Finite element analysis of residual stresses in large cold-rolled thread

As a subproject of the DFG Research Training Group Graduiertenkolleg 2075 Modelling the constitutive evolution of building materials and structures with respect to aging (GRK 2075), the fatigue behaviour of rolled-after heat-treatment and hot-dipped galvanized HT-bolts is examined. Bolted joints are one of the most frequently used joining connections in mechanical and civil engineering. HT-bolts with large diameters (M 30 up to M 72) are widely used in wind power plants both on-shore and off-shore. During the cold-rolling process, compressive residual stresses are generated at the root of the thread in combination with strain hardening. Therefore, bolts rolled-after heat-treatment under tensile loading have a much longer fatigue life than rolled-before heat-treatment ones. In order to calculate the fatigue life of rolled-after heat-treatment bolts with a notch-strain concept, it is necessary to calculate the residual stresses and the material state from the forming process. A simulation of the forming process of the M 48 thread will be presented. It is investigated if the residual stress and material state from the forming process can be captured with the transient cyclic material model of Chaboche et al.[1] implemented in the commercial FE program Abaqus. The forming process is simulated with remeshing and Mesh-to-Meshsolution-Mapping [3] on an axisymmetric model. The result will be evaluated in terms of geometry, stress and strain state. The determined stress state is compared to measured residual stresses in Unglaub [2] and Fares [4] The simulation of the forming process gives a good coincidence with the thread geometry in practice. The residual stress path corresponds qualitatively to the measured residual stresses. The plastic equivalent strain in the thread root is overestimated because of the chosen material model and the axisymmetric modeling

Modeling He-rich subdwarfs through the hot-flasher Scenario

We present 1D numerical simulations aimed at studying the hot-flasher scenario for the formation of He-rich subdwarf stars. Sequences were calculated for a wide range of metallicities and physical assumptions, such as the stellar mass at the moment of the helium core flash. This allows us to study the two previously proposed flavors of the hot-flasher scenario ("deep" and "shallow" mixing cases) and to identify a third transition type. Our sequences are calculated by solving simultaneously the mixing and burning equations within a diffusive convection picture, and in the context of standard mixing length theory. We are able to follow chemical evolution during deep-mixing events in which hydrogen is burned violently, and therefore able to present a homogeneous set of abundances for different metallicities and varieties of hot-flashers. We extend the scope of our work by analyzing the effects of non-standard assumptions, such as the effect of chemical gradients, extra-mixing at convective boundaries, possible reduction in convective velocities, or the interplay between difussion and mass loss. Particular emphasis is placed on the predicted surface properties of the models. We find that the hot-flasher scenario is a viable explanation for the formation and surface properties of He-sdO stars. Our results also show that, during the early He-core burning stage, element diffusion may produce the transformation of (post hot-flasher) He-rich atmospheres into He-deficient ones. If this is so, then we find that He-sdO stars should be the progenitors of some of the hottest sdB stars.Comment: 13 pages, including 8 figures and 6 tables. Accepted for publication in A&A. Replaced to match the final version, including a note added in proof regarding PG 1544+48

Results of using the global positioning system to maintain the time and frequency synchronization in the Deep Space Network

There are two hydrogen maser clocks located at each signal processing center (SPC) in the DSN. Close coordination of the time and frequency of the SPC clocks is needed to navigate spacecraft to the outer planets. A recent example was the Voyager spacecraft's encounter with Uranus in January 1986. The clocks were adjusted with the goal of minimizing time and frequency offsets between the SPCs at encounter. How time and frequency at each SPC is estimated using data acquired from the Global Positioning System Timing Receivers operating on the NBS-BIH (National Bureau of Standards-Bureau International de l'Heure) tracking schedule is described. These data are combined with other available timing receiver data to calculate the time offset estimates. The adjustment of the clocks is described. It was determined that long range hydrogen maser drift is quite predictable and adjustable within limits. This enables one to minimize time and frequency differences between the three SPCs for many months by matching the drift rates of the three standards. Data acquisition and processing techniques using a Kalman filter to make estimates of time and frequency offsets between the clocks at the SPCs and UTC(NBS) (Coordinated Universal Time realized at NBS) are described

On the DB gap of white dwarf evolution: effects of hydrogen mass fraction and convective overshooting

We investigate the spectral evolution of white dwarfs by considering the effects of hydrogen mass in the atmosphere and convective overshooting above the convection zone. We notice the importance of the convective overshooting and suggest that the overshooting length should be proportional to the thickness of the convection zone to better fit the observations.Comment: 13 figure

Beyond the iron group: heavy metals in hot subdwarfs

We report the discovery of strong photospheric resonance lines of Ga III, Ge IV, Sn IV and Pb IV in the UV spectra of more than two dozen sdB and sdOB stars at temperatures ranging from 22000 K to 40000 K. Lines of other heavy elements are also detected, however in these cases more atomic data are needed. Based on these discoveries, we present a hypothesis to explain the apparent lack of silicon in sdB stars hotter than ~32000 K. The existence of triply ionised Ge, Sn, and Pb suggests that rather than silicon sinking deep into the photosphere, it is removed from the star in a fractionated stellar wind. This hypothesis provides a challenge to diffusion models of sdB stars.Comment: 5 pages, 3 figures, to appear in A&A Letter

FORMAÇÃO DE PROFESSORES EM SERVIÇO NA MODALIDADE DE EDUCAÇÃO À DISTÂNCIA: UMA EXPERIÊNCIA SIGNIFICATIVA NO SUL DO BRASIL.

Este artigo busca apresentar e analisar experiências de formação de professores em serviço graduados pelo Curso de Pedagogia a Distância ofertada pela Universidade Federal do Paraná. O presente estudo centrou-se especificamente no ponto de vista dos egressos dos Centros Associados de Maringá e de Taquara. Todos os estudantes do referido curso já atuavam na área educacional da educação básica, e por vários motivos não cursaram o ensino superior. Com o objetivo de aproximar-se das concepções e experiências construídas por este grupo de alunos a respeito desta modalidade de ensino para sua prática pedagógica, foi necessário coletar dados por meio de pareceres emitidos por estes protagonistas. Seus depoimentos foram analisados na perspectiva metodológica da pesquisa qualitativa do tipo etnográfica. O curso, na concepção dos alunos, teve um papel social decisivo no atendimento aos “excluídos” da educação regular presencial, como se sentiam, a qual não conseguiria atendê-los por não poderem prescindir da atividade laboral para sua sobrevivência. Foi notório na pesquisa que os alunos do Curso de Pedagogia além de mudarem as suas concepções sobre a modalidade de EaD, evidenciaram mudanças nas práticas pedagógicas

Structure and evolution of pulsating hot subdwarfs

Hot subdwarfs are evolved low--mass stars that have survived core helium ignition and are now in (or recently finished with) the core helium burning stage. At the hot end of the Horizontal Branch (HB), many of these stars are multiperiodic pulsators. These pulsations have revealed details of their global and internal structure, and provide important constraints on the origin of hot HB stars. While many features of their structure deduced from seismic fits have confirmed what we expected from evolutionary considerations, there have been some surprises as well.Comment: 6 pages, invited review from Fourth HELAS International Conference: Seismological Challenges for Stellar Structure (February 2010