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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 investigations in the decomposition ränge of the system Na₂O-B₂O₃-SiO₂(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Eberstein, Markus; Habeck, Andreas; Brückner, RolfThree 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.
- ItemProgress in the rheology of glass melts - Α survey(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1997) Brückner, Rolf; Yuanzheng, Yue; Deubener, JoachimThe rheology of glass melts is not restricted to Newtonian viscous flow, but it involves also non-Newtonian flow and viscoelasticity The former one predominates in the steady-state flow at low deformation rates, the latter ones prevail during the initial State of stress generation and during the final State of flow and fracture under extreme deformation conditions. The whole ränge of rheological phenomena is demonstrated in this review article. Typical available experimental results and mathematical formulations are briefly given for single-phase glass melts. In this context the development of new equations gives rise to the introduction of a series of new definitions and conceptions which physically and mathematically characterize the different rheological behaviour of glass melts under various stress-strain conditions. The connection between flow behaviour and structural aspects is pointed out with a short reference to flow birefringence. Finally, an extension from the behaviour of a single-phase to a double-phase system is developed for the case of suspended crystals with defined sizes, concentrations and shapes within an inert glass melt (model for glassceramic melts).
- ItemProperties of textile glass fibers based on alkali- and boron oxide-free aluminosilicate glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Kim, Young Keun; Brückner, Rolf; Murach, JürgenThe goal of this paper is to examine the possibihty of producing fibers from alkah- and boron oxide-free glasses with properties comparable to those of commercially available glass fibers. Various compositions from the Systems SiO₂ - Al₂O₃ - CaO - RO are investigated with RO = MgO, ZnO, MnO, BaO between 0 and 10 wt%. The fibers are suitable for application as textile fibers and as reinforcement component for composites. The mechanical properties (tensile strength, Young's and shear modulus and Poisson ratio) are comparable to and/or partly better than those of commercially produced glass fibers. The chemical resistivity in 2 mol/1 NaOH is better than that of E-glass fibers. Although the diameter of glass fibers is not the only parameter which determines fiber properties and structure, it is a very important one because it influences both the mechanical and the thermal history of the fibers. For this reason, and in order to approximate commercial fiber drawing conditions, fibers were prepared with roughly the same fiber diameters. Despite this restriction the results provide information for qualitative ratings and for relative comparison of properties and structural interpretations. Of special interest is the opposite dependence of strength and elastic moduli on the fiber diameter. This is explained by means of frozen-in orientation and deformation effects of microcracks and network anisometries due to the mechanical and thermal history of the fibers produced by the fiber drawing process.
- 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.
- ItemElastic and inelastic properties of soda lime Silicate glass melts(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1999) Börnhöft, Hansjörg; Brückner, RolfThe influence of CaO on the elastic and inelastic behaviour of soda lime Silicate glass melts of the series (26 - x) Na₂O · x CaO * 74 SiO (composition in mol% with x = 0 to 16) is investigated. Longitudinal and transversal ultrasonic waves in the MHz frequency ränge are transmitted through the glass melt. Their velocities and damping behaviour are measured as functions of temperature. The elastic moduli, internal friction and relaxation times are calculated and their dependence on temperature and viscosity is shown. These properties describe the transition from an elastic to a viscoelastic and fmally to a pure viscous behaviour of the invesdgated glass melts by a large dispersion ränge. Enhanced values of the moduli and activation energies are determined with an increase of CaO content. Thus, the structure of the melt becomes strenger and stiffer i.e. the mean bonding strength and the linkage of the dynamic network of the Silicate melt is improved. The relaxation behaviour can be described by a Maxwell based model with a distribution function of relaxation times.