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Search Results: 1 - 10 of 448127 matches for " J. L.; Hallen "
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Comportamiento Mecánico de Grietas No Coplanares en Tubos Aplicando el Método de los Elementos Finitos/[title] [title language=en]Mechanical Behavior of Non-Coplanar Cracks in Pipes Applying the Finite Elements Method
Morales,A.; González,J. L.; Hallen,J. M.;
Información tecnológica , 2004, DOI: 10.4067/S0718-07642004000600005
Abstract: planar-type cracks formed at different depths within tube walls, which are generally caused by hydrogen-induced cracking were analyzed. modeling was carried out on the interaction of pressurized cracks contained within the tube wall with internal pressure prior to their coalescence, varying the radius of the defects. the finite elements method was applied under non-linear conditions of the material following the isotropic hardening law and considering the properties of the material (api 5l-x52). the results suggested the evolution of stress fields and deformations in the crack tips area as a function of the pressure of the defect. defects of less than 38.1 mm resist a pressure of 70 to 124 mpa, and those larger than 63.5 mm severely affect the mechanical integrity. the function of the critical pressure supported by symmetrical cracks is of the potential type, and this pressure produces the interaction which plastifies the region among the cracks, prior to stepping
Comportamiento Mecánico de Grietas No Coplanares en Tubos Aplicando el Método de los Elementos Finitos/[title] [title language=en]Mechanical Behavior of Non-Coplanar Cracks in Pipes Applying the Finite Elements Method
A. Morales,J. L. González,J. M. Hallen
Información Tecnológica , 2004,
Abstract: Se analizan grietas del tipo planar formadas a diferentes profundidades del espesor del tubo y que generalmente son causadas por el agrietamiento inducido por hidrógeno. Se modeló la interacción de grietas presurizadas contenidas en la pared del tubo con presión interna previas a su coalescencia, variando el radio de los defectos. Se aplicó el método de los elementos finitos en condiciones no lineales del material bajo la ley de endurecimiento isotrópico y considerando las propiedades del material (API 5L-X52). Los resultados indican la evolución de los campos de esfuerzos y deformaciones en el área de puntas de grieta en función de la presión del defecto. Defectos menores a 38.1 mm resisten una presión de 70 hasta 124 MPa, y mayores de 63.5 mm afectan severamente la integridad mecánica. La función de la presión critica que soportan grietas simétricas es de tipo potencial y esta presión produce la interacción que plastifica la región entre las grietas, previamente al escalonamiento Planar-type cracks formed at different depths within tube walls, which are generally caused by hydrogen-induced cracking were analyzed. Modeling was carried out on the interaction of pressurized cracks contained within the tube wall with internal pressure prior to their coalescence, varying the radius of the defects. The finite elements method was applied under non-linear conditions of the material following the isotropic hardening law and considering the properties of the material (API 5L-X52). The results suggested the evolution of stress fields and deformations in the crack tips area as a function of the pressure of the defect. Defects of less than 38.1 mm resist a pressure of 70 to 124 MPa, and those larger than 63.5 mm severely affect the mechanical integrity. The function of the critical pressure supported by symmetrical cracks is of the potential type, and this pressure produces the interaction which plastifies the region among the cracks, prior to stepping
Efecto del tama o de probeta y orientación en la resistencia a la tracción y a la tenacidad a la fractura
Terán, J.,González, J. L.,Hallen, J. M.,Martínez, M.
Revista de Metalurgia , 2007,
Abstract: In this work, an experimental research was conducted to determine size and orientation effects on tension and toughness properties through CTOD-R curves, using standard and miniature specimens taken from a structural steel plate. Compliance function estimation for the miniature size samples through load-displacement curves was considered. Experimental and statistical results showed that size and orientation affect tension and toughness properties. The miniature tension test specimens showed strength values slightly greater than the standard ones but with less ductility. Miniature specimen CTOD-R curves showed sensibility to load changes and measurement method of crack aperture and crack length. Inconsistency in fracture toughness for specimen orientation longitudinal circumferential (LC) regarding size effect was also observed. Short orientations showed less strength and toughness than the other directions. Este trabajo se dirigió a determinar el efecto de tama o y orientación en las propiedades de tracción y tenacidad a la fractura mediante las curvas CTOD-R, utilizando probetas estándar y miniatura de una placa de acero estructural. El estudio requirió la estimación de la función flexibilidad de las probetas miniatura a partir de la curva carga versus desplazamiento. Los resultados experimentales y estadísticos muestran que tanto el tama o como la orientación afectan las propiedades mecánicas y la tenacidad a la fractura. Las probetas miniatura de tracción mostraron valores de resistencia ligeramente mayores que las estándar pero una menor ductilidad. Las curvas CTOD-R de las probetas miniatura mostraron ser sensibles a las variaciones de carga y al método de medición de la abertura y longitud de la grieta. Asimismo, se observó inconsistencia del efecto de tama o en la tenacidad a la fractura en la orientación longitudinal circunferencial (LC), siendo las orientaciones cortas menos resistentes y tenaces que las otras direcciones.
“Ethnophilosophy” Redefined?
B Hallen
Thought and Practice: A Journal of the Philosophical Association of Kenya , 2010,
Abstract: The meaning of the term “ethnophilosophy” has evolved in both a significant and controversial variety of ways since it was first introduced by Paulin Hountondji in 1970. It was first challenged by the Kenyan philosopher, H. Odera Oruka, as based upon Hountondji’s unfair appreciation of Africa’s indigenous cultural heritage. Barry Hallen and J. Olubi Sodipo, using a form of analytic philosophy as foundational, thereafter argued that Yoruba ordinary language discourse also served to undermine Hountondji’s critique. The later work of the Ghanaian philosopher, Kwame Gyekye, and the Kenyan D. A. Masolo have further legitimized the epistemological status of elements of African culture that once would have been labeled as of no genuine philosophical significance because they were ‘ethnophilosophical’ in character. The end result of this debate seems to be that both the form and content of philosophy in culture generally must be relativized. The most significant consequence of this would be that African and non-Western philosophy generally would finally be culturally liberated from the oppressive influence, indeed dominance, of what has conventionally come to be known as ‘mainstream’ (Western) philosophy.
Characterization of a Sulfonated Polycarbonate Resistive Humidity Sensor
Carla P. L. Rubinger,Hallen D. R. Calado,Rero M. Rubinger,Henrique Oliveira,Claudio L. Donnici
Sensors , 2013, DOI: 10.3390/s130202023
Abstract: In this work; resistive moisture sensors were obtained by dip coating sulfonated polycarbonate (SPC) onto silver interdigitated electrodes. Commercial polycarbonate was sulfonated with acetyl sulphate at two different sulfonation degrees corresponding to 9.0 and 18.0 mole %. Impedance spectroscopy was used to investigate the humidity sensing properties at controlled relative humidity (RH%) environments generated from standard saline solutions in the range of 11–90 RH%. For the highest sulfonated sample; in the RH% range investigated (11 to 90%); the sensor impedance changed from 4.7 MΩ to 18 kΩ. Humidity sensors made from sulfonated polycarbonate showed exponential decay behavior of the impedance at constant frequency with the environmental relative humidity. Sample 9SPC presented dielectric relaxation response for environmental humidity between 58 and 90 RH% while sample 18SPC presented dielectric relaxation response for the entire measured range between 11 and 90 RH%. Sulfonated polycarbonate could be a promising material for the fabrication of simple and cheap humidity-sensing sensors for the assessment of relative humidity of the surrounding environment, as suggested by experimental results.
Modelado del crecimiento de picaduras en tuberías enterradas que transportan hidrocarburos utilizando técnicas estadísticas
Velázquez, J. C.,Caleyo, F.,Valor, A.,Hallen, J. M.
Revista de Metalurgia , 2011,
Abstract: New deterministic and stochastic predictive models are proposed for external pitting corrosion in underground pipelines. The deterministic model takes into consideration the local chemical and physical properties of the soil as well as the pipeline coating to predict the time dependence of pitting depth and rate in a range of soils. This model, based on results from a field study, was used to conduct Monte Carlo simulations that established the probability distribution of pitting depth and growth rate in the studied soils and their evolution over the life of the pipeline. In the last stage of the study, an empirical Markov chain-based stochastic model was developed for predicting the evolution of pitting corrosion depth and rate distributions from the observed properties of the soil. Esta investigación, resume los resultados de un estudio, enfocado en contribuir a resolver las necesidades de predicción de crecimiento de picadura en ductos que transportan hidrocarburos. Primero, se describe el desarrollo de un modelo determinista, considerando las propiedades físicas y químicas del suelo y el tipo de recubrimiento que protege la tubería. Este modelo fue utilizado para realizar simulaciones de Monte Carlo, logrando establecer las distribuciones de probabilidad de profundidad de picadura y su velocidad de crecimiento en suelos. Finalmente, se detalla el desarrollo de un modelo estocástico basado en cadenas de Markov. Este modelo tiene como objetivo, predecir la evolución en el tiempo de las distribuciones de profundidad de picadura.
Degradación de la tenacidad al impacto durante el envejecimiento acelerado de soldadura en acero microaleado
Vargas-Arista, B.,Hallen, J. M.,Albiter, A.,ángeles-Chávez, C.
Revista de Metalurgia , 2008,
Abstract: The effect of accelerated aging on the toughness and fracture of the longitudinal weld metal of an API5L-X52 linepipe steel was evaluated by Charpy V-notch impact test, fracture analysis and transmission electron microscopy. Aging was performed at 250 °C for 100 to 1000 h. The impact results indicated a significant reduction in the fracture energy and impact toughness as a function of aging time, which were achieved by the scanning electron microscope fractographs that showed a decrease in the vol fraction of microvoids by Charpy ductile failure with the aging time, which favored the brittle fracture by transgranular cleavage. The minimum vol fraction of microvoids was reached at 500 h due to the peak-aged. The microstructural analysis indicated the precipitation of transgranular iron nanocarbides in the aged specimens, which was related to the deterioration of toughness and change in the ductile to brittle behavior. Se evaluó, mediante el ensayo de impacto Charpy, fractografía y microscopia electrónica de transmisión el efecto del envejecimiento acelerado sobre la tenacidad y fractura de la soldadura en tubería de acero API5L-X52. El envejecimiento se realizó a 250 °C por 1.000 h, con control cada 100 h. Los resultados de impacto indicaron una disminución en la energía de fractura y tenacidad al impacto en función del tiempo del envejecimiento, los cuales se evidenciaron mediante fractografía, por la reducción en la fracción volumétrica de microhuecos por fractura dúctil con el tiempo, favoreciendo la fractura frágil por clivaje transgranular. Sin embargo, a 500 h, se observó la fracción volumétrica mínima debido al pico del envejecimiento. El análisis microestructural evidenció la precipitación de nanocarburos de hierro transgranulares en las muestras envejecidas, la cual se relaciono con la pérdida de tenacidad y cambio en el comportamiento dúctil a frágil, confirmado por fractografía.
Anastral spindle assembly and γ-tubulin in Drosophila oocytes
Sharyn A Endow, Mark A Hallen
BMC Cell Biology , 2011, DOI: 10.1186/1471-2121-12-1
Abstract: We show, for the first time, using a bright GFP fusion protein and live imaging, that the Drosophila maternally-expressed γTub37C is present at low levels in oocyte meiosis I spindles. Despite this, we find that formation of bipolar meiosis I spindles does not require functional γTub37C, extending previous findings by others. Fluorescence photobleaching assays show rapid recovery of γTub37C in the meiosis I spindle, similar to the cytoplasm, indicating weak binding by γTub37C to spindles, and fits of a new, potentially more accurate model for fluorescence recovery yield kinetic parameters consistent with transient, diffusional binding.The FRAP results, together with its mutant effects late in meiosis I, indicate that γTub37C may perform a role subsequent to metaphase I, rather than nucleating microtubules for meiosis I spindle formation. Weak binding to the meiosis I spindle could stabilize pre-existing microtubules or position γ-tubulin for function during meiosis II spindle assembly, which follows rapidly upon oocyte activation and completion of the meiosis I division.Anastral spindles assemble without centrosomes by a pathway that is still not fully understood. In particular, the mechanism by which microtubule nucleation occurs has not been well defined. Although chromatin has been shown to play an essential role [1], the involvement of the microtubule nucleator, γ-tubulin, is still an open question. γ-Tubulin localizes to centrosomes and other microtubule organizing centers in mitosis and is essential for nucleating microtubules in organisms as diverse as yeast, Drosophila, Xenopus, humans, and higher plants [2-5]. γ-Tubulin is also found on spindle microtubules, where it has been proposed to nucleate microtubules for spindle maintenance by functioning in a chromatin-mediated nucleation pathway that augments the dominant pathway of nucleation by centrosomes [6,7].γ-Tubulin is present in cells as a large ring complex, γTuRC, comprising 12-13 γ-tubulin molecules a
CSI-aided MAC with Multiuser Diversity for Cognitive Radio Networks
Yuan Lu,Alexandra Duel-Hallen
Computer Science , 2011,
Abstract: Cognitive Radio (CR) aims to increase the spectrum utilization by allowing secondary users (SU) to access unused licensed spectrum bands. To maximize the throughput given limited sensing capability, SUs need to strike a balance between sensing the channels that are not heavily used by primary users (PU) and avoiding collisions with other SUs. To randomize sensing decisions without resorting to multiuser sensing policies, it is proposed to exploit the spatially-variant fading channel conditions on different links by adapting the reward to the channel state information (CSI). Moreover, the proposed channel-adaptive policy favors links with high achievable transmission rate and thus further improves the network throughput.
Channel-Adaptive Sensing Strategy for Cognitive Radio Ad Hoc Networks
Yuan Lu,Alexandra Duel-Hallen
Computer Science , 2012,
Abstract: In Cognitive Radio (CR) ad hoc networks, secondary users (SU) attempt to utilize valuable spectral resources without causing significant interference to licensed primary users (PU). While there is a large body of research on spectrum opportunity detection, exploitation, and adaptive transmission in CR, most existing approaches focus only on avoiding PU activity when making sensing decisions. Since the myopic sensing strategy results in congestion and poor throughput, several collision-avoidance sensing approaches were investigated in the literature. However, they provide limited improvement. A channel-aware myopic sensing strategy that adapts the reward to the fading channel state information (CSI) of the SU link is proposed. This CSI varies over the CR spectrum and from one SU pair to another due to multipath and shadow fading, thus randomizing sensing decisions and increasing the network throughput. The proposed joint CSI adaptation at the medium access control (MAC) and physical layers provides large throughput gain over randomized sensing strategies and/or conventional adaptive transmission methods. The performance of the proposed CSI-aided sensing strategy is validated for practical network scenarios and demonstrated to be robust to CSI mismatch, sensing errors, and spatial channel correlation.
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