Tesi: "Cracking" – Grafiati

1

Acey, S. "Heavy oil cracking". Thesis, Teesside University, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.371059.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

2

Crumpacker, John R. "Distributed password cracking". Thesis, Monterey, California : Naval Postgraduate School, 2009. http://edocs.nps.edu/npspubs/scholarly/theses/2009/Dec/09Dec%5FCrumpacker.pdf.

Testo completo

Abstract (sommario):

Thesis (M.S. in Computer Science)--Naval Postgraduate School, December 2009.
Thesis Advisor(s): Dinolt, George. Second Reader: Eagle, Chris. "December 2009." Description based on title screen as viewed on January 27, 2010. Author(s) subject terms: Distributed password cracking, Berkeley Open Infrastructure for Network Computing (BOINC), and John the Ripper. Includes bibliographical references (p. 63-64). Also available in print.

Gli stili APA, Harvard, Vancouver, ISO e altri

3

Schwarzer, Roland. "CRACking the Riddle". Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät I, 2014. http://dx.doi.org/10.18452/17010.

Testo completo

Abstract (sommario):

In den vergangenen Jahren sind Lipide, Membranen und deren Organisationsformen mehr und mehr in den Fokus der biologischen Forschung gerückt. Es wurde vorgeschlagen, dass in zellulären Membranen selbstassemblierende, submikroskopische Aggregate aus Sphingolipiden, Cholesterol und bestimmten Proteinen existieren und man vermutet, dass insbesondere Viren diese “Lipid Rafts” für ihren Zusammenbau nutzen und auf diese Art ihre Proliferationseffizienz erhöhen. Gleichwohl sind die genaue biologische Funktion und auch die molekulare Basis der Assoziation bestimmter Protein mit Lipid Rafts auch weiterhin unbekannt. In der vorliegenden Arbeit wurde Fluoreszenz-Lebenszeit-Mikroskopie genutzt, um die Lipid-Raft-Anreicherung des HIV-1 Glycoproteins gp41 zu untersuchen. Förster-Resonanz-Energietransfer zwischen fluoreszenzmarkierten viralen und Raft-Marker-Proteinen wurde gemessen, um deren gemeinsame, lokale Aufkonzentrierung in Lipid Rafts nachzuweisen. Durch Verwendung verschiedener Deletions- und Mutationsvarianten des Proteins konnte nicht nur seine Lipid-Raft-Präferenz demonstriert, sondern auch das Cholesterol-Bindemotiv (CRAC) als entscheidender Faktor der lateralen Sortierung identifiziert werden. Wir haben in diesem Kontext auch eine systematische Zell-zu-Zell-Variabilität in unseren Daten bemerkt, die einen zugrundeliegenden zellbiologischen Mechanismus der Membranorganisation nahelegt. Mithilfe von Fluoreszenz-Polarisations-Mikroskopie konnte zudem eine klare CRAC-Abhängigkeit der gp41-Oligomerisierung aufgezeigt werden. Die von uns gewonnenen Daten erlauben einen tieferen Einblick in die molekulare Basis und die biologischen Folgen der cholesterol-abhängigen lateralen Proteinorganisation für Virusassemblierungsprozesse an biologischen Membranen.
In recent years, there has been a considerable interest in the molecular organization of biological membranes. It has been hypothesized that self-assembling, freely diffusing, submicroscopic domains consisting of sphingolipids, cholesterol and certain proteins exist and the prevailing view is that those lipid rafts serve as platforms for specific molecular interactions by the preferential exclusion and inclusion of proteins. It was presumed, that in particular viruses make use of plasma membrane lipid rafts to augment the infection process and spread efficiently. However, the exact biological function and physical basis of protein partitioning into microdomains remains an outstanding question in virus biology. In the present study, fluorescence lifetime imaging microscopy was used to study lipid raft partitioning of the HIV-1 glycoprotein gp41 by detecting Foerster Resonance Energy Transfer between fluorescently labeled viral and raft marker proteins in living cells. Plasma membrane microdomain association of gp41 was demonstrated and by introducing systematic mutations and truncations in different gp41 motifs, the cholesterol recognition amino acid consensus (CRAC) was identified as the crucial determinant of the lateral sorting. Interestingly, we observed a systematic cell-to-cell variability in our raft related data that suggests underlying cell-biological mechanisms of membrane organization. Moreover, fluorescence polarization microscopy revealed a distinct CRAC requirement for gp41 oligomerization whereas other properties, such as intracellular distribution and expression efficiency were clearly demonstrated to be CRAC independent. Our data provide further insight into the molecular basis and biological implications of the cholesterol dependent lateral protein sorting for virus assembly processes at cellular plasma membranes.

Gli stili APA, Harvard, Vancouver, ISO e altri

4

Attou, Abdelkader. "Cracking and stress corrosion cracking in glass fibre materials using acoustic emission". Thesis, Robert Gordon University, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.277702.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

5

Sayahi, Faez. "Plastic Shrinkage Cracking in Concrete". Licentiate thesis, Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-133.

Testo completo

Abstract (sommario):

Early-age (up to 24 hours after casting) cracking may become problematic in any concrete structure. It can damage the aesthetics of the concrete member and decrease the durability and serviceability by facilitating the ingress of harmful material. Moreover, these cracks may expand gradually during the member’s service-life due to long-term shrinkage and/or loading. Early-age cracking is caused by two driving forces: 1) plastic shrinkage cracking which is a physical phenomenon and occurs due to rapid and excessive loss of moisture, mainly in form of evaporation, 2) chemical reactions between cement and water which causes autogenous shrinkage. In this PhD project only the former is investigated. Rapid evaporation from the surface of fresh concrete causes negative pressure in the pore system. This pressure, known as capillary pressure, pulls the solid particles together and decreases the inter-particle distances, causing the whole concrete element to shrink. If this shrinkage is hindered in any way, cracking may commence. The phenomenon occurs shortly after casting the concrete, while it is still in the plastic stage (up to around 8 hours after placement), and is mainly observed in concrete elements with high surface to volume ratio such as slabs and pavements. Many parameters may affect the probability of plastic shrinkage cracking. Among others, effect of water/cement ratio, fines, admixtures, geometry of the element, ambient conditions (i.e. temperature, relative humidity, wind velocity and solar radiation), etc. has been investigated in previous studies. In this PhD project at Luleå University of Technology (LTU), in addition to studying the influence of various parameters, effort is made to reach a better and more comprehensive understanding about the cracking governing mechanism. Evaporation, capillary pressure development and hydration rate are particularly investigated in order to define their relationship. This project started with intensive literature study which is summarized in Papers I and II. Then, the main objective was set upon which series of experiments were defined. The utilized methods, material, investigated parameters and results are presented in Papers III and IV. It has been so far observed that evaporation is not the only driving force behind the plastic shrinkage cracking. Instead a correlation between evaporation, rate of capillary pressure development and the duration of dormant period governs the phenomenon. According to the results, if rapid evaporation is accompanied by faster capillary pressure development in the pore system and slower hydration, risk of plastic shrinkage cracking increases significantly.

Gli stili APA, Harvard, Vancouver, ISO e altri

6

Mitchell, Michael J. "Terrazzo cracking causes and remedies /". Connect to this title, 2008. http://scholarworks.umass.edu/theses/124/.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

7

Crump, Timothy. "Modelling dynamic cracking of graphite". Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/modelling-dynamic-cracking-of-graphite(71e81d6f-e712-458c-aa48-0a256749258a).html.

Testo completo

Abstract (sommario):

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.

Gli stili APA, Harvard, Vancouver, ISO e altri

8

Murray, Ian. "Cracking behaviour of clayey geomaterials". Thesis, University of Strathclyde, 2017. http://digitool.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=29273.

Testo completo

Abstract (sommario):

Cracking is a significant problem in the ceramics industry. It results in a substantial loss of production due to the discarded cracked wares associated. There is an understanding that cracking relates from restrained shrinkage and/or uneven drying. One of the solutions to this is to slow the rate of evaporation so that the wares dry more evenly. Such remedial measures are impractical due to the increase in drying space required. What is therefore required for the ceramics industry is a greater understanding of cracking and methods, which work within their manufacturing limitations, to reduce cracking susceptibility. Despite a number of studies, there is a lack of understanding surrounding the mechanisms of crack initiation. It is clear from experimental evidence that the vast majority of cracking occurs while the soil is still saturated or at the transition from saturated to unsaturated. Many researchers assume that cracking is governed by total stress, i.e. a crack initiates when the tensile stress generated in a soil during drying exceeds the tensile strength of the material. This research investigates the mechanism of crack initiation using an effective stress based approach. A new tensile testing device utilising high capacity tensiometers to allow for the measurement of (negative) pore-water pressure during testing and, hence, the characterisation of the effective stress state has been developed. The device has been designed to test clays in both saturated and unsaturated states. A series of tensile tests were performed on saturated clay samples prepared using non-de-aired and de-aired water. The results show that for non-de-aired tests, when suction approached the air-entry value, failure occurred at a deviatoric stress lower than the one corresponding to the critical state line derived from triaxial and uniaxial compression tests. For specimens with the slurry de-aired before slip casting the de-airing process realigned the deviator stress at failure recorded in the tensile test with the critical state line derived from uniaxial and triaxial compression tests. It could therefore be speculated that water cavitation is one of the mechanisms that can control rupture of clay when subjected to a (total) tensile stress state. Finally, failure data from tests on unsaturated specimens could be fairly modelled by the Mohr-Coulomb criterion extended to unsaturated states. These tests confirm that tensile failure is associated with failure in shear for both saturated and unsaturated states. Further validation of the shear failure mechanism is attempted via a numerical study using a simple coupled hydro-mechanical Finite Element Method (FEM) model to simulate the desiccation of a clayey soil to the point of crack initiation. The results of four laboratory desiccation tests of specimens with non-uniform geometry and different hydraulic boundary conditions are presented. These tests were simulated via FEM, and the time and location of cracking compared to test the validity of the model and the failure criterion. A greater understanding of the mechanism of cracking has then be used to test more practical remedial measures that can be used in the manufacture of ceramics to reduce cracking. These measures are based around altering the material mixes used in production process, as to this point the adjustment of the slip material relating to cracking has been done by a process of trial and error without guidance. The results of Finite Element Method simulations suggest creating a material with a more graded grain and pore-size distribution can reduce deviatoric stress development during uneven drying.

Gli stili APA, Harvard, Vancouver, ISO e altri

9

Hopkinson, Andrew Christopher Graham. "Cracking of drying ceramic dispersions". Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608101.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

10

Scetta, Giorgia. "Fatigue cracking of thermoplastic elastomers". Electronic Thesis or Diss., Université Paris sciences et lettres, 2020. http://www.theses.fr/2020UPSLS022.

Testo completo

Abstract (sommario):

Les élastomères thermoplastiques de polyuréthane (TPU) sont une classe de copolymères à blocs caractérisés par une élasticité réversible et une excellente résistance à l'abrasion. Ils sont déjà utilisés dans un certain nombre d’applications de type caoutchouc telles que semelles, roues, câbles flexibles, etc. Pourtant, le comportement en fatigue du TPU sous chargement cyclique n'a pas été étudié en détail, et plusieurs questions restent ouvertes sur la meilleure façon de prédire la durabilité à long terme des TPU. En l'absence de procédure établie pour évaluer la résistance à la fatigue dans les TPU, nous avons proposé une méthode basée sur la propagation de fissure qui permet des comparer la résistance a fatigue des TPU avec les élastomères vulcanisés. On a caractérisé les propriétés mécaniques en petites et grandes déformations de trois TPU avec modules linéaires similaires mais des comportements différents en grandes déformations : adoucissement, rhéodurcissement et cristallisation sous contrainte. Contrairement aux élastomères vulcanisés, tous ces TPU se rigidifient avec la déformation. La diffusion des rayons X a été utilisée pour caractériser les changements de structure à des échelles microscopique induits au fond de fissure pendant le chargement cyclique. La remarquable résistance à la fatigue cyclique du TPU a été expliquée comme une conséquence de la modification de la structure locale des TPU qui génère un durcissement en fond de fissure empêchant le transfert des contraintes pendant le chargement cyclique. On a enfin proposé que ce rhéodurcissement vient de la fragmentation des domaines rigides en domaines plus petits mais plus nombreux qui agissent comme des points de réticulation physiques additionnels
Soft thermoplastic polyurethane elastomers (TPU) are a class of block copolymers characterised by a low linear modulus (<10MPa), reversible elasticity and excellent abrasion resistance already used in several rubber‐like applications such as soles, wheels, flexible cables, etc. Yet, their fatigue behaviour under cyclic loading has not been fully investigated so far, leaving several open questions about how predicting long‐term durability of TPUs for a safe design. In this work we proposed a reproducible experimental protocol to assess and compare the resistance to crack propagation in cyclic conditions of TPU, with that of classical filled rubbers by using a fracture mechanics approach. Furthermore, we characterized the mechanical response under cyclic loading at large and small strain of three commercial TPUs with similar linear moduli and rheology but different large strain behaviours: extended softening, strain hardening and strain hardening enhanced by SIC. Irrespectively of their composition, all TPUs presented an unconventional strain induced stiffening in step‐cyclic experiment. Using DIC and X‐Ray in situ experiments we showed that, the strain gradient at the crack tip generates a spatial re‐organization of the TPU microstructure consistent with a volume locally stiffer than the bulk. This heterogeneity in the deformability reduces the strain intensification at the crack tip explaining the high fatigue resistance in TPU. The local stiffening was ultimately associated to the fragmentation of original hard domains in smaller but more numerous units increasing the degree of physical crosslinking

Gli stili APA, Harvard, Vancouver, ISO e altri

11

Rahbar-Rastegar, Reyhaneh. "Cracking in Asphalt Pavements| Impact of Component Properties and Aging on Fatigue and Thermal Cracking". Thesis, University of New Hampshire, 2018. http://pqdtopen.proquest.com/#viewpdf?dispub=10622119.

Testo completo

Abstract (sommario):

Cracking in asphalt pavements is one of the most common and critical pavement distresses. Cracks let the water penetrate from the surface to underlying layers resulting in shorter pavement service life and poor riding quality. There are various factors that affect the cracking potential of asphalt mixtures including the properties of asphalt components, mix design factors, loading time and loading mode, temperature, stress state, and aging. While several researchers have conducted studies investigating the cracking of asphalt mixtures, the effective parameters are not all well understood to allow engineers to design and construct more resistant pavements against cracking.

The work presented in this dissertation provides some additional insights into the effects of component properties and aging condition on asphalt cracking. The cracking susceptibility of hot mix asphalt (HMA) is evaluated through the experimental testing and numerical modeling on mixtures produced either in design (laboratory) or production (plant) stage. Various criteria and approaches for the prediction of cracking in asphalt binder and asphalt mixture are assessed and their correlation are discussed. Different levels of aging in laboratory are simulated, and the effects of long term oven aging (LTOA) on linear viscoelastic parameters, fatigue and fracture characteristics of asphalt mixtures are explored. The uniaxial tensile fatigue testing based on simplified viscoelastic continuum damage (SVECD) approach is conducted to characterize fatigue behavior, and semi circular bending (SCB), disc-shaped compact tension (DCT) testing and cohesive zone model are used to evaluate thermal cracking in asphalt mixtures.

This dissertation makes a good contribution in improvement of available approaches for evaluation of cracking potential of asphalt pavements and allows for assessment of different mixtures at early stage of material selection. The results of this study can lead to develop a new parameter to predict fatigue and thermal cracking susceptibility of flexible pavements in performance-based specifications, resulting in a better ride quality and cost saving for contractors and taxpayers.

Gli stili APA, Harvard, Vancouver, ISO e altri

12

Whigham, Jared Anthony. "Evaluation of restraint stresses and cracking in early-age concrete with the rigid cracking frame". Auburn, Ala., 2005. http://repo.lib.auburn.edu/2005%20Summer/master's/WHIGHAM_JARED_54.pdf.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

13

Gamboa, Erwin. "Stress corrosion cracking of rock bolts /". [St. Lucia, Qld.], 2004. http://www.library.uq.edu.au/pdfserve.php?image=thesisabs/absthe18302.pdf.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

14

Chan, Simon Hang Chi. "Bond and cracking of reinforced concrete". Thesis, Cardiff University, 2012. http://orca.cf.ac.uk/36698/.

Testo completo

Abstract (sommario):

Ribbed reinforcement is described as “high bond” in Eurocode 2 (EC2) and within the code serviceability checks make no allowance for variations in either the ductility or bond characteristics of these bars. In this work, this matter is explored, and the crack development and behaviour of concrete beams reinforced with various types of ribbed steel bar are investigated, using both numerical and experimental approaches. The objective of the experimental approach is to undertake a series of experiments to compare the performance of beams made with standard reinforcement with that of beams formed with a new high-ductility bar produced by CELSA UK. The relationship between the bond strength and the rib pattern of reinforcing steel was studied and the behaviour at SLS load levels of RC beams with reinforcement of different rib patterns in flexure is discussed. The cracking of beams was monitored both visually and using a non-destructive Digital Image Correlation system to trace in-plane deformations and strains on the surface of the specimens. The test results showed that specimens with bars which had the highest relative rib area (fR value) exhibited the smallest crack spacing and crack width. A numerical model was developed to explore the crack development of reinforced concrete beams under flexural loading. The model employed a non-linear material model for concrete and a smeared crack approach. In order to address the well known numerical stability problems, associated with softening models, a non-local gradient method was used. Crack widths cannot be obtained directly from such models, due to the diffuse nature of non- local simulations, therefore a post-processing procedure was developed to allow the crack characteristics to be calculated. Several numerical examples are presented to illustrate the satisfactory performance of the model. In addition, a series of numerical simulations of the BOND AND CRACKING OF REINFORCED CONCRETE Simon H.C. Chan Page vi experimental beams tested in the present study were used validate the numerical model and conversely, to provide confidence in the consistency of the experimental results.

Gli stili APA, Harvard, Vancouver, ISO e altri

15

Read, John. "Fatigue cracking of bituminous paving mixtures". Thesis, University of Nottingham, 1996. http://eprints.nottingham.ac.uk/12605/.

Testo completo

Abstract (sommario):

This dissertation is concerned with the fatigue cracking of bituminous paving mixtures. It considers both the life to crack initiation and the life for crack propagation, including the development of a method for calculating the number of wheel load applications to either critical or failure condition. The development and subsequent validation of the Indirect Tensile Fatigue Test (ITFT) are described. Both the repeatability and reproducibility of the method are examined as well as its correlation with two other fatigue test methods. The test is shown to be a suitable method for measuring the life to crack initiation of bituminous paving mixtures as well as being an economically viable test. Poisson's ratio for bituminous paving mixtures is examined showing that, provided at least 500 conditioning pulses are used in order to achieve steady state conditions, 0.35 is an appropriate value. Prior to these early load applications Poisson's ratio is shown to be variable and often in excess of 0.50, an explanation of which is given in detail. Crack propagation was experimentally simulated using beams of material with a crack initiated on the underside. The work shows that the rate of crack propagation can be described by a power relationship between the stiffness of the mixture and the number of cycles to failure, which is mixture and binder dependent. A general equation is developed which allows the critical and failure lives of bituminous pavements to be calculated and these are compared to two traditional pavement design methods, giving equivalent results for unmodified mixtures but, giving more realistic results for polymer modified mixtures. Image analysis of the cracks demonstrates that they propagate around coarse aggregate trying to separate it from the matrix and that they travel in the straightest line possible between the point of crack initiation and the point of applied load.

Gli stili APA, Harvard, Vancouver, ISO e altri

16

Lee, Wai Peng. "Cracking during drying of latex films". Thesis, University of Sheffield, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.434529.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

17

Vargas, Vidal Velez. "Bridge Deck Cracking Investigation and Repair". UNF Digital Commons, 2012. http://digitalcommons.unf.edu/etd/401.

Testo completo

Abstract (sommario):

The focus of this study is to investigate the cracking of concrete bridge decks and the sealants used in repairing transverse cracks. Cracking could occur in both hardened mature concrete and early age concrete. Several factors affect concrete cracking, such as age-dependent material properties, thermal- and moisture-related stresses and strains, material viscoelastic behavior, restraints, concrete expansion and contraction, casting sequence, formwork, material characteristics, and environmental exposure. The causes of early age cracking are primarily attributed to effects such as plastic shrinkage, temperature effects, autogenous shrinkage, and drying shrinkage. This deck cracking could greatly reduce durability, lead to a loss of functionality, loss of stiffness, and ultimately the loss of structural safety. The study investigates the deck cracking in general and also the transverse cracks developed in hardened concrete at early ages before service loads application. Both experimental and analytical investigations were performed. The experimental study included testing of 9 reinforced concrete slab specimens (18”x 48”x 5.5”). Cracks were induced in the slabs with different crack widths and lengths, sealed with 4 different materials of sealants, and tested under static loading. The study also included tensile testing of dry hardened samples of sealants. In addition, field application was performed on a bridge, where transverse deck cracks were sealed using 4 different sealant materials; cores were taken and tested according to ASTM-C496. The results of the testing showed that the 3-part HMWM was the best performing sealer for cracks between 0.01 and 0.019 inches of width with the epoxy sealer performing the best for cracks wider than 0.02 inches.

Gli stili APA, Harvard, Vancouver, ISO e altri

18

Liu, Ruofan. "Assessment of Composite Railroad Tie Cracking". University of Akron / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=akron1386688325.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

19

Albahri, Tareq Abduljalil. "Mechanistic modeling of catalytic cracking chemistry /". Digital version accessible at:, 1999. http://wwwlib.umi.com/cr/utexas/main.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

20

luo, Cheng Hong. "Early age thermal cracking of concrete". Thesis, University of Leeds, 1998. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.589517.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

21

Ducousso, Marion. "Gasification biochar reactivity toward methane cracking". Thesis, Ecole nationale des Mines d'Albi-Carmaux, 2015. http://www.theses.fr/2015EMAC0016/document.

Testo completo

Abstract (sommario):

Cette étude porte sur la compréhension et l'amélioration de la réactivité des charbons pour la catalyse de la réaction du craquage du méthane. Pour ce projet, nous avons produit des charbons à partir de la gazéification de bois de peuplier à 750°C sous vapeur d'eau. Par la suite, deux traitements de fonctionnalisation ont été appliqués. D'une part, une oxygénation en phase gaz a été réalisée pour augmenter la concentration des sites oxygénés. D'autre part, une imprégnation en phase liquide dans différentes solutions de sel de nitrate (calcium et potassium) a permis d'accroître la quantité de minéraux. Les propriétés physico-chimiques (structure carbonée, sites oxygénés, minéraux et porosité/surface spécifique) des charbons bruts et fonctionnalisés ont été caractérisées. Les résultats ont montré que les deux traitements de fonctionnalisation ont augmenté la concentration des sites actifs visés. Par ailleurs, les évolutions des propriétés texturales et de la structure carbonée lors des deux fonctionnalisations ont été mises en évidence. Les tests catalytiques du craquage du méthane sur les différents charbons, à 700°C, ont montré que les minéraux sont les sites les plus réactifs vis-à-vis de cette réaction. Les fonctions oxygénées basiques et les défauts de structure sont également des sites actifs. Une diminution de l'efficacité lors du craquage a été observée due à la désactivation progressive de la surface des biochars. Le développement d'un modèle, à l'échelle du pore, a permis de montrer que la concentration initiale de sites actifs à la surface et leur différence de réactivité étaient deux paramètres importants dans la prédiction du comportement de désactivation de la surface
This study is focused on the reactivity of biochar to catalyze the methane cracking reaction. Biochar was produced from steam gasification of poplar wood (750°C, 30 min, 20°C/min, 90%H2O/10%N2, fluidized bed) and then functionalized by an O2 gas-phase treatment and a wet impregnation into nitrate salts solutions to increase oxygen functions and minerals (calcium and potassium) concentrations at the biochar surface respectively. A set of characterization was performed on the raw and functionalized biochars to evaluate their surface physico-chemical properties. The oxygenated functions, the mineral particles, the carbonaceous structures and the textural properties (specific surface area and porosity) were analyzed. Results showed that the two functionalization treatments increased the concentration of the targeted functions and modified the carbon structures and the textural properties as well. Methane cracking tests were then performed on the biochars to compare their activities and correlate with their physico-chemical properties. It has been highlighted the minerals particles of potassium and calcium are the main active sites of the biochar surface. In fact, the reactivity of the impregnated biochars was twice to 4 times higher than the one of the raw biochar. The porosity of the biochar is the second most important criteria to notably obtain a good dispersion of the minerals particles. Basic oxygenated functions and disordered carbonaceous structures (defaults into the graphene sheets) are reactive as well. However, coke deposition progressively deactivated the biochars surface over the reaction in any case. A model at the pore scale has been proposed to better understand the surface deactivation

Gli stili APA, Harvard, Vancouver, ISO e altri

22

Kelly, Chrissy Ann. "Cracking the Code: An effectiveness study". Thesis, Curtin University, 2016. http://hdl.handle.net/20.500.11937/615.

Testo completo

Abstract (sommario):

This study employed a pre-test post-test group design to evaluate the effectiveness of the Cracking the Code (CtC) Program with students aged 3;8–5;4. CtC is a teacher implemented classroom based program, designed to target phonological awareness (PA) and alphabet knowledge. Results showed that children in the experimental condition improved significantly more in PA, alphabet knowledge, non-word reading and non-word spelling after the intervention than the control group, thus demonstrating the effectiveness of CtC.

Gli stili APA, Harvard, Vancouver, ISO e altri

23

Wysong, Zachary D. "Development and comparison of the asphalt binder cracking device to directly measure thermal cracking potential of asphalts". Ohio : Ohio University, 2004. http://www.ohiolink.edu/etd/view.cgi?ohiou1177094517.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

24

Hope, Adam T. Hope. "Development of a High Chromium Ni-Base Filler Metal Resistant to Ductility Dip Cracking and Solidification Cracking". The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1461175457.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

25

Johnson, Luke. "Solidification Cracking and Ductility-Dip Cracking Resistance of Ni-Base Filler Metal 52XL with Tantalum and Molybdenum Additions". The Ohio State University, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=osu1575462956102374.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

26

Raseroka, Mantsaye S. "Controlled chloride cracking of austenitic stainless steel". Pretoria : [s.n.], 2009. http://upetd.up.ac.za/thesis/available/etd-07032009-120615/.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

27

Theiss, Timothy J. "Preliminary investigation into the cracking of polyurethane". Thesis, This resource online, 1987. http://scholar.lib.vt.edu/theses/available/etd-04122010-083619/.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

28

Meadows, Jason Lee. "Early-age cracking of mass concrete structures". Auburn, Ala., 2007. http://repo.lib.auburn.edu/2007%20Spring%20Theses/MEADOWS_JASON_53.pdf.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

29

Meadows, Jason Lee Schindler Anton K. "Early-age cracking of mass concrete structures". Auburn, Ala., 2007. http://repo.lib.auburn.edu/2007%20Spring%20Theses/MEADOWS_JASON_53.pdf.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

30

Wells, David Brett. "Early stages of intergranular stress corrosion cracking". Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.256769.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

31

Mohammed, Farej Ahmed. "Stress corrosion cracking in duplex stainless steels". Thesis, University of Manchester, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488331.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

32

Fejzo, R. "Dynamic behaviour of concrete structures with cracking". Thesis, Swansea University, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636965.

Testo completo

Abstract (sommario):

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.

Gli stili APA, Harvard, Vancouver, ISO e altri

33

Eccott, A. R. "Environmental stress cracking resistance of phenolic compounds". Thesis, Swansea University, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636763.

Testo completo

Abstract (sommario):

The environmental stress cracking (ESC) behaviour of a series of phenolics toughened with varying proportions of thermoplastic (0-35%), has been studied. Since these materials have been designed for applications in 'under-the-bonnet' automotive components, testing took place in serveral 'in-service' environments and in some of the constituent chemicals as well as in air. Initial screening of the materials using three point bend testing highlighted the most hostile environments for further study. Tensile testing of samples in air and in methanol and immersion of samples in various environments provided a further insight concerning the diffusion effects likely to be encountered. Creep tests were conducted in selected environments at 23^oC, as well as at increased temperatures to provide more realistic 'under-the-bonnet' situations. The observed increase in creep rate in most hostile environments was related to crack initiation and growth. Within the range of added thermoplastic studied, two scales of morphology were seen to occur. In addition to a small scale morphology, only observed using TEM, there exists a large scale ribbon-like morphology. This was studied using light microscopy as well as SEM, on samples etched with permanganate and it was shown using X-ray microanalysis that the ribbons visible were thermoplastic rich regions. A good correlation was obtained between the amount of ribbon-like areas and the thermoplastic content of the sample. Samples, apparently prepared under identical conditions, vary slightly in colour. Further investigation revealed that these colour differences could be correlated with a variation in large scale morphology as well as considerable property differences. TEM, SEM and light microscopy were performed to relate the ESC behaviour with the material morphology. TEM investigations concerning the small scale morphology effect upon crack growth were inconclusive. However, from viewing etched samples subject to ESC via bend tests in methanol, using SEM and light microscopy, it was evident that the large scale morphology was responsible for deflecting microcracks.

Gli stili APA, Harvard, Vancouver, ISO e altri

34

Setiaji, A. H. B. "The preparation and characterisation of cracking catalysts". Thesis, University of Salford, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.356190.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

35

Deshais, Gerald. "Stress corrosion cracking in Al based alloys". Thesis, University of Cambridge, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.621509.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

36

Kruska, Karen. "Understanding the mechanisms of stress corrosion cracking". Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:94574eaf-4ae0-4093-bf20-3f4f4c559e7c.

Testo completo

Abstract (sommario):

Austenitic stainless steels are frequently used in the cooling circuits of nuclear reactors. It has been found that cold-worked 304 stainless steels can be particularly susceptible to stress corrosion cracking at the operating conditions of such reactors. Despite more than 130 years of research underlying mechanisms are still not properly understood. For this reason, the effects of cold-work and applied stress on the oxidation behaviour of 304SS have been studied in this thesis. A set of samples with/without prior cold-work, and with/without stress applied during oxidation, were oxidized in autoclaves under simulated pressurised water reactor primary circuit conditions. Atom-probe tomography and analytical transmission electron microscopy were used to investigate the local chemistry and microstructure in the different samples tested. Regions containing grain boundaries, deformation bands, and matrix material in contact with the environment, were extracted from the coupon specimens with a focused ion beam machine. Cross-sections of crack tips were studied with secondary ion mass spectrometry and electron backscatter diffraction. The compositions of oxides grown along the surface and the different microstructural features were analysed. Fe-rich spinels were found at the surface and Cr-rich spinels were observed along fast diffusion paths. Ni-enrichment was found at the metal/oxide interfaces and a Ni-rich phase was detected in precipitates ahead of grain boundary oxides. Li was observed in all oxidised regions and B segregation, originating from impurities in the alloy, was observed in grain boundaries and crack tip oxides. Cavities and hydrogen associated with Ni-rich regions were found ahead of the bulk Cr-rich oxide in some of the samples. The implications of these findings for the understanding of SCC mechanisms are discussed. It is suggested that Ni precipitation as well as the presence of deformation bands may play an important role in controlling SCC susceptibility in 304 stainless steel. A modification of the film-rupture model including internal oxidation and fast diffusion along H-stabilised vacancies in strain fields at the crack front is proposed.

Gli stili APA, Harvard, Vancouver, ISO e altri

37

Momeni, Amir Farid. "Y-cracking in continuously reinforced concrete pavements". Thesis, Kansas State University, 2013. http://hdl.handle.net/2097/15642.

Testo completo

Abstract (sommario):

Master of Science
Department of Civil Engineering
Kyle A. Riding
When transverse cracks meander there is a high possibility for transverse cracks to meet at a point and connect to another transverse crack, creating a Y-crack. Y-cracks have been blamed for being the origin of punchouts and spallings in CRCPs. When the direction of maximum principal stress changes, it could cause a change in the crack direction, potentially forming a Y-crack. Finite Element Models (FEMs) were run to model the change in principal stress direction based on design and construction conditions. The finite element model of CRCP using typical Oklahoma CRCP pavement conditions and design was assembled. The model included the concrete pavement, asphalt concrete subbase, and soil subgrade. The effect of areas of changed friction on the direction of principal stress was simulated by considering a patch at the pavement-subbase interaction. Investigated factors related to this patch were location of patch, friction between patch and subbase, and patch size. Patches were placed at two different locations in the pavement: a patch at the corner of the pavement and a patch at the longitudinal edge between pavement ends. A change in the friction at the corner had a large effect on the stress magnitude and direction of principal stress, while a patch in the middle did not significantly change the stress state. Also, patch size had a noticeable effect on stress magnitude when the patch was at the corner. Another model was developed to understand the effect of jointed shoulder on direction of maximum principal stress. Analysis of this model showed that the stresses were not symmetric and changed along the width of the pavement. This meandering pattern shows a high potential for Y-cracking. Also, several finite element models were run to understand the effects of different shrinkage between mainline and shoulder. In order to simulate the effects of the differential drying shrinkage between the hardened mainline concrete and the newly cast shoulder, different temperature changes were applied on the mainline and shoulder. For these models, the orientation of the maximum principal stress was not significantly changed from different amounts of temperature decreases between mainline and shoulder. Also, effect of different longitudinal steel percentages was investigated by comparing two finite element models with different steel percentage. The model with higher steel percentage (0.7%) indicated more variation in stress, potentially leading to more crack direction diverging.

Gli stili APA, Harvard, Vancouver, ISO e altri

38

Meisnar, Martina. "High-resolution characterisation of stress corrosion cracking". Thesis, University of Oxford, 2015. https://ora.ox.ac.uk/objects/uuid:6915e56d-d63b-43dc-af29-5257a21d1e4b.

Testo completo

Abstract (sommario):

The degradation of reactor grade stainless steels and their susceptibility to stress corrosion cracking (SCC) when exposed to the pressurised water reactor (PWR) primary water environment has been a topic of intense research for many decades. Nevertheless, our understanding of the underlying mechanisms of SCC remains incomplete to date. It has been generally accepted that only high-resolution (electron) microscopy techniques are capable of revealing the yet unidentified processes involved in SCC crack propagation. For this reason, one of the main objectives of this project was to make new techniques with improved spatial resolution accessible to SCC research. While low-keV energy dispersive X-ray spectroscopy (EDX) was used for the preliminary analysis of SCC cracks, transmission Kikuchi diffraction (TKD) and atom probe tomography (APT) were used for high-resolution studies of the microstructure and chemistry near the crack tip. In particular, TKD proved very beneficial for revealing the extent of the strain concentration around the crack tip. For the application of APT to SCC research, a novel method for preparing APT needles containing entire SCC crack tips was developed. The method was then used for acquiring very localised compositional measurements of the crack tip and GB oxide chemistry with extraordinary accuracy. The second objective of this thesis was to understand the impact of the SCC test temperature on the crack growth rate (CGR) in SUS316 stainless steel. It was found that after steady growth with increasing temperature, a peak in the CGR occurred at ~ 320°C, followed by a substantial drop towards higher temperatures. The inhibition of the CGR with increasing temperature between 320° and 360°C and its impact on the microstructure were studied via analytical transmission electron microscopy (TEM) and TKD. Furthermore, the potential impact of thermally activated diffusion and mechanical response-based mechanisms was investigated. It appears that higher dislocation density and strain concentrations around the crack tips at lower temperature (i.e. 320°C) lead to possibly enhanced brittle-like fracture at the crack tip. An enhanced model for the ongoing processes involved in SCC crack propagation based on the experimental results is presented at the end of this work.

Gli stili APA, Harvard, Vancouver, ISO e altri

39

Ramskill, Thomas Edward. "Effect of Cracking on Lag Bolt Performance". Diss., Virginia Tech, 2002. http://hdl.handle.net/10919/28643.

Testo completo

Abstract (sommario):

This dissertation presents the results of testing to determine the load-slip characteristics of single-shear single lag screw connections subjected to monotonic lateral loading parallel to grain. Of particular importance was the comparison of experimental capacity and 5% offset yield load resistances to load resistances as predicted by the American Forest & Paper Association's (AF&PA) publication General Dowel Equations for Calculating Lateral Connection Values, Technical Report 12 (TR-12). Additionally some other tests were conducted, including fracture, tension strength perpendicular-to-grain, lag screw connection inking, dowel embedment, specific gravity and moisture content. The results for the testing program are presented. Four hundred and forty eight lateral tests were conducted on lag screw connections. Each connection was comprised of a 2 x 6 x 14 in. long wood main member, 1/4 in. thick steel side plate, and a single lag screw. The parameters of interest were specific gravity, lag screw diameter, and pilot hole diameter. Two species of wood, Douglas-fir and spruce-pine-fir, three lag screws diameters, and three pilot hole diameters for each species of wood were implemented. Models were developed to predict lag screw connection capacity and 5% offset yield and are contained herein. Models were based on results from connection and inking tests and mechanical analysis. Recommendation for ASD and LRFD design values were derived from connection test results for connections that used AF&PA's National Design Specification for Wood Construction (NDSb) specified pilot holes. Using fracture mechanics results, work was performed to determine the effective load required to crack and separate fracture surfaces of wood main members due to the installation of lag screws with varying pilot hole diameters.
Ph. D.

Gli stili APA, Harvard, Vancouver, ISO e altri

40

Bazzo, Jeffrey D. "Analysis of Uncontrolled Concrete Bridge Parapet Cracking". Cleveland State University / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=csu1351032089.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

41

Baah, Prince. "Cracking Behavior of Structural Slab Bridge Decks". University of Akron / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=akron1417539467.

Testo completo

Gli stili APA, Harvard, Vancouver, ISO e altri

42

Gammon, M. A. "Stress corrosion cracking of nuclear grade steels". Master's thesis, University of Cape Town, 1992. http://hdl.handle.net/11427/21956.

Testo completo

Abstract (sommario):

A nuclear grade 316L stainless steel and a 508-111 quenched and tempered pressure vessel steel were studied for their stress corrosion cracking susceptibility. Cylindrical tensile specimens were subjected to slow strain rate testing at 75°C in aerated, aqueous solutions (distiled water with 1000ppm Cl⁻ or SO₄ = ions in solution) in a range of corrosion potentials. The 316L has been examined for sensitization and stress corrosion resistance. This study has shown that the peak degree of sensitization attainable in this material is well within the limits considered as safe by the nuclear power industry. This material is not susceptible to environmentally assisted cracking as long as the potential is kept below the pitting potential for the material. A single instance of intergranular stress corrosion cracking was noted when this material was tested in 1000ppm Cl⁻ solution at 440mV (SHE). Two casts of 508-111 have been examined: 508-A has been tested in the as quenched condition as well as after two tempering heat treatments, while 508-B has been tested in the fully tempered condition only. The mechanical properties of the 508 type materials are strongly influenced by the heat treated condition and mildly influenced by the service environment. In the quenched condition anodic intergranular stress corrosion cracking is severe in the chloride solution and it is argued that the absence of intergranular cracking in the sulphate solution is due to the over aggressiveness of this environment. In all three heat treated conditions loss of ductility is more pronounced in sulphate solutions than in chlorides. Transgranular cleavage is evident in strongly cathodic conditions and this is ascribed to the ingress of hydrogen. The transgranular hydrogen embrittlement seems to be independant of heat treated condition. Rising load tests on fatigue precracked specimens have indicated that environmentally enhanced crack growth of existing defects does not occur for the conditions tested.

Gli stili APA, Harvard, Vancouver, ISO e altri

43

Etoumi, Assma S. Abdalla. "Hybrid membrane-distillation separation for ethylene cracking". Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/hybrid-membranedistillation-separation-for-ethylene-cracking(aee59d9f-61a3-4edf-8d22-0123fb6e942e).html.

Testo completo

Abstract (sommario):

Gas separations are often required in chemical processes, e.g. air separation, ethylene production, etc. These are often challenging and costly processes because of the low temperature and high pressure needed if vapour-liquid phase separations are involved. This thesis focuses on hybrid membrane-distillation separations as an opportunity to develop more energy-efficient separation processes. In a typical ethylene plant, recovery, the separation and purification of the cracked product are economically important. The focus of this thesis is on the ‘C2splitter’ which separates the desired product, ethylene, from ethane. Cryogenic distillation, which is currently used to separate the binary ethylene-ethane mixture, is extremely expensive in terms of both capital and operating costs, especially because of refrigerated cooling requirements. Hybrid membrane-distillation processes are able to effectively separate low-boiling compounds and close-boiling mixtures and to reduce energy consumption, relative to cryogenic distillation. However, hybrid membrane-distillation processes present challenges for process modelling, design and operation. There are two major challenges associated with the modelling of hybrid processes for low temperature separations: i) the complex interaction between the process and the refrigeration system and ii) the large number of structural options, e.g. conventional column, membrane unit or hybrid membrane-distillation separation, where the distillation column can be integrated with the membrane unit to form a sequential, parallel, ‘top’or ‘bottom’ hybrid scheme. This thesis develops a systematic methodology to design, screen, evaluate and optimise various design alternatives. Schemes are evaluated with respect to energy consumption, i.e. power consumption of process and refrigeration compressors, or energy costs. In the methodology, process options are screened first for feasibility, based on numerous simulations and sensitivity analyses. Then, the feasible options are evaluated in terms of energy consumption and compared to the performance of a conventional distillation column. Finally, economically viable schemes are optimised to identify the most cost-effective heat-integrated structure and operating conditions. The methodology applies models for multi-feed and multi-product distillation columns, the membrane, compressor and refrigeration system; heat recovery opportunities are systematically captured and exploited. For the separation of relatively ideal mixtures, modified shortcut design methods, based on the Fenske-Underwood-Gilliland method are appropriate because they allow fast evaluation without needing detailed specification of column design parameters (i.e. number of stages, feed and side draw stage locations and reflux ratio). The modifications proposed by Suphanit (1999) for simple column design are extended to consider multi-feed and/or multi-product columns. The complex column designs based on the approximate calculations method are validated by comparison with more rigorous simulations using Aspen HYSYS. To design the hybrid system, a reliable and robust membrane model is also needed. To predict the performance of the module model, this work applies and modifies detailed membrane model (Shindo et al., 1985) and approximate method (Naylor and Backer, 1955) to avoid the need for initial estimates of permeate purities and to facilitate convergence. Heat integration opportunities are considered to reduce the energy consumption of the system, considering interactions within the separation process and with the refrigeration system. A matrix-based approach (Farrokhpanah, 2009) is modified to assess opportunities for heat integration. The modified heat recovery model eliminates the need to design the refrigeration cycle and uses a new simple, linear model that correlates the ideal (Carnot) and a more accurately predicted coefficient of performance. This work develops a framework for optimising important degrees of freedom in the hybrid separation system, e.g. permeate pressure, stage cut, side draw molar flow rate and purity, column feed and side draw locations. Heat recovery options between: i) column feeds and products; ii) the membrane feed and products and iii) the associated refrigeration system are considered. A deterministic and a stochastic optimisation algorithm are applied and compared for their efficiency of solving the resulting nonlinear optimisation problem. The new approach is demonstrated for the design and optimisation of heat-integrated sequential and parallel hybrid membrane-distillation flowsheets. Case study results show that hybrid scheme can reduce energy cost by 11%, compared to distillation, and that parallel schemes have around 8% lower energy costs than sequential hybrid schemes. These results suggest hybrid membrane-distillation processes may be competitive with distillation when applied for ethylene-ethane separations, but that further development of suitable membranes may still be needed.

Gli stili APA, Harvard, Vancouver, ISO e altri

44

Mohammad, Noor. "Desiccation Cracking Behaviour in Thin Bentonite Layers". Thesis, The University of Sydney, 2020. https://hdl.handle.net/2123/22231.

Testo completo

Abstract (sommario):

This thesis investigates the behaviour of desiccation cracks in thin bentonite layers under different conditions (initial water content, layer thickness, temperature and mixtures with kaolinite). For this, a comprehensive series of experiment was conducted as well as some theoretical models were developed. The theoretical model was developed based on the critical cracking thickness following the principles of linear elastic fracture mechanics (Chapter 4). This model was then used in the following chapters and validated with the experimental investigations. Also, hydraulic properties of the cracked soil mass were modelled and the effects of cracks on hydraulic conductivity and water retention properties were shown. In the experimental investigations, firstly, the combined effects of initial water content and layer thickness were studied in a phase diagram to observe the behaviour of desiccation cracks (Chapter 5). It was found that critical cracking thickness could be an important factor in distinguishing cracked and non-cracked samples in the phase diagram. Characteristics of cracks (crack density, total crack length, average crack length and crack width) increase with increasing layer thicknesses as well as initial water contents. The effects of temperature (Chapter 6) and mixtures of bentonite-kaolinite (Chapter 7) on desiccation cracking were investigated in the phase diagram developed in Chapter 5. At higher temperatures, the critical thickness of a clay layer ould be decreased. Results of bentonite-kaolinite mixture demonstrated that the phase boundary shifted to increase the critical thickness of the clay layers of mixtures containing less bentonite (i.e. more kaolinite). Crack morphology was also affected by the change in temperatures and properties of the mixtures. In summary, this study shows some understanding of desiccation cracking behaviour and could provide insights for better design of clay liners, which often at risk of desiccation problem.

Gli stili APA, Harvard, Vancouver, ISO e altri

45

Campet, Robin. "Simulation and optimization of steam-cracking processes". Thesis, Toulouse, INPT, 2019. http://www.theses.fr/2019INPT0009/document.

Testo completo

Abstract (sommario):

Le procédé de craquage thermique est un procédé industriel sensible aux conditions de température et de pression. L’utilisation de réacteurs aux parois nervurées est une méthode permettant d’améliorer la sélectivité chimique du procédé en augmentant considérablement les transferts de chaleur. Cependant, cette méthode induit une augmentation des pertes de charge dans le réacteur, ce qui est dommageable pour le rendement chimique et doit être quantifié. En raison de la complexité de l’écoulement turbulent et de la cinétique chimique, le gain réel offert par ces géométries en termes de sélectivité chimique est toutefois mal connu et difficile à estimer, d’autant plus que des mesures expérimentales détaillées sont très rares et difficiles à mener. L’objectif de ce travail est double: d’une part évaluer le gain réel des parois nervurées sur le rendement chimique; d’autre part proposer de nouveaux designs de réacteurs offrant une sélectivité chimique optimale. Ceci est rendu possible par l’approche de simulation numérique aux grandes échelles (LES), qui est utilisée pour étudier l’écoulement réactif à l’intérieur de diverses géométries de réacteurs. Le code AVBP, qui résout les équations de Navier Stokes compressibles pour les écoulements turbulents, est utilisé pour simuler le procédé grâce à une méthodologie numérique adaptée. En particulier, les effets des pertes de charge et du transfert thermique sur la conversion chimique sont comparés pour un réacteur lisse et un réacteur nervuré afin de quantifier l’impact de la rugosité de paroi dans des conditions d’utilisation industrielles. Une méthodologie d’optimisation du design des réacteurs, basée sur plusieurs simulations numériques et les processus Gaussiens, est finalement mise au point et utilisée pour aboutir à un design de réacteur de craquage thermique innovant, maximisant le rendement chimique
Thermal cracking is an industrial process sensitive to both temperature and pressure operating conditions. The use of internally ribbed reactors is a passive method to enhance the chemical selectivity of the process, thanks to a significant increase of heat transfer. However, this method also induces an increase in pressure loss, which is damageable to the chemical yield and must be quantified. Because of the complexity of turbulence and chemical kinetics, and as detailed experimental measurements are difficult to conduct, the real advantage of such geometries in terms of selectivity is however poorly known and difficult to assess. This work aims both at evaluating the real benefits of internally ribbed reactors in terms of chemical yields and at proposing innovative and optimized reactor designs. This is made possible using the Large Eddy Simulation (LES) approach, which allows to study in detail the reactive flow inside several reactor geometries. The AVBP code, which solves the Navier-Stokes compressible equations for turbulent flows, is used in order to simulate thermal cracking thanks to a dedicated numerical methodology. In particular, the effect of pressure loss and heat transfer on chemical conversion is compared for both a smooth and a ribbed reactor in order to conclude about the impact of wall roughness in industrial operating conditions. An optimization methodology, based on series of LES and Gaussian process, is finally developed and an innovative reactor design for thermal cracking applications, which maximizes the chemical yield, is proposed

Gli stili APA, Harvard, Vancouver, ISO e altri

46

Giovanniello, Francesca <1992&gt. "Cracking the luxury code. The Gucci Renaissance". Master's Degree Thesis, Università Ca' Foscari Venezia, 2018. http://hdl.handle.net/10579/13002.

Testo completo

Abstract (sommario):

Luxury has always been contradistinguished by the conceptual and symbolic dimension - the aura - attached to the "products" (products and services) created by luxury brands. The multisensorial sphere, lying on the increasing need of experience from consumers, characterizes the management of these creations. Luxury brands are the result of culture and business practices. They go beyond the object and they are built on reputation. Communication is the key and people, "products", passion, pleasure, purpose and price are the levers. By tradition, luxury market has been a select market run by exclusive brands and enjoyed by wealthiest individuals. Nowadays, this space shaped its boundaries due to the fast pace of change which characterizes the digital era which we are immerse in. Instability is not that uncommon and luxury brands are challenged to demonstrate their strength. For this reason, there is a new set of "survival rules" to be adopted for cracking the luxury codes and stretching brands without losing exclusivity. The most crucial one - innovating this world of meanings - is constantly knocking at the precious doors of the elected players. A successful way of how to open will be discussed illustrating the case of Gucci, which is benefitting of a renaissance after a tough period of turbulence.

Gli stili APA, Harvard, Vancouver, ISO e altri

47

Nevasmaa, P. (Pekka). "Predictive model for the prevention of weld metal hydrogen cracking in high-strength multipass welds". Doctoral thesis, University of Oulu, 2003. http://urn.fi/urn:isbn:9514271815.

Testo completo

Abstract (sommario):

Abstract This thesis studies controlling factors that govern transverse hydrogen cracking in high-strength multipass weld metal (WM). The experiments were concerned with heavy-restraint Y- and U-Groove multipass cracking tests of shielded-metal arc (SMAW) and submerged-arc (SAW) weld metals. Results of tensile tests, hardness surveys, weld residual stress measurements and microstructural investigations are discussed. The analytical phase comprised numerical calculations for analysing the interactions between crack-controlling factors. The objectives were: (i) the assessment of WM hydrogen cracking risk by defining the Crack-No Crack boundary conditions in terms of 'safe line' description giving the desired lower-bound estimates, and (ii) to derive predictive equations capable of giving reliable estimates of the required preheat/interpass temperature T0/Ti for the avoidance of cracking. Hydrogen cracking occurred predominantly in high strength weld metals of Rp0.2 ≈ 580-900 MPa. At intermediate strengths of Rp0.2 ≈ 500-550 MPa, cracking took place in the cases where the holding time from welding to NDT inspection was prolonged to 7 days. Low strength WMs of Rp0.2 ≤ 480 MPa did not exhibit cracking under any conditions examined. Cracking occurrence was, above all, governed by WM tensile strength, weld diffusible hydrogen and weld residual stresses amounting to the yield strength. The appearance of cracking vanished when transferring from 40 to 6 mm thick welds. The implications of the holding time were more significant than anticipated previously. A period of 16 hrs in accordance with SFS-EN 1011 appeared much too short for thick multipass welds. Interpass time and heat input showed no measurable effect on cracking sensitivity, hence being of secondary importance. Equations were derived to assess the weld critical hydrogen content Hcr corresponding to the Crack-No Crack conditions as a function of either weld metal Pcm, yield strength Rp0.2 or weld metal maximum hardness HV5(max). For the calculation of safe T0/Ti estimates, a formula incorporating: (i) WM strength as a linear function of either weld carbon equivalent CET or weld HV5(max), (ii) weld build-up thickness aw in the form of tanh expression and (iii) weld diffusible hydrogen HD in terms of a combined [ln / power law] expression was found descriptive.

Gli stili APA, Harvard, Vancouver, ISO e altri

48

Sullivan, William Garrett. "Calibrating triboluminescent light to cracking in concrete structures". Tallahassee, Fla. : Florida State University, 2010. http://purl.fcla.edu/fsu/lib/digcoll/undergraduate/honors-theses/2181949.

Testo completo

Abstract (sommario):

Thesis (Honors paper)--Florida State University, 2010.
Advisor: Dr. Okenwa Okoli, Florida State University, College of Engineering, Dept. of Industrial Engineering and Dept. of Civil and Environmental Engineering. Includes bibliographical references.

Gli stili APA, Harvard, Vancouver, ISO e altri

49

Crusat, Codina Laura. "Numerical modeling of cracking along non-preestablished paths". Doctoral thesis, Universitat Politècnica de Catalunya, 2018. http://hdl.handle.net/10803/671100.

Testo completo

Abstract (sommario):

The general objective of this thesis is the numerical representation of discrete cracks and their propagation using concepts of Configurational Mechanics, in such a way that their trajectory really corresponds to an energy minimum and is not predetermined by the initial mesh lines. As the first step, two existing formulations for discrete cracks and discontinuities of pre-determined orientation are revisited: the traditional zero-thickness interface elements (FEM+z), and the more recently developed eXtended Finite Element Method (XFEM). The XFEM is studied in detail, a formulation is developed and implemented in a Matlab code, and some examples of validation are also run. Some theoretical aspects are investigated such as the equivalence with the traditional interface elements, which is proven to hold under certain conditions, or the reason for pathological jumps observed in traditional implementations of the method using “natural interpolation”, which may be attributed to lack of conformity of interpolation functions between the two sides of the original element, along the intersection with the discontinuity line. Compared to existing literature, the formulation developed and implemented exhibits two non-standard aspects: a) the re-interpolation (or “two-step” interpolation) of the variables from the nodes to the discontinuity intersection points first, and from there to the interior points of the subdomains (instead of the traditional direct interpolation by means of the original interpolation functions of the element), and b) the use of additional degrees of freedom of the “overhang” or “cantilever” type (value that the nodal displacements would have on the other side of the discontinuity in the absence of jump) instead of the traditional values of the displacement “jumps” themselves. Additional new developments relate to practical aspects of implementation such as treatment of nodes lying too close to the discontinuity, which are moved onto the discontinuity itself in order to eliminate numerical problems. The second part of the thesis focuses on the use of Configurational Mechanics theory to modify the orientation of the cracks, in order to ensure that the trajectory obtained corresponds to an energy minimum and minimize its dependency on the original mesh layout. Configurational forces are defined as the energy gradients relative to the nodal position of the mesh nodes. Therefore, reorienting the fractures in the direction indicated by these forces should minimize the elastic global energy and fracture trajectories should be reproduced more realistically. Based on this idea, an additional “configurational” loop is added to the standard scheme of incremental-iterative process for “deformational” non-linear structural analysis. The change of nodal coordinates at each configurational iteration is applied selectively; only fracture tip nodes of the interface elements that begin to open are allowed to change their configurational position. FEM+z non-linear model incorporates the non-linear constitutive law for discrete cracks and a new iterative strategy of the indirect displacement control (IDC) type based on fracture dissipation has been developed. Additionally, mesh relaxation techniques and nodal variable transport have been also implemented to ensure mesh quality and consistent results. Finally, implementation in Matlab code and verification examples has let to very satisfactory results; mesh lines do succeed in reorienting themselves during configurational iterations, so that the developing crack progressively matches the known physical trajectory in all the examples presented.
L'objectiu general d'aquesta tesi és la descripció numèrica de discontinuïtats discretes i la seva propagació utilitzant els conceptes que ofereix la Mecànica Configuracional, de manera que la seva trajectòria realment correspongui a un mínim d'energia i no estigui predeterminada per les línies de la malla inicial. Com a primer pas, es revisen dues formulacions existents per a fractures amb una orientació predeterminada: els elements junta tradicionals de gruix zero (FEM+z) i el eXtended Finite Element Method (XFEM) desenvolupat recentment. En primer lloc s’estudia XFEM, es desenvolupa i implementa la seva formulació en un codi Matlab, i es realitzen alguns exemples de validació. S'investiguen aspectes teòrics, com l'equivalència amb els elements junta tradicionals de gruix zero, la qual es demostra per a certes condicions, o la raó dels salts patològics observats en les implementacions tradicionals del mètode mitjançant la "interpolació natural", que pot atribuir-se a la manca de conformitat de les funcions d'interpolació entre els dos costats de l'element original, al llarg de la intersecció amb la línia de discontinuïtat. En comparació amb la literatura existent, la formulació desenvolupada presenta dos aspectes no estàndards: a) la re-interpolació (o "interpolació en dos passos") de les variables dels nodes als punts d'intersecció de la discontinuïtat en primer lloc, i d'aquí als punts interiors dels subdominis (en comptes de la interpolació directa tradicional mitjançant les funcions d'interpolació originals de l'element), i b) l'ús de graus de llibertat addicionals del tipus "voladís" o "cantilever" (valor que tindrien els desplaçaments nodals a l'altre costat de la discontinuïtat en absència de salt) en comptes dels valors tradicionals ("salts"). A més a més, s’han realitzat altres desenvolupaments d’aspecte pràctic en la implementació, com el tractament de nodes molt propers a la discontinuïtat, per tal d'eliminar problemes numèrics aquests es traslladen fent-los coincidir amb la discontinuïtat. La segona part de la tesi es centra en l'ús de la teoria de la Mecànica configuracional per a modificar l'orientació de les fractures per tal de garantir que la trajectòria obtinguda correspongui a un mínim d'energia i es minimitzi la dependència amb la malla original. Les forces configuracionals es defineixen com els gradients d'energia en relació a la posició nodal dels nodes de malla. Per tant, reorientar les fractures en la direcció indicada per aquestes forces ha de minimitzar l'energia global elàstica i les trajectòries de fractura s'han de reproduir de manera més realista. En base a aquesta idea, s’afegeix un llaç "configuracional" addicional a l'esquema estàndard del procés iteratiu-incremental. El canvi de coordenades nodals en cada iteració configuracional s'aplica selectivament; només els nodes de la punta de fractura dels elements junta que comencen a obrir poden canviar la seva posició configuracional. El model no lineal FEM+z incorpora la llei constitutiva no lineal per a fractures discretes, per a millorar l’estratègia d’avanç s'ha desenvolupat una nova estratègia iterativa del tipus Indirect Displacement Control (IDC) basada en la dissipació de la fractura. A més a més, s'han aplicat tècniques de relaxació de malla i transport de variable nodal per a assegurar la qualitat de la malla i obtenir resultats coherents. Finalment, la implementació en el codi de Matlab i els exemples de verificació han donat resultats molt satisfactoris; les línies de malla aconsegueixen reorientar-se durant les iteracions configuracionals, de manera que la fractura propaga progressivament coincidint amb la trajectòria física coneguda en tots els exemples presentats.

Gli stili APA, Harvard, Vancouver, ISO e altri

50

Bosnjak, Daniela. "Self-induced cracking problems in hardening concrete structures". Doctoral thesis, Norwegian University of Science and Technology, Department of Structural Engineering, 2000. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-1512.

Testo completo

Abstract (sommario):

This thesis deals with prediction of early age cracking caused by restrained temperature dilation and autogenous deformation. Traditonally, prediction of early-age cracking has been based on temperature criteria. The temperature criteria alone are, however, not sufficiently accurate for cracking prediction. Stress-strain criteria are more reliable because they consider all the crucial factors: thermal dilation, autogenous deformaion, mechanical properties and restraint condtions.

The purpose of this dr.ing. project is to extend the general appliicability of stress/srain based curing technology, and to contribute to better understanding and better prediction of stresses during hardeing of concrete structures.

In the first part of the thesis the major features of hardening concrete are described and mathematical descriptions of the phenomena are given. Different models are discussed, and on the basis of such discussion, new models have been implemented in the FE program DIANA. First, the equivalent time was introduced in parallel to the degree of hydration, so that all material properties may be considered equivalent-time dependent. it is shown that the simpler and more common equivelent-time based model for the heat of hydration development can describe temperature development with the same accurancy as the degree of hydration based model. Further, the activation energy is made temperature depentent instead of being constant. A modified CEB equation for E.modulus development is implemented. The temperature influence on creep is modified by introducing a thermal transitonal creep term.

A mathematical decription of stress-and strain development in hardening concrete structures is presented, and special attention given to the viscoelastic behavior of concrete. The influence of creep on Self-induced stresses is examined. The emphasis is put on creep importance in different periods of the hardening process. In externally restrained structures, prone to through cracking, creep in the early period (heating phase) reduces the compressive stresses, and consequently increases the tensile stresses in the subsequent cooling period, thus increasing the cracking risk. For internally restrained structures, prone to surface cracking in the early period, creep obviosly reduces the cracking risk. In the cooling phase creep reduces the tensile stresses. For externally restrained structures, the effects of creep in the cooling and heating phases are counteracting, and in some cases they annul each other. The present results do not diminish the importance of creep on thermal stresses, to the contrary, it emphasises the necessity of correct prediction of creep during the whole period of hardening.

Three creep models for young concrete are compared: Double Power Law, Maxwell chain model and Burgers model. The models are calibrated against the same experimental data and used in calculation of therimal stresses. The temperature dependency of the models is also investigated. A good agreement between the Double Power Law and the Maxwell Chain was obtained, wheras the Burgers model, which consists of one Kelvin unit and one viscous damper, gave different results. Although the major reason for the deviation is due to the simplicity of the Burgers model, the deviation is also coused by differnt descriptions of the temperature dependence, and the nature of the test data used in the calibration. Test methods are also discussed, with special weight on the Temperature Stress Testing Machine tests. Although this type of test is indispensible, and very useful to check the validity of the material models and the calculation methods, the shortcomings by using it to predict the creep model parameters are also shown.

Restraint conditions and their influlence on thermal stresses were also investigated. Different ways of modelling the structure and the interaction with the subgrade were analysed and compared. Influence of the stiffness of the ground and the structural geometry on thermal stresses and cracking risk was investigated by means of 3D analyses.

The last part of the work is numerical simulation of temperature and stress/strain development in real structure, the Maridal culvert in Olso. A comprehensive laboratory test program was performed at NTNU to identify material parameters. Both thermal and mechanical properaties were tested. Material models were checked in the TSTM for three different realistic temperature histories. Within the Brite-Euram project IPACS a large field instrumentation programme has been carried out to measure temperature and strain development in the culvert. 3D analysis of the foundations and walls in two of the instrumented sections of this structure were performed with DIANA and calculated results are compared to measurements. Deviation between calculated and measured temperature is within range of ±5⁰C. Deviation between calculated and measured strains is about ±50x10^-6.Possible reasons of disagreement between calculations and measurements are discussed. When all uncertainties in materail modelling and measuring methods are sonsidered it is concluded that results of the simulation are satisfying.

Gli stili APA, Harvard, Vancouver, ISO e altri

Tesi sul tema "Cracking"

Cita una fonte nei formati APA, MLA, Chicago, Harvard e in molti altri stili