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- Itemstress generation modulus as a counterpart of the stress relaxation modulus(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Brückner, Rolf; Hessenkemper, Heiko; Habeck, Andreas; Yue, YuanzhengIn 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.
- ItemInfluence of non-homologous substitutions of chemical components on rheological properties and on isochomal workability of Silicate glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Yue, Yuanzheng; Brückner, RolfThe glass melts of a non-homologous series were investigated with respect to flow behaviour and stress generadon behaviour as well as to workability by means of the cylinder compression method. The results show that the competition between the gross viscosity effect and the bonding strength of the dynamic glass melt structure extremely deterrnines the high-temperature tensile strength, σ_ts, the critical deformation rate hc, and by that the isochomal workability. The larger hc values are not always connected to a larger non-Newtonian flow effect or to a larger gross viscosity effect as it is usually the case for homologous series. However, the rule for homologous series that smaller Emax moduli, i.e. smaller stiffness and brittleness of a glass melt, are coupled to a better isochomal workability is also vahd for non-homologous series. The special connections between σ_ts, hc and deformation, Δh/h0, are analyzed.
- ItemIsothermal and isochomal workability of glass melts - Influence of melting history: Minor composition changes of redox State and hydroxyl content - Αstudy of the sensitivity of the cylinder compression method(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Brückner, Rolf; Yue, YuanzhengThe influence of redox State and hydroxyl concentration on the pure non-Newtonian flow behaviour, on the gross non-Newtonian viscosity, on the stress generation modulus, brittleness, high-temperature tensile strength and on the critical deformation rate of an iron oxide-containing laboratory glass melt with two different melting histories, but with the same main chemical composition, is investigated by means of the cylinder compression method. The comparison of these properties between the oxidized and the reduced glass melt is made not only at equal temperatures but also at equal viscosities in order to distinguish between isothermal and isochomal workability and to examine how sensitive the applied method is. The result is that the reduced melt with a somewhat larger Fe²⁺/F_total ratio and with a larger hydroxyl content has a smaller stiffness and brittleness and therefore, a somewhat better isothermal workability than the oxidized melt. This is valid only for the comparison at equal temperatures. When the comparison is made at equal viscosities, however, the workability - or better the isochomal workability - of the two melts is the same. The reason is that a shift of the Newtonian viscosity is produced by the redox shift and hydroxyl content of the reduced melt to lower values which diminishes with increasing temperature. It is possible to construct master curves with respect to the measured properdes at four temperatures, by which a transfer to the low viscosity range is possible, i.e., one can get the isothermal workability of the glass melts over the whole working range. The measured values of the above-mentioned properties are represented by fitting with recently developed relations which give rise to interesting practical and theoretical conclusions about industrial production optimization and about some structural aspects of stressed melts.
- ItemAnisotropie glasses and glass melts - a survey(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Brückner, RolfΑ general introduction describes intentional and unintentional anisotropic phenomena resulting from chemical and thermal inhomogeneities (internal stresses) as well as anisotropies induced by external elastic stresses and strains in homogeneous glasses for special opdcal purposes. Further the paper will point out the possibilities and various phenomena of producing defmed anisotropies by means of flow stresses and strain rates in glass mehs which may be frozen-in by cooling under load. It will be distinguished between Single - and double-phase glass melts and glasses which show very different kinds and effects of anisotropies. In particular, optical anisotropies (birefringence, specific birefringence) and structural-mechanical anisotropies (densities, shrinkages and expansions during annealing) of single-phase glasses and glass melts will be investigated under defined flow condidons (in-situ measurements directly in glass melts by the classical flow birefringence) and geometries (e.g. cylindric bulk glass samples, glass fibres, glass rods). In double-phase glass Systems the following anisotropic properties will be treated: the form birefringence and the distribution birefringence of decomposed glasses, the orientation of melts with suspended anisometric crystals and birefringence effects of chemically leached microporous glasses. Finally, a new phenomenon will be reported where the frozen-in birefringence of a glass shows an other sign after deformation above Tg than the stress optical constant of the same glass at room temperature before this treatment above Tg.
- ItemRheological properties and isochomal workability of lead Silicate glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Yue, Yuanzheng; Brückner, RolfFour different industrial lead Silicate glass melts were investigated by means of the cylinder compression method with respect to the flow and stress generation behaviour, the critical deformation rate hc, and the high-temperature tensile strength, σ_ts. Increasing PbO concentration has usually a large influence on t he Newtonian viscosity, it makes the melt "shorter" (more modernly speaking: more "fragile"). On the other hand, the influence on the non-Newtonian flow and stress generation behaviour and therefore, on the brittleness is very small, however, the high-temperature tensile strength is markedly negatively influenced. At low viscosities the melts with large P bO content show larger critical deformation rates and therefore, a better isochomal workability than those with smaller P bO contents. As compared with soda lime Silicate glass melts the lead Silicate melts reach clearly smaller hc and σ_ts values at comparable degrees of deformation and therefore, they have a worse isochomal workability This unexpected behaviour is discussed under structural aspects.
- ItemInfluence of homologous substitutions of chemical components on the rheological properties and on isochomal workability of Silicate glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Yue, Yuanzheng; Brückner, RolfGlass melts of various homologous Silicate glass series were investigated by means of the cylinder compression method with respect to their flow behaviour (Newtonian and non-Newtonian), stress generadon modulus (stiffness, brittleness) and critical deformadon rate (high-temperature tensile strength, isochomal workability). The experimental results show that decreasing CaO content and increasing basicity lead to the decrease of the brittleness of Silicate glass melts and to the increase of the critical deformadon rate, i.e. to the improvement of the isochomal workability of these melts. The mixed alkali effect of sodium potassium calcium Silicate glass melts at molar fracdon [K₂O]/[Na₂O+K₂O] = 0.5 has a favourable influence on the isochomal workability The correladon between flow behaviour and workability of glass melts is also discussed in the present paper. It could be shown that the ratio β = E2.8/E3.6 (E is the extinction of the hydroxyl band at 2.8 and 3.6 ηm, respectively) indicates the structural origin of the dependence of the mentioned rheological properties on the chemical compositions, at least within the homologous series.
- ItemDetermination 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, AndreasA 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.
- ItemWhich rheological parameters control the isochomal workability of glass melts from the Container glass industry?(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Yue, Yuanzheng; Brückner, RolfTwo species of brown glass which were characterized by industrial workability and quality criteria as "good" and " bad ", respectively, were investigated by the cylinder compression method and by the Christiansen method. Significant differences between the two species were found by means of the two methods in such a manner that the "good" brown glass samples indicate a better homogeneity and the corresponding melts a larger critical deformation rate, hc and therefore, a better isochomal workability than the " bad " brown glasses and melts. Additionally, the former ones show a larger ratio Fe²⁺/F_total with the consequence that they have a lower Newtonian viscosity at the same temperature. In contrast to the brown glass melts no clear differences in the rheological properties and isochomal workability could be detected between white Container glass melts which have also been characterized as "good" or as "bad". The white Container glass melts are classified to be less workable at equal viscosities than the brown glass melts. The stress generation modulus, E_max, and the normalized non-Newtonian viscosity, η/η0T, are less sensitive properties with respect to the redox State of the melts and to the homogeneity than the hc values. With the results of this study the limits of the sensitivity of the various properties resulting from the cylinder compression method were reached.
- ItemThermal expansion behaviour of unidirectionally SiC fibre-reinforced MAS and BMAS glass-ceramics(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1995) Reich, Christian; Brückner, RolfThe thermal expansion behaviour of SiC fibre-reinforced Magnesium-Aluminium-Silicate (MAS) and Barium-Magnesium- Aluminium-Silicate (BMAS) glass-ceramics was studied in air as well as in argon furnace atmosphere. The results show that the coefficients of thermal expansion up to a temperature of 800 C are influenced to a minor degree by the furnace atmosphere and to a major degree by the phase composition of the matrices and the fibre concentration. The largest values in air and argon atmosphere are obtained from BMAS(14)/SiC fibre composites with α₁₅₀/₈₀₀ = 3.92 * 10⁻⁶ K⁻¹ and 3.87 * 10⁻⁶ K⁻¹ respectively, at a fibre content of 52 vol.%, while the lowest values are found for BMAS(9)/SiC fibre composites in air with α₁₅₀/₈₀₀ = 3.24 * 10⁻⁶ K⁻¹ and in argon with α₁₅₀/₈₀₀ = 3.11 * 10⁻⁶ K⁻¹ at a fibre content of about 25 vol.%. Increasing fibre concentration leads to increasing thermal expansion. The experimentally determined values were compared with calculated ones by the mixing rule and a good agreement was found, particularly for the BMAS(14) composites.
- ItemAnisotropic optical properties and flow behaviour of mechanically deformed single-phase glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Brückner, Rolf; Habeck, AndreasStarting 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.