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1
Thomson, Ronald D. "Ductile fracture by void nucleation, growth and coalescence". Thesis, University of Glasgow, 1985. http://theses.gla.ac.uk/6487/.
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Plastic deformation in ductile metals is limited by a mechanism in which voids, nucleated at second phase particles, grow and coalesce to form a crack. The results of a finite element solution for spherical elastic inclusions in a plastically deforming matrix are discussed. These results have been used in conjunction with experimental work using notched tensile specimens to generate multiaxial states of stress from which the local conditions leading to decohesion of the inclusion/matrix interface were determined. An important feature of these results is the statistical distribution of the interfacial strength. This distribution is bimodal, showing the presence of both weakly and strongly bonded particles. The latter have a modal strength of about 7 times the initial yield stress and the weakly bonded particles are assumed to be pre-existent. Experiments in plane and axisymmetric states of strain indicate that while the stress state is of relevance, the remote strain state is not. The absence of a macroscopic strain state effect is explained in terms of the statistical distribution of the voids nucleated from the population of randomly distributed inclusions. The stress and strain concentrations possible in local patches of high porosity have been investigated by a finite element approach based on the mechanics of a dilating continuum to determine void growth in the porous aggregate and the local conditions at failure. This investigation recognises the importance of the local hardening rate of the aggregate material and leads naturally to the idea of a size scale for failure, in the light of which the concept of a crack-like defect is re-examined.
2
Azhar, Mishaal. "2D Effects of Anisotropy on the Ductile Fracture of Titanium". Thèse, Université d'Ottawa / University of Ottawa, 2013. http://hdl.handle.net/10393/26280.
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Titanium is a widely used metal in industrial and commercial applications. It retains anisotropic mechanical properties at room temperature due to its HCP crystal structure. The effects of crystal orientation have been studied theoretically and through modeling though there is a lack of empirical data available on the topic.
The work presented here uses laser-machined voids along with EBSD analysis to study the ductility of grains in different orientations to better understand the microscale fracture process in α-titanium.
Experimental results show that hard grains with their c-axis parallel to the tensile direction behave in a less ductile manner than grains with their c-axis oriented away from the tensile direction. This is due to the basal slip systems activating in the former case and prismatic slip systems in the latter. Models utilized include the McClintock model for void growth, Brown-Embury model for void coalescence and FEM crystal plasticity simulations
3
Alinaghian, Yaser. "The Effect of Pre-strain and Strain Path Changes on Ductile Fracture". Thèse, Université d'Ottawa / University of Ottawa, 2013. http://hdl.handle.net/10393/23917.
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Industrial metal forming operations generally require several deformation steps in order to create the final product. The mechanical behavior of materials undergoing strain path changes can be very different from those deformed in a given direction to fracture.
The work presented here employed laser drilled model materials to better understand the effect of pre-strains and strain path changes on void growth and linkage leading to fracture is studied.
The experimental results show that increasing pre-strain results in faster void growth which was justified in terms work hardening rate in the sample. Scanning electron microscope images revealed that the ductility of the sample decreased with increasing pre-strain but only slightly compared to the large decrease in far field strain at failure. This suggests that pre-strain affects strain localization significantly and to a lesser extent the ductility. Finally a finite element model has been built to predict the linkage between voids.
4
Barsoum, Imad. "The effect of stress state in ductile failure". Doctoral thesis, Stockholm : Hållfasthetslära, Kungliga Tekniska högskolan, 2008. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-4667.
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Landron, Caroline. "Ductile damage characterization in Dual-Phase steels using X-ray tomography". Phd thesis, INSA de Lyon, 2011. http://tel.archives-ouvertes.fr/tel-00738820.
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Dans le cadre du développement de nuances d'aciers toujours plus performantes en termes de résistance à l'effort et à l'endommagement, les aciers Dual-Phase (DP) présentent un bon compromis résistance/ductilité. Cependant, il est nécessaire de disposer de meilleures connaissances concernant les mécanismes menant à la rupture de tels aciers. Les mécanismes d'endommagement ont ainsi été étudiés dans cette thèse à l'aide de la tomographie aux rayons X. Des essais de traction in-situ ont été réalisés sur plusieurs nuances d'aciers DP, un acier ferritique et un acier martensitique afin de caractériser chaque étape de l'endommagement ductile. Des observations qualitatives et des données quantitatives concernant la germination de l'endommagement, la croissance des cavités et la coalescence ont été recueillies lors de ces essais. Ces données quantitatives ont ensuite été utilisées pour le développement et/ou la validation de modèles d'endommagement. Une prédiction de la cinétique de germination a ainsi été proposée et la version du modèle de croissance de cavités de Rice et Tracey corrigée par Huang et prenant mieux en compte l'effet de la triaxialité a été validée expérimentalement. L'étape de coalescence des cavités menant à la rupture des matériaux a pour la première fois été caractérisée de façon quantitative dans un matériau industriel et des critères de coalescence ont été appliqués localement sur les couples de cavités présentes dans le matériau. L'utilisation de ces modèles analytiques a permis une meilleure compréhension des propriétés agissant sur les phénomènes mis en jeu. L'effet de la part cinématique de l'écrouissage sur la germination et la croissance de l'endommagement a notamment été souligné et validé par des essais de chargements complexes.
6
Jones, Matthew Kenneth. "Multiscale Analysis of Void Coalescence in Ductile Metals". MSSTATE, 2004. http://sun.library.msstate.edu/ETD-db/theses/available/etd-11112004-165827/.
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A mulitscale approach is used to model the coalescence of voids. At the microscale, cylindrical and spherical voids in nickel and the magnesium alloy AM60 are simulated through finite element analyses. The nickel cylindrical void simulations are compared to a set of experiments to validate this micromechanical finite element approach used to study void coalescence. At the macroscale, the coalescence portion of a microstructure-property material model is modified to reflect the behavior of three-dimensional spherical voids using results from the micromechanical simulations. An analysis of an automotive component illustrates the influence of void coalescence at the structural scale.
7
Griffin, Joel Sterling. "A Numerical and Experimental Investigation of Void Coalescence Causing Ductile Fracture". Thesis, Fredericton: University of New Brunswick, 2012. http://hdl.handle.net/1882/36784.
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A series of experiments and finite-element simulations were performed in order
to assess existing void coalescence criteria and propose a new model for the coalescence of cylindrical holes in a pure metal matrix during uniaxial stretching. The finite-element simulations were performed so that various plastic limit-load models could be evaluated at each strain increment during deformation, rendering predictions concerning the farfield strains required for coalescence. The experiments were performed in order to identify the actual far-field strain at the moment of incipient coalescence for the specimen geometries considered. The cylindrical-void models of Thomason (1990) and McClintock (1966) outperformed all of the other considered models in their original states. A modified form of the Ragab (2004) plastic limit-load model is proposed in the present work and is shown to have good agreement with the experimental results. The present model accounts for ligament work-hardening and ligament orientation.
8
Galvin, Kevin Patrick. "Growth and coalescence in condensation". Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/46310.
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Zhang, Hong Carleton University Dissertation Engineering Mechanical. "Fatigue crack growth and coalescence study". Ottawa, 1993.
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10
Camposagrado, Gabriel Rene. "Investigation of the cause and effect of air void coalescence in air-entrianed concrete mixes". Master's thesis, Mississippi State : Mississippi State University, 2004. http://library.msstate.edu/etd/show.asp?etd=etd-07082004-150638.
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Wang, Yong-Zhi. "Stress corrosion crack coalescence and lifetime prediction". Thesis, University of Newcastle Upon Tyne, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315623.
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Pushkareva, Marina. "Study of Void Growth in Commercially Pure Titanium". Thesis, Université d'Ottawa / University of Ottawa, 2017. http://hdl.handle.net/10393/35667.
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The ductile fracture process, which consists of the nucleation, growth and coalescence of microvoids, was extensively studied for materials deforming homogeneously. For materials with a non-homogeneous deformation behavior, such as those having hexagonal closed packed (HCP) crystal structure, experimental and numerical data is lacking. Therefore, the fracture properties of materials with such HCP structure, like titanium (used in aerospace and biomedical applications), zirconium (nuclear industry) and magnesium (manufacturing industry) are not well understood. The main research objective of this Ph.D. thesis is to better understand the mechanisms governing fracture in commercially pure (CP) titanium. In particular, the effect of grain orientation on void growth is investigated. The fracture process of CP titanium was visualized in model materials containing artificial holes. These model materials were fabricated using a femtosecond laser coupled with a diffusion bonding technique to obtain voids in the interior of titanium samples. Diffusion bonding was carried out either above or below the phase transformation temperature resulting in different microstructures. Changes in void dimensions during in-situ straining were recorded in three dimensions using x-ray computed tomography. Void growth obtained experimentally was compared with the Rice and Tracey model which predicted well the average void growth. However, a large scatter in void growth was observed experimentally and was explained in terms of differences in grain orientation which was confirmed by crystal plasticity simulations. It was also shown that grain orientation has a stronger effect on void growth than intervoid spacing and material strength. Intervoid spacing, however, appears to control whether the intervoid ligament failure is ductile or brittle. While this study showed a good agreement between experiments and simulations on average, there is no direct void growth comparison for particular grain orientations. In a follow-up study, an experimental approach was developed to directly relate the growth of a void to its underlying grain orientation. This is achieved by first annealing CP titanium samples below the α-β phase transformation temperature, then performing electron backscatter diffraction iii (EBSD) and finally diffusion bonding the samples together. Samples were then tested in x-ray tomography. This study showed the importance of the local state of strain on void growth. Crystal plasticity simulations that take into account the particular grain orientation and the local state of strain were found to predict well experimental void growth. Crystal plasticity simulations confirmed that the orientation of the voidcontaining grain is more important than the orientation of surrounding grains and more important than the volume fraction of voids, in order to determine void growth. This thesis on the growth and coalescence of voids is important to validate and improve the predictions of ductile fracture models and to design new materials with improved fracture properties.
13
Frise, Peter Richard Carleton University Dissertation Engineering Mechanical. "Fatigue crack growth and coalescence in a notch region". Ottawa, 1990.
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14
Nazarov, Andrei V., Alexander A. Mikheev, Irina V. Valikova, Aung Moe y Alexander G. Zaluzhnyi. "Kinetic of void growth in fcc and bcc metals". Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-193483.
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Nazarov, Andrei V., Alexander A. Mikheev, Irina V. Valikova, Aung Moe y Alexander G. Zaluzhnyi. "Kinetic of void growth in fcc and bcc metals". Diffusion fundamentals 6 (2007) 28, S. 1-2, 2007. https://ul.qucosa.de/id/qucosa%3A14203.
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Srivastava, Ankit. "Void Growth and Collapse in a Creeping Single Crystal". Thesis, University of North Texas, 2011. https://digital.library.unt.edu/ark:/67531/metadc84281/.
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Aircraft engine components can be subjected to a large number of thermo-mechanical loading cycles and to long dwell times at high temperatures. In particular, the understanding of creep in single crystal superalloy turbine blades is of importance for designing more reliable and fuel efficient aircraft engines. Creep tests on single crystal superalloy specimens have shown greater creep strain rates for thinner specimens than predicted by current theories. Therefore, it is necessary to develop a more predictive description of creep processes in these materials for them to be used effectively. Experimental observations have shown that the crystals have an initial porosity and that the progressive growth of these voids plays a major role in limiting creep life. In order to understand void growth under creep in single crystals, we have analyzed the creep response of three dimensional unit cells with a single spherical void under different types of isothermal creep loading. The growth behavior of the void is simulated using a three dimensional rate dependent crystal plasticity constitutive relation in a quasi-static finite element analysis. The aim of the present work is to analyze the effect of stress traixiality and Lode parameter on void growth under both constant true stress and constant engineering stress isothermal creep loading.
17
Hassan, S. A. "Particle nucleation and growth in emulsion polymerisation of styrene". Thesis, University of Manchester, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.377034.
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Hu, Jianfeng. "Grain growth by Ordered Coalescence of crystallites in Ceramics : Grain Growth Mechanisms, Microstructure Evolution and Sintering". Doctoral thesis, Stockholms universitet, Institutionen för material- och miljökemi (MMK), 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-88628.
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Grain growth and densification process play the two most crucial roles on the microstructure evolution and the achieved performances during sintering of ceramics. In this thesis, the grain growth of SrTiO3, BaTiO3-SrTiO3 solid solutions and Si3N4 ceramics during spark plasma sintering (SPS) were investigated by electron microscopy. SrTiO3 ceramics starting from nanopowders were fabricated by SPS. A novel grain growth mechanism was discovered and named as ordered coalescence (OC) of nanocrystals. This mechanism involved nanocrystals as building blocks and is distinguished from atomic layer epitaxial growth (AEG) in classical sintering theory. The results also revealed that the dominant grain growth mechanism can be changed by varying heating rates. Low rate (10°C/min) gives AEG, whereas high rates (≥ 50°C/min) yields three-dimensional coalescence of nanocrystals, i.e. OC. BaTiO3-SrTiO3 sintered bodies were made by SPS of BaTiO3 and SrTiO3 nanopowders mixtures. A novel Sr1-xBaxTiO3 “solid solution” with mosaic-like single crystal structure was manufactured by OC of the precursor crystallites. This reveals a new path for preparation of solid solution grains or composites. Si3N4 ceramics were prepared from α- or β-Si3N4 nanopowders at the same SPS conditions. The anisotropic OC of precipitated β-Si3N4 crystallites gives elongated β-Si3N4 grains at 1650°C using α-Si3N4 nanopowder. In contrast, AEG leads to the equi-axed β-Si3N4 grains using β-Si3N4 nanopowder. The metastable α- to β-Si3N4 phase transformation and OC accelerates anisotropic grain growth. Grain motions contribute to the densification process during pressureless sintered 3Y-ZrO2 (>87%TD) or SPS of SrTiO3 (>92%TD) ceramics. This extends the sintering range for active grain re-arrangement over that predicted by classical theory. In this thesis a new grain growth mechanism (OC) is proved by using SPS and nanopowders. By OC the microstructural evolution can be manipulated.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Accepted. Paper 3: Submitted. Paper 4: Manuscript. Paper 7: Accepted.
19
Mukherjee, Sunit. "Quantitative characterization of void nucleation and growth in HY-100 steels". Thesis, Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/19574.
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Dib, Jawhar Marie-Claire. "Study of the Coalescence Mechanisms during Silicone Foaming". Thesis, Lyon 1, 2012. http://www.theses.fr/2012LYO10051.
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Le principe d’élaboration des mousses de silicone est basé sur une compétition entre deux réactions mettant en jeu les fonctions hydrogénosilane portées par le polymère précurseur : la première provoque le dégagement d’un gaz (à l’origine de la formation des cellules) et la deuxième, l’hydrosilylation, bien connue et maîtrisée dans les silicones, conduit à la réticulation du milieu réactionnel. De ce fait, le contrôle des propriétés finales de la mousse nécessite la maîtrise de ces deux cinétiques de synthèse du gaz et de réticulation. D’autre part, les propriétés finales des mousses (type de porosité, densité apparente,…) dépendent également de la rhéologie (propriétés élongationnelles adéquates) ainsi que des charges ajoutées à la formulation. L’étude de la nucléation et la croissance cellulaire a été faite sous microscope optique. Les résultats montrent que le phénomène fondamental contrôlant la croissance des bulles est la coalescence. Sous l’influence couplée des effets de surface et viscoélastiques, les bulles se rapprochent et se déforment pour donner naissance à une forme intermédiaire avant d’atteindre leur forme finale. D’autre part, on a montré que l’air initialement dissous dans les formulations ainsi que l’air introduit par agitation permettent de réduire l’effet de peau, d’avoir une distribution de taille homogène et une meilleure structure cellulaire. D'autres facteurs ont également été étudiés dans le but de rendre la distribution de taille plus homogène et améliorer certaines propriétésA foam sample is assumed to be a set of bubbles embedded into a polymeric matrix with an initial gas overpressure. Silicon foams are produced by a competition between two reactions involving the hydrogenosilane functions carried by the polymer precursor: the first reaction generates gas (initiating cell formation) while the other one, hydrosilylation, well known and controlled in silicon, leads to the crosslinking of the rising foam. Thus, obtaining enhanced foam properties requires a good balance between two reactions, crosslinking and gas generation. On the other hand, the final characteristics of the foam (porosity, bulk density…) largely depend on the rheology of the mix (appropriate elongational properties) as well as the added fillers. Nucleation and cell growth were carried out under optical microscopy. The experiments show that the main phenomenon controlling cell growth is bubble coalescence. Due to the surface effects and the viscoelastic properties, bubbles approach from each other and get deformed giving birth to an intermediate shape before reaching their final geometry. Many parameters have direct effect on foam properties. In fact, dissolved gas in formulas as well as the air introduced during manual mixing, reduce the skin effect and guarantee a homogeneous cell size distribution and a better foam structure. Other factors have also been studied in order to render size distribution more homogeneous and improve certain properties
21
Witts, Nigel P. "Numerical modelling of creep crack propagation resulting from grain-boundary void growth". Thesis, University of Leicester, 1993. http://hdl.handle.net/2381/34726.
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A variational principle, and a corresponding finite-element procedure, have been developed to predict steady-state, creep crack growth rates. The cracks are assumed to propagate in a creeping material by the growth, and coalescence of micro-voids on the grain boundary, in the damage zone, ahead of the crack tip. Three mechanisms of void growth have been considered: grain-boundary diffusion, surface diffusion and power-law creep. The power-law creeping material surrounding the crack tip and damage zone has been modelled using well-established finite element methods for incompressible material; and the stiffness of the damage zone elements added to the overall stiffness matrix, to obtain a complete solution. Both stationary cracks, and by an extension of the procedure, growing cracks have been analysed. Only mode-1 type loading has been considered. Loads were applied on a circular boundary around the crack tip in accordance with the appropriate HRR field, with an amplitude given by the C* path-independent integral. Results of steady-state crack propagation rates arising from void growth by grain- boundary diffusion have been compared with published results from analytical models. These comparisons indicate that the developed technique works well, and is efficient in terms of computer resources. The transition between constrained and unconstrained void growth is particularly well defined. For the other two void growth mechanisms results are presented in a similarformat to grain-boundary diffusion, and comparisons made where possible. The relationships between crack velocity and the variables C* and damage-zone size have been established for the three void-growth processes. The methodology and numerical procedures developed have been shown to be viable, and capable of further development. This might include the consideration of more complex void-growth models, or the inclusion of the procedure within an existing finite element code.
22
Dennis, Roger James. "Mechanistic modelling of deformation and void growth behaviour in superalloy single crystals". Thesis, Imperial College London, 2000. http://hdl.handle.net/10044/1/11302.
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In this work the constitutive behaviour and the influence of casting defects on the failure of Ni-base single crystal (SC) superalloy components is investigated. It is well known that the presence of casting-related porosities can lead to the nucleation of microcracks under both creep and fatigue conditions and thus, ultimately produce component failure. A rate dependent crystallographic formulation is introduced to describe the inelastic deformation behaviour of the latest generation of single crystal superalloys. The evolution of the current dislocation and obstacle network is described through appropriate slip resistance and internal or back stress variables for each slip system. Good correlations are obtained between experimental data and numerical predictions within the 750◦C .- 950◦C temperature range and for < 001 > and < 111 > crystallographic orientations. The formulation is then numerically implemented into the FE method and used to investigate the deformation of a representative material volume containing a spherical void of approximately 20 micrometers diameter. The functional dependence of the void growth rates in terms of material anisotropy, stress state, temperature and interaction with a free surface is determined. It is shown that the rate of growth of casting defects in an infinite single crystal medium is strongly dependent on the applied triaxiality and relative orientation between the crystallographic axes and the applied stresses. Furthermore, it has been found that, for the acceleration of the defect growth rate as the result of its proximity to a free-surface to be non-negligible, the void needs to be within two diameters of the free surface. Based on the above results, a framework is proposed to describe the growth of embedded casting defects within superalloy single crystals under a given applied multiaxial stress state. The framework provides an explicit link between the mesoscopic (at the level of the voids) and the macroscopic length scales. A number of blunt notch bar creep specimens and thermo-mechanical cyclic stress-strain specimens were tested to validate both the rate dependent crystallographic formulation and the micro-mechanics void growth model. In addition microstructural analysis of the blunt notch bars provided necessary data for the development of a micro-crack initiation criterion. A life assessment methodology combining these models is developed and applied in the analysis of an actual gas turbine blade. It is expected that the understanding of defect growth kinetics and the corresponding damage accumulation will be beneficial in the design and life prediction of superalloy components.
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Rengaraj, Kishen. "Void growth mitigation in high heating rate Out-of-Autoclave processing of composites". Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/32266/.
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Increased pressure on the transport industry to reduce greenhouse gas emissions has hastened the adoption of high performance composites, particularly in the aerospace industry where the value of weight saving is very high. However, the current method of choice for manufacturing high performance composites (autoclave processing) is not cost effective for processing large (greater than 5m2) structural composite components. Developments in Out-of-Autoclave (OoA) prepreg systems have facilitated the use of vacuum only consolidation pressure to process laminates with autoclave level mechanical properties. However, owing to the low consolidation pressure, the process is heavily dependent on de-bulk quality and low cure temperatures; leading to reduced margin for error as well as long cycle times. In parallel, developments in high heating rate OoA processes have been shown to enable short cure cycle times and autoclave-level mechanical properties; albeit with a high tendency towards porosity. To date, studies on high heating rate OoA processing have been limited and the processes are not well understood. The main objectives of this self-funded study were to understand the mechanism of void growth mitigation in high heating rate OoA processes and to study the feasibility of achieving further reduction in cycle time and cost, whilst maintaining high mechanical properties. The primary mechanism of void growth was identified and an analytical model was used to predict the propensity for void growth during a given cure cycle. The model outcome highlighted a window within the cure cycle during which void growth takes place. It was hypothesised that a reduced time to resin gelation in high heating rate processes can reduce the window for void growth, leading to lower laminate porosity. A novel high heating rate pressurised tooling system (the Pressure Tool) was developed to process laminates at 15oC/min combined with the application of up to 7 Bar hydrostatic pressure. The Pressure Tool was used to verify the hypothesis that reduction in size of the window for void growth, facilitated by high heating rate, can lead to lower laminate porosity. Good agreement was observed between the model outcome and the experimental results. Studies have claimed that the reduction in resin minimise viscosity due to high heating rate can lead to gains in mechanical properties; sometimes even higher than that of autoclave cured laminates. OoA prepregs cured using up to 15oC/min heating combined with up to 3 Bar hydrostatic pressure did not result in the claimed additional gain in mechanical properties. The study confirmed earlier suggestions that additional factors such as void geometry and location within the laminate have to be taken into consideration. The final part of this thesis addresses the physical limitations to high heating rate processes; such as, the effect of tooling material, process ancillaries, laminate thickness and resin kinematics on reducing cure cycle time. The poor thermal characteristics of commonly used process ancillaries limit the dissipation of energy released by the laminate during cure. Due to which, laminate core temperature can exceed by up to 5oC, even if the laminate is processed on a highly conductive tooling material. The optimum tooling material to achieve reductions in cure cycle time whilst minimising laminate core thermal overshoot was found to have a combination of high thermal conductivity and low thermal mass. However, currently used tooling systems are not optimum for achieving further reductions in cycle time, due to unfavourable combination of thermal mass and thermal conductivity. Furthermore, the high reactivity of current resin systems and the inherently poor thermal conductivity of the polymer matrix limits the gains in cure cycle times that can be achieved.
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Traiviratana, Sirirat. "A molecular dynamics study of void initiation and growth in monocrystalline and nanocrystalline copper". Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2008. http://wwwlib.umi.com/cr/ucsd/fullcit?p3337304.
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Thesis (Ph. D.)--University of California, San Diego, 2008.Title from first page of PDF file (viewed Jan. 9, 2009). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 179-188).
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McLeod, Ronald Daniel Carleton University Dissertation Engineering Mechanical. "Investigation of subcooled void fraction growth in water under low pressure and low flowrate conditions". Ottawa, 1986.
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Bibeau, Eric Louis. "Experimental investigation of subcooled void growth for upflow and downflow at low velocities and low pressure". Thesis, University of British Columbia, 1988. http://hdl.handle.net/2429/28365.
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A two-phase experimental loop was designed and built to simulate the operating
conditions of the SLOWPOKE reactor. Void growth was measured for both upflow and downflow for velocities between 0.07 to 0.46 m/s and at a pressure of 155 kPa. The buoyancy effect causes the Onset of Significant Void, OSV, to occur at higher subcooling for downflow than for upflow. This effect is maximum close to the bubble rise velocity (0.23 m/s) and decreases at higher velocities. The results indicate that bubble detachment is not the only critical parameter affecting OSV. The OSV correlations from the literature
did not predict the experimental results well. Investigation of the heat transfer mechanisms indicates that fully developed sub cooled boiling occurs prior to OSV.Applied Science, Faculty of
Mechanical Engineering, Department of
Graduate
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Trejo, Navas Victor. "Etude numérique des micromécanismes de l'endommagement ductile dans des microstructures hétérogènes". Thesis, Paris Sciences et Lettres (ComUE), 2018. http://www.theses.fr/2018PSLEM066.
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Ce travail vise à mieux comprendre et modéliser les mécanismes de rupture ductile pour les microstructures hétérogènes. La principale méthodologie du projet repose sur la combinaison de trois techniques : la laminographie in situ par rayonnement synchrotron,la corrélation d’images volumiques (DVC) et la simulation par éléments finis (FE) avec des fonctionnalités avancées de maillage et de remaillage automatique. Ce cadre expérimental-numérique est utilisé pour réaliser des simulations numériques à l’échelle microscopique avec des microstructures immergées et des conditions aux limites réalistes. L’effet de la segmentation d’image sur les observables mécaniques finales est évalué par comparaison de trois méthodologies différentes. Ce travail permet, pour la première fois, d’accéder aux valeurs de déformation locales au début de la coalescence pour des microstructures réelles et des conditions aux limites exactes. Les résultats des simulations sont utilisés pour définir et valider des critères de rupture locaux fonctions des contraintes/déformations ou un critère de coalescence basé sur l’évolution de la distance entre les poresThis work constitutes a micromechanical study of ductile fracture.The main methodology consists of acombination of three techniques: Insitu synchrotron radiation computed laminography, digital volume correlation and finite element simulations with advanced meshing and remeshing capabilities. This experimental numerical framework is employed to carry out numerical simulations at the microscale with immersed microstructuresand realistic boundary conditions. The studied material is nodular cast iron, but this methodology has been successfully applied to other materials such as aluminum alloys.Since this is a recent methodology,attention is paid to image immersion as a source of uncertainty. More specifically, the effect of image segmentation on the final mechanical observables, is assessed through comparison of three different segmentation methodologies. The prediction capacity of the numerical simulations is then satisfactorily evaluated through a detailed comparison with experimental measurements. Finally,after evaluating different approaches,the results of the simulations are used to propose and validate a coalescence criterion based on the evolution of the intervoid distance
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Hu, Chao. "Locally enhanced voronoi cell finite element model (LE-VCFEM) for ductile fracture in heterogeneous cast aluminum alloys". The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1199209208.
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McDermott, Patrick M. "Development and implementation of a shell element with pressure variation through the thickness and void growth and nucleation effects". Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1999. http://handle.dtic.mil/100.2/ADA369167.
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Thesis (Degree of Mechanical Engineering) Naval Postgraduate School, September 1999."September 1999". Thesis advisor(s): Young W. Kwon. Includes bibliographical references (p. 107-109). Also Available online.
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Forestier-Coste, Louis. "Croissance et coalescence de bulles dans les magmas : analyse mathématique et simulation numérique". Phd thesis, Université d'Orléans, 2012. http://tel.archives-ouvertes.fr/tel-00736634.
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Cette thèse est consacrée à l'étude mathématiques et numérique d'un problème physique issu de la volcanologie. On s'intéresse à la modélisation polydisperse de croissance de bulles par exsolution, décompression et coalescence. Un modèle de croissance polydisperse a été proposé dans la litérature, mais ne prenait en compte que le volume des bulles, ce qui restreint le domaine d'application car la croissance par exsolution dépend également de la masse d'eau présente dans la bulle. Pour améliorer ce modèle, nous sommes parti d'une description monodisperse adimensionnelle de la croissance d'une bulle par décompression et exsolution, donnée par le couplage de deux EDO et une EDP. Un code numérique est proposé pour résoudre le problème monodisperse et est actuellement utilisé. Après avoir validé numériquement ce code et considéré plusieurs cas limites, nous avons étudié les solutions du problème et défini une approximation du flux qui nous permet de découpler le système d'équations. Ensuite, nous avons étendu le modèle polydisperse de une à deux dimensions. Une résolution de la coalescence est proposée et couplée avec le modèle de croissance polydisperse. La résolution de la coalescence est confrontée à d'autres schémas numériques en une et deux dimensions afin de valider le schéma numérique proposé. Les premiers test numériques appliqués au problème physique donnent de bon résultats.
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Arambula, Mercado Edith. "Influence of fundamental material properties and air void structure on moisture damage of asphalt mixes". [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-1276.
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Keralavarma, Shyam Mohan. "A micromechanics based ductile damage model for anisotropic titanium alloys". [College Station, Tex. : Texas A&M University, 2008. http://hdl.handle.net/1969.1/ETD-TAMU-2620.
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Barrioz, Pierre-Olivier. "Rupture ductile des matériaux CFC irradiés". Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLN001/document.
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Le mode de rupture des alliages CFC est généralement de type ductile par des mécanismes de germination, croissance et coalescence de cavités internes micrométriques et peut être modifié par l’irradiation. L’irradiation neutronique de ces alliages conduit à la création de défauts cristallins qui induisent un durcissement, une perte de la capacité d’écrouissage, une chute très importante de la ténacité et un mode de déformation localisé à l’échelle intragranulaire. La compréhension des mécanismes physiques élémentaires de la rupture ductile est indispensable au développement de modèles quantitatifs pour prédire la ténacité des matériaux CFC irradiés. Pour cela, trois différents points ont été étudiés dans cette thèse : (1) L’influence de la localisation de la déformation induite par l’irradiation sur la croissance et la coalescence de cavités : des expériences modèles in-situ MEB de croissance et coalescence de cavités micrométriques dans des matériaux irradiés aux protons ont été réalisées. Les résultats montrent un effet limité de la localisation pour des cavités de la taille des grains et une diminution de l’influence de la localisation avec l’augmentation du niveau de déformation pour des cavités intragranulaires. Par conséquent, les modèles homogénéisés de matériaux poreux développés pour les matériaux non irradiés pourraientt être utilisés en première approximation pour modéliser la rupture ductile des matériaux irradiés. (2) Le comportement sous chargement mécanique de nano-porosités d’irradiation et leur contribution éventuelle à la rupture : l’étude expérimentale et numérique de la déformation de cavités dans un matériau nanoporeux a permis de mettre en évidence la très forte hétérogénéité de la déformation à cette échelle et l’absence d’effet de taille significatif sur la déformation des cavités de diamètre supérieur à 10 nm en traction simple. (3) Le développement de modèles homogénéisés de matériaux poreux valides aux fortes porosités : deux nouveaux critères de coalescence obtenus par analyse limite sont proposés et validés par comparaison à des simulations d’analyses limites numériques, dans le cas de cavités de type fissures et de cavités ellipsoïdalesThe failure mode of FCC alloys is generally ductile through nucleation, growth and coalescence of micrometric voids, and can be modified by irradiation. Neutron irradiation of these alloys leads to the creation of crystalline defects that induce hardening, loss of work hardening capability, a very large drop in fracture toughness and a heterogeneous deformation mode at the grain scale. Understanding the elementary physical mechanisms of ductile fracture is essential for the development of quantitative models to predict fracture toughness of irradiated FCC materials. Thus, in this thesis, three different subjects have been studied. (1) Influence of the localization of deformation induced by irradiation on void growth and coalescence: Model experiments of growth and coalescence of micrometric voids in proton-irradiated materials have been performed based on SEM in-situ tests. Results show a limited effect of localization for grain-size voids and a decreasing influence of localization with increasing level of deformation for intragranular voids, so that homogenized models of porous materials developed for unirradiated materials may be used as a first approximation to model the ductile fracture of irradiated materials. (2) The behavior under mechanical loading of nanovoids generated under irradiation and their possible contribution to fracture: The experimental and numerical study of void deformation in a nanoporous material highlights the very strong heterogeneity of the deformation at this scale and the absence of significant size effect for voids of diameter greater than 10 nm under tensile loading. (3) Development of homogenized models for porous materials valid at high porosities: Two new coalescence criteria obtained by limit analysis are proposed and validated by comparison with numerical limit analysis simulations, in the case of penny-shaped cracks and ellipsoidal voids
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Sartori, Cédric. "Modélisation de l'endommagement dynamique avec prise en compte de l'effet de forme des cavités". Thesis, Université de Lorraine, 2014. http://www.theses.fr/2014LORR0195.
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L'endommagement des matériaux ductiles est un processus impliquant trois étapes : la nucléation, la croissance et la coalescence de vides. La phase de croissance des vides a été largement étudiée dans la littérature. Il a été montré que, durant cette étape, la forme des vides joue un rôle fondamental sur le comportement macroscopique du matériau. Dans le cas de sollicitations dynamiques, les effets micro inertiels, qui résultent des accélérations subies par la matrice au voisinage du vide, influent eux aussi fortement sur la croissance des vides. Cependant, les travaux intégrant simultanément ces deux contributions (effets inertiels et forme) sont très rares. L'objectif de ce travail est de proposer un modèle de comportement pour les matériaux poreux qui prend en compte la forme des vides et les effets micro inertiels. Dans une première partie, un volume élémentaire représentatif défini par deux ellipsoïdes allongés confocaux est utilisé pour représenter le matériau poreux. La matrice est rigide viscoplastique. En se basant sur les travaux de Molinari et Mercier (2001), la contrainte macroscopique se décompose en une partie statique et une partie dynamique. La contrainte statique est décrite par le modèle de Gologanu et al. (1997). La contrainte dynamique est obtenue en adoptant le champ de vitesse de Gologanu et al. (1993). Avec cette modélisation, il est montré que la contrainte dynamique est liée de façon quadratique au tenseur des vitesses des déformations et de façon linéaire à sa dérivée par rapport au temps. Le modèle fait l'objet d'une validation sur la base de comparaisons avec des résultats de calculs par éléments finis. Différentes forme de vides et valeurs de la porosité ont été considérées. Dans une seconde partie, le cas de matériaux contenant des vides aplatis est abordé ; le volume élémentaire représentatif est défini par deux ellipsoïdes confocaux aplatis. La contrainte statique est toujours décrite par le modèle de Gologanu et al. (1997). La contrainte dynamique est obtenue en adoptant le champ de vitesse de Gologanu et al. (1994). La procédure de validation est identique à celle mise en œuvre dans le cas des vides allongés. Une bonne adéquation entre les résultats du modèle et les résultats de calculs par éléments finis est retrouvée. L'utilisation des surfaces d'écoulement permet de mettre en lumière les effets de la forme des vides sur le comportement du matériau poreux sous chargement dynamique. En fonction du chargement appliqué, certaines géométries de vide favorisent la déformation du matériau. Le cas particulier du vide sphérique est étudié comme limite des deux modèles. La continuité des deux modèles est démontrée. L'évolution de la porosité et de la forme des vides dans un matériau poreux sous chargement dynamique est analysée. Des comparaisons avec des résultats de simulations par éléments finis sont proposées. L'influence de la triaxialité et de la vitesse du chargement sur le comportement dynamique du matériau poreux est étudiée, ainsi que celle de la forme initiale du vide. Au final, il est démontré que le modèle développé dans cette thèse permet de retrouver les tendances fournies par les calculs éléments finisThe ductile fracture mechanism involves three stages: void nucleation, void growth and void coalescence. Under dynamic loading conditions, void growth is strongly affected by microinertia effects resulting from the local acceleration of the matrix material in the vicinity of the void. Several works devoted to quasi-static conditions also show that void shape has a strong impact on the behavior of porous ductile materials. However, there exist only few works considering the combined effect of these two contributions. In the present work, we propose an original, multi-scale constitutive model of porous materials, taking into account void shape and micro-inertia effects. In a first step, a representative volume element defined by two confocal prolate spheroids is used to represent the porous material. The matrix behavior is assumed to be rigid-viscoplastic. Based on the work of Molinari and Mercier (2001), the macroscopic stress is the sum of a static and a dynamic part. The static contribution is described by the Gologanu et al. model (1997). The dynamic stress is derived by choosing the trial velocity field proposed by Gologanu et al. (1993). With the present modeling, a link is established between the macroscopic dynamic stress, on the one hand and, the macroscopic strain rate tensor and its time derivative on the other hand. To validate the proposed model, finite element computations have been performed for different void geometries and void volume fractions. The influence of micro-inertia on the macroscopic flow surface is analyzed and a good agreement between modeling and simulations is observed. In a second step, a representative volume element defined by two confocal oblate spheroids is used to represent the porous material. For this configuration, the static contribution is also described by using the Gologanu et al. model (1997), while the derivation of the dynamic stress is based on the trial velocity field proposed by Gologanu et al. (1994). As for the prolate case, a good agreement is retrieved between model predictions and results of finite element computations. The spherical void configuration is investigated as the limit case for the oblate and prolate models. The continuity between the two models is established. Finally, the proposed models are combined to investigate the porosity and void shape evolutions in a porous solid under dynamic loadings. A parametric study has been performed by varying the stress triaxiality, the initial void shape and the loading rate. Significant void shape variations are observed for low triaxiality loadings. With the present modeling, the void can evolve from prolate to oblate shapes (and the reverse). Model predictions are compared to finite element computations
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Zhou, Jun. "Numerical Modeling of Ductile Fracture". University of Akron / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=akron1384774266.
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Hrycaj, Philippe. "Modélisation de la croissance de cavités et du contact unilatéral en viscoplasticité : Application au laminage perçage". Valenciennes, 1991. https://ged.uphf.fr/nuxeo/site/esupversions/cbbf12c1-7d8c-4d44-bd00-53f61a2576af.
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Lors des grandes transformations thermoviscoplastiques des aciers, on observe souvent sous l'influence des conditions de contact, une détérioration progressive avec l'apparition de zones ou les microcavités se développent plus rapidement qu'ailleurs. La prévision de la croissance des cavités est obtenue à partir d'un potentiel viscoplastique s'exprimant en fonction des contraintes. Des schémas implicites sont proposés pour le calcul de la déformation, de la température et de la fraction volumique de cavités. Le contact unilatéral tridimensionnel est modélisé par une méthode implicite de pénalisation de la forme variationnelle associée aux équations d'équilibre. Les contacteurs, mobiles en translation ou en rotation, sont discrétisés à partir de la bibliothèque de primitives, implémentée dans le programme de calcul par la méthode des éléments finis Astrid. Les modèles sont valides par comparaison à des solutions analytiques ou à des références expérimentales puis utilises pour le laminage perçage d'ébauches tubulaires. La carte des variables d'état est présentée pour différentes configurations de perçage et la répartition de la croissance de cavités est plus particulièrement analysée. En conclusion, l'auteur montre l'apport essentiel et les performances des modèles développés.
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Grosset, Lisa. "Développement et validation d’un modèle de vieillissement thermique d’alliages d’aluminium pour application aéronautique". Thesis, Lyon, 2016. http://www.theses.fr/2016LYSEM010.
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Les exigences des clients du secteur aéronautique imposent aux entreprises de prendre en compte pour le dimensionnement les effets de la température sur toute la durée de vie du produit. Cependant, aucune loi ne permet actuellement de prévoir l’impact du vieillissement thermique sur le comportement des matériaux. Ce travail a pour objectif de comprendre le mécanisme de vieillissement thermique des alliages d’aluminium et son impact sur les propriétés mécaniques, mais surtout d’acquérir un outil performant capable d’obtenir rapidement des données matériaux après vieillissement.Au cours de cette étude, de nombreuses analyses microstructurales et mécaniques ont été réalisées sur trois alliages d’aluminium à durcissement structural. Différentes combinaisons temps-température de vieillissement ont été testées afin de disposer d’une large base de données sur ces matériaux (caractéristiques mécaniques statiques Rm, Rp0,2 et dureté et tailles des précipités durcissants). Ces données ont ensuite été compilées dans un modèle de vieillissement basé sur les théories classiques de durcissement structural, de croissance et de coalescence des précipités.Le modèle de vieillissement créé répond au besoin initial et prédit de façon conservative le comportement mécanique des alliages ayant subi un vieillissement thermique isotherme. Des axes d’amélioration sont envisagés pour ce modèle évolutif, comme l’intégration de la prévision du comportement en fatigue ainsi que le traitement de cas anisothermes pour une représentation plus réelle des conditions de service des pièces aéronautiquesIn aeronautics, customers ask companies to consider the effects of temperature over the entire life of the product in structural requirements. Indeed, aircraft parts are demanded to last longer (up to 90 000 hours) and operate at higher temperatures (up to 250°C). No laws enable to predict the impact of thermal ageing on materials behavior. Current practices are to perform mechanical testing after ageing in ovens at various temperatures, but they are expensive and incompatible with the development schedules. This work aims to understand the thermal aging mechanism of aluminum alloys and its impact on mechanical properties, but especially to acquire a powerful tool able to quickly obtain material data after aging.During this study, many microstructural and mechanical analyses were conducted on three precipitation hardened aluminum alloys. Different combinations of aging time and temperature were tested to get a large database of these materials (static mechanical characteristics Rm, Rp0,2 and hardness and sizes of hardening precipitates). These data were then compiled into a computing aging model based on the classical theories of precipitation hardening, growth and coarsening of precipitates.The created aging model responds to the initial need and can conservatively predict the mechanical behavior of aluminum alloys under isothermal aging. Improvement areas are considered for this evolutionary model, such as the integration of fatigue behavior prediction and the inclusion of thermal cycles for a more realistic representation of service conditions of aircraft parts
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Le, Vi. "Processus de branchement avec interaction". Thesis, Aix-Marseille, 2014. http://www.theses.fr/2014AIXM4743/document.
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Cette thèse se compose de quatre chapitres:Le chapitre 1 étudie la distribution du temps de coalescence (plus récent ancêtre commun) de deux individus tirés au hasard (uniformly) dans la génération actuelle d'un processus de Bienaymé-Galton-Watson en temps continu.Dans le chapitre 2, nous obtenons une représentation de la diffusion de Feller logistique en termes des temps locaux d'un mouvement brownien réfléchi H avec une dérive qui est affine en le temps local accumulé par H à son niveau actuel.Le chapitre 3 considère la diffusion de Feller avec compétition générale. Nous donnons des conditions précises sur le terme de la concurrence, pour le but de décider si le temps d'extinction (qui est aussi la hauteur du processus) reste borné ou non lorsque la taille initiale de la population tend vers l'infini, et de même pour la masse totale du processus.Dans le chapitre 4, nous généralisons les résultats du chapitre 3 pour le cas du processus de branchement à espace d'état continu avec compétition à trajectoires discontinuesThis thesis consists of four chapters:Chapter 1 investigates the distribution of the coalescence time (most recent common ancestor) for two individuals picked at random (uniformly) in the current generation of a continuous time Bienaymé-Galton-Watson process.In chapter 2 we obtain a Ray-Knight representation of Feller's branching diffusion with logistic growth in terms of the local times of a reflected Brownian motion H with a drift that is affine in the local time accumulated by H at its current level.Chapter 3 considers the Feller's branching diffusion with general competition. We give precise conditions on the competition term, in order to decide whether the extinction time (which is also the height of the process) remains or not bounded as the initial population size tends to infinity, and similarly for the total mass of the process.In chapter 4 we generalize the results of chapter 3 to the case of continuous state branching process with competition which has discontinuous paths
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Li, Duan. "Rapid sintering of ceramics by intense thermal radiation". Doctoral thesis, Stockholms universitet, Institutionen för material- och miljökemi (MMK), 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-124131.
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Sintering is an important processing step for obtaining the necessary mechanical stability and rigidity of ceramic bulk materials. Both mass and heat transfer are essential in the sintering process. The importance of radiation heat transfer is significantly enhanced at high temperatures according to the well-known Stefan-Boltzmann’s law. In this thesis, we modified the pressure-less spark plasma sintering set-up to generate intense thermal radiation, aiming at rapid consolidation of ceramic bulk materials. This approach was named as “Sintering by Intense Thermal Radiation (SITR)” as only thermal radiation contributed. Firstly, the heat and mass transfer mechanisms during the SITR process were studied by choosing zirconia ceramics as references. The results revealed that the multiple scattering and absorption of radiation by the materials contributed to the heat diffusion. The observed enhanced densification and grain growth can be explained by a multiple ordered coalescence of zirconia nanocrystals using high heating rates. Secondly, the temperature distribution during the SITR process was investigated by both numerical simulation and experimental verifications. It showed that the radiator geometry, sample geometry and radiating area were influencing factors. Besides, the change of material and geometry of the radiators resulted in an asymmetric temperature distribution that favored the formation of SiC foams. The foams had gradient structures with different open porosity levels and pore sizes and size distributions. Finally, ceramic bulk materials were successfully fabricated by the SITR method within minutes. These materials included dense and strong ZrO2 ceramics, Si3N4 foams decorated with one-dimensional nanostructures, and nasal cavity-like SiC-Si3N4 foams with hierarchical heterogeneities. Sufficient densification or formed strong necks were used for tailoring these unique microstructures. The SITR approach is well applicable for fast manufacture of ceramic bulk materials because it is clean and requires low energy consumption and properties can be controlled and tuned by selective heating, heating speed or temperature distribution.At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Manuscript.
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Raut, Vivek P. "METHODS TO QUANTITATIVELY ASSESS THE PERFORMANCE OF CONNECTIVE TISSUE PROGENITOR CELLS IN RESPONSE TO SURFACE MODIFIED BIOMATERIALS". Case Western Reserve University School of Graduate Studies / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=case1372334668.
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Fansi, Joseph. "PREDICTION PAR ELEMENTS FINIS DE LA RUPTURE DES ACIERS DUAL_PHASE EN UTILISANT UN MODELE DE GURSON AVANCE". Phd thesis, Ecole nationale supérieure d'arts et métiers - ENSAM, 2013. http://tel.archives-ouvertes.fr/tel-00869032.
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L'actuelle investigation numérique du Gurson-Tvergaard-Needleman (GTN) modèle avancé est une extension du travail de Ben Bettaieb et al. (2011). Le modèle a été implémenté à l'aide d'une sous routine (VUMAT) contenu dans le code commerciale d'éléments finis Abaqus/explicit. Le modèle d'endommagement améliore l'original en intégrant les trois mécanismes d'endommagement, la nucléation, la croissance, et la coalescence des cavités. Le modèle d'endommagement intègre les lois de nucléation et de croissance basés sur les phénomènes purement physiques. Ces nouvelles contributions incluant l'influence de l'écrouissage cinématique, ont été validées par les résultats de mesures expérimentales de tomographie à rayon X à haute résolution. Aussi, l'implémentation numérique de l'écrouissage cinématique dans le modèle modifié a contraint de proposer et de réarranger la définition de la triaxialité que l'on trouve habituellement dans la littérature. A coté de cela, un second critère d'initiation à la rupture basé sur l'ultime distance inter-cavités a été inclue afin de localiser et de quantifier avec plus de précision la distribution des déformations peu avant que le matériau ne casse complètement. L'actuel modèle d'endommagement a été appliqué dans des conditions industrielles pour prédire l'évolution de l'endommagement, l'état de contraintes, et l'initiation à la rupture pour différentes géométries de tôles et sur des essais d'emboutissage de tôles minces.
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Thakare, Amol G. "Numerical Simulations Of Void Growth In Ductile Single Crystals". Thesis, 2008. http://hdl.handle.net/2005/854.
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The failure mechanism in ductile materials involves void nucleation, their growth and subsequent coalescence to form the fracture surface. The voids are generated due to fracture or debonding of second phase particles or at slip band intersections. The triaxial stress field prevailing around a crack tip and in the necking region strongly influences the growth of these voids. In the initial stages of deformation, these microscale voids are often sufficiently small so that they exist entirely within a single grain of a polycrystalline material. Further, single crystals are used in high technology applications like turbine blades. This motivates the need to study void growth in a single crystal while investigating ductile fracture. Thus, the objectives of this work are to analyze the interaction between a notch tip and void as well as the growth and coalescence of a periodic array of voids under different states of stress in ductile FCC single crystals.
First, the growth of a cylindrical void ahead of a notch tip in ductile FCC single crystals is studied. To this end, 2D plane strain finite element simulations are carried out under mode I, small scale yielding conditions, neglecting elastic anisotropy. In most of these computations, the orientation of the FCC single crystal is chosen so that notch lies in the (010) plane, with notch front along the [101] direction and potential crack growth along [101]. This orientation has been frequently observed in experimental studies on fracture of FCC single crystals. Three equivalent slip systems are considered which are deduced by combining three pairs of 3D conjugate slip systems producing only in-plane deformation. Attention is focused on the effects of crystal hardening, ratio of void diameter to spacing from the notch on plastic flow localization in the ligament connecting the notch and the void as well as their growth. The results show strong interaction between slip shear bands emanating from the notch and angular sectors of single slip forming around the void leading to intense plastic strain development in the ligament. However, the ductile fracture processes are retarded by increase in hardening of the single crystal and decrease in ratio of void diameter to spacing from the notch. In order to examine the effect of crystal orientation, computations are performed with an orientation wherein the three effective slip systems are rotated about the normal to the plane of deformation. A strong influence of crystal orientation on near-tip void growth and plastic slip band development is observed. Further, in order to study the synergistic, cooperative growth of multiple voids ahead of the notchtip, an analysis is performed by considering a series of voids located ahead of the tip. It is found that enhanced void growth occurs at higher load levels as compared to the single void model.
Next, the growth and coalescence of a periodic array of cylindrical voids in a FCC single crystal is analyzed under different stress states by employing a 2D plane strain, unit cell approach. The orientation of the crystal studied here considers [101] and [010] crystal directions along the minor and major principal stress directions, respectively. Three equivalent slip systems, similar to those in the notch and void simulations are taken into account. Fringe contours of plastic slip and evolution of macroscopic hydrostatic stress and void volume fraction are examined. A criterion for unstable void growth which leads to onset of void coalescence is established. The effects of various stress triaxialities, initial void volume fraction and hardening on void growth and coalescence is assessed. It is observed that plastic slip activity around the void intensifies with increase in stress triaxiality. The macroscopic hydrostatic stress increases with deformation, reaches a peak value and subsequently decreases rapidly. An increase in stress triaxiality enhances the macroscopic hydrostatic stress sustained by the unit cell and promotes void coalescence. The stress triaxiality also has a profound effect on the shape of the void profile. The values of critical void volume fraction and critical strain, which mark onset of void coalescence, decrease within crease in stress triaxiality. However, the onset of void coalescence is delayed by increase in hardening and decrease initial void volume fraction.
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Xia, Qing. "Cloud drop growth by condensation and coalescence in the transition range /". 2000. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:9990607.
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Reina, Romo Celia. "Multiscale Modeling and Simulation of Damage by Void Nucleation and Growth". Thesis, 2011. https://thesis.library.caltech.edu/6165/3/Thesis_CeliaReina.pdf.
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Voids are observed to be generated under sufficient loading in many materials, ranging from polymers and metals to biological tissues. The presence of these voids can have drastic implications at the macroscopic level including strong material softening and more incipient fracture. Developing tools to appropriately account for these effects is therefore very desirable.
This thesis is concerned with both, the appearance of voids (nucleation process) and the modeling and simulation of materials in the presence of voids. A particular nucleation mechanism based on vacancy aggregation in high purity metallic single crystals is analyzed. A multiscale model is developed in order to obtain an approximate value of the time required for vacancies to form sufficiently large clusters for further growth by plastic deformation. It is based on quantum mechanical results, kinetic Monte Carlo methods and continuum mechanics estimates calibrated with quasi-continuum results. The ultimate goal of these simulations is to determine the feasibility of this nucleation mechanism under shock loading conditions, where the temperature and tensions are high and vacancy diffusion is promoted.
On the other hand, the effective behavior of materials with pre-existent voids is analyzed within the general framework of continuum mechanics and is therefore applicable to any material. The overall properties of the heterogeneous material are obtained through a two-level characterization: a representative volume element consisting of a hollow sphere is used to describe the "microscopic" fields, and an equivalent homogeneous material is used for the "macroscopic" behavior. A variational formulation of this two-scale model is presented. It provides a consistent definition of the macro-variables under general loading conditions, extending the well-known static averaging results so as to include microdynamic effects under finite deformations. This variational framework also provides a suitable starting point for time discretization and consistent definitions within discrete time. The spatial boundary value problem resulting from this multiscale model is solved with a particular spherical shell element specially developed for this problem. The approximation space is based on spherical harmonics, which respects the symmetries of the porous material and allows the representation of the fields on the sphere with very few degrees of freedom. Numerical tools, such as the exact representation of the boundary conditions and an exact quadrature rule, are also provided. The resulting numerical model is verified extensively, demonstrating good convergence results, and its applicability is shown through several material point calculations and a full two-scale finite element implementation.
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Li, Chia-Lin y 李佳霖. "The kinetic processes of color center and void growth of the polymers". Thesis, 2004. http://ndltd.ncl.edu.tw/handle/08373767208028603138.
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碩士國立清華大學
材料科學工程學系
92
Gamma-irradiation would change the physical and mechanical properties of polymer. Discoloration is a problem in the radiation processing of polymers. After γ-irradiation, the transmittance of poly(4-methyl-1-pentene) (TPX) decreases monotonically with increasing time. The cutoff wavelength moves to right side direction as time increases. The optical absorption of irradiated TPX follows a first-order kinetic process. The activation energy was slightly increased with the increase of dosage. Besides, the DSC measurement shows that the melting temperature (Tm) decreases with increasing dosages. Crack can heal above the glass transition temperature (Tg). However, when it reaches a critical temperature, instead of crack healing, it will grow to cylindrical void. The spherical defect will grow to a spherical void above a critical temperature. We find that when the polycarbonate is annealed above 177℃, the voids will grow. But a period later, some voids will shrink and the final size is approached to the original size. And some voids will combine with the other voids and final disappear. This is possible due to the rotation and movement of polymer chains above Tg.
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"3D Modeling of Void Nucleation and Initial Void Growth due to Tin Diffusion as a Result of Electromigration in Polycrystalline Lead-Free Solders". Master's thesis, 2016. http://hdl.handle.net/2286/R.I.40350.
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abstract: Electromigration (EM) has been a serious reliability concern in microelectronics packaging for close to half a century now. Whenever the challenges of EM are overcome newer complications arise such as the demand for better performance due to increased miniaturization of semiconductor devices or the problems faced due to undesirable properties of lead-free solders. The motivation for the work is that there exists no fully computational modeling study on EM damage in lead-free solders (and also in lead-based solders). Modeling techniques such as one developed here can give new insights on effects of different grain features and offer high flexibility in varying parameters and study the corresponding effects. In this work, a new computational approach has been developed to study void nucleation and initial void growth in solders due to metal atom diffusion. It involves the creation of a 3D stochastic mesoscale model of the microstructure of a polycrystalline Tin structure. The next step was to identify regions of current crowding or ‘hot-spots’. This was done through solving a finite difference scheme on top of the 3D structure. The nucleation of voids due to atomic diffusion from the regions of current crowding was modeled by diffusion from the identified hot-spot through a rejection free kinetic Monte-Carlo scheme. This resulted in the net movement of atoms from the cathode to the anode. The above steps of identifying the hotspot and diffusing the atoms at the hot-spot were repeated and this lead to the initial growth of the void. This procedure was studied varying different grain parameters. In the future, the goal is to explore the effect of more grain parameters and consider other mechanisms of failure such as the formation of intermetallic compounds due to interstitial diffusion and dissolution of underbump metallurgy.Dissertation/Thesis
Masters Thesis Materials Science and Engineering 2016
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CHANG, YI-WEN y 張怡文. "Dynamical Growth of Void in a Neo-Hookean Sphere under a periodic load". Thesis, 2011. http://ndltd.ncl.edu.tw/handle/33854729473381211062.
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碩士國立中央大學
土木工程研究所
99
This thesis studies the equation of motion of a rubber ball and the rubber ball dynamical behavior of a void in the ball. The natural frequency of rubber ball is not a fixed-value, it may be changed by assigning different initial conditions. This phenomenon reveals the character of non-linear material. The nonlinear effect on the ball with a smaller void is more obvious then that of the bigger one. The dynamical behavior of the void will change when loadings with different frequencies or speeds are applied to the surface of the rubber ball. We will focus on the unstable growth and the resonance of the void under periodic loads since such kind of growth shall cause strength degradation to the rubbers. This thesis also investigates the dynamical magnification factor for the ball.
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"Kinetics of Void Nucleation and Growth at Grain Boundaries on Shock Loaded Copper Bicrystals". Doctoral diss., 2020. http://hdl.handle.net/2286/R.I.57084.
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abstract: Shock loading produces a compressive stress pulse with steep gradients in density, temperature, and pressure that are also often modeled as discontinuities. When a material is subject to these dynamic (shock) loading conditions, fracture and deformation patterns due to spall damage can arise. Spallation is a dynamic material failure that is caused by the nucleation, growth, and coalescence of voids, with possible ejection of the surface of the material. Intrinsic defects, such as grain boundaries are the preferred initiation sites of spall damage in high purity materials. The focus of this research is to study the phenomena that cause void nucleation and growth at a particular grain boundary (GB), chosen to maximize spall damage localization.
Bicrystal samples were shock loaded using flyer-plates via light gas gun and direct laser ablation. Stress, pulse duration, and crystal orientation along the shock direction were varied for a fixed boundary misorientation to determine thresholds for void nucleation and coalescence as functions of these parameters. Pressures for gas gun experiments ranged from 2 to 5 GPa, while pressures for laser ablation experiments varied from 17 to 25 GPa. Samples were soft recovered to perform damage characterization using electron backscattering diffraction (EBSD) and Scanning Electron Microscopy (SEM). Results showed a 14% difference in the thresholds for void nucleation and coalescence between samples with different orientations along the shock direction, which were affected by pulse duration and stress level. Fractography on boundaries with strong damage localization showed many small voids, indicating they experience rapid nucleation, causing early coalescence. Composition analysis was also performed to determine the effect of impurities on damage evolution. Results showed that higher levels of impurities led to more damage. ABAQUS/Explicit models were developed to simulate flyer-plate impact and void growth with the same crystal orientations and experimental conditions. Results are able to match the damage seen in each grain of the target experimentally. The Taylor Factor mismatch at the boundary can also be observed in the model with the higher Taylor Factor grain exhibiting more damage.Dissertation/Thesis
Doctoral Dissertation Mechanical Engineering 2020
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Ahn, Deok Chan. "Mechanisms of ductile fracture : void growth by dislocation-loop emission and hydrogen-assisted crack propagation /". 2006. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3242777.
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Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006.Source: Dissertation Abstracts International, Volume: 67-11, Section: B, page: 6685. Adviser: Petros Sofronis. Includes bibliographical references (leaves 251-270) Available on microfilm from Pro Quest Information and Learning.
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Chia-HungPan y 潘嘉宏. "Study on the growth of functional crystals guided by coalescence and stacking behaviors of P(VDF-TrFE) ferroelectric lamellar crystals". Thesis, 2019. http://ndltd.ncl.edu.tw/handle/ab7q2d.
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碩士國立成功大學
材料科學及工程學系
107
The unique polarity of P(VDF-TrFE) crystalline phase below curie transition temperature is attributed to the lattice packing of all-trans molecular segments, which allocates most of substituted fluorine atoms on one side of molecular segments and hydrogen atoms on the other side. In this study, the growth habits and stacking behavior of crystalline lamellae within thin films were explored. The coalescence of lamellar crystals has been identified to occur during the secondary crystallization above Curie temperature. It is viewed that, for the residue amorphous fractions in between lamellar crystals, the further crystallization above Curie temperature causes the assembly and coalescence of present lamellar crystal. With the presence of a limited fraction of amorphous poly(methyl methacrylate) (PMMA), the continuous coalescence and growth of lamellar crystals eventually result in regular stacking arrays. Furthermore, the shrinkage and deformation of the PMMA domains is observed as a result of continuous lamellae growth above the Curie temperature, and then the PMMA domains extends along the stacking direction of lamellar crystals. The fullerene derivative of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is observed to preferentially assemble into PMMA globular domains during solution casting. Due to the stress from deformed amorphous environment, the later induced crystal growth of PCBM is identified to be generally along the PMMA extension direction. The anisotropic deformation of PMMA amorphous domains is thus realized able to modify crystal growth within deformed domains. The involved phase interactions is expected to yield fruitful influences on the evolution of thin-film structures for desired physical properties.