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Author: Brückner, Rolf × Start date: 2020 × End date: 1999 × Start date: 1990 × Type: article ×

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Now showing 1 - 10 of 11
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    SiC- and carbon-fibre-reinforced glass under alternating bending stress loadings
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Klug, Thomas; Brückner, Rolf
    Alternating bending stress experiments are described which were performed with various fibre/DURAN-glass composites reinforced by Nicalon NL 202 SiC fibres and by two different types of carbon fibres, a high-tensile strength (ht) and a high-modulus (hm) fibre. Also the influence of unidirectionally homogeneous and inhomogeneous fibre distribution as well as the bidirectional 0°/90° ply distribution are studied. In contrast to previously investigated strain-controlled experiments the present stress-controlled experiments show after the pretreatment of 1000 alternating tensile-compressive stress cycles fatigue (beginning damage) already at amplitudes below the limit of pure elasticity (below bendover stress) of a simple bending-load experiment. Despite of this difference the "training effect" of the previous investigation, the increase of the bendover stress with increasing stress amphtude after 1000 alternating-load cycles, is found to be similar and can be established also for the present experiments. The carbon-fibre-reinforced composites show a better tolerance of damage due to their smaller fibre diameters than the SiC-fibre-reinforced composites.
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    Influence of water content and basicity on redox ratio - Consequences on radiation heat absorption and emission of glass melts during fusion and processing
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Brückner, Rolf; Hessenkemper, Heiko
    It is shown that water content and basicity of glasses have an influence not only on the intensity of the absorption bands at and near the wavelengths of the characteristic vibration modes of water itself and on those of transition ions but also on other modes and even on remarkable parts of the NIR and the visible spectrum. Additionally it is shown that the water content influences the redox ratio of iron and prefers the ferrous state. An interpretation of this behaviour is given and the consequences for the industrial glass melting and forming process are discussed.
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    Elastic constants of glass melts above the glass transition temperature from ultrasonic and axial compression measurements
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1991) Hessenkemper, Heiko; Brückner, Rolf
    It will be shown that on the basis of ultrasonic measurements in connection to density, viscosity and axial compression experiments the temperature- and frequency-dependent elastic constants and the maximum of attenuation can be determined. The ultrasonic measurements are extended over the whole temperature range from room temperature up to 1300 °C. Glasses and their melts with different chemical compositions (silicates and phosphates) and properties are investigated. The results are compared and correlated with those mechanical properties of melts which are obtained from axial compression measurements of cylindrical glass specimens, such as relaxation behaviour, brittleness, influence of redox conditions, etc. It is found that the results of the two methods are compatible. The advantage of the ultrasonic method is that it covers a larger viscosity-temperature range than the cylinder-compression method. The disadvantage is that it is restricted to the linear viscoelastic region.
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    Thermal expansion behaviour of fibre-reinforced DURAN glass
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1993) Klug, Thomas; Fleischer, Veronika; Brückner, Rolf
    The thermal expansion coefficients of DURAN glass reinforced by SiC and various carbon fibres are determined in order to investigate the influence of the different types of fibres and the preparation parameters on the thermal expansion behaviour of the composites. The coefficient of thermal expansion decreases with increasing Young's modulus of the fibres and with increasing fibre volume content. Composites with high-modulus carbon fibres and/or with high fibre volume concentration show a transition from contraction at lower temperatures to expansion at higher temperatures. "Zero expansion composites" can be produced for certain temperature intervals by selection of the fibre types and the fibre volume concentration. The experimentally determined expansion coefficients are compared with the theoretically calculated ones by the mixing rule. The agreement is very good for the high-modulus fibres, less good for the high-strength fibres.
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    Nucleation and crystallization kinetics on float glass surfaces
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Deubener, Joachim; Brückner, Rolf; Hessenkemper, Heiko
    The superficial crystallization kinetics of a commercial float glass is investigated in the temperature range from 665 to 925 °C. The following phases were observed: cristobalite, devitrite, sporadically sodium metasilicate and additionally β-wollastonite if a reducing atmosphere is applied. The devitrification process is divided into a primary crystallization of cristobalite and a secondary appearance of devitrite after an induction time (t > 7 min). The growth rates of devitrite are up to 4 times larger than those of cristobalite, thus, a fractured surface perpendicular to the float glass plate shows a two-stage devitrification image after longer times of heat treatment. The growth rates depend on the furnace atmosphere as well as on the compositional variations of the two float glass surfaces. The activation enthalpies of crystal growth rates are (in kJ/mol): ≈ 110 for cristobalite, ≈ 150 for β-wollastonite and ≈ 220 for devitrite. Α superficial phase separation occurs at the tin bath side if oxidizing furnace atmosphere is applied. This can be related to a redox interaction of the tin ions and has consequences on nucleation, induction period, crystal growth mechanism and "bloom" effect. Heterogeneous surface nucleation arises from a certain number of sites which are specific for the various crystal phases. In the case of cristobalite the time and temperature dependence of the number of crystals are measured and nucleation rates were determined on the atmosphere side under oxidizing conditions.
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    Some aspects of the workability of glass melts
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Hessenkemper, Heiko; Brückner, Rolf
    Criteria are given for the isothermal or isochomal workability of glass melts and discussed by means of a schematic example which is based on numerous measurements. Α thesis for a simple connection of this kind of workability is presented and examined by means of so far existing measurements. The isothermal or isochomal workability, which becomes more important with increasing deformation rate, is in contrast to the non-isothermal or athermal workability. The differences are based on certain connections between the stiffness, the relaxation modulus of a glass mek, its high-temperature fracture strength and the value of the quantity Β in the Vogel-Fulcher-Tammann (VFT) equation if the constants are evaluated from a measurement of the whole viscosity-temperature range, from 10^1.5 to 10^12 Pa s.
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    Influence of microheterogeneities on the mechanical properties of high-viscous melts
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Habeck, Andreas; Hessenkemper, Heiko; Brückner, Rolf
    The influence of liquid-solid (Zerodur glass-ceramic melts) and liquid-liquid (PbO-B2O3 melts) two phase systems on the mechanical properties is investigated. The load-dependent mechanical properties which characterise the isothermal workability are: Newtonian and non-Newtonian flow behaviour, relaxation behaviour and therefore the stiffness as well as the high-temperature tensile strength of the melts. From the rheological point of view two systems (1 and 2) have to be regarded: 1) the liquid-solid and that part of the liquid-liquid system with the high-viscous inclusions within a low-viscous matrix on the one hand and 2) that part of the liquid-liquid system with the low-viscous inclusions within a high-viscous matrix on the other hand. These two systems show among various of the mentioned other special properties and relations a general decrease of high-temperature strength and isothermal workability with increasing portion of the distributed minor phase. System 1) shows with respect to constant equilibrium viscosities a constant non-Newtonian viscosity decrease, an increase of the relaxation modulus (stiffness) and a decrease of high-temperature tensile strength with increasing portion of solid and/or high-viscous inclusions. System 2) under the same aspect exhibits a strong increasing non-Newtonian viscosity decrease, a decrease of the relaxation modulus and a decrease of high-temperature tensile strength with increasing portion of the low-viscous inclusions.
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    Thermal shock behaviour of SiC-fibre-reinforced glasses
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Klug, Thomas; Reichert, Jutta; Brückner, Rolf
    The preparation of two SiC-fibre-reinforced glasses with very different thermal expansion coefficients and glass transition temperatures is described and the influence of long-time temperature and thermal shock behaviour of these composites on the mechanical properties is investigated by means of bending test experiments before and after thermal treatments. It will be shown from experiments and calculations on stresses due to thermal expansion mismatch between fibre and glass matrix that not only best mechanical properties but also best thermal shock behaviour are connected with low tensile intrinsic stresses produced by thermal expansion mismatch during preparation. The thermal shock resistance of the best composite (SiC fibre/DURAN glass) does not show a significant decrease of flexural strength even after 60 shocks from 550 to 25 °C in water, while the bulk glass sample of the same dimension was destroyed by one thermal shock from 350 °C.
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    Alternating bending load behaviour of unidirectionally fibre-reinforced glasses
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1992) Klug, Thomas; Bornhöft, Hansjörg; Brückner, Rolf
    Alternating bending load experiments are described and applied on SiC- and carbon-fibre-reinforced alkaline earth borosilicate glass samples. The mechanical properties (strength, bendover stress, Young's modulus) were determined by the three point bending method after 1000 strain-controlled alternations of load and compared with those of non-pretreated samples. It could be shown that all composites can be treated by an alternating bending load up to their elastic proportionahty limit (bendover stress) without failure or without any loss of their mechanical properties. Alternating bending loads above the bendover stress lead to a rapid fatigue of the SiC-fibre-reinforced glass composites while the carbon-fibre-reinforced composites show a great toleration of damage under the applied experimental conditions. In the case of the carbon-fibre-reinforced glass the homogeneity of the fibre distribution is of special importance for high bendover stresses and strengths during cyclic bending load experiments.
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    Load-dependent relaxation behaviour of various glass melts with different structural configurations
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1990) Hessenkemper, Heiko; Brückner, Rolf
    Glass melts are usually treated in literature as thermorheologically simple fluids. The temperature-time equivalence of the relaxation behaviour is described by the WLF equation. This also for silicate melts well-known behaviour will be verified in the present paper with the help of the cylinder-compression method up to 240 Κ above Tg for very different melts. In addition it is shown that the relaxation behaviour is load-dependent above a certain load limit indicating a deviation from the conception of the thermorheological simplicity. The load-dependent relaxation behaviour is investigated for glass melts with very different network structures (chains, cross-linked chains and three-dimensionally connected structures). Α clear correlation with structure is demonstrated in that way that the relaxation ability and its load dependence increase with decreasing degree of structural interconnection from a three-dimensionally to a one-dimensionally linked potential network structure.