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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.
- 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.
- Itemstructure-sensitive investigations on glass fibers from the system SiO₂-Al₂O₃-GaO(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1998) Murach, Jürgen; Makat, Andreas; Brückner, RolfGlass fibers of the system SiO₂-Al₂O₃-CaO were prepared with respect to defined drawing conditions and investigated with structure-sensitive methods (birefringence, radial and axial alterations by thermal treatment at Tg). These glass fibers, which lie in a compositional sense roughly between the disilicate and silica glasses concerning the ratio network modifiers to network formers, exhibit similar optical anisotropies. On the other hand, they show remarkably lower axial and radial alterations which are exclusively contracdons on annealing for all applied drawing conditions in contrast to the silica or the alkali disilicate glass fibers. Obviously, the filling of free volume hollows with and the reinforcement of them by the Ca²⁺ ions and the smaller polarizability of the latter as compared to alkali ions are responsible for the relatively low structural anisotropy. An increase of the optical anisotropy with increasing CaO concentration is observed at comparable viscosities. This is a consequence of the increasing incorporation of the network modifier oxide which produces a weaker network strength and a larger polarizability.
- ItemIon refraction and stress optical coefficients for di- and metasilicate glasses(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 2002) Noll, Claudius; Habeck, Andreas; Brückner, RolfGlasses of the di- and metasilicate stoichiometric composition were melted, with lithium, sodium and potassium being replaced for each other. Additionally, SiO2 and B2O3 glass samples, the DGG standard glass, a float glass and a metaphosphate glass were examined. Molar mass, molar volume and oxygen partial volume were measured and found to increase in the sequence Li < Na < K. While potassium expands the glass network, the lithium ions show a counteracting behavior. The metaphosphate and the pure B2O3 glass are those with the most compact structure (related to one structure unit) and therefore with the lowest values of the studied properties. It was shown quantitatively that the polarizabilities of the nonbridging oxygen atoms are much higher than those of the bridging oxygen atoms. The influence of the cations was usually small. The origin of the small polarizability values of the B2O3 glass may be interpreted in terms of the lack of the nonbridging oxygen atoms and in terms of the denser structure of the oxygen polyhedra as compared with those in the SiO2 glass. The values of the stress-optical coefficients increase from lithium- to sodium- to potassium-dominated glasses, presumably resulting from increasing ion refraction of the nonbridging oxygen atoms. All preloaded samples of the di- and possibly also of the metasilicate composition showed an increase of the stress-optical coefficients with increasing preload at temperatures higher than the glass transformation temperature Tg. The reason is that a certain flow condition of the glass melts is frozen-in.
- 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.
- 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.
- 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.
- 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.
- ItemOn the different descriptions of the non-Newtonian viscosity (shear-thinning effect) of glass melts with respect to heat dissipation(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1996) Yue, Yuanzheng; Brückner, RolfThe range of non-Newtonian viscosity is usually connected with high strain rates or/and stresses. Therefore, the effect of heat dissipation (or viscous heating) cannot be neglected. In order to regard this effect various correlations were given in former papers, which will be analyzed and discussed in the present paper. It turns out that through the exact thermal elimination of viscous headng one can easily obtain the quantity of the pure, realistic and exclusively structurally determined non-Newtonian viscosity, ηnN, which may be normalized by the Newtonian viscosity, η0, to yield the form, ηnΝ/η0- At small amounts of strain and stress other expressions, such as η/η0,Tmax and η/η0,Tnet, may describe the non-Newtonian flow behaviour approximately where the thermal elimination is not made for the as-measured non-Newtonian viscosity, η, but for the Newtonian values, ηο. (η0,Tmax Stands for the adiabatically corrected Newtonian viscosity, due to the maximum temperature of viscous heating by the total deformation work; η0,Tnet Stands for t h e nonadiabadcally corrected Newtonian viscosity due to the gross temperature of viscous heating, regarding also the heat flow out of t he glass sample.) In contrast to these more or less pure non-Newtonian normalized viscosities the so-called normalized non-Newtonian gross viscisity, η/η0, is that viscosity which involves the viscosity decrease due to the pure non-Newtonian flow, determined by structural orientation, as well as the viscosity decrease due to the viscous heating. The various relationschips are analyzed and discussed with the help of recently developed rheological equations.
- ItemUltrasonic measurements and complex elastic moduli of silicate glass melts in the viscoelastic and viscous range(Offenbach : Verlag der Deutschen Glastechnischen Gesellschaft, 1994) Bornöft, Hansjörg; Brückner, RolfThe temperature- and frequency-dependence of the ultrasonic velocity and attenuation in glass melts is investigated up to a temperature of 1400 C by means of an equipment for measuring the longitudinal and transversal ultrasonic waves. The range of dispersion and the attenuation maxima are determined at frequencies 0.5 to 4 MHz completely for the DGG standard glass I and for the glass melts (composidons in mol%): 7 4 Si0₂ , 16Na₂O, 10CaO and 7 4 Si0₂ , 16 K₂O, 10CaO and partly for the glass melt 7 4 Si0₂ , 16CS₂O, 10CaO and for the B₂O₃ melt. The whole number of elastic constants follows from the ultrasonic velocities and from the density in the usual way The properties of these glass melts are compared along the temperature and viscosity. The transition from the elastic solid to the viscous fluid can be characterized by the elastic modul and interpreted as a viscoelasdc response of an unrelaxed to a relaxed dynamic network which depolymerizes reversibly with increasing and polymerizes with decreasing temperature. The internal frieden, Q⁻¹ characterizes the inelastic pordon during the periodical stress of the ultrasonic waves. The activation energy of Q⁻¹ obtained via frequencies and temperature of the maxima of is nearly twice the amount of the viscosity activation energy in the same temperature region.