Academic literature on the topic 'Dynamic cracking'

Advances in dynamic fracture modelling have become more frequent due to increases in computer speed, meaning that its application to industrial problems has become viable. From this, the author has reviewed current literature in terms of graphite material properties, structural dynamics, fracture mechanics and modelling methodologies to be able to address operational issues related to the ageing of Advanced Gas-cooled Reactor (AGR) cores. In particular, the experimentally observed Prompt Secondary Cracking (PSC) of graphite moderator bricks which has yet to be observed within operational reactors, with the objective of supporting their plant life extension. A method known as eXtended Finite Element Method with Cohesive Zones (XCZM) was developed within Code_Aster open-source FEM software. This enabled the incorporation of velocity toughening, irradiation-induced material degradation effects and multiple 3D dynamic crack initiations, propagations and arrests into a single model, which covers the major known attributes of the PSC mechanism. Whilst developing XCZM, several publications were produced. This started with first demonstrating XCZM's ability to model the PSC mechanism in 2D and consequently that methane holes have a noticeable effect on crack propagation speeds. Following on from this, XCZM was benchmarked in 2D against literature experiments and available model data which consequently highlighted that velocity toughening was an integral feature in producing energetically correct fracture speeds. Leading on from this, XCZM was taken into 3D and demonstrated that it produced experimentally observed bifurcation angle from a literature example. This meant that when a 3D graphite brick was modelled that the crack profile was equivalent to an accepted quasi-static profile. As a consequence of this validation, the XCZM approach was able to model PSC and give insight into features that could not be investigated previously including: finer-scale heterogeneous effects on a dynamic crack profile, comparison between Primary and Secondary crack profiles and also, 3D crack interaction with a methane hole, including insight into possible crack arrest. XCZM was shown to improve upon previous 2D models of experiments that showed the plausibility of PSC; this was achieved by eliminating the need for user intervention and also incorporation of irradiation damage effects through User-defined Material properties (UMAT). Finally, while applying XCZM to a full-scale 3D graphite brick including reactor effects, it was shown that PSC is likely to occur under LEFM assumptions and that the Secondary crack initiates before the Primary crack arrests axially meaning that modal analysis would not be able to fully model PSC.

The subject of this thesis is the behaviour of concrete structures under dynamic loading conditions and its modelling. In particular, the modelling of material behaviour is treated and a new material model for the description of plain concrete behaviour is proposed. For the modelling of the uncracked concrete behaviour, a strain rate sensitive elasto - viscoplastic material model developed by Bicanic is used. For the modelling of cracked concete behaviour, a distributed - smeared crack representation has been adopted. Crack initiation and propagation are controlled by a crack monitoring algorithm employing a critical strain criterion, allowing multiple cracking and controlled strain softening during the first crack opening cycle, linking the shear transfer across the crack to the magnitude of crack opening and preserving the crack directionality. Implementation of the proposed material model in a finite element computer program DEGDYN is described and a computer program listing is given. An explicit time stepping scheme is used, so the computer memory requirement is small and the program may be run even on small personal computers. Material model and computer program performance are verified using simple examples. Application of the material model in the analysis of the Koyna dam is demonstrated and results of several parameter sensitivity analyses are presented.

Cette thèse propose des contributions aux méthodes éléments discrets (MED) et à l’intégration temporelle explicite avec pour objectif applicatif les calculs de fissuration et de fragmentation pour des matériaux métalliques soumis à des chargements dynamiques. Les MED, qui sont traditionnellement utilisées pour simuler le comportement de matériaux granulaires, sont ré-interprétées à la lumière des méthodes de discrétisation de gradient afin d’être appliquées à la simulation de matériaux continus. Les maillages utilisables avec la MED proposée ont été étendus des maillages de Voronoi à des maillages polyédriques généraux. Les comportements simulables par la méthode ont été étendus de l’élasto-dynamique à l’élasto-plasticité dynamique par l’ajout d’un degré de liberté tensoriel par cellule. De plus, la méthode est robuste par rap-port à la limite incompressible et ses paramètres ne dépendent que des paramètres matériau. Une méthode d’intégration temporelle explicite conservant une pseudo-énergie, même pour des comportements non-linéaires et des pas de temps variables, a également été développée afin d’éviter une dissipation numérique de l’énergie disponible pour la dissipation plastique et la fissuration. Cette méthode a été couplée avec la MED précédente. Enfin, la propagation de fissures de Griffith à travers les facettes du maillage a été intégrée à la MED pour des comportements élastiques linéaires en deux dimensions d’espace. Le taux de restitution d’énergie est calculé pour chaque mode de fissuration à partir des facteurs d’intensités des contraintes qui sont approchés près de la fissure. Enfin, un critère de maximisation de la densité d’énergie élastique sur les facettes liée à la pointe de fissure permet de simuler l’orientation de la propagation<br>The present Ph.D. dissertation proposes contributions to discrete element methods (DEM) and explicit time integration schemes with a view towards dynamic cracking for metallic materials under dynamic loading. DEM, which are usually used to simulate granular materials, are understood through the prism of gradient discretization methods in order to simulate continuous materials. The method has been extended from previous Voronoi meshes to support generalpolyhedral meshes. Material behaviours have been extended from elasto-dynamics to dynamic elasto-plasticity through the addition of a tensorial degree of freedom per mesh cell. The method is robust with respect to the incompressible limit and its parameters only depend on material parameters. Moreover, an explicit pseudo-energy conserving time integration method has been developed, even for nonlinear behaviours and variable time steps, so as to avoid thedissipation of energy available for plastic dissipation and cracking. The method has been coupled to the proposed DEM. Finally, Griffith crack propagation through the mesh facets has been adapted to the present DEM for linear elastic behaviours in two space dimensions. The energy release rate is computed for every cracking mode using the stress intensity factors approximated close to the crack. A criterion of maximization of elastic energy density is used tosimulate kinking

Cette thèse est une entreprise commune de VINCI Technologies et du laboratoire CNRS CORIA. De nombreux injecteurs comportent une zone de mélange interne dans laquelle les phases liquides et gazeuses sont toutes deux présentes dans une proportion significative. Par conséquent, cette zone appartient à la catégorie des écoulements diphasiques denses. Pour simuler la dispersion du liquide et caractériser le spray de ces injecteurs, des modèles appropriés sont nécessaires. Les points clés de cette approche sont la dispersion du liquide qui peut être associé au flux liquide turbulent et la quantité de surface liquide-gaz. En particulier, ce manuscrit rapporte, d’une part le développement théorique des modèles de la famille ELSA et, d’autre part, les approximations industrielles correspondantes. Le solveur proposé bascule dynamiquement du spray ICM au spray de sous- maille, à travers le concept ELSA et grâce à l’indicateur basé sur la résolution (IRQ). D’autre part, une fois la zone diluée se forme, l’approche ELSA est couplée à la méthode d’écoulement multiphase, qui vise à déterminer la distribution du spray à l’aide de l’équation WBE. Cette dernière équation est résolue avec une méthode hybride Euler-Lagrange. Le but est de résoudre l’équation WBE avec une approche stochastique Lagrangienne, tout en préservant la compatibilité avec la description Eulerienne de l’écoulement diphasique, basée sur ELSA, pour tirer parti des deux approches. Finalement, ces approches développées ont été utilisées pour des applications industrielles montrant leur robustesse et leur capacité à aider dans le processus de développement de nouveaux injecteurs<br>This PhD is a joint venture between VINCI Technologies and the CNRS Laboratory CORIA. For its application, VINCI Technologies, developed mainly oil-related equipments and in particular injection/atomization systems. Some of these injectors are characterized by a very big geometrical dimensions (several meters long), that leads to very high Reynolds and Weber number. In addition, many injectors incorporate an internal mixing zone, in which liquid and gas phases are both present in a significant proportion. Consequently, this zone belongs to the dense two-phase flow category. To simulate the liquid dispersion and to characterize the spray formation special from these injectors, appropriate models are required. On its side, the CORIA team, has developed a suitable approach, so-called ELSA, based on the pioneering work of Borghi and Vallet [1, 2]. Key points of this approach are the liquid dispersion that can be associated to the turbulent liquid flux and the amount of liquid-gas surface that can be used to determine eventually the Sauter mean diameter (SMD) of the spray. During this PhD, the applications proposed by V INCI Technologies, have promoted a review of a large part of the multiphase flow approaches to find the more appropriate for each case. This has been the opportunity to clarify the range of application of each approach, and therefore stress the necessity to develop coupled approaches, in order to cover the proposed application in the most suitable way. In particular, this manuscript reports, in one hand, the theoretical development of the ELSA family models, and on the other hand, the corresponding industrial approximations. Since ELSA approaches are originally developed for RANS simulation of the dense zone, it has been extended to LES description. The link of this approach to the DNS¡ ICM approach, has been studied with a special care. The resulting proposed solver, switches dynamically from ICM to subgrid spray, through the ELSA concept, and thanks to resolution based indicator (IRQ). On the opposite side, once the dispersed spray is formed, the ELSA approach is coupled to multiphase flow method, that aims to determine the spray distribution through the WBE equation. This later equation, is solved with an original hybrid Euler-Lagrange method. The purpose is to solve the WBE equation with a Lagrange stochastic approach, and at the same time, preserving the compatibility to the Euler description of two-phase flow, based on ELSA, to benefit from both approaches. This coupled approach has been tested against academic experimental data coming from ECN research initiative, a combined DNS and experimental measurement of dispersed spray on a Diesel jet, and under an air-blast atomizer numerical test case, for which the mean liquid volume fraction has been measured. Eventually, these developed approaches have been applied to industrial application showing there robustness and their capacity to help in the process of design development of new injectors

Orientador: Milton Mori<br>Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Química<br>Made available in DSpace on 2018-08-27T17:58:36Z (GMT). No. of bitstreams: 1 Pelissari_DanielCicero_M.pdf: 3433960 bytes, checksum: 82461e75c8ef3d10fb44cd8d35de3eb9 (MD5) Previous issue date: 2015<br>Resumo: A aplicação de fluidodinâmica computacional (CFD) em estudos de otimização e projeto de novos equipamentos de processos industriais vem aumentando significativamente, uma vez que apresenta custo reduzido e possibilidade de avaliar equipamentos complexos e de extremas condições de operação. Dentre os processos mais estudados via CFD está o processo de craqueamento catalítico fluidizado (FCC), onde as frações pesadas do petróleo de baixo valor são convertidas em produtos de maior valor agregado, sendo uma das aplicações de fluidização gás-sólido mais importante na indústria de petróleo. O presente trabalho avaliou especificamente a zona de injeção do FCC, na qual a matéria-prima, alimentada por bicos injetores, se mistura a sólidos quentes (catalisador) e a vapor de fluidização. A performance desses dispersores de carga para garantir uma boa distribuição das gotículas de gasóleo com o catalisador é a chave para melhorar a eficiência do riser de FCC. Desta forma, o principal objetivo deste trabalho foi avaliar o efeito de diferentes designs, ângulos (30°, 45° e 60°) e configurações dos injetores sobre o escoamento gás-sólido e o desempenho do riser. Para tal, simulou-se um escoamento gás-sólido reativo tridimensional baseado em uma abordagem Eulerian-Eulerian. Nas simulações foram utilizados o modelo cinético de 12-lumps de Wu et al. (2009), modelo de turbulência k-? e modelo de arraste de Gidaspow. Foi observado que o design, o ângulo e a configuração dos bicos injetores exercem uma forte influência sobre a fluidodinâmica e a performance do riser, sendo o ângulo a variável que apresentou maior influência. Pôde-se observar que o design de bico tipo multi-orifícios (Caso 3) foi o que apresentou os melhores resultados, sendo a partir deste avaliados os ângulos, onde notou-se que o aumento do ângulo de 30° para 60° melhorou a mistura entre as fases e o rendimento. A análise dos arranjos foi realizada utilizando o design de bico do Caso 3 e o ângulo de 45°, e observou-se que o arranjo com bicos intercalados (Arranjo 2) apresentou uma mistura mais homogênea entre as fases e, consequentemente, uma melhor conversão e rendimento de produtos desejados. Em geral, os resultados obtidos no presente trabalho salientam a importância da utilização de geometrias mais detalhadas para os bicos, uma vez que influenciam a mistura entre as fases, a qual afeta o desempenho do riser<br>Abstract: The Computational Fluid Dynamic (CFD) application in industrial process optimization and new equipments design studies has increased significantly, once it presents low cost and the possibility of evaluating complex and extreme operating conditions equipments . Among the most widely studied processes via CFD is the fluidized catalytic cracking process (FCC), where the oil heavy fractions of low-value are converted into higher value-added products and which is one of the most important gas-solid fluidization applications in the oil industry . The present study specifically evaluated the FCC injection zone, in which the feedstock fed by nozzles, is mixed with hot solids (catalyst) and fluidization steam. The nozzles performance to guarantee a good gas oil droplets distribution with the catalyst is the key to improve the efficiency of FCC riser. Thus, the study main objective was to evaluate the different nozzles designs, angles (30 °, 45 ° and 60 °) and arrangements effect on the gas-solid flow and the riser performance. For this purpose, it was simulated a three-dimensional reactive gas-solid flow based on an Eulerian-Eulerian approach. In the simulations it was used the 12 lumps kinetic model by Wu et al. (2009), turbulence model k- ? and drag model Gidaspow. It was observed that the nozzle design, angle and configuration have a strong influence on fluid dynamics and on the riser performance, and the angle was the variable with the greatest influence. It can be observed that the nozzle design of multi-orifice type (Case 3) showed the best results, and that¿s why it was used to evaluate the angle, in which was noted that the angle increase of 30 ° to 60 ° improved phases mixing and the yield. The arrangement analysis was performed using Case 3 nozzle design and the design angle of 45 °, and it was observed that the arrangement with intercalated nozzles (Arrangement 2) showed a more homogeneous phases mixture and therefore a better conversion and desired product yield. In general, the results obtained in this work highlighted the importance of using more detailed geometries for the nozzles, since they influence the mixing of the phases, which affects the riser performance<br>Mestrado<br>Engenharia Química<br>Mestre em Engenharia Química

Herein, we study the contact damage performance of two armour ceramics, alumina and silicon carbide, with varying microstructures and one particle-reinforced ceramic nanocomposite, alumina/silicon carbide, in an attempt to understand the microstructural mechanisms that affect plasticity and cracking under quasi-static and dynamic conditions. Quasi-static contact damage was imitated using Vickers indentation over a varying load regime. Numerical analysis of the indentation size effect, performed using the proportional specimen resistance model, allowed the contributions of plastic deformation and cracking to be separated into two individual values. In all three samples, higher levels of surface energy were found to correlate with increased amounts of cracking per unit area of indentation impression. Analytical modelling of crack initiation during Vickers indentation together with quantitative measurements of surface flaw populations revealed that such an increase in cracking damage was the result of higher densities of larger flaws. The hardness of the monolithic ceramics was found vary based on grain size and porosity levels, a smaller average grain size and lower porosity levels resulting in higher hardness values. In the nanocomposite materials, hardening was found to occur with further additions of silicon carbide nanoparticles. Such an effect has been attributed to the increased dislocation densities, as measured using Cr3+/Al2O3 fluorescence spectroscopy, and the impedance of dislocation movement within the lattice due to the presence of silicon carbide nanoparticles. In order to simulate dynamic contact damage, a low velocity, scaled-down drop-weight test was designed and developed. The dynamic contact damage resistance was determined based on the depth of penetration of a blunt indenter. In the monolithic ceramics, the indenter penetration was found to be shallower in materials of higher hardness. However, the nanocomposite materials displayed an opposing trend, the indenter penetration becoming deeper in the samples of higher hardness. The macro-scale fracture patterns produced during drop-weight impacts were seen to vary based on flaw populations and indenter penetration. In certain microstructures, extensive micro-cracking was also observed.

For the last decades usage of aluminium alloys have been increasing tremendously. They have been used in aerospace industry widely and now aluminium alloys are becoming more and more popular in automotive and defense industries. Consequently<br>successful welding of aluminium alloys gains importance. In this study a research is carried out on eldability of plates having different thicknesses of composition 5754 aluminium and 6063 aluminium in T-fillet geometry using Gas-metal Arc Welding technique. It was aimed to have a successful joint without using pre-weld and post-weld heat treatments. During tests welding current and voltage were the varying parameters as welding speed was held constant. Macro-examinations were performed to see the penetration of the weld metal. It was seen that the type of filler wire greatly effects weld penetration. Hardness tests, tensile tests were done to compare the mechanical properties of the welded joints with different filler wires. Despite having better penetration in 4043 filler wire used weld joints, 5356 filler wire used weld joints had higher tensile strength and ductility. In the second part of the study, a dynamic loading machine was designed and manufactured to see the behavior of the fillet welds under dynamic loading. The amount of stress and strain given to the specimen on this machine was adjustable but can&rsquo<br>t be measured. The tests that were made with this machine aimed only to compare the number of cycles of specimens before fracture. For dynamic loading tests two groups of specimens were prepared with filler wire 4043<br>each group having been welded with different heat inputs. It was aimed to see the effect of welding heat input on service lifes but no significant difference between cycle numbers of specimen groups having been welded with different heat inputs was observed. Microstructure examinations of these specimens revealed that coarsening the grains, grain boundaries, particles in PMZ and HAZ regions between Al 6063 base metal and weld zone made these areas more susceptible and favorable for crack propogation than Al 6063 base metal.