Fatigue crack propagation in a quasi one-dimensional elasto-plastic model

Fatigue crack advance induced by the application of cyclic quasistatic loads is investigated both numerically and analytically using a lattice spring model. The system has a quasi-one-dimensional geometry, and consists in two symmetrical chains that are pulled apart, thus breaking springs which connect them, and producing the advance of a crack. Quasistatic crack advance occurs as a consequence of the plasticity included in the springs which form the chains, and that implies a history dependent stress-strain curve for each spring. The continuous limit of the model allows a detailed analytical treatment that gives physical insight of the propagation mechanism. This simple model captures key features that cause well known phenomenology in fatigue crack propagation, in particular a Paris-like law of crack advance under cyclic loading, and the overload retardation effect.Comment: To be published in the International Journal of Solids and Structure

Efecto del número de indicadores por factor sobre la identificación y estimación en modelos aditivos de análisis factorial confirmatorio

La matriz multirrasgo-multimétodo (MRMM) es un diseño de investigación de larga tradición en Psicología. Las técnicas de análisis de datos adecuadas para una correcta extracción de conclusiones han estado sujetas a controversia. Parece, no obstante, que diversos modelos de análisis factorial confirmatorio resultan muy adecuados. De entre los diversos modelos, dos de ellos han recibido gran atención, el modelo completo, que apareció primero en la literatura, y el de unicidades correlacionadas, que parece una alternativa razonable a los problemas que aparecen en el primero. Los resultados de ambos modelos en la literatura se refieren a situaciones con un solo indicador por combinación rasgo-método. La presente investigación simula datos de matrices MRMM para múltiples indicadores por combinación rasgo-método y somete a prueba la adecuación de las estimaciones de ambos modelos. Los resultados apuntan a un mejor comportamiento del modelo completo, si bien los sesgos, aunque triviales en cuantía, aumentan conforme aumenta la correlación entre los métodos

A generalised model of electrical energy demand from small household appliances

Accurate forecasting of residential energy loads is highly influenced by the use of electrical appliances, which not only affect electrical energy use but also internal heat gains, which in turn affects thermal energy use. It is therefore important to accurately understand the characteristics of appliance use and to embed this understanding into predictive models to support load forecast and building design decisions. Bottom-up techniques that account for the variability in socio-demographic characteristics of the occupants and their behaviour patterns constitute a powerful tool to this end, and are potentially able to inform the design of Demand Side Management strategies in homes. To this end, this paper presents a comparison of alternative strategies to stochastically model the temporal energy use of low-load appliances (meaning those whose annual energy share is individually small but significant when considered as a group). In particular, discrete-time Markov processes and survival analysis have been explored. Rigorous mathematical procedures, including cluster analysis, have been employed to identify a parsimonious strategy for the modelling of variations in energy demand over time of the four principle categories of small appliances: audio-visual, computing, kitchen and other small appliances. From this it is concluded that a model of the duration for which appliances survive in discrete states expressed as bins in fraction of maximum power demand performs best. This general solution may be integrated with relative ease with dynamic simulation programs, to complement existing models of relatively large load appliances for the comprehensive simulation of household appliance use

Probabilidade de ocupação e separação espacial entre onça parda (Puma concolor) e onça pintada (Phantera onca) na borda oeste do Pantanal.

Populações de grandes felinos são sensíveis à intervenções antrópicas nas paisagens, habitats e populações de espécies-presa. O Pantanal constitui uma das mais importantes regiões para a conservação onças exatamente pelo seu grau de conservação até os dias atuais. O objetivo desse estudo foi estimar a probabilidade de ocupação de onça parda (Puma concolor) e onça pintada (Panthera onca), além de analisar o grau de sobreposição espacial das áreas ocupadas pelas duas espécies. Large cat populations are sensible to human intervention in the landscape, habitats and prey-species population. The Pantanal is one of the most important regions for the conservation of jaguar and puma, given its conservation status to date. The objective of this study was to estimate the occupancy by jaguars (Panthera onca) and pumas (Puma concolor), as well as to analyze the degree of spatial overlap of the areas occupied by the two species

Messenger RNA targeting to endoplasmic reticulum stress signalling sites.

Deficiencies in the protein-folding capacity of the endoplasmic reticulum (ER) in all eukaryotic cells lead to ER stress and trigger the unfolded protein response (UPR). ER stress is sensed by Ire1, a transmembrane kinase/endoribonuclease, which initiates the non-conventional splicing of the messenger RNA encoding a key transcription activator, Hac1 in yeast or XBP1 in metazoans. In the absence of ER stress, ribosomes are stalled on unspliced HAC1 mRNA. The translational control is imposed by a base-pairing interaction between the HAC1 intron and the HAC1 5' untranslated region. After excision of the intron, transfer RNA ligase joins the severed exons, lifting the translational block and allowing synthesis of Hac1 from the spliced HAC1 mRNA to ensue. Hac1 in turn drives the UPR gene expression program comprising 7-8% of the yeast genome to counteract ER stress. Here we show that, on activation, Ire1 molecules cluster in the ER membrane into discrete foci of higher-order oligomers, to which unspliced HAC1 mRNA is recruited by means of a conserved bipartite targeting element contained in the 3' untranslated region. Disruption of either Ire1 clustering or HAC1 mRNA recruitment impairs UPR signalling. The HAC1 3' untranslated region element is sufficient to target other mRNAs to Ire1 foci, as long as their translation is repressed. Translational repression afforded by the intron fulfils this requirement for HAC1 mRNA. Recruitment of mRNA to signalling centres provides a new paradigm for the control of eukaryotic gene expression

Aves ameaçadas ocorrentes no Pantanal.

O Pantanal é um ecossistema extraordinário e importante para muitas espécies de aves, principalmente as aquáticas, que ocorrem em abundância, e as migrantes em larga escala que utilizam a planície de inundação como sítio de forrageio e descanso, durante seus deslocamentos entre o Sul do continente Americano e o Hemisfério Norte (Sick, 1997; Nunes e Tomas, 2004b; Nunes e Tomas, no prelo). Brown Jr. (1986) relaciona cerca de 657 espécies de aves para a planície e áreas limítrofes e Mittermeier et al. (2003) 463 espécies. No entanto Tubelis e Tomas (2003), listam 465 espécies de aves na planície, e citam outras 50 que podem ocorrer. Publicações mais recentes revelam a ocorrência de 17 novas espécies para a planície do Pantanal (Antas e Palo Jr., 2004; Cestari, 2006). Tubelis e Tomas (2003) ressaltam a necessidade de inventários mais consistentes, especialmente em áreas pouco ou nunca amostradas. Este trabalho objetivou apresentar uma listagem das espécies de aves ameaçadas ocorrentes no Pantanal.bitstream/CPAP/55964/1/DOC83.pdfFormato Eletrônic

A study of temperature-related non-linearity at the metal-silicon interface

In this paper, we investigate the temperature dependencies of metal-semiconductor interfaces in an effort to better reproduce the current-voltage-temperature (IVT) characteristics of any Schottky diode, regardless of homogeneity. Four silicon Schottky diodes were fabricated for this work, each displaying different degrees of inhomogeneity; a relatively homogeneous NiV/Si diode, a Ti/Si and Cr/Si diode with double bumps at only the lowest temperatures, and a Nb/Si diode displaying extensive non-linearity. The 77–300 K IVT responses are modelled using a semi-automated implementation of Tung's electron transport model, and each of the diodes are well reproduced. However, in achieving this, it is revealed that each of the three key fitting parameters within the model display a significant temperature dependency. In analysing these dependencies, we reveal how a rise in thermal energy “activates” exponentially more interfacial patches, the activation rate being dependent on the carrier concentration at the patch saddle point (the patch's maximum barrier height), which in turn is linked to the relative homogeneity of each diode. Finally, in a review of Tung's model, problems in the divergence of the current paths at low temperature are explained to be inherent due to the simplification of an interface that will contain competing defects and inhomogeneities

A fully ab initio approach to inelastic atom-surface scattering

We introduce a universal and fully ab initio theory for inelastic scattering of any atom from any surface, and apply the theory to helium scattering from Nb(100). The key aspect making our approach universal is a direct first-principles evaluation of the scattering atom-electron vertex. By correcting misleading results from current state-of-the-art theories, this fully ab initio approach will be critical in guiding and interpreting experiments that adopt next-generation, non-destructive atomic beam scattering.Comment: 6 pages, 5 figure