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Author: Brückner, Rolf × Author: Habeck, Andreas × End date: 2024 × Start date: 2020 × End date: 1999 × Start date: 1990 ×

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Now showing 1 - 4 of 4
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    Anisotropic optical properties and flow behaviour of mechanically deformed single-phase glass melts
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Brückner, Rolf; Habeck, Andreas
    Starting from the stress-optical coefficient around and above Tg, the following effects are regarded in order to give a comprehensive picture of the structural response of various glass melts under flow conditions at various temperatures: anisotropy o f glass fibres, the flow birefringence and Maxwell constant of the melt directly and/or indirectly via frozen-in anisotropics by cooling the melt thermally stress-free under mechanical load. Some of the applied various rheological methods allow to study the Newtonian and non-Newtonian viscosity additionally to the birefringence. Four very different glass melts are investigated under these aspects: a potassium-calciumsilicate glass, a float glass, an alkali metasilicate glass and an alkali metaphosphate glass. The results show that there are significant differences in the specific birefringence and in the deviation from Newtonian flow behaviour depending on the special flow units of the glass types. The structure of these anisotropic glasses and melts extends from frozen-in deformations of the network to orientations of chain-like flow units depending on chemical composition, temperature, mechanical stress and deformation rate. Particularly, a close connection between the onset of the non-Newtonian flow behaviour and the alteration o f the specific birefringence was found at certain critical stresses.
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    Determination of the rheological properties of high-viscous glass melts by the cylinder compression method
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Brückner, Rolf; Yue, Yuanzheng; Habeck, Andreas
    A detailed description of the cylinder compression method and its application on the viscoelastic behaviour of glass melts is given with the help of closed solutions and equations in contrast to algorithmic methods, like e.g. the finite element method. Special attention is paid to the system deformation and system deformation rate of the testing equipment and to the dissipation of mechanical energy within the specimen and the heat flow from the specimen. The limits of this treatment are discussed with respect to the applicability of the theoretical basis (Gent and Nadai equation) and with respect to the experimental determination of the rheological properties of glass melts. On this basis the pure non-Newtonian viscosity, ηnN, can be determined. If very small differences have to be ascertained in the rheological behaviour of glass melts due to small differences in composition or in melting history (e.g. redox condition or bad/good workability), the same conditions concerning the mechanical and thermal equipment and even the same geometry of the glass specimens have to be strictly maintained. Comparison between different glass melts should be made rather on the basis of equal (Newtonian) equilibrium viscosity, η₀, (isochomal conditions) and not on equal temperature. Owing to large thermal effects, the slope of the viscosity-temperature curve at η₀ plays a certain role, particularly for the forming process.
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    stress generation modulus as a counterpart of the stress relaxation modulus
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Brückner, Rolf; Hessenkemper, Heiko; Habeck, Andreas; Yue, Yuanzheng
    In order to measure the time dependence of the stress relaxation modulus, E, a stress-strain deformation has to precede which induces a stress within the viscoelastic sample from which the sample relaxes. The generation of stress is characterized by a strain rate-dependent and relaxation rate-dependent portion which exhibits a maximum value, E_max, which is called "stress generation modulus". E_max was called the "maximum stress relaxation modulus" in earlier papers. Meanwhile, however, it turned out that a better verbal distinction should be made in future by the new term "stress generation modulus" because E_max is about one order of magnitude larger than E.
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    Rheological investigations in the decomposition ränge of the system Na₂O-B₂O₃-SiO₂
    (Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Eberstein, Markus; Habeck, Andreas; Brückner, Rolf
    Three glass melts from the Vycor-type widely distributed along the mixing gap of the pseudobinary line SiO₂-(B₂O₃:Na₂O = 84:16) were invesdgated each one with three different thermal histories. The following rheological properties were determined by the cylinder compression method: the Newtonian and non-Newtonian flow behaviour, the stress generadon modulus as a measure of the stiffness, the high-temperature tensile strength and the critical deformation rate at which the first crack appears. The influences of the fundamental glass compositions (SiO₂ content) as well as the thermal pretreatments on these properties are demonstrated and discussed on the basis of isochomal conditions (equal Newtonian viscosities). The results can be interpreted by means of the knowledge about the demixing rules and processes which lead to various rheological two-phase Systems. Most drastical changes of the rheological properties are observed in the middle of the mixing gap where spinodal decomposidon has its optimum and where he differentation between the matrix phase and decomposed phase is lost.