Simulación del proceso de rotura por tracción de alambres de acero perlítico con un modelo de fisura cohesiva

La fractura de metales dúctiles como el acero suele explicarse a partir de la hipótesis de nucleación, crecimiento y coalescencia de microhuecos. A partir de esta teoría, se han desarrollado diversos modelos numéricos, entre los que el modelo de Gurson y sus variantes son los más extendidos. Dichos modelos reproducen matemáticamente el fenómeno físico de crecimiento de huecos resultando en un desarrollo progresivo del daño en el interior del material durante un ensayo de tracción. En estos modelos, el daño comienza a desarrollarse en fases muy tempranas del ensayo, incluso anteriores a la carga máxima. Ensayos realizados por los autores parecen indicar, sin embargo, que en el caso de barras de acero eutéctico empleado en la fabricación de alambres de pretensado, el daño originado en el interior del material como consecuencia del crecimiento de microhuecos sólo es apreciable en un estado muy avanzado del ensayo, momentos antes de producirse la rotura. Además, desde hace décadas se conoce que la triaxialidad de tensiones tiene una fuerte influencia sobre la rotura de los materiales. En este trabajo se presenta un modelo de rotura para elementos de acero sometidos a tracción, basado en un comportamiento cohesivo del material y que contempla el valor de la triaxialidad de tensiones, diferente en cada punto de la sección crítica de rotura. The fracture of ductile materials, such as steel, is usually explained with the theory of nucleation, growth and coalescence of microvoids. Based on this theory, many numerical models have been developed, with a special mention to Gurson-type models. These models simulate mathematically the physical growth of microvoids, leading to a progressive development of the internal damage that takes place during a tensile test. In these models, the damage starts to develop in very early stages of the test. Tests carried out by the authors seem to point out that, in the case of eutectoid steel bars used for manufacturing prestressing steel wires, the internal damage that takes place as a result of the growth of microvoids is only noticeable in a very advanced state of the test. In addition to this, it is known that the stress triaxiality has a strong influence over the fracture of ductile materials. This work presents a fracture model for steel specimens in a tensile test, based on a cohesive behaviour and taking into account the effect of stress triaxiality, which is different in every point of the fracture plane.

A Hierarchical Tracking Strategy for Vision-Based Applications On-Board UAVs

In this paper, we apply a hierarchical tracking strategy of planar objects (or that can be assumed to be planar) that is based on direct methods for vision-based applications on-board UAVs. The use of this tracking strategy allows to achieve the tasks at real-time frame rates and to overcome problems posed by the challenging conditions of the tasks: e.g. constant vibrations, fast 3D changes, or limited capacity on-board. The vast majority of approaches make use of feature-based methods to track objects. Nonetheless, in this paper we show that although some of these feature-based solutions are faster, direct methods can be more robust under fast 3D motions (fast changes in position), some changes in appearance, constant vibrations (without requiring any specific hardware or software for video stabilization), and situations in which part of the object to track is outside of the field of view of the camera. The performance of the proposed tracking strategy on-board UAVs is evaluated with images from realflight tests using manually-generated ground truth information, accurate position estimation using a Vicon system, and also with simulated data from a simulation environment. Results show that the hierarchical tracking strategy performs better than wellknown feature-based algorithms and well-known configurations of direct methods, and that its performance is robust enough for vision-in-the-loop tasks, e.g. for vision-based landing tasks.

Autonomous Landing of an Unmanned Aerial Vehicle using Image-Based Fuzzy Control

This paper presents a vision based autonomous landing control approach for unmanned aerial vehicles (UAV). The 3D position of an unmanned helicopter is estimated based on the homographies estimated of a known landmark. The translation and altitude estimation of the helicopter against the helipad position are the only information that is used to control the longitudinal, lateral and descend speeds of the vehicle. The control system approach consists in three Fuzzy controllers to manage the speeds of each 3D axis of the aircraft s coordinate system. The 3D position estimation was proven rst, comparing it with the GPS + IMU data with very good results. The robust of the vision algorithm against occlusions was also tested. The excellent behavior of the Fuzzy control approach using the 3D position estimation based in homographies was proved in an outdoors test using a real unmanned helicopter.

A General Purpose Configurable Controller for Indoors and Outdoors GPS-Denied Navigation for Multirotor Unmanned Aerial Vehicles

This research on odometry based GPS-denied navigation on multirotor Unmanned Aerial Vehicles is focused among the interactions between the odometry sensors and the navigation controller. More precisely, we present a controller architecture that allows to specify a speed specified flight envelope where the quality of the odometry measurements is guaranteed. The controller utilizes a simple point mass kinematic model, described by a set of configurable parameters, to generate a complying speed plan. For experimental testing, we have used down-facing camera optical-flow as odometry measurement. This work is a continuation of prior research to outdoors environments using an AR Drone 2.0 vehicle, as it provides reliable optical flow on a wide range of flying conditions and floor textures. Our experiments show that the architecture is realiable for outdoors flight on altitudes lower than 9 m. A prior version of our code was utilized to compete in the International Micro Air Vehicle Conference and Flight Competition IMAV 2012. The code will be released as an open-source ROS stack hosted on GitHub.

A two-subunit type I DNA topoisomerase (reverse gyrase) from an extreme hyperthermophile.

A recently described reverse gyrase from the hyperthermophilic methanogen Methanopyrus kandleri is the only known example of a heterodimeric type I topoisomerase. The enzyme is made up of a 42-kDa subunit which covalently interacts with DNA (RgyA) and a 138-kDa subunit which binds ATP (RgyB). We have now cloned and sequenced the genes for both subunits of this enzyme. Surprisingly, the universally conserved type I topoisomerase domain [Lima, C. D., Wang, J. C. & Mondragon, A. (1994) Nature (London) 367, 138-146] which has been found as a contiguous polypeptide in the prokaryotes and eukaryotes is shared between the protomers. The subdomain with the active-site tyrosine is entirely within RgyA, whereas the subdomain implicated in noncovalent binding of the cleaved DNA strand is contained entirely in RgyB. The appearance of this unique structure in a highly conserved enzyme family supports the hypothesis that the methanogens branched from other prokaryotes and eukaryotes very early in evolution.

Effects of Institutional Factors on the Retention of Graduate Students of Color in Schools of Library and Information Science

The intent of the study was to understand the changes that have occurred over the last 25 years in library programs as far as enrollment and diversity of students, number and ethnicity of the faculty, program income and expenses, cost of attendance, and scholarship and fellowship aid, in an effort to better understand library programs granting the MLIS degree. The study also endeavored to identify institutional factors associated with the retention and productivity rates of White students and students of color in schools of library and information science. During the period studied, the proportional representation of White students decreased. For students of color, proportional representation was stable during the same time period. Results revealed a medium effect size of time with productivity rates for both groups declining over time. Retention rate differed significantly by time, with a small effect size with retention rate that initially increased over time, but is now decreasing. The final analyses were meta-regressions to determine if retention and productivity rates can be predicted by cost of attendance, scholarship and fellow aid, and program size. Results indicated that for students of color, program size in 2000 was significantly predictive of retention, cost of attendance was predictive in 2002, and scholarship and fellowship aid was predictive of retention in 2004. No variables were significantly predictive for retention of White students. The last analysis was to determine if productivity rate can be predicted by cost of attendance, scholarship and fellow aid, and program size. Results indicate that for White students in 2002, the cost of attendance was predictive of productivity rating. In 2003, scholarship and fellowship aid was predictive of productivity rate and in 2004, scholarship and fellowship aid was predictive of productivity rating. For students of color, results indicate that only scholarship and fellowship aid in 2005 was predictive of productivity rate. No other variables in any of the years studied showed any significant prediction of productivity rating for students of color.

Logistics Innovation in Small 3PLs:what is the Role of ICT?

For companies competing in highly dynamic markets, the search for new sources of competitive advantages is mandatory. Innovation is considered a fundamental component of a successful business as it allows companies to sustain profit margins and sales growth and to reduce pressures coming from competitors. In the knowledge-driven economy, information and communication technology (ICT) plays an important role as it enables innovation in product/service, processes and organisations especially in service companies. This paper investigates the role of ICT in logistics innovation. The focus of the paper is on the effects that ICT usage is having on the management of small third-party logistics service providers (3PLs). On the basis of quantitative and qualitative evidence emerging from a recent survey carried out on the Italian logistic service market, the paper analyses how ICT is used to support logistics innovation and the factors inhibiting/facilitate the usage of ICT in such companies. Implications for logistics innovation management will be outlined from both research and managerial perspectives.

Les caisses populaires haïtiennes dans le cadre du projet "ACOOPECH" face au 5e principe de l'Alliance Coopérative Internationale: La formation, l'éducation et l'information coopératives

De tout temps, les coopératives ont joué un grand rôle social en s'attaquant à des problèmes pressants. Si les équitables pionniers de Rochdale ont su établir des règles permettant de concilier l'idéal coopératif et une saine gestion, les coopératives vont continuer à poursuivre tout à travers le monde les mêmes objectifs dans le but de satisfaire les besoins de leurs membres et dans la majeure partie des cas des moins nantis. Les coopératives se donnent toujours comme objectifs d'améliorer les conditions de vie et de travail des agriculteurs, des ouvriers, des commerçants et artisans, d'hommes et de femmes par les différentes méthodes de coopération qu'elles proposent. Le mouvement par contre a beaucoup évolué et a donné naissance à d'autres modes d'exploitation de la formule coopérative. La coopérative dans les pays où elle est très développée, ajoute à ses tâches traditionnelles toute une gamme de fonctions sociales supplémentaires: soins aux personnes âgées, aux enfants et aux personnes handicapées, promotion de nouvelles technologies, exploitation des énergies renouvelables, protection de l'environnement, services funéraires à moindres coûts et ajouter à tout cela la création d'emplois pour les catégories sociales défavorisées. Les changements politiques, économiques et sociaux intervenus au cours des dernières années ont influé sur la situation des coopératives dans le monde entier. Si dans les pays en développement et en transition, l'urgence se fait sentir dans la redéfinition du rôle de l'État envers les coopératives, dans les pays industrialisés c'est à cause de l'évolution de la structure des entreprises coopératives et de l'apparition de nouvelles formes de coopératives que la nécessité de nouvelles normes se fait sentir. Les coopératives actuellement et surtout dans les pays industrialisés se sont transformées en de véritables forces économiques et leur apport dans le développement de certaines régions du monde est loin d'être négligeable, c'est le cas de la "Mondragon" en Espagne, de la "Raiffeisen" en Allemagne, de "Desjardins" au Québec, pour ne citer que celles-là. Les coopératives dans le contexte du 21 è siècle se voient obliger de modifier leur structure traditionnelle afin d'être mieux armées pour affronter la concurrence que leur livrent d'autres types d'entreprises. Certainement les coopératives dans les pays en développement pour pouvoir survivre et s'adapter aux conditions du marché doivent obligatoirement mieux se structurer tant au point de vue associatif qu'organisationnel pour enfin parvenir à offrir à leur clientèle membre et non-membre un service de choix. Cette organisation de la coopérative aussi importante soit-elle ne peut se faire sans la planification de bons moyens pouvant permettre à la coopérative d'appliquer l'ensemble des principes qui la régissent dont l'un des plus importants de la coopération demeure: l'éducation, la formation et l'information.

Tensiones y oportunidades en las multinacionales coopitalistas de Mondragón: el caso de Fagor Electrodomésticos, sdad. coop.

Las cooperativas de Mondragon, de acuerdo con su misión, reflejan el esfuerzo de combinar los objetivos de las organizaciones empresariales que compiten a nivel internacional con la creación de empleo, la utilización de métodos democráticos de gestión y el compromiso con el entorno. La multinacionalización de las cooperativas trae consigo nuevas contradicciones, paradojas y dilemas al mundo de la democracia económica. En este artículo se analiza el caso de la cooperativa-multinacional “Fagor Electrodomésticos S. Coop.” de la Corporación Mondragon. Tras años de expansión internacional mediante la inversión directa extranjera, debido a la crisis ha tenido que acometer procesos radicales de restructuración del empleo, tanto en las plantas de la matriz de Mondragón, como en sus empresas filiales extranjeras: la empresa francesa Fagor-Brandt y la antigua empresa comunista polaca Fagor Mastercook. El análisis de las medidas adoptadas para reducir el empleo en Fagor nos servirá de base para profundizar en el conocimiento de las características organizativas de las multinacionales de Mondragon y para reflexionar sobre las posibilidades de extender el modelo cooperativo a las filiales.