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Finite element and analytical modelling of roughness induced fatigue crack closureParry, M. R. January 2001 (has links)
No description available.
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A microstructural study of the extension-to-shear fracture transition in Carrara MarbleRodriguez, Erika 01 November 2005 (has links)
Triaxial extension experiments on Carrara Marble demonstrate that there is a
continuous transition from extension to shear fracture on the basis of mechanical
behavior, macroscopic fracture orientation and fracture morphology where hybrid
fractures with extension and shear fracture characteristics are formed at the intermediate
stress conditions. Extension fracture surfaces display discrete, highly reflective cleavage
planes and shear fracture surfaces are covered with calcite gouge and display grooves
and striations that are aligned parallel to slip. This study uses the fractured samples that
were formed under triaxial extension experiments to characterize 1) microscopic surface
features using scanning electron microscopy, 2) fracture surface morphology using laser
profilometry, and 3) off-fracture damage using optical microscopy. These data are used
to test the step-crack model of fracture development for the formation of hybrid fractures.
Spectral analysis of the profiles demonstrates that microscopic roughness
decreases gradually across the extension-to-shear fracture transition in both the
orientations parallel and perpendicular to slip. However, macroscopic roughness
gradually increases then decreases across the transition in the direction parallel to slip.
The greatest macroscopic roughness occurs at the transition from extension fractures to tensile-hybrid fractures and is attributed to the presence of macroscopic steps in hybrid
fractures surfaces. The treads of the steps in the hybrid fracture surfaces have
characteristics of extension fracture surfaces and the risers have characteristics of shear
fracture surfaces. The treads have a right-stepping left lateral geometry that is
consistent with the step-crack model. Thin sections of hybrid fractures display
systematically spaced, pinnate, microfractures that emanate from both sides of the
macroscopic fracture surface. The pinnate fractures on both sides correlate across the
macroscopic fracture, suggesting that they are precursory to the formation of the
macroscopic fracture surface. The spacing to length ratio of the pinnate fractures and the
macroscopic orientation of the fracture surface are also consistent the relationship
dictated by the step-crack model of fault formation.
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DEVELOPMENT OF SIMULATION TECHNOLOGY FOR FORMING OF ADVANCED HIGH STRENGTH STEELChen, Xiaoming 04 1900 (has links)
<p>Advanced high strength steels (AHSS) exhibit significant higher springback and different fracture modes in forming processes and these problems cannot be accurately predicted using conventional simulation methods in many cases. In this thesis, new simulation technologies have been developed to improve the predictability for AHSS forming. The technologies integrated various aspects of simulation techniques, including development of material models and local formability criteria, calibration of the models with experimental data, and simulation method and parameter optimisations. Both laboratory and full scale parts were used to validate the simulation technologies developed. These technologies are originally applied to solve AHSS forming problems.</p> <p>The springback predictions have been significantly improved using the newly developed simulation technology. The technologies include the implementation of the smooth contact to reduce contact errors, modification of mass scaling to reduce dynamic effect, implementation of isotropic/kinematic hardening model and optimization of simulation parameters. Shear fracture (a stretch bending fracture on a small radius) have been successful predicted using Modified Mohr Coulomb (MMC) fracture criterion. Both laboratory experiments and full scale parts have been used to validate the predictions. Shearing and pre-forming effects on hole expansion and edge stretching have been investigated. A new approach was introduced to evaluate AHSS sheared edge deformation and quality by measuring material flow line angle change on a shearing edge. Shearing processes were simulated using MMC failure criterion and the sheared edge deformation has been integrated to hole expansion simulation to produce a more accurate prediction. The pre-forming effect on edge cracking has been investigated through both experiments and simulations. The limit strains have been measured by experiments. Simulation technology was also developed to predict surface strains of pre-form and subsequent stretching. Formulation of plane stress characteristics considering normal anisotropy have been developed and applied to analyze the flange deformations and optimum blanks for cup drawing. The method of plane strain characteristics has been used to predict earing throughout the entire cup drawing process.</p> / Doctor of Philosophy (PhD)
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Friction Stir Spot Welding of Ultra-High Strength SteelHartman, Trent J. 20 August 2012 (has links) (PDF)
Friction stir spot welding (FSSW) is quickly becoming a method of interest for welding of high strength steel (HSS) and ultra high strength steel (UHSS). FSSW has been shown to produce high quality welds in these materials, without the drawbacks associated with fusion welding. Tool grade for polycrystalline cubic boron nitride (PCBN) tools has a significant impact on wear resistance, weld quality, and tool failure in FSSW of DP 980 steel sheet. More specifically, for a nominal composition of 90% CBN, the grain size has a significant impact on the wear resistance of the tool. A-type tools performed the best, of the three grades that were tested in this work, because the grain size of this grade was the finest, measuring from 3-6 microns. The effect of fine grain size was less adhesion of DP 980 on the tool surface over time, less abrasive wear, and better lap shear fracture loads of the welds that were produced, compared to the other grades. This is explained by less exposure of the binder phase to wear by both adhesion and abrasion during welding of DP 980. A-type tools were the most consistent in both the number of welds per tool, and the number of welds that reached acceptable lap shear fracture loads. B-type tools, with a bimodal grain size distribution (grain size of 4 – 40 microns) did a little bit better than C-type tools (grain size of 12-15 microns) in terms of wear, but neither of them were able to achieve consistent acceptable lap shear fracture load values after the first 200 welds. In fact only one out of five C-type tools was able to produce acceptable lap shear fracture loads after the first 100 welds.
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Estudo do desempenho mecânico e microestrutural de uniões da liga de alumínio, AA6061-T6, por solda a ponto por fricção (FSpW)FERNANDES, Camila Albuquerque 29 July 2016 (has links)
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Previous issue date: 2016-07-29 / Solda a Ponto por Fricção (Friction Spot Welding – FSpW) é um processo
relativamente novo de soldagem desenvolvido e patenteado pelo GKSS na
Alemanha, que visa complementar e/ou substituir tecnologias de uniões de
soldagem convencionais. Essa nova tecnologia de soldagem se dá pela união no
estado sólido, onde duas ou mais chapas são unidas através da transferência de
energia térmica e mecânica que é gerada pela rotação da ferramenta. O presente
trabalho visou avaliar a aplicabilidade da união da liga de alumínio 6061-T6 pela
técnica de Solda a Ponto por Fricção. Essas soldas foram produzidas em
configuração de sobreposição utilizando condições de soldagens variadas, por meio
de diferentes combinações de velocidade de rotação, profundidade de penetração e
velocidade de penetração; os quais foram estabelecidos através de uma análise
estatística, utilizando como ferramenta o Box-Behnken. A integridade da junta
soldada foi avaliada através dos efeitos dos parâmetros de processo, na geometria e
na microestrutura da junta, e também no desempenho mecânico. Realizou-se
também a otimização do processo de soldagem e um estudo sobre o
comportamento de fratura das soldas submetidas a um esforço de cisalhamento. Os
resultados mostraram que a melhor combinação de parâmetros foi encontrada na
condição de soldagem: 130 rpm, 4,0 mm/s e 1,4 mm, com um valor médio de
resistência ao cisalhamento de 6243,29 N, com boa reprodutibilidade. O botão da
solda é constituído por três elementos geométricos que se mostraram inerentes ao
processo: cunha, união principal e união secundária. Estes elementos mostraram
possuir forte influência sobre o desempenho mecânico. / Friction Spot Welding (FSpW) is a relatively new welding process patented by GKSS
in Germany. This new welding technology is given by the joining in a solid state, in
which two or more plates are joined by thermal and mechanical energy transfer that
is generated by the rotation of the welding tool. The aim of this work is to study the
integrity of the welds of AA6061-T6, by the FSpW process. These welds were
produced in an overlapping configuration using different welding conditions, through
different combinations of rotational speed, depth of penetration and penetration
speed, which were set through a statistical approach using the Box-Behnken. The
integrity of the welded joint was evaluated by the effects of process parameters on
geometry and microstructure of the joint, and also in the mechanical performance. It
was also performed the optimization of the welding process and a brief study on the
fracture behavior of the welds subjected to a shear stress. The results showed that
the best combination of parameters was found in the welding condition: 130 rpm 4.0
mm / s and 1.4 mm with an average value of the shear strength of 6243.29 N, with
good reproducibility. The metallurgical investigation revealed three geometric
elements that are inherent to the process: primary union and secondary union and
hook. These elements were shown to have strong influence on the mechanical
performance.
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Etude de l'endommagement lors du démasselotage à chaud de bielles en fonte GS ADI moulées forgées. / Study of damage during the hot-cutting of molded and forged austempered ductile iron automotive connecting rods.Martinez, Thomas 19 December 2016 (has links)
Les travaux de cette thèse ont été initiés par la volonté industrielle de lever le verrou technologique du démasselotage à chaud de bielles en fonte GS. Ces bielles sont obtenues par un procédé de fabrication hybride innovant mêlant forgeage et fonderie en moule métallique. Ce procédé exige que le démasselotage soit effectué dans la « chaude de coulée » lorsque la fonte GS est en phase austénitique. La découpe de la fonte dans ces conditions de température fait apparaître des défauts majeurs sur les surfaces découpées préjudiciables à la suite du process. Pour pouvoir répondre à cette problématique, une étude est menée sur la caractérisation du comportement et de l’endommagement de la fonte GS dans les conditions de température du process. Une attention particulière est portée sur l’influence de la microstructure nodulaire sur les mécanismes de la rupture. Afin de se doter d’outils de simulation pour mettre au point le processus de démasselotage à chaud, les paramètres du modèle de comportement et d’endommagement de Gurson-Tvergaard-Needleman appliqués à la fonte GS sont identifiés par méthode inverse. Enfin, un plan d’expérience est déployé sur un démonstrateur de découpe instrumenté afin d’obtenir la configuration optimale des paramètres process pour une découpe sans défaut. Cette dernière étude met en avant la présence d’une transition d’un mode de rupture ductile à fragile lors de l’apparition des défauts de démasselotage. / This work was initiated by the technological problematic of hot trimming of nodular cast iron connecting rods. These connecting rods are obtained by an innovative hybrid process that combines forging and metal mold casting. This process requires the trimming to be conducted at high temperatures in the is the austenitic phase of the cast iron. Hot trimming of cast iron brings up major defects on the cut surfaces which are detrimental to the continuing process. To address this problem, a study is conducted on the characterization of the mechanical behavior of nodular cast iron in the process temperature conditions. A particular attention is paid to the influence of the nodular microstructure on the failure mechanisms. To develop simulation tools for nodular cast iron hot trimming, the parameters of Gurson-Tvergaard-Needleman model are identified by inverse method for our material. Finally, a design of experiment is deployed using an instrumented demonstrator to obtain the optimum parameters configuration for a maximized cut surface quality. This latest study highlights the presence of a transition from ductile to brittle mode of failure leading to the hot trimming defects.
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The Causes of “Shear Fracture” of Dual-Phase SteelsSung, Ji-Hyun 23 August 2010 (has links)
No description available.
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