Haighdiagram. The diagramshowed that the cracks initiate in adirection perpendicular to the principal stress with thelargeststress range provided that the principal stress is tensilesometime during the load cycle. Secondly, some high cycle multiaxial fatigue criteria,including the Haigh principal stresscriterion, was evaluatedagainst the SCF crack initiation results. The surface cracklocation waspredicted by including statistical effects using aweakest-link criterion and a three-parameterWeibulldistribution. The SCF crack propagation was investigated by numericalevaluation ofJ1 andJ2integrals. The crack initiation and propagationphases were separated with a threshold criterionand a directioncriterion. It was found that during crack propagation bothsurface andsub-surface contact fatigue cracks follow thedirection with minimum mode II loading. Key words: contact fatigue mechanism; spall; spalling;surface crack; sub-surface crack;elasto-plastic indentation;contact compliance measurement; mixed-mode fatigue;fatiguecrack growth;J-integral; multiaxial fatigue; weakest-link. / QC 20100407

This study aims to develop a computer program to perform finite elements based fracture mechanics analyses of three dimensional surface cracks in T-welded joints of construction machinery. The geometrical complexity of the finite elements models and the requirement of large computer resources for the analyses necessitate the use of shell elements for general stress distribution optimization. A sub-modeling technique, together with a shell to solid conversion method, enables the user to model a local region and analyze it by defining the weld and crack parameters. It is assumed that the weld material is the same with the sheet metal material and the surface cracks are considered to occur on two weld toes and weld root. The surface cracks are assumed to have a semi elliptical crack front profile. In order to simulate the square-root strain singularity around the crack front, collapsed 20-node three dimensional brick elements are utilized. The rest of the local model is modeled by using 20-node three dimensional brick elements. The main objective of this work is to calculate the mixed mode energy release rates around the crack front for a sub-model of a global shell model by using J-integral method.

[pt] Neste trabalho são investigadas experimentalmente e numericamente as mudanças de geometria em trincas inicialmente passantes submetidas a carregamento remoto de flexão pura induzindo fechamento parcial das faces da trinca. Esse crescimento de trinca pode ocorrer numa variedade de estruturas com defeitos pré-existentes, tais como fuselagens de aviões, cascos de navios, vasos de pressões e pontes metálicas. O carregamento de flexão pura ocasiona regiões de tração e compressão na frente da trinca. É inquestionável que parte das faces da trinca sob compressão fecha independentemente de qualquer mecanismo de fechamento; e outra parte das faces da trinca, por outro lado, sob tração cresce mudando gradualmente de geometria. Após realizar ensaios em corpos-de-prova de aço ASTM A-36, foi observado que tais carregamentos geram uma quina na frente da trinca, que é a transição de uma geometria parcialmente passante e um trecho remanescente da geometria inicial. Para entender a distribuição do fator de intensidade de tensão em tais frentes de trinca, suas geometrias foram reproduzidas em um modelador tridimensional de mecânica da fratura linear elástica, o FRANC3D, acoplado a um programa de análise de elementos finitos (ABAQUS). Com este sistema acoplado, foram executadas análises considerando efeitos não lineares causados pelo contato das faces da trinca sob compressão. Verificou-se a necessidade de propor metodologias para tratamento dos resultados numéricos na quina, obtendo-se predições eficientes das mudanças na geometria da trinca. Contudo, a estimativa de vida, quando se compara taxas de crescimento da trinca obtidas em um corpo-de-prova sob tração cíclica e as taxas em um corpo-de-prova sob flexão com fechamento parcial da trinca, foi melhor reproduzida usando um fator de correção de fechamento da trinca. Adicionalmente, uma série de expressões empíricas normalizadas para geometrias da trinca e fatores de intensidade de tensão são propostas. / [en] This work investigates experimentally and numerically how the front of initially through edge cracks in plate changes after they pass to be remotely fatigue loaded under pure bending to induce partial closure of the crack faces. This type of crack growth problem can occur in a variety of structures with preexisting defects, such as aircraft fuselages, ship hulls, pressure vessels components, and steel bridges. The bending loads induce tension and compression regions along the crack front, with the part of the crack faces that work under compression undoubtedly closed by the load, independently of any other closure mechanism. The part of the crack faces that work under tension; on the other hand, crack grows by fatigue gradually changing its shape. After performing tests on ASTM A36 steel specimens, it was observed that the bending load induces a kink on the crack front, in the transition between the part through crack created on the tension side and initial crack geometry. To understand the distribution of the stress intensity factor along such crack fronts, the measured crack shapes were reproduced in a three-dimensional fracture mechanics modeler (FRANC3D) coupled to a finite element analysis program (ABAQUS). With this coupled system, linear elastic stress analysis simulations were performed considering the nonlinear effects caused by the crack face contact in the compressed region. In particular, methods had to be proposed to treat numerical noise around the kink. The proposed methodology efficiently predicts the observed crack front shape changes; although the observed fatigue lives were better reproduced using a crackclosure correction factor when compared to crack growth data obtained from standard compact tension specimens. In addition, a series of normalized empiric expressions for both crack front shapes and stress intensity factors are proposed.

Este trabalho apresenta um procedimento de avaliação para a determinação da Integral J e do CTOD para dutos com trincas circunferenciais em sua superfície externa e interna submetidos à flexão pura para uma ampla faixa de geometrias de trinca e propriedades (encruamento) de material baseados em soluções das componentes plásticas de J e CTOD. Uma descrição da metodologia sobre a qual J e CTOD são determinados estabelece o contexto necessário para a determinação das funções adimensionais h1 e h2 aplicáveis a uma grande faixa de geometrias de trinca e propriedades de material características de aços estruturais para a construção de dutos e vasos de pressão. As extensivas análises numéricas não-lineares 3-D fornecem um conjunto completo de soluções para J e CTOD, os quais entram diretamente em procedimentos avançados para a avaliação de defeitos em dutos e cilindros submetidos à flexão. Este estudo também examina uma comparação exploratória entre os resultados do procedimento proposto e os resultados de análises numéricas por elementos finitos de dutos com trincas superficiais submetidos ao processo de enrolamento. / This work provides an estimation procedure to determine the J-integral and CTOD for pipes with circumferential surface cracks subjected to bending load for a wide range of crack geometries and material (hardening) based upon fully-plastic solutions. A summary of the methodology upon which J and CTOD are derived sets the necessary framework to determine nondimensional functions h1 and h2 applicable to a wide range of crack geometries and material properties characteristic of structural, pressure vessel and pipeline steels. The extensive nonlinear, 3-D numerical analyses provide a definite full set of solutions for J and CTOD which enters directly into fitness-for service (FFS) analyses and defect assessment procedures of cracked pipes and cylinders subjected to bending load. The study also examines an exploratory comparison between the resulting fully-plastic solutions and finite element analyses of circumferentially cracked pipes subjected to reeling.

Este trabalho apresenta um procedimento de avaliação para a determinação da Integral J e do CTOD para dutos com trincas circunferenciais em sua superfície externa e interna submetidos à flexão pura para uma ampla faixa de geometrias de trinca e propriedades (encruamento) de material baseados em soluções das componentes plásticas de J e CTOD. Uma descrição da metodologia sobre a qual J e CTOD são determinados estabelece o contexto necessário para a determinação das funções adimensionais h1 e h2 aplicáveis a uma grande faixa de geometrias de trinca e propriedades de material características de aços estruturais para a construção de dutos e vasos de pressão. As extensivas análises numéricas não-lineares 3-D fornecem um conjunto completo de soluções para J e CTOD, os quais entram diretamente em procedimentos avançados para a avaliação de defeitos em dutos e cilindros submetidos à flexão. Este estudo também examina uma comparação exploratória entre os resultados do procedimento proposto e os resultados de análises numéricas por elementos finitos de dutos com trincas superficiais submetidos ao processo de enrolamento. / This work provides an estimation procedure to determine the J-integral and CTOD for pipes with circumferential surface cracks subjected to bending load for a wide range of crack geometries and material (hardening) based upon fully-plastic solutions. A summary of the methodology upon which J and CTOD are derived sets the necessary framework to determine nondimensional functions h1 and h2 applicable to a wide range of crack geometries and material properties characteristic of structural, pressure vessel and pipeline steels. The extensive nonlinear, 3-D numerical analyses provide a definite full set of solutions for J and CTOD which enters directly into fitness-for service (FFS) analyses and defect assessment procedures of cracked pipes and cylinders subjected to bending load. The study also examines an exploratory comparison between the resulting fully-plastic solutions and finite element analyses of circumferentially cracked pipes subjected to reeling.

The aim of this study is to model three dimensional surface crack problems in fillet welds. It is assumed that weld material has the same material properties with the sheet metals. The surface crack is considered to occur at two regions / one at the weld root and the other at the weld toe. The surface crack is assumed to have a semi &ndash / elliptical crack front profile. The surface crack problem is analyzed under mechanical loading and the models are built up by three dimensional finite elements. Around the crack front, strain singularity is taken into account by using degenerated 20 &ndash / node quarter &ndash / point solid elements. The main results of this work are the stress intensity factors around the crack front for the test specimen model subjected to axial and bending loads.

The main objective of this study is to model and analyze a three dimensional inclined semi-elliptic surface crack in a Functionally Graded Material (FGM) coating bonded to a homogeneous substrate with a bond coat. The parametric analyses on FGMs are based upon zirconia-yttria (ZrO2-8wt%-Y2O3) FGM coating bonded to a substrate made of a nickel-based superalloy. It is assumed that there is a nickel-chromium&amp / #8211 / aluminum&amp / #8211 / zirconium (NiCrAlY) bond coat between the FGM coating and substrate. Metal-rich, linear variation, ceramic-rich and homogeneous ceramic FGM coating types are considered in the analyses. The inclined semi-elliptic surface crack problem in the FGM coating-bond coat-substrate system is analyzed under transient thermal loading. This problem is modeled and analyzed by utilizing three dimensional finite elements. Strain singularity around the crack front is simulated using collapsed 20 &amp / #8211 / node quarter &amp / #8211 / point brick elements. Three &amp / #8211 / dimensional displacement correlation technique is utilized to extract the mixed mode stress intensity factors around the crack front for different inclination angles of the semi-elliptic surface crack. The energy release rates around the crack front are also calculated by using the evaluated mixed mode stress intensity factors. The results obtained in this study are the peak values of mixed mode stress intensity factors and energy release rates around the crack front for various inclination angles of the semi-elliptic surface crack embedded in the FGM coating of the composite structure subjected to transient thermal loading.

The main objective of this study is to model the three dimensional surface cracking problem in Functionally Graded Material (FGM) coatings bonded to homogeneous substrates. The FGM coating is assumed to be a (ZrO2) &ndash / (Ti-6Al-4V) layer. Homogeneous ceramic, metal &ndash / rich, ceramic &ndash / rich and linear variation material compositions are considered in the analyses. The surface crack is assumed to have a semi &ndash / circular crack front profile. The surface crack problem in the FGM coating &ndash / substrate system is examined under mechanical and transient thermal loading. Structural and thermal problems are modeled using three dimensional finite elements. Strain singularity around the crack front is simulated using collapsed 20 &ndash / node quarter &ndash / point brick elements. Three - dimensional displacement correlation technique is utilized to extract the stress intensity factors. The main results of the study are the stress intensity factors around the crack front for FGM coating - substrate structures subjected to uniform tension, bending, fixed &ndash / grip tension, three point bending and transient thermal loading.