Einheit von Wissenschaft und Kunst im Brückenbau: Hellmut Homberg (1909–1990) – Leben und Wirken (Teil I)

Am 5. September 2009 wäre Hellmut Homberg einhundert Jahre alt geworden. Dies ist Anlass, an sein vor allem den Brückenbau bereicherndes Wirken zu erinnern und ein, wenn auch nicht vollständiges, Werkverzeichnis zu erstellen. Das gilt sowohl für die unter seiner maßgebenden Mitwirkung entworfenen Brücken als auch für seine völlig neuartigen, die statischen Berechnungen in der Praxis erleichternden, streng theoretisch hergeleiteten und eine zutreffende Bemessung der Kreuzwerke und orthotropen Fahrbahnplatten ermöglichenden Tafel‐ und Tabellenwerke. Teil I schildert Homberg s beruflichen Weg und versucht, ein Porträt zu zeichnen; Teil II ist seinen theoretischen Untersuchungen gewidmet, und Teil III geht auf besondere Brücken ein, und. Harmony between science and art in bridge‐building: Hellmut Homberg (1909–90) – life and work (part I). Hellmut Homberg would have been 100 years old on 5 September 2009. This is an opportunity to look back on his work which so enriched the world of bridge‐building in particular, and also a chance to compile a catalogue of his work, albeit incomplete. This applies to the bridges in which he played an influential role in their design and also to his books of mathematical and design tables that enabled the accurate design of beam grids and orthotropic bridge decks. The tables with their rigorous theoretical background were at the time quite new and eased structural calculations in everyday practice. Part I describes Homberg's professional career and attempts to draw a portrait of the man; part II is devoted to his theoretical studies, and part III deals with particular bridges.     

Characterization of the ozone zero phenomenon by infrared spectroscopy

The presence of a small amount of nitrogen in the feed gas is necessary to generate ozone efficiently out of oxygen. Operating an ozone generator with ultra-pure oxygen for extended periods results in highly deteriorated ozone generation efficiency. In extreme cases, when the nitrogen levels in the feed gas are in the lower ppm range, the efficiency of the ozone generation process even drops to zero. In this article, we present our results concerning the correlation between the N₂O₅ concentration in the off-gas and the ozone generation efficiency. After the ozone generator is run for a well-defined amount of time with an oxygen–nitrogen mixture, the N₂ supply is shut off, and the behavior of the system is monitored by near-infrared spectroscopy. Different surface materials lead to different temporal behavior of both the nitrogen oxide levels and the ozone concentration after shutting down the nitrogen supply. The measurements show a good correlation between the evolution of ozone generation efficiency and the changes of the N₂O₅ concentration in the off-gas.     

Synthesis, structural and magnetic characterization of iron-zinc-borate glass ceramics containing nanocrystalline zinc ferrite

Two different glass ceramics with the composition of the (Fe₂O₃)_x·(B₂O₃)_(60−x)·(ZnO)₄₀, where x = 12.5 and 15 mol%, have been synthesized using the melt-quench method. The X-ray diffraction (XRD) patterns show the presence of nanometric zinc ferrite (ZnFe₂O₄) crystals, with spinel structure, in a glassy matrix after cooling from melting temperature. The estimated amount of crystallized zinc ferrite varies between 16 and 35%, as a function of the chemical composition. Glass transition (T_g), crystallization (T_p) and melting (T_m) temperatures were determined by differential thermal analysis (DTA) investigations. Fourier transform infrared (FTIR) data revealed that the BO₃ and BO₄ are the main structural units of these glass ceramics network. FTIR spectra of these samples show features at characteristic vibration frequencies of ZnFe₂O₄. Electron paramagnetic resonance (EPR) measurements show the presence of isolated Fe³⁺ ions predominantly situated in rhombic vicinities and as well as the Fe³⁺ species interacting by dipole–dipole interaction or to their superexchange coupled pairs in clustered formations. The magnetic properties of the studied glass ceramics were investigated by vibrating sample magnetometer (VSM). From the magnetization curves for glass ceramic containing 15 mol% Fe₂O₃ it was found that the nanoparticles exhibit ferromagnetic interactions combined with superparamagnetism with a blocking temperature, T_B. For studied samples the hysteresis is present. The coercive field is dependent on composition and magnetic field being around 0.05 μ_B/f.u for measurements performed in maximum 0.4 T.     

Untersuchungen zur Warmversprödung in der wärmebeeinflußten Zone von Schweißverbindungen austenitischer Stähle

Untersuchung von 12 stabilisierten und nichtstabilisierten austenitischen Stählen im Ausgangszustand und schweißsimulierend geglühten Zustand. Warmzugversuche und Zeitstandversuche. Licht- und elektronenmikroskopische Gefügeuntersuchungen.     

Physical and chemical effects on the dynamics of the β-relaxation of PMMA in Rhodamine 6G+PMMA+SiO<Subscript>2</Subscript> matrices

The Thermally Stimulated Depolarization (TSD) Current technique has been used to reveal confinement effects on the β-relaxation process of PMMA polymerized in-situ SiO₂ in the presence of the laser dye Rhodamine 6G/Cl^– (Rh6G). The TSD current band of the β process, associated with the rotational relaxation of the mobile carboxymethyl side groups of PMMA, presents a drastic low temperature shift with the introduction of Rh6G or the confinement of PMMA in the nanometric silica substrate. By using the fractional polarization technique we have decomposed this band into single current signals. Irrespective of the initiator type used in the polymerization process, the broad activation energy distribution associated with the β process is nearly identical in the Rh6G+PMMA+SiO₂ materials and bulk PMMA. The increase of the dye concentration enlarges the band shift, without any significant modification in the TSD current spectrum (i.e. distinct bands from the polar chromophore). The changes in the molecular dynamics of the polar carboxymethyl side groups of PMMA are discussed in terms of the hydrogen-bond interactions between the silicic acid pore surface and the basic side-groups of PMMA and the decrease of the polymer chain entanglements due to pore directed propagation of polymerization. The relative importance of the various types of interactions between the two chemically identifiable phases in the presence of the dye is discussed. Electronic Publication     

EMHD free-convection boundary-layer flow from a Riga-plate

The electro-magnetohydrodynamic (EMHD) free-convection flow of a weakly conducting fluid (e.g. seawater) from an electromagnetic actuator is considered. The actuator is a so called Riga-plate which consists of a spanwise aligned array of alternating electrodes and permanent magnets mounted on a plane surface. This array generates a surface-parallel Lorentz force which decreases exponentially in the direction normal to the (horizontal) plate. The free-convection boundary-layer flow induced by this body force is investigated numerically and analytically. It is shown that a certain length and velocity scale exists on which the flow characteristics are independent of the material properties of the fluid, as well as of the structural and functional parameters of the actuator. These universal velocity profiles are calculated numerically at different distances x from the leading edge and are discussed in some detail, both for the impermeable and the permeable Riga-plate when; in the latter case, a uniform lateral suction or injection of the fluid is applied. For the flow characteristics analytical approximations are reported. The asymptotic suction profiles approached for large values of x are given in exact analytical form. From a mathematical point of view the basic equations of the present boundary-value problem resemble those of the classical Blasius problem with an inhomogeneous forcing instead of an external flow and, accordingly, a homogeneous asymptotic condition.     

Neural networks for modelling and fault detection of the inter-stand strip tension of a cold tandem mill

This paper deals with the multilayered approach of the high-order neural network applied in a robust fault detection scheme. To introduce dynamic properties in these networks, a dynamic high-order neural unit is presented. It is shown that these networks can approximate any function with less parameters than in the case of multi-layer perceptron neural network. Such networks have good modelling properties, which make them useful for designing residuals in fault detection of dynamic processes. A method of computing a variable threshold derived from the confidence interval prediction is applied in order to obtain robustness in the fault detection process. Application of these networks for system identification and robust fault detection of the inter-stand strip tension of a continuous five stands cold mill is presented in the final part.